Abstract
This article presents a fatigue life extension procedure for subsea wells based on probabilistic fracture mechanics analysis. The dominating fatigue loading of subsea wells occurs when a drilling rig with a marine riser and blow out preventer is connected to the subsea wellhead, imposing bending loads due rig motions and wave loads acting on the riser. The challenge of life extension of these type of components is that the fatigue governing hot spots are not easily available for inspection. This makes it difficult to document safe operation in cases when the calculated fatigue damage has exceeded the acceptance criterion specified in the design code. However, during the last decade significant effort has been directed towards reducing the external loads on subsea wells. This has made it possible to document sufficient fatigue strength for continued operation by assuming the presence of relatively large cracks. This has in turn reduced the need for using inspection methods that can detect small crack and allow the use of less efficient, but practicable methods of estimating the size of potential cracks, such as pressure testing. This article presents an approach where probabilistic fracture mechanics analysis is used to estimate the size of a potential fatigue crack based on a successful pressure test and document the time required for continued operation or plug and abandonment within the required safety level.
