The Strength Analysis of Jack-up Rig by Line Leg Model

Abstract

A method is proposed to analyze the strength of jack-up rig using a model made by replacing its legs and platform with the beams and plane grid structure with equivalent stiffeness, respectively, and connecting them by equivalent springs. The expressions for the equivalent stiffeness of replaced line legs and those for the spring constant of connecting springs are introduced together with the method to calculate the axial force caused in each member of the actual legs from the axial forces, shearing forces, bending moments and torsional moments of line legs obtained by the present method. Then the accuracy of this method is studied by comparing the results calculated by this method with those calculated considering the legs as space frames and it has become clear that this method gives a practically sufficient results. Using the method of this paper, we can complete the analysis within a very short time as compared with the case where the analysis is carried out considering the legs as space frames. Therefore, this method seems to be very useful not only for the structural design of jack-up rigs but also for the investigation of many problems on the strength and vibration of them.