Thermal Effects on Subsea Wellhead Fatigue During Workover Operations

Abstract

Abstract Intervention and workover operations can significantly impact the structural integrity of the subsea wellhead system with regard to its fatigue life. The increasing frequency of these operations has called for a renewed focus on the fatigue capacity of subsea wells, which has greatly improved the overall methodology employed for wellhead fatigue analysis during the life cycle of the well. However, thermal effects along the well have yet to be incorporated. This paper addresses this issue by analyzing the wellhead fatigue damage during a workover operation considering the thermal effects. Injecting cold fluids in previously producing wells will cause the temperatures along the well to shift from a steady profile to a transient one, resulting in lower temperatures along the well throughout the workover operation. In this work, this effect has been incorporated into the wellhead fatigue analysis method issued by DNV by determining the temperature along casing strings, cement layers, annuli and tubing. Further, these temperature profiles are input into a 3-D finite element model of the wellhead system. Numerical simulations were performed for different temperature profiles, corresponding to distinct moments during the workover operation. Resulting load-to-stress curves of fatigue hotspots were combined with external load time histories to estimate fatigue damage rates. Stresses at wellhead fatigue hotspots seem to be influenced by the way in which contact forces are shared between the different structural components of the wellhead, particularly by the surface housing and the conductor housing. It was observed that the response of the tubular string inside the well towards variations of their temperature profiles, and top of cement (TOC), may cause the estimates of fatigue damage rates to change. Results indicate that the impact of thermal-related loads on the assessment fatigue damage rates is very dependent on the surface casing top of cement, being highest as the TOC approaches the mudline level. Notwithstanding the large body of research on wellhead fatigue, thermal loading and its effects on fatigue damage rates have yet to be thoroughly investigated. By incorporating the cooling of the well during intervention operations into the fatigue analysis it was possible to assess more realistically the fatigue damage of the wellhead and obtain reduced fatigue rates for some wellhead configurations.