Low-pressure fenders for ship-to-shore LNG transfers

A new fender standard is aimed at providing LNG ship owners with a flexible, competitive option for emergency ship-to-ship transfers

In recent years fendering requirements for ship-to-ship (STS) transfer operations have tended to become the preserve of high-pressure (HP) floating pneumatic fenders. The situation is supported by the current guidelines governing petroleum and LNG STS operations, which were developed on the basis of HP units that prevailed at the time of writing, but this regime may unnecessarily restrict the use of other fender types, even where these may be more appropriate for the task.

Recent testing of low pressure (LP) pneumatic fender units according to the methods laid down in the existing ISO 17357 standard suggest that these units offer a viable alternative to HP fenders for certain operations where the avoidance of high specific pressures and the ease of transportation in terms of weight and handling are priorities.

LP fenders are constructed from high-tenacity, high-strength fabric, which is coated with an abrasion and weather-resistant synthetic rubber compound. They can be manufactured to various sizes, with diameters up to 4.5m and lengths as great as 30m. LP fenders work by exactly the same principle as HP fenders. The resistance to berthing vessel momentum is provided by a reaction pressure due to compression of the air inside the fender when deformed by the vessel’s hull. The kinetic energy of the berthing vessel is absorbed during the work done to compress the air inside the fender. Fenders are sized according to the expected duty of the fender in terms of energy absorption (EA) requirements which will be, at the most basic level, a function of the vessel mass and velocity.

Where the initial pressure of a standard HP fender is in the region 500 to 800 mbar, rising to as high as 2,000 mbar when compressed, an LP fender has an initial un-deformed pressure of 70 mbar which rises to only half that of the HP unit when compressed. Because of their lower initial pressure and, therefore, lower developed pressure when compressed, LP fenders provide a ‘soft cushion’ effect whereby energy can be absorbed across a large contact area, distributing the load more effectively over the hull structure and minimizing high localized loads. This can be especially valuable for damaged or delicate hulls; for example, during emergency STS operations where offloading of a distressed tanker may be required.

Reduced developed pressure also means that tensile loads such as hoop stresses across the fender are reduced, allowing an appropriate corresponding reduction in the weight of reinforcing materials.

LP fenders of equivalent performance to HP fenders are approximately half the weight of their HP counterpart. It is important to note that given HP weights do not include the weight of the chain nets used with HP fenders. These chain nets can effectively more than double the overall HP fender weight. The reduced weight of LP fenders also means that it is possible to roll and fold such units, allowing them to be transported easily or stored away unobtrusively when not required. Some typical packed sizes for LP fenders are given in Table 1. The reduced footprint of the packed LP fender compared to HP units and the greater options for transportation mean that reduced fender inventories and inventory locations can also be considered.

The high EA capacity of LP fenders coupled with ease of handling enables ships of even the largest tonnage to be safely fendered in a wide range of weather conditions and during various berthing operations.

Any convenient air supply can be used to inflate LP fenders, such as a compressor or light-duty blower. Inflation couplings are fitted to the end of the fender and these can be supplied to suit customer preference. Maintenance requirements for LP fenders are minimal and any necessary procedures are simple. LP fenders manufactured by Dunlop GRG, based in Manchester, UK, were successfully used during the Exxon Valdez oil spill clean-up back in 1989, and they continue to be used worldwide for a wide range of operations. The Royal Navy, the US Navy and the US Coast Guard utilize LP fenders while commercial offshore companies such as Svitzer, Fuji Trading, BP and Total Marine have used and continue to use LP fenders for various operations. Such operations include STS transfers, offshore mooring, jetty mooring, naval applications, salvage and cargo recovery and emergency flotation.

Because of their robust performance and the advantage of easy handling and transportation, renewed interest in LP fenders as an alternative to HP fenders, particularly from a high profile LNG shipping company, has stimulated debate as to whether STS operations, especially emergency STS transfers, should be the sole preserve of HP fenders.

Society of International Gas Tanker & Terminal Operators Ltd. (SIGTTO), London, & Dunlop GRG Holdings Ltd., Manchester, United Kingdom
Edition: Edition 49

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