Optimisation of Wellhead Fatigue Response for Deepwater Harsh Environments

摘要

It is estimated that over 120,000 wells have been drilled offshore in the last 60-70 years. In the past decade alone there have been about 3,500 wells drilled offshore. Many of these have been exploration wells drilled in deep and ultra-deep water depths in harsh environments such as West of Shetlands and the Norwegian continental shelf. Predictions of future trends indicate the number of these deepwater wells is set to increase dramatically. Thus the validation of the structural integrity of the wellhead for deepwater and harsh environment applications has never been more critical. The offshore rig market has also adapted to meet increased demands of high pressure/high temperature and deepwater wells. Recent studies have found that the use of modern deepwater rigs can impact on wellhead fatigue and can increase the rate of fatigue when compared to older rigs. This will have a detrimental effect on wellhead loads for both exploration/development wells and workover or re-entry of older wells. Many wells in mature fields were not designed to take into account loads from larger BOP stacks which deepwater rigs are now equipped with. Due to the decreasing age of the drilling rig fleet it is likely that many future re-entries or workover operations on older wells will need to be undertaken by modern rigs with larger BOPs. This will have an impact on fatigue loading and will have implications on the feasibility of extending the life of existing wells. This paper will outline the key drivers for wellhead fatigue in deepwater harsh environment locations and a series of recommendations for mitigation of this fatigue damage will be made. Finally, the implications of the operation of modern deepwater capable drilling rigs on older wellheads will be discussed and possible mitigation techniques involving detailed wellhead assessment and/or application of real time monitoring techniques will be discussed.