Optimization of ice-class ship propellers

Transcription

1 Optimization of ice-class ship propellers Definition Study Teunis J. Huisman BSc A Delft University of Technology Master s performed at MARIN Marine Technology Science Resistance and Propulsion

2 Optimization of ice-class ship propellers Definition Study Author : Teunis J. Huisman BSc Student number : Contact Details : t.j.huisman@student.tudelft.nl j.huisman@academy.marin.nl nl.linkedin.com/in/huismanjohn/ Date : A Delft University of Technology Master s performed at MARIN Marine Technology Science Resistance and Propulsion A report submitted to the faculty of Marine Engineering at Delft University of Technology in partial fulfillment of the requirements of a Master s of Science thesis at the chair of Propulsion and Resistance of Ships.

3 II ABSTRACT This document contains a definition study aimed at the formulation of a Master s thesis research problem. Starting from a practical problem concerning ice class ships, which seldomly sail in ice infested waters, a literature review is initiated. The focus is on propeller efficiency and propeller performance in ice conditions. Ship propulsion in ice is addressed first, by considering fundamental principles to derive a power requirement as function of ice resistance. Operation in ice is a significant offdesign condition for a propeller with high loading at low speed. The conventional iterative propeller design cycle is studied together with the impact of ice class requirements on propeller geometry and efficiency. The generally accepted propeller-ice interaction process is described and governing physical processes are extracted. It is concluded that extreme loads are covered reasonably well within the regulatory framework. Ice class rules are reviewed and their background is described to study assumptions. Geometry freedom is allowed since required strength should be analyzed based on ice load cases. Recently, further design space is introduced by the repeal of any blade edge thickness requirements. Based on the literature review, research directions are indicated which make use of the design space within the current ice class rules. A computational framework is proposed in which a propeller geometry can be analyzed for strength and hydrodynamic efficiency. An automated design optimization routine should be used for that purpose. A work plan has been drafted to develop an optimization routine for ice class propellers.

4 III PREFACE This report is the first deliverable in my graduation process. I already had quite some background in ice engineering due to the Arctic minor and my bachelor project. The OMAE2014 in San Francisco was the starting point of the problem formulation. I liked to take the problem again, and focus on it which much more dedication and time. For that reason a good practical problem was needed. Eventually, this resulted in the problem of cargo ships instead of the heavy ice breakers. During a discussion with my professor, Tom van Terwisga, it was agreed that my project could be performed at MARIN. I would like to thank Tom for his confidence, critical remarks and support during the definition phase. Literature review is always time consuming, I especially underestimated the effort to understand the background of the ice class rules. Next to the literature review, the definition study and research problem were continuously in mind. I think that iterating between the two is always required to find a good research problem, although it can be frustrating once and again. I wish to thank my daily supervisor, Gerco Hagesteijn, for his support and discussions during this first stage of the graduation process. I also liked the discussions with Do Ligtelijn. His experience was of much value during the literature and early definition phase. He brought me into contact with his former colleagues at Wärtisilä, Erik van Ballegooijen and Robert Otto. I am also grateful to Ralph Moolenaar for the opportunity to see the manufacturing process and associated margins on geometry and details at Van Voorden castings. During dips in the literature and definition phase I had fruitful discussions with MARIN employees Evert-Jan Foeth, Hannes Bogaert, Joris Brouwer and Jie Dang for which I want to express my gratitude. At the propeller design department at MARIN I had some meetings with Gert-Jan Zondervan, Arjan Lampe and Jan Holtrop to study the traditional design methods and current state of research in propellers. I would like to thank them also. After the literature research, during the in-depth definition phase I had some meetings to talk about the project in order to limit the scope and have some feedback. I want to thank Henk de Koning Gans and Pieter Maljaars from the TU Delft and Erik Vroegrijk from Lloyds for their input and critical remarks. I want to thank my roommates for allowing me in the office and colleague student Reinier Bos for the daily discussions. Finally, I'm very grateful to my fiancée Jantine for her patience and understanding in good and bad times. Above all, I thank the almighty God for giving me everything that was needed in this phase of research already. John April 2015

5 IV CONTENTS Abstract... II Preface... III Contents... IV Nomenclature... V 1 Introduction Practical Problem Baltic Winter Trade Operational Profile Owner Requirements Conclusions Literature Review Approach Ship Propulsion in Ice Fundamentals of Ship Propulsion Regulatory Requirements on Shipping in Ice Ship Performance in Ice Propeller Design Conditions Propulsion Configuration Propellers in Ice Geometry Parameterization Propeller Design Methodology Optimization Principles Cavitation Ice Conditions on the Propeller Propulsive Performance Propeller Ice Loads Propeller-Ice Interaction Prediction Models Ice Class Rules Concluding Remarks Definition Study Literature Conclusions Research Scope Ice Class Propeller Design Opportunities Research Framework Optimization Algorithm Design Method Design Tools Research Problem and Workplan Problem Statement and Objectives Plan of Approach PHASE I: Starting Points PHASE II: Sensitivity Studies PHASE III: Optimization Problem Implementation PHASE IV: Full Optimization PHASE V: Analysis, Reporting and Graduation Preparations Time Planning References... 54

6 V NOMENCLATURE ABS BV BPG CPP CRS DAS DNV EEDI FPP FSICR IACS ITTC MCR NACA NSIDC NURBS SWOT TraFi ULM UPCR American Bureau of Shipping Bureau Veritas Best Practise Guidelines Controllable Pitch Propeller Cooperative Research Ships consortium Double Acting Ship Det Norske Veritas Energy Efficiency Design Index Fixed Pitch Propeller Finnish-Swedish Ice Class Rules International Association of Classification Societies International Towing Tank Conference Maximum Continuous Rating National Advisory Committee for Aeronautics National Snow and Ice Data Centre Non-Uniform Rational Basis Spline Strengths, Weaknesses, Opportunities and Threats Finnish Transport Safety Agency Unified Load Model Unified Polar Class Rules Axial losses for a self-propelled ship W Swept area by the propeller m2 Disk to calculate losses for a self-propelled ship - Stream tube area in undisturbed flow m2 Stream tube area in propeller slipstream m2 Thrust loading coefficient - Propeller hub diameter m Propeller diameter m Stream tube diameter in undisturbed flow m Stream tube diameter in propeller slipstream m Expanded blade area ratio - Kinetic energy rate W Ice thickness m Advance coefficient - Alternative quality index - Approximate quality index for ice class rules - Thrust coefficient - Torque coefficient - Mass flow Mass flow in front in undisturbed flow Mass flow in propeller slipstream Propeller rotational speed Pressure Undisturbed pressure Pressure head over actuator disk Propeller pitch Pressure losses for a self-propelled ship kg/s kg/s kg/s rev/s Pa Pa Pa m W

7 VI Added kinetic energy by actuator disk W Delivered power to propeller W Effective towing power W Shaft power required for ice class W Thrust power W Quality index - Torque Nm Radial coordinate - Ice resistance N Towed resistance N Thrust deduction factor - Thrust N Transverse losses for a self-propelled ship W Axial velocity m/s Radial velocity m/s Tangential velocity m/s Undisturbed velocity m/s Velocity over actuator disk m/s Ship speed m/s Velocity increase after actuator disk m/s Advance velocity m/s Wake fraction - x-coordinate - Number of propeller blades - Angle of attack Behind efficiency - Ideal efficiency - Hull efficiency - Open water efficiency - Relative rotative efficiency - Density of water kg/m3 Tangential coordinate - Viscous dissipation for a self-propelled ship W

8 1 1 INTRODUCTION Every winter lots of ships sail through ice infested waters. Ice conditions are important design conditions. High design demands ensure reliability and safety. However, ice going cargo ships sail only occasionally in ice infested water. These highly powered and ice strengthened ships should perform well in ice and still be competitive in summer time. Therefore, they would benefit in economic sense if their operational fuel efficiency in ice free waters could be improved. An efficiency improvement of only few percent would reduce the fuel bill significantly. These improvements should not lead to worse ice performance, longer development times and higher investment costs. Given a ship which is already assumed to be optimized for ice free water, a focus on propeller design could make this happen. Ice capable propellers already have decent research history; however, a lot of uncertainties are still to be addressed. Therefore, ice class rules govern strength requirements of ice going propellers. While it is difficult to gain efficiency by propeller design for ice free water only, it is expected that efficiency can be improved for ice class propellers. Due to strengthening and ice performance, non-optimal blade shapes are required. An optimization procedure might give valuable insight in the trade-off between efficiency, ice performance and ice strengthening. The purpose of this report is to investigate propeller design methods for ice class ships. It will suggest a Master s thesis research problem and approach on propeller design optimization for ice class ships. The scope of this study is limited to propeller design of cargo vessels designed for trade in the Baltic Area which comply with the ice navigation requirements. It should be noted that propeller design is not the only factor which influences the efficiency of ice going cargo vessels. Propeller design is inseparable from the ship and its operational conditions, otherwise design points could not be established. Also propulsion machinery is important to which a propeller should be matched. Assumptions and simplifications will be made in this respect such that the scope of the study can be focused on the propeller design itself. Limitations are also taken into account with respect to time, means, expertise and the current state of research. A research problem is defined which can be performed within a Master s thesis project. The reader should have basic knowledge about ships, their propulsion systems and sea ice conditions to read this report. For an introduction to ships, shipping and propulsion systems the interested reader is referred to Van Dokkum (2008). Also many textbooks consider propeller design, among which Carlton (2007) is easy readable. Sea ice conditions, appearances and properties have been studied in detail by Weeks (2010). A concise introduction of sea ice is given in the first chapters of Leppäranta (2011). An initial project description as accepted upon the start of this Master s thesis project is given in Appendix A as starting point. By means of a literature review the practical problem is formulated further in chapter 2. To investigate its necessary components further literature research is carried out which is given in chapter 3. A definition study with conclusions and recommendations is performed in chapter 4 after which the research problem is formulated in chapter 5.

9 2 2 PRACTICAL PROBLEM The introduction already revealed the outline of the practical problem: ship owners of ice class cargo vessels need efficient, ice capable propellers. The focus of industry is on high efficiency ships due to regulations, such as the Energy Efficiency Design Index (EEDI), and operational costs. Short return-on-investment times are required. Tight development times are needed to address new markets with suitable ships. This chapter will consider the practical problem in more detail. With regards to propeller optimization, ice class ships suitable for operation in the Baltic area have the biggest prospective increase in open water efficiency due to their operational profile. There is a difference between high powered ice breakers which proceed through any ice conditions, milling the ice with their propellers if necessary, and competitive cargo traders which should be as efficient as possible while having sufficient capabilities in ice. Ice strengthened cargo vessels are commonly not designed for ice breaking operations but for normal operation in ice free waters. An ice free water optimized cargo ship may incorporate only the minimum requirements on ice performance specified by its class notation. However, in that case, there is no consideration about the actual ice performance (Veitch, 2015). Ice operations pose significant off-design conditions. An ice capable propeller should not only withstand ice impact on its blades, it should also maintain thrust at low advance speeds during ice interactions to overcome the high ice resistance on the ship. Ice class propellers differ compared to normal propellers with respect to blade thickness and other geometry adaptations which ensure sufficient structural strength. Ice going capabilities always come at the cost of reduced open water efficiency and vice versa. A compromise for the highest overall efficiency should be searched. 2.1 Baltic Winter Trade About 40% of all cargo in the Baltic region is traded in the ice period as surveyed by Feistel et al. (2008). About 774 million tonnes of bulk were traded in Baltic ports in 2013 according to the Baltic Port List (Wahlström et al., 2014). This is divided in 41% liquid bulk, 26% dry bulk and 33% general cargo. Also container trade is present with an annual number of 9.4 million TEU. These goods are generally shipped with small to medium sized ships, due to size and draft limitations in for instance Denmark s straits and the Kiel Canal. Usually, long term trade contracts ensure trade for ice class ships. During winter, most important ports are infested with ice. Therefore, winter navigation is only lucrative because of the winter navigation system in which high powered ice breakers assist ice strengthened cargo vessels. If necessary, traffic restrictions are issued by the authorities. Only vessels suitable for ice navigation are allowed to trade in the winter season. For that reason, the Finnish and Swedish maritime authorities developed the Finnish-Swedish Ice Class Regulations (FSICR) to ensure sufficient engine output, ice performance and strength of hull and propeller, depending on the ice conditions (Trafi, 2015). The general purpose of the ice class rules is to ensure safe and smooth operations in ice conditions. Ice classed cargo vessels should be able to proceed independently in the broken channels of the Baltic area. The channels are broken by preceding cargo vessels or by an icebreaker. Ice breaker assistance is only required in and limited to very harsh ice conditions. To visualise the operating conditions, Figure 2-1 gives a photo of icebreaker

