Operational Experience of the Fatigue Performance of a Flexible Riser with a Flooded Annulus

Abstract

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.

Integrity Management of Flexible Pipes

“…In this case, corrosion fatigue of armor wires may become an issue. Charlesworth et al [13] show that generally, in their stufy, 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. They also present the fatigue testing program, and the S-N curves generated using the wires from the decommissioned riser are compared against curves for as-manufactured wire.…”

An Overview on Bonded Marine Hoses for Sustainable Fluid Transfer and (Un)Loading Operations via Floating Offshore Structures (FOS)

“…By characterisation, a marine hose is a unique type of flexible riser used in fluid transfer [10,11]. By definition, a marine hose is simply a fluid transfer conduit utilised in transferring, discharging, loading, and transporting fluids from an oil well to the platform of a floating platform or a floating structure [21][22][23]. These platforms could be an FPSO (Floating Production Storage and Offloading) vessel, a CALM (Catenary Anchor Leg Mooring) buoy, a SPAR (Single Point Anchor Reservoir), a floating semisubmersible, or a TLP (Tension Leg Platform), as depicted in Figure 1.…”

Numerical comparison between single layer woven flexible pipe and reinforced thermoplastic pipe

“…In addition to that, corrosion also becomes an issue due to the presence of water and corrosive gasses, such as carbon dioxide (CO2) and hydrogen sulphide (H2S), which can permeate through the internal sheath into the annulus area [8][9][10][11][12]. This condition is further aggravated in the event that the external sheath is breached or damaged, allowing seawater to mix with the gas to produce a corrosive environment [2,9,13]. Considering this matter, extensive research was made in replacing the steel armour layer with composite materials which were proven to be lighter and has more resistance towards corrosion [6,7,14,15].…”