CONCRETE gravity base

Description

Offshore oil development, especially in the North Sea, has brought up a completely different design of fixed platform: concrete platforms.
Concrete platforms are generally larger structure than steel jackets and are not piled but instead sit on the seabed, stabilised by their own massive weight. Apart from the North Sea, gravity concrete structures were built in Brazil (3), Gulf of Mexico and the Mississippi Delta. The concept can serve almost any offshore purpose such as drilling and exploration, oil and gas production and storage. The structure can weigh over 850,000 tonnes.

The main reasons for the offshore industry to choose such design are:

The possibility of incorporating a oil storage, especially for large field.
Cheaper installation cost.
Majority of the equipment installation and hook-up is performed onshore, limiting the amount of more expensive offshore installation work.
Larger than steel jackets, so permit greater production and in the same time reduce the total number of platforms required for the development of a field.
Concrete does not require maintenance and does not suffer any reduction in load-carrying capacity over the years.

However, there are two major drawbacks:

Relatively expensive capital cost, compared to steel jacket.
As yet no feasible means of removing (decommissioning) the structure has been defined.

Typical design

The foundation must be designed to withstand forces from the weight of the structure with deck loads, stored oil and ballast. It must also withstand to environmental parameters as waves, currents and winds and in some locations, earthquakes and ice loading.
As an example the Brent B Condeep platform in the North Sea consists of 19 interconnected 61m high cylinders, three of which extend upwards as towers to support the deck some 170 meter above the sea floor. The caisson measures 100 m across and a base area of 6300 square metres.
The wave action causes vertical and horizontal forces on the structure, as well as an overturning moment. The design also takes into account several possible loads combinations includig those on the submerged part of the structure, the ballast and live loads.
Maximum design forces for the Brent B platform are as follow:

Vertical load: around 2000 MN
Horizontal load: around +/- 510 MN
Overturning moment: around 20 GN.m

Conclusion

We have established that the concrete structures have immense strength and are in some cases more than 400 times stronger than fixed steel strucutures and can be expected to withstand the vertical, lateral and overturning moments imposed by many renewable energy devices. An individual structural analysis will have to be carried out to determine the feasibility of re-using the installation as OREC.