Design Optimization of Snorre A Tension Leg Platform (TLP) for Extreme Wave Event

Abstract

Tendons or tension legs is a vital components of a Tension Leg Platform (TLP). The platform integrity solely relies upon the tension legs. During design phase of a TLP, all components are designed for extreme event wave loads. The defining characteristics of extreme event wave can be predicted using statistical models and wave spectra but the calculation of extreme event impact forces is a tricky process. The sea at a given time and location is composed of many waves of different height and time period propagating in all the directions. The extreme event wave, in actual, generates by the superposition of many wave components of different time periods and heights. The heights of all those waves happens to be positive at that point of sea and all these heights add up to form a wave of very large amplitude. After superposition, all the individual components of that extreme event wave scatter in all directions to make the regular sea. It is very hard and time consuming to generate extreme event by modeling of irregular sea as the occurrence of extreme event is a random process. Conventionally, Stokes fifth order waves, with time period and wave height of extreme event, are used to calculate the impact loads of extreme event. The purpose of the current study is to optimize the design of tendons of a Tension Leg Platform. Study has revealed that the extreme event loads calculated by Stokes fifth waves have many conservations which results in overdesign of the tendons of TLP. Moreover, the process is also not the realization of actual wave event. The better approach is to select a wave spectrum that closely approximates the sea in which TLP is installed and then generate an extreme event wave through the superposition of individual wave components laying within the selected spectrum. Snorre A TLP, installed in North Sea, is chosen for the current study and extreme wave event load analyses using Stokes fifth order wave and superposed wave are performed. In the last sections, the results of both the analyses are compared and discussed. It is found that the extreme event loads calculated by Stokes fifth order wave have conservations and Snorre A platform is over designed for extreme events. Further, future recommendations are given for elimination of these conservations.