Optimization of Oil Outflow and Cargo Capacity of an. AFRAMAX Oil Tanker Design

Transcription

1 Optimization of Oil Outflow and Cargo Capacity of an 1) AFRAMAX Oil Tanker Design A. Papanikolaou 1, G. Zaraphonitis 2, E. Boulougouris 3, B. U. Langbecker 4, S. Matho 5, P. Sames 6 NTUA, School of NAME, Ship Design Laboratory, Athens, Greece, papa@deslab.ntua.gr 2) NTUA, School of NAME, Ship Design Laboratory, Athens, Greece, zar@deslab.ntua.gr 3) NTUA, School of NAME, Ship Design Laboratory, Athens, Greece, vboulg@deslab.ntua.gr 4) Germanischer Lloyd AG, Hamburg, Germany, uwe.langbecker@gl-group.com 5) Germanischer Lloyd AG, Hamburg, Germany, sven.matho@gl-group.com 6) Germanischer Lloyd AG, Hamburg, Germany, pierre.sames@gl-group.com Abstract The paper presents the parametric optimization of a double hull AFRAMAX tanker design, targeting reduced oil-outflow probability and increased cargo carrying capacity. The contradicting objectives of maximizing the cargo volume while at the same time minimum cargo area steel weight and mean oil outflow index are treated as part of a multi-objective decision making problem. A parametric geometrical ship model has been developed in NAPA, while the structural model was created by POSEIDON. The integration of the above software packages has been assigned to the modefrontier optimization software. The developed approach leads to an automated optimization procedure allowing an improved feedback to the designer for the trade-off between the various design parameters, optimization criteria and decision-maker preferences. Obtained results suggest notable improvements in transport capacity and oil outflow performance of known well established yard designs. Keywords Tanker design optimization; risk-based design; genetic algorithms; multi-criteria decision making; accidental oil outflow 1. Introduction March 24 th, 2009 marked the 20 years anniversary of the most striking tanker accident in the history of naval architecture, namely of the single hull tanker EXXON VALDEZ in Price William Sound in Alaska. The release of 50,000 tons of crude oil to the marine environment and the caused ecological catastrophe led the US government to the unilateral banning of single hull tankers from US waters (OPA90). In the meantime, IMO MARPOL regulations also recognize double hull tanker designs as the only acceptable solution for the safe carriage of oil. Recently introduced, accelerated phase-out of single hull tankers in the European Union has increased the pace of transformation of the world oil tanker fleet. Despite several efforts to introduce innovative tanker designs, the little acceptance of alternative tanker designs has been discouraging creativity within the industry; however, currently in force MARPOL regulations appear challengeable without increasing the risk of negative environmental impact, whereas there is room for even improved cargo capacity. Current discussions at IMO regarding tanker safety refer to the Formal Safety Assessment of tankers (MEPC, 2008) and the approval of a marine pollution assessment metric, namely the Cost for Averting one Ton Spillage (CATS); the latter, which is the subject of heated discussions at IMO, should not only consider the clean-up cost, but also some deterrence and insurance cost (MEPC, 2010). Clearly, the risk of oil spillage by tanker accidents should be kept as low as possible and as acceptable by society (ALARP); eventually, this is expected to drive new developments in tanker designs and operations. The present paper outlines the risk-based parametric optimization of a double hull AFRAMAX tanker to achieve innovative designs with increased cargo carrying capacity, reduced steel weight and improved environmental protection. The presented research is based on results of a joint industry-university project, namely between Germanischer Lloyd (GL) and the Ship Design Laboratory of the National Technical University of Athens (NTUA-SDL). The presented work is a deeper elaboration of an innovative risk-based oil tanker design procedure that was initiated by NTUA-SDL in the framework of the EU project SAFEDOR (Papanikolaou et al, 2007). Beyond the work presented earlier, the integration of the structural design software POSEIDON (Germanischer Lloyd, 2008a) in the multi-criteria optimization procedure allows a realistic estimation of the steel weight of the alternative designs, whereas the latest MARPOL regulations for the accidental oil outflow applicable to all newbuildings after January 1, 2010 have been implemented (SAFEDOR, 2005). The developed fully automated optimization procedure allows an improved feedback to the designer for the tradeoff between the various design parameters and optimization criteria. An existing AFRAMAX tanker has been selected as basic-reference design. Its main particulars and general arrangement are presented in Table 1 and Figure 1 re- 1

