Sharing lessons learned from decommissioning of a tidal energy tripod at EMEC

Ocean

Forensic analysis of the collaborative decommissioning of a tidal energy tripod at the European Marine Energy Centre (EMEC) has been completed, providing a comprehensive understanding of the end-of-life condition of the tripod, which helps assess the long-term effect of deploying components and systems in the challenging high-velocity environment at EMEC’s Fall of Warness tidal energy test site.

The tripod had been submerged for about 11 years, during which two tidal turbines had operated on it.

Significant learnings have been collated and two reports published to provide feedback and design guidelines to the offshore renewable energy industry, enabling developers to minimize decommissioning costs and de-risk future projects, EMEC said.

The first report highlights supply chain lessons regarding the tools required to cut and lift the tripod from the seabed in high tidal currents. The second report focuses on learning for the design and manufacture of the foundations themselves with regard to biofouling, corrosion and metals.

“Decommissioning is a critical part of the lifecycle of ocean energy projects, however it’s not yet well explored, as relatively few ocean energy technologies have progressed to the decommissioning stage following long-term deployment,” said John Skuse, EMEC operations and maintenance manager. “It is imperative that decommissioning is managed carefully, including design stages, installation, performance, right through to decommissioning. As the industry continues to develop and mature, the ability to decommission devices efficiently and cleanly will be instrumental in ensuring site utilization is maximized.”

EMEC contracted Orkney-based marine operations provider Leask Marine to remove the tripod foundation. During the cutting and retrieval operations, standard industry equipment used in offshore subsea oil and gas would have been unable to withhold the strong currents. Leask Marine’s engineering team therefore designed and fabricated a new cutting tool support frame to hold the tool in place on the pile with sufficient grip to control the turbulent drag forces. The frame had a buoyancy system added to help lift the 140-ton structure from the seabed in one operation.

Leask Marine’s vessel towed the frame and tripod to Hatston Pier in Kirkwall for forensic analysis, final cutting and disposal of parts. Forensic analysis was carried out by Blackfish Engineering Design Ltd, the International Centre for Island Technology (ICIT) at Herriot Watt University, Rovco Ltd, Brunel Experimental Techniques Centre, and Brunel Centre for Advanced Solidification Technology. Forensic examination activities assessed biofouling, corrosion, metallurgic (metal) defects and electrical connector longevity.

The study found that all surfaces exposed to seawater current will be subjected to vigorous fouling. Components that are expected to move, slide or require clearance in any way must be designed assuming that biofouling will occur on all steel surfaces. Components that are installed for long-term submersion should be designed to account for biofouling end-stage communities up to 15 cm thick, and the additional weight should be factored into lifting operations during decommissioning. Biofouling was markedly reduced where larvae access to surfaces was restricted due to a lack of flow and a lack of nutrient supply. Stainless steel 316L, titanium and super duplex material have all shown excellent resistance to corrosion, but all of them are susceptible to biofouling.

The full lessons learned report has been published highlighting best-practices for device and infrastructure design, deployment, operation, maintenance and decommissioning. The study was part of the EMEC-led FORESEA project, funded by Interreg North-West Europe.

EMEC was established in 2003 as a facility for testing wave and tidal energy converters in real sea conditions. The center offers independent, accredited grid-connected test berths for full-scale prototypes, as well as test sites in less challenging conditions for use by smaller scale technologies, supply chain companies and equipment manufacturers.

To date, 20 wave and tidal energy clients spanning 11 countries have tested 32 marine energy devices at EMEC.

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