The growth in oil price and improvement in techniques for recovery of oil is resulting in an increasing need to keep subsea wells that have exceeded that initial design lives in operation. Life extension will require further drilling on the wells, the feasibility of which must be assessed in terms of the fatigue damage accumulation in the wellhead system. Consideration of fatigue resistance in the design of older wells was limited, if conducted at all, which has resulted in design features that form fatigue hotspots. Such features, coupled with the additional loading applied in further operations can lead to severe challenges in demonstrating fitness-for-purpose in terms of fatigue response.
Calculation of fatigue damage from both previous and future operations is conducted to demonstrate acceptable response. This analysis typically involves a number of assuptions that err on the side of conservatism. However, the severity of loading is such that steps must be taken to minimise these conservatisms where reasonably possible in order to meet faigue life targets. This in itself may not be enough, and the use of in-service monitoring and even component fatigue testing may be needed to meet the required objectives. This paper gives an overview of the design challenges that are faced in extending the service life of older wells, the steps that can be taken to improve calculated fatigue lives and how further steps such as in-service monitoring may be used to demonstrate fitness-for-pupose.
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