utilisation volatile organic compounds 19993965

7/28/2019 Utilisation Volatile Organic Compounds 19993965

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Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Scope of This Paper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

VOC Release to the Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Shuttle tankers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Crude oil carriers/VLCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Utilisation of VOC as Engine Fuel . . . . . . . . . . . . . . . . . . . . . . . . . 5

The technical challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

The technical solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Engine design features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Combustion Tests with VOC Fuel . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Basic combustion behaviour of VOC fuel . . . . . . . . . . . . . . . . . . . . . . 11

Emission characteristics, basic tests . . . . . . . . . . . . . . . . . . . . . . . . . 12

Combustion adaptation for VOC fuel . . . . . . . . . . . . . . . . . . . . . . . . . 13

Safety System Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Verification of the VOC Fuel System on a Shuttle Tanker . . . . . . . 18

Environmental Advantages of the VOC Fuel Concept . . . . . . . . . . 18

Availability of the VOC Utilisation System . . . . . . . . . . . . . . . . . . . 18

Enhanced fuel economy in shuttle tankers and VLCCs . . . . . . . . . . . 18

Example A: Shuttle tanker with VOC utilisation system . . . . . . . . . . . 19

Example B: 300,000 dwt VLCC with VOC utilisation system . . . . . . . . . 22

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Appendix: Economy Model Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Utilisation of Volatile Organic Compoundsin Shuttle Tankers and Crude Oil Carriers

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Introduction

Shuttle tankers are widely used toserve offshore oilfields from which pipe-line connections are not feasible. Theshuttle tankers load their crude oil cargoeither from storage facilities at the oilfieldor directly from the production platformsand the loading buoys at the oilfield.The crude oil cargo is then transportedto oil refineries close to the oilfield. Thetransport distance may be fairly short.

Crude oil carriers like VLCCs mostlyserve oilfields on land and may transportthe oil over long distances from oil ter-minals in the production areas to refin-eries closer to the areas where the oilis used.

During the handling of the oil, i.e. duringloading and unloading in particular, largequantities of the light components of theoil evaporate. These oil vapours arenormally called VOC, short for VolatileOrganic Compounds. Evaporation alsooccurs during the voyage when the oilsplashes around in the tanks. To preventthe oil vapours exploding, the tanksare filled with inert gas, which normallyconsists of cleaned combustion gaswith an oxygen content below 8%. Thebulk of the inert gas is thus nitrogen.

To keep the pressure in the storagetanks below 0.14 bar gauge (which isa typical contemporary design value),the VOC is today allowed to dischargeto the atmosphere through a vent pipe

from the crude oil tanks. In addition tohydrocarbons, the discharge containsa relatively large amount of inert gas.

The discharge of the VOC represents agreat loss of energy, as well as an en-vironmental problem. Thus, the non-methane part of the VOC released tothe atmosphere reacts in sunlight withnitrogen oxide and may create a toxicground-level ozone and smog layer,which has a detrimental effect on hu-man health and the environment [1].Ozone and smog attack mucous mem-branes (in the eyes and lungs), cropsand forests.

The Norwegian authorities have com-mitted themselves to reducing VOCemissions to the environment to a sus-tainable level. It is reasonable to expectthat limitations on non-methane VOCemissions will also be introduced in thefuture by other countries. Since the dis-charged VOC, as already mentioned,represents a large amount of energy,the optimum solution will be to con-dense and collect the VOC in specialtanks and use it at high pressure as

fuel for the main engine, instead ofheavy fuel.

This will reduce the VOC release to theatmosphere and, since VOC is a cleanerfuel without sulphur, the exhaust gas ofthe main engine will be much cleanerthan when heavy fuel oil is used. Finally,the fuel oil operating costs of the shipswill be considerably reduced.

The key technologies for achieving thisare the VOC collection, storage andsupply systems developed by Statoil,and the MAN B&W high-pressure gas-injection MC-GI engine modified toburn the VOC.

Scope of This Paper

The present situation for shuttle tank-ers and crude oil carriers like VLCCstransporting crude oil, and the corre-sponding environmental problem of theVOC release to the atmosphere are de-scribed. On the basis of measurementsby Statoil of the VOC discharged fromtheir shuttle tankers, the composition,amount and energy content of the VOCare evaluated.

The paper describes an attractive tech-nical solution to the VOC emissionproblem: the VOC Fuel system whichboth reduces the VOC emission prob-lem and utilises the VOC as fuel for themain engine(s). Results of extensivecombustion tests with VOC combustionon MAN B&W Diesels 4T50MX researchengine are described together with recor-dings from the Type Approval Test forthe high-pressure, gas-injection system,performed for representatives of sixClassification Societies.

The enhanced fuel economy and thecost/benefit of installing the VOC utilisa-tion system on shuttle tankers andVLCCs have been analysed by meansof examples.

Utilisation of Volatile Organic Compoundsin Shuttle Tankers and Crude Oil Carriers

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VOC Release to theAtmosphere

The evaporation from the crude oiltanks of shuttle tankers primarily occursduring loading and discharging of cargoand during crude oil washing of cargotanks. However, in the case of othercrude oil tankers like VLCCs sailing overlong distances, the VOC emission duringthe voyage may also be significant.

Shuttle tankers

When a pipeline is not feasible to trans-port offshore crude oil ashore, shuttletankers will have to take the oil from theoilfield to the shore. Four different sys-tems that are currently in use and un-der construction are shown in Fig. 1 [2].

In Case 1 with subsea storage cellsand Case 2 without storage cells, thecrude oil is loaded directly from the oil-field on board the shuttle tanker. InCase 1 the shuttle tankers are loadedvia a buoy or a loading platform, and inCase 2 via a submerged turret.

In Cases 3 and 4, the crude oil is storedin a permanently moored tanker, while

the oil production in Case 3 takesplace on a fixed or floating platform andin Case 4 directly on the moored ship.

In Cases 1 and 2, the evaporation ofVOC occurs on the shuttle tankers, onwhich a possible VOC utilisation systemshould therefore be located. In thesetwo cases, it may be possible to recoverand utilise all VOC.

In Cases 3 and 4 the major part of the

evaporated VOC may be conductedvia a gas return line from the shuttletanker to the moored storage tanker,on which the VOC utilisation or recoverysystem could be installed. In these casesthe return VOC gas might, in principle,be used as a substitute for inert gas,which will reduce the need for produc-ing inert gas for the moored storagetanker. If all VOC is to be recoveredand utilised, a minor VOC utilisationsystem also has to be installed in theshuttle tanker.

Since 1986, Statoil, which is the leadingoperator of shuttle tankers in the NorthSea, has monitored the VOC emissionsfrom shuttle tankers, ref. the aboveCase 1. The investigations show thata substantial amount of oil vapour is

released to the atmosphere, in particu-lar during loading.

In order to reduce the environmentaleffects of the operation of the vessel,Statoil has studied ways of loweringthe VOC emissions from its fleet andhas initiated remedies such as modi-fied tank design, new loading proce-dures, lower crude oil vapour pressuresand lower temperatures during loadingas well as absorption of the VOC into

the crude oil (however, this transfersthe problem of handling the gases tothe next link in the production chain).

Measurements by Statoil showed thatthe magnitude of the energy lost byreleasing VOC to the atmosphere wascomparable to the total HFO consump-tion of the vessel, see Fig. 2, whichcompares the released VOC energyand the corresponding fuel energyneeded for a shuttle tanker during around trip in the North Sea:

Statfjord oilfield Rotterdam/return and

Gullfaks oilfield Mongstad/return.

