International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 1
Number 4
December 2007
427
Safety of Cargo Handling and Transport
Liquefied Natural Gas by Sea.
Dangerous Properties of LNG and Actual
Situation of LNG Fleet
A. Starosta
Gdynia Maritime University, Gdynia, Poland
ABSTRACT: Natural gas becomes very important source of energy. There is only one economical solution of
transport natural gas to distant destination – LNG – Liquefied Natural Gas. The LNG fleet is growing very fast
and fleet characteristic is changing. Very popular is myth that gas carriers are sailing bombs – is it true?
Properties of LNG compare with other liquid cargos show the true.
1 LNG FLEET
Nowadays, when deposits of traditional sources of
energy like coal and crude oil are close to end and
additional the Meddle East region is political
unstable, have people started to think about other
energetic medium – a natural gas. The natural gas is
produced from oil fields and natural gas fields. The
largest gas fields are probably in Russia and Iran.
Russia exports gas to West Europe countries by
pipelines. Other sources are placed for example in
Indonesia, Algeria and Malaysia, many miles from
countries which need natural gas for their industries.
There are no possibilities to transfer gas by pipelines
and only one economical solution is transport it by
sea as liquefied natural gas LNG.
1.1 History of transport LNG by see
Liquefaction of natural gas is not modern techno-
logy. British physicist and chemist Michael Faraday
liquefied methane in 1854 and the first practical
compressor refrigeration machine was build in
Munich in 1873 by German engineer Karl von
Linde.
The first LNG plant was built in the USA at
the beginning of XX century and began operation in
1917. Establishing of first commercial liquefaction
plant in Cleveland, Ohio, in 1941 gave the possibility
of transport natural gas to distant destinations.
The first transport by sea was made in 1959.
Converted “liberty” freighter was the first LNG
carrier named “Methane Pioneer”. This experiment
demonstrated that large quantities of liquefied
natural gas can be transported safety. Commercial
transport of LNG started in 1964. The British Gas
imported natural gas from Algeria to United
Kingdom. They used two carriers “Methane
Progress” and “Methane Princess” which had Conch
tanks of capacity 27 400 m
3
each. These vessels
were used till half of the nineties of XX century.
“Methane Progress” made 467 voyages and was
scrapped in 1992 and “Methane Princess” before
scrapping in 1998 had made over 500 voyages.
1.2 Evolution of LNG fleet
1.2.1 Number of LNG carriers
LNG fleet is in the midst of an unprecedented
expansion. At December 2000 there was 119 vessels
428
of summary tanks capacity 12 003 MSCM
(Thousands Cubic Meters). New ships were ordered
to replace the old ones. A lot of vessels were 15 or
more years old. The prediction was that in 2005
would be 148 LNG carriers and 172 ships in 2010.
Nobody predicted that LNG market would start
develop so quickly. Figure 1 shows how many new
vessels was build from 1970 till now each year. Till
2003 only few ships were built each year, but at the
beginning of XXI century some new players entered
LNG market. They ordered a lot of ships to serve
their new LNG projects. These ships started to be
delivered in 2003 – 17 new vessels, in 2004 – 20 and
in 2005 -29. At the end of 2005 LNG fleet
comprised 195 ships (47 more then was predicted in
2000) of summary tanks capacity 23 143 MSCM.
224 LNG carriers were sailed across the oceans
on 1
st
March 2007 and they were able to carry
27 279,5 MSCM liquefied natural gas. Ship yards
worldwide had 145 new builds on their order books
with cumulative tank capacity 25 280,7 MSCM.
0
10
20
30
40
50
60
1970
1972
1974
1976
1978
1980
1982
1984
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
Number of Ships
0
50
100
150
200
250
300
350
400
Cumulative Number of Ships
Ordered ships Existing ships
Fig. 1. Age of LNG fleet
Those new orders will boost the global LNG fleet
to over 300 vessels in 2008. (That year will enter to
service record number of 53 new LNG carriers.)
