世界上最大的LNG船

Copyright 2008, International Petroleum Technology ConferenceThis paper was prepared for presentation at the International Petroleum Technology Confe-rence held in Kuala Lumpur, Malaysia, 3–5 December 2008.This paper was selected for presentation by an IPTC Programme Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the International Petroleum Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the International Petroleum Technology Conference, its officers, or members. Papers presented at IPTC are subject to publication review by Sponsor Society Committees of IPTC. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the International Petroleum Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, IPTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax +1-972-952-9435.AbstractDescription.Korean shipbuilding is buzzing with talk around the large LNG (Liquefied Natural Gas) vessels being constructed there. The collaborative efforts of Qatargas 2 and ExxonMobil have achieved a breakthrough in the LNG shipping industry by developing, contracting and implementing a new generation of the largest, most efficient LNG ships.The first series, Q-Flex, has a capacity of 210,000 cbm, and the first of these vessels was delivered in September 2007. The next series, Q-Max, will be the world’s largest LNG ships with capacities up to 266,000 cbm. The Q-Max vessels have been ordered by Qatar Gas Transport Co Ltd (NAKILAT) for the fifth train of the Qatargas 2 project, and the first vessel is to be delivered at the time of writing in the Korean Samsung shipyard.Applications.This paper will discuss the challenges and opportunities of the biggest single directly connected LNG shipbuilding project in the world and how the project encouraged improvements in quality and productivity without compromising the safety as the highest priority.Results and Conclusions.The primary focus will be on speed, fuel consumption, maneuvering characteristics and global vibrations. Enhanced operational safety and sharing of lessons learned will also be reviewed for the LNG vessels’ future consideration. Finally, operational feedback from the trading Q-Flex vessels plus sea trial data for the Q-Max ships will be investigated and reported by comparing the results against the original design basis. Technical Contributions.With the application of several cutting edge technologies for LNG ships, along with the optimization of marine systems such as adapting the first prototype re-liquefaction plant, this new generation of ship represents a significant step change in reducing the cost for delivery of LNG to markets worldwide.IntroductionIt’s been almost five years since the concept of larger LNG vessels was initially mooted. Today the world has witnessed that the cargoes are being safely loaded and transported to discharge terminals obliging our customers in Japan, Korea, Spain and more planned exports worldwide. Our new vessels ventured with commissioning spot cargos for New Mexico, Sabine Pass (US) and were successfully delivered .This marks a significant milestone in a journey that one might argue has transformed LNG shipping and reset the redefined possibilities.The primary goal of the industry has always been to transport LNG safely in the pursuit of sustaining flawless performance. However, mitigating the environmental impact by reducing emissions along with safe, efficient, cost-effective transportation, it became apparent that those challenges would definitely require some radical rethinking.The design basis adopted at inception for the new large LNG carriers, both Q-Flex and Q-Max, was the delivery into service a series of vessels would be to meet or beat the ‘conventional’ LNG carriers in safety, reliability, efficiency, operability and maintainability.The significant technology innovations have allowed adoption of twin screw slow speed diesel engines in place of single screw steam turbines and reliquefaction for the handling of Boil-off Gas (BOG) instead of burning it for the propulsion system. These were the major highlights apart from the obvious size increase of these new Q-Flex and Q-Max vessels that has increased the capacity by approximately 50% and 80% respectively. It is these areas that have generated the most interest within the industry and are the focus of this paper.Speed.At writing, 16 Q-Flex vessels, consisting of three slightly different hull designs, have been delivered to trade and have impressively confirmed target performance. Both vessel designs were independently tested at the SSPA model basin in Malmo Sweden. The curves in Figure 1 show the speed/power relationship predicted by model tests for the two designs (dotted lines) and the average speeds at design draft (12 meters) derived from sea trial results.IPTC 12445An Insight into the World’s Largest LNG Ships Abdulla Khalid Al-Kubaisi, Qatargas Operating Company Limited2 IPTC 12445Model & Trial PerformanceDesign Draft - 12 Meters12131415161718192021222324Ship