2 spectively. It is a typical modern AFRAMAX tanker with 6 tanks along the cargo space and 2 cargo tanks across, already adequately optimized by the shipbuilder. Chapter 4 of MARPOL 73/78 (MEPC, 2004a) setting the requirements for the arrangement of the cargo area of oil tankers constructed after has been used as the regulatory basis in the present work. In particular, the following regulations have been implemented: Regulation 18 requirements for the minimum capacity of segregated ballast tanks (SBT) Regulation 19 requirements for the double hull arrangement Regulation 23 requirements for Accidental Oil Outflow along with the procedure for its calculation Regulation 27 criteria for Intact Stability Regulation 28 criteria for Damage Stability Table 1: Main particulars of the reference design Length, oa m Length, bp m Breadth, moulded m Depth, moulded m Width of double skin m 2.50 sides Width of double skin m 2.50 bottom Draught scantling m Deadweight tons 112,700 Cargo capacity cbm 127,271 Slops cbm 2,890 HFO cbm 3,380 DO cbm 260 Water ballast cbm 41,065 Peaks cbm 3,500 Classification Lloyds Register Propeller Diameter m 7.2 No of Cargo tanks (6x2) 12 plus 2 slop tanks Cargo Block length m In ballast condition, including the conditions consisting of lightweight plus segregated ballast only, the ship's draughts and trim should meet the following requirements: Moulded draught amidships, d m L Trim by stern 0.015L Draught aft (Taft) should always lead to full immersion of the propeller(s) For oil tankers of 5,000 tons deadweight and above, delivered on or after , Regulation 19 requires ballast tanks or spaces other than tanks carrying oil along their entire cargo tank length, effectively protecting the cargo space with the following minimum dimensions: Wing tanks or spaces, w = min { DWT/20,000 ; 2.0m} > 1.0m Double bottom tanks or spaces, h = min {B/15 ; 2.0m} > 1.0m It should be noted that the requirements of Reg. 19 regarding the minimum spacing of wing and double bottom from the outer shell (2.0m for AFRAMAX) was herein challenged, namely kept flexible during the optimization runs and set equal to min. 1.7m for AFRA- MAX size tankers. Regulation 23 applies to oil tankers delivered on or after 1 January For oil tankers of 5,000 tonnes DWT and above, it sets the limits for the mean oil outflow parameter (OM), along with the procedure for its calculation. For the vessel of this particular study, with a total volume of cargo oil less than 200,000m3, an OM value not exceeding is required. The mean oil outflow parameter is calculated independently for side and bottom damages and then combined in nondimensionalized form as follows: O = ( 0. 4O O ) C (1) M MS MB / where O MS and O MB are the mean outflows for side and bottom damage respectively and C is the total volume of cargo oil in m 3 at 98% tank filling. The bottom damage mean outflow is calculated independently for zero and minus 2.5m tide conditions and averaged as follows: O = + (2) MB 0. 7OMB( 0) 0. 3OMB( 2. 5) The calculation of the mean outflows for side and bottom damage is based on a probabilistic approach. The side damage outflow is calculated by the following formula: Fig. 1: General arrangement of the reference design For crude oil tankers of 20,000 tons DWT and above as well as for product carriers of 30,000 tons DWT and above delivered after , Regulation 18 requires a sufficient capacity of segregated ballast tanks. O MS n 3 3 s( i) s( i) m 1 = C P O ( ) (3) where P S(i) is the probability of penetrating cargo tank i from side damage, O S(i) is the corresponding outflow in m 3 while C 3 is an appropriate coefficient. Accordingly, the bottom damage outflow, either for zero or minus 2.5m tide condition is calculated by the following formula: 2

