Environmental Event Combination Criteria for Tension Leg Platforms
Authors: Wen Y.K. 
Published: October 2008

ABSTRACT
The performance of TLPs in the Gulf of Mexico (GOM) s evaluated in terms of annual probability of expedience of a given response level due to hurricanes.A reliability-based design method is also presented. Modern methods of reliability analysis and GOM hurricane statistics since 1900 are used. To properly model the nonlinear response behavior of the platforms, a simulation-based procedure is used in both performance evaluation and calibration (determination) of the environmental event combination corresponding to a target probability of platform performance. The responses considered are maximum offset. maximum tendon tension, minimum tendon tension, and minimum deck clearance. Three platforms, a light deck, Jolliet type; a heavy deck, Hutton type; and a very large deck, Heidrun type; in various depths of water are used forthe calibration. The effect of platform orientation is also investigated. The design winds, significant wave heights, and currents corresponding to responses of return periods of 50 yrs, 100 yrs, and 200 yrs are respectively: 40.4 m/s, 10.1 m, and 0.87 m/s; 45.2 m/s, 11. 3m, and 1.15 m/s; and 49.6 m/s, 12.4 m, and 1.31 m/s. As an approximation, the above design events can be used in the direction of the diagonal of the platform disregarding platform orientation.

INTRODUCTION
Combination of environmental loads such as those due to winds, waves, and currents, has been an important consideration in performance evaluation and design of offshore platforms. It was the subject of a previous study of fixed bottom platforms in water depths of 1000 ft or less in the Gulf of Mexico (GOM)"2. A risk model was developed for the prediction of the jointprobability of the events. Reliability-based design criteria were proposed and adopted in API Recommended Practice3.
For tension leg platforms in deeper waters, both the environmental forces and structural response behavior are significantly different. The contribution form winds is higher and the current is comparatively less important; furthermore, nonlinear response behavior and more limit states need to be considered, e.g., maximum and minimum tether tension, maximum platform offset, and minimum deck clearance.
The objective of this study is to develop environmental event combination criteria for TLPs in the Gulf of Mexico. The sensitivities of the response to major storm parameters such as wave height, wave period, wind and current speed are investigated. The risk model previously developed for the evaluation of the probability of a given limit state being exceeded due to the combined action of winds, waves, and currents is extended to TLPs. The annual risks of various TLP esponses are also evaluated. The results are then used to develop the design criteria corresponding to given TLP response return periods. Three different TLP configurations are used in this study, i.e., a light deck, Jolliet type platform in a water depth of 1760 ft (536 m); a heavy deck, Hutton type platform in a water depth of 1640 ft (500 m); and a very large deck, Heidrun type platform in a water depth of 1148 ft (350m).