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5510 0026-00ppr.indd

  1. 1. LNG Carriers with ME-GI Engine andHigh Pressure Gas Supply SystemContents: Introduction ......................................................................... 3 Propulsion Requirements for LNG Carriers with Dual-Fuel Gas Injection ...................................................... 4 Fuel Gas Supply System – Design Concept ...................... 5 Fuel Gas Supply System – Key Components .................... 6 Capacity Control – Valve Unloading .................................. 9 Compressor System Engineering – 6LP250-5S ................. 10 ME-GI Gas System Engineering .......................................... 11 ME-GI Injection System ....................................................... 12 High-Pressure Double-Wall Piping ..................................... 13 Fuel Gas System - Control Requirements ......................... 15 Machinery Room installation – 6LP250-5S ....................... 18 Requirements for Cargo Machinery Room Support Structure ..................................................... 19 Requirements for Classification ......................................... 20 Actual Test and Analysis of Safety when Operating on Gas .......................................... 20 Main Engine Room Safety ................................................. 20 Simulation Results .............................................................. 21 Engine Operating Modes ................................................... 22 Launching the ME-GI .......................................................... 23 Machinery Concepts Comparion ........................................ 24 Concluding Remarks ................................................................... 28 References ................................................................................... 28 Appendices: I, II,III, IV, V, VI,VII ................................................... 28 MAN Diesel A/S, Copenhagen, Denmark
  2. 2. LNG Carriers with ME-GI Engine andHigh Pressure Gas Supply System Introduction • low speed, heavy fuel oil burning die- sel engine combined with a relique- The latest introduction to the marine faction system for BOG recovery market of ship designs with the dual- fuel low speed ME-GI engine has been • medium speed, dual-fuel engines very much supported by the Korean with electric propulsion. shipyards and engine builders, Doosan, Hyundai, Samsung and Daewoo. A further low speed direct propulsion alternative, using a dual-fuel two-stroke Thanks to this cooperation it has been engine, is now also available: possible to introduce the ME-GI en- gines into the latest design of LNG car- • high thermal efficiency, flexible fuel/ riers and get full acceptance from the gas ratio, low operational and instal- Classification Societies involved. lation costs are the major benefits of this alternative engine version This paper describes the innovative de- • the engine utilises a high-pressure sign and installation features of the fuel gas system to supply boil-off gas at gas supply system for an LNG carrier, pressures of 250-300 bar for injection comprising multi-stage low temperature into the cylinders. boil-off fuel gas compressor with driver and auxiliary systems, high-pressure Apart from the description of the fuel piping system and safety features, gas supply system, this paper also controls and instrumentation. The discusses related issues such as re- paper also extensively describes the quirements for classification, hazardous operational control system required to identification procedures, main engine provide full engine availability over the room safety, maintenance requirements entire transport cycle. and availability. The demand for larger and more energy It will be demonstrated that the ME-GI efficient LNG carriers has resulted in based solution has operational and rapidly increasing use of the diesel en- economic benefits over other low gine as the prime mover, replacing tra- speed based solutions, irrespective of ditional steam turbine propulsion plants. vessel size, when the predicted criteria Two alternative propulsion solutions for relative energy prices prevail. have established themselves to date on the market: 3
  3. 3. Propulsion Requirements manufacturer Burckhardt Compression, Redundancy in terms of propulsion isfor LNG Carriers with AG (BCA), the classification society not required by the classification socie-Dual-Fuel Gas Injection and MAN Diesel has been mandatory ties, but it is required by all operators on to ensure a proper and safe design of the LNG market. The selection of theIn 2004, the first diesel engine order the complete gas distribution system, double engine ME-GI solution resultswas placed for an LNG carrier, equip- including the engine. This has been not only in redundancy of propulsion,ped with two MAN B&W low speed achieved through a common Hazid / but also of redundancy in the choice of6S70ME-C engines. Today, the order Hazop study. fuel supply. If the fuel gas supply fails, itbacklog comprises more than 90 en- is possible to operate the ME-GI as angines for various owners, mainly oil Configuration of LNG carriers ME engine, fuelled solely with HFO.companies, all for Qatar gas distribution utilising the boil-off gasprojects. For many years, the LNG market has The superior efficiency of the two- not really valued the boil-off gas, as thisWhile the HFO burning engine is a well stroke diesel engines, especially with a has been considered a natural loss notknown and recognised prime mover, directly coupled propeller, has gained accounted for.the low speed dual-fuel electronically increasing attention. On LNG carriers,controlled ME-GI (gas injection) engine the desired power for propulsion can Today, the fuel oil price has been at ahas not yet been ordered by the market. be generated by a single engine with a high level, which again has led to con- single propeller combined with a power siderations by operators on whether toAlthough the GI engine, as a mechani- take home system, or a double engine burn the boil-off gas instead of utilis-cally operated engine, has been avail- installation with direct drive on two pro- ing 100 % HFO, DO or gas oil. Vari-able for many years, it is not until now pellers. This paper concentrates on the ous factors determine the rate of thethat there is real potential. Cost, fuel double engine installations boil-off gas evaporation, however, it isflexibility and efficiency are the driving estimated that boil-off gas equals aboutfactors. 2 x 50 %, which is the most attractive 80-90 % in laden voyage, and in ballast solution for an LNG carrier of the size voyage 40-50 % of the energy neededThe task of implementing the two- 145 kcum and larger. By selecting a for the LNG vessel at full power. There-stroke ME-GI engine in the market has twin propeller solution for this LNG car- fore, some additional fuel oil is requiredfocused on the gas supply system, rier, which normally has a high Beam/ or alternative forced boil-off gas mustfrom the LNG storage tanks to the high- draft ratio, a substantial gain in propeller be generated. Full power is defined aspressure gas compressor and further efficiency of some 5 % for 145 kcum a voyage speed of 19-21 knots. Thisto the engine. A cooperation between and larger, and up to 9 % or even more speed has been accepted in the marketthe shipyard HHI, the compressor for larger carriers is possible. as the most optimal speed for LNG car-Table I: Two-stroke propulsion recommendations for LNG carriers in the range from 145-270 kcum LNG carrier size Recommended Propulsion power Propulsion Beam/ Estimated gain in efficiency (cum) two-stroke solution (kW) speed (knots) draft ratio compared to a single propeller 145,000- 2 x 6S60ME-GI 2 x 14,280 19-21 3.8 5% 150,000 2 x 5S65ME-GI 2 x 14,350 160,000- 2 x 5S70ME-GI 2 x 16,350 19-21 4.0 > 5% 170,000 2 x 7S60ME-GI 2 x 16,660 200,000- 2 x 6S65ME-GI 2 x 17,220 19-21 4.2 9% 220,000 2 x 6S70ME-GI 2 x 19,620 240,000- 2 x 7S65ME-GI 2 x 20,090 19-21 4.5 > 9% 270,000 2 x 7S70ME-GI 2 x 21,7704
