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the order of several hundred thousand tonnes. GBS have been installed in

water-depths of up to 300 m. Most gravity based structures are made from

concrete although one steel gravity base platform, Maureen, was installed in

the North Sea.

* Concrete platforms are built and installed in a different way from steel

jackets. The construction commences in a dry dock adjacent to the sea. The

structure is built vertically from the bottom up, in a similar manner to

onshore buildings. When the structure is complete, the dock is flooded and

the structure floats under its own buoyancy. The topside structures are

normally installed at an inshore location by deck mating or any other

suitable method. Multiple tugs are used to tow the structure to its offshore

location (fig. 3.5). Once on location, the structure’s tanks are filled with sea

water to a predetermined ballasting plan and the structure is sunk down to its
final position on the seabed. The GBSs are typically trial-ballasted prior to

tow to site.
GBS are towed at a large draft and their towing requires very detailed analyses

and marine procedures including the following aspects:
* Available water depth, underkeel and horizontal clearances in the tow route.

* Stability and freeboard.

* Required number of tugs, bollard pull and design of the towing attachments.

Given its size, several tugs tow the GBS at a very slow speed of 2 knots or less.

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