Dynamic Analysis of a Jack-Up Rig, SDOF Approach: Design Consideration — I

Abstract

A jack up rig, such as those used for oil drilling in the Gulf of Mexico, is a dynamic sensitive system subjected to random and periodic environmental loads (wave, wind, current, etc.), in which the inertia forces cannot be ignored. However, a static load analysis approach can be justified only if an extra inertial load set, due to the dynamic effect response, is included in the analysis. The Society of Naval Architects and Marine Engineers, SNAME T&R 5-5A, “Guidelines for Site Specific Assessment of Mobile Jack-Up Units”[1], addresses the calculation of the inertial load set by using the classical Single Degree of Freedom (SDOF) analogy to calculate such dynamic effect. This study evaluates how far apart crucial structural members’ stresses/loads obtained by using the SDOF analogy method are from those values obtained from a more realistic modal dynamic analysis. The analysis is performed for wave loads at different heights and frequencies such as those existing on extreme or severe design conditions as a storm. Although a jack up rig structure presents some non-linearities, especially in the legs-hull contact areas and the spud-cans interaction with the soil; a modal superposition analysis can be used if proper linearization is considered. The results of this study for a jack show that the SDOF analogy method tends to underestimate the base shear loads for high wave periods and conversely overestimate some stresses on crucial members, such as the legs’ chords close to the hull for all the wave periods. This study shows that the stresses on one of the legs’ chord of the most loaded leg, due to the dynamic effect produced by the harmonic loads calculated with the SDOF analogy developed in this study tend to be overestimated as the wave period decreases. Conversely, the calculation of the base shear of the structure employing a quasi-static analysis with inertial load set (ILS) as calculated in this study shows that the base shear forces difference between the modal dynamic analysis and the quasi static analysis tends to decrease as the periods of the wave decreases.