Verification of Jackup Leg-to-Hull Beam Model Interaction Using Super Elements

Abstract

ABSTRACT: The design of leg members for a mobile offshore drilling unit (jackup) with guides, is driven by the loads introduced at the lower guide. The selection of the optimum leg member sizes for the many environmental loading possibilities represents a major effort in the design of jackups. In an attempt to control the time and cost of analysis, a simplified beam model of the leg to hull connection has been used. This "pitch fork" beam model developed and calibrated in the late 1970's, combines the effect of different leg to hull positions into a single representative case. The evolution in finite element technology allows for re-examination of the "pitch fork" beam model. New rig design and the desire to extend the operational envelope of existing designs have stimulated a reappraisal of the simplified beam model approach. The accuracy of the beam model is investigated using a detailed finite element plate model of an entire leg of a jackup. The effects of structural details including member eccentricities and gusset reinforcements are accurately accounted for in the analysis. The FEA superelement method makes the problem computationally manageable. A typical load case for the GORILLA Class jackup operating m the North Sea is studied. The loading is applied to the simplified beam model and to two superelement models that represent the extreme conditions of leg to hull position. In one superelement model the lower guide on the hull aligns with a main horizontal leg member. The second model has the lower guide vertically positioned mid way between horizontals. The "pitch fork" beam model is calibrated against the results of the superelement derived loads. Optimization methods are used to adjust the properties in the beam model to match the results of the superelements. BACKGROUND: The structural design of a jackup depends upon the load transfer between the hull and the supporting legs. The means of load transfer directly affects the size of leg members and thus drives the weight and cost of the jackup. The load transfer mechanism for the GORILLA Class jackup is used for illustration, although the concepts are generally applicable to other designs. The jackup GORILLA is configured with a triangular shaped hull and three support legs. Each of the legs is square in plan view. The leg is constructed from vertical members positioned at each corner and crosslinked tubulars. A "K" brace lattice design of main horizontal and vertical diagonal tubes is used. Other smaller pipes provide horizontal support to the joint intersection of the main diagonal and horizontal members. The main horizontal and vertical diagonal tubular members are the same size. A typical leg construction is shown in Figure 1. The vertical post at each corner of the leg is a built-up member, triangular in plan view, with one flat surface (back plate) facing the leg center. This surface is extended to form a lip which slides through channel shaped guide members attached to the top and bottom of the hull.