The Effect of a Fatigue Failure on the Wellhead Ultimate Load Capacity

A subsea well will experience external loading during drilling operations that can lead to the development of a fatigue fracture in the primary load bearing structural members of the upper well construction. Such a fatigue fracture can occur at several fatigue hotspots which all are located in the upper part of a subsea well. There are two main load sharing structural members; the outer tubular string named the conductor (structural) casing and the next tubular string named the surface casing. Both these strings have a circumferential load bearing weld close to the top. The load sharing between these 2 tubular strings are affected by the supported weight from further tubular strings placed inside the well.This paper discusses the residual ultimate load capacity of a typical North Sea subsea well assuming that a fatigue fracture has developed. The discussion is based on FEM analysis results where a fully developed fatigue fracture has been introduced to the analytical model of a typical well either to the conductor part of the well or to the surface casing string. Then the residual ultimate load capacity is evaluated assuming a fully developed fatigue fracture. Evaluations presented herein can be important and necessary tools in considering the consequences of a possible fatigue failure of a subsea well.A reduction in ultimate load capacity due to a fatigue fracture may reduce the safety margin should an accidental or extreme loading occur. The results indicate that the location of the potential fatigue failure is important when assessing the residual ultimate load capacity. If the factored fatigue life of a subsea well is approaching its limit the presence of a fatigue fracture should be assumed. The most prudent approach would then be to perform a permanent P&A operation of the well. Planning of such operations should comprehend the possibility of reduced structural capacity of the well due to a fatigue fracture. This paper also discusses the results in an operational context. The applied methodology is outlined and illustrative results are presented from a typical North Sea well.Copyright © 2012 by ASME