slow steaming a viable long term option

8/17/2019 Slow Steaming a Viable Long Term Option

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WÄRTSILÄ TECHNICAL JOURNAL 02.2010

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Slow steaming– a viable long-term option?AU T HO R : Andr eas Wi es m ann, G e n e r a l M a n a g e r I n n o v a t i o n & B u s i n e s s D e v e l o p m e n t , Tw o - s t r o k e , W ä r t s i l ä S e r v i c e s

The slow steaming of merchantships, particularly container ships,

has become a much discussed topic.

It is a topic that affects the entire

industry, from cargo owners, carriers,

ship owners and operators, to

equipment manufacturers.

Background for slow steaming A combination of factors have led to thepast two years becoming hugely challengingfor certain sectors of the global merchantshipping industry. Tese factors include:

Te downturn in the global economy,resulting in reduced transportationcapacity demand

Te substantial global order book fornew tonnage, a legacy from the boom

years, resulting in record-highdeliveries of new ships

Te global financing crisis Te sudden fall in ship values High fuel costs Increasing operating costs (manning,lube oil, maintenance)

Falling freight rates, which in turn alsoimpacts charter rates.

All of these things have put the entirevalue chain – starting from cargo ownersto the carriers, ship managers, ship owners,financing institutes and equipment suppliers

– under big pressure. Te industry has hadto quickly adjust, which has resulted ina sharp slow-down in new ship orders,cancellations of already confirmed orders,the delaying of new ship deliveries, laying-

up and idling of vessels, and all kinds ofcost reduction measures.

■ Fuel consumptionTe biggest single cost factor in merchantshipping, particularly for container andother large vessels, is the fuel oil. And theeasiest way to reduce this cost is to reducethe ship’s speed. Te typical propulsionsystem for larger merchant ships is a low-speed two-stroke main engine, directlydriving the fixed-pitch propeller via thepropeller shaft. Te ship’s speed is, therefore,

reduced by lowering the speed of theengine and propeller. Te power requiredfrom the main engine, however, correlatesdisproportionately with the ship’s speed(Figure1).


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[ MARINE / IN DETAIL ]

[ M A R I N E / I N D

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For example, reducing the nominalship speed from 27 to 22 knots (-19%)

will reduce the engine power to 42% ofits nominal output (CMCR). Tis resultsin hourly main engine fuel oil savings ofapproximately 58%. A further reductiondown to 18 knots saves already 75% of thefuel. Te reduced speed however resultsin a longer voyage time; therefore the fuelsavings per roundtrip (for example Asia-Europe-Asia) are reduced by 45% at22 knots, or 59% at 18 knots. Tese arecalculated values, and the actual valuesdepend also on a number of externalfactors, such as the loaded cargo, vessel

trim, weather conditions, and so on.

■ Effect of slow steamingSuch savings cannot, of course, beneglected, which is why carriers have usedslow steaming as an immediate means ofcutting fuel costs and reducing capacity.Carriers can choose between laying up someof their vessels or applying slow steaming.Slow steaming is preferred because it offersgreater flexibility to increase the capacityagain when the market situation changes.

And there are other big advantages coming

as a free side effect of slow steaming,namely that for every ton of fuel saved,the industry reduces its carbon dioxideemissions (CO2) to the atmosphere bythree tons, and the cylinder lubricating oilconsumption of the main engine isreduced at almost the same percentagesas the fuel, which also reduces solidparticle emissions.

General considerationsTe change to a long-term slow steamingscenario needs, however, a number of

considerations.

■ Te cargo owner’s perspectiveFirst of all, cargo owners have to acceptthat the transportation time of their goods

will be increased slightly. Reducing thevoyage speed from 27 to 22 knots, forexample, will increase the time from Asiato Europe by 3–4 days. Similarly, a voyagespeed of 18 knots rather than 27 knots willrequire one week's more sailing time. Forsome goods this requires changes to thecargo owner’s logistics, and might increase

the costs for “goods in progress”.

