Drilling operations for subsea wells are typically carried out using drilling risers with subsea BOP stacks deployed from mobile drilling units. The drilling risers are subject to wave-induced motions both from vessel motions and direct wave loading, as well as vortex-induced vibration (VIV) due to current flow past the riser.

The motions are transferred down the riser to the wellhead, conductor and casing system and can cause fatigue issues at critical connectors and welds. The potential for severe fatigue loading of the wellhead system has increased over the last few years due to the use of 5th and 6th generation drilling vessels with larger BOP’s and considerably longer well operations, required to maximise output. 

Equipment selection is often based on what has been used on previous wells and is not always application specific. Analyses done during the conceptual design stage often focuses on the drilling riser and the fatigue performance of the wellhead and conductor are often overlooked. 

This paper describes the parameters that influence the riser motions transferred to the wellhead, conductor and casing system, and highlights those that are most damaging to fatigue performance. The options available to mitigate the fatigue issues through improved equipment configuration and specification are also discussed, together with various monitoring and fatigue tracking strategies to address these potential concerns.