Punch-Through Structural Analysis of Jack-Up Rigs During Preloading of the Foundations

A jack-up rig has to be designed for extreme storm conditions in its elevated mode during operations. Guidelines of ISO 19905-1 and SNAME TR-5-5A for site specific assessment of jack-up rigs explain in detail such analysis and assessment requirements. The stability of the foundation is extremely important for a jack-up rig during its operations when it may have to withstand harsh environments. The legs of the jack-up are preloaded to achieve the maximum expected vertical loading and penetration in the soil during the entire duration of operation at that location. This preloading is done to prevent any unusual settlements to a spudcan in the event that it exceeds the bearing capacity of the soil during operations, particularly in extreme environments. During installation of a jack-up rig, the hull is normally raised few feet above the waterline and the preload is applied by pumping in water to the preload tanks and the legs are allowed to penetrate. The reason this is normally done with the hull only a few feet above the waterline is that, if required, the hull buoyancy may be able to counter excessive uneven penetration of the different legs and tilting of the hull. The preloading process may be repeated until the required penetration is achieved. As the leg penetrates, possibility exists that differential penetration occurs among the legs and the rig settles in with some inclination. This differential penetration can cause additional penetration and inclination of the hull due to shift in the center of gravity and may lead to rapid penetration of a leg, until the hull reaches the water. This rapid penetration is called “punch-through”. This can happen due to various causes of the soil strata such as a weak layer of soil, most likely clay, underlying a strong layer, possibly sand. There may be other scenarios of soil strata that can cause punch-through. These geotechnical considerations are not discussed in the present paper. Punch-through may lead to overstressing of legs (chords and diagonals) or the overloading of the jacking units. The present paper describes the structural analysis that can predict the maximum allowable hull inclination angle or the depth of penetration of a leg during a punch-through event. The unit considered is a cantilever type Self-Elevating jack-up drilling rig. The rig unit is modeled with the hull, jack house and three independent truss legs. The nonlinear analysis is performed by incrementing the gravity load and assuming only one leg penetrates further, while the other two legs do not penetrate. Several cases are studied as described below. Preload Condition with a small air gap for three water depths for the following cases: • Preload condition without any buoyancy effect on the hull and with no Rack Contact (R.C) reversal • Preload condition with buoyancy springs for a selected water depth but without Rack Contact (R.C) reversal • Preload condition with buoyancy springs and considering Rack Contact (R.C) reversal for a selected water depth Preload and Elevated jacking conditions are also studied.