The uncertainty associated with the nature and extent of the corrosion threat to different parts of the system presented the duty holder with a considerable challenge and business risk. Increased time-of-flight-diffraction (TOFD) inspection would give confidence in the ability to monitor preferential weld degradation but the costs associated with such a low risk strategy were considerable, several £100k’s. A significant reduction in TOFD inspection would have exposed the field partners to a significant risk of production losses through further failures.

CAPCIS applied the uncertainty modelling approach, supported by specialised testing of weld corrosion and inhibitor performance, to understand the risk of preferential weld attack throughout the topsides pipework system. By developing an understanding of the damage mechanism and taking account of the transport of free acetic acid and corrosion inhibitor through the system, clear recommendations could be made to minimise the risk of future failures.

The conclusions from the study lead to:

• A change in the location of injection of the inhibited acetic acid and a modified inhibitor formulation.
• A targeted inspection strategy based upon prioritisation of the risk of failure.
• Installation of weldment probes at key locations to enable the early warning of upset conditions that could increase the risk of weldment failure.

Subsequent to implementing the above recommendations, the corrosion threat is now under control and costly inspection activities and the risk of production loss through future failures have been reduced significantly. This Case Study demonstrates how an uncertainty modelling approach can provide operators with the necessary rational and justification for making key integrity decisions.