Hydroelastic Responses of a Submersible Ring Structure for Offshore Seaweed Cultivation under Wave Action

Abstract

This paper investigates the hydroelastic response of a submersible circular ring structure, designed for offshore seaweed cultivation, under wave action and during the submergence process. The ring structure comprises two circular HDPE pipes connected to each other by equally spaced brackets. The structure carries seaweed grow-out lines, and is kept in position by a mooring-line system used for fish pens. The HDPE collar is equipped with multiple inlet and outlet valves, allowing it to be submerged to avoid strong waves and to be raised to the water surface when the strong waves die down. The software AquaSim was used for the hydroelastic analysis of the moored structure. It is found that we can significantly reduce the von Mises stresses in the ring structure as well as the mooring-line forces by submerging. However, the structure can experience significant increase in stress during the submergence process due to bending from combined wave action and non-uniform distribution of filled water in the ring structure. This stress increase may cause structural damage or even failure. Therefore, it is important to submerge the ring structure in calm waves ahead of predicted storms and to control the distribution of seawater into the ring structure. For the latter, it is best to use at least two inlet valves and two outlet valves to minimize the likelihood of damage of the ring structure during the submergence process.