This led to the idea of using the VOCas the main fuel for the engines, as this

Production

VOC

Transport

Shuttle tanker

Case 4: Floating Production,

Storage and Offloading (FPSO)

Fixed orfloatingproductionplatform

Storage

Storage

VOCProduction

Transport

Moored tanker forfloating productionand storage

Case 3: Floating Storageand Offloading (FSO)

Shuttle tanker Moored storage tanker

VOC

Storage/transport

Case 1: Gravity Based

Structure (GBS) Storage

Production

Submerged turretloading shuttle tanker

Case 2: Submerged

Turret Loading (STL)

ProductionVOC

Transport

Fixed orfloatingproductionplatform

Shuttle tanker Buoyloadingplatform

Gravitybasedproductionplatform

Fig. 1: Crude oil production, storage and transport systems shuttle tankers

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would substantially reduce the environ-mental effects of VOC release, providesignificant savings on HFO costs and aconsiderable reduction in the toxic gasemissions from the engines. A shuttletanker with its frequent loading andshort sailing distance to port will bene-fit substantially from this new VOC utili-sation technology.

Crude oil carriers/VLCC

Crude oil is shipped in large bulk con-signments and carried in huge tanks atclose to atmospheric pressure. As thegas in the tanks has to be kept aroundatmospheric pressure, large amountsof VOC evaporate and are subsequentlyemitted to the atmosphere during trans-port and handling.

Measurements have shown that the

amount of evaporated VOC may bearound 0.6% of the total cargo volumefor a 300,000 dwt VLCC transportingIranian light crude oil from the Persian

Gulf to north-west Europe [3]. This cor-responds to some three weeks heavyfuel oil consumption for the ship, andrepresents a significant savings potential.

However, the type and behaviour ofthe various types of crude oil on boardtankers may vary and so may theamount of VOC. The amount of VOC

may also depend on the vessels tankdesign, the ambient conditions (a hotclimate promotes increased VOC emis-sions), and the vessels sailing sched-ule (trade pattern).

According to the above, therefore,crude oil carriers will also benefit fromthe new VOC utilisation technology,though on a smaller scale. The VOCemissions from such vessels may beregulated in the future, in which casethis technology will ensure compliancewith new rules and, at the same time,

reduce the engines toxic gas emissionsand provide a substantial reduction inthe vessels consumption of heavy fueloil.

Utilisation of VOC asEngine Fuel

The majority of main engines in theworlds shuttle tanker fleet are of MANB&W design, so it was natural for Stat-oil to contact us about the possibility ofadapting our engines to utilise VOC asthe main fuel. A cooperation agreementhas been signed for the joint develop-ment and demonstration of the relevanttechnology for use, not only in Statoils

future vessels, but also by other inter-ested shipowners.

The technical challenges

As mentioned above, most VOC releaseoccurs during loading, when the crudeoil splashes into the inert-gas-filledcargo tanks of the vessel. The splash-ing, as well as the presence of non-hydrocarbon inert gas, promotes vapori-sation of the light fractions, in particularmethane, ethane, propane, butane, pen-tane and some higher hydrocarbonsC6+, see the example for Statoil shuttletankers serving the Norwegian oilfieldsof Statfjord and Gullfaks in Fig. 3. Theexample shows that the mole fractionsof the various hydrocarbons in thedischarged VOC vary greatly with theoilfield in question.

The discharged inert gas mainly con-sists of nitrogen (about 83%) withsmaller amounts of carbon dioxide(12%) and oxygen (5%).

The discharged gas (on todays ves-sels vented to the atmosphere) thuscontains the above-mentioned hydro-carbons as well as inert gas. Duringloading, the proportion of hydrocar-bons varies from about 20% of theemitted volume at the start of loadingto about 70% when the cargo tanksare nearly full, see Fig. 4.

The composition of the VOC from dif-ferent oilfields, as shown above, variesconsiderably. The propane and heavierhydrocarbons account for 87% of thetotal energy of the VOC at the Statfjordoilfield and only 46% at the Gullfaks oil-

0

1

2

3

4

10 MJ6

Rotterdam/ return

Mongstad/return(west coast

of Norway)

VOC energy Statfjord

VOC energy

Gullfaks

Energy balance

(no contributionfrom methane

and ethane)

5

Assumptions:288,000 Nm VOC emitted during loading with average ALFA = 0.4Engine efficiency = 0.37

.

.

3

VOC energyGullfaks10 MJ

6

Energyconsumption

Mongstad/return10 MJ

6

Energyconsumption

Rotterdam/return10 MJ

6

VOC energyStatfjord10 MJ

6

VOC-processing

Return to North Sea

Unloading

Sailing

Loading

Total energy consumption

Energy content VOC

0.116

0.228

0.475

0.228

0.432

1.479

0.116

1.458

0.475

0.458

0.432

3.939

1.4403.670

Fig. 2: Comparison of energy content in the VOC discharge with the energy requirement of

a shuttle tanker during a round trip in the North Sea (Source: Statoil)

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field. It also varies during the produc-tion lifetime of the oilfields.

It should be mentioned that Statfjordand Gullfaks are old oilfields. Neweroilfields have less total VOC emissionwhich, however, tends to increase withage.

There are two main challenges in utilis-ing the VOC. First, the emission rate ofVOC during loading far exceeds thefuel consumption rate needed in thesame period, so it is necessary to col-

lect the VOC (or as much as possibleof it) during that period and store it un-til the engine needs it. Secondly, thecombustion of a variable and to someextent unpredictable fuel compositionmakes great demands on the flexibilityand adaptability of the engines com-bustion system. These demands mustbe met, along with the demands of thevessel on the engine.

A shuttle tanker or any other crude oilcarrier requires that the main engines

must, at any time, be capable of sup-plying the power required by the ves-sel. This applies equally if there is noVOC supply, or if the VOC collection,

storage and supply systems fail. Thus,engines utilising VOC must at any timebe able to switch over to operation onHFO, supplying the same power soonly dual-fuel type engines can be con-sidered for this application.

In principle, the VOC could be used tofuel the auxiliary engines. However, thetotal amount of VOC emitted from ashuttle tanker or other crude oil carri-ers far exceeds the total amount of fuelneeded for the auxiliaries so most ofthe VOC must still be burned in the

main engines.

Among the various types of dual-fuelengines, lean-burn gas engines em-ploying stratified-charge combustion ofa premixed gas/air charge in the cylin-der, ignited by a pilot injection of fueloil, are often used for stationary appli-cations. The gas used is normally natu-ral gas (mainly methane) of a constantand known composition. The fuelgases used in such engines normallyhave methane numbers 1 in the range

80-100 and normally no lower than 70.If the methane number of the fuel gasvaries during operation, the enginemust be designed for the lowest meth-

ane number that can occur during op-eration.

In the actual VOC fuel gas in question,the high content of propane and, inparticular, the higher hydrocarbons C4 -C6+ lead to methane numbers whichare very low (close to zero) and whichvary with the oilfield, making it unrealis-tic to utilise this in a lean-burn dual-fueltype of engine.

Thus, the only technology available forusing VOC as fuel is high pressure

injection of the VOC directly into thecylinders. This is what is used on ourhigh-pressure gas-injection MC-GIengine. Fig. 5 shows a 12K80MC-GIengine which uses this technology toburn natural gas in a power station inChiba, Japan. The engine started op-eration in 1994 and serves as a peakshaving plant, supplying electricity toTokyo during the daytime.

1The methane number expresses the

knock resistance of gaseous fuels, similarto the octane rating of gasoline for motor

vehicles. Methane is rated 100 while hydro-

gen is used for zero.

Hydrocarbons

Inert gas

ALFA =Hydrocarbon volume

Total gas release volume

ALFA

0 0.2 0.4 0.6 0.8 1.0

0

0.2

0.4

0.6

0.8

1.0

0.19 0.20.21

0.24

0.32

0.43

0.6

0.670.7

Relative loading time

0.27

Fig. 4: Relative volume content ofhydrocarbons in the discharged gas

0

0.2

0.4

0.6

0.8

0.404

0.210

0.106

0.060

0.067

0.049

0.021

0.083

1.000

Statfjord

VOC-specification from loading at Statfjord and GullfaksBased on Statoil measurements and recordings 1986-1996

GullfaksHC-spec.Gullfaks

Molefraction1.0

methane

ethane

propane

i-butane

n-butane

i-pentane

n-pentane

C6+

0.060

0.160

0.370

0.060

0.180

0.040

0.050

0.080

1.000

Mole fractions of HC gas

StatfjordHC-spec.

