The Brage Platform: A New Generation of Gravity-Base Structures

ABSTRACT This paper describes the main features of the concrete gravity base platform designed for the Brage field in the Northern North Sea. This design concept encompasses several new design features. The traditional caisson is not used and the necessary strength and stiffness is provided by the base slab and skirts. The well conductors are guided outside the towers which are slimmer than previous concrete platforms. The global wave forces are thus reduced and the foundation can be simplified. The platform can be installed over predrilled wells. Necessary skirt penetration is achieved and the soil strength improved by the application of suction under the skirts. The paper describes the platform and the investigations conducted to substantiate its feasibility. It is shown that the large caisson, normally a prime element of gravity base structures, can be deleted for multishaft platforms and soft soil. Buoyancy in floating phases during construction is achieved by compressed air under the skirts. Main dimensions of the platform are governed by requirements of floatability during offshore tow and dynamic behaviour in operation rather than strength in operation. Thus a reduction in material quantities and construction time of about 40% is achieved. Towout without a heavy topside load is necessary to take full advantage of the platform concept. INTRODUCTION The majority of offshore concrete gravity structures has been of the so called caisson and tower type. This concept is composed of a deck supported by a number of towers projecting from a voluminous caisson. The caisson is a closed box with an upper and a lower level slab or assembly of domes. It is subdivided in cells by straight (Sea Tank, Andoc) or circular (Condeep) partition walls. The caisson is used to serve three or four purposes:Act as structural element of adequate strength and stiffness to absorb the tower bending moments with acceptable deformations. The top and bottom slabs act as beam flanges and the partition walls as webs.Act as a foundation which transfers the functional and environmental forces on the platform to the subsoil with adequate safety. On soft soils the caisson will be fitted with skirts which penetrate down to soil layers of adequate bearing capacity.Act as a buoyancy volume in the temporary phases of floating construction, transportation and installation. Towout with the topsides installed is thereby possible.In many cases the caisson is used for temporary crude storage. Recent platforms in very soft soil e.g. Gullfaks C, have been fitted with very deep skirts, i.e. more than 20 m. Other platforms have been designed to obtain the necessary buoyancy from the volume of the towers alone, independent of the caisson. New offshore cranes of huge capacity can now be used to install the topsides and inshore deck mating is not a major asset. For Norsk Hydro's 136 m deep Brage field in the Northern North Sea, several platform designs of traditional tower and caisson type were conceived.