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A gravity-based structure (GBS) is a support structure held in place by gravity. A common application for a GBS is an offshore oil platform. These structures are often constructed in fjords since their protected area and sufficient depth are very desirable for construction. A GBS intended for use as an offshore oil platform is constructed of steel reinforced concrete, often with tanks or cells which can be used to control the buoyancy of the finished GBS. When completed, a GBS is towed to its intended location and sunk. Prior to deployment, a study of the seabed will have been done in order to ensure it can withstand the vertical load exerted on it by that structure.
Gravity-based structures are also used for offshore wind power plants. By the end of 2010, 14 of the world's offshore wind farms were supported by gravity-based structures. The GBS are suited for water depths greater than 20 m. The deepest registered offshore wind farm with gravity-based structures is Thornton Bank 1, Belgium, with a depth up to 27.5 m. As offshore wind power plants are growing in size and moving towards deeper waters, the GBS is considered competitive in comparison with other support structures.