Total

Fig. 3: Example of VOC discharge during the loading of a shuttle tankerin the North Sea: Mole fraction of the hydrocarbons

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The gas injection system and its safetysystem have been type-approved formarine use by five major classificationsocieties. The type-approval applies tothe use of this system on any of MANB&W Diesels type-approved two-strokeengines. The MC-GI technology is de-scribed in detail in our paper LargeDiesel Engines using High PressureGas Injection Technology [4].

Fig. 5: A 40 MW 12K80MC-GI-S engine utilising natural gas

in dual-fuel mode in Chiba, Japan

Crudeoil

VOCgas

4. High-pressureVOC supply pump

Exhaust gaslow on SO , NO

and particulatesx, x

Crude oilsupply

2. VOC gascondensationsystem

1. VOC gascleaning

system

6.VOC

injectionsystem on

engine

Vent to atmosphere

(Mostly nitrogen)

3.VOC

storagetank

(CondensedVOC gas)

5.VOC

preheatingsystem

VOC

Tanker

AirFuel oil

VOC treatmentand collectionsystem on deck

Fig. 6: Principle of the VOC utilisation system

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The technical solution

The VOC utilisation system shownschematically in Fig. 6 consists of themain equipment mentioned below.Statoil is responsible for Items 1, 2, 3,4 and 5 while MAN B&W Diesel is incharge of Item 6 (patents are pendingfor the proprietary technology devel-oped by the two companies):

1. VOC gas cleaning system

2. VOC gas condensation system3. VOC storage tank4. High-pressure VOC supply pump5. VOC preheating system6. VOC injection system on the engine.

As mentioned above, the tendency torelease VOC is greatest during thehandling of the crude oil, i.e. in shuttletankers especially during loading at theoilfield. The VOC has to be convertedto a form that can easily be collectedand stored until the engine can use it.

The VOC and inert gases emitted fromthe crude oil tanks are therefore con-ducted through gas pipes to the VOCtreatment and collection system con-sisting of (1) a cleaning system and (2)a gas condensation system. The con-densed hydrocarbons are separatedfrom the lighter hydrocarbons + inertgas (which are currently emitted to theatmosphere) and transported to a stor-age tank. Liquid VOC is taken from thetank, pressurised and supplied to theengine, where it is injected directly into

the cylinders by an electronically con-trolled mechatronic injection system.The various stages/phases of the totalVOC collection, storage, supply and in-jection systems are briefly described inthe following:

1. VOC gas cleaning system. All rustetc. that has peeled off from the crudeoil tanks must be removed before thegas enters the gas condensation sys-tem.

2. VOC gas condensation system.The gas is compressed before it iscooled. During this process the pro-panes, butanes (liquefied petroleumgas, LPG) and the higher hydrocar-

bons may condense and become liq-uid. The inert gas and the light VOCgas (methane and ethane) remain ingaseous form and are vented to theatmosphere. The liquid VOC is sepa-rated and transported to a storagetank. The condensation of liquids andseparation from the gaseous phasemay take place in several steps de-pending on the available technology.

3. VOC storage tank.The liquefied

VOC may be stored in a tank which,depending on the VOC amount to bestored, may be either:

a pressurised tank storing the VOCat ambient temperature. If the vol-ume to be stored is less than about300 m3, a pressurised tank storingthe liquid VOC at ambient tempera-ture at a pressure of about 10-15bar gauge may be the most eco-nomical; or

a cooled tank at atmospheric pres-sure. If the volume to be stored ismore than about 300 m3, an insu-lated and cooled tank storing the liq-uid VOC at atmospheric pressure atlow temperature may be the mosteconomical.

4. High-pressure VOC supply pump.From the VOC storage tank the VOCmay be delivered to the diesel engineat a pressure of about 400 bar. A high-pressure reciprocating diaphragmpump with quantity control to ensure a

stable pressure at the engine inlet maybe used. This type of pump seals 100%tight and ensures that the VOC cannotenter the lube oil system of the pump.It must be possible to stop the pumpwith a signal from the engine in theevent of a shutdown of operation onVOC.

5. VOC preheating system.To avoidthe risk of ice formation on the outsideof the high-pressure VOC pipes (whichmay happen in the case of decompres-sion of the pipes, involving flash-boilingof the VOC), the VOC is heated toabout 50

oC before inlet to the engine.

The preheater may, for example, utilisesteam or heat from the jacket cooling

water system on the engine but, in anycase, a separate circuit (exchangingheat with the jacket cooling water)must be used to ensure that VOC can-not enter the cooling water system ofthe engine if there is a leakage in theVOC heat exchanger.

6. VOC injection system on the engine.The preheated pressurised VOC is in-jected directly into the combustionchamber immediately after the injection

of a small amount of fuel oil (8% of theoil amount at 100% load), acting aspilot oil and securing stable, safe com-bustion. The special VOC injectionvalves are operated by a mechatronicsystem which features computer con-trol to allow for the greatly varying prop-erties of the VOC fuel. The system isdescribed in detail in the following.

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Engine design features

The internal and external systemsneeded for operation of the engine onVOC are shown schematically in Fig. 7.As can be seen, these systems are verysimilar to the well-proven systems usedon the natural gas burning 12K80MC-GI-Shigh-pressure gas-injection engine, de-scribed in detail in [4], the major differen-ces being that the VOC (being a liquid) iscompressed to 400 bar by means of a

membrane type pump (not a multi-stagegas compressor) and that the injection ofVOC is controlled by a computer control-led mechatronic system (not by a cam-shaft driven control oil pump).

The cylinder cover (Fig. 8) has boresfor two fuel oil valves and two VOCinjection valves. Furthermore, there arebores for two more valves which canbe used for injection of the gaseousVOCs (methane and ethane) or forinjection of water in order to reducethe NOx emission, if required. On thecamshaft side, the cylinder cover hasa face for fitting the valve block.

The valve block (Fig. 9) houses a VOCaccumulator, a fast acting shutdownvalve, a non-return valve at the VOCinlet pipe, blow off and purging controlvalves and the fast acting NC valvebelonging to the Mechatronic VOCinjection system (see below). The pre-heated and pressurised VOC is injecteddirectly into the combustion chamberimmediately after the injection of a small

amount of fuel oil (8% of the oil amountat 100% load), acting as pilot oil andsecuring stable, safe combustion at allengine loads.

The VOC injection valves (Fig. 10) areoperated by a Mechatronic systemwhich features computer control to allowfor the greatly varying properties of theVOC fuel. They are supplied with seal-ing oil (fine filtered system lube oil fromthe engine) from a separate sealing oilsystem at a pressure some 25-50 barabove the VOC pressure in order toprevent the VOC from leaking into thecontrol oil system and in order to lubri-cate the moving parts of the VOC injec-tion valves.

Engine room Outside

Outside

Pilotpump

Sealing oil systemVOC system on the engine

Exhaust receiver

Cylindercover

Valve blockInert gas line

Actuator ECSGovernorfunction

Ventilation system VOC supply system

Double wall pipe

VOC pipe

Air flow direction

VOC flow direction

Pressure oilsupply formechatronicsystem

Mechatroniccontroloil system

HPpump

Airheater

Fig. 7: Internal and external systems for diesel engine operation on VOC

Fuel oil

Gas

VOC

Fig. 8: Cylinder cover with bores for fuel oil valves, VOC valves and valves for gas or waterinjection

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The mechatronic system is a com-puter controlled and hydraulically acti-vated system. The concept is shown inFig. 11. A pump station on the enginesupplies lube oil at high pressure to anelectronically controlled hydraulic valve(NC valve) for each cylinder. VOC is in-jected by opening the NC valve, admit-ting high-pressure lube oil to the VOC

injection valves. The lube oil pressureopens the VOC injection valves and al-lows injection of pressurised VOC intothe cylinder. When sufficient VOC hasbeen injected, the lube oil pressure isreleased to the tank by shifting the NCvalve to its other position and, as a re-sult, the spring-loaded spindle in theVOC injection valve closes, and VOCinjection is terminated. By virtue of thissystem, the timing of the VOC injectioncan be freely controlled in relation tothe injection of pilot oil so as to adaptto the actual combustion behaviour of

the VOC.

