51314-4162-The Excluded Element: Corrosion Challenges of Removing Dissolved Oxygen in Seawater from Gulf to Reservoir

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Product Number: 51314-4162-SG

ISBN: 4162 2014 CP

Author: Ridha Abbas

Publication Date: 2014

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At a facility operating since 1978 a first of its kind corrosion-related failure occurred in 2012.Facility “A” is a seawater treatment plant that leys on the shores of the Arabian Gulf. Its mission is to produce filtered disinfected non-corrosive and non-scaling water for oil reservoir injection a process known as water flooding. Excluding dissolved oxygen from water is fraught with challenges from treatment plant to injection points.At the plant a design production capacity of 14 MMBD seawater is treated through twenty-eight identical “modules.” Each module consists of four horizontal multi-media filters from which seawater passes through to a shared deaerator column.At the deaerator column dissolved oxygen is removed; it enters at 6 to 8 mg/l and exits at less than 10 ppb. At the upper section filtered seawater gushes from the top and travels down through fourteen trays while pure nitrogen gas surges from above the bottom tray. The escalating nitrogen and purged oxygen gases escape the deaerator through a vent open to the atmosphere. At the lower section an oxygen scavenger is provided residence contact time with the water.The deaerator shell is protected from corrosion by a coating system and nearly one hundred sacrificial anodes. At one module three pin-hole leaks developed in areas of vertical corrosion extending from top tray level to four inches higher. Of the ten anodes placed there only four were intact on the shell and completely consumed. All others had fallen just partially consumed.This paper details what happened reconstructs how it happened and addresses why it happened from a technical point of view. It investigates tray vibration anode integrity and possible excessive water turbulence. Furthermore it outlines the system of operating conditions maintenance practices and engineering support that have been recommended to prevent recurrence in other modules.