Fatigue analysis and reliability assessment of tether system of an offshore oil and gas platform operating in extreme sea-state conditions

The rise in energy demands has increased hydrocarbon exploration and production-related activities in deep-water and ultra-deep-water settings. In this context, Tension Leg Platforms (TLPs) play a significant role in offshore hydrocarbon exploration and production projects. They are compliant platforms that are vertically moored permanently to the seabed with the help of taut mooring lines. They are appropriate for water depths ranging from 1000 to 5000 ft. The environmental loads acting on the structure can induce dynamic variation in the tether tension, resulting in tether failure. The present study focuses on investigating the dynamic response pattern of the platform’s restraining system under extreme sea-state conditions. With the transition in sea-state condition from rough to phenomenal, a significant increase in the variation in stress cycle range and cycle average was observed. The dynamic analysis under phenomenal sea-state conditions highlighted an increase of almost 28.57% in the tether tension. Estimating fatigue damage on the tethers and predicting fatigue life were carried out. The results indicate that as the sea state intensifies, the fatigue life of the tethers tends to decrease substantially.