Subsea Wellhead Fatigue Analysis With Focus on Thermal Conditions

Abstract

The structural integrity of subsea wellhead systems has to be maintained during the life cycle of a well and is of concern when frequent intervention and workover operations are performed to extend its service lifetime. Recent efforts have been made to improve and standardize methods for modeling wellhead fatigue-related loads and expected service life. Nevertheless, no comprehensive study of thermal loads and their implications on wellhead fatigue calculation has been published so far. This paper addresses the significance of thermal-related loads on wellhead fatigue damage assessment during the drilling phase of a subsea satellite well. Subsea wells are exposed to dynamic loads during drilling operations that are transmitted to the wellhead through the marine riser, which may reduce their functionality over time. The wellhead housing is located on top of a shoulder in the conductor housing. As a result, forces on the wellhead datum are transferred between the wellhead housing, casing strings, the template and soil. The way in which forces are exerted on the different wellhead structural components is strongly dependent on factors such as wellhead design, soil support, cement level and bonding between conductor and surface casing. To investigate the effects of thermal loading, numerical analyses have been performed based on the finite element method. First, the temperature profiles along the wellbore were estimated during the drilling operation, given the circulation of drilling fluids. Temperature data at selected moments were input to a 3-D structural local analysis of the wellhead. Then, its response to imposed mechanical and thermal loads was investigated. A beam proxy model of the wellhead was established and coupled to the global riser analysis, including the marine riser subject to environmental loading and the corresponding top-end vessel motion, which gave the load history on the wellhead according to the environmental loading. Local and global analysis results combined yielded the fatigue damage incurred in the wellhead during the drilling operation. Four different top of cement configurations for the surface casing were considered in the study. It was found that the induced thermal variation on the wellhead may lead to significant variation in the numerical estimation of fatigue damage rates during a drilling operation. The results indicate that the impact of thermal loading is strongly dependent on the cement level.