The punch-through of self-elevating unit is the accident during the period of preloading, which may cause platform over tilt, equipment damage, and even cause personal casualty and platform overturning. In order to analyze the punch-though process conveniently, the four stages of initial, balanced, continued and terminal are considered according to the dynamic feather of punch-through and applying the energy conservation method, the gravitational potential energy is converted to the buoyancy potential energy, leg deformation potential energy and potential energy absorbed by seabed soil which can be calculated by the soil bearing capacity for spud can. An equilibrium equation is built based on the above theory to find out the maximum punch-through distance; Then the allowable punch-through distance is obtained by finite element method, considering the leg strength, jacking system capacity and hull structure altogether; Finally, the safety of platform is checked by the comparison of maximum punch-through distance and the allowable distance. As an example, a Jack-up with 400ft working water depth is analyzed for a location punch-through operation. The maximum punch-through distance and the allowable distance are calculated and safety judgement is made for leg penetration. Based on the analysis result, optimized preloading program and risk control measures are advised. The above conclusion is verified in practical jack-up leg penetration operation. The calculation theory and method proposed in this paper for punch-through of self-elevating unit have high application value, which is significant for deep water Jack-up to reduce the risk of punch-through, and will provide strong guidance to the designers and operators of self-elevating unit.
Self-elevating unit is the platform whose main hull can be elevated along the leg to the certain height above the sea for drilling operation. The Jack-up has been widely used in offshore oil and gas exploration because of its high efficiency, convenient operation and low cost (Chen and Li, 2007; Chen, 2008). When the spud can of a Jack-up encounters a stiff-over-soft soil layer during preloading, it is likely that the leg will rapidly drop through the upper stiff layer into the softer underlying layer, which is punch-through (Kellezi and Stadsgaard, 2012). For a Jack-up operating in deep water, punch-through during preloading process may cause high speed tilting of the platform, casualties, equipment damage, or even overturning of the whole platform. HSE (International Conference on Health, Safety and Environment) has found (Burd and Frydman, 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 lost for one accident. In October 2009, when the "Sapphire Driller" carried out the preloading in West Africa's Ivory Coast, punch-through of the bow leg happened and several leg nodes were damaged by punching shear, so that the platform tilted 7.8 degrees. In May 2009, in the course of preloading operation for "HYSY941" in the South China Sea, the bow leg piercing led bow inclination of 6.2 degrees and large deformation of 4 diagonal braces and two internal horizontal braces. Therefore, before the jack-up preloading operation, leg penetration analysis should be carried out. Especially in case of the risk of punch-through, more detailed study must be executed and response measures be prepared to avoid structural damage by punch-through.