10 3 assisted cargo vessels. The cargo vessels operate in the newly broken channel by the icebreaker. Figure 2-1: Photo of cargo vessels sailing in a newly broken ice channel by icebreaker IB Urho in the Baltic area. [Courtesy of Arctia Shipping, Helsinki, Finland] Ship owners have to make a choice in the trade-off between high dues for ice breaker assistance and high investment and operational costs due to high ice performance requirements. The number of icebreakers is too few to properly assist vessels trading to Finland and Sweden. Independent ship operations might therefore be required by ship owners. Ideally, ice class is chosen based on scenario and risk assessments in addition to the predicted ice conditions as argued by Hindley and Shepherd (2009). For higher efficiency during the life, ice class should be based on the trade pattern. 2.2 Operational Profile Ice conditions in the Baltic area are only present during winter time, roughly from December till May. Moreover, during a voyage up north, open water is encountered most, ice is encountered during a smaller percentage of the total voyage distance. Also, ice class ships do not always have a trade in the Baltic area or other ice infested areas. Trade of cargo in ice infested waters is the most important motivation for ship owners to choose an ice class. However, unforeseen future market developments and saleability of their ships may be additional arguments. Sometimes the ship is not even required by the owner to ever sail in ice conditions or to perform well in ice. Yet, an ice class is assigned to obtain a better EEDI. The EEDI requirement contains correction factors for ice class ships (Westerberg, 2014). The ship is fully optimized for open water, while just complying with the ice class rules. Hence, most ice class ships do not enter the ice frequently and only sail in ice for 10% of their lifetime or much less. Some ships with ice class never entered the ice during their operational lifetime. Still, good ice performance is required, both by authorities and ship owners as Riska and Kämäräinen (2011) explain: traffic to the northernmost ports in Finland and Sweden ran very slowly for several weeks in the winter of 2011 due to bad

11 4 performance of some ships. Ship-owners claimed that they lost several hundred shipdays in delays. 2.3 Owner Requirements Typical owner requirements concerning propulsion are quite straightforward. To have a competitive ship it should be as fuel efficient as possible to reduce the operational expenditure. This, amongst machinery specifications, requires a highly efficient propeller which should meet some additional requirements which are listed below: The ship should be designed for operations world-wide. Ship owner Wagenborg (2014), for instance, writes in the leaflets: Solidly constructed to Finnish/Swedish Ice Class IA, these vessels are fully fitted to trade worldwide, including in the Saint Lawrence Seaway, Panama Canal and Suez Canal. Notwithstanding the ice class, efficient open water performance is required. Ice class IA should enable shipping without significant restrictions in the Baltic area during average winters. High cargo transport reliability is expected without a need for icebreaker assistance in design ice conditions. High propeller performance in ice conditions is required. It should be possible to deliver maximum engine power to the propeller in all operating conditions. Both maximum speed and maximum bollard pull are required. High bollard pull is beneficial due to high ice resistance. The propeller should be designed for the expected operational profile. An optimal overall performance throughout the operational life of the ship is required. In the ideal case of perfect ship design there should be, compared to other designs, an improvement of the efficiency in all operational conditions, including ice infested waters and heavy weather conditions while having a reduction of vibrations and cavitation hindrance such as noise and erosion. Ship-owner, captain and crew should be comfortable with their competitive ship while sailing through any environmental conditions. In summary, besides optimum open water efficiency, the owner needs reliability on his ship in ice conditions. Hence adequate structural integrity, both with regards to ultimate strength and fatigue is required. Also acceptable noise, vibration and cavitation erosion levels are expected. Finally, the propeller should be able to maintain thrust to propel the ship through ice at sufficient speed as required by the FSICR. 2.4 Conclusions Based on the operational profile and owner requirements it can be concluded that ice class ship propulsion should always be capable of independent ice operations while being optimized for open water behaviour. The practical problem can be formulated as: High propeller efficiency is expected for ice class cargo ships while featuring good ice performance and full compliance with the Baltic winter trade system. Literature review is required to investigate the possibilities and approach for an in-depth research problem. The research problem should be part of a solution to address the practical problem.

12 5 3 LITERATURE REVIEW The introduction in chapter 1 already introduced the problem which was formulated in the previous chapter 2. This chapter elaborates on the literature review approach in section 3.1 after which the literature review is reported in the subsequent sections. Review of literature is necessary to arrive at a research problem within the scope of a Master s thesis. Below the other purposes of literature review are described first. Literature review is needed for a correct interpretation and original processing of existing information conform the practical problem. Current state of research should be investigated, summarized and evaluated. Relationships and dependencies between research projects and individual researchers are needed to place scientific literature in its context. In the line of this Master s thesis, literature review is the basis of the project. It sets the focus and starting point of study. It also prevents duplicating or overlapping research. It should confirm the practical problem and give the basis for the formulation and definition of the research problem. Also the feasibility of the Master s thesis should be supported. Ideally, this definition report should contribute to the research field. By drawing conclusions from literature it should be made sure that new insights are obtained which are not already mentioned or observed. There should be a contribution to new technology development, besides obtaining personal knowledge in the field. This gives the basis for an innovative research project. 3.1 Approach The literature review is approached by supposing that the practical problem can be addressed by an optimization routine as indicated in the introduction chapter. A lot of study has already been performed on ice class propellers regarding the ice loads and performance. From the practical problem it is deduced that there is a need of using this research in propeller design. For design purposes optimization routines may come in beneficial, especially when trade-offs are to be quantified. Design choices and advanced compromises can be directed by the optimization routine. A literature research question is formulated as: How can propeller efficiency of ice class cargo ships in operational conditions be improved by means of an automated propeller design optimization routine taking design constraints and simplifications into account? The terms in this question are considered in more detail in the itemization below: Propeller efficiency is defined as a measure of the relation between torque and thrust. In ice conditions additional factors have to be taken into account to quantify the propeller performance. Operational conditions are to be interpreted as conditions which the ship will face during its lifetime and their respective duration. An optimization is envisioned in which all conditions are addressed. Special focus is on ice conditions. Automated refers to minimal user interaction during both the propeller analysis for performance and strength and the optimization itself. The user should specify design objectives, parameters and constraints. Design is to be conceived in this context as the development of shape and scantlings of the propeller blades. Neither the matching to the main engine nor a detailed analysis of propeller-hull interaction is to be taken into account to limit the scope of work.

13 6 Optimization Routines are existing iterative algorithms which could be used to assess the current state of knowledge by means of a design optimization problem. No extensive literature reviews will be carried out on the algorithms, only the structure of optimization problems will be reviewed. Constraints such as blade strength, cavitation and pressure pulses will be considered shortly. Ice loads, however, will be considered in more detail to study the applicability of the current state of research for practical propeller design. Compliance with classification societies and other regulatory frameworks is required. Simplifications refer to the amount of simplification which can be or should not introduced in propeller analysis tools without losing crucial sensitivity to design parameters. The literature question can be addressed by fist considering ship propulsion in ice in general in section 3.2. Operational aspects and design points for the propeller are studied. Also the ice performance of ships is reviewed shortly to estimate, amongst others, the ice conditions on the propeller. A comparison for different propulsion configurations is made as well. Section 3.3, subsequently, zooms in to the propeller. Its geometry is briefly described and parameterization methods are reviewed. It is also noted that propeller design is a specialism which requires experience. Therefore, common propeller design methodology is studied as well. After that, the focus is shifted to ice conditions again. Ice conditions and scenarios are described. Common ice interaction processes are described. Finally, propeller efficiencies are considered both in ice and in ice free conditions. Since the interaction of a propeller with ice induces additional loading, section 3.4 focuses on propeller ice loads. First the origin of blade loads is discussed. After that, load prediction models are reviewed. Ice class rules are considered with their background and assumptions. Finally, the propeller design consequences due to ice are explored. Conclusions about the current state of research and the gaps in knowledge are given in chapter 4. Research directions are indicated, after which the research problem and its intended approach are formulated in chapter Ship Propulsion in Ice Ship propulsion in ice should be addressed before focusing on the propeller to study design points and operational profiles. The ice performance of ships is reviewed to estimate, amongst others, the working point and ice conditions on the propeller. First the fundamental principles of ship propulsion and the corresponding definitions are given. Then ship performance in ice conditions is considered. Definitions of different ice regimes and conditions are summarized. Also the ice breaking process is explained. Furthermore, regulatory requirements for shipping in the Baltic area are addressed since they give an indication of a required design point for the propeller. After a discussion about scenario based design to find the design points, this section is closed with a consideration about the ice performance of different propulsion configurations.

14 Fundamentals of Ship Propulsion In this work the conventional standard definitions will be used to express the power and efficiency of a ship propeller. These are summarized in Figure 3-1 (MAN Diesel & Turbo, 2015a, p.14). Figure 3-1: Basic definitions of ship propulsion. [Courtesy of MAN Diesel & Turbo] Many factors influence the total efficiency of ship propulsion; however, focus is on propeller efficiencies and in this report. Propeller efficiency is related to the performance of a propeller in a homogeneous inflow. The behind efficiency also takes the actual velocity of the behind hull inflow into account with the relative rotative efficiency. Non-constant and rotational flow in the wake of the ship influence propeller efficiency. As indicated in Figure 3-1, the propeller efficiency can be written as the relation between the delivered thrust power by the propeller and the required power to overcome propeller torque : (1) where propeller thrust, propeller torque and propeller rotational speed. Furthermore, in the last equality the following standard non-dimensional definitions are used:

15 8 (2) Thrust coefficient, torque coefficient and advance coefficient are made nondimensional by a combination of density, rotational speed and diameter. It should be noted that the incoming velocity is a circumferentially averaged quantity in behind hull conditions Actuator Disk Theory By means of mass, momentum and energy conservation, propeller efficiency can be derived if the propeller would work in an ideal, incompressible, inviscid flow with infinite blades without inducing rotation into the slipstream. The thrust is assumed to be uniformly distributed over the propeller disk and uniform inflow and outflow are supposed. Only axial kinetic energy losses are considered while rotational losses, viscous losses and non-uniformities in the inflow are neglected. This method is referred to as actuator disk theory since the propeller is represented with a thrust generating disk as visualised in Figure 3-2 (Klein Woud & Stapersma, 2008, p.391). Figure 3-2: Actuator disk representation of propeller action. [Courtesy of Klein Woud & Stapersma] The disk acts in a circular fluid column bounded by streamlines. Due to increased velocity by propeller action, the streamlines contract. No mass flow is allowed over a streamline boundary. Conservation of mass and momentum can be applied. A short derivation is given here, details can be found, if needed, in Carlton (2007), Klein Woud & Stapersma (2008) and many others. Rankine and Froude were the first to apply momentum theory to propellers. Amongst other properties of ideal propeller flow, the ideal efficiency can be derived by means of momentum and kinetic energy considerations. Also conservation of mass applies for the three disks in Figure 3-2 such that the mass flow : Conservation of momentum says that the exerted force on the fluid should equal the net outflow of momentum where momentum is the product of mass flow and velocity: (3)

16 9 (4) where Equation (3) has been used for the last step. Subsequently, the energy conservation equation in the form of Bernoulli s equation may be applied, since it is assumed that the flow is incompressible, inviscid and irrotational. Bernoulli can only be applied in front and after the actuator disk. The pressure is discontinuous over the disk. Also note that the pressure at the inlet and outlet is the same and equal to the undisturbed pressure. Bernoulli is only valid when no external force is applied. (5) (6) where Equation (5) is valid after the propeller action while Equation (6) is valid in front of the propeller. These equations can be solved for the pressure rise over the propeller which can be used to form an alternative formulation for the thrust: (7) Equating Equation (4) and (7) and solving for gives a relation between the advance velocity and the velocity increase over the propeller disk : (8) Substituting Equation (8) into Equation (4) gives a definition for the thrust: (9) Traditionally, this equation is rewritten in the form of a non-dimensional thrust loading coefficient which is defined as: (10) where Equation (9) has been substituted. Alternatively, when substituting Equation (2), can be expressed as: (11) Equation (10) is a simple quadratic equation for quadratic formula according to: and can be solved with the

