technology – marine, rolls- propulsion propulsion/2010-propulsion...marine transport has generally...

Marine transport has generally beenseen as having a lowerenvironmental impact than otherforms of transport. The increasingdemand for economical yet rapidmovement of both passengers andfreight has brought renewedmomentum to the development ofmarine propulsion systems. Newtechnologies are aiding theproduction of propulsion systemsthat are capable of driving vessels athigher speeds; that are moreefficient; that provide bettermanoeuvrability; and are quieter,with less vibration. Here, the latestdevelopments in marine propulsionare brought into focus.

Introduction

The start of the twenty-first centuryhas brought a resurgence ofinterest in marine propulsion.

Major navies world-wide are assessingand updating their fleets. The Internetoffers international shopping to everyhousehold, fuelling demand for fast andeconomical freight transport. Travel is astressful element of many people’s jobs,so the cruise increasingly offers a

■ increasing demand for speed, forships to arrive on time whatever theweather

■ outstanding manoeuvrability to allowvessels to keep station above a well-head or berth at a busy port.

These challenging requirements, alongwith the overall need to reduceemissions and energy usage, aredriving developments in technology.The role of the marine propulsionsupplier is changing, from the supply ofindividual items of equipment to thedelivery of sophisticated, integratedsystems with guaranteed performanceand through-life support.

Speed and powerThe continuing internationalisation oftrade and production, combined withincreasing congestion on land and inthe air, is generating interest in novelconcepts for fast cargo and passengervessels. In the naval arena, arequirement for rapid force deploymentcapability, while avoiding the need tostation troops close to sensitive areas,is providing further stimulus and fundingfor fast vessel developments.

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DR JULIA KING CBE FREngDIRECTOR OF ENGINEERING &TECHNOLOGY – MARINE, ROLLS-ROYCE PLCDR IAN RITCHEYHEAD OF RESEARCH &TECHNOLOGY – MARINE, ROLLS-ROYCE PLC

TECHNOLOGY

propulsionThe transport

technology for the 21st century?

Marine

pampered and relaxing way to visit manydestinations from a single hotel room.Prior to the attack on the World TradeCentre on 11 September 2001, marketpredictions indicated healthy growth inworld shipbuilding, supporting thispositive view of opportunities in marinetransport (see Figure 1). The tragicevents in the United States gave uscause to reflect, both personally andprofessionally. Whilst it is still early tojudge the long-term effects on marinemarkets, the initial impact on cruisebookings has now been reversed andthere appears to be no reason why thelong-term fundamentals will be changed.

Changing customerrequirementsMarkets as apparently diverse asdeepwater offshore production, cruiseships, fast cargo and advanced navalvessels share many commonrequirements:

■ safe, reliable and cost effectiveoperation and maintenance

■ high levels of ride comfort andstability, for the care of passengers,crew, cargo and complex equipment

This document, and more, is available online at Martin's Marine Engineering Page - www.dieselduck.net

The first challenge for the propulsionsystem, in terms of increasing thespeed of a ship, comes from the cubelaw relationship between speed andpower for a conventional propellerdriven system. Put simply, the problemis that doubling the speed requires aneight-fold increase in power.

Around 95% of ships are poweredby diesel engines, where the relativelylow power-to-weight ratio isassociated with intermittentcombustion, with each cylindercontributing in turn to the powergeneration process. Marine gasturbines, derived from aeroengines,provide prime movers with between

four and ten times the power-to-weight of a marine diesel through acombination of a continuouscombustion process and design tominimise weight (see Figure 2).Figure 3 shows a comparison of theMarine Trent engine and the Trent 800aeroengine, where some 80% partscommonality is retained between themarine and aeroengine variants.Additional benefits of the modernaeroengine parentage include theexcellent reliability, good turndowncapability for harbour manoeuvringand excellent emissions performanceas a result of significant and continuingaeroengine technology investment.

The second propulsion challenge iscavitation, which limits the maximumspeed of propeller-driven ships tobetween 30 and 35 knots. Aspropellers are driven faster, cavitationstarts to develop in low pressure areason the blades, until at high speed thelow pressure faces of the propellers arecovered in tiny bubbles. These proceedto coalesce and collapse, creating rapidsurface damage to the propeller,pressure pulses that are experienced asnoise and ship vibration, and a markedreduction in propeller efficiency.

