Conceptual design, numerical and experimental investigation of a SPM cage concept for offshore mariculture

Abstract

Offshore mariculture is gaining more attention as expansion of the nearshore aquaculture, led by increasing demand for food fish, is faced with stiff resistance from other nearshore stakeholders, including the environmentalists. Due to the necessity to move into offshore waters for aquaculture, single point mooring (SPM) fish cage concepts in offshore mariculture are presently being considered as viable candidates for achieving such an objective. Economic and environmental advantages over the traditional grid mooring systems have been reported for SPM cages. In addition, from an engineering point of view the ability of the SPM cage systems to align with the prevailing current or weather direction implies a smaller projected area, and correspondingly, a reduction of environmental loads and stresses which are associated with the mooring components.

Harnessing these advantages of the SPM cage concept, during the PhD research work outlined in this thesis, a SPM cage concept that allows selfsubmergence was developed. This new design concept offers to minimize the complexity and operating costs compared to existing SPM concepts that require ballasting and de-ballasting manually or through automatic control systems for submergence or surfacing. Since the success of such a system depends on the operational performance in the offshore environment, the submergence characteristics of the self-submersible SPM cage system were analyzed through numerical simulations to show its effectiveness in relation to regular waves with following and oblique current directions. Regular as well as random wave cases were analyzed. Subsequently scaled physical model testing was conducted in order to verify the results from the numerical simulations with focus on the submergence characteristics of the self-submersible SPM cage system.

The numerical simulations performed during the PhD research showed that the self-submersible SPM cage system responded effectively to increasing wave heights, by submerging deeper for increasing wave heights for a given current speed. It was found that the system is tuned-in to a particular (or a range of) intermediate wave heights and current levels in the sense that more pronounced submergence was observed for these loading conditions. By changing the reserve buoyancy of the cage system, the behavior could be manipulated in order to achieve a certain level of submergence. In random waves the submergence characteristics of the cage system were less effective than in regular wave cases. However, whether in regular waves with following currents, regular waves with oblique currents or in random waves with following currents, the self-submersible SPM cage system responded with additional submergence for increasing wave heights. In all the simulations and model tests, the cage system was found to be very stable and reaching equilibrium at a depth corresponding to the applied loading conditions. Although the scaled physical model and test conditions did not exactly match those of the numerical and the associated response simulations, the general trends of submergence characteristics of the self-submersible SPM cage concept were well captured by the experiments. Therefore, the numerical simulation results were verified, and the self-submersible SPM cage system is shown to have a potential for being used for the purpose of offshore mariculture.

In random waves the submergence characteristics of the cage system were less effective than in regular wave cases. However, whether in regular waves with following currents, regular waves with oblique currents or in random waves with following currents, the self-submersible SPM cage system responded with additional submergence for increasing wave heights. In all the simulations and model tests, the cage system was found to be very stable and reaching equilibrium at a depth corresponding to the applied loading conditions. Although the scaled physical model and test conditions did not exactly match those of the numerical and the associated response simulations, the general trends of submergence characteristics of the self-submersible SPM cage concept were well captured by the experiments. Therefore, the numerical simulation results were verified, and the self-submersible SPM cage system is shown to have a potential for being used for the purpose of offshore mariculture.