17 10 (12) Only one solution is physically feasible. The relation is needed for an expression of the delivered thrust power of the propeller, as defined before in the expression of the open water efficiency of the propeller. Moreover, kinetic energy has been added to the fluid. Hence, the added power can be written as: (13) The ratio between the effective delivered thrust power and the power needed for delivering this thrust can be regarded as a quantification of the ideal efficiency of a propeller: (14) From this equation can be seen that the ideal efficiency of a propeller decreases with higher loading on the propeller disk area due to kinetic energy in the slipstream which cannot be used effectively. It is more efficient to give a large mass of water a small acceleration than vice versa Quality Index A measure of the energy conversion and involved losses can be obtained from the relation between and. Traditionally this is called the quality index. (15) As seen, this equation is not zero for like. Hence it can also be used as a measure of the performance in bollard pull conditions: (16) In order to derive an alternative equation for the delivered power as function of the required power use is made of the standard definitions. The definition of can be solved for rotational speed :

18 11 (17) which can be substituted in Equation (2): (18) Torque can be expressed in the required power where Equation (17) can be used as well. (19) Substitution into Equation (18) gives an expression for the thrust as function of delivered power: (20) The factor can be regarded, similar to Equation (16), as a kind of quality index for the performance of the propeller in bollard pull conditions where. Sometimes it is referred to as the merit coefficient of a propeller (Carlton, 2007). It should be noted that no considerations about the relation between ship resistance and required thrust have been made. Expression (20) will be used in a subsequent paragraph to explain the power requirement in the ice class rules. To do that, propellerhull interaction effects are considered first in the next subsection Propeller-Hull Interaction Propeller-hull interaction and hull efficiency is regarded according to Kerwin & Hadler (2010). The suction from the propeller of the flow in the aft ship can be regarded as thrust deduction due to low pressure or as an increase in resistance. The thrust deduction factor is defined as: (21) in which the thrust and the towing resistance of a hull. In most cases is assumed to be constant. In reality it will vary as function of loading conditions and ship speed. A second effect is the reduction of inflow into the propeller due to the boundary layer on the hull. The wake is highly non-uniform with high turbulence and rotation levels. The wake fraction expresses the difference in ship velocity and averaged propeller inflow speed :

19 12 (22) Larsson & Raven (2010) indicate that momentum loss in the boundary layer may be compensated by the slipstream of the propeller to increase propulsive efficiency. The wake fraction and thrust deduction factor can be used to express this effect in the hull efficiency (23) in which the effective towing power. This efficiency can be larger than one Energy Losses Traditionally, Equation (1) is used to compare different propellers. Olsen (2004) critically remarks that this method gives no details about the background of differences between propellers. Olsen indicates that more insight can be obtained by comparing different energy losses for the propeller based on momentum flux considerations. Terwisga (2013) summarizes these ideas and expresses the required power in the losses in the wake: where axial losses, the loss of effective energy by transversal losses due to fluid rotation, pressure losses and the viscous dissipation. The subscript refers to the wake of a self-propelled ship. The energy balance gives the expression for the losses at a certain disk after the propeller: (24) (25) in which and the axial, radial and tangential velocities at respectively, while represents the pressure. Undisturbed conditions, in front of the selfpropelled ship, are represented by velocity and pressure. The first term contains the axial and transversal losses while the second term accounts for the pressure losses. Pressure losses and viscous dissipation are related and should both be considered in terms of heat production. Kinetic energy is dissipated into heat due to the turbulence cascade and skin friction. Axial losses compromise both retarded flow due to hull friction and accelerated flow due to propeller action. Axial losses of propeller action in ideal fluid are represented by in Equation (14). In non-ideal conditions the combined losses of hull and propeller should be considered. Transverse losses are mainly due to hull and propeller induced rotation in the flow Regulatory Requirements on Shipping in Ice Shipping is subjected to all kinds of regulations concerning safety and environment. Ice poses significant ship demands, not only on safety but also on performance as explained in chapter 2. Ice class ships are ships with such structure, engine output and other properties that they are capable of navigating in difficult ice conditions, with the

20 13 assistance of icebreakers when necessary as formulated by the Finish Swedish Ice Class Rules (FSICR) issued by Trafi (2010, p.4). The most common ice conditions for ice class cargo ships are old ice channels covered with brash ice. Brash ice is the wreckage from other forms of ice (NSIDC, 2015). It is formed due to the repeated breaking of a newly frozen consolidated layer on top of the broken channel. The channel becomes filled with brash ice which may even be thicker than the surrounding level ice as sketched in Figure 3-3. Cargo vessels should be able to push the brash ice aside to proceed. Sometimes, the resistance in a brash ice channel can be higher than in the surrounding level ice, depending on ice breaking capabilities of the bow. Old ice channels, however, are often found in the navigation channels in shallow areas. Figure 3-3: Sketch of a possible cross section of a brash ice channel [Sandkvist (1978), obtained from Kujala (2007)] Harsher ice conditions, such as strong level ice and ice ridges, are managed by ice breakers after which the broken channel can be used by commercial ships. If no assistance would be provided, the ships would need much more power and strengthening for continuous ice breaking in all kind of ice regimes. Level ice is floating ice with a flat surface which never has been broken or deformed. Ice ridges, however, are formed when wind, ocean currents, and other forces push sea ice around into piles of broken ice pieces (NSIDC, 2015). Although the FSICR prescribe engine power and assume certain ship behaviour a more formal statement on ice performance is given. If ice model tests are used to define the engine power, it should be based on the design requirement of a minimum speed of 5 knots in brash ice channels with a thickness of 1.0 m for ice class IA (TraFi, 2010, p.9). Ships designed without ice model tests are assumed to have this performance based on a minimum required engine power Ship Performance in Ice The background of the Baltic trading system is thoroughly reported by Juva and Riska (2002) in a research report. They explain the basis of the power requirement. Riska et al. (1997) are focussed on the explanation of the ice resistance. Ice performance of a ship is influenced by a number of factors. First there is an added resistance due to ice while ice conditions on the propeller may influence its ability to provide adequate thrust. Also the machinery installation and propulsion concept are important. It should be able to deliver high thrust at low speeds due to the high resistance Ice Breaking Principles Traditional ice breaking principles in level ice are described by for instance Lindqvist (1989). Figure 3-4 gives a quick overview of the ice breaking process. In the breaking phase the ice will be bent and to some extent crushed until the bending moment is high enough to break the ice in one or more pieces. The broken ice floes will then be turned in the rotative phase until the piece is aligned with the bow surface. Due to forward

21 14 motion the ice pieces will submerge further and slide along the hull. Due to buoyancy of the ice, pieces will float up depending on hull shape, ice thickness and size and speed. Figure 3-4: Illustration of ice breaking principle. [Obtained from Jalonen (2012)] Hull design could play an important role with regards to ship reliability in ice. A spoon shaped bow will be more effective in breaking the ice than a bulbous bow which encounters the ice perpendicularly. Considering the operational profile as sketched in chapter 2, an open water optimized hull shape is to be preferred. Optimization procedures can help to make a good compromise between the performance in ice and regular performance in open water. Moreover, hull design can also be such that ice conditions on the propeller are avoided. Large ice breakers, for instance, barely face ice conditions on their propellers due to their large draft as Airaksinen and Marttila (1976) indicate. Due to draft limitations with respect to operational area and hull form, cargo ships generally have limited draft. Propellers are exposed to ice conditions especially in ballast conditions. A new development is the Double Acting Ship (DAS) concept in which ships proceed astern through ice conditions as patented by Aker Arctic (Harjula & Salmi, 1993). The bow can be optimized for open water while the aft ship is optimized for both high hull efficiency and good ice performance. In this concept the propeller is also designed as a milling tool. Conventional merchant ships do not feature this ability since an expensive podded propulsion concept has to be applied while efficiency in ice free waters may be lower than normal shaft line propulsion. Extensive research is available on the ice breaking resistance and its fundamentals. However, within the scope of this report, ship resistance due to ice is only to be approximated to define design points for the propeller. Therefore, the ice resistance formulations for the Baltic area are addressed in their final formulations only. Other formulations are not considered here Ice Resistance Formulations Based on ice resistance measurements, a linear relationship with speed is assumed both for level ice resistance and brash ice channel resistance. Level ice resistance is assumed to depend mainly on the ice thickness. Three groups can be distinguished

22 15 further: assumed properties of the ice, ship shape and ship size. Based on earlier formulations of the resistance by, amongst others, Lindqvist (1989), the rule ice resistance has been formulated. Brash ice resistance is based on the bow shape and mid ship length, buoyancy of the brash ice, ship speed, friction and the thickness and shape of the channel. A consideration of the resistance components has been made by Riska et al. (1997). The resistance is mainly due to mass inertia of ice blocks which have to be pushed away. A much simplified form of ice resistance is used in the FSICR. Level ice resistance and brash ice resistance are combined with the assumption that the consolidated layer on top of the brash ice resembles level ice. Speed dependency, almost linear relationships and physical constant factors such as density and ice strength are replaced by constants. The final equations are presented in Power Requirement In the development phase of the FSICR the assumption has been made that ship performance in ice can be completely quantified in terms of propulsion power. Furthermore, it is assumed that sailing through ice can be compared with bollard pull conditions. Hence, Equation (20) is used to develop a power criterion. Due to high resistance and low speed in ice conditions, the total thrust in Equation (20) may be assumed to be equal to the total ice resistance. The quality index is assumed to be constant. Together with the other constants involved, an approximation for this factor,, is given based on the propeller arrangement to include hull efficiency. The factor is also assumed to take thrust decrease due to ice interaction and the difference between bollard pull conditions and five knots forward speed into account. After rearrangement of Equation (20), the required power in ice conditions can be given as: (26) Final approximations for have been derived based on full scale measurements. For single fixed pitch propellers = 2.26 is assumed. Since a controllable pitch propeller (CPP) is able to deliver more bollard pull than a comparable fixed pitch propeller due to engine limitations, should be multiplied with 0.9 for a CPP according to the FSICR and Riska et al. (1997). As given by Equation (26), the current rules base engine power on resistance and propeller diameter. No requirements are given about the machinery plant. Big diameter fixed pitch propellers require high torque at a low rotation rate. Normal direct driven diesel engines cannot deliver this demand. Hence, the ship is not really capable of ice conditions. A large diameter propeller is good for efficiency and for required installed power, but is not suited for sufficient ice performance. This problem is also found in a brochure by MAN Diesel & Turbo (2015b) which states that modern ships with larger fixed pitch propellers, lower rotational speed and ultra-long stroke two-stroke engines will comply easier to the FSICR. Therefore, the requirement of Equation (26) does only work when the engine is well matched. The biggest reason (55% on average from 2000 to 2011) for poor ice performance and need for ice breaker assistance in normal ice conditions is the nonavailability of sufficient power in ice conditions as Vedenpää (2012) surveyed.

23 16 Unfortunately, no distinction has been made in fixed or controllable propellers in this report. It can, however, be assumed that controllable pitch propellers are more capable of full power absorption at low speeds and high ship resistance. A minimum thrust requirement in ice conditions would maybe work out better than a power requirement since engine performance is excluded and will be left to the creativity of the designer Propeller Design Conditions A proper propeller design is matched to its main engine and optimized for the intended operations of the ship. First the matching principle is addressed, and then definitions about the design points are given. Furthermore, the mission profile of a ship is considered and the choice of design points is discussed Propeller Matching Design requirements on ship propulsive performance can already be determined without a propeller design. There may be a required ship speed, power or desired rotation rate for a power take off (PTO). The propeller should be matched to a certain engine with its specific characteristics. Usually, the engine load diagram and propeller curves are used to design and match the propeller. The engine load diagram specifies the operational limits of the engine with respect to power as function of the rotational speed. A propeller curve represents the performance of the propeller in a certain operating condition of the ship. Engine load limits are not discussed in detail in this report. It suffices to describe Figure 3-5 in this respect. Line 3 represents the speed limit of the engine; line 7 is the power limit. Line 4 and 5 are properties of the diesel engine. In this case the engine is matched to deliver maximum power in all conditions between line 2 and 6. It should be noted that the axis are logarithmic. Line 1, 2 and 6 represent propeller curves. For a short derivation, note that ship resistance can be assumed to be proportional to the square of ship speed. Hence, power will be proportional to the third power of ship speed. For constant pitch propeller operation, ship speed will be proportional to the propeller rate of revolution. In a logarithmic diagram this relation will be presented as a straight line. In heavy conditions the propeller is more heavy loaded and will require more power. Calculations or measurements are needed to find the exact relationships for a certain ship type. The propeller should be designed such that sufficient power is available in all operating conditions, during acceleration and in ice conditions. Imagine that the conditions will be harder than line 2, for instance due to ice. The propeller curve will move to the left. In case of a FPP, full power is not available. The engine load diagram can aid in this process to find a correct balance and matching points. For controllable pitch propellers the diagram can be used as well. It is extended with constant pitch lines. Also the combinatory curve is usually given. This curve presents the optimum pitch and rotational speed settings as function of the ship speed.

