Dynamic Response of a Generic Self-Elevating Unit in Operation With Hull in Water

Abstract

Self-elevating units (SEUs), with a water-tight hull fitted with long support legs and spudcans, are widely used in offshore drilling and operations, as well as offshore wind turbine installations. SEUs are also known as jack-up rigs. A jack-up rig undergoes several stages of operations involving different leg configurations, such as legs retracted, legs suspended in the water, spudcans pre-loaded into the soil, and legs deployed in the seabed with the hull lifted clear above water. The hull and the legs will therefore be subjected to various external environmental actions. Transit operation (when the hull is in water) is only carried out in mild environmental conditions, due to safety concerns. The dynamic response of the SEU in the transit operation is less investigated in contrast to normal operation when the hull is in elevated condition supported by the legs. In this paper, we investigate the dynamic behavior of a generic (in-house designed) three-legged SEU. The configuration is such that the hull is in the water while the spudcans are secured in the seabed. A nonlinear time-domain model is established for the coupled hull and legs through Cummins’s equation. The hull is assumed as a rigid body with motions in six degrees of freedom, and the hydrodynamic coefficients are calculated from radiation and diffraction analysis. The legs are simplified as lumped mass models with equivalent stiffness value as the prototype, and Morison-type hydrodynamic loads are applied. Various scenarios of boundary conditions are considered, i.e., constant spudcan constraint stiffness, pin, fixed boundary conditions, and incidental cases when up to two spudcans are released while the other is still secured in the seabed. The dynamic responses of the SEU under operating sea conditions are examined. The results are compared to those from the conventional quasi-static analysis where the legs are simplified as linear springs. It is found that the dynamic response of the SEU with the hull-in-water condition can be as large as that in the elevated condition, despite the much milder sea conditions. The operational limit can be significantly reduced if the resonant motion occurs. These results show the importance of a full coupled dynamic analysis for a rational design of an SEU and may serve to guide operations for mobile offshore drilling units. It is even more crucial for certain SEUs where the hulls are intended to be in the water for a longer period, such as offshore wind turbine installation vessels. It may also allow the transit operations to be performed under slightly more severe conditions by better defining safe operational limits and reducing uncertainty.