Wartsila SP a Id LNGPac

8/11/2019 Wartsila SP a Id LNGPac

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Enabling the safe storage of gasonboard ships with the Wrtsil LNGPacA U T H O R S : S r e n K a r l s s o n , G e n e r a l M a n a g e r , G a s S y s t e m s , W r t s i l S h i p P o w er a n d

L e o n a r d o S o n z i o , P r o j e c t M a n a g e r D e v e l o p m e n t P r o j e c t s , W r t s i l S h i p P o we r

The use of liquefied natural gas (LNG)

as a marine fuel has taken on added

significance as a result of the IMOsstringent requirements concerning

emissions from ships. The safe and

convenient onboard storage of LNG

is ensured by the Wrtsil LNGPac

system.

In October 2008, the IMO nalized itsrevision of the Marpol Annex VI thePrevention of Air Pollution from Ships.Te stringent requirements thus introducedconcern mostly the sulphur oxide (SOX)and nitrogen oxide (NOX) emissions from

the exhaust gases. However, the IMO isalso working on other measures intended

to reduce greenhouse gases from shipping.Wrtsil dual-fuel engines in gas mode

produce roughly 80% less NOXcomparedto IMO ier I levels and practically zeroSOXand particulates, and are, therefore,compliant with the most stringentregulations. Moreover, when gas is used ina dual-fuel engine, CO2emissions arereduced by about 20% compared toliquid fuels.

In addition to the environmental issues,the use of LNG as a marine fuel haspositive effects on a ships operating costs.Depending on the initial purchase price,the LNG used to power ship engines can

be expected to have a similar, or slightlyhigher, price per energy content than

heavy fuel oil.Te handling of gas in a safe way is, of

course, of great importance. It requires theadequate integration of the entire chain,from the bunkering stations at shipsideto the engine inlet, until the storedhydrocarbon energy is nally convertedinto power. Wrtsils LNGPac systemensures that the safety aspects onboard shipare handled by one single qualied supplier.Te development of LNGPac is supportedby Wrtsils Ship Power system integrationcapabilities.

Key features

Te design philosophy of the LNGPachas been to focus on safety and simplicity.

Fig. 1 The LNGPac system layout.

Bridge control panel Switch gear

Engine control room

Bunkering stations

Bow thrusters

Manoeuvring thruster

Main engine generatorsets (4)

Gas valve units Tank room

LNG Storage tank

Azimuth mainpropulsion units

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WRTSIL TECHNICAL JOURNAL 01.2010

From the beginning, the emphasis has beenon a complete system approach and on

achieving a seamless interface with otherWrtsil products and systems.Te LNGPac is designed according to

the IMOs recently published InterimGuidelines on Safety for Natural Gas-FuelledEngine Installations in Ships(Resolution MSC285-86).

Te system includes the key componentsdescribed below and shown in Figure 1.

Bunkering station(s)Tis is the ships connection with theLNG terminal on shore or with the LNG

bunkering barge. Each station includesone bunkering line (LNG line), one returnline, and one nitrogen purging line withrespective control/thermal relief valves(pressure safety valves) and anges. Tereturn line is used in case the bunkeringoperation takes place with two hosesconnected, and the evaporated gas isreturned to the bunkering terminal orbarge. During bunkering operations,LNG could evaporate due to heatleakages in the piping and/or due to thehigher temperature in the storage tank

onboard compared to the relling tank.

1IMO IGC code = International code for the construction and equipment of ships carrying liquefied gases in bulk.

LNG vacuum insulated pipesFrom the bunkering station, LNG is

led to the tank via insulated pipes.Vacuum insulation is selected for itsexcellent insulation properties, and tominimize LNG evaporation duringbunkering.

LNG tankTe pressurized storage tank is cylindricallyshaped with dished ends.Te tank is designed in accordance withthe IMO IGC Code, the InternationalCode for the Construction and Equipmentof Ships Carrying Liqueed Gases in

Bulk, and EN 13458-2 "Cryogenicvessels. Static vacuum insulated vessels".