24 17 Figure 3-5: Standard engine load diagram of a turbocharged diesel engine. [Courtesy of MAN Diesel & Turbo, 2015a, p.30] Definition of Design Conditions Before designing the propeller and its matching to the main engine a proper selection of design conditions should ensure that the propeller can be designed and optimized for its intended operational conditions. Typical operational requirements, or design conditions, set the demand on powering, manoeuvring, propulsion performance and strength. Connections are made towards the propeller and its arrangement, the propulsion configuration and power delivery. The design conditions have a big impact on the balance between design and off-design properties of the propeller. The importance of the off-design conditions depends on ship type, mission profile and the preferred optimizing conditions. Van Beek (2004) stresses the importance of the evaluation of all environmental and operational conditions and their impact throughout the whole design cycle. If loading conditions, speed, engine performance and weather and sea conditions are also considered, the propeller can be optimized for all conditions, such that optimal efficiency is obtained during commercial usage of the ship. This concept is referred to as scenario based design by Hindley and

25 18 Shepherd (2009) for which they argue that the ship and its propeller can be optimized for intended operations only. This will contribute to safety, lower emissions and better performance in the envisioned conditions. Scenario based design is not commonly used in propeller design. In other branches, however, scenario based product design is widely accepted and more and more used as Anggreeni and Van der Voort (2009) indicate. The design condition is the most common operating condition, usually this is the fully loaded condition at 90% of the maximum continuous rating (MCR) of the engine. The propeller should be optimized for a required ship speed, at specific draft and sea margin. Revolutions and engine load will be calculated. Also a required rotation rate and power take off may be specified. Alternatively, speed may be calculated when power is prescribed. Often the main engine is already chosen. Off-design conditions are conditions, different to the design condition, in which the propeller should still perform well. These conditions change the operational point of the propeller and should be studied on their hydrodynamic performance. Due to increased ship resistance the operational point of the propeller may change. Resistance may increase due to fouling, high sea states and ice conditions. Sailing in ballast conditions and towing operations are also regarded as important off-design conditions. Ligtelijn (2010) mentions that off-design conditions are in fact additional design conditions. Often, the off-design conditions are more critical with respect to propeller design than the dominant design condition. Special conditions are those in which the operation of the propeller is not stationary. For instance during shaft reversal, acceleration and manoeuvring. Sufficient power should be available for such conditions Mission Profile The mission profile of a ship indicates the intended operational usage and is mainly influenced by ship type, trade and environmental conditions (Carlton, 2007). Time distributions of speed, loading condition and ship resistance are specified in the mission profile. A typical mission profile and chosen design conditions is not needed in this text since it is noted by Verhulst et al. (2012) that there is a large diversity in mission profiles and operational conditions for ships within the same deadweight range. Also the required minimal operational speed and design points differ per ship owner. For propeller design, the unique mission profile of each ship should be considered. Also ice conditions are part of the mission profile for ice class ships. In principle, the traditional design methods do not change as ice resistance can be regarded as a strong off-design condition with a heavy propeller curve. An ice class propeller should be designed for at least two design conditions, one in open water and one in ice conditions Propulsion Configuration A controllable pitch propeller (CPP) is mostly used for heavy ice classes, ships which should be capable of independent operations in ice conditions. Light ice classes, however, will have a fixed pitch propeller (FPP) for optimized open water efficiency and lower investment costs. FPP can have higher efficiency in the dominant design condition due to streamlined smaller hub geometry. It should be also noted that the pitch distribution of CPP blades is usually also optimized for the dominant design condition. The choice depends on the mission profile. If good performance is required in multiple off-design conditions without a dominant design condition a CPP is advantageous. The

26 19 pitch settings can be optimized over the operational range such that full power is available in all conditions. A FPP cannot adapt its pitch and would require very high torque at low rpm to deliver the required thrust. Normal diesel engines cannot provide this torque due to surge limitations (Stapersma & Klein Woud, 2008). Ghose and Gokarn (2004) mention several advantages and disadvantages of a CPP compared to a FPP which are not related to ice conditions such as improved manoeuvring, no need for shaft reversal for backward thrust and the possibility to adapt the pitch to the most efficient operational point of the main engine. A CPP, however, is more vulnerable to damage due to the pitch mechanism. Additionally, as Ligtelijn (2010) indicates, reduced pitch operation with a CPP reduces efficiency. Also pressure side cavitation is likely to occur. In extreme off-design conditions the propeller design pitch is chosen in between the conditions to avoid erosive cavitation at heavily loaded, reduced pitch operation. Some ships, after the reduction of ship speed since 2008, feature a nozzle which gives higher thrust at low speed. Since ice conditions can be compared with bollard pull conditions, a nozzle can be beneficial in ice. The nozzle also acts as a propeller protection against large ice blocks. However, the nozzle may become clogged with ice blocks, which will cause thrust breakdown due to violent cavitation (Veitch, 2015). Also, nozzle performance in ice may be reduced by circulation disturbances due to ice blocks and blockage. For general cargo ships with limited draft a nozzle is not preferred. Big ice breakers do have nozzles; however, their draft is large enough to avoid regular ice interaction (Wind, 1983). 3.3 Propellers in Ice Following the ITTC (1999) a propeller is defined as a ship propulsor which consists of a central hub fitted with a number of foil shaped blades. Lift is generated by the blades when the propeller is rotated. One component of the lift force produces the desired thrust to propel the ship and the other component creates drag. This drag, manifested as torque, must be overcome by the engine to sustain rotation. Propeller performance can be quantified using the relation between thrust and torque. For a given thrust, the propeller should be designed for minimum required torque. In this report the standard ITTC propeller terminology is used (ITTC, 1999). A complete description of propeller geometry is not given here. The interested or unfamiliar reader is referred to Carlton (2007) or for an initial introduction to Kuiper (2002). This section not only considers the parameterization of a propeller and its design methodology, also its design constraints and performance are addressed. Especially when interacting with ice, the propulsion efficiency and strength need additional consideration Geometry Parameterization This section shortly considers the basics of propeller parameterization which should ensure a complete specification of propeller geometry. Parameterization is required for systematic variations and optimization algorithms. Propeller parameters can be divided into three main groups: 1. Global parameters such as diameter, hub size and number of blades. 2. Distribution functions to describe the chord, chamber, pitch, rake, skew and thickness distributions in radial direction.

27 20 3. Parameters to describe local details such as hub shape, root fillets, anti-singing edge and tip geometry. The standard ITTC definition of the coordinate system is used. The propeller is represented in 2D foil sections over the propeller radius. The distribution function should be described in as few parameters as possible. Nowadays, the Non-uniform Rational Basis Spline (NURBS) has become the standard to describe complex geometries in as few control points as possible. A NURBS always produces a smooth geometry with continuous second derivative such that flow separation is delayed. Sectional foil shapes may also be described using a closed NURBS. Alternatively, standard shapes, such as the NACA profiles may be used. For analysis purposes the parameterization is converted into a point cloud or 3D object, depending on the required input, level of detail and smoothness Propeller Design Methodology Propeller designers often have to make numerous choices and compromises. Also, propeller design usually is performed when hull form and propulsion configuration have already been defined. Therefore, a propeller should be optimized by taking into account numerous constraints. Figure 3-6 gives a simplified representation of the propeller design process. In reality the process is highly iterative, especially between phase III and IV in order to converge to the optimum design, tailored to a specific ship and ship owner demands. Phase I is needed to set the starting point of the propeller design. Together with phase II the estimated propeller performance is determined. A datum propeller is designed based on existing propeller designs and systematic propeller series. If needed, the main characteristics can be evaluated with simplified numerical or empirical tools. PHASE I: Preparation PHASE II: Initial design PHASE III: Detailed geometry PHASE IV: Detailed evaluation Propeller questionnaire: Ship data and design conditions Owner requirements Consultancy on propellerhull interaction Comparable projects Selection and optimization of main parameters, based on estimated performance and expierence Definition of design constraints and objectives Investigation of design space Evaluation of main characteristics Adaption to ship wake while maximizing efficiency Preferred pitch distribution while constraining the cavitation extent and possible harmful behaviour. Sectional shape and radial camber distributions Iteration between propeller geometry parameters until a suitable compromise is found between main objectives Figure 3-6: Different phases in the propeller design process. Evaluation of off-design conditions to find the best compromise Detailed flow analysis Hull Interaction Performance optimization by fine tuning the design parameters and exploring the design space which is left Compatibility with class rules regarding strength and performance Alternatively, when focusing on design constraints in more detail, the iterative propeller design process may be represented as in Figure 3-7. Five main topics are presented with their keywords which need attention during the propeller design process. In this figure also ice performance is included. Not only hydrodynamic analysis is important, also the strength and matching to the main engine should be taken into consideration.

28 21 Hydrodynamics Thrust, Torque Cavitation Pressure fluctuations Noise Ice Performance Operation Ship constraints Operational profile Design conditions Transient operations Machinery Matching Drive train Pyramidal Strength Structure Ultimate stress Fatigue Spindle torque Weight Manufacturability Figure 3-7: Iterative Propeller Design Spiral The terms in Figure 3-7 are considered in more detail: Hydrodynamics is the study of the interaction of the solid propeller blades and the liquid water. Thrust & Torque are properties of the propeller in given operational conditions from which propeller efficiency is derived, Equation (1). Cavitation is a source of noise, vibration and erosion and should be considered as a constraint. The propeller induces hydro-acoustic pressure fluctuations due to the passing blades in an uneven wake field and cavitation dynamics. Pressure fluctuations are an important source of on-board vibrations. As rough estimate the first harmonic of thrust variation may be used according to Foeth & Lafeber (2013). A short comment on cavitation is drawn in section Ice Performance should be studied for ice class propellers to meet the requirements of the FSICR. The propeller should be able to develop the required thrust in ice conditions while torque due to ice cutting should be minimized. Ice performance is considered in more detail in sections and Structure refers to the structural integrity of the propeller to withstand the hydrodynamic and ice loading. Ultimate stress is the amount of stress that propeller material is expected to bear without failure. Fatigue is the weakening of material due to cycling loading. Since both hydrodynamic loading and ice loading are cyclic to some extent, fatigue analysis is important in propeller design. Spindle torque is the torque acting about the spindle axis about which a CPP is rotated for pitch adaption. Spindle torque is limited by the actuator forces of the pitch adaption mechanism. Weight is important for propeller investment costs, especially in case of a FPP. Although sufficient strength is important in propeller design, no further consideration besides required compliance with class rules is deemed necessary in this literature review. Load cases and strength analysis according to class society will be considered in section Manufacturability should be kept in mind. Adaptations in propeller design should always be bigger than production allowances.

29 22 Propulsion machinery should be taken into account as already seen in section The propeller should be matched to a certain drive train which consists of shafting, gear box and main engine. It should be noted that pyramidal strength is required in which the propeller blades are the weakest link. Operation is part of the propeller design spiral as already explained in more detail in section Traditionally, the propeller design cycle of Figure 3-7 requires significant user interaction. Automated optimization routines could help the designer in the design choices. Ultimately, the complete design can be automated with well-chosen objectives, analysis tools, design parameters and constraints Optimization Principles Optimization is the process of determining the best solution to a certain problem. Or, as Papalambros & Wilde (2000) define: improve the design so as to achieve the best way of satisfying the need, with all the available means. A formal definition of optimization is also given in this text: The determination of values for design variables which minimize the objective, while satisfying all constraints. Optimization provides a systematic problem solving approach with minimal human interaction. It also provides insight in design problem characteristics, the underlying physics and their sensitivities and weaknesses. A short comparison with a traditional design method is presented in Figure 3-8. Figure 3-8: Comparison between traditional design process (bottom) and optimization-based design process (top) Design variables are variables by which the design problem is parameterized. The design space is the set of all possible designs for which where and are the lower and upper bound of the design variables respectively. Usually, an optimization problem is given in its Negative Null Form (NNF). An objective is a quantity as function of the design variables that is to be optimized. Optimization is a minimization of the objective,, in case of a NNF. A constraint is a condition that has to be satisfied. Inequality constraints or equality constraints should be defined for a NNF such that and respectively. Typically, an optimization problem consists of an iterative procedure with a model and a optimization algorithm like indicated in Figure 3-9a. An optimal solution should be achieved using the smallest number of function evaluations. Multi-objective optimization problems, with objective vector where, are often used to quantify design trade-offs. A possible design space is given in Figure 3-9b. Pareto points are points on the Pareto set for which no other feasible point exists that has smaller

30 23 without having a larger as Papalambros & Wilde (2000) define. There is no improvement of an objective possible without worsening another objective. The designer has to make the choice between these multiple objectives. Often, the weighted sum with proper, designer preferred weight factors, is used for this purpose. Constants Model Design Variables x Optimization Responses f, g, h Optimizer Figure 3-9a: Iterative optimization cycle Figure 3-9b: Visualisation of a Pareto set Cavitation This section considers the basic principles of propeller cavitation. Cavitation is regarded as a constraint which should be taken into account during propeller design. Cavitation is formed when the liquid s pressure drops below its vapour pressure (White, 2011, p. 34). Cavitation can have several appearances as seen in a typical propeller cavitation sketch in Figure Cavitation should be taken into account in every propeller design. Cavitation may influence propeller performance in terms of vibrations, noise and erosion. A well balanced design allows as much as possible cavitation such that no hindrance is experienced.