Safety system.A full MC-GI safetysystem is incorporated (see a detailed

description of this in [4]). The systemensures redundancy, i.e. the enginechanges over to diesel mode in caseof any abnormality in the VOC system,

maintaining the same power output.Recordings of the functionality of thesafety system will be shown below.

VOC Double wallventilation system

Control oil

Ventilationsystem

Seal oil

Fig. 10: VOC injection valve

Gas

Blow off

Gas

Gas

VOC accumulator

Shut down valve

Double wallventilationsystem

Fig. 9: Valve block with shutdown valve and VOC accumulator

VOCinjection

valvesElectroniccontrolsignal

Highpressure

pump

PressurisedVOC supply

NC valve

Lube oil reservoir

Accumulator

Valve block

Fig. 11: Concept of the Mechatronic VOC injection system

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Combustion Testswith VOC Fuel

The combustion tests with VOC in-clude tests with dual fuel operation aswell as pure VOC operation on a largebore research engine (the 4T50MX In-telligent Engine, a 10,000 bhp, 4-cylin-der 50-cm-bore engine) in Copenhagen.The engine was equipped with aMechatronic VOC fuel injection system.The tests carried out served to identify

the demands which would be made onthe engine and fuel injection control sys-tem when using VOC as the main fuel.

As mentioned above, the liquefiedVOC consists mainly of propane andhigher hydrocarbons for which reasona rather low methane number can beexpected. Consequently, the self igni-tion properties of the VOC might allowoperation on pure VOC, i.e. withoutpilot injection of HFO as the source ofignition, thus opening an attractive pos-

sibility for nearly eliminating particulateemissions and the complete replace-ment of HFO by VOC.

Some of the main results of the initialinvestigations are briefly outlined in thefollowing, accompanied by results ofadaptation tests in which the modernresearch engine was adapted to per-form similar to the 14 year old mainengines of the selected test vessel (seebelow).

Basic combustion behaviour ofVOC fuel

The above mentioned possibilities foroperating the engine in single fuelmode on VOC are illustrated in Fig. 12.The VOC composition used for thesetests was a 70/30 mixture of propaneand butane (in fact a commercial LPGbrand) which was considered to be atthe poor ignition quality end (or the rela-

tively high methane number end) of theactual range of VOC compositions. Ifstable ignition could be obtained at lowengine load with this VOC composi-tion, it was considered that all the avail-able VOCs could be burned in pureVOC operation. In this phase of devel-opment, methane and ethane are notincluded as fuel.

The tests were all carried out at 123r/min, corresponding to the research

engines MCR point (Maximum Con-tinuous Rating, i.e. 100% load andrated engine speed), since the shuttletanker engines in question operate atconstant engine speed due to the useof shaft generators and ControllablePitch (CP) propellers.

As can be seen from Fig. 12, combus-tion at high load is very satisfactory,with a smooth pressure rise in the cylin-der and a smooth ROHR (Rate ofHeatRelease) with an almost negligible igni-

0

10

20

30

40

50

60

70

80

170 180 190 200 210 220 230 240

TDC

Rate of heat release

25%50%

75%

100%

MJ/s

Deg. C.A.

Start

of

VOCinjectionat

100

,7

5and50%

load

StartofVOCinjectionat

25%

load

0

20

40

60

80

100

120

140

160

bar

120 140 160 180 200 220

180

240

25%

50%

75%

Cylinder pressure

Deg. C.A.

100%

Fig. 12: Cylinder pressure and Rate of Heat Release in pure VOC operation

11


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tion delay. The ROHR starts rising im-mediately upon start of the VOC injec-tion, and the ignition delay is significantlyless than 1 oCA. Though the ROHRdoes not rise to the same peak valueas for diesel operation, combustion isbasically completed at the same time,and the indicated efficiency is almostthe same.

At lower loads, the situation is com-pletely different and combustion char-

acteristics resemble those of a smallhigh speed engine running on a mar-ginal ignition quality fuel: there wasaudible knock from the engine, thereason being evident from the cylinderpressure curve (steep pressure rise)and from the ROHR: a nearly stepwiseinitial rise to a very high peak-value ofthe ROHR, reflecting the pre-mixedburning period following the rather longignition delay period (in the order of 6oCA, equal to some 4.4 ms). The injec-tion timing at 25% load was retardedsome 1.5 oCA, with injection starting

around TDC, in order to limit the audi-ble knocking to an acceptable value.

Though ignition was rather stable (thecurves shown are average values of 50consecutive cycles), it is evident thatthe engines cylinder condition (in par-ticular the piston ring condition) wouldsuffer from the hard combustion in partload operation. The absolute require-ment of shuttle tankers for reliableoperation at any load without time limi-

tations, together with the logic of thepertaining safety system, mean thatpure VOC operation is not a straight-forward possibility.

Consequently, dual fuel operation usingVOC as the main fuel and diesel fuel asthe source of ignition must be used.The cylinder pressure and ROHR curvesfrom 25% to 100% load shown in Fig.13 indicate that stable ignition andquite normal diesel type combustioncan be obtained in this way. An interes-ting feature is, however, that ignition at

all loads takes place before the start ofthe pilot oil injection. The reason forapplying such a late injection timing wasthat an early injection of the pilot fuelseems to disturb combustion of themain VOC fuel under the given circum-stances. The VOC composition testedignites easily before the pilot oil, how-ever, this may be due to a local hotspot or another ignition source createdby the pilot oil injection since pure VOCoperation at low load is significantly dif-

ferent from this dual fuel mode.

Emission characteristics, basic tests

The tests were carried out on one cylin-der only (with the remaining three cylin-ders operating on diesel fuel), so theaccuracy of the emission measurementsis limited, even when gas samples aretaken in the exhaust pipe from the VOCcylinder, before the exhaust gas receiver.Thus, the measurements should beconsidered only as a rough indication

0

10

20

30

40

50

60

70

80

170 180 190 200 210 220 230 240

MJ/s

TDC

Star

to

fp

ilo

to

ilin

jec

tion

Startof V

OC

injection

Rate of heat release

25%

50%

75%

100%

Deg. C.A.

0

20

40

60

80

100

120

140

160

bar

120 140 160 180 200 220

180

240

25%

50%

75%

100%

Cylinder pressure

Deg. C.A.

Fig. 13: Cylinder pressure and Rate of Heat Release in dual fuel operation with 10% pilot oil

12


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of what can be expected from the en-gines in a shuttle tanker (see below).

The results from these first tests (at 75%load) show a reduction in NOx emis-sions in the dual fuel mode of around27%, compared to the diesel mode,confirming the expected values. In pureVOC mode (without pilot oil injection),the NOx emission is only around 5%lower than in the diesel mode.

Emissions of carbon monoxide and un-burned hydrocarbons were expectedto increase in the dual fuel mode dueto the increase in the fuel nozzle sacvolume (four fuel valves per cylinderversus two in pure diesel operation).CO and HC emissions were found toincrease by some 25 and 40%, respec-tively, while in the pure VOC mode, theCO and HC values were unchanged.The final verification of the emissioncharacteristics will be obtained in early1999 when the shuttle tanker entersservice with the complete VOC collec-tion, storage and utilisation system.

Combustion adaptation for VOC fuel

As mentioned above, the engines ofthe selected test vessel are around 14years old and represent the state ofthe art at that time. Modern engineshave higher compression ratios andoperate at much higher mean effectiveand combustion pressures, as illus-trated in Table 1. As a consequence,

the ignition and combustion behaviourmight be somewhat different and in or-der to prepare properly for the demon-stration test, the research engine wasmodified extensively so as to approachthe performance characteristics of theL55GUCA engine installed in the ship.

Initial tests in this configuration clearlyindicated that pure VOC operationwould not be acceptable, not even athigh load, and the tests continued indual fuel operation, to investigate theeffects of a number of parametersrelevant for the actual engines. Someresults are outlined in the following.

