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.
Presentation to CCS 5th December 200612th June 2007
Risk Assessment of LNG Tanker Operations
• 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.
Presentation to CCS 5th December 200612th June 2007
Risk Assessment of LNG Tanker Operations
• 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
Presentation to CCS 5th December 200612th June 2007
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.
Presentation to CCS 5th December 200612th June 2007
Sloshing and membrane LNG ships
• 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.
Presentation to CCS 5th December 200612th June 2007
Sloshing and membrane LNG ships
• 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
Presentation to CCS 5th December 200612th June 2007
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.
Presentation to CCS 5th December 200612th June 2007
Risk Assessment of LNG Tanker Operations
• 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?
Presentation to CCS 5th December 200612th June 2007
Risk Assessment of LNG Tanker Operations
• 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.
Presentation to CCS 5th December 200612th June 2007
Risk Assessment of LNG Tanker Operations
• 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
Presentation to CCS 5th December 200612th June 2007
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.
Presentation to CCS 5th December 200612th June 2007
Membrane over-pressurisation
• 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
Presentation to CCS 5th December 200612th June 2007
Membrane over-pressurisation
• 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!
Presentation to CCS 5th December 200612th June 2007
Membrane over-pressurisation
• 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
Presentation to CCS 5th December 200612th June 2007
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
Presentation to CCS 5th December 200612th June 2007
Protection against brittle fracture.
• 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
Presentation to CCS 5th December 200612th June 2007
Protection against brittle fracture.
• 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.
Presentation to CCS 5th December 200612th June 2007
Protection against brittle fracture.
• 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.
Presentation to CCS 5th December 200612th June 2007
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
Presentation to CCS 5th December 200612th June 2007
Thank You