3 O MB n = P O C ( m ) (4) 1 B( i) B( i) DB( i) In the above equation, C DB(i) accounts for the capturing of oil flowing out from a tank in the double bottom. Major parts of the presented research may be also found in the recent journal publication: Papanikolaou et al. (2010). 2. Design Optimization The main objective of this study was to improve the accidental oil outflow performance of the reference cargo tank arrangement, while at the same time minimizing the steel weight and maximizing the cargo capacity. The improvement of the performance of a ship in terms of oil outflow, the maximization of cargo capacity and minimization of steel weight are contradicting objectives; for example, the former requires an increased distance of the cargo space from the outer shell, resulting in a reduction of the cargo tanks volume; also, the reduction of the mean outflow parameter can be achieved with a more thorough subdivision, decreasing the average size of cargo tanks, and at the same time increasing the steel weight with a corresponding increase of construction cost and a reduction of payload. For meeting optimally the above contradicting objectives, a formal multi-objective optimization procedure has been developed and applied. 2.1 Optimization Framework A generic optimization framework of a system S incorporates the following main elements, see Figure 2: Input Ε Ι Design Variables D Design Parameters P Merit functions Μ Constraints G Fig. 2: 3 Generic Optimization Framework At the core of the developed optimization framework there is a parametric design tool, developed within the well known ship design software NAPA (NAPA Oy). It consists of a set of macros, developed in NAPA Basic, facilitating the fully automatic generation of the detailed layout of the cargo block of a vessel, based on the values of a series of design parameters and design variables. The design pool is then created by the systematic variation of design variables, while using predefined (user-selected) values for the design parameters. The procedure evaluates the fulfillment of a set of constraints, while at the same time a set of objectives is optimized. The approach is holistic in its nature and allows integrating of an arbitrary number of objective functions and constraints as necessary for the design problem at hand (Papanikolaou, 2009a). The generic optimization framework developed by NTUA-SDL has been applied previously in a variety of problems, including the optimization of the watertight subdivision of RoRo-Passenger ships (Boulougouris, 2004), or the external hullform optimization of highspeed ships (Zaraphonitis, 2003). This generic procedure was adapted to the present optimization problem, by adding methods and the corresponding software tools for the structural design of the steel structure of the ship and for the probabilistic assessment of oil outflow performance. 2.2 Multi-objective Optimization Ship design is a typical optimization problem of multiple, frequently contradicting objective functions and constraints. The easiest way to address such a multiobjective problem would be to combine the objective functions into one, assuming that relative weights and relationships between the objectives are known. In most cases, however, these weights and relationships are unknown and there is little knowledge about the space of feasible solutions. Hence, a truly multi-objective methodology is required, leading to a set of best designs, i.e. designs for which no objective can be improved without sacrificing the performance of another. This set of best designs is known as the Pareto Set. Its graphical representation is the Pareto Frontier. For the present problem the multi-objective Genetic Algorithms (GA) have been selected as the most suitable optimization method (Sen, 1998). GAs are stochastic, non-linear optimization methods adopting principles of biological evolution (Goldberg, 1998). In particular they utilize populations of solutions using methods of selection, reproduction and mutation, in contrast to more traditional optimization methods which use gradient information to move between successively better points in the solution space. This makes them uniquely adaptive to multi-objective problems such as finding Pareto frontiers. With the Pareto set of non-dominated designs in hand, the designer can select an optimal solution according to his preferences. This can be done in a number of ways by, e.g. using a utility function for ranking the different designs, using scatter 2D and 3D diagrams for visually identifying the more attractive designs, comparing them on the basis of his criteria-preferences and experience-based selection, using other visual tools (parallel plots, histograms, frequency plots, Student plots etc.) and deciding according to his experience 3

