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Q: Propulsion system for LNG Carriers ( Answered 5 out of 5 stars,   2 Comments )
Subject: Propulsion system for LNG Carriers
Category: Science > Technology
Asked by: infoseekerr-ga
List Price: $140.00
Posted: 04 Apr 2003 16:09 PST
Expires: 04 May 2003 17:09 PDT
Question ID: 186174
I need a detailed and well presented evaluation of the propulsion
systems that can be used in LNG Carriers..You should take into account
the particular operating problems associated with this type of ships.
The two options are:

-Traditional Steam plants using the boil off.
-Diesel engines+reliquefaction, also the new Dual-Fuel diesel engines.

You should consider the following:
1) A brief history of propulsion systems used in this type of ships
(i.e.steam plant).

2) An investigation into the economic background of LNG prices,
including historic, current and projected future trends.

3) An engineering evaluation of the two types of propulsion plant,
including specifications, economic issues, safety issues and
reliability performance issues.

4) An opinion consisting of a balance reasoned argument over which
system you have chocen!

Clarification of Question by infoseekerr-ga on 05 Apr 2003 02:04 PST
Can you please include in-line references within your answer
i.e. the URLs of the web sites used.

Subject: Re: Propulsion system for LNG Carriers
Answered By: reeteshv-ga on 15 Apr 2003 03:13 PDT
Rated:5 out of 5 stars
Dear infoseekerr-ga,

Good day!

1. Introduction
Over the past three years, there has been a increased interest in the
LNG business:
- A large number of orders have been placed for new vessels
- New supply projects are coming up
- New terminals are being constructed and the existing ones are being

A big factor behind this volatility in the market is new technologies
that are bringing costs down and making (previously) uneconomic
projects viable. Nowhere is this change more evident than in ship
design where new propulsion systems are trying to displace the
traditional steam turbine engines with smaller, more efficient units
that will:
- Reduce fuel costs
- Increase cargo carrying capacity

Drewy Insights - Feature Pieces
November 2002
Drewry Annual LNG Shipping Market Review and Forecast 2002

2. Steam Plants
Since 1964 all LNG ships have used steam boilers to generate power for
both propulsion and ship services with a high degree of reliability.
The steam
- Drives the main engine and generators
- Powers auxiliary equipments like compressors, fans, pumps, etc.
- Provides the heat source for fuel tanks, air conditioning, etc.
Generally, a couple of diesel generators are also carried along for
back-up (when manoeuvring at sea and in port) and for cold-starting

The major advantages of the steam turbine engines are:
- High reliability (as it is widely adopted)
- Total use of boil-off gas (BOG) during the voyage (clean burning)

The major disadvantages of the steam turbine plant are:
- Low fuel efficiency
- High fuel cost
- Need for experienced crew capable of handling the steam turbine in
good condition

3. LNG Prices and Trends
The following data excerpted from the BP statistical review of world
energy 2002 shows the LNG prices:

Year  LNG Japan cif (cost+insurance+freight; average price)
         US dollars per million Btu (British thermal unit)
1984  –
1985  5.23
1986  4.10
1987  3.35
1988  3.34
1989  3.28
1990  3.64
1991  3.99
1992  3.62
1993  3.52
1994  3.18
1995  3.46
1996  3.66
1997  3.91
1998  3.05
1999  3.14
2000  4.72
2001  4.64

A report by The Institute of Energy Economics, Japan (IEEJ) makes the
following predictions as regards to LNG pricing options over the next
10 years:
- Crude oil price will remain the price indicator for LNG; however the
share of fixed elements will increase while the portion linked to the
crude oil price will be curtailed in pursuit of further price
- The price indicators for electricity generation could be coal,
coal+heavy fuel oil+crude oil, and the retail price of electricity
- For town gas production, the price indicator will be petroleum
products such as heavy fuel oil and kerosene
- The price of natural gas futures contract (Henry Hub price) at the
New York Mercantile Exchange (NYMEX) can be a price indicator
- New pricing in the Asia/Pacific LNG market is likely to be heavily
influenced by market changes (privatization of state oil companies,
the entry of new suppliers, etc.)

