Challenging Ground Conditions for Saint-Brieuc’s Offshore Substation

03 August 2022

Photo credit: Saipem

With the Saint-Brieuc offshore substation (OSS) jacket foundation recently installed off the coast of France, independent engineering consultancy, SLPE, who designed the substructure, have reflected on some of the project’s challenges, including its complex ground conditions and how these were overcome.

“It has been a privilege supporting this project, from working on the substation’s initial design scope right through to the installation”, said Sam Lawes, Head of OSS Substructures at SLPE. “The project came with many challenges, primarily due to the conditions of the site where the substation is being installed.

“The ground conditions comprise interbedded sands and silts overlying very weak to strong blocky, disturbed dolerite. The dolerite rock conditions are considered unusual in the context of the conventional design of drilled and grouted piles offshore”.

“Our jacket design had to be robust enough to cope with swell waves from the Atlantic but was constrained by the need for a piling template for the drilled and grouted piles pre-installed into the dolerite dyke present at the OSS location”, added Thomas Horsley, Senior Engineer at SLPE. “Furthermore, careful secondary steel design was necessary to ensure safe access and egress to the platform, contending with one of the largest tidal ranges in the world.

“As the French offshore wind industry continues to develop, where potential sites are often characterised by more challenging ground conditions, we anticipate that the experience gained on the Saint-Brieuc project will be invaluable. We look forward to applying our knowledge and expertise to future engineering challenges in this sector”.

The 1,600-tonne jacket has been installed 16.3 km off the coast of France. The 496 MW offshore wind farm is expected to be operational by the end of 2023.

Ailes Marines, a company 100% owned by Iberdrola, is in charge of the development, construction and installation of the Saint-Brieuc offshore wind farm. The engineering, procurement and construction (EPC) of the offshore substation has been delivered by a joint venture between Iemants (subsidiary of Smulders) and Equans. SLPE was subcontracted to Iemants to design the substation support structure.