31 24 Figure 3-10: Typical cavitation sketch. Several forms of cavitation are present on this propeller blade. [Courtesy of Carlton, 2007, p.239] Carlton (2007, p. 219) states that the cavitation number should be one of the design constraints for cavitation in propeller design. The cavitation number can have several forms in which the location of consideration is different. The difference between the local pressure and the vapour pressure is an indication for cavitation inception. Bernoulli s equation, neglecting gravity effects and assuming steady, incompressible, inviscid flow, can be used to express the pressure variation along the streamline of a body, or in this case a cavity surface: (27) The cavitation number is formed from this relation. It gives the relation between the static and dynamic pressure head: (28) in which and are characteristic pressure and velocity, respectively, is the density of water and is the vapour pressure. The cavitation number is a measure for the vulnerability to cavitation. Higher numbers indicate that cavitation is less likely to occur. White (2011) states that depending on geometry, a flow has a critical cavitation number below which the flow will cavitate. Hence, the cavitation number is suitable for a rough prediction of cavitation. Cavitation inception properties can be expressed in an inception diagram. This gives the risk on cavitation as function of the cavitation number and different load levels of the propeller. The assumption can be made that cavitation inception will occur when the pressure on a blade is equal to the vapour pressure. Based on operating conditions a diagram can be constructed which gives an indication off the risk on cavitation, as given in Figure 3-11.

32 25 Figure 3-11: Typical cavitation inception diagram as function of thrust loading and cavitation number. [Courtesy of Kuiper, 2012] Alternatively, the inception pressure may be given as function of the dimensionless advance coefficient or the angle of attack for two dimensional sections. This diagram is referred to as the cavitation bucket. Normally, distinction is made between sheet cavitation and bubble cavitation only. The cavitation extent is not specified. This should be determined with more advanced tools or measurements. Cavitation may be more severe in ice conditions. Flow blockage due to ice induces very low pressures which will induce cavitation. Extensive cavitation studies for propellers in blockage and milling conditions have been performed by Sampson et al. (2009). He concludes that cavitation is an additional significant factor to take into account for propeller-ice interaction. Cavitation contributes to a loss in mean thrust and torque and leads to an increased dynamic loading as compared to propeller-ice interaction tests without cavitation. At the start of the propeller design the power density, cavitation number and wake quality should be optimized to reduce the dominance of the cavitation constraint (Ligtelijn, 2010) Ice Conditions on the Propeller Ice conditions on the propeller depend on several factors. Not only the environmental conditions, but also the breaking of the ice by the hull is important as highlighted in section Ice blocks beneath a ship eventually come into contact with the propeller if not cleared to the sides or blocked and deflected by some propeller protection. There is also an influence of propeller suction. Large draft or aft trim give less ice on the propeller. Ships in consideration are shallow drafted. Especially in ballast conditions the propeller will interact with ice, since the propeller may partly even be surfaced. Ice conditions on the propeller depend on the operational mode as indicated in Figure 3-12 as function of advance velocity and propeller rotational speed. Subsequent propeller blade sections are sketched.

33 26 Figure 3-12: Ice interaction as function of operational mode. [Copied from Soininen & Veitch, 1996, p.9] Interaction of a propeller with ice is a term to indicate the mutual effect of ice on the propeller. This interaction can be divided into contact interaction and non-contact interaction. For the description of the contact interaction the following definitions are required: Contact occurs when the propeller comes into contact with an ice block in any way whatsoever. Impact is the contact with ice on a flat part of the propeller blade, i.e. on the face or back of the blade. Impacts occur most in quadrant II and IV. (Soininen, 1998) Milling is the ice erosive contact between ice block and the leading or trailing edge of the propeller blade in quadrant I and III respectively. The ice is milled and scraped by the propellers edge part due to its rotary motion. (Soininen, 1998) Crushing occurs when the ice is pressed by the propeller blade until the crushing pressure of ice is reached. The ice will pulverize in small particles. Spalling is the mechanism due to contact which produces cracks and subsequently flakes of ice. Flaking is the process of pushing away flakes from the ice block by the propeller blade. The difference with spalling is illustrated in Figure Shearing often occurs in combination with crushing. If the shear strength of the ice in a certain direction is higher than the crushing strength, the ice will fail by a shear process, i.e., parts of the ice slide along each other in a deforming motion. Figure 3-13: Sketch of the spalling process at the left and the flaking process at the right. [Adapted from Soininen, 1998, p.86, 48] Non-contact loads arise due to the presence of ice blocks in the vicinity of the propeller (Soininen, 1998). Two different aspects are generally distinguished as per Veitch (2015): Proximity effects and blockage. These effects will be described below. A summary of a typical, generally accepted ice interaction process is given based on Veitch & Laukia (1993), Veitch (1995, 2015) and Soininen (1998). It is established using full scale observations of the interaction and laboratory tests of the failure of ice.

34 27 Ice interaction can take innumerable forms since the inflow and the interaction are highly stochastic. Many ice pieces approach and pass through the propeller in an unsteady, discontinuous, irregular stream where contact and non-contact loads occur simultaneously. If ice pieces approach the propeller, their transient, local flow disturbances will affect the wake field. The propeller responds with a transient hydrodynamic loading into thrust variation and vibration. This is the proximity effect. Veitch (2015) states that the main issue during the approach phase is the effect of the ice wake on hydrodynamic performance, rather than propeller strength. Since ice pieces have positive buoyancy the ice pieces tend to flow in the top half of the propeller plane only, right where the wake already is disturbed most in normal conditions. The propeller designer should take these effects into account. It will lead to a lower efficiency propeller due to more stringent cavitation and pressure pulses constraints. The hydrodynamic disturbance due to an ice block reaches a maximum until blockage and contact. Blockage occurs when the ice blocks come very near the propeller blade before and during contact. During milling, part of the blade is not in contact with the ice. The inflow to the blade, however, is blocked by the ice. Due to the lack of inflow, a low pressure is generated which may cause severe cavitation. Also a complete ice block may become stuck upstream the propeller. The highly turbulent flow can cause large vibrations due to increased unsteady, cyclic propeller loading. Veitch (2015) discusses that some have argued that blockage may increase efficiency due to the extra low pressure on the suction side which would improve the contribution of the wake fraction. However, blockage conditions induce severe flow separation, thrust breakdown and cavitation. Note also that the wake fraction is assumed to be steady while the blocked conditions are highly unsteady. The ship operator will be forced to flush the blockage away by thrust reversal. In addition to proximity and blockage loading, the propeller comes also into contact with the ice. Large ice pieces, larger than the propeller radius, will be progressively milled by consecutive blade passes. Each blade cuts into the ice with its leading edge and spalls, shears, flakes and crushes the ice into a burst of small broken ice particles which may be mixed with cavities. These loads are cyclic and will induce large blade loads and increased torque. Depending on the ice block size, whether it is constraint by the hull, the momentum of the propeller blades will move and rotate the ice blocks. Small ice blocks, compared to the propeller radius, may pass through the blades while impacting without being milled. In brash ice channels the concentration of small ice blocks may be very high such that hydrodynamic performance is also significantly compromised. Sliding ice blocks on the blades cause severe flow circulation disturbances. Temporarily, the flow encounters a different shape. Also, the slipstream of the propeller is disturbed with shedded vortices due to the interaction with ice blocks.

35 28 Figure 3-14: Sketch of ice contact on a propeller blade section. 1) Local crushing, shearing, spalling, 2) Cracks prior to spalling, 3) Crushing and extrusion, 4) Flaking, 5) Extrusion and cavitation, 6) Clearing [Adapted from Veitch, 2015, p.9 & Jones et al., 1997, p. 401] The contact process may be summarized by Figure 3-14 in which the following processes can be distinguished: 1. Leading edge contact processes. The ice is locally crushed and sheared. Due to the pressure, cracks are initiated. The propeller blade mills through the ice, widening the gap by its leading edge and flat suction side depending on the forward speed of the ice block en the milling depth. It may become clear from the left photo in Figure 3-15 that ice contact is not only oriented at the leading edge, but also on the flat blade area. 2. Spalling due to initial cracks. Once spalling occurs, the blade proceeds further into the ice, flaking and clearing the broken spall, after which the cycle is started again. 3. Extrusion of crushed ice through the gap between the ice and suction side. Once the ice is crushed at position 1, the ice has to be removed to enable to blade to proceed. 4. Flaking of ice is possible because the ice can be pushed away at the pressure side. No extrusion process is present here. 5. Cavitation in the gap due to blockage effects. The cavities are mixed with the extruded ice. 6. Clearing of broken ice particles into the wake. The bottom-right sketch shows in more detail that clearing may involve a large amount of ice particles. Subsequent conceptual sketches, similar to Figure 3-14, for the whole ice interaction process, are given in Figure A milled ice block is shown in the right photo in Figure 3-15 to give an impression of the dimensions of the ice blocks.

36 29 Copyright by Aker Arctic Technology Inc. Figure 3-15: Left: Propeller model test for ice milling. [Courtesy of Sasajima, 1981, p.264], Right: Milled ice block. [Obtained from Kari Laukia, Aker Arctic Inc.] A summary of the ice interaction process is given in the hierarchy of Figure First the propeller processes are divided into a part which is not influenced by ice and a part which is influenced and interacts with the ice. Three process can then be distinguished elaborated on above: disturbed hydrodynamic flow, direct contact interaction and indirect contact. As illustrated above, the propeller ice interaction process is highly dynamic and should be simplified for design and optimization purposes. Design loads and the influence on propulsive performance as function of the ice conditions are to be specified. Design loads are governed by ice class rules. Performance, however, is not considered in any regulatory framework. Figure 3-16: Ice milling process according to Veitch (1995). Sketch 1-10 show the first blade cutting, show the next blade which continues cutting. [Copied from Veitch, 1995, p.55]

37 30 Physical processes on a propeller operating in ice Undisturbed hydrodynamics Interaction Disturbed hydrodynamics turbulent, discontinuous flow Direct contact of propeller blade and ice Indirect contact mix of hydrodynamics & ice mechanics Proximity Failure Impact Extrusion Blockage Spalling Clearing Circulation Disturbance Flaking Ice block wakes Shearing Crushing Figure 3-17: Hierarchy of the propeller-ice interaction processes Propulsive Performance The effectiveness of available power will largely depend on the capabilities of the propeller and its interaction with ice. In this section it is assumed that the main engine is able to deliver the required power. As seen in section this is not always the case. Torque and thrust in ice conditions can be evaluated with a focus on the open water efficiency according to Equation (1). However, especially the milling process tends to cause sharp peaks in torque, in magnitude several times higher than the average hydrodynamic torque load. Besides the loss in efficiency in the main engine due to the unsteady loading, also the average torque will be higher. Both yield a lower overall efficiency. Also thrust is influenced by the interaction loads. Depending on the contact geometry, the thrust peak may be forward or backward. It should be noted that the loads are impact type of loads such that their momentum is very limited compared to the propulsive momentum. These large ice loads are especially important for strength, but of minor importance for performance due to their low probability of occurrence. Figure 3-18 presents an overview of factors which influence propeller ice performance. Thrust production is hindered by the highly turbulent, discontinuous flow which induces cavitation and possible thrust breakdown. Also the continuous impact of small ice pieces, with higher drag than water particles, will create energy losses. Milling capability should be sufficient to avoid blocking and continue rotation during severe ice impacts. Besides loss of thrust, static loading may become much higher than design ice loads. In

38 31 summary, more power is needed to develop the required thrust compared to open water conditions. Circulation Disturbance Propeller Ice Performance & Efficiency Ice accumulation in front of propeller Cavitation Limits Sensitivity to ice impacts Thrust Breakdown Erosion, Vibrations, Noise Milling Capability Propeller rotation block Figure 3-18: Summary of components of ice performance. There are several options to quantify propeller performance in ice conditions compared to ice-free characteristics. The ratio between the hydrodynamic efficiency in ice-free water with and without ice strengthening only gives an indication of the impact of the ice class rules on propeller design; no actual ice performance considerations are performed. A second option would be to define empirical factors based on measurements to specify the reduction of efficiency in ice conditions. Besides the lack of enough measurements, limited dependencies on propeller geometry and ice conditions can be derived. Third, quantification of all efficiency reducing factors including milling, impact loads, circulation disturbance, proximity and blockage effects might be performed, preferably as function of the ice conditions, propeller arrangement and propeller geometry. In the end, if needed, these effects might be combined into an overall performance factor. Options in this respect could be the prediction of propeller performance during: a) unsteady wake field to simulate proximity and blockage, b) blockage conditions including cavitation to study thrust breakdown, c) ice milling and d) high frequency small ice block impacts as encountered in ice channels The first three options are addressed by for instance Walker (1996) and Sampson (2009). To put the above into the perspective of the practical problem, it should be noted that it is assumed that the propeller is exposed to ice conditions due to for instance environmental ice conditions, hull shape, propeller immersion and clearances. Common practise, however, is to neglect propeller ice performance considerations at all. No guidelines are available. Hence, the ice off-design condition is solely considered based on open water. The off-design condition is formed by an ice resistance curve from the ship. Normally, no effective wake is calculated for this condition. The practical problem agrees with these observations, since ships operate only limited in ice conditions which has negligible effect on the overall efficiency. Hence ice performance for the practical problem can be summarized in only three components, neglecting efficiency in ice: 1. Sufficient structural integrity, both with regards to ultimate strength and fatigue, 2. Capability to maintain thrust to propel the ship through ice at sufficient speed as required by the FSICR and 3. Acceptable noise, vibration and cavitation erosion levels in ice conditions.

