ABSTRACT

The concrete Gravity Based Structure (GBS) is an attractive concept for shallow water oil and gas developments. The present paper discusses 3 subjects related to the concept:

  • Wave amplification by the large elements of a GBS causes significant problems in setting deck elevations. Physical model tests have usually been required for accurate results. Linear diffraction theory combined with second order simulations of crests heights gives predictions of crest heights with useful accuracy. The simulations tend to be somewhat conservative since they ignore the effects of wave breaking.

  • The hydrodynamics of LNG carriers moored to GBS type structures are complex: this relates to the multi-body interaction in the wave forces, added mass and damping, but also to the drift forces in shallow water. With an optimum orientation of the GBS, a shielding can be achieved for the moored LNG carrier, reducing the weather downtime. However, a wave field still exists behind the GBS due to diffraction, which depends on the wave direction and wave period. For some motions (such as roll) there is very little shielding.

  • A GBS used as an LNG terminal will be oriented to shelter the carriers from the dominant sea direction. The survival conditions will often be beam to the GBS as well. This means that the wave run up and possible green water on the deck of the GBS is a problem that needs serious evaluation. With the improved Volume Of Fluid (iVOF) method it is possible to simulate the run up again the side of a GBS.

INTRODUCTION

A concrete Gravity Based Structure (GBS) for offshore oil production usually consists of a base caisson supporting several vertical columns which in turn support a deck containing productions facilities. A GBS has many advantages when used in the proper situations. Some of the first were built for deep water fields in the Norwegian sector of the North Sea. The Troll platform in 303 m water depth remains the tallest object ever moved over the face of the earth. The Norwegian fjords provided a perfect and almost unique location for the construction of these platforms. Their deep, sheltered waters allowed for slip-forming the columns as they were gradually ballasted lower in the water. Soils in the Norwegian sector were strong enough to support the massive structures and their mass is sufficient to resist the overturning forces caused by environmental loads.

Since the base of a GBS is large, it is easy to adapt it for oil storage. Production can then be stored until a load large enough to fill a tanker is accumulated. This method of transportation can be the most cost effective solution in remote areas far away from existing pipelines.

Figure 1 Gravity Based Structure as LNG import terminal (Available in full paper)

Malampaya is a good example of the advantages of a GBS in shallow water (Chudacek et al., 2002 [1]). The Malampaya gas field is actually in water about 820 m deep, but the production is sent from the subsea wells by pipeline to the GBS in 43 m depth where it is processed.

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