Lifted versus Floated Gravity Base Structures Study

Descriptions

The Carbon Trust is working on a major offshore wind technology accelerator initiative focussed on reducing the cost of energy (GBP/MWh) from offshore wind by more than 10 %. This initiative, the Offshore Wind Accelerator (OWA), is funding large-scale research, development and demonstration (RD&D) projects to unlock technological barriers to advance the industry, and thereby accelerate its development so that significant carbon emissions reductions are achieved. Projects have been selected based on their potential impact on the cost of offshore wind farms and on their ability to deliver operational improvements in the short to medium term. The OWA has been structured as a funding collaboration between the Carbon Trust and a number of international energy companies active in offshore wind farm development.Gravity Base Structures (GBS), in terms of potential cost reduction, are very promising foundations concepts for wind turbine generation in deeper water sites competing against steel jacket structures for large offshore wind farms.Within the GBS family, two major types of gravity foundations can be differentiated:— Float-out-and-sink concepts (“Floated concepts”). These types of GBS are made with high concrete volumes (usually cone-shaped) leading to a buoyant element during transport. They only require tugboats to transport them from the construction site to the offshore wind site. There they are lowered to the seabed by ballasting the inside of the structure with water. Once they settled down at the sea bed the structure is usually ballasted with sand to provide higher final stability.— Lifted concepts. These types of GBS are optimised in terms of concrete volume for its final operational requirements (not also transportation requirements). In order to transport and deploy this structures form the construction site to the offshore wind site a transportation vessel / heavy lift crane or barge is required. This auxiliary mean can be one single element or a combination of different elements such as a barges and a heavy lift crane.The Foundations Technical Working Group of the OWA has performed a Concrete Benchmarking Study with the aim of down-selecting the most promising concepts. This study has shown that there is a differentiation between the overall costs of Floated concepts against Lifted concepts.A more detailed study is required analysing the cost assumptions of both the manufacturing/construction and the marine operations (transport and lowering/ballasting).Results of the work previously done can be potentially shared with the awarded contractor once the project starts.