Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines

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159

C h a p t e r | s i x

Engine and Plant

Selection

Choosing a propulsion engine or engines and the most suitable plant conﬁguration

for a given newbuilding or retroﬁt project is not a simple decision. It dictates care-

ful study of the machinery options available and the operating proﬁle of the ship.

In the past, the ship owner or the designer had the straight choice of a

direct-coupled low-speed two-stroke engine driving a ﬁxed pitch propeller or a

geared medium-speed, four-stroke engine driving either a ﬁxed or controllable

pitch (CP) propeller. Today, ships are entering service with direct-coupled (and

sometimes geared) two-stroke engines driving ﬁxed or CP propellers, geared

four-stroke engines or high-/medium-speed diesel–electric propulsion plants.

Diverse diesel–mechanical and diesel–electric conﬁgurations can be consid-

ered (Figures 6.1 and 6.2).

II - Medium-speed, PR

IIB - Medium-speed, PR

I - Medium-speed, as-built

III - Medium-speed, PRS

IV - Low-speed, non-redundant

Low-

speed,

8R22

6R46

8R22

6R22

6R46

8R22

6R46

8R22

6R46

8R22

6S42MC

5S70MC

FigurE 6.1 Low- and medium-speed diesel-based propulsion machinery options for

a 91 000 dwt tanker and associated auxiliary power-generating (g) source

160 Engine and Plant Selection

Low-speed engines are dominant in the mainstream deep sea tanker, bulk

carrier and container ship sectors while medium-speed engines are favoured

for smaller cargo ships, ferries, cruise liners, RoRo freight carriers and diverse

specialist tonnage such as icebreakers, offshore support and research vessels.

The overlap territory has nevertheless become more blurred in recent years:

mini-bore low-speed engines can target short sea and even river/coastal vessels

while new generations of high-powered large bore medium-speed designs con-

test the traditional deep sea arenas of low-speed engines.

Many ship owners may remain loyal to a particular type or make and

model of engine for various reasons, such as reliable past-operating experi-

ence, crew familiarity, spares inventories and good service support. But new or

reﬁned engine designs continue to proliferate as enginebuilders seek to main-

tain or increase market share or to target a new sector. In some cases an owner

who has not invested in newbuildings for several years may have to consider a

number of models and plant conﬁgurations which are unfamiliar.

Much decision making on the engine focuses on cost considerations—not

just the initial cost but the type of fuel which can be reliably burned, main-

tenance costs, desirable manning levels and availability/price of spares.

The tendency now is to assess the total life cycle costs rather than the pur-

chase price of the main engine. Operating costs over, say, 20 years may vary

signiﬁcantly between different types and makes of engine, and the selected

8 Cyl.

450 V switchboard

Geared propulsion

6600 V switchboard

7,5 MW

Cyclo

6 Cyl.

5430 kW

4 Cyl

3620 kW

4 Cyl.

6 Cyl.

8 Cyl.

3240 kW

Low-voltage

electric system

4 Cyl.

3620 kW

6 Cyl.

5430 kW

6 Cyl.

4 Cyl.

6 Cyl.

Electric propulsion

Medium-voltage

powerstation

FigurE 6.2 Schematics of geared medium-speed diesel propulsion plant with

gensets (top) and diesel–electric plant serving propulsion and auxiliary power

demands (bottom)

plant conﬁguration in which the engine has to function. Key factors inﬂuencing

the choice of engine may be summarized as:

l Capability to burn heavy fuel of poor quality without detrimental

impact on the engine components and hence maintenance/spares

costs.

l The maintenance workload: the number of cylinders, valves, liners,

rings and bearings requiring periodic attention in relation to the number

of crew carried (bearing in mind that lower manning levels and less

experienced personnel are now more common).

l Suitability for unattended operation by exploiting automated controls

and monitoring systems.

l Propulsive efﬁciency: the ability of the engine or propeller shaft to be

turned at a low enough speed to drive the largest diameter (and hence

most efﬁcient) propeller.

l Size and weight of the propulsion machinery.

l Cost of the engine.

