Analysis of riser-induced loading on well-head

Abstract

Fatigue of wellhead at subsea-completed wells is a growing problem in the offshore industry. It has been a large technology development in the offshore industry. This has resulted in an extension in the expected life for subsea wells. This partly due to new technology which allow for drilling of more and more complex side-tracks in already existing wells and the considerably well maintenance performed to extend the production phase of the subsea wells. Despite the technology development the subsea wellhead has remained the same for decades. The industry also has been moving to deeper and deeper water depth. This has led to larger drilling rigs and equipment suited for deep water and now challenges regarding the use of deep water rigs and equipment in shallow waters have appeared in the industry. It is therefore of interest to investigate the effects of using equipment designed for deeper water in operations on shallow water.

Recently the industry has become more and more aware that the transition area between the upper casing and the wellhead is subjected to large bending forces of such an extent that it provides a limited fatigue life for the wellhead. Cracks in this area could lead to a leakage and the well will be lost. A leakage could both have large environmental as economic consequences.

In this thesis computer software as Riflex, Gensod and Abaqus is used to perform a riser system analysis in order to carry out a fatigue life assessment of the subsea wellhead. A global analysis model is created in Riflex. The Gensod software is used to create the soil interaction between the conductor and the different soil layers. A Matlab code is generated to apply the P-y curves as nonlinear springs in Riflex. The drilling rig Deepsea Atlantic and its riser system is the system modeled in this thesis. The rig has experienced fatigue problems in a drilling operation at the Gullfaks field in the North Sea. The modeled environment therefore is modeled as the Gullfaks field with corresponding sea depth, current, waves and soil condition. The results from the global analysis carried out in Riflex makes up the load basis for a local analysis carried out in the FEM analysis software Abaqus Cae. The results are post processed in order to evaluate the safety margin in relation to the fatigue capacity of the wellhead.

The investigation of forces in the global analysis shows that the transition area in the wellhead is subjected to large bending moments and corresponding bending stress. The fatigue life assessment results in a fatigue life for the wellhead that is not sufficient to be classified as safe regarding fatigue for all the drilling programs carried out in the industry today.

It has been carried out parameter studies regarding use of a light weight BOP and so-called high fatigue wellhead. Also the effect the different effective level of cement between the 22” casing and the conductor have on the fatigue life has been investigated. The fatigue assessment gives a resulting fatigue life that is not sufficient to carry out all the drilling programs in the industry today in the wellheads safe life regarding fatigue. The implementation of a light weight BOP has small impact on the fatigue life for the wellhead. A high fatigue wellhead gives an increase in the fatigue life and the combined light weight BOP and high fatigue wellhead gives almost twice the fatigue life as the heavy BOP system today. For all the parameter studies, it appears that the effect of the effective level of cement has large impact on the fatigue life. The cement life providing the largest fatigue life is cement level in-between a high or low cement level. However dependency on the level of cement is not recommended.