Speed KnotsS h a f t H o r s e p o w e rFigure 1The designs performed virtually identically in the model basin but slightly different on sea trials despite that the designs differ slightly in form at the bow and stern. But speed trial results at ballast draft gave slightly lower speed than model test prediction. After an extensive study, SSPA’s key finding was that the optimised bulbous bow that was not fully submerged at ballast draft, made it very difficult to accurately predict the ‘form factor’ which is used for scaling of the viscous resistance component using conventional Prohaska plot. A new modified method for prediction of the form factor was proposed by SSPA. The description of the new method is beyond the scope of this paper. In summary, • The new method is believed to give more objective and consistent estimates of the form factor. • In ballast the form factors will often be higher than what has previously been used. • Predictions using the new method are believed to correlate well with sea trials as shown in the next figure. • A correction of 1-2% is also proposed for design draft.Single screw operation with the idle shaft either locked or free wheeling has been considered through design by providing clutches to allow such option for the Large LNG (LLNG) ships with twin screw. Model tests predicted that the vessel would achieve up to 15 knots on trials in single screw mode with the idle propeller free wheeling and about 12 knots with the idle propeller locked. The vessels demonstrated about 13 knots with the idle propeller free wheeling and about 11 knots in locked configuration on trials, and in service. The difference is due to torque limitations of the main engine which could not be modeled in the test basin.The ballast trial results in twin screw mode show the propellers running with an operating margin of 5% over the trial speed range compared to a specification minimum of 3%. However, in single screw mode, maximum available power was limited due to torque-speed considerations to about 90% of NCR for the free wheeling mode and 80% for the locked mode. Figure 2 illustrates an engine RPM/Power layoutdiagram for one of the vessel designs with ballast trial results plotted on the diagram.Engine Layout MAN-B&W 6S70ME-C500070009000110001300015000170001900021000230002500027000Engine RPME n g i n e B H PFigure 2The provision of clutches in the shafting presented unforeseen problems with shaft alignment. The Renk clutches installed on these vessels are gear tooth type sliding clutches with internal sleeve bearings that allow the idle propeller to free wheel while maintaining alignment of the fore and aft shaft sections. On two early sea trials one of the clutches on each ship exhibited a sharp rise in temperature soon after starting of free wheeling tests. Later inspection showed the sleeve bearing to be heavily damaged. The damage was consistent with misalignment, although the vessel’s shafting had been carefully aligned in accordance with alignment calculations using traditional gap-and-sag and bearing weight methods.After reviewing the clutch manufacturer’s tolerances of allowable bending moment and shear across the clutch and the actual results achieved, it was determined that traditional ‘gap & sag’ alignment methods did not provide the accuracy necessary for clutch alignment. After switching to strain gauge alignment techniques there have been no further clutch incidents due to installation methodology. In addition the project has followed the maker’s operational recommendation to slowly raise the speed to the intended knots to gradually warm up the clutch components and avoid thermal spikes.Vibration and Noise.A joint research program by the shipyards, Class societies and independent analytical studies commissioned by the project was carried out with a single objective of minimising hull and engine induced vibrations in Q-Max and Q-Flex designs. The findings from the first principle approach using Computational Fluid Dynamics (CFD) analysis were looped back into the design process to find the optimum solution to the vibrations problem.One of the noteworthy examples was the identification of resonance in the pump tower of one design. After the combined review and analysis, it was agreed that a five bladed propeller would not only mitigated resonance issue but alsoIPTC 12445 3reduced pressure pulses from the propeller on the hull by some 30%.On another design, external electric vibration dampers were installed to mitigate known effects of six cylinder engine. Further, additional hydraulic side bracings were fitted to the engine tops and tuned on sea trials to minimize X-moment forces.On yet another design, optimally designed ‘SAVER’ fins were fitted (See Figures 3a & 3b). CFD analysis indicated that by directing the flow outside the propeller tips, excitation forces generated by the propellers would be reduced by as much as 30%, while increasing the powering performance by about 1%.Figure 3aFigure 