The size of the machinery space is largely governed by the size of the main

engine which may undermine the cargo-carrying capacity of the ship. The

available headroom is also important in some ships, notably ferries with vehi-

cle decks, and insufﬁcient headroom and surrounding free space may make it

difﬁcult or impossible for some engines to be installed or overhauled.

DiESEL–mEChaniCaL DrivES

The direct drive of a ﬁxed pitch propeller by a low-speed two-stroke engine

remains the most popular propulsion mode for deep sea cargo ships. At one

time a slight loss of propulsive efﬁciency was accepted for the sake of simplic-

ity but the introduction of long stroke and, later, super- and ultra-long stroke

crosshead engines has reduced such losses. For a large ship a direct-coupled

speed of, say, 110 rev/min is not necessarily the most suitable since a larger

diameter propeller turning at speeds as low as 60 rev/min is more efﬁcient than

one of a smaller diameter absorbing the same horsepower at 110 rev/min. The

longer stroke engines now available develop their rated outputs at speeds rang-

ing from as low as 55 rev/min (very large bore models) up to around 250 rev/

min for the smallest bore models. It is now possible to specify a direct-drive

engine/propeller combination that will yield close to the optimum propulsive

efﬁciency for a given ship design.

Large bore low-speed engines develop high speciﬁc outputs, allowing the

power level required by many ship types to be delivered from a small number

of cylinders. Operators prefer an engine with the fewest possible cylinders, as

long as problems with vibration and balance are not suffered. Fewer cylinders

obviously inﬂuence the size of the engine and the machinery space, the main-

tenance workload, and the amount of spares which need to be held in stock. In

most deep sea ships the height restriction on machinery is less of a problem than

Diesel–mechanical drives 161

162 Engine and Plant Selection

length, a larger bore engine with fewer cylinders therefore underwriting a shorter

engineroom and more space for cargo. Larger bore engines also generally return

a better speciﬁc fuel consumption than smaller engines and offer a greater

tolerance to heavy fuels of poor quality.

A direct-coupled propulsion engine cannot operate unaided since it

requires service pumps for cooling and lubrication, and fuel/lube oil handling

and treatment systems. These ancillaries need electrical power which is usu-

ally provided by generators driven by medium- or high-speed diesel engines.

Many genset enginebuilders can now offer designs capable of burning the

same heavy fuel grade as the main engine as well as marine diesel oil or

blended fuel (heavy fuel and distillate fuel mixed in various proportions,

usually 70:30) either bunkered as an intermediate fuel or blended onboard.

‘Unifuel’ installations—featuring main and auxiliary engines arranged to burn

the same bunkers—are now common.

auxiliary Power generation

The cost of auxiliary power generation can weigh heavily in the choice of main

machinery. Developments have sought to maximize the exploitation of waste

heat recovery to supplement electricity supplies at sea, to facilitate the use of

alternators driven by the main engine via speed-increasing gearing or mounted

directly in the shaftline, and to power other machinery from the main engine.

Gear-based constant frequency generator drives allow a shaft alternator to

be driven by a low-speed engine in a ﬁxed pitch propeller installation, with full

alternator output available between 70 per cent and 104 per cent of propeller

speed. A variety of space-saving arrangements are possible with the alterna-

tor located alongside or at either end of a main engine equipped with compact

integral power take-off (PTO) gear. Alternatively, a frequency converter system

can be speciﬁed to serve an alternator with a variable main engine shaft speed

input in a ﬁxed pitch or CP propeller installation.

The economic attraction of the main engine-driven generator for electrical

power supplies at sea is that it exploits the high thermal efﬁciency, low speciﬁc

fuel consumption and low grade fuel-burning capability of the ship’s diesel

prime mover. Other advantages are that the auxiliary diesel gensets can be shut

down, yielding beneﬁts from reduced running hours in terms of lower fuel and

lubricating oil consumptions, maintenance demands and spares costs.