■ Te carrier’s perspective At the same time, the carrier needs toadapt his trade schedule, and should he

want to maintain a weekly service on acertain trade route, he will need to add

vessels to his fleet.However, this has a positive effect onthe over-capacity situation of transportationtonnage. Vessels that are idle or laid-up canbe utilised for these fleet additions, as toocan new ships whose delivery time has beenpostponed. Tis is exactly, what wehave seen since the beginning of 2010.Since January, the inactive containership fleet has been globally reduced byapproximately 450 ships, and the totalmerchant fleet by more than 800 ships.Te establishment of slow steaming,

and the resultant trade and fleetadaptations, have greatly contributedto this reactivation of inactive vessels.

Te carriers are definitely the main driversfor the introduction of slow steaming, asthey have the most to gain from largereductions in fuel consumption. Of course,the calculated percentages of fuel savingsdescribed above do not reflect the overalloperational savings to the carrier. Tecapital and operating costs of theadditional ships – or the additionalchartering costs – need to be taken into

account, as do many other additional andfixed costs. Furthermore, slow steamingis not possible for all services, nor is itappropriate for all times during operations.Several carriers, for example, operate atalmost nominal speeds on some legs ofa service, while using slow steaming onothers. And sometimes ships will have tocatch up with delays, or add a port call toits service. All this requires high operationalflexibility for both the vessel and itspropulsion system. aking all theseconsiderations into account, it is not

possible to make a general statementconcerning the overall cost reductionpotential of slow steaming. However, anumber of industry players have indicatedthat the overall savings could be in therange of 10–25%, depending on theproportion of vessels in a fleet that areslow steaming, and on the averageachieved speed reduction.

■ Te ship owner and manager’sperspective

Ship owners and their technical managers,

who are not the carrier themselves, providethe carriers with fully operated andmaintained ships, which the carriers utilisefor their shipping business. Under long-or short-term charter contracts, the ship


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owner and manager basically have tofulfil the carrier’s requirements, including

instructions relating to ship speed andthe introduction of slow steaming. Tisdemand from the carriers was met withconcern by many ship owners, and has ledto discussions within the industry as to howto address these concerns.

Te ship owner’s interest is two-fold: ■ a) o have attractive ships forthe charter market, which fulfilthe requirements of a carrierand lead to high utilisation andprofitable charter income, and

■ b) o keep his assets in good shape

with a sustainable value. Teship manager’s interest is to fulfila variety of requirements fromthe carrier, the owners, as wellas from legal and environmentalbodies, at the optimal cost.

In this context, concerns have been raisedthat the owners and managers might endup carrying the potentially consequentialcosts of slow steaming operations, whereasthe carriers reap the benefits. In addition,during the shipping crisis charter rates

were severely depressed.

Next to these commercial concerns,owners and managers have mainly expressedtechnical concerns. Tese concerns arediscussed in the next section.

■ Te commercial perspectiveConcluding this general discussion, ascenario whereby slow steaming wouldcontinue in certain industry sectors orcertain trades has to take into considerationthe perspectives and interests of everyonein the value chain. Of course, commercialdiscussions and negotiations between

carriers and their customers, and betweencarriers, ship owners, suppliers and serviceproviders will be done on a business tobusiness basis. Tese negotiations will inthe long-term lead to a new balance,

wherein all players able to cope with thenew requirements of high operationalflexibility, will benefit from slow steaming.

Technical concerns andrecommendationsShips are designed and built for a certainspecified load and speed range, at which

the system’s total efficiency is optimised.Because of the fixed-pitch propeller’s directdrive by the main engine, the main engineitself is then also laid-out for that optimisedoperating range.

Fig. 1 – Correlation between ship speed, required engine power and fuel consumption.

Fig. 2 – Ship operation costs for Europe – Far East trade with different vessel speeds.

100%

90%

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60%

50%

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Engine power Ship speed

Fuel consumption

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Voyage

M/E fuel

Mgmt Capex

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A/E fuel

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Opex Knots

11 ships

15 knots 18 knots 22 knots 27 knots

M i o U S D

10 ships9 ships

8 ships

k n o t s a n d n u m b e r o f s h i p s


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Te optimal load range of the two-strokeengine lies between 70-85%. Te fuelefficiency of the engine, its operationalparameters, the specification of theturbochargers, coolers, auxiliary systems,exhaust gas boilers, and so on, are chosenand optimised for that normal load range.It is natural, therefore, that when theengine is operated continuously in a loadrange below or even far below 60%, the

overall system is no longer fully optimised. As the industry didn’t use slow steamingduring the previous 20 years, nobody hadreally good long-term experience withcontinuous low-load operation of today’sgeneration of new engines. Terefore,marine engineers who daily operate theengines, technical managers, as well asengine builders, were initially reluctant tofully embrace the concept.

