Spectral fatigue analysis of jacket platform under wave load equipped with viscous damper

Abstract

Offshore jacket platforms are exposed to environmental loads such as wind, wave, current, and earthquake throughout the lifetime of operation. Due to dynamic and periodic nature and fatigue phenomenon in the structure, wave forces are the most important loads among others. There are diverse methods to explore the fatigue life of jackets, including deterministic, spectral, and time domain analysis. Among these methods, spectral method is a reliable method, which considers the random nature of sea waves in fatigue analysis. In the current study, a spectral method is introduced for assessment and rehabilitation of jacket platform structure. To this end, a computer program has been developed, meanwhile, probability spectral density functions of displacement and stress are calculated in each joint of jacket elements. Furthermore, using S–N curve approach, cumulative fatigue damage in critical members of an example jacket is obtained. Finally, several configurations of viscous dampers are applied to jacket, and damage and fatigue lifetime are discussed with and without dampers. Consequently, the best arrangement of dampers is achieved. Results indicate the best cumulative fatigue damage with dampers which was about 0.01 times of cumulative fatigue damage without a damper.