Ordered ships will raise this number to 369 at the
end of 2010. There is a big probability that it could
be over 400 vessels, because only during two first
months of 2007 shipyards got 14 new build orders.
1.2.2 Region of LNG shipping
One of the first LNG importers was Japan.
According to geographic location, LNG was the only
one solution of natural gas supplies. This is the
reason that the main region of LNG shipping has
been the Pacific region and the main LNG route
leads from Indonesia and Malaysia to Japan.
On 1
st
March 2007 50% of LNG fleet had long
term charters on Pacific region. And only 42% of
LNG carriers operated on Atlantic routes. Those
number doesn’t show clearly the actual situation.
The most of new import terminals are being built in
USA and Western Europe so new ships are ordered
to serve those projects. In the last 3 years over 80%
vessels were built for Atlantic region. 74% of
ordered vessels are designated to particular long
term charters. 52% of new carriers will carry LNG to
USA and Europe and only 22% are ordered for
Pacific region. The rest, 26% of newbuildings, are
still waiting for definitive trading route. Maybe this
is the chance to find LNG carriers for planned LNG
terminal in Poland.
1.2.3 Main types of cargo tanks of LNG carriers
LNG carriers are double-hulled specially designed
to prevent leakage or rupture in an accident. The
main problem witch had to be solved by designers is
extremely low temperature of cargo. The inner hull
of tanks and other parts of containment system
contacts with liquid gas in temperature -161,5°C.
This extremely low temperature forces usage of a
special construction materials: stainless steel or invar
– 36% nickel steel.
Existing LNG carriers cargo containments
systems reflect one of two main designs: spherical
design produced by Kvaerner-Moss, and membrane
design by two firms: Technigaz and Gaz Transport.
There are some other designs of LNG cargo
containment systems, for example self-supporting
structural prismatic design, but their account for
about few percent of all LNG fleet.
Ships with spherical tanks are most characteristic
as LNG ships because tank covers are visible above
the deck. The figure 2 shows that this design was
used by most of LNG ships till 2003. Nowadays
there are ordered 21 of 145 new vessels with Moss
tanks only and the rest are being built with
membrane design. Technigaz technology will be
installed in 44% of newbuildings and 41% new LNG
carriers will be equipped with Gaz Transport
membrane. The membrane ships look more like oil
tankers, because their tanks structure is less visible
above the deck.
Membrane technology is the most popular
because it is cheaper then spherical, but the more
important is the fact that gas carriers with spherical
tanks have higher Suez Canal Tonnage. New vessels
are dedicated on Atlantic region so they will travel
through Suez Canal very often. Vessel with
membrane tanks of capacity the same as carrier with
Moss tanks has 20% less Suez Canal Tonnage.
429
Other
Fig. 2. Changes of LNG cargo tanks types.
1.2.4 Types of propulsion machinery
Until now, steam turbines have dominated as
propulsion machinery for LNG carriers. 97% of
existing ships are burning the boil-off gas from the
ship’s cargo tanks to power two boilers supplying
steam to steam turbine plant driving a propeller. This
solution is very old and was used on firsts LNG
carriers in 1964, but it is a simple and reliable
solution for these ships. However the relatively low
efficiency of this kind of propulsion system (<30%)
and increasing size of new ships has motivated
investigations into utilizing alternative propulsion
systems.
New propulsion systems are used on modern
LNG vessels. Figure 3 presents propulsion system of
ordered ships. Only 47% will have traditional steam
turbine. 31% will have option employs conventional
low speed diesel engine technology for propulsion
purposes and a reliquefaction plant to turn the boil-
off gas back to liquid and return to the cargo tanks.
That concept is similar to the practice for fully
refrigerated LPG vessels, but till now there were
technical problems to construct economical LNG
reliquefaction plant.
The other concept is to use dual fuel medium
speed diesel generators to provide all the vessel’s
power requirements. The electricity will be used for
main propulsion and a “power station” principle.
Development of Dual Fuel engines, which can
operate both on gas and diesel oil, electric
propulsion has become very attractive solution. Boil-
off gas is used directly in the medium speed engines,
which will be used in 21% of ordered vessels.