  4. 4. riers when both first cost investment In order for the ME-GI to achieve this Fuel Gas Supply Systemand loss of cargo is considered. superior efficiency of 50 % (+/− 5 %fuel – Design Concept tolerances) during gas running, the gasTo achieve this service speed, a two- fuel requires a boost to a pressure of The basic design concept of the fuel gasstroke solution for the power require- maximum 250 bars at 100 % load. supply system presented in this paperment for different LNG carrier sizes is At lower loads the pressure required considers the installation of two 100 %suggested in Table I. decreases linearly to 30 % load, where fuel gas compressors. Full redundancy a boost pressure of 150 bars is re- of the fuel gas compressor has beenWith the high-pressure gas injection quired. To boost this pressure, a high- considered as a priority to satisfy classifi-ME-GI engine, the virtues of the two- pressure compressor solution has been cation requirements (see Fig. 2).stroke diesel principle are prevailing. developed by BCA, which is presentedThe thermal efficiency and output re- in this paper. Each compressor is designed to delivermain equivalent to that obtained when the boil-off gas at a variable dischargeburning conventional heavy fuel oil. Fig. 1 shows an example of an LNG pressure in the range of 150 to 265The high-pressure gas injection system carrier with the recommended ME-GI bar g (15–26.5 MPa g), according tooffers the advantage of being almost application. required engine load to two 50 % in-independent of gas/oil fuel mixture, as stalled ME-GI engines A and B. Thelong as a small amount of pilot oil fuel is selected compressor runs continuously,injected for ignition. and the standby compressor is started manually only in the event of malfunc- tion of the compressor selected. LNG Tank Compressor Oxidiser The amount of boil-off gas (BOG), and hence the tank pressure, varies consid- erably during the ship operating cycle. The design concept therefore requires ME-GI that the compressors be able to oper- ate under a number of demanding con- ditions, i.e. with: • a wide variation of BOG flow, as ex- perienced during loaded and ballast Compressor voyage, High pressure gas • a variation in suction pressure ac- FPP ME-GI cording to storage tank pressure, ME-GI PSC Clutch • a very wide range of suction tempera- tures, as experienced between warm start-up and ultra cold loaded opera- tion, andFig. 1: LNG carrier with the recommended ME-GI application. • a variable gas composition. The compressor is therefore fitted with a capacity control system to ensure gas delivery at the required pressure to the ME-GI engine, and tank pressure con- trol within strictly defined limits. These duty variables are to be handled both simply and efficiently without compro- mising overall plant reliability and safety. 5
  5. 5. The compressor is designed to effi- Fuel Gas Supply System The fuel gas compressor with the des-ciently deliver both natural boil-off gas – Key Components ignation 6LP250-5S_1 is designed(nBOG) and, if required, forced (fBOG) to deliver low-temperature natural orduring the ballast voyage. Fuel gas compressor forced boil-off gas from atmospheric 6LP250-5S_1 tank pressure at an inlet temperatureFinally, the basic design concept also as low as −160°C, up to a gas injectionconsiders compressor operation in The compression of cryogenic LNG pressure in the range of 150 to 265 bar.alternative running mode to deliver low boil-off gas up to discharge pressures A total of five compression stages arepressure gas to the gas combustion in the range of 10-50 barg (1.0 to 5.0 provided and arranged in a single verti-unit (GCU). Operation with gas delivery MPa g) is now common practice in cal compressor casing directly drivensimultaneously to both GCU and ME-GI many LNG production and receiving by a conventional electric motor. Theis also possible. terminals installed world wide today. guiding principles of the compressor design are similar to those of API 618Alternative fuel gas supply system Compressor designs employing the for continuous operating process com-concepts, employing either 2 x 50 % highly reliable labyrinth sealing prin- pression applications.installed compressors and a separate ciple have been extensively used forsupply line for the GCU, or 1 x 100 % such applications. The challenge for The compressor designation is as follows:compressor in combination with a BOG the compressor designer of the ME-GIreliquefaction plant, are currently being application is to extend the delivery 6LP250-5S_1considered by the market. pressure reliably and efficiently by add- 6 number of cranks ing additional compression stages to L labyrinth sealing piston,These alternative concepts are not de- achieve the required engine injection stages 1 to 3scribed further in this paper. pressure. In doing so, the compressor’s P ring sealing piston, physical dimensions must consider the stages 4 to 5 restricted space available within the 250 stroke in mm deck-mounted machinery room. 5 number of stages S cylinder size reference 1 valve design A unique compressor construction al- lows the selection of the best applicable cylinder sealing system according to the individual stage operating tempera- ture and pressure. In this way, a very high reliability and availability, with low maintenance, can be achieved. Oil-free compression, required for the very cold low pressure stages 1 to 3, employs the labyrinth sealing principle, which is well proven over many years on LPG carriers and at LNG receiving terminals. The avoidance of mechanical friction in the contactless labyrinth cylin- der results in extremely long lifetimes of sealing components (see Appendix 1). The high-pressure stages 4 and 5 em- ploy a conventional API 618 lubricated cylinder ring sealed compressor tech- nology (see Fig. 3).Fig. 2: Basic design concept for two compressor units 100 %, type 6LP250-5S_16
