Risk Assessment of Operation of LNG Tankers. LNG Conference Copenhagen

Transcription

1 Risk Assessment of Operation of LNG Tankers LNG Conference Copenhagen

2 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.

3 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.

4 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

5 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.

6 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.

7 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

8 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.

9 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?

10 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.

11 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

12 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.

13 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

14 Membrane over-pressurisation The first is that the pressure in the secondary space may become greater than the primary by misoperation 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!

15 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

16 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

17 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

18 Protection against brittle fracture. The cargo tanks are normally filled to levels of about % 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.

19 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.

20 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

21 Thank You