Natural gas prices (BP statistical review of world energy 2002)

"Current Situation and Future Prospects of Japanese LNG Market" by
Akiyuki FUJITA, Takeharu UEDA, Nobuya NAGASAKA, Satoshi SANO, Gas
Group, 1st Research Department (The Institute of Energy Economics,

4. Propulsion Alternatives
We start the section with brief descriptions of the various options
available. Later on, we compare a few of the alternatives with the
conventional steam turbine plant.

4.1 Diesel Engines
Diesel engines work on the Diesel cycle in which the fuel is
compressed so as to heat it beyond its auto-ignition temperature and
therefore diesel engines are also known as Compression Ignition (CI)
engines. They are amongst the most common and most efficient ship
engines. Diesel engines are of two types:
- 2 stroke engines are very simple but also the biggest and the
heaviest. They use low quality oil and have poor fuel efficiencies
- 4 stroke engines occupies less space and are more flexible to
changes in load. They make less noise than 2 stroke engines and are 2
times more efficient than the latter

"Diesel Engines" (

4.2 Gas turbines
Derived from aircraft engines, they provide high power in the smallest
possible area. They are 4 times as light as the best modern high pwer
diesel engines and yet deliver 6 times the power of the latter! These
turbines require 200 g of fuel to generate 1 kWh (compared to 160-170
g/kWh for diesel engines). In short, compared to the diesel engines,
gas turbines supply much more energy but at the cost of fuel

"Gas Turbines" (

4.3 Diesel Electric Systems
Here a large number of diesel engines are connected in parallel and
generate electicity that is provided for ship propulsion and for
auxiliary services (e.g., the HVAC system). At optimum loads, these
systems allow smoother transients and constant speeds. The major
advantages are:
- Low noise and vibration disturbances
- Lower energy consumption 
- Higher flexibility in the ship design

"Diesel Electric Systems" (

4.4 Azimuth Thrusters
They are electric podded drive systems mounted on a 360 degree
rotating shaft under the ship. The absence of rudder improves the
underwater dynamics and increases manoeuvrability. By allowing
flexible machinery arrangements, azimuth thrusters make possible a
smaller vessel with the same cargo space. Some other advantages of
these systems are:
- High energy savings (> 10%)
- Easier modularisation 
- Standardisation of machinery units 
- Reduced maintenance costs

"Azimuth Thrusters" (

4.5 Water Jet Propulsion Engines
They are similar to the aeroplane jet engines in the sense that both
use Newton's third law of gravity to propel the craft forward. The
engine uses an impeller to "suck" water in and then forces it out
through a nozzle at a high pressure. This force generates an equal and
opposite force accelerating the craft forward.

"Water Jet Propulsion Engines" (

5. Choosing a Propulsion System
The following table taken from the Mitsubishi report compares some of
the propulsion plants for LNG carriers:

Propulsion Steam turbine Dual fuel     Diesel engine with   Gas
plant      engine        diesel engine reliquefaction plant cycle
---------- ------------- ------------- --------------------
Adv.       - Widely      - Better fuel - Better fuel        - Fuel
cost is
           adopted       efficiency    efficiency           better
w.r.t. steam
           - High        - BOG can be  - Cargo and engine   turbine
           reliability   used          parts can be 
           - 100% BOG                  separated
           can be fired  
Disadv.    - Low fuel    - Exclusive   - High HFO           - High
quality fuel
           efficiency    BOG burning   consumption          oil is
                         impossible    - Electric power     - Dual
fuel burning
                                       reuired for          impossible
Initial    100           105           105                  104
Fuel       100           67            65                   79
efficiency (BOG + HFO)   (BOG + HFO)   (HFO)                (BOG or Ga
Discharge gases       
CO2        100 (87)      66            77                   73
NOx        4 (3)         100           99                   10
SOx        67 (0)        43            100                  0

Note: Numerals in parentheses are those in the case of BOG exclusive

From the comparison table, it is clear that any of the alternatives
can be used as far as reliability and safety are concerned.

The steam turbine plant is highly reliable and widely adopted. The
ability to use both BOG and HFO ensures clean combustion. However,
these plants suffer from inferior fuel efficiency and high fuel costs.