Based on requests from carriers andoperators, Wärtsilä has investigated thevarious concerns that have been raised,

including concerns on componenttemperatures, fouling of exhaust systems,turbochargers, etc., and has addressedthem in field investigations and throughdiscussions with operators. Te conclusions

and recommendations were summarized inthe beginning of 2009 in a Wärtsilä ServiceBulletin about low-load operation.

In general, it was concluded that themodern Wärtsilä two-stroke engines areable to reliably operate in all load rangesbetween 10% CMCR and 100% CMCR

without major modifications, if theoperational parameters and precautions, asdocumented in the Operating Instructions

and in the mentioned Service Bulletin, areproperly followed. By adhering to theserecommendations, the potential risksinherent to such operation will be mitigated.

■ Expressed technical concernso varying degrees across the low loadrange, different engine conditions can beobserved. Te possible consequences ofcontinuously operating at reduced load

without taking the recommendedprecautions are:Lower air flows

A problematic area after the auxiliaryblowers cut out / before they cut in

Te possibility of very high exhaust,and thus component temperatures.

Poor combustion Poor atomisation

Higher sac volume: injected volumeratio, increased likelihood of dripping

Increased fouling and carbondeposits likely.

Cold corrosion Caused by condensation of corrosivevapours

Possible when observing very low

engine temperatures during very lowload operation.

Fouling Of the exhaust system, turbochargers,exhaust boilers

Of the scavenging air space due toexcess cylinder oil.

Apart from these engine related concerns,concerns have also been voiced aboutefficiency losses (e.g. propeller, turbo-chargers, shaft generators, heat recoverysystems) and the accelerated deteriorationof condition and performance (e.g. fouling

of the hull and propeller due to reducedship speed, stern tube seals, shaft bearings).

■ Summary of recommendations Wärtsilä R-flex engines are better suited

Fig. 3 – The Wärtsilä Slow Steaming Upgrade Kit.


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than the Wärtsilä RA engines forcontinuous loads down to 10% due to theirunique electronically controlled commonrail injection system and flexible exhaustvalve control. Te selective fuel injectorcut-off at low load enables improvedinjection characteristics, resulting inreduced carbon deposits and, therefore,less fouling of both the exhaust gas boilerand turbocharger.

However, for both engine designs therecommended measures in the OperatingInstructions and Service Bulletin have tobe implemented and followed to ensurereliable continuous low or ultra low loadoperation. In short, the recommendedprecautions are as follows:

Ensure the injector nozzle conditionis correct. Tis is standard engineeringpractice but should be given moreattention than in normal operation

Maintain higher fuel temperatures andaim to achieve lower viscosities,

12 / 13 cSt Keep the L cooling water temperatureat 36°C in order to maintain theoptimum scavenge air temperature, andthe jacket cooling water temperature

at the upper limit (85–95°C). A highcooling water temperature will reducecondensation and thermal stresses. By-passing of the fresh water generator willmost likely be necessary to maintainthe cooling water temperatures.

Normally the cylinder oil feed rate isload dependant, and no adjustment isneeded. However, frequent pistonunderside inspections are recommended

to monitor piston running conditionsand signs of over- or under-lubrication.In a recently released Wärtsilä ServiceBulletin, we have described thischaracteristic and shown certainobserved symptoms on the piston ringsduring low load operation. When thesesymptoms occur, a temporary increasein the cylinder lubricating feed rate willhelp stabilise the situation and recoverthe reliable piston-running performance.

It is important that the temperatureof the exhaust gas after the cylinders is

kept above 250ºC in order to reducecold corrosion. If the exhaust gastemperature drops below this value,the engine load should be increased

High exhaust gas temperatures, above

450ºC, after the cylinders should beavoided during the period following theauxiliary blower cut out or before cutin. Tis may cause hot corrosion andburning of the exhaust valve seats.