1%
31%
47%
21%
Di esel
Dual f uel Diesel + Re-liquefacti on Steam
Fig. 3. Propulsion system of ordered LNG carriers
2 PROPERITIES OF LNG
The natural gas is a mixture of gases that is produced
with oil in gas or gas fields and consists mainly of
methane. Composition of natural gas depends on
production area. (For example natural gas from
Alaska contains: Methane 99,5%, Ethane 0,1%,
Nitrogen 0,4%; and natural gas from North Sea
contains: Methane 85,9%, Ethane 8,1%, Propane
2,7%, Butane 0,9%, Pentane 0,3%, Nitrogen 0,5%,
CO
2
1,0%.) To achieve liquefied natural gas the
methane is cooled down to below minus 160°C
under normal atmospheric pressure. Process of
liquefaction eliminates some of natural gas
components, so in general LNG is more reach of
methane than the its original natural gas. Standard
LNG contain Methane 91-92%, Ethane 6-7% and
others hydrocarbons 2%. Natural gas in liquid form
is about 600-620 times less in volume then its
gaseous equivalent, and that is the economical
reason to transport LNG.
LNG is clear, non-corrosive, non-toxic, cryogenic
liquid. It is odourless and well known odour of gas
from our kitchens is obtained by adding special
odorants to enable detection of gas leaks by end
users. LNG specific gravity is one half of water
specific gravity. In case of spillage LNG floats on
water and vaporised very fast. Vapours at low
temperatures are heavier than air and make visible
white cloud. The cloud is white because the water in
the air is frozen by cold vapours. The cloud
disperses quite quickly, because vapour of methane
is heated and at the temperature of about minus
100°C it has the same weight of air. At higher
temperature vapour becomes lither than air.
The very high percentage of methane in LNG
(natural gas) may permit us to assume the methane
characteristic also valid for LNG, at least as first
approximation. The followings are its main physical
characteristics:
Diesel
Dual fuel
Diesel + Re
-liquefaction
Steam
430
− Chemical composition: CH
4
− Boiling point at atmospheric pressure -161.5°C
− Specific gravity of liquid at -160°C 458 kg/m
3
− Specific gravity of gas at 30°C 0.67 kg/m
3
− Critical pressure 44.7 bar
− Critical temperature - 82.5°C
− Heat of vaporization at boiling point 121 kcal/kg
3 HAZARDS OF LNG
The main hazard of LNG is flammability of liquid
gas vapours. However the health effect of transport
substance is also very important. The health hazards
of substances are their toxicity, carcinogenic, oxygen
deficiency and other specific characteristic. In case
of liquefied gases, LNG specially, the problem is
extremely low temperature. Comparison, in table 1,
LNG with other substances, like LPG, VCM and
gasoline, transported in bulk by see, shows if LNG is
very dangerous cargo.
Table 1. Comparison of properties of liquid cargos
Properties
LNG
LPG
VCM
Gasoline
Toxic
-
-
+
+
Carcinogenic
-
-
+
+
Narcotic
-
-
+
+
Irritant
-
-
+
+
Asphyxiant
+
+
+
+
Frostbite
+
+/-
+
-
Flash point °C
-175
-105
-78
-40
Boiling point °C
-162
-40
-14
60
LFL % (in air)
5
2
3,5
1,5
UFL % (in air)
15
9
33
7,5
3.1 Flammability and explosions hazard
Flammability is the main hazard of transport natural
gas. LNG as a liquid is not flammable. Only vapours
released from LNG, as it returns to a gas phase, can
become flammable, but explosive only under specific
conditions.
Flammable vapours can ignite and will burn only
when mixed with air in certain proportions. For LNG
lower flammability limit (LFL) is 5% by volume and
upper flammability limit (UFL) is 15% by volume.
When vapour concentration exceeds its UFL, it
cannot burn because too little oxygen is present. This
situation exists in cargo tanks where the vapour
concentration is approximately 100% methane.
