diesel electric systems

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Authors: Hans Niessink, IMTECH and Björn von Ubisch, Candies Shipbuilders, L.L.C.

Diesel Electric Systems for Offshore Vessels

1. Introduction and Summary

This paper discusses the various types of dieselelectric systems and components applicable to

the offshore industry and gives a couple ofexamples how the diesel electric system can beintegrated with other systems onboard.

The various aspects of diesel electric propulsion

are discussed as well as some of the pit-fallswhen designing a diesel electric system. The paper also gives an overview of some typicaldiesel electric offshore vessels, like divingsupport vessels, drilling rigs and supply vessels.

2. Diesel electric propulsion

Why would one want to change a time provensimple system like diesel propulsion andintroduce complex parts in a vital power-train?

This paper would like to give some basic info

Modern ships fulfill more tasks, more deepwatertechnology is required. Vessels make longer

trips, more power is installed. Fuel prices are aconcern.

Diesel electric vessels with an integrated powermanagement system bring the following:

Pro’s

• -lower Total Cost of Ownership

• -lower fuel consumption (especially on part load)

• -better maneuverability (DynamicPositioning)

• -less diesels required

• -space saving, no long inconvenientdrive shaft

• -easy lay-out of machine room

• -improved redundancy (no loss of hire)

• -smooth running, low noise

• -environmentally friendly

Contra’s:• -More equipment

• -More expensive

• -Complex for crew.

•

-Harmonic distortion

We will address these points.

A diesel electric system for a new build vesselcompared to a conventional diesel direct systemmay result in a cost increase from 2 % to 20 %.A completely integrated electric vesseleventually may cost less

One of the major advantages with a dieselelectric system is less diesels to maintain the

diesels are operating at their design optimum power. The many prime movers are, in the caseof an integrated electrical system, all replaced byreliable electrical motors.

Efficiency:A well laid out Diesel Direct propulsion system,

working on full power, in a vessel sailing onlyfrom A to B is more efficient than Diesel-Electric, and has a better fuel economy.

Diesel Electric propulsion is not the solution for

all ships, but if the operational philosophy ofyour vessel is not requiring full power of yourmain engines for all the time,(variable loaddemands) and you have auxiliary engines forother operations, you may want to look at adiesel electric integrated vessel.

Fuel:A diesel generator is most efficient whenworking under full load.At part load specific fuel-consumption rises, and

the diesel may suffer from black sludge.


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G.air = flow-rate through engine

P.z = maximum cylinder-pressure

α = air / fuel ratio

be = specific fuel consumption.

Simultaneity:Dynamic positioning (station keeping) will

normally not require full power of the Maindiesel generator sets. Full speed at transit for

propulsion normally requires the highest loadfrom the diesel generators.

You can install less auxiliary diesel power whenyou can use the power of the main diesel

generator sets for other purposes as well.

3. Lay-out of the vessel.

No long drive shaft through the whole shipwhich would normally interfere with space fortanks and equipment, and which has to be

supported and exactly in-line with the maindiesels.

• No transmission-gear, clutch, thrust- blocks

• No Rudders, steering gear, (whenazimuth thrusters are used)

• A drive replaces the CPP(controllable pitch propeller) or the direct dieseldriven FPP

• No tunnel thrusters aft, if azimuthingthrusters aft are used

A cluster of diesel generator sets is also easier tolocate in the engine room.

4. Electric System layout

Voltage.440 V - 480 V - 690 V - 3.3 kV – 6.6 kV – 9

kV – 11 kV etc…

Each case will require a study for optimumlayout, starting with the load balance, the powerrequired, availability of generator sets and

transformers, and the power the drive itself canhandle.

A very practical approach would also look at theshort-circuit current of the main switchboard.The main switchboard must always have at leasttwo sections according the rules and regulations.The short man bus tie breaker will have to beable to break the circuit current. This breakermay be a limiting factor when selecting the proper voltage; if it is commercially availableand if a class approval is available.

If the currents become too large, the voltage will

have to rise to the next level.


