Technological Feasibility Study and Fish Welfare Considerations for Novel Structures in Open Ocean Aquaculture
Authors: Tsarau Andrei , Su Biao , Moe Føre Heidi , Tobias Slette Hans , Vennell Ross , Janssen Gerard , Cook Denham , Black Suzy 
Published: August 2024

The potential future of open ocean aquaculture (OOA) lies in its ability to provide low-impact, sustainable seafood production offshore, offering a solution to meet the rising global seafood demand while addressing climate change and reducing the pressure on coastal areas. However, the development of OOA structures is costly, which emphasises the crucial importance of thorough concept evaluation during the design phase, where any risks associated with the new structure during its operation must be addressed. This involves not only ensuring structural reliability in challenging offshore environments but also ensuring that fish thrive in the enclosed space provided by the structure. A comprehensive, integrated evaluation of novel OOA concepts and design solutions may require special analysis tools and methods, which are found in neither today’s offshore engineering nor traditional coastal aquaculture technology.

This paper presents methods for a comprehensive analysis of a novel, submersed, flexible enclosure system developed for open-ocean finfish aquaculture in New Zealand. Herein, the enclosure structure is simplified and generalised by considering it as a horizontal fabric cylinder with ends covered by nets. The structure has outer ring structures to stiffen the enclosure and help maintain its cross-sectional shape. A single-point-mooring (SPM) system with either catenary-like or taut moorings allows the structure to freely rotate with the flow while being submerged at depths below which wave actions are reduced.

The performed analyses consisted of three major steps needed to evaluate the OOA structure from different perspectives. In step (i), a computational fluid dynamics (CFD) modelling of steady flows through the structure with porous end covers was carried out to assess the internal flow reduction, which depends on the structure’s shape and solidity of the net. Here, a CFD approach based on measured hydrodynamic properties of net material was employed. This modelling is necessary for ensuring optimal flow conditions for fish contained within the structure. It also provides water-velocity data needed for predicting the dissolved oxygen (DO) transport. In step (ii), time-domain simulations of the moored OOA structure in dynamic environments with waves and varying currents were conducted. Here, extreme conditions were modelled to evaluate the reliability of the structure and its mooring system under possible design loads. To model the flexible enclosure, a new finite element model based on so-called rotation-free shell elements was developed within the framework of FhSim, which is the simulation software at SINTEF Ocean for modelling flexible marine structures in waves and currents. In step (iii), the FhSim model was used to predict the motion of the structure in a harmonic tidal current. Building upon the results from the steady-state CFD analysis, these transient simulations provided the basis for predicting the DO dynamics within the enclosure depending on water temperature and respiration rates of fish (considering Atlantic salmon as an example). This helped identify when the dissolved oxygen levels within the enclosure may drop and how long it may take for them to recover during the tidal transitions, which is important for fish welfare. It was observed that the motion of the structure connected to an SPM mooring system may have a large impact on internal oxygen levels, offering insights for optimising the mooring design.

Overall, the proposed methodology represents an integrated solution for concept evaluation during the design phase of OOA structures, potentially enabling the optimisation of new designs to handle offshore environmental loads while promoting fish welfare.