As a countermeasure, the auxiliaryblower may be switched to “continuousoperation”.

Another concern during continuouslow load operation is the accumulation

of unburned fuel and lubricating oil inthe exhaust manifold, as such depositscan ignite after the engine load isincreased again. Tis may result insevere damage to the turbochargerdue to sudden over-speeding. Wärtsilätherefore recommends that the engineload be periodically (twice a week)increased to as high as possible (at leastto 70%) for a minimum of one hour inorder to blow through any accumulatedcarbon deposits. Whilst operating atthese increased loads, turbocharger

washing and soot blowing of theeconomiser should be undertakenin order to reduce fouling

Wärtsilä continues to observe severalparameters, by collecting customer feedback,

Fig. 4 – Additional fuel savings at different speeds using the Slow Steaming Upgrade Kit.

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vessel speed (knots)

Fuel consumption and savings versus vessel speedExample: very large container vessel

Fuel savings with Upgrade Kit normal BSFC

CO2 reduction with Upgrade Kit BSFC with Upgrade Kit

15 16 17 18 19 20 21 22 23 24 25 26 27

S p e c i fi c f u e l c o n s u m p t i o n ( g / k W h )

R e d u c

t i o n o f f u e l a n d C O

2 ( t o n s / 2 4 h r s )


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service attendance feedback, and throughstructured field testing. In this way new

trends can be detected early, and in depthexperience on the long-term behaviour ofthe engine and components at low andultra low load operation can be gained.If required, we will publish additionalrecommendations and solutions viaService Bulletins.

Optimisation solutions As mentioned above, the Wärtsilä R-flexengines are better suited for continuouslow-load operation, due to their highflexibility in engine control, which allows

optimisation of the parameters in thelower load ranges as well. Such featuresinclude the selective fuel injector cut-out atvery low loads, as well as the Delta-uning,

which optimises the specific fuel oilconsumption below 75% load. For newengines, Low-Load uning providesadditional flexibility and a further reductionin specific fuel consumption at lower loadsby optimising turbocharger efficiency forthe lower load range, and through by-passing a part of the exhaust gas flow athigh loads.

In addition to these inherent or optionalfeatures in new engines, Wärtsilä is alsocontinuously seeking upgrade- and retrofitsolutions that fit specific operationalrequirements, and which provide optimalengine performance and efficiency.

Te owners and operators of Wärtsilätwo-stroke engines have the followingsolutions available:

■ Wärtsilä Slow Steaming Upgrade KitTis automated flexible turbochargercut-out solution extends the optimised and

reliable load range of the engine forcontinuous low load operation, andsignificantly reduces the specific fuelconsumption in the low load range. Tesefuel savings are achieved by cutting-off oneof the turbochargers, which in turn leadsto increased scavenge air, and thus betterfiring pressures. Te turbocharger cut-off isdone in a controlled and fully automated

way. Te fuel savings, as well as the load at which the turbocharger can be cut-off,depends on the number of turbochargers.For example, for a Wärtsilä R-flex96

engine with three turbochargers, the loadrange with a cut-off turbocharger isabout 10–60%.

In addition to a major reduction inBSFC (Brake Specific Fuel Consumption)

in the low-load range, this solutionprovides full flexibility (the engine can be

operated from 10 to 100%) and decreasesthe risk of engine fouling and excessivecomponent temperatures. Tis solution is,therefore, best in a long-term scenario,

whereby both slow steaming and nominalspeeds are, or might be, required as theengine can operate at any time up to itsmaximum installed power for full sea speed.

Tis retrofit is available for ships fitted with Wärtsilä RA and R-flex low-speedengines having more than one turbocharger.One such installation of the Slow SteamingUpgrade Kit took place in October 2009

onboard a vessel with a 12-cylinder Wärtsilä R-flex96C main engine withthree turbochargers. Te measured fuelsavings were 8–12 g/kWh in the optimalload range.

Te benefits and operational flexibilityof this solution have been recognised byseveral major operators and carriers, and

Wärtsilä has received a number of ordersfor the Upgrade Kit.

