Due to failure of internal coatings in the high pressure and test separators on offshore production facilities, the option to install an internal cathodic protection system was considered. The background to the system selection (covering literature review, discussions with operators, laboratory study and computer modeling) are described. The initial concept was to install a sacrificial anode system based on high-temperature zinc alloys. However, the prolonged shutdown required to install nearly 3000 kg of anodes in high-pressure separator was not economically viable (due to deferred production costs). Hence an impressed current system was designed. This required considerable engineering work to overcome objections on electrical safety grounds in the hazardous environment of an oil production facility. Detailed HAZOP studies were carried out and special protection systems incorporated to ensure safe operation. The final system installed is described together with preliminary data from commissioning program.
INTRODUCTION & BACKGROUND
The Shell Expro Brent offshore complex in the North Sea lies some 140-km east of the Shetland at approximately 61°N 1.5°E and comprises 4 separate production facilities; Alpha, Bravo, Charlie and Delta. In the mid 1990's the complex was extensively upgraded to convert it from a mainly oil production to an essentially gas production facility. As part of this refurbishment new process separators (for High Pressure, Low Pressure and Test operation) were installed on Bravo, Charlie and Delta. The separators were constructed from carbon- manganese steel and to NACE TM0175. The separator sizes, together with the design & present operating conditions were are summarized in Table 1. The internal fittings of the vessels were constructed from grade 316L stainless steel. Corrosion protection for the separator vessels was provided by a factory applied glass flake epoxy coating.
Unfortunately, after a period of operation, the internal coating started to disbond from the vessel surface, to varying degree for all the separators on all three platforms - hence compromising the corrosion protection for the vessel walls. It was concluded that for the carbon steel High Pressure and Test Separator vessels some form of corrosion protection system was necessary. Re-coating of the vessels offshore was considered impractical and uneconomical since it would have required an extensive shutdown of the facility, and since the factory applied coating had failed, it was unlikely that any in-situ coating would perform any better. Consequently the option of installing an internal cathodic protection system was considered.
