Reliable prediction of bollard- and tow-pull characteristics of AHTS

Transcription

1 No. 2 Aug 2014 Reliable prediction of bollard- and tow-pull characteristics of AHTS >> Senior Research Scientist Egil Jullumstrø >> Research Scientist Ørjan Selvik Photo: Ulstein MARINTEK is participating in a development programme initiated by Ulstein Design & Solutions AS, which focuses on the bollard- and tow-pull characteristics of anchor-handling tug supply vessels (AHTS), specifically, a large tug design. The programme is evaluating the following topics: CONTENTS MARITIME Norwegian Marine Technology Research Institute Propeller/duct performance Interaction hull-propulsion system Effect of speed Propulsion unit performance in waves 1 Reliable prediction of bollard- and tow-pull characteristics of AHTS 3 Manoeuvrability and minimum required power under adverse conditions ISSN A large matrix of propeller/duct combinations has been studied, followed up by open-water tests, propulsion tests (free speed), bollard-pull and tow-pull tests. Interaction between the hull and the propulsion system are very important for high-powered ships as described here. The thrust deduction factor depends on hull and skeg clearance to the duct/propeller, brackets, tunnel thrusters and other appendages, and finally on the total wake and the pressure distribution on the duct. Cont. on page 2 4 Internet at sea: Does it work? 6 The autonomous ship where does it go? 8 Hybrid Power Laboratory at MARINTEK

2 REVIEW Reliable prediction... Cont. from page 1 Example: Measurements in head seas, bollard pull. Air suction is clearly dependent upon wave length, wave encounter frequency, heading and vessel speed. As the figure shows, the average thrust is significantly reduced when severe suction occurs. Calm-water tests at forward speed documented that the pull is gradually reduced as forward speed increases. This effect is not surprising, and two factors are involved here. The first is that resistance rises with increasing speed. Secondly, thrust and torque decrease with increasing speed; i.e. propeller loading is reduced. These development studies have provided new insight into tug propulsion performance. More knowledge is still needed regarding the correlation between model predictions and full-scale performance, and this will be the subject of further investigations. Performance in waves was investigated for different speeds, irregular wave conditions and in head and following seas. Air suction to ducts and propellers can reduce pull in calm water to a certain extent, while in waves, losses due to air suction are more pronounced. Losses are also generated by the fact that propellers and ducts are moving in the actual wave field. Manoeuvrability and... Cont. from page 3 MARINTEK s vessel simulator VeSim is under continuous development e.g. within the framework of the ongoing research project SimVal (Sea Trials and Model Tests for Validation of Shiphandling Simulation Models, funded by the Research Council of Norway) and the Rolls-Royce University Technology Centre (UTC) Performance in a Seaway. Further important work on manoeuvrability in adverse conditions is being carried out at MARINTEK as part of the ongoing research project SHOPERA (funded by the European Union within the Seventh Framework Program under agreement SCP2-GA ), where the goal is to develop proposals for improved regulations that will ensure both safe and environmentally friendly ship operation. Example: One set of propellers, rudders and ducts. 2

3 No. 2 Aug 2014 Manoeuvrability and minimum required power under adverse conditions >> Research Scientist Florian Sprenger >> Research Scientist Ørjan Selvik >> Research Manager Dariusz Fathi >> Research Scientist Edvard Ringen MARINTEK s vessel simulator VeSim offers unique opportunities to assess manoeuvrability and minimum required power to advance in head seas as early as at the design stage, in order to ensure that the vessel satisfies current IMO regulations. Since January 2013, all new-builds and vessels that have undergone a major conversion have to fulfil the requirements of MARPOL Annex VI, which defines reference lines in terms of an Energy Efficiency Design Index (EEDI). This index is an indicator of the vessel emissions, given in grams-co2/ tonne-mile, and its implementation in the IMO regulations is intended to be a step towards green shipping. One way to meet the EEDI is to reduce installed power, which reduces the powering margin and may lead to significant safety issues for certain types of ship, since their manoeuvring capabilities in adverse conditions may no longer be sufficient. In order to ensure safe operation in adverse conditions, the interim resolution MEPC.232(65) was released by the IMO in 2013, but so far it covers only tankers and bulkers. It is obvious that more detailed investigations in t his field are required to ensure that shipping is both green and safe. Full-scale trials to systematically assess vessel performance in adverse conditions are hardly possible, while model testing is time-consuming and expensive. The development of reliable sophisticated numerical tools such as MARINTEK s vessel simulator software VeSim is therefore extremely important. VeSim is an efficient time-domain simulator based on Fossen s unified model for seakeeping and manoeuvering. It takes into account environmental forces from wind, waves and currents as well as forces from the propulsion system, including loss effects, e.g. due to propeller ventilation. Since both hydrodynamic coefficients and manoeuvring properties are precalculated by the plugins VERES (VEssel RESponses) and HullVisc to MARINTEK s software workbench ShipX, the actual timedomain simulations can be performed very efficiently. While the calm water IMO criteria regarding the turning and course keeping ability are so far not defined in adverse conditions, there is a clear definition of the minimum advance speed in head wind and waves in the interim IMO resolution MEPC.232(65). This speed is the greater of the minimum navigational speed of 4.0 kn and the minimum course keeping speed, which is a function of the rudder area and the vessel s windage area. VeSim can be efficiently applied to assess this criterion for any vessel. Cont. on page 2 Example of the capabilities of VeSim: Power loss due to a ventilation event observed during model tests (top) and simulations (bottom) in irregular waves (Hs=5.3m, Tp=9.7s). 3