If the operational profile of a vessel ischanged for a longer period to slowsteaming, and engine loads above 60–70%

are not required at all, the blinding of oneof the turbochargers can be more costeffective, achieving the same BSFCreductions as the flexible Slow SteamingUpgrade Kit solution. However, this clearlylimits the engine’s load range (10–60%,upper limit depending on the amount ofturbochargers), and the vessel has nopossibility to achieve full sea speed ifrequired. Also, the recommended periodicaloperation at higher loads to blow out anyaccumulated carbon deposits in the system,cannot be carried out with full effectiveness

due to the upper load limits with a blindedoff turbocharger.

■ Permanent de-rating of the engineIf the operational profile of a vessel ischanged for long-term operation at reducedspeeds and lower engine loads (e.g. 5–15%reduction), a de-rating of the engine mightbe the best solution considering fuelconsumption, reliability and operationalflexibility. Te scope of the solutiondepends upon the required de-rating. Tissolution is applicable for all engine types.

■ Combination with a propeller

modification When the engine is de-rated orcontinuously running at low-load, an


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Fig. 5 – Slow steaming of merchant ships affects the entire industry.

optimised propeller to better match thenew operation conditions might achieve

additional fuel efficiencies. Te optimalspecification can be offered by WärtsiläServices, combining the support from itstwo-stroke engine and propulsion experts.

■ Pulse Lubricating System Wärtsilä introduced the Pulse LubricatingSystem (PLS) a few years ago as its standardcylinder lubricating system for all newengines. Te system was also introducedas a retrofitting solution – known as theRetrofit Pulse Lubricating System (RPLS)– and was installed widely on large bore

engines.Earlier lubricating systems might have a

problem in achieving optimal timing anddistribution of the cylinder oil, especiallyin the lower load ranges when combined

with a cut-out turbocharger. Te RPLSsystem ensures precisely timed injection ofthe lube oil into the piston ring package,

which optimises the lubrication, while thepiston ring pressure is increased and thespeed decreased during low load operation.

In addition, the PLS and RPLS will reduce the specific lubricating oil

consumption by 20–40% and in so doing,further increase lube oil savings (low-loadoperation in itself already saves a substantialamount of cylinder lube oil).

■ Monitoring and control optionsFor the continuous monitoring of cylinder

liner wall temperatures, Wärtsilä has formany years already offered the so calledMAPEX-PR system, which allowsthe temperature trends of the liners(corrected by actual engine loads) to bemonitored. Te system also gives early

warnings should they exceed the upper orlower limits.

Recently introduced by Wärtsilä, theIntelligent Combustion Monitoring systemcontinuously monitors cylinder pressuresand several parameters during the fullcombustion cycle. It enables the trending

and analysis of the monitored data in orderto understand the engine’s performance,as well as that of the condition ofcomponents in the combustion chamber.Tis system gives additional informationabout combustion performance,particularly during low load operation.

Wärtsilä offers also an integratedmonitoring system, which transmitsspecified data from ships to Wärtsilä’soperation data server; the data is regularlyanalysed and evaluated by engine expertsand reports, together with expert

recommendations, are provided tothe technical managers of the ships.

For R-flex engines, Wärtsilä will alsointroduce the Intelligent CombustionControl system in early 2011, which takes

elements from the monitoring system andcombines the measured data with an

intelligent logic in the Wärtsilä EngineControl System (WECS). Tis enablesoptimal cylinder pressures to be controlledand adjusted automatically. Using thisautomatic control, further fuel efficiencyoptimisation in the range of 1–2% ispossible.

CONCLUSION

Looking at slow steaming from a widerperspective, this article describes severalimportant considerations covering theglobal shipping industry’s view, the drivers

for slow steaming, the commercial aspects,and the technical concerns for achievingfurther efficiency solutions whilst slowsteaming. We can conclude that slowsteaming has a number of very obviousadvantages, and will, therefore, probablycontinue to be used by the industry fora long time. Conversely, slow steamingthrows up several challenges, which needto be properly addressed, and theoperational recommendations need alsoto be followed. Wärtsilä is supporting itscustomers with all required advice and

instructions. Furthermore, Wärtsilä’supgrade and retrofit solutions offercustomers the opportunity to furtheroptimise the overall efficiency of the ship’sengine and propulsion.

slow steaming a viable long term option

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