4 2.3 Implemented Optimization procedure The optimization procedure applied herein is schematically shown in Figure 3. It integrates the following software packages: NAPA (NAPA Oy), a naval architectural software POSEIDON (Germanischer Lloyd, 2008a), a structural design and analysis software developed by GL modefrontier (E.STE.CO.), a general optimization software Within NAPA a set of macros was developed in order to: create the parametric 3D model of the hullform and internal compartmentation, calculate loading conditions, perform intact and damage stability calculations, calculate the accidental oil outflow, prepare the necessary geometric data for the software tools (POSEIDON ) performing the structural design Fig. 3: Implemented optimization procedure POSEIDON implements GL s rules for classification of a ship s structure (Germanischer Lloyd, 2008b). It allows the automatic calculation of the scantlings for all structural components based on rule requirements for the particular vessel parameters, class notation, global bending, cargo loads and external sea pressure. Note that an additional module has been developed/implemented to create POSEIDON models from a set of parameters. The same set of parameters is used for both defining compartments in NAPA and creating the structural model in POSEIDON hence ensuring consistency between the two models. modefrontier is a general purpose optimization scheduler. It provides several optimization algorithms: genetic algorithms, conjugate gradient method, quasi- Newton method, sequential quadratic programming, simplex etc. The various optional algorithms can be combined, e.g. genetic algorithms for global search and another algorithm for local search (refinement). Software modules on different platforms can be integrated via a network. 2.4 Design Variables A parametric definition of the layout and structural arrangement of the cargo area of a ship requires a large number of parameters, controlling the details of the arrangement and of the various structural components. In the present study some of these parameters were kept constant during each optimization run, while others were treated as free variables and their values were selected (in a predefined range) by the optimization scheduler. More details on the employed design parameters and variables are given in the following section describing the geometric model. 2.5 Objectives The following objectives were used: Maximization of the cargo capacity Minimization of the accidental oil outflow parameter according to MARPOL Annex I Regulation 23 Minimization of structural steel weight in the cargo area while fulfilling the requirements of GL Rules for the construction of Double Hull Oil Tankers (non-csr) 2.6 Constraints The following constraints were employed: MARPOL Regulation 18 for mean draft, trim, propeller immersion etc. MARPOL Regulation 23, except for the min. spacing of wings and double bottom, which was set here for AFRAMAX tankers equal to 1.7m MARPOL Regulation 27 requirements for intact stability MARPOL Regulation 28 requirements for damage stability 3. Geometric Model The geometry of the reference hullform was modeled in NAPA using available offsets. A series of NAPA macros were developed to parametrically define the internal compartmentation of the design alternatives. In the geometric modeling, the external hullform, the length and position of the cargo block area were kept fixed. Fig. 4: Hullform modeled in NAPA Typical examples from the variety of configurations that 4

5 may be parametrically defined are illustrated in Figure 5 to Figure 8. The details of the internal layout and of the structural arrangement of the ship along the cargo area are controlled by a series of 41 design parameters. The most important of them are summarized in the following: Compartmentations with one (central) or two longitudinal bulkheads over the entire cargo block may be developed. The number of longitudinal bulkheads is controlled by the corresponding parameter. The number of transverse bulkheads in the cargo area may be controlled by the user by assigning the value of the corresponding parameter. A set of parameters is used to define the position of the transverse bulkheads in the cargo area. A set of parameters is used to define the double bottom height within each main transverse zone. An additional set of parameters is used to define the inner hull clearance within each main transverse zone. A set of parameters is introduced to control the type of inner hull and double bottom. Depending on the values of the corresponding parameters, the inner hull side and double bottom may be: o Parallel to the centre-plane and bottom o Inclined (see Figure 6) o Stepped (see Figure 7) The transverse and longitudinal bulkheads can be either flat or corrugated. The type of bulkheads is controlled by the corresponding design parameter. A set of parameters is used to control the details of the geometry of the hoper plates of the inner hull. In the case of two longitudinal bulkheads, the width of central tank as percentage of the ship s breadth is specified by the corresponding design parameter. A set of parameters is introduced to control the details of the geometry of the upper and lower stools for