VOC injection pressure: Fig. 14shows the influence of the VOC injec-tion pressure (with constant injectiontiming) on cylinder pressure, ROHRand NOx emissions. Thanks to the

slower injection and mixing of the VOCat lower injection pressures, the ROHRis lower, leading to a lower firing pres-sure and lower NOx emissions. Com-bustion, however, is smooth andbasically satisfactory in all cases.

Pilot oil amount: Fig. 15 shows theinfluence of the pilot oil amount (withconstant injection timing) on cylinderpressure, ROHR and NOx emissions. Itis desirable to use as low an amount ofpilot oil as possible in order to replaceas much fuel oil by VOC as possible.On the other hand, stable injection andignition of the pilot oil must be ascertai-ned to ensure stable and reliable opera-tion of the engine.

As can be seen from the figure, thecombustion process of the VOC(ROHR) is almost identical for the threetested amounts of pilot oil: 5%, 8%(normal value) and 10% of the amountat full load, with a tendency to slightlylower NOx emission for low amount ofpilot oil. The initial part of the ROHR-

curve is slightly different, reflecting thedifferent amount of pilot oil injectedinitially.

It can be concluded that the lower limitfor pilot oil amount is determined bystable functioning of the pilot injectionvalves and not by the ignition processor the combustion of the VOC: at 5%

pilot oil, the pilot oil valve spindle isonly partly lifted, and a small variationin pump index, opening pressure forthe pilot oil valves or fuel pump wear(leakage) may lead to failure to injectpilot oil through one injection valve orin one cylinder in this event the safetysystem will trigger a gas system shutdown and revert to fuel oil only opera-tion. Thus, 8% pilot oil will be used soas to ensure stable operation.

Pilot oil/VOC amount: Fig. 16 showsthe influence of the ratio between pilotoil amount (with constant injectiontiming) and VOC amount (with variableinjection timing) on cylinder pressure,ROHR and NOx emissions. To ensureflexible operation of shuttle tankers(and LNG carriers as well) it is neces-sary to be able to use the availableVOC irrespective of the actual loaddemand on the engine. Thus, if there isnot sufficient VOC (or boil off gas) avail-able, the pilot oil amount must be in-creased so as to provide the requiredpower output from the engine. If this is

done with fixed timing for pilot oil andVOC (or gas), the cylinder pressure willincrease substantially (by some 15 bar

Design features 6L55GUCA 7S50MC 4T50MX

Maximum firing pressure 98 bar 140 bar 180 bar

Mean effective pressure 13 bar 18 bar 21 bar

Scavenge air pressure 2.90 bar 3.55 bar 3.7 bar

Mean piston speed 7.1 m/s 8.1 m/s 9.0 m/s

Stroke to bore ratio 2.51:1 3.82:1 4.40:1

Rated engine output per cylinder 1100 kW/cyl. 1430 kW/cyl. 1840 kW/cyl.

Rated engine speed 155 r/min 127 r/min 123 r/min

Table 1: Design features of the main engines on board the test vessel, of a typical standardmain engine for shuttle tankers of today, and of the 4T50MX research engine

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010

20

30

40

50

60

70

80

90

100

140 150 160 170 180 190 200 210 220

0

5

10

15

20

25

30

35

40

45

50

170 180 190 200 210 220 230 240 250

175 185 190 195 200 205

Deg.CA.

Cylinder

pressure

(bar)

Pilot oil

VOC (5% pilot)

8% pilot

VOC (8% pilot)

10% pilot

VOC (10% pilot)

180

Deg.CA.

Deg.CA.

Rate

of

heat

release

(MJ/s)

Needle

lift

0

0,2

0,4

0,6

0,8

1

0

4

8

12

16

5% pilot 8% pilot 10% pilot

NO

(g/kWh)

x

5%8%

10%

5% pilot

Pilot oil

5%8%

10%

Fig. 15: Effect of pilot oil amount on cylinder pressure, Rate of Heat Release and NOx emission shown together with the lifting curves forpilot oil injection valve and VOC injection valve

0

10

20

30

40

50

60

70

80

90

100

140 150 160 170 180 190 200 210 220

400 bar350 bar300 bar

0

5

10

15

20

25

30

35

40

45

50

170 180 190 200 210 220 230 240 250

Deg.CA.

Cylinderpressure(bar)

Deg.CA.

Rateofheatrelease(MJ/s)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

Pilot oil valve

180 185 190 195 200 205 210

Deg.CA.

Needlelift

0

4

8

12

16

300 bar 350 bar 400 bar

NO

(g/kWh)

x

175

VOC valve:



Fig. 14: Effect of VOC injection pressure on cylinder pressure, Rate of Heat Release and NO x emission shown together with the liftingcurves for pilot oil injection valve and VOC injection valve

14


14/24

at 50% VOC/50% fuel oil), thus over-loading the engine.

The figure shows the combustion be-haviour of the engine with optimisedcontrol of VOC injection (but fixed tim-ing of the pilot oil, which is injected bythe conventional camshaft operatedfuel pumps). As can be seen, the

mechatronic system makes it possibleto control the VOC injection in such away that the cylinder pressure andcombustion (ROHR) remain virtuallyunchanged, independent of the ratiobetween pilot oil amount and VOCamount. The figure also illustrates thatNOx emission in the dual fuel mode islower than in the pure diesel mode.

It is obvious from the diagram withneedle lift for the pilot oil injectionvalves and the VOC injection valves

that this optimal control would hardlybe possible with a conventional cam-shaft operated system for VOC injection.

0

20

40

60

80

100

120

140 160 180 200 220

Cy

lin

der

pressure

(bar)

100%80%60%40%20%10%

0

0,2

0,4

0,6

0,8

1

60

170 180 190 200 210 220 230 240 250

Nee

dle

lift

200185 205195 210175 190

Pilot oil

20%40%60%80%

100%80%60%40%20%10%

90%

0

10

20

30

40

50

Hea

tre

lease

(MJ/s)

0

4

8

12

16

20

10 20 40 60 80 100

% DO

NO

(g/kWh)

x

[Deg.CA.]

Deg.CA

180

100%

80%

60%

40%

20%

10%

100%

80%

60%

40%

20%

10%

Deg.CA

Pilot oil valve lift

Pilot oil

VOC valve lift

Fig. 16: Effect of VOC/pilot oil ratio on cylinder pressure, Rate of Heat Release and NOx emission shown together with the lifting curves forpilot oil injection valve and VOC injection valve

15


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Safety System Performance

The utilisation of VOC is controlled bythe mechatronic system as illustratedabove and monitored by a dedicatedsafety system with the same functional-ity as that of the natural gas burningMC-GI engines, described in detail in [4]The engines normal safety system fordiesel operation is fully maintained andis complemented by the VOC safetysystem.

The main features of the VOC safetysystem are briefly outlined in the follow-ing and illustrated by recordings fromthe Type Approval Test for the system,performed on 24 and 25 June 1998 onthe 4T50MX research engine in Copen-hagen to the full satisfaction of six ma-jor Classification Societies.

Operational precautions. In compli-ance with the demands from the Clas-sification Societies, all start, stop andmanoeuvring takes place in the dieselmode (HFO) only. Also, operation be-low a certain load limit will only takeplace in the diesel mode, the limitbeing determined by the demand forstable operation of the VOC injectionsystem, i.e. stable minimum injectionamount of VOC.

Thus, the limit will be decided fromcase to case on the basis of the actualengine layout and propeller type (fixedpitch or controllable pitch propeller),typically resulting in a lower limit of

some 20-30% engine output. If the en-gine load drops below the relevantlimit, the engine automatically changesover to diesel operation and maintainsthe demanded power.

Fuel supply system. It is essential toprevent VOC in the engine room, espe-cially VOC gases which, becausethese gases are heavier than air, wouldtend to accumulate at the bottom ofthe engine room. Therefore, VOCpipes and all VOC containing enginecomponents are of double wall de-sign meaning that any VOC leakagewill go directly into the ventilatedannular space surrounding the highpressure VOC pipes.

