Fatigue Assessment of Subsea Tree Connectors and Wellheads

Abstract

Drilling and workover operations offshore impose fatigue loads on the subsea tree and wellhead due to the riser dynamics. In particular, the connector between the tree composite valve block and the wellhead components may be subjected to fatigue damage. This is primarily due to specific mechanical connector designs, girth welds and thickness or section transitions commonly used in trees and wellheads. The fatigue assessment methodology of subsea tree connectors and wellheads requires a holistic approach involving sequential analyses of the floating vessel, riser, subsea well stack-up, and the wellhead and downhole casing assemblies including cement and soils. In this paper, a methodology for the analysis of each of those assemblies and their interactions is discussed. The methodology reflects ExxonMobil’s experience and offers a balance among analysis efficiency compatible with project and operational constraints, availability and uncertainty of the inputs, and accuracy of the outputs. In general, the methodology is based on dynamic analysis of models to obtain global response loads of the floating unit, riser, and wellhead. Finite element analyses are then used to translate global responses to local stresses and to define SCFs for structural details of interest. The conventional cumulative damage approach is adopted, which provides fatigue lives based on local stresses and fatigue resistance defined by S-N curves. The fatigue resistance of machined components is based on a strain-life model that provides material specific S-N curves.