In order to prevent the overturning of the platform caused by punch-through accident, the deep water jack-up may preload with hull close to the water surface which is called "preloading in wave". The platform operates under wave impact loads, which exert influence on legs, local structure of the hull, jacking system and so on. Under severe condition, it will cause damage to the structure. Therefore, it is necessary to analyze the performance of jack-up preloading in wave and determine the critical sea state. In this paper, for example, a 400ft jack-up preloading in wave is analyzed. First, the example jack-up is simulated as a floating body with spring restraint during preloading operation in analysis. Then using three dimensional potential flow theory and considering non-linear effect such as second-order wave force caused by water surface fluctuation and local slamming pressure, the short term statistics of motion response and wave load in random waves are calculated for two conditions which include the certain draft and positive air gap. Then the strength of legs, hull structures and jacking system is analyzed to determine bearable wave load level for example jack-up. Finally, using the iterative searching method, the critical sea states during "preloading in wave" can be obtained, and compared with the model test data, several useful conclusions are drawn. The above method of preloading in wave, may have a high practical value which is important for deep water jack-up to reduce the risk of punch-through, and will provide useful guidance to the designers and operators.
Jack-up drilling unit is the platform whose hull can be elevated along the leg to the certain height above the sea for drilling operation. Jackups have been widely used in offshore oil and gas exploration because of their high efficiency, convenient operation and low cost (Chen and Li, 2007; Chen, 2008). When jack-up during preloading encounters an upper stiff layer and below this a softer layer soil stratum, it is likely that the leg rapidly penetrates through the upper stiff layer into the soft soil layer, which is called punch-through (Kellezi, 2012). For jack-up operating in deep water, punch-through in preloading process may cause high speed tilting of platform, casualties, equipment damage, or even cause the whole platform overturning. HSE (International Conference on Health, Safety and Environment) has found (Burd, 1997) that punch-through damage accounted for 53% of the total number of accidents, and every year occurrence of punch-through accident is increased by 1 or 2. About 1 million USD to 10 million USD are lost for one accident. In October 2009, when the "Sapphire Driller" carried out preloading in west Africa ivory coast, punch-through for the bow leg happened and several leg nodes were damaged by punching shear, making the platform tilted 7.8 degrees. In May 2009, during the preloading operation of "HYSY941" in the South China Sea, the penetration of the bow leg caused the hull to tilt 6.2 degrees, and the four diagonal braces and two internal horizontal braces were greatly deformed.
