Subsea wellheads are the male part used for connecting subsea components such as drilling BOP, XT or Workover systems equipped with a female counterpart. Subsea wellheads have an external locking profile for engaging a preloaded wellhead connector with matching internal profile. When the connector is locked subsea a metal-to-metal sealing is obtained and a structural conduit is formed. Due to the nature of subsea drilling operations a wellhead connector will be subjected to external loads. Fatigue and plastic collapse are therefore two potential failure modes. These two failure modes are due to the cyclic nature of the loads and the potential for accidental and extreme single loads, respectively.Life extension of wellhead systems provide huge financial savings for the operator. Much attention has therefore been directed towards more refined fatigue analysis methods to reduce the degree of conservatism. The development of industry practice is also reflected in design guidelines. In the 2016 edition of DNVGL-RP-C203 a new section related to fatigue analysis of subsea connectors was introduced.Subsea wellheads have historically been considered as un-inspectable components. One important reason why the use of inspection has not been considered effective is that fatigue cracks in base material are smaller than the detection limit most of the fatigue life. Thus, the benefit of an inspection has been considered very modest in terms of extended operation. However, in recent years, measures have been implemented that significantly reduce the external load on the wellhead system. With a significantly reduced load, one is no longer dependent on detecting cracks at such an early stage of the crack growth process in order to be able to document safe operation over a time period corresponding to the length of e.g. a drilling campaign. A concept using a pressure test was introduced in the article ‘Risk based integrity assessment and life extension procedure for application of probabilistic fracture mechanics analysis for subsea wellhead connectors’ (OMAE2020-19330). The article presented a procedure based on fracture mechanics to the problem of achieving life extension of a wellhead external locking profile while connected to a wellhead connector. The method is based on the growth of an initial crack, determined by the largest crack which remains stable after the pressure test, until it becomes unstable due to an accidental load.In this article the concept introduced in OMAE2020-19330 has been further developed. Instead of only considering the pressure test event separately, the probability of the presence of such a crack is also considered. In the proposed methodology the probability of failure of the assumed crack determined from the pressure is set to be inversely proportional with the probability of such a large crack to be present. This probability is considered to be well estimated by probability of failure based on probabilistic S-N analysis. Finally, safety factors to be used with deterministic fatigue crack growth analysis are calibrated and presented as a function of the accumulated damage from a deterministic S-N analysis.

Fatigue Life Extension Procedure for Subsea Wells Based on Pressure Testing

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.