Assessment of Corrosion Damage in a Finger-Type Slug Catcher

摘要

Corrosion pits of up to 6 mm depth, associated with areas of high solid deposition were discovered in the finger-type slug catcher and glycol sump during routine inspection (first in 2005 and then in 2013) following lengthy periods of below-threshold CI injection and partial implementation of the corrosion management plan. Corrosion rates were predicted to have increased from 0.46 mm/year in 2005 to 0.71 mm/year in 2007 and then decreased to 0.41 mm/year in 2008. Subsequent integrity assessments concluded that the slug catcher was not fit for continued operation at the current rate of pitting and was therefore shut down to prevent potential loss of containment. Results of API 579 fitness for service assessments of the corroded areas justified the subsequent decision to de-rate the system and that no further corrosion can be tolerated at the original design pressure, at least until the rates of corrosion and its mechanisms were fully understood. Based on the damage morphology, chemical and monitoring data and theoretical concepts, a synergy of mechanisms (under deposit corrosion, CO_2 corrosion, preferential weld corrosion and microbial-induced corrosion) is identified as being responsible for the observed damage. The short-term mitigation plan is to routinely inspect the damaged locations (by UT wall thickness checks) while simultaneously implementing a corrosion management and control plan which consists of: continuous inhibitor dosing, pH stabilisation (with MEA/DEA) to above pH 6, batch biociding, sampling of residuals, chlorides, iron, SRB/GHB count, water cut, water content and pH of lean glycol, glycol content of pipeline fluid and online monitoring. The inspection, sampling and monitoring results should then be reviewed initially monthly, subsequently quarterly by the corrosion and integrity team in order to assess the rate of pitting. If the target (<0.125 mm/year) is being breached frequently, then alternative but more costly mitigation should be considered such as repair or outright replacement of the finger-type by vessel-type slug catchers which have built-in sand jetting systems designed to deal with high deposition rates and are therefore considered less prone to associated corrosion damage.