2013.TEL.7807
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Summary
Jack-up barges are barges equipped with legs that are able to jack themselves out of the water. When jacked, a stable work platform is available and operation is not influenced by wave motion. Jack ups can be used as working platform for, among other utilities, installation and servicing structures such as offshore wind turbines, long bridges, and drilling platforms.
The legs and a jacking system are used to lift the barge out of the water. Jacking up a barge is a dangerous operation, especially during the transition phase between floating and standing on the legs. During this phase there will be a repeated impact between the leg and the seabed when the barge follows the wave motions. The workability of the barge is limited by the allowable magnitude of the impact.
The impact of the leg during touch down depends on the vessel specifications and environmental parameters. Jack-ups are present in a large variety of size and shape. Two main groups were distinguished. Flat barge type and jack-ups with the shape of a vessel. The barge type is often not self-propelled while the vessel type often is self-propelled and equipped with dynamic positioning system. The length of barge types ranges from 30 to 100 m. Ship type has range in length of 90 to 160 m. The barge type jack-ups are usually built for shallower water, up to 45 m, while the ship type is used in water to 80 m depth.
Several leg types are used with jack-ups; cylindrical legs, square tubular legs, and lattice legs. The lattice legs are often used for larger water depth. The length of the leg determines the operating depth which is important in the calculation of the impact load.
Jacking of the units can be achieved in several ways. Two types are most common: rack and pinion and pin-hole. The first is by use of hydraulic jacks and the second uses electric motors driving the pinion over racks along the legs. The jacking system has lower influence on the impact load but determines the possibility of a leg impact reducing device.
Besides dimensions of jack-ups, environmental conditions are important to determine the impact load. Maximum operating conditions are prescribed in specifications sheets for each vessel. It often concerns the maximum significant wave height. The corresponding wave period is often not provided but is important for determining the impact load. Another important parameter which is not prescribed is the seabed condition. Soft clay results in much lower reaction loads than dense sand and hard clay. Some classification societies provide calculation method for the impact load. Of all the members the International Association of Classification Societies (IACS) only three provide a method. They all provide more or less the same method. There are some difference in presentation, but the principles are the same. The three societies are: Bureau Veritas, Det Norske Veritas and Russian Maritime Register of Shipping.
The DNV calculation method was used to perform calculations to determine leg impact for the range of jack-ups concerned in this research. Impact loads were calculated and compared for three situations. The first was according to prescribed allowable operating conditions. The second was calculated to determine the effect of a shock absorber. And the third calculation was to compare higher wave conditions, which represented a larger weather window, with allowable values when a shock absorber was used. The impact loads for lattice legs were different from tubular legs. Loads were higher and the shock absorber had less effect than for tubular legs. The conclusion for all jack-ups was that adding only a small damping stroke resulted in large allowable wave conditions.
The calculation was simplified on vessel motion and leg specifications. This means that for accurate load values more detailed information should be used.
In the past several patented designs were made to reduce the leg impact load during installation of jack-ups. Designs consider shock absorbing at location of the jack house or at the bottom of a jack-up leg. Designs for the leg bottom often use compressive members, while designs for jack housing often use compressible gas or hydraulics to reduce the shock.
Although there exist several patented designs for shock absorbing devices for jack-up legs, no information was found about actual applied devices on the selection of jack-ups.
