Fatigue Considerations for Subsea Well Systems

Fatigue assessments are a central component of subsea well system analysis and are essential in understanding overall well system performance. In recent years there has been an increased focus on understanding fatigue in well systems. This focus has taken on many forms — a broad industry JIP, technical studies by individual operators, development of a variety of technologies for subsea instrumentation and monitoring and case studies implementing this technology. An overarching objective of all these efforts is to clarify some of the complexity that is inherent to well system fatigue. As with any analysis, well system fatigue analysis benefits from appropriately defined inputs. These inputs affect both sides of the analysis equation — loading considerations and resistance considerations. A majority of the efforts to date have examined loading considerations. Inputs such as metocean criteria and soil properties can have a dramatic impact on the analysis results. Accurate characterization of these inputs is desired, though often difficult to achieve. As a result, large safety factors are often employed to provide a margin against this uncertainty. Often overlooked, however, is the resistance half of the assessment, particularly the accurate characterization of fatigue hotspots such as welds and connectors. Conservative analysis of hotspots is frequently employed as these components are not amenable to inspection, however the conservative assumptions can result in non-realistic assessment of fatigue performance. This paper presents some additional considerations for minimizing the uncertainty in well system fatigue analysis, through thorough materials characterization and design analysis, focusing on connector hotspots. The information presented is representative of actual considerations utilized for ExxonMobil projects. Fatigue performance is improved through finite element characterization of the connector design and “hand picking” component match-ups to minimize the resulting stress amplification. Attention to the total life fatigue properties, including experimental assessment of the material used, is needed to provide an accurate representation of fatigue performance. Finally, in the absence of this information, mitigation of fatigue risks can be obtained by displacing fatigue hotspots.