  6. 6. Labyrinth piston – oil-free compression Ring piston – lubricated designFig. 3: Highly reliable cylinder sealing applied for each compression stageSix cylinders are mounted on top of a Stage 4/5 Stage 4/5 cooled cooledvertical arranged crankcase. The double lube lubeacting labyrinth compression stages 1 Pa= 265 bar a Pa= 265 bar ato 3 are typical of those employed at anLNG receiving terminal. Stage 1 Stage 3 Stage 2 Stage 1 not cooled cooled cooled not cooledThe single acting stages 4 and 5 are adesign commonly used for compres-sion of high-pressure hydrocarbon Ps= 1.03 bar a Ps= 1.03 barprocess gases in a refinery application(Fig. 4).The two first-stage labyrinth cylinders, Heat barrierwhich are exposed to very low tem- stage 1 onlyperatures, are cast in the materialGGGNi35 (Fig. 5). This is a nodular castiron material containing 35 % nickel,also known under the trade name of Ni-Resist D5.This alloy simultaneously exhibits re-markable ductility at low temperaturesand one of the lowest thermal expan-sion coefficients known in metals.The corresponding pistons are madeof nickel alloyed cast iron with laminargraphite. Careful selection of cylindermaterials allows the compressor to be Fig. 4: Main constructional features of the 6LP250-5S compressor 7
  7. 7. Cylinder gas nozzieValve portsFig. 5: Cylinder blockstarted at ambient temperature condi-tion and cooled down to BOG tempera-ture without any special procedures.Second and third stage labyrinth cylin-ders operate over a higher temperaturerange and are therefore provided witha cooling jacket. Cylinder materials arenodular cast iron and grey cast iron re-spectively.The oil lubricated high-pressure 4thand 5th stage cylinders are made fromforged steel and are provided with acoolant jacket to remove heat of com-pression.In view of the smaller compressionvolumes and high pressure, the pistonand piston rod for stages 4 and 5 areintegral and manufactured from a singleforged steel material stock. Compres-sion is single acting with the 4th stagearranged at the upper end and the 5thstage at the lower end and arranged instep design. Piston rod gas leakage ofthe 5th stage is recovered to the suc-tion of the 4th stage (see Fig. 6). Fig. 6: Sectional view of the lubricated cylinder 4th and 5th stage8
  8. 8. Double-acting Capacity Control – Valve Piston rod guiding labyrinth or ring Unloading Piston rod guidance is provided at the lower crank end by a heavy Cylinder nodular cast iron crosshead an Compression at the upper end by an additional section, oil-free Capacity control by valve unloading is or lubricated Packing oil-free guide bearing . Both these or lubricated extensively employed at LNG terminals components are oil lubricated and where very large variations in BOG Heat barrier water cooled. Distance piece pro- flows are experienced during LNG vides separation Oil shield transfer from ship to storage tank. These key guiding elements are therefore subjected to very little Gulde bearing The capacity of the compressor may wear. be simply and efficiently reduced to Piston guide 50 % in one step by the use of valve Heat barrier system lubricated Crosshead unloaders. The nitrogen actuated un- The cold first-stage cylinders loaders (see Fig. 8) are installed on the are separated from the warm compressor motion work by Gas-tight casing lower cylinder suction valves and act means of a special water jacket to unload one half of the double-acting situated at the lower end of cylinders. the cylinder block. This jaket is supplied with a water/glycol Additional stepless regulation, required coolant mixture and acts as a thermal heat barrier. to control a compressor capacity cor- responding to the rate of boil-off andFig. 7: Design principle of vertical gas-tight compressor casing the demand of the engine, is provided by returning gas from the discharge to compressor suction by the use ofMotion work – 6LP250-5S bypass valves. The compressor control system is described in detail later in thisThe 6-crank, 250 mm stoke compres- paper.sor frame is a conventional low speed,crosshead design typically employedfor continuous operating process du-ties. The industry design standard forthis compressor type is the American Compressed gasPetroleum Industry Standard API 618 Cylinder Suction gasfor refinery process application. Valve disc gas nozzleThe forged steel crankshaft and con-necting rods are supported by heavytri-metal, force lubricated main bear- Diaphragmings. Oil is supplied by a crankshaft actuatordriven main oil pump. A single distancepiece arranged in the upper frame sec- Valve seattion provides separation between thelubricated motion work and the non-lubricated compressor cylinders.The passage of the crankshaft throughthe wall of the crankcase is sealed off N2 controlby a rotating double-sided ring seal gas inlet/outletimmersed in oil. Thus, the entire insideof the frame is integrated into the gascontaining system with no gas leakage Fig. 8: Cylinder mounted suction valve unloaderto the environment (see Fig. 7). 9
  9. 9. Compressor System The P&I diagram for the compressor The design of the gas system com-Engineering – 6LP250-5S gas system is shown in Appendix III. prising piping, pulsation vessels, gas intercoolers, safety relief valves and ac-A compressor cannot function correctly Bypass valves are provided over stage cessory components follows industryand reliably without a well-designed 1, stages 2 to 3, and stages 4 to 5. practices for hydrocarbon processand engineered external gas system. These valves function to regulate the oil and gas installations.Static and dynamic mechanical analy- flow of the compressor according tosis, thermal stress analysis, pulsation the engine set pressure within defined Process duty – compressoranalysis of the compressor and auxiliary system limits. Non-return valves are ratingsystem consisting of gas piping, pulsa- provided on the suction, side to preventtion vessels, gas intercoolers, etc., are gas back-flow to the storage tanks, The sizing of the fuel gas compressor isstandard parts of the compressor sup- between stages 3 and 4, to maintain directly related to the “design” amountplier’s responsibility. adequate separation between the of nBOG and, therefore, to the capacity oil-free and the oil lubrication compres- of the LNG carrier.A pulsation analysis considers upstream sor stages, and at the final dischargeand downstream piping components in from the compressor. The fuel gas system design conceptorder to determine the correct sizing of considers compressor operation notpulsation dampening devices and their Compressor safety only for supplying gas to the ME-GI en-adequate supporting structure. gine, but also to deliver gas to the gas Safety relief valves are provided at the combustion unit (GCU) in the event thatThe compressor plant is designed to discharge of each compression stage the engine cannot accept any gas.operate over a wide range of gas suc- to protect the cylinders and gas systemtion temperatures from ambient start- against overpressure. Stage differen- The compressors are therefore rated toup at +30°C down to −160°C without tial relief valves, where applicable, are handle the maximum amount of naturalany special intervention. installed to prevent compressor exces- BOG defined by the tank system sup- sive loading. plier and consistent with the design rat-Each compressor stage is provided ing of GCU.with an intercooler to control the gas Pressure and temperature instrumenta-inlet temperature into the following tion for each stage is provided to en- Design nBOG rates are typically in thestage. The intercooler design is of the sure adequate system monitoring alarm range of 0.135 to 0.15 % per day ofconventional shell and tube type. The and shutdown. Emergency procedures tanker liquid capacity. During steady-first-stage intercooler is bypassed when allow a safe shutdown, isolation and state loaded voyage, a BOG rate ofthe suction temperature falls below set venting of the compressor gas system. 