System options for electricity generation have been extended by the arrival

of power turbines which, fed with exhaust gas surplus to the needs of modern

high-efﬁciency turbochargers, can be arranged to drive alternators in conjunc-

tion with the main engine or independently.

These small gas turbines are also in service in integrated systems linking

steam turbo-alternators, shaft alternators and diesel gensets; the various power

sources—applied singly or in combination—promise optimum economic elec-

tricity production for any ship-operating mode. Some surplus electrical out-

put can also be tapped to support the propulsive effort via a shaft alternator

switched to function as a propulsion motor.

Such a plant is exploited in a class of large low-speed engine-powered

container ships with signiﬁcant reefer capacity whose overall electrical load

proﬁle is substantial and variable. Crucial to its effectiveness is a computer-

controlled energy management system which co-ordinates the respective

contributions of the various power sources to achieve the most economical

mode for a given load demand.

Integrated energy-saving generating plants have been developed over the

years for application mainly to large container ships. The systems typically

exploit waste heat (from low-speed main engine exhaust gas, scavenge air and

cooling water) to serve a steam turbo-alternator, air-conditioning plant, heaters

and distillers. System reﬁnements were stimulated by the diminishing amount

of energy available from the exhaust gas of low-speed engines, in terms of both

temperature and volumes, with the progressive rise in thermal efﬁciencies. The

ability of the conventional waste heat boiler/turbo-alternator set to meet elec-

trical demands at sea was compromised, any shortfall having to be plugged by

supplementary oil ﬁring of an auxiliary boiler or by running a diesel genset

and/or shaft alternator. The new integrated systems, some also incorporating

power gas turbines, maximize the exploitation of the waste heat available in

ships whose operating proﬁles and revenues can justify the added expense and

complexity.

Wider application of ‘cold ironing’—shutting down the diesel gensets in

port and tapping shoreside electrical power supplies—will be stimulated by

concerns over emissions in highly populated areas, a trend initiated in US west

coast container terminals.

geared Drives

The most common form of indirect drive of a propeller features one or more

medium-speed four-stroke engines connected through clutches and couplings

to a reduction gearbox to drive either a ﬁxed pitch or CP propeller (Figures 6.3

and 6.4). The CP propeller eliminates the need for a direct-reversing engine

while the gearing allows a suitable propeller speed to be selected. There is

inevitably a loss of efﬁciency in the transmission but in most cases this would

be cancelled out by the improvement in propulsive efﬁciency when making

a comparison of direct-coupled and indirect drive engines of the same horse-

power. The additional cost of the transmission can also be offset by the lower

cost of the four-stroke engine since two-stroke designs, larger and heavier, cost

more. The following generic merits are cited for geared multi-medium or high-

speed engine installations:

l Ships with more than one main engine beneﬁt from enhanced

availability through redundancy: in the event of one engine break-

ing down another can maintain navigation. The number of engines

engaged can also be varied to secure the most economical mode for a

given speed or deployment proﬁle. Thus, when a ship is running light,

partially loaded or slow steaming, one engine can be deployed at its

Diesel–mechanical drives 163

164 Engine and Plant Selection

normal (high-efﬁciency) rating and some or all of the others shut down.

In contrast, in similar operational circumstances, a single direct-coupled

engine might have to be run for long periods at reduced output with

lower efﬁciency.

l The ability to vary the number of engines deployed allows an engine

to be serviced at sea, easing maintenance planning. This ﬂexibility

is particularly valued in an era when port turnround times are mini-

mized. Engines can also be overhauled in port without the worry

of authorities demanding an unscheduled shift of berth or sudden

departure.

l By modifying the number of engines per ship and the cylinder numbers

per engine to suit individual power requirements the propulsion plant for

a ﬂeet can be standardized on a single engine model, with consequent

savings in spares costs and inventories, and beneﬁts in crew familiarity.