4 REVIEW Internet at sea: Does it work? >> Research Director Ørnulf Rødseth For several years, MARINTEK has been involved in a number of projects that investigate the quality of service provided by various types of broadband communication at sea. The MARENOR project 1 is currently contributing the most concrete results to these studies. Satellite communication and positioning systems use different frequencies, as shown in the table below. Band GHz Services L 1-2 GNSS, Inmarsat C/Fleet, Iridium C 4-8 Some VSAT systems, e.g. Sea Tel Inc. Ku Most VSAT systems Ka Newer VSAT systems, Inmarsat Global Xpress In general, higher frequencies offer more bandwidth, but are more susceptible to atmospheric attenuation, which may lead to loss of connection or transmission delays. This is normally expected to apply mainly to Ka and Ku band transmissions. Our studies have focused on Iridium and VSAT over Ku, as these are often used to provide communication services in the North-Atlantic. Iridium is typically used as backup as it has a relatively limited bandwidth (134 kilobits per second), but has world-wide coverage including the Arctic. VSAT can normally supply much higher bandwidth, but this is dependent on the subscription type used and the user s willingness to pay. MARENOR Measurement campaign 1 See The MARENOR project has installed a measurement system on several ships that operate in different parts of the world, but with emphasis on the Arctic. The system measures roundtrip times (RTT) for short and long data messages over a typical Internet link and as the user experiences it. For short messages (around 100 bytes), a measurement is done each 5 minutes. The investigation shows that the data links are relatively stable and well suited for most applications. However, certain limits to service quality need to be taken into account for some applications. VSAT data has been collected in the area indicated on the Google Earth (above), between 78 and 69 degrees north and between 11 and 20 degrees west. Iridium data has been collected from a station located on ground, close to Longyearbyen (78 13ʹN 15 33ʹE). This area has good coverage of both Iridium and VSAT and our currently available data provide little evidence that latitude has a strong influence on RTT in this area for either system. Some measurement results Measurements have been done on an Iridium data link (Iridium Pilot) and a commercial VSAT service over Ku band. In this article we report on data that was collected between 21st April and 31st May This period was selected in order to maximise data for VSAT, as the ship containing the measurement system moves in and out of the coverage region for the satellite quite often. The results are summarised in the table below. The difference in the total number of messages is due to the complete loss of VSAT connection when the ship moves 4

