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12th June 2007

Risk Assessment of Operation of LNG Tankers

LNG Conference

Copenhagen

Presentation to CCS 5th December 200612th June 2007

Risk Assessment of LNG Tanker Operations

• Liquefied Gas ships are unlike any other in the risks that they pose to

the ship and environment.

• The boiling point of methane at atmospheric pressure is about -163°C

and this low temperature causes considerable design issues for the

ship’s hull, the cargo containment system and the associated cargo

safety systems that are required to be fitted.

• The risks associated with the carriage of LNG at sea need to be

addressed in the design, construction and operation and we shall look

at some of the issues.



• The majority of LNG ships at sea and on

order are of the membrane type

containment system.

• That is the cargo is contained by a

system comprising primary and

secondary membranes supported by an

insulation system attached to the ship’s

inner hull.

• It should be noted that the membranes

themselves have no inherent strength

but rely on the insulation and the ships

hull.



• The membrane systems are therefore delicate and susceptible to damage from

amongst others:

• Sloshing damage

Water ingress

Over pressurisation

Mechanical damage

• Likewise the hull can be damaged from contact by the LNG itself due to brittle

fracture.

We shall examine some of these issues to more fully understand the rsks

associated with LNG ship operations


Sloshing and membrane LNG ships

• Sloshing describes the motion of liquid within the cargo tanks whilst at sea and

these liquid motions can create large forces on the boundaries of the tank.

• For most ship types the sloshing forces are reduced by the fitting of wash

bulkheads in the fore and aft or transverse directions.

• For the membrane LNG ship these are not provided and as such alternative

methods must be used to prevent damage to the containment system.



• Sloshing damage has occurred and following

these instances design changes were made to

:-

• The shape of the cargo tank

• The design of the containment system

(reinforced)

• The imposition of prohibited filling ranges for

sea going conditions.

• In normal operations the ship’s cargo tanks will

either be full or empty. A small amount of cargo

is carried as fuel and to keep the tanks cool

ready for loading but this is normally only 2 or 3

% of the tank capacity.



• However the ship may be forced to go to sea in conditions other than full or

empty:-

• There is insufficient cargo to carry, charter requirements etc

• Insufficient capacity at the receiving terminal

• Emergency conditions on board or at the terminal forcing the vessel to depart

with the tanks partly filled.

• For these situations the Master must be aware of the potential risk of damage to

the containment system and take appropriate actions such as transfer of cargo,

change of course and or speed, choosing a least onerous loading condition


Integrity of the inner hull

• As previously mentioned the cargo containment system is attached to the inner

hull and as such the hull can impose loads on the membranes in addition to the

thermal stresses caused by the cargo. The systems either absorb these loads

or are designed around the predicted fatigue life of the system.

• The design of the ship and in particular the inner hull must be carefully

considered – the (predicted) area of operation will influence the fatigue life of

the hull.

• Get this wrong and the ship could be out of service for considerable periods of

time.



• Consider the following photograph of a crack in the hopper knuckle joint of an

oil tanker.

• How would this be different if it occurred on a membrane LNG ship?

• The containment system is directly in way of the inner hull – in order to carry

out hotwork it would be necessary to remove the cargo containment system

before any hull repair is possible.

• What else could happen?



• If the crack is in way of a ballast tank

then water will enter the insulation

spaces.

• Not only will this damage the

insulation but if it is allowed to freeze

then the resulting expansion will

completely destroy the containment

system.



• By using predictive methods of fatigue analysis based on the expected voyages

the critical locations can be examined by Lloyd’s Register’s Shipright FDA

methodology and these areas subject to special attention during construction.

• The impact of the route can have a great influence on the fatigue factors as is

shown.

• In short, attention must be paid at an early stage to assess the impact the

intended trade may have on the ship and to ensure that sufficient attention is

paid to the construction of the ship


Membrane over-pressurisation

• In normal operation the cargo tank pressure is in the region of 50 ~ 180 mbar

with the membrane spaces operating at about 3 ~ 8 mbar and as such the

membranes are pressed into close contact with the supporting insulation.

• The membrane spaces are provided with a pressure control system and relief

valves.

• There are instances when over-pressurisation of the membranes can occur:-

in dry-dock

in emergency conditions following primary membrane failure.

• We shall look at the situation in dry-dock when over-pressurisation can (and

has) occurred.



• Whilst the ship is in dry-dock the usual protection systems for the membranes

may be out of service.

• It is a requirement to test the secondary barrier and this is achieved by

monitoring the vacuum decay of the space.

• Relief valves may be removed for overhaul and the automation system may be

out of service with the pressure transmitters isolated.

• There are two potential dangers



• The first is that the pressure in the

secondary space may become

greater than the primary by mis-

operation of valves etc.

• The second is that when breaking a

vacuum by using the nitrogen supply

over-pressurisation of the

membranes can occur.

• In either case significant damage

can result!



• In order to prevent such damage a

careful plan of how to carry out

membrane operations should be

made:-

• Identification and locking of valves

controlling the pressure in the

spaces

• Provision of additional safety

devices


Protection against brittle fracture.

• Should LNG come into contact with

any part of the ships hull then brittle

fracture will occur instantaneously.

• The damage may not just be limited

to the shell plating but the primary

members may also fail leading to a

loss of strength



• LNG could come into contact with the hull by the following methods:-

• Over flow of a cargo tank

• Failure of a cargo pipe flange or connection,

• Leakage through a defective secondary barrier in event of primary barrier failure

• Leakage from the shore terminal loading arms or piping system



• The cargo tanks are normally filled to

levels of about 98.5 – 99.4% of capacity.

• The alarm and safety system is such that

automatic isolation of the cargo tank

occurs at the higher levels. The safety

system also stops the ship’s cargo

pumps and compressors.

• However an override switch is normally

provided in order to prevent unwanted

alarms whilst at sea.



• Mistakes have been made in:-

• leaving the isolation on during loading

operations.

• In-correct operation of valves opening the

cargo piping system to the vent mast.

• Carrying out maintenance to the ship’s

equipment with cargo in the tank.

• The secondary barrier is required to contain

liquid cargo in the event of a failure of the

primary and as such routine testing is required

to prove that it is intact.

• In order to prevent liquid contact by leaking

pipes (ship or shore) drip trays and water

curtains are provided where required.


Risk Assessment of LNG Tanker Operations.

• We have looked at a few of the risks in the design and operation of LNG

tankers pose risks to the ship, the crew and the environment not found on other

ship types.

• Careful assessment of these risks must be made at each stage in order to have

a successful through life operation by examining the:

• Design

• Construction

• Operational procedures

• Extensive Crew training


Thank You

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