Operational Experience of the Fatigue Performance of a Flexible Riser with a Flooded Annulus
Authors: Charlesworth David , D'All Bill , Zimmerlin Cecile , Remita Elias , Langhelle Nina , Wang Ting 
Published: October 2011

The integrity of BP's West of Shetland (WoS) flexible riser systems has been actively managed using a risk-based approach since they were installed in the late 90s. A failure mode that was identified as part of this process was the fatigue of armour wires in those risers with flooded annuli. Annulus flooding WoS was caused by external sheath breaches that occurred during installation and to mitigate the risks of both corrosion and corrosion fatigue, BP sealed the breach locations, displaced the sea water in the annuli with a corrosion inhibitor, and implemented a programme of regular fatigue life reassessments for the affected risers.

In 2008, informed by the integrity management process BP put in place, the decision was taken to replace one of the WoS high pressure gas risers which had a flooded annulus. To complete the integrity management cycle, the decommissioned riser was dissected and the various metallic and polymer layers inspected. In addition, armour wire samples were taken from the fatigue critical region of the riser, and fatigue tested to establish S-N curves in an environment representative of the annulus.

This paper describes the dissection process and the general condition of the pipe following decommissioning. It shows that generally the condition of both the polymer and metallic layers was good and the introduction of the corrosion inhibitor into the annulus of the pipe had a positive effect. The fatigue testing programme is also presented, and the S-N curves generated using the wires from the decommissioned riser are compared against curves for as-manufactured wire. This indicates a significant level of conservatism in current fatigue life prediction methods.

The inventory of flexible risers in the major offshore basins that have flooded annuli is large and the fatigue performance of armour wires in this situation can be a key integrity management risk. Currently assessment of this risk is based on calculation and small scale testing alone, rather than actual operational experience. This paper seeks to address that gap in industry knowledge and to start building a database of actual fatigue performance.

Introduction

BP's Foinaven and Schiehallion fields are located approximately 180km West of Shetland (WoS) in water depths ranging from 300 to 500m. Both fields were developed in the late 90s using subsea wells tied back, via rigid flowlines and Pliant Wave® flexible risers, to separate Floating, Production, Storage and Offloading vessels (FPSOs). The WoS environment is particularly challenging for flexible riser systems and as a consequence BP has actively managed riser integrity WoS using a risk-based approach. A failure mode that was identified as part of this process was the fatigue of armour wires in those risers with flooded annuli. Riser annulus flooding was caused WoS by external sheath breaches that occurred during installation, and to mitigate the risks of both corrosion and corrosion fatigue, BP sealed the breach locations, displaced the sea water in the annuli with a corrosion inhibitor, and implemented a programme of regular fatigue life reassessments for the affected risers.