Increasing demand for seafood globally is pushing capture fisheries to their limits. Aquaculture is a viable, sustainable alternative to capture fisheries that is seeing significant growth worldwide. As near- shore sites are limited, new infrastructure is needed to support aquaculture further offshore in the open ocean. While several countries have developed large-scale structures to enable this open-ocean aquaculture, New Zealand is uniquely placed in that its aquaculture production is predominantly bivalves (shellfish); this provides the need for infrastructure to expand New Zealand’s bivalve aquaculture into the open ocean. Cawthron Institute has been conducting research into enabling this expansion, which produced a farm structure named the Shellfish Tower.

While the Shellfish tower was proven to be a viable structure for open-ocean farming of shellfish, it lacked an efficient deployment and retrieval method. Unlike near-shore aquaculture, the Shellfish Tower was required to be submerged at significant depths to survive the high-energy environment of the open ocean. Therefore, there existed a need for a system to deploy and retrieve the Shellfish Tower.

A deployment and retrieval system was designed through a structured design process, resulting in a capstan-winch climber device and a releasable wedge clamp. The climber was a novel application of existing marine technology by extending a capstan-style device into a submerged rope tensioner. All other commercial-scale aquaculture depth management systems currently use a ballasting system, which is of limited use in the application of the Shellfish Tower due to design criteria such as speed of deployment. The clamp design was less successful than the climber, as it required significant geometry optimisation of the clamp jaw cleat after production to enable correct grip and release functionality. It was recommended that alternative concepts be evaluated for future design developments of the clamp.

Ocean force modelling was undertaken to determine the loads acting on the deployment and retrieval system. This modelling was completed using the Ansys AQWA package, which provided many tools ideal for ocean modelling. However, the analysis was complex, and the results were not as accurate as desired. The model provided a considerable understanding of the dynamics of the deployment and retrieval process, where it was discovered that snap loading caused peak loads during high sea states, which was in agreement with previous experimental research.

This system was tested both on land and in the ocean, where it successfully submerged and retrieved a 1200 kg positively buoyant structure similar to the Shellfish Tower, proving the viability of the design. While significant design improvements remained before the system could be used commercially, the design was still a successful deployment and retrieval system that allowed for further development of the Shellfish Tower into a unique commercial solution that could further open-ocean aquaculture in New Zealand and globally.