Innovation in offshore foundation design

An innovative offshore foundation, designed and constructed by a consortium of European engineering companies, is to be installed as part of a windfarm in the English Channel.

Offshore oil rig foundation
Offshore oil rig foundation

The Cranefree® Gravity (CFG) foundation design concept allows the foundation to be fully constructed on a quay, lowered into the harbour and ballasted, before being towed to its location. It's then installed by ballasting with seawater, without the need for offshore cranes.

At the final stage of the project, a 725-tonne base was constructed on the quay at Le Havre in France. The final location of the completed foundation is approximately 17km from the French coastal town of Fécamp, at over 30m mean sea depth.

Part of a windfarm project for energy client consortium, Eoliennes Offshores des Hautes Falaises, the foundation will support a meteorological mast and act as a prototype for other structures planned for the site.

The detailed design of the foundation was done by Norwegian company Seatower, assisted by Danish contractor MT Højgaard, which is also responsible for installation. It's being constructed by French company Eiffage.

Benefits of design

Eliminating the need for large installation vessels with heavy offshore cranes significantly reduces costs and benefits project logistics and planning. These vessels are scarce and getting them to the project site can cause delays, particularly when weather in unpredictable. Tugboats offer a cheaper and more flexible resource.

The windfarm includes many foundations, so these factors deliver considerable overall benefits to the project dynamics.

This installation method is better for the environment because of the reduced impact on the seabed and disruption to and noise to sea life.

Design detail

The foundation comprises a reinforced and post-tensioned conical base, with a closed compartmented chamber which can be variably ballasted for towing and sinking the foundation into its final position. It also has a steel tower bolted down to the top of the concrete section, and a working platform is positioned at the top of the steel tower. The meteorological mast is bolted to the top of the steel tower, at the platform level.

The concrete part of the foundation is approximately 18m tall and 23m in diameter at its base. The inner compartment is subdivided to create a stable ballasting and sinking sequence. The foundation base has a steel (sheet pile) skirt which extends below base level to penetrate the seabed under the action of its self-weight.

The steel part of the foundation is approximately 26m tall, typically 2m in diameter (3m at the connection to the concrete) and typically uses 25mm and 20mm thick plates. The meteorological mast is a further 40m tall.

By combining normal weight concrete, lightweight concrete, hollow steelwork and extra-dense concrete ballast, the design is carefully optimised to have a low centre of gravity and ensure stability during towing.

The total weight of the foundation, plus the mast to be lowered from the quay into the water, will be around 1,400 tonnes.

Acknowledgements:

Ed Garvey (ICE member and ICE representative for Denmark) is key account manager (design) to MT Højgaard's offshore department.

John Currie (ICE member) is senior project manager for Fécamp project in MT Højgaard's offshore department.

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