Pull-Down Analysis of Jack-Up Rigs

Abstract

During demobilization of jack-up rigs, when the rig’s legs are stuck in the seabed due to resistance from the surrounding soil, the hull is lowered more than the neutrally buoyant condition to draw the legs up. This operation of pulling down the hull to provide net upward buoyancy force to extricate the legs is called ‘pull-down’ operation. The hull, being partly underwater, attracts considerable wave forces even during relatively calm weather. Due to the changing support condition at bottom of the leg, while it is being extricated, the natural period of the system changes continuously. The dynamic amplification may be high when the wave periods are close to the natural periods of the structure. This makes it imperative to consider dynamic analyses of the rig. A unique but simplified ‘pull-down’ analysis procedure is developed in this study considering the harmonic wave forces, added mass of the hull in water, boundary condition of the legs in soil, and distributed buoyancy springs under the hull. Wave excitation loads and added mass for the hull are computed using diffraction-radiation analysis of the hull in water. A number of steady state dynamic analyses of the complete jack-up rig structural model have been performed for a range of wave periods, water depths, and drafts of the hull. Three different bottom boundary condition scenarios have been considered — three legs supported, one leg free but two legs still stuck in soil, and two legs free but one leg still stuck. The static and the dynamic load cases are combined to get the maximum effects on the stresses of the leg members. Using this procedure the allowable safe wave heights are predicted for a range of wave periods for a particular water depth, and draft of the hull. Results are presented for one class of jack up rig. The results show that the leg stresses are strongly dependent on the wave periods, indicating the importance of including dynamic effects in the pull-down analysis.