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 "Going-off-location" operation of pulling down the hull to provide net upward buoyancy force to extricate the legs is called a 'pull-down' operation. The hull, being partly underwater, attracts considerable wave forces even during relatively calm weather. Due to the changing support condition at the 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 described here considering the harmonic wave forces, added mass of the hull in the water, the 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 the water. Several steady-state dynamic analyses of the complete jack-up rig structural model have to be performed for a range of wave periods, water depths, and drafts of the hull. Three different bottom boundary condition scenarios have to be considered:
1. three legs supported,
2. one leg free, but two legs still stuck in the soil
3. two legs free, but one leg still stuck.
As the operation is likely to take several days or more, the possibility of the occurrence of wind, current and wave loads need to be considered for the jack-up structural safety. The static and dynamic load cases must be combined to get the maximum effects on the stresses of the leg members.
This procedure predicts the allowable safe wave heights for a range of wave periods for a particular water depth and the hull's draft.
Such a methodology was examined in a previous paper (OMAE2007-29083, PULL-DOWN ANALYSIS OF JACK-UP RIGS) by the author, Dr. Partha Chakrabarti, VP Engineering, Zentech Inc.