Subsea Wellhead Life-Cycle-Fatigue Analysis and the Role of Well Temperature

Abstract

Pushing the frontier of oil exploration and production into more-challenging environments and more-frequent interventions and workover operations in subsea wells has led to an increased focus on the structural integrity of a subsea wellhead (WH) and its capability to withstand the fatigue damage it will be subjected to throughout the well’s life cycle. The renewed interest led, among other works, to the publication of a WH-fatigue-analysis method statement (MS)1 in 2011, followed by a recommended practice (RP) (DNVGL-RP-0142 2015), both by Det Norske Veritas Germanischer Lloyd (DNVGL).

The MS aimed to reflect the best practices in the industry and to provide a consistent analysis procedure that would ensure the comparability of the results. However, although it thoroughly described the modeling of the WH, the MS deferred the incorporation of wellbore thermal effects, whereas the RP (DNVGL-RP-0142 2015) only briefly mentions that the effect of temperature should be considered. This perhaps responds to a lack of studies on how this may be taken into account or scarce knowledge about how the variations of wellbore-temperature distribution might affect fatigue damage on the WH.

This study describes a numerical approach to increase the scope of the current WH-fatigue-assessment methodology by incorporating the well-temperature distribution into the fatigue analysis. A representative case study of a typical drilling operation in the North Sea has served to investigate how temperature may affect the WH cyclic stresses and the estimates of fatigue-damage rates over time for different cement-sheath levels between the conductor and surface casing.

1Grytøyr, G., Hørte, T., Lem, A. I. 2011. Wellhead Fatigue Analysis Method. Technical Report No. 2011-0063, JIP Structural Well Integrity, DNVGL, Høvik, Norway. This MS has been referred to by several studies but is no longer available in the public domain.