When fuel concentrate is lower then LFL there is too
little methane to burn. An example is leakage of
small quantities of LNG in well-ventilated area, for
example deck. The LNG vapour will rapidly mix
with air and dissipate to less then 5% concentration.
Comparison LNG with other liquid cargos shows
that its LFL is generally higher, which means that
more LNG vapours is needed to ignite as compared
to gasoline or LPG. Additionally vapour of LNG is
lighter than air and it disperses quickly so is not easy
to ignite. LNG fire is smokeless and leaves no residue.
However LPG when spilled forms an explosive
vapour cloud because it is heavier than air and does
not disperse. LPG and gasoline burn very hot and
black. In consequence fire-fighters consider LPG and
gasoline fires more dangerous than LNG fire.
3.2 Health hazards
Liquefied gases and their vapours displace air from
cargo tanks and enclosed spaces. Human body
requires air containing about 21% oxygen by volume
for normal breathing. Effects of oxygen deficiency
are loss of muscle movement, mental confusion,
unconsciousness and finally respiratory arrest. This
effects may occur when oxygen concentration
decreases below 19,5% by volume at normal
atmospheric pressure. A lot of liquid cargos carried
by sea have poison, narcotic, irritation or toxic
properties additionally.
LNG in comparison with other substances
transported by sea is quite safe. However natural gas
that is vaporized from LNG can cause asphyxiation
due to lack of oxygen if a concentration of gas
develops in an unventilated, confined area, but other
cargo may do the same. Very important is fact that
methane is non-toxic and non-irritant and for example
gasoline, transported every day by numerous tankers,
is toxic, irritant and have carcinogenic properties.
Extremely low temperature is one of the main
hazards specific connected with the transportation of
LNG. Direct contact with liquid cargo, cold vapour
or non-insulated pipes or equipment may cause a
cold burn. Cold burn, frostbite, causes damages
similar to those when skin contact with hot materials
of similar temperature difference. The symptoms are
extreme pain in the affected area and may cause even
fainting of victim
4 CONCLUSIONS
LNG is not as dangerous as people think. LNG
carriers are not sailing bombs, and the best argument
is that LNG has been safety delivered across the
oceans for over 40 years. In that time there have
been over 40 000 LNG carriers voyages. In all of
those voyages and associated cargo transfer operations
no fatality has been recorded for a member of any
LNG ship’s crew. This excellent safety record is a
result of several factors:
− physical and chemical properties of LNG are well
understood,
− industry has technically and operationally evolved
to ensure safe and secure operation,
431
− the standards, codes and regulations that apply to
the LNG business further ensure safety,
− crews of LNG carriers are experienced and good
educated.
The fleet of LNG carriers are changing and
growing up. New ships are bigger, have new types of
propulsion and usually membrane cargo tanks.
Technology is well understood but very short
constructions time may bring some little mistakes
and errors. The last two accidents involving LNG
carriers were connected with inaccuracy of tank
isolations performance.
The bigger problem is lack of experienced crew
to operate complicated cargo handling system of
LNG carriers. To keep excellent safety record good
educated and experienced crew members are need.
The estimations shows that LNG ship owners need
about 2300 deck officers, 1200 engineers, 1200
steam engineers and about 4500 ratings. It is
essential to prepare special syllabus for LNG crews
and start educates new officers and ratings especially
for this kind of ships as soon as possible. Otherwise
when ships would had uneducated crews without
necessary experience the LNG carriers might
become “sailing bombs”.
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Foss M., 2003, LNG safety and security, University of Huston
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Kabaciński J., Kicińska M., Wolski A., 1993, Eksploatacja
statków do przewozu gazów skroplonych, WSM Szczecin.
Mc Guire and White, 2000, Liquefied Gas Handling Principles
on Ships and Terminals, London, Whiterby.
Molenda J., 1996, Gaz Ziemny. Paliwo i Surowiec., Warszawa,
WNT.
Włodarski J.K., 1994, Safety of transport of liquefied gases on
tankers, Gdynia.