3bAs a consequence, none of the vessels have barred speed ranges and have improved manoeuvring performance.Effectiveness of this approach during the design process has been confirmed by local and global vibration measurements. The results in Tables 1a and 1b below indicate not only specification limits were met, in some cases found to be considerably better:Noise VibrationLocation Spec Maximum Db(A) Trial Result Db(A) SpecMaximummm/sec 2 Trial Result mm/sec 2Bridge 65 55 107 38.5Bridge Wings 90 70 107 - Captain’s Day Room 55 49 107 46.0 Cargo Control Room 55 52 143 24.6 Crew’s Mess 55 53 107 38.1 Engine Control Room70 69 143 43.4 Steering Gear Room - 0.8mm/sTable 1aNoise VibrationLocation Spec Maxi-mum Db(A) Trial Result Db(A) SpecMaxi-mum mm/secTrialResultmm/se cBridge 65 59 3 0.8 Bridge Wings 70 69.2/71 - 3.4 Captain’s Day Room 55 47 3 2.9 Cargo Control Room 55 50 3 1.5 Crew’s Mess 55 56.8 3 2.3 Engine Control Room 70 69 3 1.2 Steering Gear Room - 6.9Table 1bManeouvering.The design of the Q-Flex and Q-Max LLNG Carriers of twin screw and twin rudders is demonstrating several advantages. In ship handling and maneuvering the advantages include: redundancy in propulsion and steering plus very good heading control and turning ability. The performance of these vessels, from a ship handling and maneuvering perspective, meet or exceed the project team’s expectations. Recent operational experiences at Suez Canal and Sabine (US) proved the vessels handled better than predicted through the virtual reality simulations carried out in labs. Table 2 below shows the results obtained vs. IMO / Project standard.Table 24 IPTC12445Concept and Development.The concept and development of larger LNG Carriers, to maximize cargo carrying capacity to customers, necessarily took into consideration the physical limitations of the export and import terminals of their intended trade. This resulted in the vessel classification design called Q-Max. The Q-Flex concept design, as is implied in its name, provides an increased flexibility in delivery points. Once the principle dimensions and concept design of the Q-Flex and Q-Max class of ships were identified, ship handling studies were conducted to verify that these ship sizes could safely enter, maneuver, berth and depart the targeted ports.A wide range of propulsion system options, with different rudder and propeller configurations were carefully studied before the final project selection of the use of twin screw and twin rudders. This was considered to be the more challenging propulsion system of those under review by the project team. However, studies confirmed that the propulsive efficiency of the twin screw ship is higher than the single screw design due to wake flow characteristics of the relatively wide but shallow draft ships. The improved efficiency and reduced fuel consumptions compensated for the extra capital cost of a twin screw ship.Additionally, from a ship handling and control point of view, the twin rudder and twin propeller design also provides advantages over a single propeller vessel for the size.Preliminary ship handling studies were carried out for several concept designs of the Q-Flex and Q-Max ships for maneuver in Ras Laffan Harbour and the port of Milford Haven in Wales, UK. These studies verified the feasibility of being able to safely maneuver the large LNG Carriers into and out of these ports with both twin and single screw configurations. They also gave the project team confidence in the design concept and handling ability of these ships with their length over all, beam, draught, and wind sail area.Throughout the project there were numerous studies involving ship handling looking at various aspects of berth locations, configuration, and layout. The ship models used always performed well and resulted in no changes in the berth designs being required due to ship handling characteristics. Trials and Service.The first fleet of eight Q-Flex vessels have been delivered and are in service. Their outstanding ship handling and manoeuvring performance has been proven while delivering cargoes to LNG Receiving Terminals around the world including in: Japan; Korea; the United States; Mexico and Spain, as well as during transits of the Suez Canal. Feedback from pilots and the ships’ captains, on the handling properties of these ships have been very positive and reinforce the findings during the concept design maneuvering studies and in sea trails.The first Sea Trials of the Q-Max design ships have recently been completed. These have demonstrated the effectiveness of the twin rudders and screws on this class of ship with results similar to the Q-Flex ships performance and within the design specifications. The Full Speed Turning Circle Tests for the Q-Max resulted in starboard turning Advance and Tactical diameter of up to 3.15 ship lengths and a Transfer of 1.13 lengths and port turning slightly lower. The full turns were completed in less than thirteen minutes. (See Figure 4 and Table 3 below)Figure 4Table 3Split Engine Turn.It’s been proven from sea trials that these large LNG vessels can easily turn within their own length splittingIPTC 12445 5 engines due to twin-screw capabilities during harbourmanoeuvring. (See Figure 5 and Table 4 below).Table 4Crash Stop Tests.Full ahead to full astern at sea speed on two engines, theresults showed that the twin screw Q-Flex ships stop within anadvance of about 12 Lbp or less with a travel distance of lessthan 15 Lbp.