The drop keel concept: a semi-submersible-spar foundation adpated for ease of assembly for the floating offshore wind turbine market

This paper provides an overview of the analysis methods, results and conclusions reached during an Innovate UK funded research program into a novel 10MW wind turbine floating foundation structure. Current foundation designs are developments of concepts established in the offshore oil and gas sector: semi-submersible, spar and tension leg platforms. Each have their own particular technical and operational drawbacks. The project set out to develop an alternative hybrid solution to take advantage of the benefits of the semi-submersible and spar designs while removing their disadvantages. The concept considered is referred to as the Drop Keel and applied a solid ballasted keel elevated in the launch and transit conditions and deployed to depth in the operation condition. Thus, the hybrid would exhibit the semi-submersible advantages of assembly and launch at a quayside location while possessing the spar advantage of a low centre of gravity in operation. Results from independent numerical and wave tank tests provided consistent results that proved the concept exhibited stable operating performance for the simulated offshore wind and wave conditions. However, the initial Drop Keel concept lacked commercial appeal due to a high steel and ballast weight estimate, complex assembly method, dependency on deep draft submersible barge for assembly and launch, and use of multiple mechanical lift devices that presented logistical challenges for removal during installation. Fortunately, identification of these drawbacks provided a basis for design improvement and led to a final design outcome that resolved all of these disadvantages and improved the design’s commercial appeal.