The concept can also be extended to the auxiliary power plant through

‘uniform machinery’ installations embracing main and genset engines of

the same model.

l Compact machinery spaces with low headrooms can be created,

characteristics particularly valued for RoRo ferries.

Designers of marine gearing, clutches and couplings have to satisfy varying

and sometimes conﬂicting demands for operational ﬂexibility, reliability, low

noise and compactness from transmission systems.

FigurE 6.3 a typical multi-engine geared twin-screw installation

Diesel–mechanical drives 165

Advances in design, materials and controls have contributed to innovative

solutions for versatile single- and multi-engine propulsion installations fea-

turing PTOs for alternator drives and power take-ins (PTIs) to boost the pro-

pulsive effort. In many propulsion installations a gearbox is expected to: (a)

determine the propeller speed and direction of rotation, and provide a reversing

capability; (b) provide a geometric coupling that can connect and separate the

ﬂow of power between the engine and propeller shaft or waterjet drive; and (c)

absorb the thrust from the propeller.

An impressive ﬂexibility of operating modes can be arranged from geared

multi-engine propulsion plants which are, in practice, overwhelmingly based

on four-stroke machinery (a number of geared two-stroke engine installations

have been speciﬁed, however, for special purpose tonnage such as offshore

shuttle tankers).

Father-and-Son Layouts

Operational ﬂexibility is enhanced by adopting a so-called ‘father-and-son’

(or ‘mother-and-daughter’) conﬁguration: a partnership of similar four-stroke

engine models, but with different cylinder numbers, coupled to a common

reduction gearbox to drive a propeller shaft. Father-and-son pairs have been

Leroy Somer 1600 kW alternator

Leroy Somer 1550 kW alternator

Ulstein Bergen BRM 9

(max 3970 kW at 750 rpm)

Ulstein Bergen KRG 9 engine

(max 1660 kW at 750 rpm)

Ulstein CP propeller

Volda gearbox

FigurE 6.4 One of the medium-speed geared propulsion/auxiliary power lines of a

norwegian coastal ferry

166 Engine and Plant Selection

speciﬁed to power large twin-screw cruise vessels and ferries. An example

is provided by the 1995-built P&O liner Oriana whose 40 000 kW propul-

sion plant is based on two 9-cylinder and two 6-cylinder MAN B&W L58/64

medium-speed engines. Each father-and-son pair drives a highly skewed Lips

CP propeller via a Renk Tacke low-noise gearbox which also serves a 4200 kW

shaft alternator/propulsion motor.

Propulsion can be effected by either: the father-and-son engines together;

the father engines alone or the son engines alone; and with or without the shaft

alternators operating as propulsion motors (fed with electrical power by the

diesel gensets).

DiESEL–ELECtriC DrivE

An increasingly popular form of indirect drive is diesel–electric propulsion

featuring power stations based on multi-medium speed main gensets. New gen-

erations of AC/AC drive systems exploiting cycloconverter, synchroconverter

and pulse width modulated converter technology have widened the potential

of electric propulsion after years of conﬁnement to specialist niches, such as

icebreakers, research vessels and cablelayers. Converters allow a propulsion

motor to be driven over its whole speed range from zero to nominal speed in

forward and reverse modes.

Diesel–electric propulsion is now dominant in cruise ships and new gen-

eration liqueﬁed natural gas (LNG) carriers, and references are established in

North Sea shuttle tankers, short sea and deep sea chemical carriers, and Baltic

RoRo passenger/freight ferries. The mode is also increasingly popular for off-

shore supply and support tonnage.

A number of specialist groups contest the high-powered electric drive mar-

ket, including ABB Marine, Alstom, STN Atlas and Siemens, which highlight

system capability to cope with sudden load changes, deliver smooth and accu-

rate ship-speed control, and foster low noise and vibration. Applications have

been stimulated by continuing advances in power generation systems, AC drive

technology and power electronics.