the case of corrugated bulkheads. A set of parameters is used to define the various structural details, such as the number and position of the stringer decks, stiffener spacing on shell, inner bottom, strength deck, transverse members, and longitudinal bulkheads etc. For practical purposes and considering the importance of the various design parameters, it has been decided to select a subset of them and treat them as free variables, while the others were assigned constant values. The values of the free variables (herein called the design variables) were systematically varied by the optimization scheduler, searching for the optimal solutions; the most important design variables are those defining: the position of the transverse bulkheads in the cargo area, the double bottom height within each main transverse zone, the inner hull clearance within each main transverse zone, the number of longitudinal bulkheads (either one or two), the width of central tank as percentage of the ship s breadth, in the case of two longitudinal bulkheads, the distance of transverse frames the distance of longitudinal stiffeners, and the inclination of the hopper plate connecting the double bottom with the inner hull. For example, for the typical case of a vessel with 6x2 or 6x3 tanks (i.e. with five transverse bulkheads inside the cargo block and one or two longitudinal bulkheads) this results in a total of 26 to 27 design variables. Fig. 5: Fig. 6: Fig. 7: Arrangement with 6x2 tanks, corrugated bulkheads, constant double bottom height and inner side clearance Arrangement with inclined double bottom and constant inner side clearance Arrangement with stepped double bottom and inner hull 5

6 Fig. 8: 4. Structural Model Arrangements of reference design longitudinal and transverse members. The following simplifications were made for the POSEIDON model: Local structural details required for structural continuity (i.e. brackets etc.) are not included in the model Holes and cut-outs were not considered The material for the whole structure is steel Grade A (mild steel) Scantlings were calculated from longitudinal strength assessment without taking into account global FE calculations, local buckling or fatigue assessment 4.1 Typical AFRAMAX Structure Oil tankers of AFRAMAX size (80,000 t DWT to 119,999 t DWT) are commonly longitudinally framed ships over the full length of the cargo block. They usually dispose a large number of continuous longitudinal, closely spaced stiffeners and a small number of web frames spaced more sparsely. A centerline bulkhead separates across the two cargo tanks. A hopper slopping plate at the lower part connects the longitudinal girder with the first stringer and provides strength and rigidity at the double bottom wing space interface. There are typically 3 stringers in the wing space, connecting the inner hull with the side shell. Floors, vertical webs in the wing tanks and at the longitudinal bulkheads, and deck transverses are arranged at every web frame. A structural model was created within POSEIDON for the reference design based on available structural information (Germanischer Lloyd, 2008a). The model was more detailed in the cargo area and limited in the bow and stern region, see Figure 9. The structural model was created in such a way that all layouts and topologies addressed in the previous section on geometry modeling could easily be built up in an automatic way. Next to 15 (16) design variables, necessary for the geometry modeling, additional 21 structural design parameters were introduced for the parametric structural model, see for example, Figure Classification Rules Germanischer Lloyd Rules [15] have been applied to calculate the minimum scantlings for the structural arrangements of the design according to class notation GL +100A5 Oil Tanker. Common Structural Rules (CSR) were herein not implemented, as the reference ship was not designed under CSR rules and the optimized designs should remain comparable to the reference design 1. Two modules have been developed/ implemented on top of POSEIDON. The first one creates a POSEIDON model from a set of parameters, while the latter invokes POSEIDON to determine minimum scantlings for plates and stiffeners according to GL Rules. This allows the calculation of the structural weight of 1 In a continuation of the original Germanischer Lloyd-NTUA-SDL collaborative project, CSR rules are being currently implemented in the developed optimization procedure. Fig. 9: Sample POSEIDON model with outer shell Fig. 10: Sample POSEIDON model without outer shell 5. Case Studies 5.1 Alternative Configurations Five different configurations have been considered, with 6 or 7 tanks in the longitudinal direction, 2 or 3 tanks in the transverse direction and flat or corrugated bulkheads. The five different combinations are summarized in Table 2. A total of designs were examined in the present study. In the following figures only the feasible designs are shown. The open circles correspond to dominated designs, while the full circles correspond to designs on the Pareto front. For comparison, the refer- 6

