Session: 05-02-02 Aquaculture and Related Technology II

Paper Number: 125990

125990 - The Influence of Design and Environmental Factors on the Hydrodynamics and Dissolved Oxygen Exchange of Structures for Open Ocean Aquaculture 

Aquaculture production is projected to expand from land-based or inshore areas (e.g. semi-enclosed bays, harbours) to the open ocean as demand for seafood grows, competition increases for inshore space, and there is an emphasis to minimise environmental effects. Open ocean aquaculture (OOA), however, poses many challenges due to the high energy environment, inaccessibility of power supply and supporting services, thus, requires various novel technologies as those that are used in traditional aquaculture may not be appropriate. The structures for OOA, for example, might require different designs compared to conventional sea pens. Conventional sea pens are designed to withstand environmental loads (currents, waves) so that they do not collapse, shrink or float away, therefore minimising their ability to move. In the open ocean where loads can be significant, it could be too expensive to use these more rigid and robust pens to withstand the high energy environment. Furthermore, the low mobility of sea pens also results in challenges in ensuring healthy liveable zones for the fish when adverse environmental conditions occur (e.g. increasing number of heatwaves, rising sea level temperature, rapid development of biofouling). Therefore, to develop this industry, new OOA structures need to be designed to allow the structure and fish in it to survive in this complex environment including the ability to move to safe zones. The design of OOA structures also needs to maximise the Dissolved Oxygen (DO) exchange via current flows to avoid the use of expensive artificial DO supply systems. To date, however, the appropriate designs for such OOA structures have rarely been investigated.

The current study first examines the influence of the new design of OOA structure and various environmental factors on the hydrodynamics and DO exchange using Computational Fluid Dynamics (CFD). The structure is a 1,500 m³ square cross-section cylinder with fabric nets placed at the front and back of the body. There are slots on the body of the structure with densities varying between 0% (i.e. no slots) to 12%. Both parallel and perpendicular to the flow direction slots are examined. Bio-fouling is simulated using a flow reduction ratio through the nets that varies between 5% and 50%. For all CFD simulations the fish density is 15 kg/m³ and water temperature is 15^o with current speed scenarios are 0.1 m/s, 0.05 m/s, and 0.01 m/s. 

CFD modelling results indicate that the structure design strongly influences the flow hydrodynamics and DO distribution. Increasing slot density facilitates flow ventilation through the structure and increases DO inside the structure. Parallel slots are more effective than perpendicular slots in terms of supporting flow ventilation. Increasing the flow reduction ratio through the nets to simulate bio-fouling effects results in a decrease of DO inside the structure. Increase of current speed results in an increase of DO inside the structure. Overall, we found that CFD modelling is an useful tool to explore the complex relationship between OOA structure design and environmental factors on the hydrodynamics and DO exchange. The methodology and results of this study, therefore, can assist in developing appropriate designs for OOA structures.

Presenting Author: Duc Nguyen University of Otago

Presenting Author Biography: I completed a Bachelor of Environmental Engineering in Vietnam in 2009; Master of Environmental Management in Australia in 2015; and Ph.D in Geography and Mathematics in New Zealand in 2022.
I am interested in using Computational Fluid Dynamics (CFD) to examine the environmental fluid flow to support environmental management. My previous Ph.D research examined the wind flow dynamics over the coastal sand dunes and implications for sand transport and dune evolution. My postdoctoral research examines the water flow and oxygen through the innovative designs of salmon cages to provide healthy habitat for open ocean aquaculture.

Authors:

Duc Nguyen University of Otago
Sarah Wakes University of Otago
Ross Vennell Cawthron Institute
Si Thu Paing The New Zealand Institute for Plant and Food Research Limited
Scott Rhone The New Zealand Institute for Plant and Food Research Limited
Louise Kregting The New Zealand Institute for Plant and Food Research Limited
Suzy Black The New Zealand Institute for Plant and Food Research Limited
Gerard Janssen AQUI-S New Zealand Limited