Hydraulic and earth engineering – Marine structure or fabrication thereof
Patent
1998-12-16
2000-08-15
Taylor, Dennis L.
Hydraulic and earth engineering
Marine structure or fabrication thereof
114122, 114126, 114264, 405224, E02B 1700, B63B 3900
Patent
active
061026255
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a wave dampener for floating structures, and particularly, for dampening a vertical motion component of the floating structure.
U.S. Pat. No. 5,330,293 describes an oil platform of annular cross-section. At its lower end, the platform has a skirt which extends horizontally and serves to restrict wave-induced motion. Mooring means are also secured to the skirt. This wave dampening skirt is completely closed.
U.S. Pat. No. 3,673,974 teaches a platform having a large horizontal, perforated plate a small distance below the platform. It was in fact proposed that this type of dampener should be used on Norwegian production and drilling ship Petrojarl about 10 to 12 years ago. During experiments with models, use was made of whole horizontal plates suspended a small distance below the model. It was proven that these plates gave very little effect. They would nowhere near be able to justify the extra costs that would be incurred by equipping the ship which plates of this kind. Nor indeed was the Petrojarl built with dampening plates of this kind. To equip the plates with openings would increase the dampening effect a little, but would not be of any crucial significance. For a plate of this kind to be effective it must be located at a depth of at least 40 m, where the sea is considerably calmer than on the surface. It is self-evident that this is far too expensive to be profitable.
GB 2 200 082 teaches a similar dampening means, but here large ballast tanks are also provided in connection with the dampener. These ballast tanks serve no purpose other than to make the structure heavier, and thus have a marginal effect on the dampening of the wave-induced motion of the platform.
U.S. Pat. No. 3,224,401 teaches a platform equipped with two different types of dampeners. One type comprises horizontal perforated plates located along horizontal struts. Experiments with models carried out by the present applicant show that a dampener of this kind will have some effect during motion at speeds of 7 to 8 meters per second. However, normal speed of motion for a platform is in the range of between 1 and 4 meters per second. In this range, the dampener according to this U.S. patent will have very little effect. The second type of dampener comprises passages through vertical struts. This dampener causes the platform to lose buoyancy, but otherwise has very marginal effect on the dampening of motion. The dampeners according to U.S. Pat. No. 3,224,401 are only suitable for use in relatively fine weather, as is often found in the Gulf of Mexico, and would have an almost insignificant effect under the tough weather conditions experienced in, e.g., the North Sea.
GB 2 009 693 teaches a concrete tower equipped with passages which extend in an arch from the horizontal direction to an upward direction of about 45.degree.. This means will dampen the heaving motion and prevent the tower from heeling over. However, the dampeners have a highly complex design, which means that they must be cast in concrete. This results in a considerable maintenance problem, and the dampeners will only be suitable for a concrete tower, and not a platform The concrete tower will, in fact, have a very small loading capacity. At the bottom of the concrete column a number of cells are constructed, which have very little effect on the dampening since these merely help to make the structure heavier in that the cells will hold water masses.
U.S. Pat. No. 3,921,408 teaches a device comprising vertical and horizontal beams located on the outside of a structure that is to be protected. This device is first and foremost suitable for dampening spray from waves, and will not have any effect in terms of dampening wave-induced motion of a floating structure.
U.S. Pat. No. 4,232,623 teaches a barge which is equipped with a plurality of openings in the side. These opening will absorb wave spray, but will only slightly dampen wave-induced motion, and this is because the extra water mass which the barge carries with it in the hull causes the barge to beco
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igarden Hans
Olsen Thomas Fred
Olsen Fred.
Taylor Dennis L.
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