39 Propeller Ice Loads The governing ice failure processes during propeller-ice interaction have been described in section This section contains an overview of load prediction models and associated research. Also the current ice class rules are reviewed. Their background is explained. Also the design space in the ice class rules is investigated Propeller-Ice Interaction Prediction Models Jussila and Soininen (1991) made an extensive literature review about then existing state-of-the-art research into propeller-ice interaction. A short summary of this review is given below. Some references to the original early model papers are based on Jussila and Soininen (1991) Early Models Although ice breakers already operated from the beginning of the previous century, the first propeller-ice interaction model was published by Jagodkin in This work is regarded as the basis of all ice models by Jussila and Soininen (1991). An ideal milling situation is considered with a fixed pitch propeller on a rigid ice block. Two loads are distinguished. The first is ice crushing in compression when the leading edge contacts and penetrates the ice. The second consists of a shearing load due to the sliding motion between the blade and the ice. Full scale measurements are used to tune the coefficients in the model. The model is intended to only calculate the added torque due to ice. Ignatjev (1964) extended the research with considerations about propeller blade strength while Wind (1983) also takes elementary dynamic effects into account based on traditional momentum theory. Belyashow and Shpakov (1983) studied the propeller blade as two-dimensional segments. Forces on the segments are just simply integrated over the whole blade. Ice forces are based on laboratory experiments with a cutting tool, no failure mechanisms are studied. These were studied by Sajajima and Mustamäki (1984). Energy balance equations are used to equate the dissipation due to the interaction and the external work. Failure is divided in cutting and splitting, shearing, crushing and friction. Also Kotras et al. (1985) divide the propeller into two-dimensional, highly simplified strips. The model deviates from the simple milling case and considers all four quadrants of propeller operation. Therefore, impact loads and milling loads are considered separately. Moreover, Kotras et al. (1985) introduced the shadowing effect of the milled paths of successive propeller blades. Laskow et al. (1986) studied ducted propellers in ice which required the evaluation of ice induced hydrodynamic loads due to blockage in addition to milling and impact loads. Ice failure is studied using the assumption of elastic behaviour of ice outside the contact zone which fails in compressive crushing. Neither ice model considers the actual blade geometry in the models. Impact surfaces are approximated as wedge-shaped sections. The models are intended to predict the overall loading on the propeller blade. There is no clear distinction or transition from milling to impact type of loading. In view of possible optimization, no pressure distributions due to the ice loading are prescribed. Loading is applied by means of forces. Blade geometry is taken into account by means of averaged angles and thicknesses. Chernuka et al. (1989) consider the milling process in different stages for which pressure distributions are defined based on contact length and blade curvature. Hence

40 33 the actual blade geometry can be studied. Static Finite Element Method (FEM) calculations were used based on a nonlinear concrete material model. Unfortunately, details about the pressure distributions are not explained. Even that this model is the most extensive model, Jussila and Soininen (1991), miss details about the contact phenomena and governing failure processes Joint Research Project Arrangement no. 6 The goal of the early ice load models in previous paragraph was to understand and describe propeller ice interaction in more detail. However, each class society used its own rules, complementary to the FSICR, whether or not based on ice load models or experience. These ice class rules were based on the use of ice torque as dimensioning load. Full scale test measurements are converted to blade loads using assumptions and simplifications. However, applications outside the measurement data such as highly skewed propellers, large diameters, severe arctic ice conditions and nozzle propellers cannot be extrapolated from these rules as Soininen & Veitch (1996) conclude. Ideas for updated, unified FSICR were already initiated by the Finnish Board of Navigation in 1971 as indicated by the ABS guidance notes (2005). Norhamo et al. (2009) indicate that this was at first intended for the development of new FSICR only, however, the International Association of Classification Societies (IACS) joined in the process which was the actual drive behind a new project. The Canadian and Finnish Swedish Authorities established a joint research project (JRPA#6) to develop a propeller-ice interaction simulation model (Soininen & Veitch, 1996). The goal of the JRPA#6 expanded to develop unified propeller strength requirements for both Baltic and Arctic conditions based on direct expressions of the load (Soininen & Veitch, 1996). As part of this project it was concluded by Jussila and Soininen (1991) that the current state of research at that time was still not sufficient for a physically justified development of new ice class rules. For further developments Canada acquainted full scale data while Finland developed a contact model and an interaction model for propellers in ice. The data and simulations were calibrated into a unified load model (ULM) as explained by Browne et al. (1998). The ULM is a propeller-ice interaction simulation program which calculates the contact geometry at each time step as visualised in the diagram of Figure It was developed at the Technical Research Centre of Finland (VTT) by Koskinen et al. (1996). A force balance is used on contact loads and hydrodynamic disturbance loads. Momentum theory is used to update the simulation. The ULM uses an early version of the contact model of Soininen (1998). Hydrodynamic disturbance loads due to proximity and blockage and non-contact loads at the suction side are based on full-scale measurements which were analysed by Browne et al. (1998). The contact model of Soininen (1998) considers local contact processes in much more detail than earlier models. The model is based on experimentally observed failure phenomena in laboratory conditions with a propeller blade alike impact tool. It simulates the ice contact scenario which is described in section The model consists of the subsequent combination of solid ice failure and extrusion of crushed ice. A simplified geometry of the blade and the ice block are taken into account. Finally, this resulted in an averaged pressure distribution on a propeller blade section which can be used for design loads on propeller blades.

41 34 Figure 3-19: Flow-Chart of the simulation process of the JRPA#6 model [Reproduced from Jones et al., 1997, p.411]. A concise summary of the JRPA#6 which contains the main assumptions, the contents of the ULM and its results is reported by Jones et al. (1997). It is explained that the ULM and full scale measurements are calibrated after which a large number of simulations have been performed. Ice block size, location and strength, propeller diameter, blade thickness, blade area, rotational propeller speed and ice block speed have been systematically varied. A regression analysis resulted in draft versions of the design ice loads which are nowadays found in the Unified Polar Class Rules (UPCR) and FSICR. Veitch (1995) took a similar approach by coupling a time domain propeller-ice interaction model to hydrodynamic propeller analysis tools. Veitch also did full scale cutting tests, however, with different types of impact tools. He made the explicit distinction between pressure and suction side tools and stressed that a propeller blade can be regarded as a combination of the two. In further work, Veitch (1997) incorporated his model in a panel method to study the hydrodynamic loads including blockage and proximity loads. Both Veitch (1995) and Koskinen et al. (1996) take ice motion into account due to contact forces, weight, buoyancy, drag and added mass. A complete overview of the parameters is given in Figure Veitch assumed spherical ice shapes while Koskinen et al. implemented disk shapes. No definitive description or distribution of the shapes of ice pieces reaching a propeller disk had been made in 1995 as Veitch indicates. To the knowledge of the author this conclusion is still valid as per date. Also the relationship between ice in a channel and the conditions at the propeller disk has not been reported. (Model) test video results are usually not obtainable or proprietary. Veitch (1992) gave ice piece sizes in brash ice channels according to high speed video results. The ice pieces are highly irregular and randomly shaped. However, it is reported by Veitch (1995) that old brash ice pieces become kind of circular due to repeated scraping of hull and propellers. This explains his choice for spheres, while Koskinen et al. might have focussed on pieces from newly broken level ice as Wind (1983) proposed.

42 35 Figure 3-20: Parameters in an ice simulation model Further research A literature review by Wang et al. (2007) provides an overview of the research which was conducted after the JRPA#6. Canadian research was granted to investigate the operation of marine propellers in ice blocked flow as was recommended and agreed on within the JRPA#6. Their focus is on hydrodynamic loads, advanced measurement techniques and podded propulsion. Since Canada was responsible for non-contact loads in the JRPA#6, Walker (1996) indicates that this follow up research is extended responsibility about the development of the non-contact loads. Numerical and experimental work was done on propeller operation in disturbed flow conditions due to the presence of ice. Bose (1996) proposed a theoretical model of propellers in ice blocked flow. Walker (1996) did experimental work on the hydrodynamic loads and the effect of cavitation while he also gives numerical predictions of blockage effects based on a Boundary Element Method (BEM). Blockage was modelled as a heavy disturbance in the wake field (Walker, 1997). Searle (1999) experimentally investigated a high skew propeller in ice conditions in all four quadrants. He concluded that highly skewed propeller behave similar in ice conditions, however, he noted that strength requirements on the tip section were not sufficiently covered within the then existing ice class rules. Wang (2006) and Wang et al. (2007) also performed ice model tests to study the interaction phenomena in more detail on a working propeller. Based on advanced experiments in which the contact and hydrodynamic load components were investigated

43 36 separately, he made a comparison with existing ice load models, also the JRPA#6 model was used in this work (Wang, 2006). The experiments were in the range of the JRPA#6 formulations. Also his computational predictions were included in these comparisons. Wang also made computational predictions with a BEM including blockage and ice milling. BEMs were already applied by the Canadians to study blockage effects. Wang extended this research by including shadowing effects and milling contact loads. Ice crushing is taken into account by applying a pressure to each panel on the propeller which is in contact with an ice piece. Ice shearing is applied externally by means of a force. Sampson et al. (2009) designed another experimental setup in a cavitation tunnel and observed violent cavitation due to the ice blockage effect. Large variations in loading are reported. The propeller became also more heavily loaded on average due to the hydrodynamic blockage effect, both thrust and torque increased. Small unfinished research is not investigated in details here. No further finished projects concerning propeller ice interaction, which served for development of ice class rules, are published to the knowledge of the author Ice Class Rules Ice class rules govern the design of ships and their propellers. Recently, basically only two formulations are used, the Unified Polar Class Rules (UPCR) and the Finnish Swedish Ice Class Rules (FSICR). Most class societies have adopted both formulations, sometimes still supplemented with own experience. The UPCR do not prescribe a minimum power requirement for operations in ice. Propeller strength requirements for UPCR and FSICR are almost the same. Minor differences include propeller edge thickness requirements and prescribed fatigue calculations. Actual ice performance is not taken into account at all in the ice class rules Background The earlier explained JRPA#6 is the background of the current propeller ice class rules. Propeller requirements are based on the ULM in which a design ice block has been chosen. Ice block shape, orientation and location were selected to represent extreme loading events (Jones, 1997, p.414). The dimensions are based on maximum ice thickness depending on ice class. The results of the ULM, supplemented with full scale data, served as basis for the maximum blade force, torque and thrust formulations in the ice class rules. They are based on normal ship operation in the first quadrant. The backward force is due to impact processes and based on the extreme events from the ULM by means of regression analysis. The forward force is based on an empirical formula from full-scale measurements. Load intensity, loaded area and inertia effects were taken into account in the regression analysis. A complete explanation is given in Browne and Norhamo (2006). They also show that some dependencies are neglected in the final rule equations. The equations in the class rules are to be used as maximum design loads, not depending on ice block size or advance velocity. Also the angle of attack, for instance, which is highly uncertain and hardly measurable, has been chosen such that the simulation results from Koskinen et al. (1996) and full scale data agree Summary of Requirements The general formulation of the ice class loads is (29)