LNGPac tanks are insulated withperlite/vacuum. Te tank consists ofa stainless steel inner vessel, which isdesigned for an internal pressure, andan outer vessel that acts as a secondarybarrier. Te outer vessel can be made ofeither stainless steel or carbon steel.

According to the current IMOGuidelines, the LNG fuel tanks have tobe selected from among the Independentypes A, B, or C. Te LNGPac is designed

according to ype C requirements.

Table 1 - Comparison of IMO IGC1independent tanks.

TankType

Description Pressure Pros Cons

A Prismatic tankadjustable tohull shapes.Full secondarybarrier

< 0.7 bar(g) Space efficient Boil-off gashandling

More complexfuel system(compressorrequired)

B Prismatic tankadjustable tohull shapes.Partialsecondarybarrier

< 0.7 bar(g) Space efficient Boil-off gashandlingMore complexfuel system(compressorrequired)

Spherical(Moss type).Full secondarybarrier

< 0.7 bar(g) Reliable/proven system Boil-off gashandlingMore complexfuel system(compressorrequired)

C Pressure vessel(cylindricalshape with

dished ends)

> 2 bar Allows pressureincrease (easyboil-off gas handling)

Very simple fuelsystemLittle maintenanceEasy installation

Space demand onboard the ship

A summary of the main characteristics of theindependent tank types is shown in able 1.

As summarized in able 1, the pressurevessel (as selected for LNGPac) allows easyhandling of the evaporated gas (boil-off),since the tank is designed to withstanda signicant pressure increase and thepressure relief valves are set at 9 bar(g).In practice, vessels can operate for a longtime in liquid fuel mode (HFO or MDO)before having to take care of the pressureincrease in the tank. Te handling of theboil-off is done very simply by a temporaryswitch over of the engines to gas mode,and the gas is taken from the vapor phase

in the upper part of the tank. As anindication, a 200 m3pressurized typeC tank, lled at 50% could hold LNGfor about 25 days, even without anygas consumption from the tank.

A Wrtsil dual-fuel engine requiresapproximately 4 5 bar(g) at the inlet ofthe gas valve unit. In case LNG is storedat atmospheric pressure (ype A and ypeB tanks), the fuel system should includeeither compressors or cryogenic pumpsto deliver the fuel at the correct pressure.

Process skid and tank roomTe process skid includes all theconnections and valves between the tankand the Pressure Build-Up Unit(PBU)and the Product Evaporator, together

with the evaporators themselves.Te PBU consists of an insulated pipe,

an evaporator, valves, a single wall pipe andsensors. Te mission of the PBU is to buildup the pressure in the tank after bunkeringLNG and to maintain the requiredpressure in the tank (around 5 bar(g)),during normal operation. Maintaining the

correct pressure in the tank ensures thatthe Wrtsil dual-fuel engines are able tomeet the maximum power (100% MCR)at any time. Since the LNGPac systemdoesnt have any cryogenic pump orcompressor, the engine gas inlet pressurerequirements are met by achievingthe correct storage pressure inside the LNGtank. Te circulation of LNG to the PBUevaporator is achieved by the hydrostaticpressure difference between the top andbottom of the tank, with LNG fromthe bottom of the tank being fed to the

evaporator. Te evaporated gas is thenreturned to the top of the tank. Te naturalcirculation through the PBU continuesuntil the required pressure in the tankis achieved.

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Fig. 2 LNGPac simplified P&ID.

Tank room

LT-water heat

exchanger

To GVU

LNG

Gas

Anti-freeze heating media

LT-water

PBU

Product

evaporator

Stop valve &

master valve

Table 2 The LNGPac tank range.