7 ence design is also included, marked by a full triangle 2. It should be noted that the steel weight of the reference vessel is not its actual weight as built, but the weight calculated by the POSEIDON software. This ensures full comparability with the generated optimal designs. Table 2: Alternative configurations Tanks Bhd type No of des. Config.1 6x2 flat 7287 Config.2 6x2 corrugated 1738 Config.3 6x3 flat 6147 Config.4 6x3 corrugated 3270 Config.5 7x2 flat Configuration 2 The second configuration considers the change of the structural design from flat to corrugated bulkheads. The results are given in the figures below. Fig. 14: Oil outflow vs. cargo volume for config Configuration 1 This configuration corresponds to the tank arrangement of the reference design. This is the standard configuration for most AFRAMAX vessels. Comparing the obtained designs with the reference design we may identify whether the reference design was already on the Pareto front and whether improvements are still. The results for the three selected objective functions (cargo volume, structural weight and oil outflow index) are shown in the figures below. Fig. 15: Oil outflow vs. steel weight in cargo area for configuration 2 Fig. 11: Oil outflow vs. cargo volume for config. 1 Fig. 16: Cargo volume vs. steel weight in cargo area for configuration Configuration 3 The third configuration was created by introducing an additional longitudinal bulkhead (flat) in the cargo area. The results are shown in the figures below. Fig. 12: Oil outflow vs. steel weight in cargo area for configuration 1 Fig. 17: Oil outflow vs. cargo volume for config. 3 Fig. 13: Cargo volume vs. steel weight in cargo area for configuration 1 2 It should be noted that the reference design, which has been optimized by a major shipbuilder for Far East, disposes a double hull side clearance and double bottom height of 2.5m (thus beyond the minimum MARPOL limits of 2.0m); it proves particularly environmental friendly, with a calculated mean outflow index of abt , compared to the max according to MARPOL. Fig. 18: Oil outflow vs. steel weight in cargo area for configuration 3 7

8 Fig. 19: Cargo volume vs. steel weight in cargo area for configuration 3 Fig. 24: Oil outflow vs. steel weight in cargo area for configuration Configuration 4 Configuration 4 was derived from configuration 3 replacing flat bulkheads by corrugated ones. The results are given in the figures below. Fig. 25: Cargo volume vs. steel weight in cargo area for configuration 5 Fig. 20: Oil outflow vs. cargo volume for config. 4 Fig. 21: Oil outflow vs. steel weight in cargo area for configuration Discussion of Results The five alternative configurations were selected in view of a systematic assessment of the characteristics of the reference design and in order to identify possible improvements through an analysis of the respective Pareto frontiers. Putting all Pareto frontiers into a single diagram provides a better insight of the relative importance of the design objectives, design parameters and alternative configurations. Figure 26 clearly shows that the 6x3 flat Pareto designs dominate all the other. Furthermore, there are several Pareto designs with significantly better oil outflow and cargo volume performance than the reference design. This is very interesting result, considering, simultaneously, the associated steel weights in the following graphs that are comparable or even lower than that of the reference design. Fig. 22: Cargo volume vs. steel weight in cargo area for configuration Configuration 5 Finally, for configuration 5 an additional transverse bulkhead (flat) was introduced leading to the results shown in the figures below. Fig. 23: Oil outflow vs. cargo volume for config. 5 Fig. 26: Outflow vs. cargo volume Pareto designs from different configurations As expected, Figure 27 shows that for the same cargo volume, most generated 6x2 flat Pareto designs have less steel weight than all the other configurations, noting that the structural weight of both the generated Pareto designs and of the reference ship has been calculated by the same model, namely here POSEIDON. The reference design is here again dominated by several 6x2 flat and 6x3 flat designs. 8