In the dual fuel diesel engine, the fuel efficiency is better.
However, these plants require high pressure injection (of BOG) and
require plot burning of fuel. Also, they are not very flexible (as far
as fuel mixture is concerned) and have high NOX emissions..

The diesel engine with a reliquefaction plant enables perfect
separation of the engine and the BOG handling system. The fuel
efficiency is better but at the cost of high initial investments, high
consumption of HFO and high levels of NOX and SOX emissions.

The gas combined cycle offers better fuel efficiency and clean
emissions as compared to the conventional steam turbine, emission gas.
On the flip side, it requires a high quality petroleum fuel and does
not permit duel fuel combustion.

Considering the above factors, many of the present operators continue
to use the convention steam turbine engines for LNG propulsion. In the
medium future, electric propulsion is likely to be adopted more
widely, as it (like steam) allows a common power source of electricity
to be used for all main and auxiliary services on the ship.

"Future Development Options for LNG Marine Transportation" by Ian
Harper (Wavespec Limited Maldon, CM9 4PZ, United Kingdom)
Paper presented at the American Institute of Chemical Engineers,
Spring National Meeting in New Orleans, March 10-14, 2002,
Second Natural Gas Utilization Topical Conference, Session TCa02

"Key Technologies of Mitsubishi LNG Carriers - Present and Future -"
by Kazuaki Yuasa, Katsuya Uwatoko and Junshiro Ishimaru Mitsubishi
Heavy Industries, Ltd., Technical Review Vol.38 No.2 (Jun. 2001)

Note: The Mitsubishi paper gives the layouts of the various
propulsions plants and the specifications of the various containment
systems for LNG carriers.

Search Strategy:
Diesel propulsion systems for LNG carriers

LNG Carriers Propulsion Systems

LNG future trends


Hope this helps!


Request for Answer Clarification by infoseekerr-ga on 15 Apr 2003 07:42 PDT
Good effort!
Can you please provide typical propulsion plant specifications for
large LNG carriers including (power output, No. of engines required,
etc) for
- Steam plants.
- Mechanical Diesel.
- Electrical Diesel.
Forget about Gas turbine option.

I will let you know if I have any further questions as I explore your
answer..Thanks for the job done so far

Best regards

Clarification of Answer by reeteshv-ga on 18 Apr 2003 05:14 PDT
Dear infoseekerr-ga,

Good day!

Extremely sorry for not being able to reply to your clarification
request promptly :( In the following paragraphs, I have tried to
address the issues raised by you.

Typical LNG carriers have a capacity between 125,000 - 138,000 cubic
meters of LNG and have dimensions of about 900x140x78 (LxWxH) feet.
The water draft is usually between 33 - 36 feet. There are about 5
spheical (Moss) cargo containment systems on board. If the membrane
design is used, there are 4 such tanks on the carrier.

1. Steam Plant
A steam turbine driven LNG ship with the above measurements will
usually have a cross compound turbine with reduction gear that is able
to generate 40,000 shaft horse power at 96 rpm.
The gear is a dual tandem articulated, double reduction, double
helical type gear. The main boiler is a two sets, two-drums water tube
natural circulation boiler with a capacity of about 60,000 kg/hr. The
propellor has six blades and is of solid keyless type made with Ni-Al
bronze. It has a diameter of around 8,000 mm and a mean pitch of about
6,700 mm.
The bow thruster provides a nominal thrust of 30 tons (using a motor
rated at 2,000 kW) to one set of controllable pitch propeller type
with four blades
Eight electric pumps, two per tank, are used to fill the tanks at the
rate of 1,500 cu.m. per hour.

2. Mechanical Diesel
LNG ships are notorious for their conservative stand in retaining the
use of steam as the main source of power. The diesel engine has for
decades dominated all other sectors of merchant shipping except LNG
carriers. While the LNG carriers always have either one or two diesel
generators, they have only been used for backup and maneouvering
purposes. To my knowledge, the LNG Carrier Al Hamra uses one Wartsila
Vasa 32 engine to generate 2960 kW of power at 720 rpm. This HFO
diesel engine (also capable of burning gas) powers the 135,000 cu.m.
carrier at speeds of 19.5 knots.