0.10 to 0.12 % may be expected.limits (approx −80°C). Carrier capacities in the range 145 toTable II: Rated process design data for a 210 kcum carrier 260 kcum have been considered, re- sulting in the definition of 3 alternative Volume LNG tanker cum 210,000 compressor designs which differ accord- Max. BOG rate LNG tanker % 0.15 per day and liquid volume ing to frame rating and compressor speed. Density of methane liquid at 1.06 bar a kg/m3 427 assumed basis for design BOG mass flow kg/h 5,600 Rated process design data for a carrier LNG tank pressure low / high bar a 1.06/1.20 capacity of 210 kcum are as shown in Temperature BOG low °C −140 during loaded voyage Table II. Temperature BOG high °C −40 during ballast voyage The rating for the electric motor driver Temperature BOG start up °C +30 is determined by the maximum com- Delivery P to ME-GI pressure low / high bar a 150/265 pressor power required when consider- Temperature NG delivery to ME-GI °C +45 ing the full operating range of suction temperatures from + 30 to −140°C and Compressor shaft power kW 1,600 suction pressures from 1.03 to 1.2 bar a. Delivery P to GCU bar a 4.0 to 6.510
  10. 10. ME-GI Gas System Exhaust recieverEngineeringThe ME-GI engine series, in terms ofengine performance (output, speed,thermal efficiency, exhaust gas amountand temperature, etc.) is identical to the Large volumewell-established, type approved ME en- accumelatorgine series. The application potential for Gas valvesthe ME engine series therefore also ap-plies to the ME-GI engine, provided that ELGI valvegas is available as a main fuel. All ME en-gines can be offered as ME-GI engines. High pressure double wall Cylinder cover with gas pipesSince the ME system is well known,the following description of the ME-GI gas valves and PMIengine design only deals with new ormodified engine components.Fig. 9 shows one cylinder unit of aS70ME-GI, with detail of the new modi- Fig. 9: Two-stroke MAN B&W S70ME-GIfied parts. These comprise gas supplydouble-wall piping, gas valve control The GI system also includes: • They act as flexible connections be-block with internal accumulator on the tween the stiff main pipe system and(slightly modified) cylinder cover, gas in- • Control and safety system, compris- the engine structure, safeguardingjection valves and ELGI valve for control ing a hydrocarbon analyser for check- against extra-stresses in the main andof the injected gas amount. In addition, ing the hydrocarbon content of the air branch pipes caused by the inevitablethere are small modifications to the ex- in the double-wall gas pipes. differences in thermal expansion ofhaust gas receiver, and the control and the gas pipe system and the enginemanoeuvring system. The GI control and safety system is desig- structure. ned to “fail to safe condition”. All failuresApart from these systems on the en- detected during gas fuel running includ- The buffer tank, containing about 20gine, the engine and auxiliaries will ing failures of the control system itself, times the injection amount per strokecomprise some new units. The most will result in a gas fuel Stop/Shut Down, at MCR, also performs two importantimportant ones, apart from the gas and a change-over to HFO fuel operation. tasks:supply system, are listed below, and Blow-out and gas-freeing purging of thethe full system is shown in schematic high-pressure gas pipes and the complete • It supplies the gas amount for injec-form in Appendix IV gas supply system follows. The change- tion at a slight, but predetermined, over to fuel oil mode is always done with- pressure drop.The new units are: out any power loss on the engine. • It forms an important part of the• Ventilation system, for venting the The high-pressure gas from the com- safety system. space between the inner and outer pressor-unit flows through the main pipe of the double-wall piping. pipe via narrow and flexible branch Since the gas supply piping is of com- pipes to each cylinder’s gas valve block mon rail design, the gas injection valve• Sealing oil system, delivering sealing and accumulator. These branch pipes must be controlled by an auxiliary control oil to the gas valves separating the perform two important tasks: oil system. This, in principle, consists of control oil and the gas. the ME hydraulic control (system) oil sys- • They separate each cylinder unit from tem and an ELGI valve, supplying high-• Inert gas system, which enables the rest in terms of gas dynamics, utili- pressure control oil to the gas injection purging of the gas system on the sing the well-proven design philoso- valve, thereby controlling the timing and engine with inert gas. phy of the ME engine’s fuel oil system. opening of the gas valve. 11
  11. 11. ME-GI Injection System Sealing oil inlet Sealing oil inletDual fuel operation requires the injectionof both pilot fuel and gas fuel into thecombustion chamber.Different types of valves are used forthis purpose. Two are fitted for gasinjection and two for pilot fuel. The aux- Cylindercover Cylinder coveriliary media required for both fuel andgas operation are as follows: Connectionto the Connection to the ventilatedpipe system ventilated pipe system Controloil Control oil• High-pressure gas supply Sealing oil Sealing oil Gas inlet Gas inlet• Fuel oil supply (pilot oil) Gas spindle gas spindle• Control oil supply for activation of gas injection valves• Sealing oil supply.The gas injection valve design is shown Fig. 10: Gas injection valve – ME-GI enginein Fig. 10. This valve complies withtraditional design principles of com-pact design. Gas is admitted to the system, in order to detect any malfunc- on fuel oil, without stopping the engine,gas injection valve through bores in the tioning of the valve. this can be done. If the demand is pro-cylinder cover. To prevent gas leakage longed operation on fuel oil, it is recom-between cylinder cover/gas injection The designs of oil valve will allow oper- mended to change the nozzles andvalve and valve housing/spindle guide, ation solely on fuel oil up to MCR. lf gain an increase in efficiency of aroundsealing rings made of temperature and the customer’s demand is for the gas 1% when running at full engine load.gas resistant material are installed. Any engine to run at any time at 100 % loadgas leakage through the gas sealingrings will be led through bores in thegas injection valve and further to spacebetween the inner and the outer shield Low pressure fuel supply Gaspipe of the double-wall gas piping sys-tem. This leakage will be detected by Fuel returnHC sensors. Injection Measuring and limiting deviceThe gas acts continuously on the valve Pressure booster (800 - 900 bar)spindle at a max. pressure of about Position sensor250 bar. To prevent gas from entering Bar abs 800the control oil activating system via theclearance around the spindle, the ELFI valve 200 bar hydraulic oil. 600 Pilot il pressure ospindle is sealed by sealing oil at a Common with exhaust valve actuatorpressure higher than the gas pressure ELGI valve 400(25-50 bar higher). Control oil pressure The system provides: 200 Pressure, timing, rate shaping,The pilot oil valve is a standard ME fuel main, pre- & post-injection 0oil valve without any changes, except 0 5 10 15 20 25 30 35 40 45 Deg CA .for the nozzle. The fuel oil pressure isconstantly monitored by the GI safety Fig. 11: ME-GI system12