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5. Drives or Frequency

Converters

A drive or a frequency converter, is a heavy dutyelectric speed control of an AC electric motor.A drive controls the flow of power to the motor.(torque and speed)

It consists mainly out of three sections:

a) The front end: (converter) takes AC(Alternating Current) power out of themain grid, and converts it to pulses ofDC (Direct Current).

b) the Mid, DC section: temporarily buffer of DC Power

c) The rear end, (inverter) builds it up toany required AC (Alternating Currentwith variable frequency) again for the

electric motor.

A drive minimizes the voltage drop whichoccurs when an electric motor comes online.

This effect is especially noticeable in smallernetworks like the network of a vessel.

Pulse drives.The passive diode bridge at the front end of a pulse drive takes power out of the ships network

by slicing the incoming sinus in a fixed numberof pulses. Per cycle 6-12-18-24.

The pulsating surge however creates power grid pollution. The sinus of the main grid showsripple effects. This is called harmonic distortion.

The power surge is the same in per cycle, but themore pulses, the smoother the effect.

This is comparable to a ride on a 500 ccmotorbike with one cylinder, two cylinders, or

four cylinders. More cylinders, less vibration.

At full power the effect is bigger than at low power.

A 24- pulse system is smoother, but more

expensive than a 6-pulse, and it is not alwaysnecessary.

Active Front End drives. (AFE)An AFE has actively switching electronicsystems at the front, at the intake. The way thisfront end drains power from the grid greatlyreduces the ripple effect.

Advantages of an Active drive:

• Much less harmonic distortion,

• Power-factor improvement,

• Power feed-back capability into shipsnetwork.

• A large splitting transformer is notrequired.

6.

FEEDBACK systemsAn electric A-synchronous motor, when under braking conditions, can generate power.


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Certain active front end converters can inject this power back in to the network.

This should be carefully calculated, (powerdemand, load balance) otherwise other problemsmight be introduced. (Voltage variations)

The normal way to burn off this power is by airor water-cooled resistor-banks, simply wasting itin the form of heat.

7. The effects of Harmonic

Distortion, and its mitigation.

The power-peaks will heat up and age theinsulation of transformers etc. The power peakswill also cause extra heat losses in all electric

equipment. This will result in power overload ofelectronic equipment.

Electromagnetic interference will affect sensitiveinstruments and can cause false readings ofsensors.

Counter measures are:

• Filters, (active or passive)

• Sizing of the equipment.

• Splitting the network into a “dirty grid /

clean grid”

Most of the heavy duty equipment on board isindifferent about harmonics. The large propulsion motors or pump motors will notnotice.

It is the electronic equipment in ROV’s(Remotely Operated Vehicles), radars and othersmaller electronic devices which will be affected

by the harmonic distortions.

A rotating converter (electromotor driving agenerator) can be used to create a clean gridfrom a dirty one. (This device is frequently used

for the electronics of the ROV)

Careful power demand calculations arenecessary. Future expansions may becomedifficult.

Power factor

W: Watt, Real PowerVA: Apparent PowerVAr: Reactive power

Power factor is the ratio of Active Power (W) to

the Apparent Power (VA). The apparent powercan be greater than the real power.

A distribution system that is designed to handlethe higher currents caused by loads with low power factor will cost more than a distributionsystem that delivers the same useful energy toloads with a power factor closer to 1.

Active front end drives can improve the powerfactor and thus reduce certain components.

8. Economical “24” Pulse

systems.Due to the economy of large series, industrial 12 pulse systems are becoming less expensive.

If you can trick two 12- pulse systems into firingout of phase, the grid will see it as a 24 pulse

system. This will require two transformers whichhave special windings causing the phase shifting.

This results in a system with Total HarmonicDistortion figures according to the class rules.


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9. Diving Support Vessel

The Stena Seaspread type of vessel was one ofthe first integrated diesel electric vessels. Fourwere built. One was taken over by the UK Royal Navy and the other three are still around in the

oil industry.

The concept of the vessel was transporting Oilfield supplies and Diving Support.

When acting as a Diving Support Vessel this

type of vessel will spend the majority of the

time on station. A dual redundant Dynamic

Positioning system was installed.

These vessels has a power plant forward,

consisting of five diesel generators and is

fitted out with two tunnel thrusters forward,

one centre screw with rudder aft and two

azimuthing thrusters aft. The vessels also has

a firefighting system with a total of four

electrically driven fire pumps.

