Analysis of methods for reducing subsea wellhead fatigue

Abstract

This master thesis has been executed to analyze methods for reducing the fatigue of the subsea wellhead system. The subsea wellhead is locked to the seabed with a mobile marine riser indirectly connected to it, thus making the subsea wellhead an exposed part of the well. The bending moments generated combined with the static loads exposed to the subsea wellhead, can generate detrimental stress levels in the subsea wellhead system. Operators dwell on performing interventions due to the fear of fatigue failure in the subsea wellhead, and several wells have been abandoned because of fatigue failure. For several decades and to the present day, solutions and new developments have been presented to reduce the fatigue of the subsea wellhead fatigue. The large focus on subsea wellhead fatigue emphasizes the seriousness of the issue.

The work of this master thesis has consisted in identifying the loads and bending moments affecting the subsea wellhead along with the resulting stresses, and a method for finding the combined effective stress in the subsea wellhead has been elaborated. The traditional subsea wellhead design has been studied, and the fatigue vulnerable areas in the subsea wellhead have been identified. Several methods of reducing subsea wellhead have been studied and analyzed to find the best suited methods of fatigue reduction in the subsea wellhead. A distinction has been made between existing and future wells, as some of the methods are not applicable for existing wells.

The work of this master thesis has resulted in an acknowledgement of the importance of proper monitoring and cementing to ensure the reduction of subsea wellhead fatigue. The Force Element has been considered as the most effective method to reduce the subsea wellhead fatigue in existing wells. For future wells, implementation of the Conductor Anchor Node is recommended to keep the fatigue of the subsea wellhead system at an acceptable level.