Impact Loads on the Legs of a Jack-up Rig in Installation Operation in Waves

Abstract

In the installation operation of a jack-up rig, the legs will be subjected to impact loads when colliding with the sea bottom due to the rig's motion in waves. To avoid the damages of legs and jacking mechanisms, several operational manuals restrict an operation up to 1.5m in wave height. This standard, however, is not reasonable because it is determined with no relation to an wave period and a bottom rigidity. Excessively conservative standards may lead to much down-time in operation. Then, both from structural and operational viewpoints, it is important to clarify the characteristics of impact loads on legs.
In this study, experiments on a model rig subjected to both vertical impact load and horizontal frictional loads from bottom are carried out in regular waves. Theoretical analyses in time domain are also performed using newly developed analytical model where legs are treated as elastic bodies. Based on these results, the occurrence mechanisms and response characteristics of vertical impact loads and impact bending moment at platform bottom are investigated. The accuracy and the reasonableness of conventional simplified formula to estimate impact loads based on the energy concept are also discussed, and finally, an allowable wave height on a proto-type rig is investigated.