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Abstract

Fish farming operators are seeking suitable offshore sites as an inevitable choice for sustainable and high-quality fish production. However, offshore fish farming has its challenges due to its being a relatively high energy environment with poor accessibility in the more remote sites. Nevertheless, a combination of fish farming with and other marine activities are desirable from an economic viewpoint. The overall infrastructure and operational procedure will no doubt be more complex, and the increased functionalities will bring more risk and require more rigorous assessments for warrants and insurance coverage than solely fish farming activity. More research and developments are is needed in this spacearea.
Offshore Fish Farming: Challenges and Developments in Fish Pen Designs
Authors:
*C.M. Wang (The University of Queensland, cm.wang@uq.edu.au)
Y.I. Chu (Griffith University, y.chu@griffith.edu.au)
J. Baumeister (Griffith University, j.baumeister@griffith.edu.au)
H. Zhang (Griffith University, hong.zhang@griffith.edu.au)
D.S. Jeng (Griffith University, d.jeng@Griffith.edu.au)
N. Abdussamie (University of Tasmania, nagi.abdussamie@utas.edu.au)
* corresponding author
1. Introduction
1.1. Background
Captured fisheries have become unsustainable because most of wild captured species have been
overfished or fully fished with no potential for increase in production. On the other hand, in
recent decades, farmed aquaculture has taken an increasing role in filling the gap between
seafood supply and rising demand as shown in Figure 1 (FAO Report, 2020). A recent report
published by DNV (DNV, 2021) gave an estimation of marine aquaculture production by 2050.
According to the report, among aquatic animals, farmed finfish will dominate marine
aquaculture production (more than 50%) when considering edible weight versus total live
weight. The edible weight for finfish is expected at 14 million tonnes, while crustaceans take
up 7 million tonnes and molluscs at 6 million tonnes by 2050. However, the farmed fish
production has been slowing down due to less nearshore (i.e. sheltered) sea space been licensed
for fish farming, emergence of sea lice problem (especially in Norway, Scotland and Chile
farm sites) and public and environmental oppositions towards expansion of nearshore fish
farms. If this trend is not reversed, farmed fish will not be able to meet global seafood demand.
Figure 1: World captured fisheries and aquaculture production (FAO, 2020)
To date, almost all the marine water fish farms are located at nearshore sites where are
sheltered (in bays, coves and fjords), shallow water depth and hugging the shorelines, mainly
for safe operation and easy access to service facilities such as power supply, feed, hatchery,
storage, maintenance, and fish processing. With an increasing demand for a higher production
target and cost-effective operation of fish production, many suitable nearshore sites have
already fully exploited and most farming pens have reached their allowable fish stock density
(Huguenin, 1997; Stickney, 2002). The current nearshore fish farming practice has led to
conflicts with local communities, conservation and environmental groups. The criticisms
hurled towards nearshore fish farming is environmental degradation due to water pollution,
noise pollution and unsightly appearance (Colbourne, 2005; Noroi et al., 2011; Shainee et al.,
2013; Tidwell, 2012). The competition for common sea space in coastal areas has been
intensified, not only among fish farmers but also with other marine sectors such as shipping,
tourism, conservation and recreation. Moreover, incidents of farmed fish escapement and
spread of diseases have seriously threatened native sea life population (Beveridge, 2008;
Huguenin, 1997; Taranger et al., 2015; Tidwell, 2012; Verhoeven et al., 2018).
In response to environmental concerns and pressures from regulatory authorities, fish
farming companies have started exploring offshore sites in their quest to expand fish production
in a more sustainable and environmental friendly way (Bjelland et al., 2016; Buck, 2007; Holm
et al., 2017; Kankainen and Mikalsen, 2014; Kapetsky et al., 2013). Offshore sites offer more
spacious and pristine sea water and less contests with other sea space users (Holm et al., 2017;
Huguenin, 1997; Kankainen and Mikalsen, 2014; Tidwell, 2012). The offshore environment
with stronger waves, currents and deeper waters helps in waste dispersal and preventing the
accumulation of fish wastes (i.e. uneaten feed or faeces) under fish pens as well as containing
less parasites and diseases. Consequently, fish farming operators are seeking suitable offshore
sites as an inevitable choice for sustainable and high-quality fish production. However,
offshore fish farming has its challenges due to relatively high energy environment with poor
accessibility in the remote sites. These challenges will be discussed further in Section 2.
1.2. Definition of offshore for fish farming
Definition of offshore” for fish farming takes on different forms according to different
stakeholders that include fish farmers, legislators, government agencies, researchers,
technology providers and classification societies. The terms to define offshore” for fish
farming vary and include various parameters such as met-ocean conditions, bathymetry,
geographical distances, technologies used or any combination of these. In addition to the word
offshore”, such words as exposed”, “high energy” and “remote” are also used as synonyms
(Morro et al., 2021).
