On the Jackup Platform In the North Sea

Abstract The adaptability of the jackup drilling platform to the severe environment of the North Sea requires knowledge of its capabilities both Afloat and while standing on location. Through extensive computer analysis, this paper surveys the drilling mode strength envelope of a Marathon LeTourneau type jackup platform. The results form a family of curves which express acceptable combinations of water depth, wave height, wave period, and wind speed for given air gap, storm direction, and current. 1. Introduction The jackup platform can be designed to operate in the North Sea. While this statement is partially supported by the presence of several jackup platforms in that demanding environment, the limits of these platforms' capabilities need precise definition. Such definition involves multiple disciplines, including naval architecture, materials, and structures among others. This paper concentrates on the structural aspect of jackup platform design and analysis. More specifically, it attends to computer structural modeling techniques and a procedure to provide meaningful data about the standing-on-1ocation strength envelope of a mobile self-elevating offshore drilling platform. The Marathon LeTourneau type jackup platform is roughly triangular in plan with three legs which may be either triangular or square in cross-section. These legs are open space frames consisting of tubular bracing members and chords of circular or built-up cross-section. Built into each chord are continuous racks which engage electrically driven pinions located in a substructure above the top deck level. These independent electric motors elevate and lower the platform at a rate of 90 feet an hour. The computerized analysis for determining leg stress levels under various combinations of storm loads requires a mathematical model to simulate the real platform, a definition of the loads on this computer model, and finally a suitable structural analysis computer program. There are many ways to develop an analysis sequence. This paper outlines one approach. For simplicity, only the elevated condition during a simulated storm from 60° off the bow is considered. 2. Statement of Structural Modeling Approach Analysis of the storm response of a mobile self-elevating offshore drilling platform (nonmat supported) leads one to the conclusion that the portion of the leg within and near the hull is the strength critical portion. Consequently, the philosophy underlying the mathematical modeling of the jackup platform outlined below seeks to simulate accurately the hull-leg interaction. This is accomplished by using a three dimensional beam element model of the platform of sufficient complexity to appraise the actual storm response of the platform. It is noted that any calculated results, whether by slide rule, desk calculator, or large digital computer, are only as good as the assumptions forming the basis of the calculation. Hence, computerized calculations must provide answers which reflect experience and are not just the results of an abstract computation.