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Accelerated corrosion technology trial optimises steel removal post-decommissioning

Project Summary

3-SCI Ltd conducted a Technical, Economic, Legal, Operational and Scheduling feasibility study exploring accelerated corrosion techniques for steel. The project, supported by NZTC, was completed over a 14-week period, exploring a new method of removing subsea metallic structures via an accelerated corrosion technique, which can cut or fully dissolve steel in the marine environment. This would have both cost and environmental benefits compared to traditional methods, where steel is either left subsea or returned to shore.

Initial laboratory activities were conducted by the 3-SCI team to identify and demonstrate the accelerated corrosion method. The study also covered the outline configuration of subsea tools, known as C-Beaver, the primary deployment options, operational and energy supply options, legal and environmental issues, as well as exploring the commercial implications such as cost and savings to industry.

Industry value:
The removal, long-term surveillance and potential intervention associated with steel structures remaining subsea post-decommissioning is a significant cost to industry. This technology has the potential to increase the rate at which steel is currently eliminated, enabling a reduction in time and cost efficiency.

Key results:
Workshop data suggest that C-Beaver tools could dissolve steel at the rate of circa 100 Kg per (approximately 10 watts eliminating 1 gram of steel per hour).

For example:

  • Applying 10 KW to 100 feet of 38lb/foot steel tubular using the C-Beaver method could reduce it to its corrosion product within two months
  • A 6 MW capacity offshore wind turbine (assumed creating average power at approximately 25% of capacity, could help eliminate steel at the rate of approximately 1000 per year

Accelerated corrosion may provide some significant benefits in reducing costs and environmental impacts, compared to traditional methods. It may also offer options for disposing of steel structures, which cannot be disposed of by existing practice, such as large footings and inaccessible subsea pipelines.

The successful feasibility study provided proof of concept, which in turn attracted industry support for field trials.

Lessons learned:

  • Validation of the accelerated corrosion technique on a larger scale (e.g. using large water tanks) is required
  • Additional studies are required to confirm the precise approaches for provision of power
  • The approaches to effectively eliminate the various coatings and electrically-isolating barriers on steels needs to be defined
  • Design and early prototyping work should commence for the production of a large scale accelerated corrosion tool prototype for sea trials
  • Assuming a positive outcome of recommendation 1, and no early ‘blockers’ being identified through recommendations 2 and 3, an Environmental Impact Assessment (EIA) should be conducted. This should focus on the possibility of discharging the accelerated corrosion products directly into sea. The outcome of this EIA may influence the nature of prototyping work and the scope of application of accelerated corrosion.

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