Authors: Jeroen Kokshoorn, Björn von Ubisch, Imtech Marine and Offshore

Diesel Electric Systems for Offshore Vessels and certain types of Yachts

1. Introduction and Summary This article discusses the various types of diesel electric systems and gives examples of the ways in which such systems can be integrated within modern offshore vessels and installations. The various aspects of diesel electric propulsion are discussed as well as some of the difficulties which may be encountered when designing such a system are highlighted. This article also gives examples of some typical diesel electric installations for pipe-layers, diving support vessels, drilling rigs and offshore supply vessels. An example of a diesel electric hybrid system for a super-yacht is also given.

2. Diesel electric propulsion Why change a time proven simple system like diesel propulsion and introduce complex parts in a vital propulsion-train? Modern ships combine multiple tasks. A typical offshore vessel may be equipped with cranes, pumps, winches and other power consuming equipment. Fuel prices are a major concern even if the fuel is often supplied by the oil company, the Client. Diesel electric vessels with integrated power management are characterized by the following features:

Advantages: • Lower Total Cost of Ownership • Lower fuel consumption • Better maneuverability (Dynamic

Positioning) • Better load sharing between the diesels

due to multiple use of the power plant • Space saving, greater freedom in the

positioning of power plant and propulsion equipment – long and inconvenient shaft lines are eliminated

• Simple lay-out of machinery spaces • Improved redundancy

• Improved smooth running, less vibration and low noise levels

• Reduction in maintenance costs – a major advantage of diesel electric systems is that the diesel engines can be made to operate at their design optimum power and generally installed within one area of the vessel. Power is distributed to the consumers through cables, not through shaft-lines

Disadvantages:

• More complex equipment • Complex for crew • Harmonic distortion, electromagnetic

interference and earthing of equipment is more complex due to common mode voltages and spurious currents

Fuel Efficiency: Many offshore support vessels, research vessels, yachts and drilling units must be able to operate with a wide range of load demands. A multiple diesel-generator installation allows the selection of the appropriate number of generators working at their optimum fuel efficiency point with respect to the power demand.

(Source R. Durković)• n [min-1] engine speed • pe [bar] mean pressure • Pe curves represents constant Power • be curves represent constant specific fuel

consumption • L optimum fuel consumption

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The conclusion of the diagram above is that a diesel engine at low load consumes relatively more fuel than at the optimum working point, normally around 75 – 95 % of maximum power. In the diagram above the specific fuel consumption increases from 235 g/kWh at 1,600 rpm and 100 % load to 360 g/kWh at 600 rpm and 15 % load. Load sharing and arrangements:

The above diagram compares the traditional arrangement (one diesel coupled to a propeller and two diesel generators with switchboard for the domestic power demand) with a typical diesel electric configuration (with multiple diesel generator installation with one central switchboard and two electrically driven azimuthing thrusters). With diesel direct drive the diesel will have to run at all loads from very low to full power as the power demand from the propeller requires. With the diesel electric solution the appropriate number of diesels may be selected and the fuel consumption will be optimal. Vessels with Dynamic Positioning (station keeping) will have a varying power demand depending on the weather conditions and the activities onboard. The diesel electric solution is ideal for this type of application. The diesel electric principle gives a more efficient use of the installed power. All consumers onboard will benefit from the centralized power-plant and separate power generation units are not required. The diesel electric solution gives the designer greater freedom to distribute the equipment in order to achieve optimum layout and performance of the vessel. For example the main diesels do not require to be directly attached to the propulsion propellers by extended shaft lines. The power plant can be placed forward and the

propulsion plant can be located aft with only power cables in between. Long drive shaft through the major part of the ship, which would normally interfere with space for tanks and equipment, are not required.

The top figure depicts a typical supply vessel with shaft-lines and conventional propellers. The cargo tanks must be elevated in order to give space for the shaft-lines. The lower figure depicts the typical diesel electric application where the full volume of the midship part of the ship can be utilized for cargo.