Type LNGPac 105 LNGPac 145 LNGPac 194 LNGPac 239 LNGPac 284

Geometric volume [m3] 105 145 194 239 284

Net volume (90%) [m3] 95 131 175 215 256

Diameter [m] 3.5 4.0 4.3 4.3 4.3

Tank length [m] 16.7 16.9 19.1 23.1 27.1

Tank room [m] 2.5 2.5 2.7 2.7 3.0

Total length [m] 19.2 19.4 21.8 25.8 30.1

LNGPac empty weight [ton] 47 62 77 90 104

Tank full weight [ton] 92 125 161 195 228

LNGPac max operating weight [ton] 94 127 164 198 231

Theoretical Max. Autonomy [MWh] 244 318 427 525 625

* Includes an estimate of the process skid weight.

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WRTSIL TECHNICAL JOURNAL 01.2010

Te Product Evaporatorcircuit consistsof an insulated pipe, an evaporator, valves,

a single wall pipe, and sensors. Te task ofthe product evaporator is to evaporate theLNG into gas and heat it to a minimum of0C as per engine specications. Te gasis then fed to the gas valve unit beforethe engines.

Both the PBU and Product Evaporatorare heated by a water/glycol mixture, whichis re-circulated to an external cooler. Here,the mixture is heated by the waste heatfrom the low temperature engine cooling

water circuit.Te process skid has a modular design,

making it easy to be selected and assembledfor the entire LNGPac product range.Te key parameters inuencingthe modularization of the process skid arethe sizes and volume of the tank, andthe engines (model and number) connectedto the tank.

Te tank room is a stainless steel barrierwelded to the outer vessel of the tank.Te structure contains the process skid andall the pipe penetrations to the tank. In theunlikely event of an LNG leakage,the tank room acts as a barrier that avoids

damage to the external compartments, andfacilitates the quick ventilation of theevaporated gas. Te tank room andventilation system are to be re protectedto A-60/A-0 insulation class, depending onthe safety designation of the adjacent space.

AutomationTe LNGPac control system is based

on Wrtsils vessel automation systemplatform. When combined with theWrtsil Integrated Automation System(IAS), the same hardware and HumanMachine Interface can be used throughoutthe vessel to operate the LNGPac,the dual-fuel (DF) engines, and thepropulsion system. In addition, separatelydelivered features typical of DF-engineapplications, have been incorporatedinto the Wrtsil IAS. Tese include the

Wrtsil Operators Interface System(WOIS), Condition Based Maintenance

(CBM), and monitoring of IMO ier III*compliance in Emission Control Areas.

Te core of the control system is aPLC cabinet placed in a safe area near thetank room. All LNGPac transmitters andintrinsically safe sensors, as well as allinterfaces to external systems such as re& alarm, gas detection, etc., originate fromthe cabinet. Te pneumatic valves arecontrolled by solenoids placed in a safe areaadjacent to the tank room and bunkeringstation(s).

In the case of ships retrotted with

LNGPac, the control system is able tooperate as a separate system with monitorson the bridge and ECR (Engine ControlRoom), or by being integrated intothe existing system.

The LNGPac portfolioLNGPac will be offered in various

congurations that allow the installationof multiple tanks and gas valve units(GVUs), i.e. multiple engines. able 2below illustrates the most straightforwardconguration (i.e. single tank installation).Inherent redundancyof the dual-fuel technologyTe superior redundancy and reliabilityof the dual-fuel technology has been takeninto consideration by authorities whendeveloping their rules for the use of gas asfuel on board ships. Te possibility ofswitching over from gas to liquid fuel is

considered as a valid methodology to achieve100% redundancy in case of a leakage ora failure in the gas supply system.

Te main redundancy requirements aresummarized as being: For single fuel installations (gas only),

the fuel storage should be dividedbetween two or more tanks ofapproximately equal size. Te tanksshould be located in separatecompartments (Resolution MSC 285-86,2.6.2.3).

In the case of leakage in a gas supply

pipe making shutdown of the gas supplynecessary, a secondary independent fuelsupply should be available. Alternatively,in the case of multi-engine installations,independent and separate gas supplysystems for each engine or group ofengines may be accepted. (ResolutionMSC 285-86, 2.6.2.2).