The centre screw was driven by four

electrical motors via a gear box. The

thrusters all had one electrical motor each as prime mover.

Main distribution voltage was 6 kV, AC 60

Hz.

The main power consumers are as follows:

• Main centre screw 4 x 1,100 kW

• Azimuth thrusters 2 x 1,100 kW

• Tunnel thrusters 2 x 1,100 kW• Fire pumps 4 x 1,100 kW

The original philosophy behind this was to

use one type of electrical motor for all

consumers on board. In practice it did not

work out so well as the speed requirements

for the motors were different. All motors

were 6 kV, 60 Hz, asynchronous, squirrel

cage, constant speed:

• Centre screw 885 rpm

• Azimuthing thrusters 885 rpm

• Tunnel thrusters 1,180rpm

• Fire pumps 1,760 rpm

Total installed power is 5 x 2,250 kW =

12,600 kW about 17,000 HP, in two separate

engine rooms.

Power consumption in the different

operational modes was estimated to,

according the original electrical balance:• Transit, free sailing 5,440 kW

• Fire fighting 8,911 kW

• Discharge at rig 6,303 kW

• DP and diving 4,692 kW

The advantage with an electric power plant

is that the electrical consumers can be fed

from one central power plant. This power

plant can be sized to maximum simultaneous

power demand. This power demand is less

than the total installed power of all pumps propellers, thrusters cranes etc. The various

power consumers on board can be located at

the most logical position and are not

restricted to a certain location because of a

diesel engine as prime mover.

All that connects the consumer with the

energy source is an electrical cable. The

power-plant is in this case concentrated

forward where all the exhaust uptakes and

ventilation trunking can be worked away in

the deckhouse and will not obstruct the aft

deck. “Real Estate” on these types of vessels


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is of premium and should be as obstacle free

as possible.

This vessel was designed in 1978 and

delivered in 1980.

10. Constant Speed thrusters

with Controllable Pitch

Today the constant speed/variable pitch

thruster will be changed to the variable

speed/fixed pitch type of thruster. The same

type of asynchronous squirrel cage motor

will be used but with a drive (frequency

converter) for controlling the speed instead

of the direct on-line feed. The voltage of the

motor will determined by the output voltage

of the frequency converter.

A constant speed thruster motor requires a

variable pitch thruster. When the thruster is

started up and running with zero pitch, about

25 % of the nominal power is consumed. A

dynamically positioned vessel will very

often have thrusters with less thrust demand

than 25 %. This has to do with the

redundancy requirements.

The DP-operator on the Bridge prefers to

have all thrusters up and running, in case

something will happen. The requirement is

to maintain heading and position for as long

as possible, whatever happens.

11. Cooling systems

Traditionally, in the past, the diving support

vessels were DC vessels with 600 V maindistribution voltage. The rectifiers and the

electrical motors were all air-cooled. This

resulted in huge air ducts for cooling air,

supply and exhaust. The DC motors also

require certain maintenance.

The asynchronous squirrel cage motor can

very easy be totally enclosed and made

water cooled. The modern frequency

converters are also water-cooled. This

results in a very compact installation with a

minimum of air cooling requirements.

12. Well Stimulation Vessel

A well stimulation vessel is a vessel that eitherhooks up directly to a subsea completion or isvia hoses and a fixed or mobile drilling rigdirectly connected to the oil well. The example

given here is a vessel with a subsea coiled tubing

unit onboard that hooks up to the well and do allthe logging and well stimulation directly,without the intermediary drilling rig.

This particular ship has the following main

power consumers:

• Thrusters

• Deep water deployment system

• ROV system

• Well stimulation pumps

The total installed power is 4 x 2,350 kW,

total 9.4 MW.

Thrusters, the deployment system and the

various well stimulation pumps plus various

transfer pumps are all speed controlled via

frequency converters.