3. Electric System layout Voltage. Typical voltage ranges for AC generation and distribution systems are normally from 400 V to 690 V and 3.3 kV to 11 kV. The voltage used is generally determined by the short-circuit current in the main switchboard. The limiting factor is the capacity of the outgoing breakers and the current they can handle. Higher voltage reduces the current. The main switchboard must always have at least two sections according the rules. Number of diesel-generators. In determining the optimum number of diesel generators to be installed as part of a system, the following points should be considered:

• Minimum of two • A load around 80 – 95 % of the

maximum continuous rating (MCR) per diesel for the various operating profiles

• At least two diesel generators in each engine room in case of multiple engine rooms (redundancy requirements)

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AC – DC. AC distribution systems in ships are normally preferred for the following reasons:

• AC power circuits are easier to break than DC power circuits

• DC system power capacities are limited in power due to the size of commercial circuit breakers

• AC distribution systems are simpler to make safe than DC distribution systems

4. Total Harmonic Distortion A Variable Frequency Drive (VFD) can control both the speed and the torque of the associated electric motor.

A VFD controls the flow of power to the motor. (torque and speed). An induction motor has a high starting current when started direct on-line. A drive minimizes the voltage drop which occurs when an electric motor comes on-line. This effect is especially noticeable in smaller networks like the network of a vessel. The starting current for an AC motor is as follows:

• Direct on-line start; 6 - 10 times nominal current

• Star-Delta start; 2 – 3 times the nominal current

• Soft starter; 2.5 – 5 times nominal current (depending on required starting torque)

• VFD start, 0 – 100 % of nominal current The introduction of a VFD also introduces harmonic distortion in the distribution system.

The typical VFD (6 pulse) consists of the following main components:

• Rectifier bridge with AC input and DC output (AC – DC converter)

• Intermediate circuit with a DC link • Inverter with DC input and an AC

output with a variable frequency (DC – AC inverter)

Direct Front End Drives (DFE). The DFE consist of a passive diode rectifier bridge with 6 pulse, 12 pulse or 24 pulse configuration.

The figure above depicts a 12 pulse system. In order to reduce the harmonic distortion a 12 pulse system is introduced. The phase shifting (introduced by the phase shifting transformer) in one line counteracts the Harmonic distortions in the other line. As a result the Harmonic distortion is reduced. The 24 pulse system gives an even better result, but requires additional phase splitting transformers. Active Front End drives. (AFE) The AFE incorporates (active) bi-directional controlled rectifiers in the AC-DC converter circuit. This leads to a significant reduction in the level of the harmonic distortion produced by the drive.

Advantages of an Active Front End drive: • Less cabling • Less harmonic distortion (up to the 50th

harmonic distortion) • AFE has the ability to control the power

factor for the incoming power (voltage – current). This will improve the power factor of the total grid.

• Less sensitive to voltage variations on the input side

• Power feed-back capability into ship’s network

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• A heavy and voluminous phase splitting transformer is not required

Disadvantages with the AFE are:

• Isolation monitoring of grounded systems are complicated

• The higher harmonic distortions (> 50th ) can give electromagnetic interference problems

• Additional semi-conductors make the AFE solution more expensive than a DFE solution.

5. Reversing power capability Under certain circumstances the AC induction motor will act as a generator. This can happen when rendering lines on an electrically driven winch or when a propeller is free-wheeling while the ship still has speed. This generated power is usually absorbed in a resistor bank or can be fed back into the net. In the feed-back situation the net must be able to absorb the power. This can only be done by other consumers on the net like propellers, winches etc. An AFE variable frequency drive is capable to feed the reversed power back into the net. This will consequently reduce the power demand from the diesel generators.

6. Counter measures on harmonic distortion (THD)

Harmonic distortion results from the switching of the alternating current waveform by non-linear conducting elements such as diodes. The resulting waveform is non sinusoidal in nature. The higher harmonic currents can cause several problems:

• In rotary machinery and in transformers, internal losses result in increased heating

• Increased cable sizes may be required in order to accommodate the non-sinusoidal part of the current.

• High harmonic currents (due to capacitive coupling) may follow unwanted routes throughout the ship and can cause interference problems on communication lines, radars etc.

The Classification Societies have defined a limit to the THD which are not to be exceeded.

