Methodology for Assessing Reliability of Tension Leg Platform Tethers

Abstract

The tension leg platform (TLP) is a large, floating offshore platform for producing oil and gas in deep water. The TLP is moored to the ocean floor by vertical tethers. Loads in the tethers have several components that are timevariant and non‐Gaussian. Therefore, conventional methods of reliability analysis typically applied to fixed offshore platforms are not applicable to TLP tethers. This paper presents a method for probabilistic combination of time‐variant (Gaussian and non‐Gaussian) and time‐invariant tether load components. The procedure is applied to a Gulf of Mexico TLP design where the environmental hazard is due to hurricane events. Two modes of failure are considered for the TLP tethers: (1) The maximum lifetime tension exceeds the yield load; and (2) the minimum lifetime tension is less than zero, i.e., the tethers experience the loss of tension. The resulting TLP tether reliability is compared to the calculated reliability of conventional fixed offshore platforms, which form the basis of industry experience.