Cathodic Protection Custom Retrofit Ensures Integrity of Subsea Manifold
- Gordon McKinnell (Stork Technical Services UK)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- December 2011
- Document Type
- Journal Paper
- 28 - 29
- 2011. Copyright is retained by the author. This document is distributed by SPE with the permission of the author. Contact the author for permission to use material from this document.
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Preventing corrosion on subsea architecture is critical for operators in their effort to maintain an asset’s integrity and extend field life. Failure to do so can lead to unscheduled maintenance, deferred production, and possible punitive costs should environmental damage occur.
Cathodic protection (CP) can be a cost-effective means of preventing corrosion and should be considered as part of any long-term integrity management strategy. With accurate datasets, it is possible to predict the most likely future state of a structure’s integrity, which aids in planning preventive measures.
When a CP system reaches the end of its design life, a retrofit system is often required to ensure that an asset’s integrity is not compromised. A recent example was a custom retrofit system designed, engineered, and supplied by Stork Technical Services to Marathon Oil for the cathodic protection of its West Brae field subsea manifold in the UK North Sea. The system needed repair and the measures had to be carried out without causing a major disruption to oil and gas production.
Developing a CP System
A CP system comprises an anticorrosion coating and either sacrificial anodes, the most commonly used subsea, or impressed current anodes with a power unit to drive them. A system is designed to operate for a fixed period, usually between 10 and 30 years. However, the extension of field life and addition of facilities to an asset can compromise the original design by increasing the surface area and members, which may reduce the effectiveness of the anodes installed as part of the original design.
The aim of a CP system is to polarize a structure as quickly as possible and maintain the optimum protection for the design life. When depolarization begins, the system has reached the end of its design life. Either a retrofit system is installed or the structure is allowed to operate in an underprotected state. Underprotection may be allowed because installing a new system may overprotect another connected component, and that could create other failure risks with a higher impact.
Overprotection and the materials used must be evaluated in the CP system design. Hydrogen blistering, cracking, and embrittlement can occur when a system is overprotected. Susceptibility to such damage depends on the structure and hardness of the metal in question. For example, super duplex steels are the metals most commonly affected by overprotection hazards in subsea oil and gas applications. As the supply of a high current at low voltage is central to CP, a proper consideration of the equipment and its use is essential to ensure that repeatability, historical traceability, and accurate measurement in the range of <10 mV are achieved.
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