Several methods can be employed to reduce the levels of THD to meet these requirements, such as:

• Intentional routing of disturbance currents

o Requires a proper earthing and EMC/EMI plan

• Separation of the “dirty” and “clean” network by:

o Passive filtering o Using 12-pulse system (often

configured as a pseudo 24 pulse system) or AFE drives

o Clean net rotating or static converter

• EMC measures/filters in the VFD cabinets

A network quality study and a network design analysis should be carried out at an early stage in the engineering process. The calculation of THD for various load cases, short circuit calculation and the generation of a proper earthing plan, will provide information essential to the accurate costing and design of a new installation. The figure below shows an example of a “clean” and a “dirty” network separation and is typical of the electrical layout on an equipment class 3 DP pipelay vessel. The VFDs are directly connected to the high voltage system and introduce a significant amount of harmonic distortion into the supply network. Essential equipment on the low voltage side is protected by using a passive filter, creating a low impedance path to earth for the dominant fifth and seventh harmonic. Note: only deck equipment (which does not require special care) is connected to the mid low voltage switchboard.

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7. Economical “24” Pulse systems.

Due to their increased usage, industrial 12 pulse systems are becoming less expensive. True 24 pulse systems are quite expensive. Using two 12-pulse systems with a small (± 7.5º) additional phase shift winding it is possible to benefit from the symmetrical loading on the grid. Even if the loading is not fully balanced a considerable part of the harmonics will be cancelled out. The typical level of harmonic distortion stays well below 5 %.

“Basic Design” (12-24 pulse)

G~

Main Generator2000kVA1600 kW 1800 rpm

Retractable Thruster800 KW

G~MCC/ DP

Port

Emergency Generator150 KW 480 V / 60 Hz

ESB

480 V / 60 Hz MSB

G~G~ G~

Main Generator2000kVA1600 kW 1800 rpm

Main Generator2000kVA1600 kW 1800 rpm

Main Generator2000kVA1600 kW 1800 rpm

Bow Thruster910 KW

MCC/ DPstarboard

M3

M3

Main propulsion port2000 KW

0-1800 rpm12 pulse

Main propulsion starboard2000 KW

0-1800 rpm12 pulse

Shoresupply

pump

pump

pump

pump

pump

pump

Not in scope

Not in scope

Not in scope

Not in scope

8. Benefits of Energy Storage

Certain types of vessels will require very high “power-bursts” during a short moment and other types of vessels will require very low power for extended periods. In either case a battery bank may prove to be a cost-effective solution. The additional battery capacity, which is drawn on only as required, can be used to ensure that the diesel generator operates at optimum efficiency. The above figure depicts a simple application with a diesel generator feeding a DC bus through the associated rectifier. The electro motor driving the propeller is speed controlled via

VFD. As the battery can supply power instantly, the overall stability of the supply network will improve. A previously critical case of diesel generator tripping becomes less of a concern. There are two main types of batteries, lead acid (LAB) and Lithium ion (Li-ion). The lead acid batteries have been around for the last 150 years and represent well proven technology - especially their use in diesel electric submarines as a primary source of power. Lead acid batteries are also used in UPS (Un-interrupted Power Supply) for computer systems, control systems etc. The Li-ion is a new type of lightweight high energy density, rechargeable battery type that is widely used in consumer electronics and in various types of vehicles and in aerospace applications. Comparison Battery Chemistries

Spec. Energy Whr/kg

Energy density

Whr/dm³ Lead Acid 15 50 Li-ion 150 300

The lead batteries may during charging and in particular when overcharging release Hydrogen. This implies that the batteries must be located in a well ventilated space and the battery space is to be regarded as hazardous area with a potential explosive atmosphere. The Li-ion batteries can, when mistreated, catch fire and this fire cannot be extinguished. Mitigating measures must be taken in order to reduce the risks. Li-ion battery has a very high power density compared to all other power sources and the power can be delivered instantaneously.

9. 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 be determined by the optimum output voltage of the frequency converter.

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A constant speed thruster motor requires a variable pitch thruster. When the thruster is started up and running with zero pitch, about 20 % of the nominal power is consumed. A dynamically positioned vessel will very often have thrusters running 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.