  12. 12. As can be seen in Fig. 11 (GI injection High-Pressure Double- supply pipes to the main supply pipe,system), the ME-GI injection system Wall Piping and via the suction blower into the at-consists of two fuel oil valves, two fuel mosphere.gas valves, ELGI for opening and clos- A common rail (constant pressure) gasing of the fuel gas valves, and a FIVA supply system is to be fitted for high- Ventilation air is exhausted to a fire-safevalve to control (via the fuel oil valve) pressure gas distribution to each valve place. The double-wall piping systemthe injected fuel oil profile. Furthermore, block. Gas pipes are designed with is designed so that every part is ven-it consists of the conventional fuel oil double-walls, with the outer shielding tilated. All joints connected with seal-pressure booster, which supplies pilot pipe designed so as to prevent gas ings to a high-pressure gas volume areoil in the dual fuel operation mode. This outflow to the machinery spaces in the being ventilated. Any gas leakage willfuel oil pressure booster is equipped event of rupture of the inner gas pipe. therefore be led to the ventilated part ofwith a pressure sensor to measure the The intervening space, including also the double-wall piping system and bepilot oil on the high pressure side. As the space around valves, flanges, etc., detected by the HC sensors.mentioned earlier, this sensor monitors is equipped with separate mech-anicalthe functioning of the fuel oil valve. If ventilation with a capacity of approx. 30 The gas pipes on the engine are de-any deviation from a normal injection air changes per hour. The pressure in signed for 50% higher pressure thanis found, the GI safety system will not the intervening space is below that of the normal working pressure, and areallow opening for the control oil via the the engine room with the (extractor) fan supported so as to avoid mechanicalELGI valve. In this event no gas injec- motors placed outside the ventilation vibrations. The gas pipes are further-tion will take place. ducts. The ventilation inlet air is taken more shielded against heavy items fall- from a non-hazardous area. ing down, and on the engine side theyUnder normal operation where no mal- are placed below the top-gallery. Thefunctioning of the fuel oil valve is found, Gas pipes are arranged in such a way, pipes are pressure tested at 1.5 timesthe fuel gas valve is opened at the cor- see Fig. 12 and Fig 13, that air is suck- the working pressure. The design is torect crank angle position, and gas is ed into the double-wall piping system be all-welded, as far as it is practicable,injected. The gas is supplied directly from around the pipe inlet, from there using flange connections only to the ex-into an ongoing combustion. Conse- into the branch pipes to the individual tent necessary for servicing purposes.quently the chance of having unburnt gas valve control blocks, via the branchgas eventually slipping past the pistonrings and into the scavenge air receiveris considered to be very low. Monitoring Protective hose Solderedthe scavenge air receiver pressure safe-guards against such a situation. In theevent of high pressure, the gas modeis stopped and the engine returns toburning fuel oil only.The gas flow to each cylinder duringone cycle will be detected by measur-ing the pressure drop in the accumu- Bonded seallator. By this system, any abnormal Ventilation air Ventilation airgas flow, whether due to seized gasinjection valves or blocked gas valves, Fuel Gas flow Fuel Gas flowwill be detected immediately. The gassupply will be discontinued and the gaslines purged with inert gas. Also in thisevent, the engine will continue running Ventilation air Ventilation air Outer pipe High pressure gason fuel oil only without any power loss. Ventilation air High pressure gas pipe Fig. 12: Branching of gas piping system 13
  13. 13. Gas stop Ventilation air Fuel gas valve Control air Nitrogen Cylinder Control Oil cover Purge valves Fuel gas accumulator volume One way valve Fuel gas inlet Control oil buffer volumeFig. 13: Gas valve control blockThe branch piping to the individualcylinders is designed with adequateflexibility to cope with the thermal ex-pansion of the engine from cold to hotcondition. The gas pipe system is alsodesigned so as to avoid excessive gaspressure fluctuations during operation.For the purpose of purging the systemafter gas use, the gas pipes are con-nected to an inert gas system with aninert gas pressure of 4-8 bar. In theevent of a gas failure, the high-pressurepipe system is depressurised beforeautomatic purging. During a normalgas stop, the automatic purging will bestarted after a period of 30 min. Time istherefore available for a quick re-start ingas mode.14
  14. 14. Fuel Gas System - When considering compressor control, The main control input for compressorControl Requirements an important difference between cen- control is the feed pressure Pset re- trifugal and reciprocating compressors quired by the ME-GI engine. The feedThe primary function of the compres- should be understood. A reciprocating pressure may be set in the range of 150sor control system is to ensure that the compressor will always deliver the pres- to 265 bar according to the desired en-required discharge pressure is always sure demanded by the down-stream gine load. If the two ME-GI engines areavailable to match the demand of the user, independent of any suction con- operating at different loads, the highermain propulsion diesel engines. In do- ditions such as temperature, pressure, set pressure is valid for the compressoring so, the control system must ade- gas composition, etc. Centrifugal com- control unit.quately handle the gas supply variables pressors are designed to deliver a cer-such as tank pressure, BOG rate (laden tain head of gas for a given flow. The If the amount of nBOG is insufficient toand ballast voyage), gas composition discharge pressure of these compres- satisfy the engine load requirement, andand gas suction temperature. sors will therefore vary according to the make-up with fBOG is not foreseen, the gas suction condition. compressor will operate on part load toIf the amount of nBOG decreases, the ensure that the tank pressure remainscompressor must be operated on part This aspect is very important when within specified limits. The ME-GI en-load to ensure a stable tank pressure, considering transient starting conditions gine will act independently to increaseor forced boil-off gas (fBOG) added to such as suction temperature and pres- the supply of HFO to the engine. Prima-the gas supply. If the amount of nBOG sure. The 6LP250-5S_1 reciprocating ry regulation of the compressor capac-increases, resulting in a higher than compressor has a simple and fast start- ity is made with the 1st stage bypassacceptable tank pressure, the control up procedure. valve, followed by cylinder valve unload-system must act to send excess gas to ing and if required bypass