(See Figure 6 below).Figure 6Manoeuvring Stability.The responsiveness and manoeuvring stability of the vesselis captured by the ‘zigzag’ test and the plots in the Figures No.7 & 8 below attest to measured outcome significantly betterthan IMO resolution requirements.Figure 7Figure 8Even at low speeds the ships have proven to be verysensitive to the twin rudders and screws at speeds down toabout three knots and have maintained steerage withoutengines down to about four knots. Ship heading has beenmaintained while on Auto Pilot at speeds as low as four knots.In at least one instance a pilot maintained vessel positionand heading outside the breakwater for an extended period oftime by operating engines and rudders independently. Theseships can easily be turned, with engine and tug assist, in abouttheir own length.In summary, the handling characteristics and ability tomaintain heading of these twin screw and twin rudder vesselsis already proving to be outstanding.Reliquefaction Plant.Unlike all other cargo ships, LNG carriers have continuedto use steam turbine propulsion plant despite more efficientdiesel engines being available. This is because the gas thatnaturally evaporates from the cargo BOG is used as fuel forthe steam turbines, and until recently there was no alternative.The ability to re-liquefy the BOG now makes it possible toincrease the amount of LNG delivered to the discharge port, Initial speed 0.0ktsTravel Distance 211.5mAhead Reach 193.8mSide Reach 19.8mRpm 26Rudder angle 35°6 IPTC 12445which is more of interest for the LNG buyers globally.Reliquefaction paves the way for the installation of more efficient propulsion systems on LNG carriers. The efficiency of diesel engines is up to 50 per cent, compared with approximately 30 per cent for a steam turbine plant. The economic advantage of diesel engine propulsion translates to minimum savings of 2 to 5 million USD per year for a LNG carrier depending on size of vessel and LNG price.This all leads to higher efficiency and economy which enhances profitability significantly. However, it’s also considered as one of the most difficult challenges for such new technology to be adopted; since its use in a marine environment to date has been very limited.This paper addresses the two designs that have been adopted by the project as the main reliquefaction plants from “Hamworthy” and “Cryostar”, installed onboard the Q-Flex and the Q-Max vessels respectively. Both the concept and the actual performance as witnessed on the maiden voyage is also discussed.Main Plant Concept.The LNG is kept in a liquid state at -163°C in the tanks. Due to heat leak during transportation, gas naturally evaporates from the cargo raising tank pressure, in order to control this pressure the boil-off gas produced can be utilized within in a boiler for production of steam required for a steam-turbine propulsion plant. Or alternatively it can be used in a gas turbine or dual-fuel diesel engine to provide electric power for the propulsion of the vessel, a further option is to reliquefy the BOG and return to the cargo tanks, resulting in increased cargo quantity delivered .The reliquefaction plant process is based on the simple Brayton cycle, selected for its simplicity over a mixed refrigerant type, it uses a single closed refrigeration cycle extracting heat from the BOG through a series of heat exchangers, the preferred choice for refrigerant medium is nitrogen, due to its bulk production capability onboard standard LNG vessels it is also suitable for the reliquefaction of LNG, and as non-toxic/flammable properties.The basic plant concept (Hamworthy Mark I design) consists of cryogenic compressors, Compander and heat exchangers, whose duty is to re-liquefy BOG and control tank pressure onboard the LNG vessel. (See Figure 9)The green loop in the Figure 9 above shows the integrally-geared compressor-expander (Compander) which provides three stages of compression and one stage of expansion. Thehigh-pressure nitrogen stream is pre-cooled in a counter-current heat exchanger against the cryogenic nitrogen stream that is generated by the expansion of the pre-cooled stream. The cryogenic nitrogen refrigerant also serves to liquefy the boil-off gas, which is compressed by a two-stage integrally geared compressor (BOG Compressor) -in the red loop above- to increase its boiling point. After the boil-off gas is reliquefied within the separator, it is returned to the LNG tanks using the pressure generated by the