10. Cooling systems Traditionally, in the past, the diving support vessels were DC vessels with 600 V main distribution voltage. The rectifiers and the electrical motors were all air-cooled. The air cooling required huge air ducts for cooling air, supply and exhaust. Compared with a squirrel cage AC motor, the DC motors requires certain amount of maintenance. The asynchronous squirrel cage motor can easily be totally enclosed and made water cooled. The modern frequency converters are also water-cooled and this combination results in a very compact installation, requiring the minimum of air-cooling.

11. Application - Pipe Lay Vessel The Audacia Pipe Lay vessel is a converted bulk-carrier. The “stinger” for pipelay operations is located forward and when laying pipe, the vessels advances astern.

The old propulsion plant has been kept and is used for transit. A new power plant has been installed consisting of two separate engine rooms with each three diesel-generators, feeding the pipe-lay installation and the new thrusters for position keeping. Each diesel-generator has a power of 6.5 MW and each of the six thrusters has an installed power of 5 MW.

12. Application - Well Stimulation Vessel

A well stimulation vessel is a vessel that either hooks up directly to a subsea completion (well head) or is via a fixed or mobile drilling rig directly connected to the oil well via hoses. The example given here is a vessel with a subsea coiled tubing unit onboard that hooks up to the subsea production tree on top of the well-head and do all the 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.

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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:

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 set to feed a DC bus which in turn supplies the converter based consumers. There are three groups: DC Link 1

• 50 % of main propulsion motor PS (tandem motor)

• Steering motors for azimuthing thrusters PS

• Various well stimulation pumps located aft (on line pumps)

DC Link 2: • 50 % of main propulsion motor SB

(tandem motor) • Steering motors for azimuthing thruster

SB • Various well stimulation pumps located

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:

• 50 % azimuthing thruster SB • 50 % azimuthing thruster PS • Retractable thruster forward

The vessel is also fitted with two constant speed tunnel thrusters forward as a backup for the retractable thruster forward. The philosophy behind this is that the backup tunnel thrusters will not be used very much and the constant speed option is the least expensive. This vessel will have a dual redundant Dynamic 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 of the subsea coiled tubing unit is disconnected after landing of the unit on the well head. 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. Application - Semi Submersible Drilling Rig

A typical semi submersible drilling vessel looks something like this:

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

Among the large consumers are the thrusters, located in the far ends of the pontoons and the drilling equipment concentrated around the moon pool in the centre of the unit.

The feeder cables to the thrusters will have a considerable length. There are certain advantages to select a high distribution voltage, 6 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 main distribution systems and all auxiliary systems are split up in three. The unit is laid out according DP equipment class 3. In the event of worst possible failure case, 2/3 of the installed power will be available. The electrical distribution system is in the form of a ring line system, which implies that 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. The closed bus-tie breaker philosophy gives a better load distribution across the installed diesel generators.

14. Application - Supply Vessel Diesel electric supply vessels have certain advantages over conventional diesel direct supply vessels.

Traditional Gulf Coast Supply vessels have the engine room aft and the uptakes for the exhaust and the engine room ventilation is often combined with smoke stacks half way down the aft deck. A diesel electric arrangement gives the freedom to locate the engine room where it is the less obstructive, considering the main function of the vessel.

The engine room is forward, and the propulsion room / thruster room aft. The centre square part of the hull is for the various cargo tanks. There are three diesel generators. One of the diesel generators can be connected to either SB or PS of the switchboard. Normal operation is with two diesel generators. This type of ship is safer than the traditional Gulf supply vessel as the machinery ventilation can be located high up and access to below deck spaces are all forward and well protected.

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The engine room can also be placed forward with diesel direct propulsion. In this case there will be long shaft lines passing through the hull, from the engine room forward to the thrusters aft and the shaft lines will reduce cargo space. Diesel electric propulsion will have as result that the cargo volume can be increased compared to the diesel direct solution. The advantages of diesel electric propulsion for a supply vessel are:

• Less total power installed • Reduced Fuel consumption when

maneuvering and when on DP at the rig • Fewer restrictions in the arrangement of

cargo spaces. DC-Bus configuration The DC-Bus will save space and weight in particular on an offshore supply boat where space and tank volumes are at premium.