44 37 for force and torque in which the expanded blade area, the number of blades, the ice thickness, the propeller diameter, rotational speed, hub diameter and the pitch at. Coefficients are used to tune the forces with respect to angle of attack, ice strength and inertia effects. These design loads are applied on the propeller blade by means of load cases which should be considered in a Finite Element Method (FEM) stress analysis. The load cases are given as a uniform pressure applied on a certain area of the propeller blade. The shaded area in Figure 3-21 should be considered for both backward and forward bending. In case of an FPP also the trailing edge should be loaded to account for operation in the third quadrant. (30) Figure 3-21: Load cases as defined by the ice class rules The total force should be equal to the maximum blade forces, uniformly applied on the area. A milling condition of the design ice block is considered as the extreme load case during normal propeller operation. It should be noted that extreme special conditions are not taken into account; only normal operating conditions with working propeller are considered. Especially the backing operations with a FPP appear to give critical loads. Furthermore, if a propeller is stopped, high static ice loads may occur. The basis of load cases are points of application based on full scale measurements. Also damage cases for high skew propellers such as in Figure 3-22 contributed to the developments. Reverse engineering with FEM showed that a tip load case should be considered (Norhamo et al., 2009). Differences have been introduced for FPP, CPP and nozzle propellers. Requirements on spindle torque and blade bending moment are a function of the maximum blade force and the moment arm. Spindle torque is important for strength of the root section and the pitch mechanism in case of a CPP. Load distributions are prescribed based on statistical data. The number of ice loads is important for fatigue design. Blade failure is defined as the occurrence of plastic deformation. In case of a seemingly harmless damage like in Figure 3-22, extreme imbalances require immediate temporary repairs by cutting of the blade tip and the opposite one to avoid damage to other parts of the propulsion system. Loss of propulsion might lead to dangerous situations.

45 38 Figure 3-22: Bent blade of high skew propeller in old ice channel. [Courtesy of DNV, 2011, p.8] For early design approximations a rule formula for blade thickness is given to avoid plastic bending around the blade root. Final approval of propeller design is always based on strength assessments by means of FEM calculations in which the load cases are applied. Best practise guidelines are issued by ABS (2005) and DNV (2011) for instance. A blade stress criterion to assess FEM results is based on a safety factor of 1.5 and a reference stress based on material properties. Local details such as tip details, anti-singing edge, hub and root fillets are not required. The load is governed by bending, not by local stress concentrations. DNV (2011, p.9) recommends to check details separately. A blade edge thicknesses requirement is removed from the current FSICR to provide a unique opportunity for designers to optimize the blade edges and profiles individually (DNV, 2011, p.13). The old blade edge requirements were based on bending considerations of high local forces as derived by Koskinen et al. (2006). The edge thickness requirement is only included in the UPCR, since the FSICR are intended for first year ice conditions only. This type of ice is relatively weak compared to multiyear hard glacier ice for which the UPCR are suited Design Procedures An alternative design procedure might be accepted if it includes both fatigue and maximum load design calculations and fulfils the pyramid strength principle. Loads on the propeller blade and propulsion system shall be based on an acceptable estimation of hydrodynamic and ice loads as stated in the FSICR (TraFi, 2010, p.43). Class societies as ABS (2015), BV (2014), DNV-GL (2013), Lloyds (2014) and others have adapted the FSICR and UPCR for propellers one-to-one as design basis, as required by the Finnish Maritime Authorities (FMA) and agreed within the International Association of Classification Societies (IACS) respectively. DNV-GL, for instance, added some clarifying notes on direct calculations. Also notes on propeller edge thickness differ. DNV-GL leaves it for the propeller designer, while old methods are still mentioned. Some propeller designers prefer to have quick starting points for an ice class propeller design. BV, ABS and Lloyds, on the other hand, still include requirements for the minimum edge thickness based on beam theory.

46 39 Since the FSICR or UPCR do not consider the complete propulsion plant class societies developed their own rules concerning the design of nozzles, propeller hubs, podded propulsors and other individual components Concluding Remarks Both from experience and calculations it is found that the ULM, its regression model and the ice class rules over-predict the loads for small ice blocks. This has a consequence for the design loads of small ships. Small ice class propellers need more ice strengthening and bigger components due to pyramidal strength than before with the old ice class rules. In practise these propellers performed already without damages. Also the developers admit this, since Koskinen et al. (2006, p.2) state: calculated stresses for Gudingen propeller indicate that blades should have been destroyed. The ship has been operated 20 years in occasionally difficult ice conditions without visible damage on propeller blades. The Gudingen is a 49 meter car ferry. Comparison of the ULM with full scale measurements is carried out as validation. The angle of attack is tuned to obtain similar results. Mass of the ice pieces is estimated. Only few cases are validated and only two ships have been considered. Ship shape and ice conditions during the measurements are implicitly taken into account in this approach. Still, the results are generalized for all ship types. In general, validation of the ice class rules is difficult since the ice class rules govern the extreme events which implies that a long measurement campaign is required. Damage cases, or the absence thereof, should indicate the value of the ice class rules. It might appear that the rules are too conservative for an efficient and safe propeller design. The discrepancy between the loads based on the rules, the measured loads and actual loads on a propeller is considerable. There is already a discrepancy between rule loads and actual loads since ice class loads are based on maximum loads during life time. Additionally, safety factors are applied on ice strength, propeller material strength and load cases. Ice strength and load cases are chosen to represent the extreme events. Measured loads on the propeller shaft might be full of noise due to shaft vibrations. Ice models and ice class rules are based on both empiricism and first principles. Observations and experiments serve to establish theories about the propeller ice contact process. Uncertainties are covered by means of extreme cases and safety factors. Only significant parameter dependencies are taken into account, assumptions and generalizations are made for other detailed parameters. Ice models are not suitable for optimization purposes since they are generally based on empirical results, highly simplified propeller geometry, extreme conditions and loads or extensive simulations. Assumptions and uncertainties in these factors obscure crucial parameter sensitivities for optimization. Pressure distributions to represent ice loads, for instance, are prescribed without taking the shape of the leading edge into account. Instead, it is assumed that all propeller leading edges have a circular shape with constant circularity. Ice class load cases are mainly based on full scale observations on damage cases. The loads which are applied are maximum loads during the lifetime. Hence, only structural considerations can be drawn from the ice class loads. No hydrodynamic performance relationships can be derived. For a representative propeller analysis in ice conditions extreme loads should be taken into account for strength while all loads should be considered for performance.

47 40 4 DEFINITION STUDY The literature review in chapter 3 addresses the problem of ship propellers in ice. Based on conclusions from the literature research in section 4.1, research directions are indicated in section 4.2. Limitations and scope of the intended work are given in the last section of this chapter. The research directions serve as basis for a further definition of the research problem in chapter Literature Conclusions Currently, the practical problem is already partly addressed by other researchers. Podded propulsion and the double acting ship concept are studied at classification societies and research institutes to develop Best Practise Guidelines (BPG) for design and load cases. Thanks to the development process of the UPCR and the updated FSICR there is a consensus on relevant issues in propeller engineering for ice conditions among researchers. However, most ship owners and propeller designers were satisfied with the old ice class rules. Barely any damage cases where encountered for traditional propellers. Not only the design process is more complex for the new ice class rules in the early design phase, small propellers also need to be stronger. Due to pyramidal strength principles which couples other components and system design, this impacts throughout the whole ship. The ice load prediction for low powered ice class propellers is currently under discussion at classification societies. Recently, also approximation formulae for strength requirements in ice conditions have been developed for initial design purposes or to easily make offers for propeller designs without the usage of FEM. State-of-the-art in propeller ice loading are the models of Veitch (1995), Koskinen et al. (1996) and Wang (2007). Work after the JRPA#6 was mainly focussed on the development of the ice class rules. This joint arrangement project recommends to focus on the hydrodynamic performance of a propeller in ice conditions, with a special focus on the load due to blocking and cavitation. No comparable big projects have been carried out since. Recommendations from the ice class propeller literature are almost completely governed by the JRPA#6. Especially the updated ice class rules, which include design by means of FEM based on the JRPA#6, are a big step forward. Previously, no governing requirements about propeller strength were specified. Rule formulae only specified a certain thickness, without giving actual criteria on the strength of propeller blades. More design freedom is allowed when the load is based on a physical, first principles model, like in the current ice class rules. This freedom, however, is seldom utilized in practise due to time limitations. The propeller design process is already complex as it is for a good propeller design with limited resources. Research on the practical smart usage of the ice class rules in the scientific context is needed, aimed at the improvement of the propeller design process. Propeller parameterization and automatic meshing procedures become more and more standard among propeller designers. Coupling with automatic optimization algorithms is not yet performed in the design cycle. Although there is also a lack of experience with such algorithms, it is not straightforward to specify the objectives, design parameters and constraints in an efficient way. It is expected that research will focus on this area in the near future.

48 41 Ice models are not yet available as fully established theory, and hence not suited for implementation in an optimization routine which requires all details. Only the strength requirements are available with only limited dependency on propeller geometry and its detailed shape. Ice failure is described by Soininen (1998) in great detail. However, he had to use numerous assumptions about the failure process and contact geometry. The pressure he prescribes in his model is still not fully tailored to specific propeller blade geometries but established on a semi-empirical basis. There is no dependency on the shape of the leading edge for instance. Scenario based design will become more important in order to develop methods to optimize the whole ship for its intended operations. Shipping in ice infested water should be included. However, a performance criterion of propellers in ice conditions is not yet available and should be developed to specify efficiency, risk on damage and additional vibrations. Performance in ice conditions is still based on ice free propeller analysis. Direct calculations of the loads and the effect on performance are difficult. Some statistical probability based methods have been proposed. Ice class rules do not take this into account. Risk based design for propellers is not commonly applied, partly because strength is prescribed in the ice class rules based on extreme events which implicitly includes numerous risk assessments. Further usage of CFD is envisioned in hydrodynamic predictions of propellers. Systematic series based on CFD are expected to supplement existing systematic series. Especially cavitation analyses are sparsely covered in the traditional series. Not to mention a propeller s performance in ice conditions. Therefore, propeller performance in ice conditions should be researched in more detail. Time simulations should be more generic such that realistic ice blocks can be added. Ultimately, a hydrodynamic solver with the inclusion of ice blocks and particles can aid in this process. Ice blocks can be shaped and distributed conform typical ice conditions such that the interaction dynamics can be computed and visualised. A next step is the coupling with an ice failure analysis method and the subsequent updates of ice geometry due to damage. 4.2 Research Scope One of the goals for this Master s thesis definition study is to further define its scope. Based on the literature review several directions can be pointed out, no specific in-depth choices are given. The main framework is proposed. An initial focus is given in the infographic of Figure 4-1. yes Propeller design, detailed or general no Ice properties & characteristics High efficiency with sufficent ice performance BPG for practical usage Detailed propeller interaction model Datum propeller Detailed ice material models In depth failure mechanics Ship design Scenario based design Optimization and sensitiviy studies Figure 4-1: Initial scope of the Master s thesis definition study. Propeller-hull interaction

49 42 An elucidation on Figure 4-1 starts by mentioning that propeller design is the focus of this study. New openings in ice class rules give opportunities for propeller design which are not fully utilized yet. Therefore, general design directions or BPG for ice class propeller design are the objective of this study. Additionally, ice models are not yet fully suitable for in-depth optimization. Interaction models, however, are based on established momentum theories and could be suited. Sensitivity studies are needed to prove these statements while indicating design directions. Optimization is a tool which can be used to study sensitivities on the design objectives. A datum propeller is required to have a correct reference point to indicate the improvements in the propeller design. The focus of this study is limited with respect to the right side of Figure 4-1. Firstly, ice class rules are based on extreme events such that the uncertainties in ice characteristics are implicitly covered. The same reasoning applies for detailed ice failure mechanics and material models. Secondly, ship design is left out of the scope of the definition study. A propeller should be matched to a given ship in two conditions only. Scenario based design should be considered in a next phase. Also propeller-hull interaction, although very important for the overall efficiency, will not be considered in full details. Finally, the problem should be defined in further details to avoid ambiguity and misguided interpretability in the research problem definition. This will be done subsequently in the next subsections Ice Class Propeller Design Opportunities An important starting point is the design freedom within the current FSICR as summarized in Figure 4-2. Two different paths can be distinguished depending on whether the ice class rules are followed or an alternative method is chosen. Design Opportunities in the current 2010 Finnish Swedish Ice Class Rules for given ship characteristics Design trade-offs Efficiency Strength Milling Forces Cavitation Ice Class Rule Formulations Alternative Design for Propeller Operation in Ice 1. Optimization of General Outline and Thickness 2. Optimization of Blade Profiles and Edges 3. Estimation of Ice Loads, Thrust and Milling Torque 4. Minimization of Ship Power in Ice Blade Scantlings based on FEM Conservative Rule Loads and FEM modelling Removal of Edge Thickness Requirement Alternative Propeller Design Procedure Ice Performance Alternative Ship Power Requirement Design Point Figure 4-2: Design space within the FSICR of 2010.

