Offshore fish pen design

What is it about?

In this paper, a novel offshore fish pen design named SeaFisher has been proposed. The SeaFisher comprises modular cubic pens that are assembled to form a large 2 x n array offshore fish pen. Its frame structure is made from HDPE, making it flexible and durable against the harsh sea environment. Specially tailored connection brackets and connector pods are designed to assemble bundles of HDPE pipes forming the SeaFisher structure. The SeaFisher is moored using a single point mooring to minimize environmental and collision loads, and for improved waste dispersal. More importantly, the SeaFisher possesses ballast tubes positioned on the top surface to allow it to submerge to a desired water depth to dodge the strong surface waves during severe weather events. This paper presents the engineering design details and hydroelastic analysis of the SeaFisher. Based on a hydrostatic analysis, suitable materials were chosen for the various components of the SeaFisher, and the components were appropriately sized up. By using the software AquaSim, the SeaFisher’s hydroelastic responses under different sea-state conditions were investigated. It is found that the designed SeaFisher structure and mooring system are adequate with respect to strength and stiffness for the considered sea-state conditions of up to 8 m significant wave height and 0.8 m/s current speed. It is expected that the SeaFisher will be a game changer for offshore fish farming due to its cost-effectiveness and ability to survive in severe storms.

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Why is it important?

Global demand for fish products continues to grow due to population increase. However, capturing fish has become unsustainable due to overfishing, and 90% of wild captured species are already overfished or fully fished with no potential for increases in production. On the other hand, farmed fish production has shown impressive growth over the past three decades to fill the gap between fish demand and production. Most marine fish farms are primarily located in sheltered, shallow nearshore waters for safe operation, easy access, and quick transportation. However, in recent years, the annual growth rate of farmed fish production is slowing due to public and environmental opposition to water, seabed and shoreline pollution, and competing uses of sheltered sea space such as shipping, water leisure activities, conservation, and tourism. In response to these criticisms and constraints, offshore fish farming is gaining traction as an alternative solution. Moreover, offshore sites offer larger water space, better water quality, cooler temperatures and better waste dispersion from deeper waters, less sea lice infestation, stronger waves, and higher current speeds. Moving fish farms offshore, however, poses several challenges that include having to contend with a harsher sea environment, an uncertain environment for the farmed fish, operational requirements regarding workers’ safety and support vessels, and the lack of experience and standards in designing offshore fish pens. The most challenging aspect of offshore fish farming is the survivability of the fish pen and the well-being of fish during severe storms that are accompanied by huge surface waves, winds, and shear current actions. Therefore it is important to design offshore fish pens that are storm proof and cost effective.