3. Electrical Diesel
The first LNG carrier to be powered by electric propulsion is
currently under construction at the French shipyard Chantiers de
l'Atlantique. It is one of the few carriers using the IC engines
instead of the usual steam turbine plant. Gaz de France has
commissioned this carrier capable of carrying 75,000 cu.m. of LNG. The
carrier will be powered by 4 dual-fuel diesel engines that will meet
all the ship's propulsion and shipboard electrical requirements. Each
engine is capable of developing 5,700 kW at 514 rpm. The cargo
containment system uses the membrane design
I hope that this answers your doubts. Do let me know if you want any
further clarification / information.


Request for Answer Clarification by infoseekerr-ga on 18 Apr 2003 07:28 PDT
Dear reeteshv-ga,

Thanks for your reply..can you tell me what references have you used
for the last bit of your answer..Al Hamra uses one engine with an
output of 2960 kW? it sounds really unbelievable


Clarification of Answer by reeteshv-ga on 18 Apr 2003 22:41 PDT
Dear infoseeker-ga,

Good day!

The information about Al Hamra ws taken from the following

"Propulsion systems for future LNG carriers", Wartsila Marine News
articles (Issue 3/2000)
In the said article, there is a photograph of the Al Hamra LNG carrier
and the caption reads, "Fig. 1 The LNG Carrier Al Hamra is equipped
with one Wärtsilä Vasa 32 engine with an output of 2960 kW at 720

Here I may add that in a report titled, "JANUARY 1997 TODAY'S NEWS
ARCHIVE" for the date 1/2/97, Al Hamra is stated to have a machinery
output of 29,600 kW!


As there were two conflicting figures differing by a factor of 10, I
chose to go with the official version.

Let me again clarify that LNG carriers do not seem to go for the
mechanical diesel option. The dual-fuel diesel option is getting a
gradual acceptance among the community. Even the Vasa 32 is capable of
taking a dual-fuel mixture!

Hope this clarifies your doubts. Please let me know if you need any
further clarification / information.



Request for Answer Clarification by infoseekerr-ga on 20 Apr 2003 16:09 PDT
Dear reeteshv-ga,
Thanks for the last clarification..I am putting all the questions I
have at once before rating your answer, I really appreciate your work
so far,

- Can you clarify and expand the bit of the 'economic background of
LNG prices' of the original is not very clear.

- the amount of boil off quantity in modern LNG carries is decreasing
owing to advances in tank insulation technology and design, can you
get an approximate figure for a typical amount of boil off during a

- What exactly are the particular operating problems with this type of
vessels(LNG Carriers)

Please don't forget referencing..Thank you once again

Request for Answer Clarification by infoseekerr-ga on 20 Apr 2003 17:23 PDT
Dear reeteshv-ga,
Please take note of Kemlo's comment,and can you get an economic
background of construction costs of LNG carriers, historic, current,


Clarification of Answer by reeteshv-ga on 21 Apr 2003 06:40 PDT
Dear infoseekerr-ga,

Good day!

In the next few paragraphs, I have tried to provide all the
clarifications desired by you.

1. Economic background of LNG price
Though LNG has been around since the 40's, it was not considered
economically viable in the USA because of the prohibitive cost of
producing, shipping, and re-gasifying. (between $ 3.50 - 4.00/mmbtu
range). The first shipment of LNG arrived in the USA in 1968. Then,
owing to the "oil shocks" of the 70's, the LNG imports rised
continually till 1979 (when 253 billion cubic feet was imported).
However, in the early 80's, the boom eneded partly due to the
stabilising oil prices and partly due to moe and more gas reserves
being discovered in the USA and the Canada. The imports declined
sharply reaching a nadir in 1986 when there was no import of LNG! This
"bust" was very soon over and LNG imports started rising from 1988 as
more and more power plats started switching to gas turbines. It is
predicted by the EIA (Energy Information Agenecy) that LNG imports by
the USA will increase at the rate of 8% per year on average over the
next 20 years. The EIA estimates the medium-term gas prices to range
from $3.25 - 3.50/mmbtu, and long-term gas prices to be about

