Gravity Based Structure

As I mentioned in December, I would provide some educational blogs in-between my normal blog postings here is a definition of a Gravity Based Structure and links to the Drillship and the Oil Platform definition are below.

http://davidnealis.blogspot.com/2011/12/definition-of-offshore-oil-platform.html 

http://davidnealis.blogspot.com/2012/01/definition-of-drillship.html

A Gravity Based Structure or GBS can either be made of steel or concrete and is usually anchored directly onto the seabed.

 Steel GBS are predominantly used when there is no or limited availability of crane barges to install a conventional fixed offshore platform, for example in the Caspian Sea. There are several steel GBS in the world today (e.g. offshore Turkmenistan Waters (Caspian Sea) and offshore New Zealand). Steel GBS do not usually provide hydrocarbon storage capability.

It is mainly installed by pulling it off the yard, by either wet-tow or/and dry-tow, and self-installing by controlled ballasting of the compartments with sea water. At low depths Gravity Based Structures are very affordable, but above 10 meters depth it is not as cost competitive with other types of structures. To position the GBS during installation, the GBS may be connected to either a transportation barge or any other barge (provided it is large enough to support the GBS) using strand jacks. The jacks shall be released gradually whilst the GBS is ballasted to ensure that the GBS does not sway too much from target location. The concrete Gravity Based Structures can last up to 100 years with minimal maintenance. 

Gravity Based Structures can also be used for alternative energy sources like sea based wind farms. The first offshore wind farm in the world was placed on gravity bases. It was 11 large concrete structures weighing in average 908 tons placed in the water near the shore of Lolland, Denmark in1991to hold the wind turbines at the Vindeby offshore wind farm.