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University of Technology
Gothenburg
SHIPPING AND MARINE TECHNOLOGY
MARTIME ENVIRONMENT AND ENERGY SYSTEMS
Cecilia Gabrielii Lecturer, PhD
What is LNG: properties and facts
LNG - what is it?
Production, storage, transport
Properties and behavior
What is LNG?
Natural gas that is converted to liquid for ease of storage/transport
LNG takes up about 1/600th of the volume of natural gas.
Natural gas becomes a liquid (LNG) at approximately -162°C
Transported and stored at around -162°
LNG’s extremely low temperature makes it a cryogenic liquid.
LNG as marine fuel – a way to reduce air pollution
LNG – produced for transportation purposes
Gas market is often far from the natural gas source
LNG offers greater flexibility than pipeline gas
There are challenges with storing / transporting LNG…
Outside of storage tanks
LNG quickly warms back into natural gas
Volume expansion 600
Inside storage tanks
LNG eventually warms back into natural gas
so called Boil Off Gas – BOG
Volume expansion 600
Tank pressure increases
20°C
-162°C
LNG´s principal hazards result from its
Cryogenic temperature
Vapor expansion and dispersion characteristics
Flammability characteristics
If an LNG release occurs, there is an immediate potential for a
range of different outcomes and types of consequences.
Exploration/production
Natural gas reservoirs
Conventional gas - porous reservoir with sufficient permeability to allow
gas to flow to producing well
Unconventional gas - Deposits in relatively impermeable rock formations
artificial pathways have to be created
What is natural gas?
Crude oil and natural gas constitute of hydrocarbons
Gas from different sources have different chemical composition
Methane is by far the major component, over 80%
Typical natural gas composition
And impurities such as hydrogen sulphide, water and mercury
From the natural gas production/exploration plant
Natural gas is transported in pipelines to the processing and liquefaction plant
Composition before
Composition after
Processing and liquefaction plant
Processing and liquefaction plant
-162°C
Processing before liquefaction
Removing of impurities
hydrogen sulphide (H2S) and mercury
Removing components which would freeze at the liquefaction
water vapour and carbon dioxide
Removing heavier hydrocarbons
raw materials to industry or used as fuel at the plant
Liquefaction
The gas is cooled down in stages until it is liquefied, at -162°C
Nitrogen removal
Nitrogen decreases the energy content in LNG
Nitrogen
Typical LNG composition
Others:
Ethane < 4%
Propane < 1%
Butane < 0.5%
Nitrogen < 0.5%
LNG can now be transported wherever needed
Transport of LNG
By truck
very short distances
specialised, double-skinned tank trucks
By ships
long distances in a special purpose LNG carrier
good insulation and a double hull design
LNG Carriers – 50 years ago
Methane Princess - the first ship built purposely for transport of LNG
Capacity of 27 000 m3
LNG Carriers - today
350 carriers - average capacity of 150 000 m3
Specifically designed to contain LNG at or near atmospheric pressure
at a cryogenic temperature of approximately -162°C
How to keep LNG in its liquid form?
Tank insulation
will not keep the LNG cold enough to remain as a liquid by itself
Auto-refrigeration
LNG stays at near constant temperature if kept at constant pressure.
Achieved if the LNG vapour boil off (BOG) leaves the storage tank
20°C
-162°C
How to handle the boil-off?
Use it as fuel in the propulsion machinery
Steam turbine propulsion
Dual fuel engines
Re-liquefy and send back to the LNG tanks
Disposal of boil-off gas to atmosphere
Only in an emergency situation
LNG Carriers – two different tank systems
Spherical
Membrane
Spherical - self-supporting - tank
Tank shell: 30 mm aluminium or 9% nickel steel
Insulation: 220 mm of e.g. polystyrene foam
Spherical; high degree of safety against fracture or failure
Membrane tanks
not self-supporting - the inner hull forms the load bearing structure
very thin primary barrier - 0.7 to 1.5 mm stainless steel or nickel alloy
utilize the hull shape more efficiently – larger cargo capacity
must always be provided with a secondary barrier
Which type is ”the best”?
Spherical
Simple construction
Independent from the ship's hull
Can be partially pressurised
Membrane
Larger cargo capacity
Less tank weight
Lower windage area
LNG has arrived at the import terminal!
Onshore terminal or floating (FSRU)
Feeder vessel to intermediate terminal
Feeder vessel
Regional distribution of LNG
Typical cargo capacity: 7000 - 20000 m3
At the receiving terminal (import / intermediate)
LNG is either
re-gasified into natural gas
delivered to the gas grid
or LNG is
delivered to a truck or bunker vessel
which deliver it to a LNG fuelled vessel
LNG is re-gasified just before entering the engine
”Some components” at the terminal
Cryogenic pipelines
Storage tanks
Boil-Off Gas (BOG) compressors and re-condensers
Pumps
Vaporisers (re-gasifiers)
Onshore terminal storage tanks
Flat bottomed tanks (FBT) - stores LNG under atmospheric pressure
Semi-pressurised tanks - stores LNG under pressure (approx. 10 bar)
withstand cryogenic temperatures
maintain the liquid at low temperature
minimize the amount of evaporation (BOG)
Example of LNG bunkering terminal
From the atmospheric FBT :
High pressure pump
Regasification
To the natural gas grid
From the semi-pressurised tanks:
To truck or bunker vessel
Storage tanks – how to prevent leakage of LNG?
LNG tanks have more than one means of containment.
