An Evaluation of the Fatigue Performance of Subsea Wellhead Systems and Recommendations for Fatigue Enhancements
Authors: Evans John , McGrail John 
Published: May 2011

Abstract
The specification and selection of subsea wellhead systems is typically based on the use of generic system designs which have a defined extreme load capacity. However the fatigue performance of the wellhead system is also a critical aspect of the equipment which may not always be fully evaluated at the specification and selection stage, as the fatigue life is specific to field design parameters. These parameters can include local environmental loading, drilling rig motions, marine riser stack-ups and BOP configurations, and soil conditions, as well as operational parameters such as riser tensions and mud weights which can vary during different stages of the drilling, completion and work-over operations.
This paper will highlight through the use of analytical results the sensitivity of typical wellhead system designs to changes in these design parameters. This will provide an indication of the significance of these parameters on fatigue life predictions for subsea wellhead and conductor systems, and identify scenarios where fatigue is most likely to be of concern.
Where fatigue lives are marginal, restrictions on usage such as environmental or connected riser duration limits may need to be imposed for operations. Alternatively fatigue enhancements to the conventional wellhead and conductor system designs and configurations may be required. If identified at an appropriate stage, some of these enhancements can be readily achieved and implemented during the equipment design and manufacture process.
This paper shall discuss the potential for the use of these enhancements, identify how best they can be achieved, and show how making these enhancements in the conductor system design can have significant effects on the fatigue life of the wellhead installation. Designing the correct conductor system by determining the fatigue life of a wellhead installation can be critical, whether in shallow water, deep water or in TLP/SPAR applications.

Introduction
Subsea wellhead systems are used worldwide for wells drilled in both shallow and deepwater, in a wide a range of environmental conditions, and using many different types of drilling rig. The industry has a number of established well system suppliers, who can offer a range of wellhead products based on designs which have been field proven in many worldwide applications over a number of years.
When selecting a wellhead system for a new well, the specifications defined for such equipment typically focus on parameters such as size, pressure rating and extreme load capacity. These parameters are able to be defined based on the well designers understanding of the well geology and expected reservoir characteristics, and their required well configuration. Relatively straightforward analysis can also be undertaken to evaluate the maximum extreme loading expected to be applied to the wellhead and conductor system during service from the reservoir, drilling rig and environmental conditions, and the result of this can be used to confirm the suitability of the selected subsea system from an overall strength and well stability perspective.
However, when attempting to define the required fatigue capacity for a wellhead system, operators and wellhead system suppliers are faced with far greater uncertainty. The fatigue damage rate for a wellhead can be sensitive to a number of uncontrollable parameters, and, as is often the case, even the drilling duration can be highly uncertain. This uncertainty can lead to difficulty in determining exactly what fatigue capacity should be defined for a wellhead and conductor system.