Development of a Reliability-Based Design Code for the Structure of Tension Leg Platforms

The paper outlines the development of a state-of-the-art reliability based ''model code'' for the structural design of TLP (tension leg platform) production platforms for deep water. The origin and organization of the activities is described. The technical advances made include development of lifetime extreme environmental loading models (formal descriptions) which account for jointly occurring wave heights and periods, wind, current and tide; the development of ultimate strength limit state formulations for stiffened cylinders and rectangular grillage stiffened cross-section components as might be used in columns, braces, pontoons and decks; and finally the extension of previous advanced Level 2 safety algorithms to include correlation effects. These algorithms are used to derive the safety indices (beta) for calibration purposes in order to recommend a target safety index for design. For this target, partial safety factors are derived for a Level 1 code format which is relatively straight forward to use. The associated studies are very briefly outlined and the scope for weight saving is indicated. This is the result of joint work initially between CONOCO and the University of Glasgow, and later with the American Bureau of Shipping.