In contrast, LNG is the staple fuel in Japan which doesn't have
abundant natural resources and can't import natural gas through
pipelines. The original (1969 - 73) pricing system used in the
Japanese markets was a "cost plus" fixed and unindexed prices.
Surprisingly, during the oil shock, the Japanese buyers allowed LNG
prices to go up even though the contracts were of fixed price type.
This led to further development of the LNG makret in Japan. Post the
oil shock years, Japanese LNG buyers started purchasing at a price
that depends upon a crude oil price complex, usually the Japanese
Crude Composite Index (the "JCCI"). The followig simplified formula
was used to calaculate the prices:
P = 14.85 JCC +a
where P is the price in US cents/MMBTU and JCC (or the "Japan Crude
Cocktail" which represents the weighted average delivered price of the
top 20 crudes by volume imported into Japan) is in dollars per barrel.
This formula gives an increasing premium over crude parity at lower
oil prices and a diminishing one at higher oil prices.

Additional links:
"LNG: What Is It All About?" by Joseph P. Mathew,

"Pricing Philosophy", article

"The Tide Rises for LNG", by Wanda Avila (March / April 2002),

Search strategy:

2. Boil-off rates
It is important to understand that the amount of natural boil-off-gas
available (to be used as afuel) depends on the ship design
specification as well as the operating conditions. For a typical LNG
carrier (with a capacity of between 125,000 - 145,000 cu.m.), the
accepted design BOR is 0.15 % per day as it has been found to be the
most economical from the viewpoint of practical use (in terms of tank
design considerations). A BOR of 0.10% per day is also technically
feasible wherease, under unfavourable conditions a BOR higher than the
0.15% has also been reported.

Additional links:
"Propulsion systems for future LNG carriers", Wartsila Marine News
articles (Issue 3/2000),

"Key Technologies of Mitsubishi LNG Carriers - Present and Future -"
by Kazuaki Yuasa, Katsuya Uwatoko and Junshiro Ishimaru Mitsubishi
Heavy Industries, Ltd., Technical Review Vol.38 No.2 (Jun. 2001)

3. Operating problems
The safety and reliability record of LNG carriers is unmatched in the
shippinf industry. This has been made possible by paying close
attention to detail in the development of new ideas and concepts and
procedures. Also, the stringent standards set by countries like the
USA, Japan, Australia and the European nations have played a
significant role in ensuring the safe transportation of LNG over the
seas. Till date, there have been no cargo fires on LNG carriers. Only
8 LNG spillages have been reported over the age of the industry (~ 40
years). A few offshore incidents have resulted in fatalities, e.g., at
Cove Point, Maryland in October 1979. The safety record sugests that
the normal operating hazards (viz., fire, explosions, spillages, etc.)
are quite manageable. In the next few paragraphs we discuss the
operating problems of LNG carriers by considering the various
components of such ships:
- The most widely used liquid containment systems (tanks) - the Moss
and the membrane - have been adopted because of their economy and
reliability. The spherical tanks have proved the design concept, "No
LNG leaks" in various experiments. This is owing to the simple
configuration and structure of axial symmetry of the spherical tank
and cylindrical skirt. The membrane tanks reduce the metallic material
exposed to low temperatures.
- As the cargo handling systems on all LNG ships are very similar,
there are few problems experienced or changes foreseen.
- Steam turbines are the most widely exercised power generating
options on LNG carriers. These require experienced crew familiar with
the technical challenges of maintaining a steam plant in good
condition. Such people are becoming fewer as almost all the other
types of ships have switched to diesel engines.
- The current propulsion designs (steam turbines driving a single main
propeller through a gearbox)do not allow for continued operation after
any single failure of equipment items. Multiple propulsion drives,
enhancing redundancy and maneouvarability, will have to be built into
future ships
- Advances made in integrated automation systems (IAS) enable more
effective control of the ship, either from a central location, or from
distributed points

Additional links:
"Future Development Options for LNG Marine Transportation", by Ian
Harper (March 2002),

"LNG - the past, present and future", by Anthony Bingham (October
2002), Shipping World & Shipbuilder

"Introduction to LNG", Institute of Energy, Law & Enterprise (January
2003), IELE_introduction_to_LNG.pdf 