According to Drumm (2010), offshore aquaculture is defined as taking place in the open sea
with significant exposure to wind and wave actions, and requires equipment and servicing
vessels to survive and operate in severe sea conditions. Spanish law defines offshore as the sea
area outside the straight line joining two major capes or promontories. The sea space within
these capes is correspondingly defined as inshore waters (CabeIIo, 2000). Holmer (2009)
defined three classes for fish farming sites: Class 1 - coastal farming; Class 2 off-coast farming;
and Class 3 - offshore farming, based on physical and hydrodynamic settings as shown in Table
1. It can be seen that distance from shoreline, water depth, and significant wave height are the
key-parameters for defining offshore fish farming. Also shown in Table 2 as referred from the
Norway Standard of Marine fish farms (i.e. NS 9415 (2009)), the Holmer’s Class 1 can be
regarded as moderate exposure sites with respect to the significant wave height, whilst Classes
2 and 3 can be regarded as severe exposure sites. Figure 2 presents the definition of inshore
and offshore waters based on the Spanish law together with Holmer’s three classes for fish
farming.
Table 1: Definitions of coastal, off-coast and offshore farming according to Holmer (2009)
Defining setting
Class 1
Coastal farming
Class 2
Off-coast
farming
Class 3
Offshore farming
Physical
setting
Distance
< 500m from
shore
500m to 3km from
shore
> 3km from shore
Depth
< 10m
10m to 50m
> 50m
Visibility
from shore
Within sight of
shore users
Usually within
sight
Not visible from
shore
Exposure
Significant
wave height
< 1m
3m to 4m
Up to 5m
Accessibility
100%
90%
80%
Legal definitions
Within costal
baseline
National waters
Within coastal
baseline
National waters
Outside coastal
baseline
National/internatio
nal waters
Major countries with fish
farming under the various
classes
China,
Chile,
Norway
Chile,
Norway,
Mediterranean
USA (Hawaii),
Spain (Canaries)
Note: Accessibility <100% refers to limitations in access to the farm due to weather conditions
Table 2: Classification of wave by wave height according to NS 9415 (NS 9415, 2009)
Wave
classes
Peak wave
period Tp (s)
Current
classes
Current
velocity (m/s)
Degree of
exposure
A
0.0-2.0
a
0.0-0.3
Low
B
1.6-3.2
b
0.3-0.5
Moderate
C
2.5-5.1
c
0.5-1.0
Large
D
4.0-6.7
d
1.0-1.5
High
E
5.3-18.0
e
>1.5
Severe
Figure 2: Definition of inshore and offshore waters with Holmer’s classes for fish farming sites
There are also some restrictions related to the selection of offshore site for fish farming. For
example, Cardia and Lovatelli (2016) recommended that the water depth should be at least
three times deeper than the open net pen depth and no less than 15m between the pen bottom
and the seabed. In addition, Kapetsky et al. (2013) pointed out realistic restrictions imposed on
conditions for offshore fish farming such as:
Offshore fish farming should take place within Exclusive Economic Zones (EEZ) (i.e.
up to 200 nautical miles or 370.4 km from the low water mark) in order to ensure
national governance and to provide for the legal protection of investors. Maximum
distance from the coastline to an offshore site is recommended to be 25 nautical miles
(46.3km) for economic feasibility; considering installation and operation as reported by
Jin (2008).
Depth threshold for conventional sea pens is about 25m to 100m based on actual practice
and feasible mooring method and costs.
Current velocity is within 0.1m/s to 1m/s for cultured fish in the confined open net pen
The operation will be dependent on onshore facilities to support offshore grow-out
installation (e.g. feed, holding seed, storage, maintenance, set-up for processing and
transporting harvested fishes)
After studying the various definitions, parameters and viewpoints on offshore fish
farming, the offshore fish farming can be categorized by;
(i) an unsheltered site which is seaward of a straight line joining the closest
two major capes or promontories, and at least about 3km seaward of the
shoreline but within the EEZ,
(ii) a water depth greater than 50 m,
(iii) current velocity within 0.1m/s to 1m/s, and
(iv) significant wave heights exceeding 3m.
Note that the above definition of offshore fish farming is meant for a preliminary
engineering design of offshore fish pen. There are other factors such as environmental,
ecological, other regulatory issues and fish health that have to be considered in the final
offshore fish pen design.