The DC-Bus configuration reduces the number of electric motor control components to be installed as well as all the sensitive high-cost electrical components are concentrated to the switchboard room. The DC Bus configuration is not sensitive to frequency variations from the generators. Fuel can be saved by optimizing the diesel r.p.m. and the generator frequency to the actual load on the generators. The frequency converters forward and aft together with the phase splitting transformers will in this configuration be part of the switchboard. The heat load in the thruster room aft will also be less as the major heat sources are deleted from the space all together. The DC-Bus works on the following principle:

• Each of the standard AC generators feeds a DC-Bus via a rectifier

• All inverters (VFD) are connected to the DC-Bus and derive their power from the DC-Bus.

• The inverters (VFD) will control thrusters, cargo pumps, main ventilation fans, main cooling water pumps, compressors for cargo and the compressors for the AC units.

• The normal auxiliary distribution network is fed by two static inverters with sinus filter creating a “clean” supply with a minimum of harmonic distortions.

This equipment will occupy approximately the same space as a conventional switchboard in the switchboard room. The installation for the shipyard will be simpler as there is just one switchboard to install, no VFD with transformers forward and aft. The ventilation in the thruster room aft will be simpler as there is less heat to dissipate.

15. Hybrid Propulsion for a Yacht

A hybrid propulsion system has been introduced in one of the “super-yachts”, consisting of four diesel-generators and two Lithium Ion battery arrays. One of the battery arrays will always act as an emergency generator while the other array will be used for night time power supply in port and for silent cruising or two shave off peak-loads form the generators when cruising under diesel power.

Ghost Yachts' Ghost G180 hybrid super-yacht The main consumers are the HVAC (heating, ventilation, air-conditioning) plant and the propulsion consisting of two thrusters aft and one forward. The electrical installation consists of the following main components:

• Four diesel-generators each 450 kW • Two thrusters aft, each 750 kW • One thruster forward, 200 kW • Four chiller units, each 45 kW

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• Two Li-Ion Battery arrays, each 800 Ah at 500 V or approximately 400 kWh

Ghost Yachts

Dec, 2010

Shore power

=

To MSB To ESB

M3

=

Prop750 kW

==

NXP 16405

Power Electronics Panel(water cooled)

M3

=

Chiller2* ~45 kW

NXP 05205

M3

=

To MSB

M3

=

Prop750 kW

==

BatteryCompartment#2

(air cooled)

M3

=

Chiller2 * ~45 kW

M3

=

To MSB

=

Battery800 Ah~500 V

400 kWh

To MSBTo ESB

NXI 05205µGrid

100 kW(250 kW)

SIN 01805

Transf.120 kVA

MCCB250A

Battery800 Ah~500 V

400 kWh

NXP 02505

SIN 01805

400A

Power Electronics Panel(water cooled)

BatteryCompartment#1

(air cooled)

Bi-directional chopper~250A

400A NXP 05205

SIN 01805

NXP 02505 NXP 16405

M3

=

Bow200 kW

NXP 05205

Bi-directional chopper~250A

G

Generator450 kW

G

Generator450 kW

G

Generator450 kW

G

Generator450 kW

700 – 740 V 700 – 740 V

500 V AC motors500 V AC motors

All main consumers and the two battery arrays are connected to the DC-Bus, one SB and one PS. Domestic consumers like pumps, lighting, fans etc. are fed via a static converter giving a very clean and harmonic distortion free net. The battery arrays are always “hot-stand-by” and can be utilized at any time as required. When running the diesel-generators, the batteries are charged. The charging is monitored by a power management system that monitors each individual cell end ensures that no overcharging can occur.

16. Conclusion Diesel Electric system need not be more expensive than conventional diesel direct alternative. Integrated diesel electric systems for offshore vessels of various types are more economical and cheaper in operation than the conventional diesel direct approach. This is mainly due to fuel savings and less maintenance. More or less all large offshore support vessels today are diesel electric, as well as all DP drilling rigs. A number of super-yachts are also being fitted out with diesel electric propulsion and sometimes with battery back-up for ultra silent operations. The best approach to an integrated cost effective diesel electric system is to spend time and effort upon engineering the system properly at the beginning of the project, prior to ordering components. Of vital importance are the total harmonic distortion calculations, the short circuit calculations, cable routing and the earthing plan of the electrical equipment.