Analysis Approach for Estimating Wellhead Fatigue
Authors: Aronsen Kristoffer H. , Kuzmichev Sergey , Grytøyr Guttorm , Gregersen Kathrine , Kirkemo Finn , Reinås Lorents 
Published: September 2018

A structured technology development process targeting to combine industry and Statoil’s experience has produced an engineering approach for wellhead fatigue analysis that is verified against measurements of load and load effects in actual subsea wells.

This paper outlines Statoil’s wellhead fatigue analysis approach, which is based on the new industry standard for wellhead fatigue analyses, DNVGL-RP-E104, ref. [1].

Parts of the methodology has been presented in previous papers. The present paper provides a birds eye view, putting all the pieces together into one coherent methodology.

The development and validation of an engineering approach for estimating the bending moment in the surface casing, between the wellhead housing and top of cement, will be presented in detail; this has previously been referred to as load sharing between wellhead and conductor.

The wellhead fatigue analysis approach is based on a “coupled model”, which in this case means that the conductor with PY-soil springs are included in the model, compatible with industry recommendations [1], with the following main characteristics:

• The lower boundary condition is modelled as a conductor in soil with a bending stiffness equivalent of the well system.

• Soil and template interaction is modelled by discrete springs.

• The global riser load analysis is run with long crested waves and head sea. Directionality of the waves are handled by reduction factors applied to the damage rate. Alternatively, directionality effects may be included by running multiple wave directions with short crested waves.

• Fatigue capacity of the hotspots in the well system is represented by ΔM-N curves generated from detailed FE models. Typically, ΔM-N curves are established for connectors, welds between housings and casings, and for the wellhead housings.

The paper includes validation against full scale measurements for a wellhead of preloaded type. In addition, it is demonstrated how the approach can be used for wellheads where the high-pressure housing may rotate inside the low-pressure housing. For this case, the validation is performed against a full 3D solid element model.

The analysis approach presented is computationally effective and it will hence enable increased focus on sensitivity analyses. Analysis work is moved from time consuming local- and global analysis, to effective post-processing of data.

Uncertainty in the input parameters has been found to significantly influence the fatigue estimate. Understanding these effects is considered vital for making conscious decisions on the fatigue life of a well. See e.g. [8], [10] and [20].

As pointed out already in 1985 by Valka et.al., ref. [5], and also by Milberger et. al., ref. [6], the cement level, and the relative motion of the two housings, represent large uncertainties. Macke et. al, ref. [10], showed that the additional uncertainty due to cement level and friction between housings exceeds the levels covered by the traditional fatigue safety factor of DFF = 10. A method is proposed to handle this in a consistent manner.