To avoid such problems, fast vesselsare propelled by waterjets – hugepumps which suck in water below thevessel and pump it out at the stern (seeFigure 4). The construction and flowthrough a waterjet are illustrated inFigure 5. Because the impeller of thewaterjet is enclosed, in contrast toexposed propeller blades, higherpressures are achieved and the impellerand stator designs are optimised tosuppress cavitation. The shaft speedsof waterjets are typically double thoseof propellers at similar power levels.Rolls-Royce is now applying the CFDcodes developed for the analysis ofcomplex multi-row aeroenginecompressors to the design of advancedwaterjet pumps. The use of such codes

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0

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Ro/Ro Ships

Offshore Service Vessels

LNG

Chemical

Accumulated

Segmentk CGT

Accumulatedk CGT

Source: Drewry Shipping Consultants Ltd August 24, 2000

Accumulated2000

1987

1989

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1999

2001

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Figure 1: Shipbuilding market trends

Rolls-Royce and marine enginesThat marine transport has long been an area of interest is evident in theoriginal Memorandum of Association of Rolls-Royce Ltd, dated 1906:

‘The objects for which the Company is established are: (i) To manufacture,sell or let or hire or in any manner dispose of or turn to account, motorvehicles for use on land, water or in the air, and any parts or accessoriesto the same’.

With the acquisition of Vickers in 1999 Rolls-Royce has become a world-leading marine equipment and systems supplier, with a product rangespanning prime movers, propulsors, steering and stabilisation systems, deckmachinery, propulsion and electrical systems and controls, right through towhole ship design in specialist sectors.


of service for long distance transportof high value goods.

The technical challenges in suchdevelopments are considerable.FastShip has to be able to maintain aspeed of 36 knots in 7.5 metresignificant waves. The impact of airingestion into the waterjets is a keyfactor in the mechanical design of boththe jets and the gas turbines – in7.5 metre waves air ingestion in theouter waterjets is likely to occur, and, inmore extreme sea conditions,emergence of all five waterjet inlets is apossibility. In order to ensure that theloads experienced in such events areunderstood, special measuringequipment, including a 6-componentdynamometer attached to the impellerhub, has been developed to enabledynamic recording of all forces andmoments acting on an impeller during

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Weight/Power Ratio

GasTurbine

MediumSpeedDiesel

SlowSpeedDiesel

SpeedP

ower

Figure 2: Power-to-weight ratio: the diagrams of the reciprocating engine and thegas turbine are at approximately the same power level

Figure 3: The Marine Trent and the Trent 800 (top) aeroengine

offers significant reduction in both thecost and lead time for the developmentof new designs, with the potential tolook at a wide range of variants beforeprogressing to model testing. Waterjetpropelled vessels operate withmaximum speeds of 35 to 70 knots,depending on the hull design andnature of the operation.

Several projects to develop fast seatransport services are currently inprogress around the world. Rolls-Royce has been selected to supplyboth the gas turbines and thewaterjets for the transatlantic projectknown as FastShip (see Figure 6).FastShip features an innovative semi-planing hull which will be powered byfive 50 MW Marine Trent gas turbines,each driving a 3 metre diameterKamewa waterjet. The service isdesigned to operate between speciallydeveloped port facilities at Philadelphiaand Cherbourg, crossing the Atlanticin 3.5 days, so that 7-day door-to-door delivery can be achieved. At acost of around 25% of that of airfreight, and with delivery times typicallyfour times faster than conventional seafreight, FastShip will offer a new type


normal operation and air ingestion. Thisprovides the information required toverify the waterjet design and meetclassification society requirementsrelating to loads on the shaft andsurrounding structure. The gas turbineneeds to be able to respond to thesudden drop in power demand: as theload torque drops the engine must bedesigned to be able to reduce powersufficiently rapidly to avoid anoverspeed trip. Dynamic simulation ofthe entire propulsion train is used toverify the functionality of the controlsystem during air ingestion andemergence events.

Better use of spaceOne of the greatest contributors toincreasing the useable space in a ship,most notably a warship or a cruise liner

where the premium on space isgreatest, is the concept of the ElectricShip. Integrated Full Electric Propulsion,IFEP, lies at the heart of the ElectricShip. IFEP also offers valuable runningcost advantages, combined, in somecases, with purchase cost savings. Inan IFEP system, the ship’s propulsorsare driven by electric motors alone, andthe power for the electric motors isdrawn from a unified electrical powersystem that also provides all of theship’s electrical services. The powerand propulsion systems are thereforeintegrated, because there is only oneelectrical power system where moreconventionally there might have beentwo.

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Figure 4: Waterjets in operation at thestern of a fast ferry

Figure 5: Construction and water flow through awaterjet

Figure 6: (Above) Two artist’s impressions of theFastShip


A major benefit of IFEP is the layoutflexibility offered through removal of the‘tyranny of the shaft line’. Since theprime movers are no longer directlycoupled to the propulsors, there isconsiderable freedom in their location,permitting more effective use of theavailable space, as illustrated inFigure 7. In a warship this opportunityto distribute the power and propulsionsystem around the ship offers addedbenefits in terms of preservation offunctionality and reconfigurability in theevent of battle damage.

