Abstract

A three-by-three grid of submersible mussel rafts was analyzed using an experimentally validated dynamic numerical modeling approach. When submerged, the rafts’ pontoons are flooded, and they are held vertically by lines attached to surface floats and horizontally by a mooring grid. The rafts’ decreased waterplane area and increased inertia reduce the heave and pitch natural frequencies so that they are below the frequencies associated with the greatest wave energy. This has been found to significantly reduce the motion of the rafts compared to the surfaced configuration.

The nine submersible rafts were anchored with 16 anchors and mooring lines. These mooring lines were connected to a grid of adjacent rectangular bays, with each corner (node) supported by a grid float. Each bay contains a raft connected to the submerged nodes of the grid by four bridle lines. The dynamics of the full system were modeled using a combined multibody and Finite Element Analysis (FEA) approach with dynamic loads computed using a modified Morison formulation. This model was implemented in the commercial code OrcaFlex. A similar model for a single submersible raft was previously validated with full-scale field experiments. The full dynamic system was simulated in the maximum expected waves and currents. Mean and maximum tensions in each grid line were quantified. Accelerations and velocities at the mussel rope attachment points were also examined, since these relate to mussel drop-off.

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