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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.

From gas field to users

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…