over stages 2the gas combustion unit (GCU). Compressor control – to 5. With this sequence, the compres- 6LP250-5S_1 sor is able to operate flexibly over theTank pressure changes take place over full capacity range from 100 to 0 %.a relatively long period of time due to Overall control conceptthe large storage volumes involved. If the amount of nBOG is higher thanA fast reaction time of the control sys- Fig. 15 shows a simplified view of the can be burnt in the engine (for exampletem is therefore not required for this compressor process flow sheet. The during early part of the laden voyage)control variable. system may be effectively divided into resulting in higher than acceptable a low-pressure section (LP) consisting suction pressure (tank pressure), theThe main control variable for compres- of the cold compression stage 1, and a control system will send excess gas tosor operation is the feed pressure to the high-pressure section (HP) consisting of the GCU via the side stream of the 1stME-GI engine, which may be subject to stages 2 to 5. compression stage.controlled or instantaneous change. Anadequate control system must be ableto handle such events as part of the Control of gas delivery pressure General Data for“normal” operating procedure. Gas Delivery Condition: Pressure: Gas pressure Set point (bar)The required gas delivery pressure var- Nominal 250 baries between 150-265 bar, depending Max. value 300 baron the engine load (see Fig. 14 below). Pulsation limit ± 2 bar Set point tolerance ± 5%The compressor must also be able Temperature : Approx. 45 oCto operate continuously in full recycle Quality:mode with 100 % of delivered gas Condensate free, without oil/waterreturned to the suction side of the droplets or mist, similar to thecompressor. In addition, simultaneous PNEUROP recommendation 6611delivery of gas to the ME-GI engine and ‘‘Air Turbines’’GCU must be possible. Engine Load ( % of MCR ) Fig. 14: Gas supply station, guiding specification 15
  15. 15. Fig. 15: Simplified flow sheetIn the event of engine shutdown or sud- Pmin suction Prevents under-pressure 1st stage bypass valve, which will openden change in engine load, the com- in compressor inlet mani- or close until the actual compressorpressor delivery line must be protected fold - tank vacuum. discharge pressure is equal to the Pset.against overpressure by opening by- With this method of control, BOG de-pass valves over the HP section of the Phigh suction Suction manifold high- livery to the ME-GI is regulated withoutcompressor. pressure - system safety any direct measurement and control of (GCU) on standby. the delivered mass flow. If none of theDuring start-up of the compressor with above control limits are active, the con-warm nBOG, the temperature con- Pmax suction Initiates action to reduce troller is able to regulate the mass flowtrol valves will operate to direct a flow inlet manifold pressure. in the range from 0 to 100 %.through an additional gas intercoolerafter the 1st compression stage. Pmax Prevents overpressure of The following control limits act to over- ME-GI feed compressor discharge rule the ME-GI controller setting andThe control concept for the compres- manifold. initiate bypass valve operation:sor is based on one main control modewhich is called “power saving mode”. A detailed description of operation with- Pmin suction (tank pressure belowThis mode of running, which minimises in these control limits is given below. set level)the use of gas bypass as the primarymethod of regulation, operates within Power saving mode The control scenario is falling suctionvarious well defined control limits. pressure. If the Pmin limit is active, the 1st Economical regulation of a multi-stage stage recycle valve will not be permittedThe system pressure control limits are compressor is most efficiently executed to close further, thereby preventing fur-as follows: using gas recycle around the 1st stage ther reduction in suction pressure. If the of compression. The ME-GI required pressure in the suction line continues set pressure Pset is therefore taken as to decrease, the recycle valve will open control input directly to the compressor governed by the Pmin limiter.16
  16. 16. Action of Pressure will fall at the burned simultaneously in the GCU.ME-GI control compressor discharge No action is taken in the ME-GI controlsystem: requiring the HFO system. injection rate to be increased. Pmax ME-GI feedfBOG: If a spray cooling or The control scenario is a reduction of forced vaporizer is the engine load or closure of the ME-GI installed, it may be supply line downstream of the com- used for stabilising the pressor. The pressure will rise in the suction pressure and delivery line. Line overpressure is pre- thereby increase the vented by a limiter, which acts to direct- gas mass flow to the ly open the bypass control valve around engine. Such a sys- stages 2 to 5. As a consequence, the tem could be activated controller will also open the 1st stage by the Pmin suction recycle valve. pressure limit. The control range of the compressor isPhigh suction (tank pressure above 0 to 100 % mass flow. set level) GCU-only operating modeThe control scenario is increasing suc-tion pressure due to either reduced The control scenario considers a situa-engine load (e.g. approaching port, tion where gas injection to the ME-GI ismanoeuvring) or excess nBOG due to not required and tank gas pressure is atliquid impurities (e.g. N2). the level of Phigh.The control limiter initiates a manual The nBOG is compressed and deliveredstart of the GCU (the GCU is assumed to the GCU by means of a gas take-offnot to be on standby mode during nor- after the 1st stage.mal voyage). The following actions are initiated:There is no action on the compressorcontrol or the ME-GI control system. • manual start of the GCUPmax suction (tank pressure • closing of the bypass valve too high) around 1st stageThe control scenario is the same as de- • fully opening of the bypassscribed above, however, it has resulted valves around stages 2-5.in even higher suction pressure. Actionmust now be taken to reduce suction In this mode, the compressor is oper-pressure by sending gas to the GCU. ating with stages 2-5 in full recycle at a reduced discharge pressure of approxi-The high pressure alarm initiates a mately 80 bar. The pressure setting ofmanual sequence whereby the 1st the GCU feed valve is set directly by thestage bypass valve PCV01 is closed GCU in the range 3 to 6 bar a.and the bypass valve PCV02 to theGCU is opened. When the changeover There is no action on the ME-GIis completed, automatic Pset control controller.is transferred to the GCU control valvePCV02. The gas amount which can-not be accepted by the ME-GI will be 17