Search strategy:
LNG Carriers Propulsion Systems

4. Construction costs
The LNG ships accounts for between 20 - 40% of required up-front
project financing for a LNG project. The capital cost of LNG carriers
depend on two factors:
- Shipyard contract price: this is the directly visible factor
affecting the price of a new tanker. Here, the important facotrs to
consider are the competitive strategies of the various shipyards and
of their respective governments. The two important issues in this
context are the ability and the willingness of a shipyard (and its
government - through finance institutions) to offer low prices. The
kind of technology and the production process used is not very
important as these are standardized throughout the industry. The new
technological developments (e.g., diesel electric propulsion, new
types of propellers, etc.) may have significant cost implications over
the next decade.
- Project finance costs: the factors that need consideration include
the term of the finance, repayment schedules, interest rates, amount
of finance and the currency hedging costs. Unfortunately, while the
LNG ships are 	used for 20 - 25 years, they have to be amortized over
a period of 10 - 15 years. Thus, financing is sub-optimal. In
addition, there are relatively few institutions that understand the
LNG market  and this further reduces the capital available for new
ship building

The following table shows the cost of constructing a new LNG carrier
(capacity between 125,000 - 135,000 cu.m.) in the last decade:

Year  Price (million USD)
1990  255
1991  280
1992  265
1993  250
1994  245
1995  250
1996  220
1997  235
1998  190
1999  165
2001  170
2002  170
Note: The prices quoted are approximate and represent the general
prices (not specific to any particular carrier). The current price of
USD 170 million is lower by 40 % of the peak price.

Additional links:
"Reducing the Capital Cost of LNG Shipping", by Kimball C. Chen (16
May 1995),

"Introduction to LNG", Institute of Energy, Law & Enterprise (January
2003), IELE_introduction_to_LNG.pdf 

Search strategy:
LNG Carriers Propulsion Systems

5. kemlo-ga's comment
I would like to take his word for it but, alas, the scientifc
community is not willing to accept it :( Given below is the's definition of British Themal Unit,
"A British thermal unit (Btu) is an English standard unit of energy.
One Btu is the amount of thermal energy necessary to raise the
temperature of one pound of pure liquid water by one degree Fahrenheit
at the temperature at which water has its greatest density (39 degrees

Coming to the Board of Trade Unit, the following definition is taken
from "Wikipedia - The Free Encyclopedia",
"The Board of Trade Unit or B.O.T.U. is a non-metric unit of
electrical energy, defined by a former department of the UK
government...The B.O.T.U. should not be confused with the British
thermal unit or Btu, which is a much smaller quantity of thermal

Additional links:
British Thermal Unit,,sid9_gci213563,00.html

Board Of Trade Unit

Search strategy:
"British Thermal Unit"

"Board of Trade Unit"

I hope that I have been able to answer all your questions.


Request for Answer Clarification by infoseekerr-ga on 21 Apr 2003 08:09 PDT
Dear reeteshv-ga,

Good Job :-)

One last question, what is the reference you used for the 'steam plant
specification' in your clarification on 18 April?

Clarification of Answer by reeteshv-ga on 21 Apr 2003 21:10 PDT
Dear infoseekerr-ga,

Good day!

The details of the steam plants were taken from the specifications of
various LNG carriers available on the site of Oman LNG

In this section of their web site, detailed specification sheets are
available of the LNG carriers that transport Oman LNG to customers in
Korea, Osaka and Dabhol.

I have taken these as representative of steam plants for the typical
LNG carriers considering that the design of LNG carriers has been
fairly standardized throughout the world.

Additional links:
LNG Ships at a Glance

Search strategy:

I hope this addresses your query in a satisfactory manner :)

infoseekerr-ga rated this answer:5 out of 5 stars
Sure it does :-)
Good effort !

Subject: Re: Propulsion system for LNG Carriers
From: neilzero-ga on 05 Apr 2003 13:53 PST
Dual fuel engines is a good idea. The ship will often need to return
for an other load, empty, when boil off is not available.   Neil
Subject: Re: Propulsion system for LNG Carriers
From: kemlo-ga on 20 Apr 2003 16:55 PDT
A minor point but 
B.Th.U.  is British thermal Units  
B.T.U. is Board of Trade Units
Take my word for it I have no idea for the link

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