9 revenue) is considered more important than the initial cost (steel weight) and environmental impact (outflow), see Figure 30, Equ. (6), Table 4. w 1 = w sw = wout (5) 3 cv = Fig. 27: Cargo volume vs. steel weight in cargo area Pareto designs from different configurations In Figure 28 the 6x3 flat designs as well as the 6x2 flat designs dominate all other designs. The reference design is again clearly dominated by several 6x3 flat designs. At the same time practically all 6x2 flat Pareto designs have less steel weight than the reference design at acceptable oil outflow performance. 3 w cv = and 4 w 1 = w out (6) 8 sw = where w cv, w sw, and w out are the utility functions at saturation for cargo volume, steel weight and oil outflow respectively. Fig. 28: Outflow vs. steel weight in cargo area Pareto designs from different configurations In addition to the above, the following observations can be made: None of the corrugated arrangements proves better than flat bulkhead designs. This does not mean that the corrugated geometries should be in general disregarded as alternative configurations. They have important advantages with respect to the ease of production and maintenance, which have not been considered in this study. The 7x2 flat arrangement is performing poorly since the steel weight increases without any significant gains in the outflow or the capacity, respectively. The reference design appears to be on the Pareto front of the 6x2 flat designs. It was already noted earlier that the reference design is a proven design in practice, which was optimized with respect to steel weight (by the yard designer, most likely by use of FEM). The proof of dominance of the 6x3 flat designs holds for the particular AFRAMAX vessel size, which is on the border to SUEZMAX. 5.3 Multi-criteria Decision Making (MCDM) Two different MCDM scenarios were examined by use of the Utility Functions Technique (E.STE.CO.): Scenario #1: the same preference to all objectives is assumed; see Figure 29, Equ. (5), Table 3. Scenario #2: the cargo volume (corresponds to Fig. 29: Design Ranking according to scenario #1 Table 3: Comparison of optimum and reference design according to scenario #1 Ref. des. 6X3 Flat 6X3 Flat ID #1710 #2122 Rank 1 2 Cargo Vol % % Oil Outflow % % Wst % % When all three objectives are considered to be equally important (scenario #1), design # 1710 with the characteristics shown in Table 3 turns out to be the optimal one. This is due to the significant reduction of the oil outflow in case of collision or grounding accident (- 23%). At the same time the cargo volume is also increased (+2%) and the steel weight reduced by 2%. This design is in every respect dominating the reference design. It is interesting to note that design # 2122 that is ranked second achieves less reduction of oil outflow (- 6%) but a significant increase in cargo volume (+7%), whereas the steel weight is reduced by 1%. Based on the results of the first assumed assessment scenario, the preferences were modified in the second scenario #2, namely the increase of the cargo volume is considered much more important than the other objectives (relative weights 0.75: 0.125: 0.125). Design # 9

10 2069 with the characteristics shown in Table 4 is now the optimal one. This is due to a significant increase of the cargo capacity (+8%). For this design, the accidental oil outflow is increased by 10% (but still remaining well below the regulatory requirements), while the steel weight is reduced by 2%. This design is also dominant over the reference design. Design # 2122 is ranked here again second due to a smaller increase in cargo volume (+7%) and the steel weight reduction by 1%. Its arrangement is shown in Figure 31. Fig. 30: Design ranking according to scenario #2 Table 4: Comparison of optimum and reference design according to scenario #2 Ref. des. 6X3 Flat 6X3 Flat ID #2069 #2122 Rank 1 2 Cargo Vol % % Oil Outflow % % Wst % % Fig. 31: Tank arrangement of design # The shown design has characteristically a varying cargo tank length, which may be associated to additional production costs; the likely additional production costs need to be rationally assessed in a Coast Benefit Analysis on the basis of an economic model, which is currently developed. Independently, the developed optimization procedure allows the easy filtering of alternative Pareto design of equal tank length or the setting of the tank length as a fixed design parameter. 