Session: 02-12-01 Reliability Based Maintenance

Paper Number: 104800

104800 - Probabilistic Approach for Fatigue Life Extension of Subsea Wells Using Pressure Testing 

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, primarily because the governing hot spots are very difficult to access. Another important reason is that the use of inspection has not been considered effective because fatigue cracks in base material components are smaller than the detection limit for a majority 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 concept was further developed in the article ‘Fatigue life extension procedure for subsea wells based on pressure testing’ (OMAE2021-63003). These articles 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.

Experience from using the approach presented in the two referenced articles has shown that there are some practical limitations associated with the concept. In some cases it has not been possible to carry out the pressure test with sufficiently high pressure. Consequently, it became more challenging to document a sufficiently small crack to justify safe future operation. An alternative method for assessing the crack size was therefore developed which is based on the principles behind inspection planning using probabilistic fatigue crack growth analysis as described in DNV-RP-C210. The approach is based on the principle that a probabilistic fatigue crack growth analysis is applied to the historical loading, which leads to a probability distribution of the current crack size. This distribution is then updated based on a successful pressure test that confirms no crack through the wall thickness is present. This distribution is further used to document future operations within the desired safety level.

Presenting Author: Arne Fjeldstad DNV

Presenting Author Biography: Arne Fjeldstad is a Principal Engineer working in the Offshore Structures Section in DNV. He has a MSc in mechanical engineering and a PhD in fracture mechanics from the Norwegian University of Technology and Science. He has more than 15 years of experience with strength and fatigue analysis, Risk Based Inspection (RBI), project management, fracture assessment, laboratory testing ,verification and technology qualification. His work is currently directed towards fatigue analysis and RBI of offshore structures. He is currently responsible for DNV-RP-C203 and DNV-RP-C210.

Authors:

Arne Fjeldstad DNV
Torfinn Hørte DNV
Gudfinnur Sigurdsson DNV
Stian Sætre Equinor