Subsea wells are typically drilled and completed using marine risers with subsea BOP stacks deployed from mobile drilling units. Wave and current induced riser motions are transferred to the wellhead, conductor and casing system which can then cause fatigue issues at the critical connectors and welds. In recent years, the risk of fatigue failure has increased due to the use of 5th and 6th generation rigs with larger BOP stacks, and longer well drilling and completion durations. During the design stage, the fatigue performance of the wellhead, conductor and surface casing is often overlooked.
During well construction, variations from design can occur in terms of wellhead stickup, wellhead lockdown and cement levels, both around the conductor and in the annulus between the conductor and surface casing. This paper describes analysis results showing the effect of these variations on the fatigue performance of the wellhead, conductor and surface casing. The recommended analysis load case matrix to account for these uncertainties is also presented, together with the need for as-installed assessments.
Recommendations on fatigue resistant designs with equipment configuration and specification options capable of meeting these challenges are presented. Wellhead monitoring and fatigue tracking strategies that can be implemented during drilling and workover operations to address these potential concerns and manage the risk are also discussed.