Two key AC technologies emerged to supersede traditional DC electric

drives: the Cycloconverter system and the Load Commutated Inverter (LCI)

or Synchrodrive solution. Both are widely used in electric propulsion but,

although exploiting the same basic Graetz bridge prevalent in DC drive sys-

tems, they have different electrical characteristics. Development efforts have

also focused on a further AC system variant, the voltage source Pulse Width

Modulation (PWM) drive.

A variable-speed AC drive system comprises a propulsion motor (Figure 6.5)

and a frequency converter, with the motor speed controlled by the converter alter-

ing the input frequency to the motor.

Both Cycloconverter and PWM drives are based on advanced AC variable-

speed technology that matches or exceeds the performance characteristics of

conventional DC drives. Each features motor speed control with maximum

torque available from zero to full speed in either direction, facilitating operation

with simple and robust ﬁxed pitch (rather than CP) propellers. Smooth con-

trol underwrites operation at very low speeds, and vector control yields a rapid

response to enhance plant and ship safety. Integrated full electric propulsion

(IFEP) is now standard for cruise liners and speciﬁed for a widening range of

other ship types—including naval vessels—in which the economic and opera-

tional beneﬁts of the central power station concept can be exploited.

Electric propulsion requires motors to drive the propellers and gensets

to supply the power. It seems somewhat illogical to use electric generators,

switchgear and motors between prime movers and propeller when direct-

coupling or geared transmission to the shaft may be all that is necessary. There

are obviously sound reasons for some installations to justify the complication

and extra cost of electric propulsion, Lloyd’s Register citing:

Flexibility of layout: The advantage of electric transmission is that the prime

movers and their associated generators are not constrained to have any parti-

cular relationship with the load—a cable run is a very versatile medium. In a

propulsion system it is therefore possible to install the main diesel gensets and

their support services in locations remote from the propeller shaft (Figure 6.6).

Diesel gensets have even been installed in containers on deck to provide pro-

pulsive power, and other vessels equipped with a 10 000 kW generator mounted

in a block at the stern above the RoRo deck.

Another example of the ﬂexibility facilitated by an electric propulsion

system is a semi-submersible rig installation whose gensets are mounted on

the main deck and propulsion motors in the pontoons. In cruise ships, ferries

Diesel–electric drive 167

FigurE 6.5 One of two 15 000 kW Stn atlas propulsion motors installed on the

cruise liner Costa Victoria

168 Engine and Plant Selection

and tankers the layout ﬂexibility allows the naval architect to create compact

machinery installations releasing extra revenue-earning space for passenger

accommodation/amenities and cargo.

Opportunities to design and build tankers more cost-effectively are also

offered. The optimized location of the main machinery elements allows the

ship’s overall length to be reduced and steel costs correspondingly lowered for

a given cargo capacity; alternatively, the length of the cargo tank section can be

extended within given hull dimensions.

Diesel–electric solutions facilitate the modularization and delivery of

factory-tested turnkey packages to the building yard, with the complete main

gensets mounted on common bedplates ready for coupling to their support

systems. The compactness of the machinery outﬁt fosters shorter runs for the

cabling and ancillary systems, and the engine casing and exhaust gas piping

are also shortened.

It is difﬁcult for a diesel–electric plant to match the fuel economy of a

single direct-drive low-speed main engine which is allowed to operate at its

optimum load for the long transoceanic leg of a tanker’s voyage. But some

types of tanker (oil products and chemical tonnage, as well as North Sea shut-

tle carriers) are deployed in varied service proﬁles and spend a considerable

time at part loads: for example, in restricted waters, during transit in ballast

and manoeuvring.

1. Diesel gensets

2. Main switchboard and cycloconverters

3. Propulsion motor

4. Stern thrusters

5. Cargo pumps

6. Engine control room

FigurE 6.6 machinery arrangement in a diesel–electric tanker