Evaluation of Stress Distribution in Subsea Wellheads Installed in Caisson

The wellhead is a pressure containing vessel at the top of well and acts as the mechanical connection between well and blowout preventer (BOP), or Christmas tree. Wellheads in subsea wells are subjected to dynamic loading, and consequently fatigue damage, whenever a riser connects a well to a floating vessel. This paper investigates the load distribution and stresses along the components of subsea wellheads installed in a caisson, as an alternative to mitigate wellhead fatigue. To prevent fatigue-related incidents, operators, suppliers, and classification societies have collaborated to improve the evaluation of wellhead fatigue. A detailed finite element model (FEM) of the wellhead is used to simulate the mechanical response of the system and calculate stress levels at relevant spots. The simulations presented in this study follow the industry's recommended practices to investigate the potential of reducing the loading on wellhead fatigue hot spots by having a caisson installed around the conductor. Different parameters were established to evaluate the sensitivity of the system's response and determine the optimal mechanical configuration. Caissons have been used to place the wellhead and the Christmas tree below the mudline level, as a protective measure against icebergs in Arctic regions and trawl nets in fishing areas. The results of the presented study case indicate that this practice may be of benefit to subsea wellheads in general. The interactions with the caisson and the thick cement ring between conductor and caisson near the mudline may alter the response of the wellhead to the environmental loads transferred by the riser. While the magnitude of the loading itself is not reduced, its distribution along the wellhead components is. As a result, stresses on wellhead spots particularly vulnerable to fatigue failure, such as welds, may be reduced, while stresses on other, less vulnerable, locations of the wellhead may increase, which still results in a net gain to the overall service life of the product. Installation of a subsea wellhead in a caisson is a potential fatigue mitigating measure applicable to new wells, either in satellite or clustered configuration. The industry's established experience with similar equipment may expedite the adoption of this measure.