50 43 Two opportunities in the design space within the ice class rule formulations can be identified. Firstly, there is a freedom in propeller outline, blade profiles and material as long as stress criteria are fulfilled. These are checked by means of FEM analyses in which the ice load cases are superimposed on the ice free hydrodynamic loading. Even in the FEM analysis there is conservativity due to the replacement of the root fillet and hub with a clamped boundary condition. Secondly, even more freedom is allowed when edge thickness requirements are disregarded. Either better cavitation favourable profiles could be designed or the focus could be on sharp edges to mill the ice more easily. An elaboration on the edge thickness requirement opportunities is given in subsection Both options have their trade-off for efficiency, strength, milling capability and cavitation. Especially the milling capability poses an additional objective compared to traditional propeller design. Proper modelling of the governing processes is required, but not easily achieved. In addition, correct physical sensitivity of the models on propeller geometry and its details is questionable, but worth to investigate. Hence, there appears a link between the ice class rules and the ice models: ice models should give an indication of milling capability while the ice class rules still govern the strength. If it can be proved that improved milling capability reduces ice loading, the alternative design route can be exploited such that improved propeller design is possible. Two opportunities are also given these alternative design methods within the FSICR. In the first place, rather than accepting prescribed power based on Eq. (26), the power is determined such that sufficient speed in an ice class brash ice channel can be obtained. Usually, self-propulsion model tests are performed in order to take hull form and all interaction effects with the propeller into account. In this approach the propeller is actually of minor importance, the required thrust is governed by ice resistance on the hull. In this approach, hull resistance is usually optimized, without any considerations to the propeller. However, if propeller efficiency of the propeller in ice conditions can be improved, there might be a reduction of required power. The second opportunity concerns the propeller design procedure as mentioned in section It should be demonstrated that the propeller can operate safely and reliably in ice. If it can be shown that extreme loads are reduced because of smart propeller design, more optimization freedom is allowed. The next subsections elaborate shortly on possible projects. Subsection explores a possibility of alternative edge designs due to the removal of the edge thickness requirements. Milling loads can be estimated with a simulation model which is the focus of subsection Edge Thickness Requirements A possible research problem could be a two-dimensional optimization of blade profile sections. The assumption is that a sharper leading edge will reduce the ice milling load. Traditionally, section shapes are chosen based on maximum efficiency and minimum cavitation. Each propeller designer uses own, sometimes proprietary, section shapes. Often, they are the same for open water and ice conditions. The importance of the sections in ice conditions is not yet investigated in detail although leading edge shapes have been investigated indirectly by numerous laboratory indentation and wedge cutting tests, e.g., Veitch (1995). Application for propeller design purposes is not published to

51 44 the knowledge of the author besides a summarily patent by Daley & Bulat (1995) in which they propose several leading edge modifications to reduce the ice loading. Cavitation will be incepted earlier; the cavitation extent will probably not change. A conceptualization of the idea is given in Figure 4-3 below. The changes are exaggerated because the blade is still subject to the ice load cases from the ice class rules which apply on the edges. Traditionally, only the thicknesses were prescribed, while currently the stresses are to be evaluated. Figure 4-3: Edge adaptations for better efficiency and improved ice performance. Since the FSICR excluded edge thickness requirements, this option not only has an opportunity for ice performance, but also for open water performance. Its goal is twofold: create better ice class sectional shapes for performance in ice free water and reduce the milling load. A focus is on the leading edge; however, section profiles and 3D propeller shape are needed for global strength considerations. A trade-off is sought between cavitation, strength, efficiency and ice milling forces. An evaluation of Strengths, Weaknesses, Opportunities and Threats (SWOT) of this problem is given in Figure 4-4. Strengths Usage of existing design space and tools Possible coupling to an optimization algorithm Approach is useful for ice class propellers, regardless of their ice performance Quantification of actual design space within the ice class rules Weaknesses Milling force cannot be quantified in such detail. There is no ice performance quantification. It can only be estimated based on laboratory tests Hard to prove the gains with respect to ice performance without sensitivity of the tools to the leading edge shape Opportunities Can be practically applied in propeller design methods. Modular, can be extended with more advanced constraints Threats Practical functioning with respect to manufacturing accuracy, maintenance and repairs Sharp leading edges could be against common sense Sharp leading edges are not allowed for polar conditions Figure 4-4: SWOT evaluation of edges optimization Advanced ice failure models, sensitive to leading edge shapes, are required while an accurate prediction of cavitation inception and stalling behaviour is needed. Probably,

52 45 existing ice models are not suited to properly perform this research. The gain in objective cannot be quantified if ice failure models are insensitive to leading edge shapes. A short study with systematically varied leading edge shapes can be carried out to investigate whether existing ice models are suitable for leading edge optimization. After that a recommendation can be given for further work. This leading edge problem is not considered in further detail in this work Milling Load Estimates A second option deals with the minimization of milling loads. A milling process is the governing condition for ice induced torque. If this condition can be gentled for extreme conditions, less torque and installed power will be required. In that case, the alternative design opportunities of Figure 4-2 can be utilized. Not only the leading edge mills the ice, also the flat suction side of the blade scrapes the ice as seen in Figure 3-14 and Figure This might induce the highest loading and bending moment. An optimal combination for skew, pitch, chord length and RPM should be sought. Also, in case of existing CPPs, pitch and RPM can be tuned in ice conditions for a reduction of power Research Framework Figure 4-5 abstractly proposes a framework in which a datum propeller is improved by means of a standardized design method. Different models and tools may be utilized. Based on the practical problem the main objective is on efficiency, while ice performance should be sufficient to comply with the FSICR. Smart choices of design parameters, objectives and constraints are required to tune the framework for its intended optimization direction within the feasible design space. Results are compared on certain measures for efficiency, ice performance and simplified cavitation. Possibly, the optimized propellers will be checked, if time would permit, with advanced tools for a more detailed analysis with respect to cavitation, flow separation and quality of the slipstream. Datum Optimization Comparison Stock Propeller Typical Ship Wake Field Design Point Modelling Design Parameters Objectives Constraints Detailed analysis Efficiency Cavitation Ice Performance Best Practise Guidelines Figure 4-5: Framework of the research scope An advantageous effect of this approach is the opportunity to derive a better quantification of the impact of ice class on propellers. Ice class rules pose significant constraints. A comparison between propeller design without ice class and with ice class can be performed within this framework. Among ship owners, ship designers and propeller designers there is a deficit of knowledge on the impact of ice class with respect to efficiency or cavitation behaviour. This is seldom checked, let alone published. If the impact of ice class on a certain propeller can be quantified, ship owners can make an informed choice while weighing the costs and benefits. This would of much aid to the practical problem.

53 Optimization Algorithm An optimization algorithm which drives and couples analysis tools could perform the design analysis as given by the cycle in Figure 4-6. Note the similarity to Figure 3-9a. Based on the results from the analysis tools, the objectives and constraints are checked within the algorithm. Design parameters are updated after which a new propeller geometry is created. This iterative process should converge to the global optimum design for a given objective function. Optimization Algorithm Objectives Design Parameters Constraints Strength Ice load Hydrodynamic load Geometry Fully Parametrized Smart, modular choice of design parameters Dynamics Thrust and Torque Cavitation Pressure variations Ice Interaction Figure 4-6: Cycle to iterate through feasible designs to find the optimal propeller. Alternatively, if the computational cycle of Figure 4-6 is not available, the cycle can be iterated by hand. A human factor is introduced which should be minimized by means of prescribed steps at different stages of the iteration. To save computational effort, the design cycle should be modular. Initial design can be performed with less design parameters and constraints than the final iteration in which, ideally, full details have to be taken into account with more advanced tools Design Method An outline of a design method for ice class propellers is given in Figure 4-7. The structure is inspired by the standard optimization problem formulation of design space, design parameters, objectives and constraints as given in section and Figure 4-6. Firstly, choices are important to set the overall focus of the design procedure. They are threefold. There are global choices first, to study the influence of ice class, cavitation and milling performance on propeller geometry. Second, design parameters can be chosen as well. Parameters such as pitch and RPM can be varied, while it is also possible to only change the thickness distribution or leading edge shape. Third, preference of objective influences the design direction. Secondly, starting points for the design are given by fixed data such as the design point, design space and computation settings. These should not be varied for comparison reasons. It should be noted that these are also choices in an earlier stage, while defining the design method.

54 47 Design method applied to datum propeller Ice Class (y/n) Edge Requirement (y/n) Cavitation (y/n) Ice Milling (y/n) Design Point Fixed Geometry Parameters Computation Settings Geometry Design Space Design Parameters Objectives Constraints Global Propeller Parameters Distributions Efficiency Ice Milling Equality Required Thrust Inequaltiy Stresses Thrust Variation Blade Profile Shape Cavitation Extent Spindle Torque Figure 4-7: Outline of a design method for ice class propellers. Green ovals give choices, blue parallelograms give fixed data, striped blue rectangles give objectives and red trapeziums give constraints on the design method. Finally, constraints ensure compliance to ice class rules and owner requirements. The propeller is tuned to the design point with the equality constraint on thrust. Effectively, this requires an iteration to specified thrust. 4.3 Design Tools All components of Figure 4-6 and Figure 4-7 are available, however, iterative feedback coupling has not been fully established yet. Cavitation, ice interaction and strength analyses are not included in the current state-off-the-art optimization tools within MARIN as indicated by Foeth & Lafeber (2013) and Foeth (2015). In their case, objectives are efficiency and pressure pulses. Full propeller parameterization with predefined sectional shapes is already used in the daily commercial design process. Sectional descriptions are also available separately. Furthermore, a boundary element method for the hydrodynamic analysis of unsteady

55 48 cavitating lifting bodies, PROCAL, is available within MARIN from the Cooperative Research Ships consortium (CRS). Additionally, efforts have been made to implement a propeller ice interaction model within PROCAL by Peddle et al. (2012) and Peddle (2013). Validations with model tests at Aker Arctic have been performed which indicate that the model gives a correct estimation of the magnitude of milling loads. Full details of PROCAL and the implementation of the ice model are available only within the CRS. Within PROCAL it is already possible to superimpose ice loads from the simulation model or the ice class rules on the hydrodynamic loads for further strength analysis by means of FEM. Strength analysis can be performed automatically in the commercial ANSYS FEM package. However, design checks are performed visually only, based on a representation of Von-Mises stresses over the propeller blade. Not all components of propeller design can be coupled or automated yet. A modular approach can be imagined in which cavitation is only taken into account in a next optimization phase. Iterative cavitation analysis for each design in the optimization phase is computationally very expensive. First the propeller is optimized on main parameters and simplified cavitation constraints. Further optimization can then be carried out including more advanced cavitation analysis. This project s scope will focus on the first optimization phase. For this phase a focus on the strength analysis coupling is required.

56 49 5 RESEARCH PROBLEM AND WORKPLAN The definition study in the previous chapter revealed the choice of the research problem, based on the literature study of chapter 3. These observations lead to the objectives of the research are proposed first, after which the problem is formulated further in detail in a main problem statement. A plan of approach is proposed together with a time planning. All work in this thesis will be focussed on the proper definition of a design optimization problem based on the design space within the FSICR. Before any optimization can be performed, an in-depth study of the optimization problem is required. This provides insight in the problem and paves the way for efficient implementation and solution methodology. A summary is given by Figure 5-1 below. Not only the choice of objectives, constraints and design parameters is to be studied, also the model responses and sensitivities are required to check the well-posedness of the problem. Section 5.2 explains this figure in further detail by means of the plan of approach for this thesis project. x shape, size tayloired choices dependencies, relationships c f weighting factors possible pareto fronts... model analyses, preparation & simplifications response analyses sensitivity studies formulation errors optimality conditions well-constrainedness suitable algorithm g & h constants requirements constraints modelling Figure 5-1: Summary of optimization problem analysis and characteristics study First the objectives of the study are defined and explained in section 5.1. Then the work plan is proposed for the remainder of the thesis work in section 5.2 after which this report is closed with a time planning and estimate of workload in section 5.3.