Wellhead fatigue response affected by seabed interaction

Subsea wellhead system assessment is a vital aspect of well integrity assurance. This thesis focuses on the fatigue damage of the wellhead which has become an important design consideration in the design of offshore structures. Subsea wellheads are generally exposed to fatigue, due to environmental loads such as waves, winds and currents, which implies cyclic loading to the wellhead. The conductor-soil interaction may have significant impact on stress range variation in wellhead system and consequently on accumulative fatigue damage. In practice, the seabed soil is usually replaced with linear elastic springs to define lateral force-displacement curves. There are several models proposed in the literature to simulate the lateral response of soil to force applied by the conductor. However, the selection of an appropriate model depending on seabed soil condition and the nature of soil-structure interaction may have significant influence on fatigue performance of wellhead system. In this thesis, the influences of a range of different lateral force-displacement models were examined through implementation into global riser analysis. Finite element analysis software, OrcaFlex, was used to model the riser configuration and global load analysis. Both static and dynamic analyses of the drilling riser system were conducted. The loads of particular interest with contribution to the accumulated wellhead fatigue damage were investigated with focus on assessment of the impact of conductor-soil interaction and soil properties. The result showed that the selection of lateral P-y curve and the soil properties governing the soil stiffness may have significant influence on fatigue damage. It was observed that, the stiffer soil models will greatly reduce the bending moment at the wellhead datum leading to decrease in fatigue damage and increase fatigue life of the wellhead, whereas, the softer soil model leads to more damage and decrease fatigue life of the wellhead. This implied the significance of developing new models to incorporate the cyclic soil stiffness degradation for fatigue analysis of the drilling risers