This annular space is kept at lowerpressure than the engine room pressureand ventilated at a minimum rate of 30times per hour. The ventilation air isfiltered and preheated to some 50 oCand monitored at the outlet by twohydrocarbon sensors. The monitoringsystem sets off an alarm at a VOCconcentration of 30% of the LowerExplosion Limit (LEL) and triggers aVOC system shut down at 60% LEL.

The performance of the safety systemin this respect is illustrated in Fig. 17.To trigger the shut down, propane gashas been blown into the double wallpipe system and, as can be seen, thesafety system closes the shut downvalve in the valve block (see Fig. 9) andthe engine changes over to dieselmode. A very similar picture is seen forthe VOC shut down when the VOCsupply pressure becomes too low (indi-cating high pressure pump failure, frac-tured pipes or lack of VOC supply), toolow sealing oil pressure, too low engineload and indication of VOC in the en-

gine room (the latter being rather un-likely).

Combustion monitoring system.Tomonitor the combustion process, eachcylinder unit of the engine is providedwith sensors for cylinder pressure, fueloil injection pressure and pressure inthe VOC accumulator in the valveblock.

The cylinder pressure is monitored in

order to detect faults such as a stickingexhaust valve, leading to ignition failurebecause of the lack of compressionpressure, or any combustion irregularity.

The fuel oil injection pressure is moni-tored in order to ensure that pilot oilhas been injected prior to the start ofVOC injection, thus ensuring safe com-bustion of the VOC. If the pressuredoes not reach the opening pressurelevel for the fuel oil injection valves, themechatronic system will not allow VOCinjection, and the engine immediatelychanges over to diesel fuel mode on all

Engine revolutions

Mechatronic NC valve lift

VOC injection valve lift

Pilot oil injection valve lift

0 1 2 3 4 5

Shut down valve lift

Fig. 17: Demonstration of safety system features: VOC system shut down due to too highVOC concentration in the double walled pipe simulated by introducing propane in the pipe

16


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cylinders. This is illustrated in Fig. 18where the VOC shut down is triggeredby the pilot oil injection pump indexsuddenly being pushed to zero. As canbe seen, no VOC is injected right fromand including the first failure cycle,and the load is quickly re-establishedin the diesel mode.

The pressure in the VOC accumulatoris monitored in order to ensure that aVOC injection valve sticking in the fully

open position does not lead to a dan-gerous situation due to the largeamount of VOC injected. It is hardlylikely that sticking will occur from onenormal cycle to the next most likelythe valve will operate increasingly slug-gishly over a period of some length,leading to higher load on the pertainingcylinder and thus increased exhaustgas temperature. This is monitored aswell, and a too large deviation for acylinder from the average exhaust gastemperature triggers a VOC shutdown.

However, even the situation of a sud-den sticking in the fully open position ismanaged by the safety system, and theengine safety will not be endangered:Fig. 19 shows the safety system reac-tion to a sticking VOC injection valve,simulated by suddenly increasing theVOC injection amount substantiallyabove the normal value. When thepressure drop in the accumulator ex-ceeds a limit value, indicating that injec-tion continues beyond the permitted

amount, the shut down valve in thevalve block is immediately closed bythe safety system, thus preventing fur-ther injection.

Engine revolutions

Pilot oil injection valve lift


0 1 2 3 4 5

Pilot oil injection pressure

Cylinder pressure

Fig. 18: Demonstration of safety system features: VOC system shut down due to missingpilot oil injection simulated by reducing the pilot pump index to zero

Shut down valve lift


0 1 2 3 4 5 6

Pressure in VOC accumulator

Cylinder pressure

Engine revolutions

Fig. 19: Demonstration of safety system features: VOC system shut down due to sticking

VOC injection valve simulated by introducing a very large VOC injection

17


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Verification of the VOC FuelSystem on a Shuttle Tanker

As the next step, a full-scale demon-stration of the technology and conceptfor shuttle tankers is being prepared.The test will be carried out on M/TNavion Viking, a shuttle tanker servingthe Norwegian Statfjord oilfield, whichprovides large amounts of VOC. Thevessel has two 6.6 MW 6L55GUCAmain engines, both equipped with

shaft generators.

During a planned dry-docking of thevessel in May 1998, equipment wasfitted for converting the two main en-gines for VOC burning. At the sametime, the vessel was prepared forfitting the full-scale VOC collection,storage and supply systems, whichwill be used to supply VOC fuel to theconverted main engines. The systemswill be fitted during the first quarter of1999, after which one of the main en-gines will start operating on VOC. Theother main engine will follow some sixmonths later, subject to satisfactoryoperation on VOC of the first engineand satisfactory operation of the VOCcollection, storage and utilisationsystems.

The full scale trials comprise one yearsoperation on VOC. Statoil will use theresults as a basis for deciding on thefull implementation of the VOC fueltechnology in its tanker fleet.

Environmental Advantagesof the VOC Fuel Concept

Depending on the composition andamount of the VOC as well as theships sailing schedule, up to some90% of the shuttle tankers fuel oilconsumption may be replaced by theVOC, leading to substantial fuel costsavings (considering the formerly dis-charged VOC vapours to be free ofcharge) as well as cleaner exhaust gas:

up to some 90% reduction of SOxemissions, directly proportional tothe percentage of fuel oil substitu-tion. A further economic advantage

is that the use of VOC may replacelow-sulphur fuels in IMO Specialareas that require operation onlow-sulphur fuels

up to some 90% reduction in par-ticulate emissions, due to the lighterand more volatile fuel, which causesless smoke formation

20-30% reduction in NOx emissionsdue to the dual fuel combustion

process and more uniform mixing offuel and air in the cylinders

some reduction in CO2 emissionsdue to the higher hydrogen/carbonratio in VOC fuel than in fuel oil.

Availability of the VOCUtilisation System

The concept will be generally availableto interested shipowners after the suc-cessful termination of the demonstra-tion test mentioned above. It might,however, be of interest to shipownersto have new shuttle tankers preparedfor this technology even before then.For the engine, this means the use ofthe MC-GI design for the exhaust gasreceiver and the cylinder covers:

The exhaust gas receiver needs tobe made of thicker plates (pluschanges in some minor designdetails) to comply with the require-ments of the Classification Societies

The cylinder covers have to beprovided with extra bores for theVOC injection valves and faces forthe fitting of a valve block on thecamshaft side.

The extra cost of preparing the enginefor VOC operation is some 1-2% ofthe engine cost. This preparation allowsthe engines to operate on normal fueloil until it becomes feasible to carry outthe full conversion of the vessel so asto be able to collect, store and burnthe VOC. This option has already beenselected by a number of owners order-ing shuttle tankers, and more than adozen engines (mainly of the 7S50MC

type) are currently on order or in serv-ice prepared for VOC as describedabove.

Enhanced fuel economy in shuttletankers and VLCCs

With the system described above, theVOC can be utilised and will thus re-duce the fuel bill of the ship as well asthe pollution of the environment.

The advantages of the enhanced fueleconomy and the cost/benefit of install-ing the VOC utilisation system oncrude oil tankers have been analysedin terms of net present value (NPV).For guidance, this economy model isdescribed in detail in the Appendix.We have evaluated the economy of theVOC system, using two examples, viz.A) a shuttle tanker, and B) a 300,000dwt VLCC.

The fuel cost in particular will be dis-cussed, because of its great influenceon the operating costs of the ships,but the differences in spare parts con-sumption, overhaul work, and lube oilcosts have also been taken into consid-eration.

The investment cost for the VOC utilisa-tion system greatly influences the netpresent value and the payback time,while differences in price relating tomain engines and electrical power pro-ducing equipment, also included, have

a minor influence.

The investment cost also depends onthe actual requirements for the limita-tion of VOC emissions. Thus, if thereare no such VOC limitation require-ments, the whole investment cost ofthe VOC utilisation system should beincluded. However, if there is a require-ment for reduction of the VOC emis-sions, the cost of the pertaining systems(for instance a VOC recovery system)should be considered, so that only thedifference in investment cost is included.