5 No. 2 Aug 2014 Measurement Iridium VSAT Total number of messages measured Expected RTT (seconds) E (see note) Percentage message with RTT > 2E 11.3% 14.0% Percentage message with RTT > 4E 3.9% 8.3% Percentage message with RTT > 10E 2.8% 1.4% Percentage message with RTT > 20E 2.2% 0.5% Percentage message with RTT > 100E 1.2% 0.0% Connection breaks < 400E (see note) 62 0 Connection breaks > 400E 56 0 above about 79 degrees north. In these measurements, data from positions north of 78 degrees have been removed for the VSAT connection, as these are influenced by the satellite coming and going out of the ship s line of vision. Below 78 degrees there is no discernible correlation between latitude and VSAT performance in these measurements. The Iridium test station is stationary, but other measurements show no significant correlation between latitude and performance. The expected RTT (E) represents a best fit mean value to the distribution of 90% of the messages. This is the round-trip time that the user would expect in most communications. As can be seen, the expected RTT for Iridium is twice as long as for VSAT. The distribution of messages with much longer RTT than E is also different between VSAT and Iridium: Iridium has a much higher percentage of messages with very long RTT. Connection losses in the data link in our experiment occur when a message is delayed more than 300 to 500 seconds. Connection losses also occur with VSAT, but these are always associated with the ship moving north beyond degrees. They are therefore probably due to losing sight of the geostationary satellite. As the table shows, Iridium has connection losses that are more random in nature and which occur relatively frequently. Some of these breaks are also of substantial length. Some consequences for Internet users at sea The analyses of the measurements reported on here are not complete as we do not have space to include all details, and also because the project has not yet been completed. Some of the results may therefore be changed in future publications. There are also other important findings that we cannot discuss in detail at this stage. However, the data do represent the general impression of our experiences with satellite communication in the high north: Iridium is functional in all areas, but there are a relatively high degree of very long RTTs and breaks in connection that must be taken into consideration when operations are being planned. VSAT offer lower RTTs than Iridium, but there is obviously a trade-off between geographic availability and speed. There are significant variations in RTT that must to be taken into consideration when operations are being planned. Fast closed-loop control may not always be possible via a satellite link. These variations have little impact on large file transfers, except that in relatively rare cases (once or twice a day for Iridium) transfers will be interrupted if the connection is lost. There are also some asymmetric properties of Iridium that causes longer RTT in certain cases, e.g. when starting up transmissions after an idle period and when starting the transmission from an Internet node rather than from an Iridium terminal. This is only a small selection of the results of the analysis and there are also other operational characteristics of both systems that may cause problems in special operational settings that we have not reported on here. However, one conclusion is that although satellite communication is becoming more reliable and more widely available, its characteristics need to be well understood to use data links effectively and safely in practical applications. The MARENOR Project The MARENOR project is studying the performance of satellite-based maritime navigation and radio communication systems in the High North. The duration of the project is February 2012 until February Photo: Shutterstock 5

6 REVIEW The autonomous ship where does it go? >> Research Director Ørnulf Rødseth MARINTEK is technical coordinator in the European research and development project MUNIN1, which will perform a feasibility study on an unmanned merchant ship, using a Handymax dry bulker as case. This article provides an overview of the background and some results of the project. In particular, we want to look at the potential applications of the technology involved and make some guesses at what could develop in the future. Autonomous does not necessarily mean the same as unmanned. A ship may have some form of intelligence to be able to operate without full crew attention for some time, without actually being unmanned. The strategic basis for autonomous ships Shipping, shipbuilding and related areas make up an important sector of the European economy. It is also a strategically important societal function as about 80% of Europe s imports and exports are transported by sea. To cover the industrial interests, the European Maritime Industries Forum (MIF) has taken the initiative to develop the Waterborne Technology Platform 2, which includes a strategic research agenda. The autonomous ship had a prominent place in the original strategy, although in later versions it has been changed to partly autonomous ships. We will return to this later in this article. In general, Europe is meeting strong competition in areas where labour costs are an issue. This has already led to a high degree of automation and a dramatic reduction in the size of ship crews. However, there may still be efficiency and safety gains to be had by further automation, although the completely unmanned ship may not be the best solution. More automation could help to strengthen the European deep-sea shipping industry and even increase Europe s share of this strategically important business. Making European shipping more attractive to mariners and thereby maintaining local expertise is also important for landbased industry. Access to expert knowledge on operations and vessel maintenance is vital to competitive shipbuilding, equipment and marine operations industries. The legal basis Legislation and international rules will obviously be a problem for unmanned ships, as will insurance and general confidence. However, where legislation is concerned, there are already two cases that are relatively easy to handle: Unmanned operation only in international waters, which is the case in the MUNIN project. In principle, this would only require the approval of the flag state. This solution would require crew to be on board during passage in national waters. Unmanned operation only in national waters, e.g. shortsea shipping or supply services in the offshore industry. In this case the coastal, port and flag state would be one and the same and approval of operation would be up to this state alone. To allow operation in international traffic more generally, where ports in different countries are called on, international legislation would need to be harmonised. The commercial trust will have to be gained by proper documentation of safety and reliability. Piracy and hijacking This could be a problem for completely unmanned ships, as there is no crew onboard to prevent hijackers or pirates from boarding. However, the ship will also be less attractive to pirates, as ransoming the crew is normally their main objective. One can also take technical measures to hinder unauthorized people from operating the ship. Electronic control of propulsion machinery is relatively simple to implement. Tracking the ship by AIS satellite or other means also makes recovery possible. 6