6. Conclusions A multi-objective optimization procedure for the development of efficient and environmentally friendly tanker designs has been developed. The implemented procedure is largely automated and combines the naval architectural software package NAPA, the optimization software FRONTIER and the structural design software POSEIDON. The application of the optimization procedure to an AFRAMAX design, already optimized by the yard, showed that: the reference design was close to the Pareto designs (optimal solutions) generated, which confirms the validity of the set-up modeling, several Pareto designs were of improved oil outflow performance and comparable steel weight and capacity, whereas other designs had improved capacity but slightly worse oil outflow performance, particular design features of optimal designs observed in (Papanikolaou et al, 2007) with respect to the increase of double bottom height and decrease of size of tanks towards the bow were confirmed, and fine-tuning of the hull-form around the cargo block is expected to further improve the oil outflow and cargo carrying capacity performance of generated designs The way ahead may include: The enhancement of optimization procedure by inclusion of: o Optimization of local structural design for least structural weight o The implementation of Common Structural Rules (CSR) for the structural design o Other design criteria (e.g. ease of production and maintenance etc.) o Economic criteria (building, maintenance and operating costs, RFR, NPV) o Optimization of hull form to best fit to the cargo block, along with a minimization of powering (fuel consumption) and emissions The extension to other ship sizes such as ULCC, VLCC, SUEZMAX, PANMAX etc. The refinement of the probabilistic assessment of oil outflow by introduction of probabilities of more recent damage statistics (beyond MARPOL) in the framework of a Risk-based Design procedure, challenging existing regulations (Papanikolaou, 2009b). References Boulougouris E. K., Papanikolaou A. and Zaraphonitis G. (2004). Optimisation of Arrangements of Ro-Ro Passenger Ships with Genetic Algorithms, Ship Technology Research, Verlag Heinrich Söding, 51:3, pg E.STE.CO, modefrontier software. 10

11 Germanischer Lloyd (2008a) POSEIDON ND v Germanischer Lloyd (2008b). Rules for Classification and Construction, Ship Technology, Seagoing Ships, Hull Structures, Edition Goldberg, D. (1998). Genetic Algorithms in Search, Optimization, and Machine Learning, Addison- Wesley Publishing Company, Inc., USA. Marine Environment Protection Committee (2003). Resolution MEPC.110(49) Revised Interim Guidelines for the Approval of Alternative Methods of Design and Construction of Oil Tankers Under Regulation 13F(5) of Annex I of MARPOL 73/78. MEPC 49nd Session, Agenda Item 22, Annex 16, International Maritime Organization, Adopted on July 18. Marine Environment Protection Committee (2004a). Resolution MEPC.117(52) - Amendments to the Annex of the Protocol of 1978 Relating to the International Convention for the Prevention of Pollution From Ships, MEPC 52nd Session, Agenda Item 24, Annex 2, International Maritime Organization, Adopted on October 15. Marine Environment Protection Committee (2004b). Resolution MEPC.122(52) Explanatory Notes on Matters Related to the Accidental Oil Outflow Performance Under Regulation 23 of the Revised MARPOL Annex I. MEPC 52nd Session, Agenda Item 24, Annex 11, International Maritime Organization, Adopted on October 15. Marine Environment Protection Committee (2008) MEPC 58/17/2 & MEPC 58/INF.2 Formal Safety Assessment Crude Oil Tankers, submitted by Denmark on behalf of SAFEDOR. Marine Environment Protection Committee (2010) MEPC 60/WP.11 Formal Safety Assessment Report of the Working Group on Environmental Risk Evaluation Criteria with the context of Formal Safety Assessment NAPA Oy, NAPA software. SAFEDOR (2005). Integrated Project on Design, Operation and Regulation for Safety. EU-funded project, contract TIP4-CT , Sen P., Yang J-B (1998). Multiple Criteria Decision Support in Engineering Design, Springer-Verlag London Limited. Papanikolaou A., Tuzcu C., Tsichlis P., Eliopoulou E. (2007). Risk-Based Optimization of Tanker Design, Proceedings 3rd Int. Maritime Conference on Design for Safety, Berkeley, September Papanikolaou A. (2009a) Holistic ship design optimization, Computer-Aided Design, doi: /j.cad Papanikolaou, A. (ed.) (2009b). Risk-based Ship Design Methods, Tools and Applications, Springer, ISBN Papanikolaou, A., Zaraphonitis, G., Boulougouris, E., Langbecker, U., Matho, S., Sames, P. (2010), Multi-Objective Optimization of Oil Tanker Design, accepted for publication at the Journal Marine Science and Technology, Springer Verlag, Tokyo, April Zaraphonitis G., Papanikolaou A. and Mourkoyiannis D. (2003). Hull-form Optimization of High Speed Vessels with Respect to Wash and Powering, Proc. of IMDC 03, Athens, Vol. II, pg