Root Cause Failure Analysis of Corrosion in Wet Gas Piping

Abstract

This paper discusses the root causes and operational mitigations of corrosion anomalies reported for the wet gas system of a floating production storage and offloading (FPSO) vessel. This system is classified in general into the flowing and dead leg sections and operated below 62 °C, where a protective FeCO₃ film will not form. Corrosion which is temperature dependent is general. The root cause of the anomalies in the flowing wet gas system was found to be due to CaN and solid deposition in the LP separator, increasing levels of corrosive condensate, which drips down the pipe walls, resulting in wall losses of less than 50%. Evacuation of deposits in the LP separator and subsequent control of its formation rate would mitigate anomalies attributed to increasing levels of corrosive condensates. The corrosion pattern at elbows, modified by turbulent flow of emergent fluid streams through the complex-geometry elbows, is mitigated by velocity control. The root cause of the inspection anomalies at dead leg locations was due to the lifting of the PSV and/or infrequent flow through a bypass valve and directly to flare. Infrequent transportation of condensed, corrosive water thereby resulted in dead leg corrosion. Proposed mitigation entails replacement of defective PSVs and modifying flow frequencies. This paper summarizes all the identified root causes and proposed mitigations and provides a greater understanding of the internal degradation mechanisms operating in wet gas systems in general.