7 No. 2 Aug 2014 There are also security issues related to communication and positioning systems. Satellite systems rely on very weak signals that are easy to fake or jam. Full remote control of a ship on autopilot by spoofing GPS signals has already been demonstrated. While it is difficult to safeguard against jamming, spoofing can be avoided by using additional ship sensors to validate data input. Communication will need to use strong encryption to avoid interception or faking. Cost reduction as a driving factor? When the MUNIN project was conceived, cost reduction through less crew was one argument for investigating the subject. Crewing costs are particularly significant on slow or ultra-slow steaming on long voyages. More detailed studies have shown that direct reductions in crewing costs will probably not be a factor in considering the use of autonomous ships. Additional equipment, higher fuel quality and the cost of operating a shore control centre will offset many of the savings. Many tasks such as cargo hold cleaning, machinery maintenance and ship and paint repair need to be done on ships during voyage as part of regular operation and upkeep. These tasks cannot easily be automated and may make it difficult to realise the concept of the completely unmanned ship, at least for longer voyages. On the other hand, slow and ultra-slow steaming place a significant burden on the crew, since they are confined to the ship for up to four weeks at a time, with limited Internet and family access while doing three-shift watches throughout the voyage. Such considerations will certainly make it difficult to attract people to the job. Automated watch-keeping and steering at night and shore control centres will allow working shifts to be reduced to a single daytime watch. At night, the crew would only be called when the control centre is unable to handle a situation remotely. Shore control will also require higher capacity Internet, which would also be available to the crew when not in use for vessel operation. The crew would also work together during the single shift, giving them more free time together, which would greatly improve working conditions. Cost savings in this scenario would come from lower fuel consumption and to a lesser degree from crewing reductions. Improved technical systems Autonomous ships will require much improved technical systems, mainly for watch-keeping and steering, but also to improve the monitoring and maintenance of technical systems. This will reduce costs related to off-hire and damage to ship and cargo. Exactly how the figures add up will be investigated in the coming year, when the main cost-benefit analysis for MUNIN will be performed. 7 Other types of autonomous ships In the longer term, completely unmanned ships will probably appear on certain long voyages, mainly with dry cargoes. This would require more purpose-built systems onboard with much more emphasis on operation without continuous care and maintenance. Our estimate is that around 3500 ships could be in this category, which is 7% of the international fleet. These ships would have an ordinary crew onboard for entering and leaving port. Another possibility would be supply services to offshore installations, where the ships operate in national waters, thus reducing legal problems. High capacity and reliable wireless communication will be available near the shore base and around the installations to allow direct remote control of vessels during the most critical parts of the voyage. The number of ships in this category is also limited. Around 1000 ships worldwide would be candidates for this solution. Impact on the maritime profession The number of crew affected by autonomous shipping will be limited. On the other hand, the new job positions and the new work environment created by autonomous ships will attract more people to the maritime profession. For Europe at least, this may well create an environment capable of recruiting more maritime professionals who can also assume management and supervision positions on shore. This will help to stop the movement of the deep-sea shipping industry to the Far East, where it is currently easier to obtain experienced personnel. Relation to e-navigation Thomas Porathe The IMO e-navigation activity has finalised its strategic implementation plan. Autonomy in shipping is closely related to many of the objectives in the plan, particularly those related to better integration of activities between ship and shore. Applications at the interface between e-navigation and autonomy primarily concern more advanced traffic management systems. Several projects in this area are also under way and we can expect to see an interesting cross-fertilisation between these initiatives.

8 Hybrid Power Laboratory at MARINTEK >> Senior Research Engineer Erik Hennie A hybrid test facility, made feasible by cooperation between ABB, MARINTEK and NTNU, is now operative in MARINTEK s Energy Laboratory. The facility comprises: diesel engines connected to variable-speed generators rectifiers multidrive DC lineup (with inverters and transformers) electro motors, connected to a eddy current brakes capacitor bank battery bank control and management system. A schematic overview is shown in Figure 1, while Figure 2 illustrates some of the main components of the facility. The set-up is capable of simulating real-life operating profiles, e.g. typical OSV profiles. Advanced power management Figure 1. Hybrid facility systems and control algorithms will be developed and tested, and by utilising all available options and operational flexibility, the potential for fuel savings and reducing emissions will be shown. The test facility will also be a useful tool for educational and PhD projects. Figure 2. Some of the facility s main components: diesel generator sets, load units, rectifiers and drive line-up Authors Egil Jullumstrø Senior research scientist Ørjan Selvik Research scientist Florian Sprenger Research scientist Dariusz Fathi Research manager Phone: Egil.Jullumstro@ Phone: Orjan.Selvik@ Phone: Florian.Sprenger@ Phone: Dariusz.Fathi@ Edvard Ringen Research scientist Ørnulf Rødseth Research director Erik Hennie Senior research engineer Phone: Edvard.Ringen@ Phone: OrnulfJan.Rodseth@ Phone: Erik.Hennie@ Norwegian Marine Technology Research Institute Otto Nielsens veg 10, P.O.Box 4125 Valentinlyst NO-7450 Trondheim, Norway Phone: Fax: marintek@