One of the key technologies enablingthe realisation of the benefits of IFEP,particularly in naval vessels, is thecompact electric motor. Over the pastten years there have been significantimprovements in the torque density ofelectric motors, moving fromconventional motors to advancedinduction motors and permanent-magnet propulsion motors (see Figure8). The transverse flux motor, currentlybeing developed by Rolls-Royce, is apermanent magnet motor with a noveltopology in which the magnetic and

electrical circuits do not compete forspace, so allowing the development ofparticularly high torque densities (seeFigure 9). Figure 10 indicates theimportance of developments in motortechnology to the application of IFEP tosmaller ships. The future has more tooffer in this area. Over the next ten

years we may well see the developmentof superconducting motors, offering afurther step in the evolution of compactmotors and further extending theirapplication.

The greatest flexibility in ship layoutis achieved if the motor is actuallymoved outside the vessel, into a pod

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Conventional Propulsion

IFEP

Podded IFEP

Figure 7: Layout of mechanical and electrical propulsion systems

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Figure 8: Comparative size of different propulsion motor technologiesAIM – advanced induction motorPM – permanent magnetTFM – transverse flux motor


below the ship, directly driving apropeller. Such podded propulsorshave already been taken up by thecruise industry, including in the newQueen Mary 2 Cunard liner, wherethey offer increased space, reducednoise through complete elimination of

the shaft penetrating the hull andoutstanding manoeuvrability, as theycan be rotated through 360° and actas both rudder and propulsor. Anexample of such an arrangement, theMermaid™ pod, is shown inFigure 11.

Reduced fuel consumptionIn addition to layout flexibility, IFEP offersfuel savings because the ability to runany part of the propulsion or powersystem from any prime mover, combinedwith the base load of the ship’s servicepower demand, can be used to ensurethat the load on the prime movers neverfalls to inefficient levels. Vessels such aswarships and cruise liners, with relativelyhigh service loads and operationalprofiles that frequently leave thepropulsion system operating at fractionalloads, offer an ideal platform for the IFEPsystem to generate fuel savings.

Improvements in prime movers,however, are key to reducing the fuelconsumption of any propulsionsystem, mechanical or electrical. Inboth cases the goal is to offer acombination of high power density andlow fuel consumption. The nextgeneration of marine prime movers isexemplified by the WR-21, illustratedin Figure 12, the most advancedmarine gas turbine currently available.It incorporates both intercooler andrecuperator heat exchangers, thecombined effect of which is to allow

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Figure 9: Layout of the transverse flux motor

Figure 10: Motor technologies required for application of IFEP in different ship sizes

Conventional Motors

Advanced Induction Motor

Radial Flux Motor

Axial Flux Motor

Transverse Flux Motor

Ship Tonnage3,000 5,000 7,000 9,000 11,000+

16 MW @220 rpm

18 MW @200 rpm

20 MW @165 rpm


‘waste’ heat to be recovered from thegas turbine exhaust, providingsignificant fuel savings across theentire power range.

Although the WR-21 was originallydeveloped for naval applications and willgo into service in the Navy’s new Type45 destroyer, there are some strongsimilarities between the requirements ofwarships and cruise liners.Consequently, there is considerableinterest in the use of complex cycles incruise applications, illustrated by theselection of a combined gas turbine and

steam turbine electric drive system(COGES) for Celebrity Cruises’Millennium class ships. In this instance asteam bottoming cycle rather thanintercooling and recuperation has beenchosen as an alternative solution toreduce fuel consumption throughrecovery of exhaust heat.

Manoeuvrability andstationkeepingManoeuvrability is a basic safetyrequirement for all vessels, as well as

being an intrinsic element of theoperational capability in someapplications. These requirements aremet through a combination of compact,powerful and efficient thrusters and thecontrol systems that manage them.Podded propulsors provide a high levelof manoeuvrability (for example theMillennium class has a tactical diameterof less than two ships lengths from aninitial speed of 24 knots). There areequally innovative designs formechanical drive applications, such assteerable mechanical thrusters.

In demanding offshore applications,such as floating production platforms,vessels must maintain station within afew metres. This is validated at thedesign stage using simulations of theperformance of the ship and its powerand propulsion systems in a range ofwind, current and sea states. Navalvessels have similar requirements sothat they can be replenished at sea.

Noise and vibrationMuch hydrodynamic research, bothcomputational and experimental, isfocused on noise reduction. Key areasinclude tip vortex cavitation, hullpropulsor interaction and hull pressurepulses, and blade-guidevane interactionin waterjets. Alongside these efforts toreduce propulsor noise at source, gasturbines offer an in-built reduction ofprime-mover noise, since they emit highfrequency noise that is readily damped

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Pressurised fuel cell

Fuel

Air

Exhaust

Generator

Combinedelectricoutput

Figure 11: Mermaid pods on theMillennium cruise ship

Figure 12: WR-21 gas turbine airflow diagram

Figure 13: Solid oxide fuel cell schematic

● Fuel cells transform chemicalenergy directly into electricity, usinghydrogen as a fuel

● Reformer technology required toconvert methane or diesel tohydrogen

● A fuel cell consists of an ion-conducting electrolyte between twoporous electrodes through which airand fuel flow

● Thermal efficiency of 40–50%stand-alone and 60–70%pressurised by a gas turbine

● Ultra-low NOx emissions ● Initial small-scale applications 0.2–1

MW – expected to grow to over20MW


to ensure passenger comfort even inclose vicinity to the engine room.