compressor. The return pump is also provided as an alternative means to push the condensate back to the tanks if the pressure induced by the BOG compressor is not sufficient.The project opted for full redundancy of all rotating elements consistent with the base design philosophy (see Figure 10 for typical layout) in a configuration that meets full IGC requirements for back-up. Additionally a Gas Combustion unit (GCU) back up system to the reliquefaction plant is provided.Figure 10Step out in Technology.Over forty years have passed since the construction of the first LNG carrier. Of the more than 150 LNG carriers currently in service, nearly every one has been equipped with a steam turbine propulsion system. This propulsion arrangement offers notable advantages for this application. The LNG cargo continuously boils and if left unchecked would lead to unacceptable rises in tank pressure. Combustion of the boil off gas in the main boiler to produce steam provides a safe, convenient and reliable means for disposal of this gas to control tank pressure.The economic disadvantages of this propulsion system are, however, significant. Steam turbine propulsion systems are thermodynamically inefficient, converting only about 30% of the fuel's energy into useable power. Considering alternative propulsion systems can offer thermodynamic efficiencies of 40% to 50%, fuel savings provided by their introduction can be significant. In an effort to capture these savings, the QGII Development project undertook a thorough study of alternative propulsion systems for LNG carriers.IPTC 12445 7 Some of these propulsion options, such as slow-speeddiesel, were not at the time compatible with the use of thisboil-off gas as a fuel and therefore required an alternative tankpressure control system. Reliquefaction of the LNG boil-offprovides one such means. Economics may also suggest thatreliquefaction systems be installed even on LNG carriers withpropulsion systems that allow the consumption of boil-off gasas a fuel. This would provide the flexibility to select thepropulsion system's primary fuel depending on the relativeprices of the fuel options at a given time.Experience with LNG reliquefaction is, however, verylimited. Only one LNG carrier, the LNG Jamal, has ever beenbuilt with a reliquefaction system. The plant on the LNGJamal was installed as a test bed for future system development. This is evidenced by the fact that the plant was installed in association with a steam turbine propulsion system, which limits the criticality of the operation of the reliquefaction plant.The QGII Shipping Team decided that the ExxonMobil Technology Qualification Management System (TQMS) should be applied for this qualification effort. TQMS provides a method to evaluate the introduction and adequacy of new technologies. For this process, the TQMS procedure seeks consensus from ExxonMobil's and Qatar Petroleum's technical and operating organizations that this new technology, which offers significant potential but whose performance is not established and outside existing applied norms, is qualified for such marine application.Reliquefaction capacity.The initial plant design capacity was set to equal the guaranteed boil off rate of the specified vessel with the process concept for the partial reliquefaction of the QG2 cargo composition, any waste gas generated (nitrogen rich) is disposed of within the gas combustion unit (GCU). However, an early result from the dynamic model simulations was an indication that the plant would be capable of full reliquefaction at QG2 LNG design composition. This has been subsequently proven in actual service with a composition that is slightly more nitrogen rich than the original design basis. Implementing advanced concepts.Later on, in the project, HGS introduced the improved Mark III reliquefaction plant to the project for evaluation. The main feature of the new plant is that it uses a pre-heater instead of the pre-cooler as in Mark I (See Figure 11). The new concept is based on the interesting thermal phenomena of the refrigeration process when the heat exchanging operation of the hot BOG takes place in the cold box, the energy efficiency is improved by some 800kW. Figure 11The implementation of the Mark III LNG reliquefaction system will also have effect on other equipment than that delivered by HGS. Since the BOG cycle pressure may become higher than the N2 refrigeration loop, all equipment should be placed in hazardous area zone 1. This implies that all electrical equipment must be (Ex) classified in accordance with this zone.The bulkhead which previously separated the motor room from the compressor room can be removed. Equipment can therefore be arranged more efficient in the room, and it consequently seems to be a potential for reducing the room size in total. When removing the bulkhead, all bulkhead seals are removed. This results in reduced instrument air/N2 consumption. (See Figure 12 HGS Mark III design).Figure 12Due to a