Hydrodynamics aspects of Gravity Based Structures in shallow water

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.

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