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The massive Hibernia Gravity Base Structure (GBS) is currently under construction while floating in Bull Arm, Newfoundland, Canada [2, 3]. It consists of a large concrete caisson that is about 106-m diameter and 85-m high (figure 1). The caisson acts as a support for an oil production platform [1]. The caisson's base slab, which is up to 1.4-m thick, will rest on the seabed when the platform is finally installed on the Grand Banks. Beneath the base slab are a series of concrete skirts that are designed to penetrate into the ocean floor.
The exterior of the caisson (figure 2) consists of a 1.4-m thick, toothed ice wall supported by a series of walls (XVT) that act like a circular truss (called the ice belt). The internal area is divided into independent oil storage areas by walls of varying thickness. This area contains four shafts; two used for drilling, one riser shaft, and one utility shaft that contains most of the mechanical equipment. In addition, in order to reduce the span and, therefore, thickness of the base slab, there are several 8-m deep beams above the slab.
The weight of the structure must be minimized because of the criticality of the draft both during tow to and installation at the Hibernia field. The concrete sections are, therefore, kept as thin as possible.
This, combined with the large forces acting on the structure, ensures that the concrete sections are heavily reinforced.
The base raft portion of the GBS was constructed in a dry dock. The dock was then flooded and the GBS towed 3-km to the deep water site where the remainder is being built while afloat.
The contract for GBS design and construction was originally awarded in 1990 to Newfoundland Offshore Development Constuctors (NODECO), a consortium of mainly British and French companies. Because of various problems with the construction of the GBS, the owners decided early in 1994 to bring in a new construction management team. The owners realized that on a mega project employing up to 6,000...