more energy efficient system, N2 compander motor can be significantly reduced along with the associated frequency converter, cables, and similar. The BOG compressor motor becomes larger. Overall motor power is reduced while number of motors remains the same. Cooling water system (including pumps, pipes, valves, heat exchangers, etc.) can in total be reduced since the cooling water demand for the LNG Reliquefaction system will be reduced proportionally with the rated power. See Table 5 below for the estimated power demand for the Mark I and Mark III:8 IPTC12445 Equipment/system MarkIMarkIIIN2 Compander 4681 3572BOG Compressor 381 870Total kW 5062 4442Table 5The power demand is approximately 13-15% higher forMark I than for Mark III. This will consequently represent amajor advantage to the owners in terms of operational costsfor the LNG Reliquefaction plant. Mark III was then the bestchoice for the rest of the 11 Q-Flex’s.Promoting commercial competiveness.Following signing of the Shipbuilding Contract (SBC)with the three shipyards, different vendors were put forwardby the yards for consideration as Reliquefaction vendors. At that time with HGS being sole supplier, the project was well aware that such a situation did not necessarily promote design development and/or commercial competiveness and so a project decision was made to identify at least one additional suitable vendor.Having appraised the initial submissions it was clear, careful evaluation was required. In the pursuit of finding another qualified vendor; the project team with specialist support in both rotating equipment and process applications fields being provided by ExxonMobil Upstream Research Company (EMURC) has subjected both hardware and software aspects of the various proposals to:•Close scrutiny and inspection, with particular emphasis being placed on the software/control applications. •Process evaluations using sophisticated Dynamic Modeling System (DMS) techniques.•Presentation of proven track record system in marine cryogenic system applications,•Demonstration of having adequate resources, technical ability and capabilities to produce a working plant. Following this strategy, Cryostar attained vendor qualification in 2005 and have been awarded the contracts for the Q-Max vessels now being constructed.The Ecorel system that has been proposed by Cryostar is for full reliquefaction of LNG BOG with up to 1% N2 content, with no venting or waste gas handling required. Design highlight of the system include:•Upstream of the compressor, part of condensate stream from the condenser outlet is recycled into a spray cooler in order to reduce and control the vapor inlet temperature •The condensed stream after the expansion valve is remixed with an LNG stream taken from spray pumps inorder to sub-cool the mixed stream before returning this stream into the tank.•The system design relies upon specialized distribution manifold within each tank to ensure good mixing of the condensate into tank.A schematic of the Cryostar system is presented in Figure 13 Figure 13Functionality assurance.All plant designs are a combination of major cryogenic components supplied by different vendors and onshore testing of the complete plant prior to installation was not practicable. In recognition of the interface complexity the Project developed an alternative testing schedule to mitigate any risk: •All major rotating equipment subject to full performance Factory Acceptance Testing (FAT).•Adoption of an integrated Mechanical Acceptance & Completion (MAC) matrix based on established upstreamproject methodology.•Process verification using snapshot and DMS techniques. •DMS model subject to functionality testing, while integrated with vessels Distributed Alarm & Control System (DACS) to confirm control algorithms before anylive tests being conducted onboard•Prior to Gas Trial each yard to undertake a N2 onboard reliquefaction test on the first vessels.Adopting a consistent project philosophy for all PC based sys-tems installed has significantly reduced the potential for the introduction of faults and errors. Thus a guiding principle of a single point of contact (generally the DACS vendor) to adopt responsibility for all integrated solutions was instigated.Reliquefaction operation overview.Minimise thermal cycling and improve plant availability.Minimizing plant exposure to thermal cycling is one of the major design concept and philosophies to be achieved. This was addressed by ensuring that at all times the nitrogen (N2) cycle would be maintained in cold conditions. Other than for equipment changeover and/or maintenance, the N2 Compander will be running continuously, including during vessel loading and unloading, for which a standby mode is adopted with the BOG cycle shutdown and N2 cycle on minimum load. In order to reduce the BOG start-up times upon completion of cargo loading; the project has investigated the possibility of using BOG free flow mode without running BOG compressor.。