  17. 17. Machinery Room Instal-lation – 6LP250-5SThe layout of the cargo handling equip-ment and the design of their supportingstructure presents quite a challenge tothe shipbuilder where space on deckis always at a premium. In conjunctionwith HHI and the compressor maker, anoptimised layout of the fuel gas com-pressor has been developed.There are many factors which influencethe compressor plant layout apart fromlimited space availability. (See Fig. 16.)External piping connections, adequateaccess for operation and maintenance,equipment design and manufacturing 15mcodes, plant lifting and installation are 27mjust a few. 34mThe compressor together with acces-sory items comprising motor drive, Fig. 16: Typical layout of cargo machinery roomauxiliary oil system, vessels, gas cool-ers, interconnecting piping, etc., aremanufactured as modules requiringminimum assembly work on the shipdeck. Separate auxiliary systems pro- Compensator E. mortorvide coolant for the compressor frameand gas coolers. Discharge lineIf required, a dividing bulkhead mayseparate the main motor drive fromthe hazardous area in the compressorroom. A compact driveshaft arrange-ment without bulkhead, using a suitablydesigned ex motor, is however pre-ferred. Platforms and stairways provideaccess to the compressor cylinders forvalve maintenance. Piston assembliesare withdrawn vertically through man-holes in the roof of the machinery house(see Fig. 17). Oil System Suction line Fig. 17: Fuel gas compressor with accessories18
  18. 18. Requirements for Cargo Foundation deflection due to ship Major intervention for dismantling andMachinery Room Support movement must, furthermore, be con- bearing inspection is recommendedStructure sidered in the design of the compressor every 2-3 years. plant to ensure stress-free piping termi-Fig. 18 shows details of the compressor nations. Average availability per compressorbase frame footprint and requirement unit is estimated to be 98.5 % with bestfor support by the ship structure. Maintenance requirements availability approximately 99.5 %. With - availability/reliability an installed redundant unit, the com-Reciprocating compressors, by nature pressor plant availability will be in theof movement of their rotating parts, The low speed, crosshead type com- region of 99.25 %.exhibit out-of-balance forces and mo- pressor design 6LP250-5S, like thements which must be considered in the ME-GI diesel engine, is designed for Any unscheduled stoppage of thedesign of the supporting structure for the life time of the LNG carrier (25 to 30 6LP250-5S compressor will most likelyacceptable machinery vibration levels. years or longer). Routine maintenance be attributable to a mal-function of a is limited purely to periodic checking in cylinder valve. With the correct valveAs a boundary condition, the structure the machinery room. design and material selection (Burck-underneath the cargo machinery room hardt uses its own design and manu-must have adequate weight and stiff- Maintenance intervention for dis- facture plate valves) these events will beness to provide a topside vibration level mantling, checking and eventual part very seldom, however a valve failure inof (approximately) 1.2 - 1.5 mm/s. Sat- replacement is recommended after operation cannot be entirely ruled out.isfactory vibration levels for compressor each 8,000 hours of operation. Annualframe and cylinders are 8 and 15 mm/s maintenance interventions will normally LNG boil-off gas is an ideal gas to com-respectively (values given are rms – root require 50-70 hours work for checking press. The gas is relatively pure andmean square). and possible replacing of wearing parts. uncontaminated, the gas components are well defined, and the operating tem- peratures are stable once “cool-down” is completed. These conditions are excellent for long lifetime of the compressor valves where an average lifetime expectancy for valve plates is 16,000 hours. Therefore, we do not expect any unscheduled inter- vention per year for valve maintenance. Such a maintenance intervention will take approx. 7-9 hours for compressor shutdown, isolation and valve replace- ment. A total unscheduled maintenance in- tervention time of 25 hours, assuming 8,000 operating hours per year, may be used for statistical comparison. On this basis compressor reliability is estimated at 99.7 %. Our experience in many installations shows that no hours are lost for un- scheduled maintenance. The reliability of these compressors is therefore com-Fig. 18: Compressor base frame footprint parable to that of centrifugal compres- sor types. 19
  19. 19. Requirements for Actual Test and Analysis Main Engine RoomClassification of Safety when Operating Safety on GasWhen entering the LNG market with the The latest investigation, which wascombined two-stroke and reliquefaction The use of gas on a diesel engine calls recently finished, was initiated by asolution, it was discovered that there for careful attention with regard to number of players in the LNG marketis a big difference in the requirements safety. For this reason, ventilated dou- questioning the use of 250 bar gas infrom operators and classification socie- ble-walled piping is a minimum require- the engine room, which is also locatedties. ment to the transportation of gas to the under the wheel house where the crew engine. is working and living.Being used to cooperating with theclassification societies on other com- In addition to hazard considerations Even though the risk of full breakagemercial ships, the rules and design re- and calculations, it has been necessary happening is considered close to neg-commendations for the various applica- to carry out tests, two of which were ligible and, in spite of the precautionstions in the LNG market are new when carried out some years ago before the introduced in the system design, MANit comes to diesel engine propulsion. installation and operation of the Chiba Diesel found it necessary to investi-In regard to safety, the high availability power plant 12K80MC-GI engine in gate the effect of such an accident,and reliability offered when using the 1994. as the question still remains in part oftwo-stroke engines generally fulfil the the industry: what if a double-wall piperequirements, but as the delivery and A crack in the double-wall breaks in two and gas is released frompick up of gas in the terminals is carried inner pipe a full opening and is ignited?out within a very narrow time window,redundancy is therefore essential to the The first test was performed by intro- As specialists in the offshore industry,operators. ducing a crack in the inner pipe to see DNV were commissioned to simulate if the outer pipe would stay intact. The such a worst case situation, study theAs such, a two-engine ME-GI solution test showed no penetration of the outer consequences, and point to the appro-is the new choice, with its high effi- pipe, thus it could be concluded that priate countermeasures. DNV’s workciency, availability and reliability, as the the double-wall concept lived up to the comprised a CFD (computational fluidtraditional HFO burning engines. expectations. dynamics) simulation of the hazard of an explosion and subsequent fire, andCompared with traditional diesel Pressure fluctuation an investigation of the risk of this situa-operated ships, the operators and ship- tion ever occurring and at what scale.owners in the LNG industry generally The second test was carried out tohave different goals and demands to investigate the pressure fluctuations in As input for the simulation, the volumetheir LNG tankers, and they often apply the relatively long piping from the gas of the engine room space, the positionmore strict design criteria than applied compressor to the engine. of major components, the air ventilationso