Offshore-to-Harbour Model Chaining for Floating Pontoon Sizing: Case Study of Larache Port

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Abstract Floating pontoons are widely used in ports and marinas due to their modularity, cost efficiency, and ability to accommodate variations in water level. In semi-enclosed basins, conditions are strongly controlled by harbour agitation, which depends strongly on local wave transformation processes including entrance diffraction, multiple reflections, wave interference, and basin-scale resonance. Directional wave information inside ports is often scant, and pontoon responses computed using frequency-domain boundary element method solvers typically involve spatially uniform forcing that may not accurately represent the actual excitation. This paper presents an offshore-to-harbour model-chaining framework for floating pontoon sizing and siting, combining TOMAWAC, ARTEMIS, and NEMOH. The framework is applied to the Port of Larache, Morocco, in three offshore sea states with increasing severity, corresponding to return periods of about four, 10 and 50 years. The results show pronounced spatial gradients in the in-harbour wave conditions, with significant wave heights ranging from 0.22 to 0.40 m and local maxima of about 0.55 m near reflective boundaries. These variations are found to be primarily controlled by the local wave conditions within the basin. The restoring stiffness for the mooring does not affect the relative ranking of candidate locations. A first-order sensitivity analysis of the offshore forcing, boundary reflection coefficients, spectral discretisation, mooring stiffness, incident-wave direction and pontoon geometry confirms that the relative ranking of the candidate locations is preserved under all tested perturbations. This framework establishes a link between offshore forcing, harbour-scale excitation, and structural response, thereby supporting more robust pontoon siting and design in semi-enclosed ports.

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