The investment cost for the VOC utilisa-tion system used as the VOC Fuelreference case in the following two

18


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examples is probably on the low side ifthere is no environmental requirementto reduce VOC emissions, while it isprobably much too high if there is sucha requirement. However, we feel thatthe feasibility study illustrates the char-acteristics of various systems and indi-cates the magnitude of the potentialbenefits quite well.

Example A: Shuttle tanker with

VOC utilisation system

A shuttle tanker requires quite uniquemanoeuvrability. Loading of the shipincludes long periods of accurate dy-namic positioning at the oilfield, usingside thrusters and main engine drivenpropeller(s).

The large side thrusters installed andused mainly for dynamic positioningcall for equipment that can generatesufficient electrical power, i.e. largediesel generators or large shaft gener-ators. With the installation on board ofa large power generating capacity,owners often decide to install cargopumps driven by electric motors.

Whether power is supplied by dieselgenerators or shaft generators, trans-formers are needed to provide voltageregulation for the generators, theswitchboard, the thrusters and thecargo pumps. For more details as re-gards shuttle tankers, please see ourpaper Shuttle Tanker Propulsion [5].

The electrical power production equip-ment normally used today includeslarge diesel generators. An alternativeis large shaft generators driven by themain engine(s), see Figs. 20 and 21,respectively. The investment cost ofthe electrical power producers is al-most the same in these two cases.Using these engine room arrangements,we have evaluated the VOC utilisationsystem for a shuttle tanker with twomain engines of the 7S50MC type,each driving a CP propeller.

Measurements of the amount of VOCfrom a shuttle tanker with the roundtrippattern Statfjord Rotterdam/return in

DG

ME

ME

DG

ME: Main engineDG: Diesel generator

Pumproom

ME

Pump room

DG

DG

DG

Fig.. 20: Engine room arrangement with diesel generators shuttle tanker

Disconnectablethrust bearing

DG

ME

ME

DG

ME: Main engine

SG: Shaft generatorDG: Diesel generator

Pump

room

ME

SG

Pumproom

Fig.. 21: Engine room arrangement with shaft generators shuttle tanker

19


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Fig. 2 have shown that the energy lostby the VOC evaporation during around trip is of the same magnitude asthe total consumption of heavy fuel bythe main engines. The trade patternand the power consumption of thisround trip are illustrated in Fig. 22,together with three engine roomalternatives.

Using a contemporary shuttle tankerwith diesel generators (DG) and with-out a VOC utilisation system as thebasic Alternative No. 1, two otherarrangements with the VOC utilisationsystem are compared, the one alterna-tive, No. 2, with diesel generators (DG)and the other alternative, No. 3, withshaft generators (SG).

Since the main engine, by driving ashaft generator, can meet the relativelyhigh electrical power demand, Alterna-

tive 3 will have the highest proportionof VOC-based power (see Fig. 23)and, therefore, as seen in Fig. 24, willrepresent the lowest annual fuel costs.

Propulsion power, ME

Main engines:

SMCR:2 x 7S50MC

2 x 10,010 kW at 127 r/min

Electrical power production, DG or SG

Buoy-

loadingat oilfield

Voyage to

port

Unloading

at portVoyage to

oilfield

1

2

3

4

4'

1

1

1

2

0

5,000

10,000

15,000

5,000

10,000

El. power consumers

1 Accommodation and aux. machinery2 Cargo pumps3 Side thrusters

4 VOC system, step 1

4' VOC system, step 2

days/year

hours/trip

Engine room alternatives

Alt. 1 ME + 4 x DG (3,000 kW)

Alt. 2 ME + VOC + 4 x DG (3,000 kW)

Alt. 3 ME + VOC + 2 x SG (7,500 kW)+ 1 x DG (1,500 kW)

kW

Propulsion power

Electrical power

65

20

117

36

65

20

117

36

kW

Fig. 22: Power estimate for a shuttle tanker (Typically: Statfjord Rotterdam/return)

Alt. 1 ME + DG

Alt. 2 ME + VOC + DGAlt. 3 ME + VOC + SG

HFO based power

VOC based power

Power produced by ME

kW

15,000

10,000

5,000

12 3 1 23 3211 2 3

Buoy-loadingat oilfield

Voyage to port Unloading

at port

Voyage tooilfield

0

5,000

Power produced by DG

kW

Fig. 23: Heavy fuel and VOC-based power for a shuttle tanker(Typically: Statfjord Rotterdam/return)

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On the basis of the above round-trippattern, the VOC utilisation system willhave a payback time of 3.5 to 5.2years, the shortest payback time beingachieved with the shaft generator alter-native, see Fig. 25.

Figs. 26 and 27 illustrate the sensitivityof this feasibility study to the parame-ters used, especially to developmentsin fuel oil prices and investment cost.It will be seen that if, for instance, the

fuel oil price is 140 USD/t instead of100 USD/t, the payback time will de-crease to some 2.5 and 3.7 years, andif the investment cost increases from5.6 million USD to 8.1 million USD, thepayback time will increase to some 5.2and 8.0 years.

Fig. 27 also shows that even if theshaft generator Alternative 3 was up to2.5 million USD more expensive thanthe diesel generator Alternative 2, the

Alt. 2 ME + VOC + DG

Alt. 3 ME + VOC + SG

Real payback time

Years8

7

6

5

4

3

2

1

0

80 10060 120 140

Fuel price

Alt. 1 ME + DG

Alt. 3

Alt. 1

Alt. 2

USD

Fig. 26: Payback time sensitivity to fuelprice for a shuttle tanker with VOCutilisation system



Alt. 1 ME + DG

1 2

Buoy-loading

at oil field

3 1 2 3 1 32 1 2 3

Voyage to port Unloading

at port

Voyage to

oil field

Annual fuel costs

800,000

600,000

400,000

200,000

Diesel generators

Main engines

Fuel costs (100 USD/ton)

USD1,000,000

0

Fig.24: Fuel oil costs for a shuttle tanker



Alt. 1 ME + DG

12

10

8

6

4

2

0 2 4

Real payback time

Years

0

2.5 Mill USD

Alt. 1

Mill USD6 8

Alt. 2

Alt. 3

Investment cost

Fig. 27: Payback time sensitivity to invest-ment cost for a shuttle tanker with VOCutilisation system

Years after investment

20

16

12

8

4

0

2 6 10 14 18 22

Alt. 1

Alt. 2

Alt. 3

-4

Net present value

Million USD

In normal sea service days/year: 364.0

Fuel oil price USD/t: 100.0

Rate of inflation %/year: 4.0

Rate of interest/ d iscount %/year: 8.0

Alt. 1 ME + DG



Fig. 25: Net present value for a shuttletanker with VOC utilisation system

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payback time would still be shorter forthe shaft generator solution.

Example B: 300,000 dwt VLCC withVOC utilisation system

Very large crude oil carriers (VLCCs)often have a relatively simple operatingprofile, which includes many days innormal continuous service running at70 and 80% engine load with the tanksin ballast, and with fully loaded tanks,respectively. On the other hand, load-ing and unloading of the cargo at oil ter-minals and in port only takes a few days.

The normal electrical power consump-tion used for the accommodation quar-ters and for auxiliary machinery isrelatively low, so the greater part of thefuel consumption is for the main engine.

We have made an evaluation of theVOC utilisation system for a contempo-

1 1 1 1

22

2

15

2.7

14025

15

2.7

14025

Electrical power production, DG

Port,loading

Voyage, full loaded Voyage in ballast

days/yeardays/trip

Propulsion power, ME

Main engine: 7S80MC

SMCR = 25,480 kW at 79 r/min

El. power consumers

1 Accommodation andauxiliary machinery

2 VOC system

kW

25,000

15,000

10,000

5,000

0

5,000

Engine room alternatives

Alt. 1 ME + 3 x DG (1,000 kW)

Alt. 2 ME + VOC + 4 x DG (1,200 kW)

Propulsion power

Electrical power

Portunloading

kW

20,000

Fig. 28: Power estimate for a 300,000 dwt VLCC

Alt. 1ME + DG

Alt. 2ME + VOC + DG

1 2 1 2 1 2 1 2

25,000

20,000

15,000

10,000

5,000

kW

VOC

based powerHFObased power

Power produced by ME

Port,loading

Voyage, fully loaded Voyage in ballastPort,unloading

80% SMCR

70% SMCR

kW

Electrical power produced by DG

5,000

0

Fig. 29: Heavy fuel and VOC-based power for a 300,000 dwt VLCC

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rary 300,000 dwt VLCC having a con-ventional engine room arrangement witha 7S80MC main engine, and diesel gen-erators for electrical power production.

