OPEN OCEAN AQUACULTURE: Why Humans and Fish should not inhabit the same offshore structures.

Due to COVID-19, my family and I are ensconced at home. Strange times. To keep our spirits up, one activity we took an interest in was tracking the International Space Station. From our garden, each evening we marvel at a bright speck streaking across our skies at 28,000km/hr, 403km overhead, wondering if those up there feel smug now that we are also confined to our capsules down here on Earth. I bet they would have some good advice.

This led me to reflect on the lengths to which engineers must go in order to make hostile environments safe and habitable for humans. It is perhaps no surprise that more distant space exploration is undertaken not directly by humans, but by human ingenuity - through robotics and automation. Taking human limitations out of the equation, makes the near impossible realistic and affordable. This applies to my own technical projects also – creating a safe platform in highly exposed ocean environments is a significant engineering challenge – but for aquaculture, should the objective be to provide a safe and habitable place just for fish or for humans as well?

There are now some really exciting developments in offshore, open-ocean aquaculture – with new structures rolling out of Chinese shipyards. But they all have one thing in common: their design pedigree comes from conventional offshore engineering, in particular from Norway’s extensive oil and gas experience. Because of this, I wonder if these developments have inherited a critical, but potentially unnecessary design requirement: to provide a safe and habitable structure for humans? They have all, without explanation, abandoned the pre-dominant flexible structural solutions used in fish farming to date, in favour of adopting standard offshore platforms to create fish pens. Examples include:

**Semi-Submersible Structures & Ocean Farm 1**

Semi-submersible structures are designed to have reduced surface-piercing structures, which attenuate the dynamic response to waves in order to make a better work platform for drilling operations. Larger buoyant elements or “pontoons” below the surface can be flooded to match payload and freeboard needs, as well as raise platforms for transport to/from shallower harbours. Key design requirements include maintaining the “Air Gap” between large waves and the operating deck to avoid large wave slam loads on critical structures and equipment. Salmar’s Ocean Farm 1 has now been operating for several months and utilises such a structure to enclose a pen environment within its vertical “columns”. Costing some €100m, the structure has approximately 7,000 tonnes of steel, with a containment volume in excess of 200,000 m3. It is designed on the basis of a near-permanent human operational presence.

Nordlaks’ Havfarm1 designed by NSK Ship Design has just been floated from a shipyard in China. In addition to having semi-submersible features, this 33,000 tonne ship-like structure incorporates a bow-turret mooring, frequently used by Floating Production, Storage and Offloading (FPSO) installations for the oil and gas sector. It is understood that future variants will also have a propulsion system to “dynamically position” the structure with smaller temporary anchors. Overall, the ability to move the structure may allow storm avoidance and could be very beneficial, if licencing systems allow for such “roaming” fish farming operations. But if 33,000 tonnes of steel is required to host 10,000 tonnes of fish, will cost metrics be competitive?. How will fish fare contained in a ship structure in larger waves? Time will tell.

**Gravity Base / Spar Structures & Pan Ocean Aquaculture**

Another interesting project just rolling out of a Chinese shipyard comes from Dutch company Pan Ocean Aquaculture, in which a containment pen is oriented around a central gravity base fixed spar structure, similar to an offshore wind gravity base foundation. The net structure can be raised or lowered below the water surface. Such submersible fixed structures can typically only withstand exposure to moderate storm wave conditions, but do provide a stable fixed platform for human operations.

These are fantastic and ambitious projects, underlining just how big a prize offshore aquaculture is. But I wonder if they rely too much on the human frame of reference. These structural solutions are all born from designs to overcome ocean dynamics and provide a safe and comfortable working platform for human operations. While human operations will be an inevitable part of fish farming, they can be undertaken during calm weather windows (especially with emerging automation and monitoring technology). Furthermore, the more intensive feed supply and harvesting operations can be undertaken on structures that are separate to the fish containment pen itself. These strategies are already widespread in fish farming and allow fish pen engineering to focus on the needs of fish only. In this respect, I think two crucial design considerations have been lost in the rush to adopt conventional offshore structures:

1. When the ocean moves, fish move with it – the structure must also move.

Conventional offshore structures are designed to attenuate or resist the motion response due to ocean waves. This results in relative motions between the structure and the ocean. In aquaculture, this may have consequences for fish health in storms. From the fish’s frame of reference, nets and structures will be accelerating through their habitat requiring them to swim against accelerating flows to avoid impacts. The freeboard of structures like Havfarm must also be sufficiently large to avoid common ship phenomena like overtopping or “greenwater”, where waves spill onto decks. “Sloshing” in moonpools or overtopping through the ceiling of net structures must be avoided also. In fully open ocean aquaculture where waves can reach 35m, managing this requirement could be extremely onerous for large rigid structures. From the fish’s perspective, it is far better that a fish pen structure flexes and moves with the ocean in so far as that is possible.

2. Structural Loads are dominated by waves:

Hydrodynamic forces are induced on structures when there is relative motion between structural elements and the ocean. While it is always necessary to resist “steady” ocean movements like currents in order to maintain station, it is often the “oscillating” movements from waves that impart by far the greatest structural loads. The secret to cost-effective mooring of structures is to resist steady loads from currents, but have enough remaining “compliance” to allow structures to simply move dynamically in response to waves. Structures such as fish pens are large relative to wavelengths and must themselves flex to follow wave movements. Therefore, there is a huge advantage to structures that can flex in response to wave-induced movements. It reduces, if not eliminates the associated wave loads, such that the structures themselves can be much lighter. Flexible structures also attenuate “shock” loads like mooring snatching as well as wave slamming from breaking waves. On the contrary, for rigid steel structures, there is frequently a design “runaway” problem, where more structure is added to resist dynamic wave loads, the increased structure attracts more wave loads, which in turn requires more structure, and so on. Exponential cost increases relative to flexible structures can result.

These are two key reasons I believe why Akva’s early Polar Cirkel fish cages and others that use HDPE pipes have evolved to be the solution of choice over many rigid alternatives in conventional aquaculture. While surface-reference walkway pipes may seem flimsy in comparison to steel structures, they follow the surface of waves such that nets are never overtopped. They flex and reduce loads and reduce relative motions between nets and accelerating sea flows. They have performed remarkably well, even in moderately exposed ocean locations – more exposed than the current deployment site of Ocean Farm 1, for example.

*Gael Force / Fusion Marine Fish Pen (courtesy twitter @gaelforcefusion )*

While state-of-the-art plastic pen solutions will reach their limit in large, steep, plunging breaker waves, the core principle still stands. Impact-9 works on the assumption that in the same way flexible solutions out-performed rival solutions in early aquaculture developments, this will also be the case as the industry moves further offshore. There are certainly problems to be solved because open ocean conditions imply a step change in environmental loading - but the principle advantages of flexibility remain. In fact, Impact-9 intends to “double-down” on flexible solutions and use such features to enhance not just cost and fish health performance but also other operational aspects. We are encouraged by recent market interactions and are moving to fund detail design work on our first-of-a-kind hardware. We are always keen to work with like-minded partners so please feel free to reach out!