far by the classification societies. rate, and the location of the gas pipe By estimation of the necessary buffer and control room were the key inputA Hazid investigation was therefore volume in the piping system, the stroke parameters.found to be the only way to secure that and injection of gas was calculated toall situations are taken into account see when safe pressure fluctuations are Realistic gas leakage scenarios werewhen using gas for propulsion, and that achieved within given limits for optimal defined, assuming a full breakage ofall necessary precautions have been performance of the engines. The piping the outer pipe and a large or small holetaken to minimize any risk involved. system has been designed on the basis in the inner fuel pipe. Actions from the of these calculations. closure of the gas shutdown valves, theIn 2005, HHI shipyard, HHI engine ventilation system and the ventilationbuilder, BCA and MAN Diesel therefore conditions prior to and after detec-worked out a hazard identification study tion are included in the analysis. Thethat was conducted by Det Norske Ver- amount of gas in the fuel pipe limits theitas (DNV), see Appendix V. duration of the leak. Ignition of a leak causing an explosion or a fire is further- more factored in, due to possible hot20
  20. 20. spots or electrical equipment that can Simulation Results This new engine room design is basedgive sparks in the engine room. on the experience achieved by HHI The probability of this hazard happen- with their first orders for LNG carriersCalculations of the leak rate as a func- ing is based on experience from the equipped with 2 x 6S70ME-C and reli-tion of time, and the ventilation flow offshore industry. quefaction plant. The extra safety thatrates were performed and applied as will be included is listed below:input to the explosion and fire analyses. Even calculated in the worst case, no structural damage will occur in the HHI • Double-wall piping is located as LNG engine room if designed for 1.1 far away as possible from critical bar over pressure. walls such as the fuel tank walls and switchboard room walls. • No areas outside the engine room will be affected by an explosion. • In case of an engine room fire alarm, a gas shutdown signal is sent out, the If this situation is considered to repre- engine room ventilation fans stops, sent too high a risk, unattended machin- and the air inlet canals are blocked. ery space during gas operation can be introduced. Today, most engines and • During gas running it is not possible equipment are already approved by the to perform any heavy lifting with the classification societies for this type of engine room crane. operation. • A failure of the engine room ventilation • By insulation, the switchboard room will result in a gas shutdown. floor can be protected against heat from any jet fires. • HC sensors are placed in the engine room, and their position will be based • No failure of the fuel oil tank structure, on a dispersion analysis made for the consequently no escalation of fire. purpose of finding the best location for the sensors. The above conclusion is made on the assumption that the GI safety system • The double-wall piping is designed is fully working. with lyres, so that variation in tem- peratures from pipes to surroundings In addition, DNV has arrived at a differ- can be absorbed in the piping. ent result based on the assumption that the safety system is not working. On In fact, any level of safety can be the basic in these results DNV have put achieved on request of the shipowner. up failure frequencies and developed The safety level request will be achieved a set of requirements to be followed in in a co-operation between the yard case a higher level of safety is required. HHI, the engine builder HHI, the classifi- cation society and MAN Diesel A/S. After these conclusion made by DNV, HHI has developed a level for their The report “Dual fuel Concept: Analysis engine room safety that satisfies the of fires and explosions in engine room” requirements from the classification so- was made by DNV consulting and can cieties, and also the requirements that be ordered by contacting MAN Diesel are expected from the shipowners. A/S, in Copenhagen. 21
  21. 21. Engine Operating Modes Gas Main Operating Panel in the control the 30% limit for minimum-fuel mode room. In this mode, the control system will be challenged taking advantage ofOne of the advantages of the ME-GI will allow any ratio between fuel oil and the increased possibilities of the ME fuelengine is its fuel flexibility, from which gas fuel, with a minimum preset amount valves system to change its injectionan LNG carrier can certainly benefit. of fuel oil to be used. profile, MAN Diesel expects to lowerBurning the boil-off gas with a varia- this 30% load limit for gas use, but fortion in the heat value is perfect for the The preset minimum amount of fuel oil, now no guaranties can be given.diesel working principle. At the start of hereafter named pilot oil, to be useda laden voyage, the natural boil-off gas is in between 5-8% depending on theholds a large amount of nitrogen and fuel oil quality. Both heavy fuel oil andthe heat value is low. If the boil-off gas marine diesel oil can be used as pilotis being forced, it can consist of both oil. The min. pilot oil percentage is cal-ethane and propane, and the heat value culated from 100% engine load, andcould be high. A two-stroke, high-pres- is constant in the load range from 30-sure gas injection engine is able to burn 100%. Below 30% load MAN Diesel isthose different fuels and also without not able to guarantee a stable gas anda drop in the thermal efficiency of the pilot oil combustion, when the engineengine. The control concept comprises reach this lower limit the engine returnstwo different fuel modes, see also to Fuel-oil-only mode.Fig. 19. Gas fuels correspond to low-sulphur• fuel-oil-only mode fuels, and for this type of fuel we rec- ommend the cylinder lube oil TBN40 to• minimum-fuel mode be used. Very good cylinder condition with this lube oil was achieved from theThe fuel-oil-only mode is well known gas engine on the Chiba power plant.from the ME engine. Operating the A heavy fuel oil with a high sulphur con-engine in this mode can only be done tent requires the cylinder lube oil TBNon fuel oil. In this mode, the engine is 70. Shipowners intending to run theirconsidered “gas safe”. If a failure in the engine on high-sulphur fuels for longergas system occurs it will result in a gas periods of time are recommended to in-shutdown and a return to the fuel-oil- stall two lube oil tanks. When changingonly mode and the engine is “gas safe”. to minimum-fuel mode, the change of lube oil should be carried out as well.The minimum-fuel mode is developedfor gas operation, and it can only be Players in the market have been fo-started manually by an operator on the cussed on reducing the exhaust emis- sions during harbour manoeuvring. When testing the ME-GI at the MAN Diesel research centre in Copenhagen,Fuel 100% Fuel-oil-only mode Fuel 100% "Minimum fuel" mode Fuel gas Fuel oil Min. Pilot Fuel oil oil 5-8% 100% load 30% load 100% load Min load for Min. fuel modeFig. 19: Fuel type modes for the ME-GI engines for LNG carriers22