Rolls-Royce is leading a majorEuropean Union (EU) funded project todevelop validated design tools forsystem simulation, vibration isolation andfluid-borne noise modelling. The majortechnical areas to be addressed include:

■ reduction of propulsor noise atsource

■ isolation of propulsion machinerynoise

■ control of intake and exhaust noise.

The challenge is to develop vessels thatcan exceed new comfort classrequirements being developed byclassification societies for cruise shipsand fast ferries, while meeting theconflicting requirements for low costsand high power-to-weight ratios,particularly with fast ferries.

New waterjet impeller designs thatminimise blade order and harmonicpressure fluctuations, to reduceimportant low frequency (<100 Hz) hullvibrations, are being developed, andreduction of noise transmission from thewater jet tunnel through double skinisolation is being investigated.Propulsion machinery vibration isolationhas been extensively developed in navalprogrammes and full-scale noisetransmission trials on naval vessels havedeveloped an in-depth understanding ofpropulsion system vibration isolationrequirements. This has enabled finite-element models of complete propulsionsystem installations to be used toanalyse noise transmission paths anddevelop effective mounting and isolationsystems.

Environmental impactMarine transportation is generallyperceived to have a lowerenvironmental impact than most otherforms of transportation. However,increasing attention is being paid tothe emissions from marine dieselengines (in Japan, for example, it isestimated that 17% of domestic

nitrogen oxide is due to coastalshipping), particularly with regard tonitrogen oxide, sulfur oxide, soot andvisible smoke, whether driven bylegislation or less tangible but equallyvalid issues of public perception. Thisis an area where gas turbines have aninherent advantage, emitting no sootor visible smoke in the exhaust andsignificantly lower levels of nitrogenoxide and sulphur oxide than diesels.Marine gas turbines benefitsignificantly from investments beingmade in combustion research for land-based and airborne applications.Despite the difference in fuels the drivetowards lower emissions is common,and the basic understanding ofcombustion processes, as well as thedry low emissions combustor designsthat result, have broad application.

Carbon dioxide emissions from gasturbines, and hence their contribution toglobal warming, are generally higherthan diesels. However, more efficientadvanced cycle gas turbines such asthe WR-21 significantly reduce thelevels of carbon dioxide producedtowards the levels emitted by the dieselengine. In the long term, Rolls-Royce isdeveloping fuel cell systems, such asthe solid-oxide system illustratedschematically in Figure 13 for land-based applications. Such systemscould ultimately be applied in marinesystems, particularly in electric shipapplications, subject to thedevelopment of reforming technology toenable them to operate on typicalmarine fuels. With thermal efficienciesof up to 60–70% (potentially achievedin hybrid fuel-cell gas turbine systems)and almost no reduction in efficiency atpart-load, this would yield a 40–50%reduction in fuel consumption, andcarbon dioxide emissions below thoseof the most efficient diesel engines.

Concluding remarksAs our world becomes increasinglycrowded, integrated transport systemsexploiting a broader range of modes willbecome essential to reduce pressure

on roads and airports. This will bringincreasing opportunities for newdevelopments in the marine sector.New technology in areas such as cleancombustion technology, computationaldesign and analysis methods, powerelectronics, advanced electricalmachines, fuel cells and magnetic andsuperconducting materials will findapplication in marine vessels of thefuture. However, if the UK is to maintainand develop its position to exploit thesedevelopments, we need to find ways toimprove the perception of the marineindustry amongst our schoolchildrenand young engineers, convincing themthat it offers exciting and challengingcareer opportunities. ■

Julia King spent 16 years as anacademic researcher and universitylecturer workingin the field offracture ofstructuralmaterials, beforejoining Rolls-RoyceAerospaceGroup as Headof Materials. She has continued towork for Rolls-Royce in various roles,and from 2000, for its MarineBusiness as Director of Engineering &Technology – Marine. In July 1999Julia was awarded the CBE forservices to materials engineering. InSeptember 2002, Julia will start a newrole at the Institute of Physics as itsChief Executive.

Ian Ritchey is Head of Research andTechnology in Rolls-Royce's MarineBusiness. Hehas been in thisrole for twoyears, precededby two years asa Professor atNewcastleUniversity andsix years invarious roleswith Rolls-Royce before that.

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