Root Cause Failure Analysis and Mitigation of Corrosion in Seawater Injection Piping

Abstract

This paper illustrates the methodology for performing a root cause failure analysis of corrosion anomalies reported in the low pressure section of an FPSO seawater injection piping system. Specific practical actions are recommended for mitigating the identified root causes of failure. The primary root cause of failure is oxygen corrosion due to oxygen ingress at components upstream (u/s) and downstream (d/s) of the sulfate removal (SRM) pumps. Oxygen corrosion was worsened by underdeposit corrosion caused by unavailability of coarse straining of abstracted seawater. The secondary root causes are likely due to: deterioration of graphite-filled gaskets on the flanges; occasional bisulphite overdosing; incorrectly specified oxygen scavenger; non-optimized scavenger injection rates; incorrect risk assessment; and unavailability of hypochlorination of abstracted seawater. This paper recommends that the primary and secondary root causes should be further investigated and mitigated. The less difficult but big-impact mitigations which should reduce significantly the number and severity of reported anomalies include: modeling corrosion (based on dissolved oxygen concentration inputs) with predicted velocities (V) guiding operational adjustments within the target range 2 < V < 10 m/s and the predicted cumulative corrosion rates used to target and optimize inspection requirements; inspecting and testing each component u/s and d/s of the SRM pump, in order to identify locations of oxygen ingress; and checking deaeration performance especially possible leaks along the vacuum pump system. Results of KPIs should comply with the specified targets and be managed as part of the corrosion management system.