The main distribution system is depicted

below:


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The main distribution voltage is 690 V, 60

Hz, AC. In order to save money and space

the frequency converters have been split up

in three groups. Each group has a rectifier

that is feeding a DC bus. From this bus the

consumers are fed. Each DC consumers has

a converter converting the DC to AC withvariable frequency. There are three groups:

DC Link 1

• ½ of main propulsion motor PS(tandem motor)

• Steering motors for azimuthingthrusters PS

• Various well stimulation pumpslocated aft (on line pumps)

DC Link 2:

• ½ of main propulsion motor SB(tandem motor)

• Steering motors for azimuthingthruster SB

• Various well stimulation pumpslocated aft (back-up pumps)

DC Link 3:

• Deep Water deployment system

• Various transfer pumps for the well

stimulation system located forward

There are also three frequency converter

panels for the propulsion:

• ½ azimuthing thruster SB

• ½ azimuthing thruster PS

• Retractable thruster forward

There are also two tunnel thrusters forward

as a backup for the retractable thruster

forward. The tunnel thrusters are of constant

speed type. The philosophy behind this isthat the backup tunnel thrusters will not be

used very much and the constant speed

installation is cheaper than the variable

speed installation.

This vessel will have a dual redundantDynamic positioning system. The four main

diesel generators are located in one engine

room forward with the auxiliary systems

split up SB and PS. The vessel will receive

no hydrocarbons onboard and is only

attached to the well with an umbilical for the

chemicals and power/data transfer, also with

a quick disconnect. The hoisting wire is

disconnected after landing of the subsea

coiled tubing unit.

Power consumption during the various

operations is expected to be:

• Transit 6.0 MW• Landing the Tool 4.5 MW

• Acidicing 4.1 MW

• Acidicing and squall 7.2 MW

13. Semi Submersible Drilling

Rig

A typical semi submersible drilling vessel lookssomething like this:

In this case the power plant is located in the deck box forward and the various consumers aredistributed around the unit.

One of the large consumers is the thrusters,

located in the far ends of the pontoons.


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The feeder cables to the thrusters will have a

considerable length. There are certain

advantages to select a high distribution

voltage, 9 or 11 kV and to have the

frequency converters as close to the thrusters

as possible. This will save considerable

amount of money and weight due to lighter

and smaller power cables. The AC

distribution system is in this case an obvious

advantage compared to the traditional DC

system.

In this case the main distribution

switchboard is split up in three and the

distribution system and all auxiliary systems

are laid out according DP equipment class 3.

The electrical distribution system is in the

form of a ring line system, which impliesthat during normal operation, the bus tie

breakers are closed. Certain Classification

Societies have some difficulties with this,

while other Classification Societies accept to

operate under DP equipment Class 3 with

closed bus tie breakers. In case of worst

possible failure, you will then have 2/3 of the

installed power available instead of ½ the

installed power. This saves weight and

money. Weight savings have high priority on

a semi sub.

14. Supply Vessels

Diesel electric supply vessels have certainadvantages over conventional diesel directsupply vessels.

Traditional Gulf Coast Supply vessels have the

engine room aft and the uptakes for the exhaustand the engine room ventilation is often

combined with smoke stacks half way down theaft deck. A diesel electric arrangement gives thefreedom to locate the engine room where it isless obstructive, considering the main functionof the vessel.

The engine room is all the way forward, and thethruster room all the way aft. The centre square part of the hull is for the various cargo tanks.

There are three diesel generators. One of thediesel generators can be connected to either SBor PS of the switchboard. Normal operation iswith two diesel generators.

This type of ship is safer than the traditionalGulf supply vessel as the machinery ventilation

can be located high up and access to below deckspaces are all forward and well protected.

The engine room can also be placed forwardwith diesel direct propulsion. In this case therewill be long shaft lines passing through the hull,from the engine room forward to the thrusters aftand the shaft lines will reduce cargo space.

The advantage of diesel electric propulsion for asupply vessel is:

• Less total power installed

• Reduced Fuel consumption whenmaneuvering, waiting on the rig to getready for cargo operations and when on


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DP and discharging cargo and backloading cargo

• Fewer restrictions in the arrangement ofcargo spaces.

15.

Conclusion

Diesel Electric system need not be moreexpensive than conventional diesel directapplications. Integrated diesel electric systemsfor offshore vessels of various types are moreeconomical and cheaper than the conventionaldiesel direct approach. More or less all largeOffshore support vessels are today dieselelectric. The best approach to an integrated costeffective diesel electric system is to spend timeand effort to engineer the system properly at the

beginning of the project, prior to orderingcomponents.

diesel electric systems

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