Wave Dynamics Of Jackup Rigs

Abstract

ABSTRACT A study was conducted to investigate the dynamic response to wave loading of a typical mid-sized jack-up drilling rig in the elevated position. Results of the study revealed the potential for significant dynamic amplification of wave loads in water depths exceeding about the 150 ft range. Major parameters studied were water depth, damping ratio, and current velocity. The increase in wave-induced leg stresses due to the inclusion of dynamic effects was calculated to be on the order of 20% and 150% for the subject rig in 150 and 300 ft of water, respectively. Sensitivity to damping ratio was found to be quite high, while the effect of currents was minimal. The dynamic analysis procedure used in the study incorporated a time domain simulation method with a simplified wave-load model. This procedure allowed for explicit treatment of wave-load nonlinearities while maintaining reasonable computer costs. The simulation results were cast in the form of probability distributions from which extreme dynamic response parameter were evaluated. These extreme response parameters were then used to define dynamic amplification factors which were easily incorporated into structural evaluations of the jack-up rig. INTRODUCTION Recent structural assessments of several jack-up rigs demonstrated the potential for significant dynamic amplification of wave loads in water depths exceeding about the 150 ft range. Incorporation of the wave dynamic effects into structural analyses resulted in substantial reductions in calculated limiting environmental conditions for the rigs in the 150 to 200 ft water depth range. Had the effects of dynamics been ignored in the structural analyses of these rigs, different conclusions would have been reached regarding the structural suitability of several of the rigs at the proposed drilling sites. Because of these findings, a study was initiated to investigate the potential impact of dynamic effects in even deeper water. A single jack-up rig, selected as typical of the mid-sized jack-up rig fleet, was chosen for analysis in this study. The major parameters studied were water depth, current velocity, damping ratio and degree of spud can fixity. These latter two parameters were included since there exists considerable discrepancy in the offshore industry as to what levels of damping and what degree of spud can fixity are appropriate for dynamic analyses of jack-up rigs. The purpose of this paper is to present the findings of this parameter study by, first, presenting results in terms of base shear and overturning moment dynamic amplification factors (DAF?s) and, secondly, by presenting the effects of a given DAF on the limiting environmental conditions calculated for the rig. DESCRIPTION OF ANALYSIS RIG The rig analyzed in this study is a typical mid-sized, three-legged, independent leg jack-up rig capable of operating in water depths of 300+ ft. The specific rig details are not of consequence to dynamic amplification except as they affect the fundamental natural period of the rig.