Measurements show that the amountof VOC emitted from the cargo may beas high as 0.6%, but may depend onthe type of crude oil carried. For a7S80MC main engine operating at80% MCR, this amount of VOC corre-sponds to the fuel needed for about 21

days of operation.

The economic evaluation thereforedepends on the length of the voyage,and we have used the voyage patternshown in Fig. 28 for an economy evalu-ation of the VOC utilisation system.

As Alternative 1, we use the 7S80MCmain engine in combination with dieselgenerators as the basis and then com-pare the economy for a similar engine

Alt. 1 ME + DG

Alt. 2. ME + VOC + DG

Fuel costs

(100 USD/ton)

Diesel generators

Main engine

Annual fuel costs

0

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1 2 1 2 1 2 1 2

USD

port,

loading

voyage,fully loaded

port,

unloading

voyage,

in ballast

Fig. 30: Fuel oil costs for a 300,000 dwt VLCC

14

10

8

6

4

2

060 80 100 120 140

Alt. 2

Alt. 1

USD

12

Fuel price

Alt. 1 ME + DG


Real payback timeYears

Fig. 32: Payback time sensitivity to fuelprice for a 300,000 dwt VLCC with VOCutilisation system

0

2

4

6

4 8 12 16 20

Net present value

Million USD

Alt. 2

Alt. 1

Years after investment

Alt. 1 ME + DGAlt. 2 ME + VOC + DG

Rate of interest/discountRate of inflation

Fuel oil price

In normal sea service

%/year: 8.0

%/year: 4.0USD/t: 100.0

days/year: 310.0

-4

-6

-2

Fig. 31: Net present value for a 300,000dwt VLCC with VOC utilisation system

Real payback time

Years

Investment cost

Alt. 1 ME + DG


16

12

10

8

6

4

2

0 2 4

0

Alt. 2

Alt. 1

Mill. USD

14

6 8

Fig. 33: Payback time sensitivity to invest-ment cost for a 300,000 dwt VLCC withVOC utilisation system

23


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configuration featuring the VOC utilisa-tion system, Alternative 2. As the elec-trical power requirement is relativelylow, the shaft generator case has notbeen investigated.

For the two alternatives, the fuel oil andVOC-based power is shown in Fig. 29,and the fuel oil costs in Fig. 30.

As the amount of VOC is only sufficientto cover part of the fuel consumption

on the fully loaded voyage, the paybacktime of the VOC utilisation system forthis example is as high as some 8.6years, see Fig. 31. When demands tolimit VOC emissions are introduced,extra investment costs will be necessaryfor all vessels to comply with such lim-its, which will reduce the payback timefor the VOC Fuel option.

This study is also sensitive to fuel oilprices and investment cost, a fact thatis illustrated in Figs. 32 and 33, whichshow that if, for instance, the fuel oilprice is 140 USD/t instead of 100 USD/t,the payback time will decrease from8.6 to 5.8 years, and if the investmentcost increases from 6.2 million USD to8.2 million USD, the payback time willincrease from 8.6 to 12.0 years.

Summary

Environmental friendliness will be oneof the dominant development goals inthe years to come. By introducing theVOC utilisation system developed byStatoil and MAN B&W Diesel, using thehigh-pressure gas-injection MC-GI en-gine, adapted to burn the VOC, thisgoal can be met for shuttle tankers(and other crude oil tankers). The com-bustion tests carried out on the research

engine confirm the feasibility of utilisingVOC as the main fuel for large marinediesel engines with fuel redundancy anda very high safety level.

In particular for shuttle tankersequipped with large shaft generators,the installation of the VOC utilisationsystem will, at the same time, have avery beneficial effect on the ships op-erating costs. Thus, the reduced fuelcosts could relatively quickly pay backthe extra investment cost involved incomplying with environmental require-ments.

However, the installation of the VOCutilisation system may also be benefi-cial for other crude oil tankers likeVLCCs, in particular when VOC emis-sion requirements are introduced and,at the very least, a VOC recovery sys-tem will have to be installed. A furtheradvantage is that the utilisation of VOCas fuel may replace the use of low-sul-phur fuel in areas designated by IMOas Special areas (requiring the use of

HFO with less than 1.5% sulphur).

Acknowledgements

The demonstration project is supportedfinancially by most oil companies inNorway in a joint effort to achieve acleaner environment, and by the Euro-pean Commission through its Thermiedevelopment programme under contractnumber OG/147/97/NO/DK. TheNorwegian shipowners RasmussenMaritime Services and Navion haveplayed a definite role in the development

project. Gas emission recordings andanalyses from a number of crude oilcarriers have been performed over sev-eral years by the SINTEF-organisationin Norway. This support is highly appre-ciated.

References

[1] Per R. Larnholm: VOC RecoveryTests Successfully Completed by KPSand Statoil, SINTEF VOC Seminar,Oslo, 1997.05.14

[2] Otto M. Martens: Control of VOCEmission from Shuttle Tankers andFloating Storage Systems,SINTEF VOC Seminar,Oslo, 1997.05.14

[3] Anders J. Steensen: ImportantCrude Oil Research will Reduce Pollu-tion Technology Review Weekly,August 1996

[4] Large Diesel Engines using High

Pressure Gas Injection Technology.MAN B&W Diesel A/S,Copenhagen 1996,Publication No. P.206-96.02

[5] Shuttle Tanker Propulsion.MAN B&W Diesel A/S,Copenhagen 1997,Publication No. P.335-97.04

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For the purpose of evaluating the econ-omy of alternative projects, we use thenet present value method. This methodis preferred because, irrespective ofthe payback time, it compares the totalgain after a certain number of years inoperation, and thus also incorporatesthe investment costs.

Definition of Net Present Value (NPV)

The net present value method is usedin order to get an evaluation of the profit-ability of investing an extra amount of in-itial capital in an alternative project,compared with the basic project.

It is assumed that the alternative projectnecessitates an extra investment of Co

at the project start, and that this invest-ment gives an annual saving on the fuel,lubricating oil and maintenance cost billequal to So, based on todays prices,see the Figure.

To determine the annual savings ob-tainable during the subsequent yearsn, So must be corrected for inflation,i.e. Sn = So (1+i)

n, in which i is infla-

tion and n is the number of years afterthe investment.

To put these savings in relation to Co,Sn must be calculated back into to-days prices at the discount rate d,assuming that the discount rate isequal to the interest rate for financingr, as normally done in the shippingtrade, i.e. Sn/(1+d)

n= Sn/(1+r)

n.

As d = r, the investment cost after nyears Cn = Co(1+r)

ncalculated back

to todays price level is still equal to Co.

For the alternative project , the NPVnshows, compared with the basic project,how much extra money you will havein your pocket i.e. the accumulatedsavings obtained by making the extrainvestment in todays prices after nyears.

The result of the calculation for the al-ternative project is shown as an NPV-curve as a function of years afterinvestment. The intersection pointwith the abscissa (basic project) is thealternative projects real payback time,compared with the basic project.

Definition of Net Present Value (NPV)

The net present value is then defined as:

NPVn =

n=1

nSn

(1+d)

n

CO (1+r)n

(1+d)

n =

n=1

n

SO

1+i

1+r

n

CO

SnSavings the nth year afterinvestment

CoExtra investment at projectstart

nNumber of years afterinvestment

i Rate of inflation

r Rate of interest for financing

d = r Discount rate

S2S3

S4

Co

S /(1+d)nn

S1

C /(1+d)nn

C = C x (1+r)n on

S = S x (1+i)n on

Appendix

Economy Model Used

utilisation volatile organic compounds 19993965

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