The Carbon Trust is looking for an offshore wind farm consultancy services to study which gravity base structures could reduce offshore wind cost more, lifted or floated.
Gravity Base Structures (GBS), in terms of potential cost reduction, have 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 have 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).
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Offshore WIND Staff, September 3, 2013; Image: deltares