* The Wrtsil DF engine portfolio fulfils IMO Tier III in gas mode.

LNGPac 280 LNGPac 308 LNGPac 339 LNGPac 402 LNGPac 440 LNGPac 465 LNGPac 520 LNGPac 527

280 308 339 402 440 465 520 527

252 277 305 362 396 419 468 474

4.8 4.8 5.0 5.0 5.6 5.0 5.6 5.0

21.3 23.4 23.5 27.5 23.8 31.5 27.8 35.5

3.0 3.0 3.0 3.0 3.0 3.5 3.5 3.5

24.3 26.4 26.5 30.5 26.8 35.0 31.3 39.0

105 113 119 135 142 152 162 168

229 248 267 312 336 357 392 401

233 252 271 316 340 362 397 406

616 677 745 884 967 1022 1143 1159

Note: Data reported in the table are indications only and can be subject to change.

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For an installation with engines able to runonly on gas, the redundancy requirements

can be met either by installing doubleLNG tanks and fuel systems, or byinstalling additional diesel generators. Tegenerators must provide sufficient powerduring shut-down of the gas system.

If there is sufficient space onboard thevessel for one LNG tank, dividing the tankvolume will result in increased, highercomplexity, and reduced holding time dueto increased heat leakage into the tank.

For installations without dual-fuelengines, the requirement of a secondaryindependent fuel supply system has been

typically fullled by installing back-upgenerators running on liquid fuel (dieselor HFO). Depending on the vesselpropulsion conguration, for instancediesel electric or mechanical propulsion,the classication rules of redundantpropulsion or emergency propulsion canbe applied in order to achieve the requiredredundancy level. As a consequence,the total installed power onboard a ship

without dual-fuel engines might increaseconsiderably, since separate gas burning

engines and engines running on diesel fuelare installed simultaneously. Te amount

of auxiliary power may, therefore, increaseconsiderably as compared to where thepower requirements would be those ofthe hotel load only.

A reference group of installations whereredundancy has been met by installingboth engines capable of burning gas, andengines running on diesel is shown inable 3. For this group of vessels, the ratiobetween installed diesel engines power andinstalled gas engines power varies from10% up to 161%; the emergencygenerators have been excluded from the

calculation. Despite the higher amount ofinstalled power, the complete propulsionplant cannot be run at full load on bothfuels.

Conversely, in LNG carriers andoffshore supply vessels equipped with

Wrtsil dual-fuel engines, no primemovers other than the DF engines wereinstalled. Tanks to the inherent fuelexibility of the dual-fuel technology, thecomplete vessel power needs can befullled by gas or liquid fuel (even liquid

biofuel if requested).Furthermore, there are a number of

operational reasons that favour a highdegree of fuel exibility: During bunkering, local authorities

may require that the vessel not beoperated on gas.

Te availability of LNG duringoperation cannot be guaranteed.

Inadequate LNG storage volume fortransit from shipyard to end user (forexample from China to Europe).

Options of fuel choice to the operatoror owner decision based on fuel prices.

As a conclusion, it is possible from botha technical and regulatory standpointto build a vessel without diesel back-upgenerators and without redundant tanks/gas supply system. A safe and cost efficientapproach is to select Wrtsil dual-fuelengines and the LNGPac storage system.

Table 3 Reference group of gas-fuelled ships with diesel generators installed for redundancy.

Vessel-type Gas engines [kW] Diesel engines [kW] Propulsion power [kW]Ratio diesel engine/gas engine

installed power [%]

A 2 x 26002 x 3400

2 x 600 4 x 2750 10%

B 1 x 5250 2 x 720 5250 27%

C 2 x 900 1 x 1000 2 x 2000 56%

D3 x 9001 x 676

1 x 40001 x 400

6500 77%

E 2 x 2380 2 x 3840 2 x 5000 161%

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Wartsila SP a Id LNGPac