Primary
the tank which holds the LNG, with insulation
Secondary
dikes, impoundment dams around storage tanks, or:
second tank around the primary storage tank
Atmospheric pressure tanks (FBT tanks)
Large tanks (>10 000 m3)
Built on site on flat-base concrete foundations
A system for BOG is needed
Different ”types”
Double containment
Full containment
Double containment tank
In case of failure/leakage:
LNG is contained by a concrete bund wall
But…uncontrolled release of LNG vapour to the ambient
Full containment tank
In case of a failure / leakage
The outer tank - concrete wall - is capable of containing both LNG
and LNG vapour (controlled venting of the vapour)
Handling of the Boil-off Gas (BOG)
0.05 - 0.1% of the total tank content per day
This gas is captured and
sent to the pipeline (gas grid), or
re-injected into the LNG carrier during the unloading, or
re-condensed and sent back to the storage tank, or
sent to the flare - only in abnormal or accidental situations
-162°C
Handling of the Boil-off Gas (BOG)
BOG
Regasification
LNG is warmed back to natural gas and delivered by pipeline to consumer.
The atmospheric storage tanks are equipped with submerged
pumps that transfer the LNG towards high-pressure pumps.
Regasification
The pressurised LNG is then turned back into a gaseous state
in vaporizers (warmed by seawater).
Regasification
Example of LNG bunkering terminal
From the semi-pressurised tanks:
To truck or bunker/vessel
Pressurised tanks
Small scale tanks - typically 1000 m3
Cylindrical tanks designed to resist pressures of up to approx. 10 bar.
The BOG can “remain” in the tank
Simple tank arrangement
Two ”types”
Double integry
Full integry
Double integry pressure tank
In case of a failure/leakage
a pool / collection basin directing any spillages away in a safe location.
Full integrity pressure tank
In case of a failure/leakage
Both the inner and the outer containment are constructed from
cryogenic steel being able to hold the LNG.
Example of LNG bunkering terminal
From the semi-pressurised tanks:
To truck or bunker/vessel
Bunker vessel
Smaller and more manoeuvrable compared to an LNG feeder vessel
Typical cargo capacity: 500- 6000 m3
Floating LNG Terminal (FSRU)
FSRU - Floating Storage and Regasification Unit
170 000m3 membrane tanks
Klaipeda
FRSU
Bunker vessel loading at FSRU
Chemical and physical properties
fundamental for understanding and predicting LNG behaviour
distinguish between the properties as a liquid and as a gas/vapour
the properties which make LNG a good source of energy can also
make it hazardous if not adequately contained
properties, characteristics and behaviour of LNG differ significantly
from conventional marine fuels
LNG´s principal hazards result from its
Cryogenic temperature
Vapor expansion and dispersion characteristics
Flammability characteristics
If an LNG release occurs, there is an immediate potential for a
range of different outcomes and types of consequences.
Chemical and physical properties
Cryogenic liquefied gas
Chemical composition
Boiling point
Density – volume expansion
Flammability
LNG is a cryogenic liquefied gas
Gas:
“a fluid that is in gaseous form at normal pressure and ambient
temperature”
Liquefied gas:
a gas that has been turned into a liquid by cooling or compressing it
Cryogenic liquefied gas:
liquefies at a temperature below –150°C at atmospheric pressure.
How to liquefy a gas:
Vapour pressure (barg)
Gas
Liquid Compress: Increase pressure
Cooling
Common cryogenic liquefied gas
Nitrogen (LIN) -196°C
Oxygen (LOX) -183°C
Helium (LHe) -269°C
Hydrogen (LH2) -253°C
Natural gas (LNG) -162°C
”Typical” chemical composition of LNG
others:
Ethane 4%
Propane <1%
Butane < 0.5%
Nitrogen < 0.5%
Hydrocarbons in LNG
Name of the hydrocarbon
Number of carbons
Boiling point atm. pressure
Methane
1
-162°C
Ethane
2
-89°C
Propane
3
-42°C
Butane
4
0°C
Nitrogen: -196°C
Boiling point at atmospheric pressure
Outside of storage tanks LNG quickly warms back into natural gas
water 100°C
LNG -162°C
Vapour pressure curve Higher pressure – higher boiling point
Vapour pressure (barg)
Gas
Liquid
LNG is always stored at the vapour pressure curve, thus at
its boiling point!
Density
A liquid has a higher density than a gas/vapour!
Liquid
Gas
Volumetric expansion
1 m3 LNG corresponds to around 600 m3 natural gas
The reason why natural gas is stored and transported as LNG
A small leakage of LNG results in a large (flammable) gas cloud
Density – LNG (liquid)
Lower density than water, i.e. LNG floats on water
But becomes a vapour pretty soon…
Water: 1000 kg/m3
LNG: 450 kg/m3
Density – LNG vapour (natural gas)
At temperature below -110°C: LNG vapour is heavier than air
LNG vapour blankets the ground, the cloud travels with the wind
At temperature above -110°C: LNG vapour is lighter than air
LNG vapour will rise when sufficiently warmed by ambient air
How to recognize LNG (natural gas)?
colourless and odourless
The white clouds that forms at a leakage of LNG is not LNG/natural gas
Cold LNG vapours will make the moisture in the air condense
causing the formation of a white cloud (fog)
This is not a leakage of LNG…
The cold LNG pipe will make the air moisture to condense
What about flammability?
Yes, natural gas burns - this is why it can serve as a fuel!
LNG (liquid form) does NOT burn
But, LNG begins vaporising immediately upon its release…
Flammability properties
Flashpoint
Flammability range
Auto ignition temperature
Minimum ignition energy
Flammability properites - summary
LNG MDO/HFO
Flammability range (in air) 5 - 15% 1 - 7.5%
Flashpoint -187°C >60°C
Auto ignition temperature 537°C 250 - 450°C
Minimum ignition energy in air 0.27 mJ 20 mJ
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To be continued…