2. Challenges faced by offshore fish farming
There are many challenges associated with moving fish farms to offshore sites which have not
been clearly identified and rigorously studied. As a result, fish farmers are not fully confident
about moving to offshore for fish farming. It is crucial to identify the challenges in offshore
fish farming as they affect running costs, productivity, fish mortality, and HSE (health, safety
and environment) for workers.
An appropriate offshore fish pen design should consider environmental risks associated with
the selected farming site such as wave, current and wind. In addition, offshore fish farming
system should provide not only sufficient farming space but also allowance of stable
positioning for easy operation and maintenance as well as guarantee of fish’s well-being for
optimum growth (Shainee et al., 2013). Occurrences of structural damage: sinking of pens and
failures of mooring system can bring massive fish escapes that threaten biosecurity and
profitability of farming business. Moreover, a relatively poor accessibility in the remote sites
put farming operators in a difficulty situation maintaining facilities, monitoring fish’s
behaviour, and carrying out planned feeding and anti-parasite/disease treatments (Morro et al.,
2021; Taranger et al., 2015).
After a thorough literature review, the following environmental and operational challenges
together with design challenges are identified:
(1) Environmental challenges:
water depth
current velocity
wave action
seabed condition
adverse weather and storms
(2) Operational challenges:
conducive environment for fish welfare
vessel collision with fish pens
marine animal invasion
infrastructure for offshore fish farming (e.g. utility vessels, power supply)
economic sustainability for operations (including material selection)
(3) Design challenges:
lack of experience in designing mega/submerged offshore fish pens
lack of standardized and comprehensive design guidelines/codes
2.1. Environmental challenges
2.1.1. Water depth
Water depth directly affects installation and maintenance costs for the anchoring and mooring
system. The length of the mooring lines is usually three to five times the water depth. Therefore,
deeper water means more costs for anchoring and mooring systems (Cardia and Lovatelli, 2016;
Forster, 2013). The cost for surveying the seabed by using autonomous or remote vehicles at
such larger water depths will also be high. In addition, a greater water depth may influence the
fish growth rate due to a deficiency of illumination and oxygen saturation, or a wide variation
of water temperature.
On the flip side, a large water depth can lessen the concentration of waste sediment in the
area around fish pens (see Figure 3). The deeper water allows for greater dilution potential for
dissolved waste and time for detrital consumers to act on solid waste. Since water gets into the
pen not only through the sides but also through the bottom, keeping the pen bottom clear is
essential to ensure pristine water for fishes (Chacon-Torres et al., 1988). Moreover, deeper
water allows one to have a much taller fish pen that gives more space for fish movement and
lessens the probability of fish disease. It also allows fish to swim to deeper and calmer water
zone during a storm.
Figure 3: Influence of depth in solid waste displacement on seabed below pens (Cardia and Lovatelli,
2016)
2.1.2. Current velocity
Although an adequate current flow is essential for farming fish in pens for replenishment of
oxygen and removal of organic waste, high flow rates may result in detrimental impact on both
pen system and fishes. The maximum drag loads on nets are in most cases caused by current
and not by waves. Especially for a flexible open net pen system, horizontal drag forces exerted
by current on the pen can reduce the internal volume of the pen. These cause excessive strain
on pen collar and increases tension on mooring lines. Moe-Føre et al. (2016) conducted
experimental tests and observed significant volume reductions of fish pen with increasing
current velocity as shown in Figure 4. Klebert et al. (2015) performed a full-scale field
measurement for the current flow field with multiple fish pens. The test results show that the
maximum reduction in volume of the fish pen is up to 30% when the current velocity exceeds
0.6 m/s.
Figure 4: Net pen models subjected to increasing flow velocity (Moe-Føre et al., 2016)
Moreover, under an excessive current flow, fish may spend too much energy on swimming,
as well as suffering from unacceptable losses of feed (Beveridge, 2008). Consequently, fish
growth is curbed and the risk of mortality increases. For example, Solstorm et al. (2015) tested
post-smolts of Atlantic salmon (98.6 g, 22.3 cm) in water velocities ranging from slow (0.04m/s)
to fast (0.33 m/s) over six weeks. They found that fish subjected to fast water velocity showed
5% lower weight gain when compared to fish subjected to moderate and slow velocities. In
practice, current velocities in the range of 0.1m/s to 0.6m/s have been found to be satisfactory
for salmon fish farming (Beveridge, 2008; Faltinsen, 2015; Gowen and Edwards, 1990; Hvas
et al., 2017; Hvas and Oppedal, 2019; Kapetsky et al., 2013; Oldham et al., 2019; Remen et al.,
2016; Yuen et al., 2019).
2.1.3. Wave action
Wave plays a significant role in determining whether fish can be farmed in offshore sites. Some
farmed fish species, whose habitat is in sheltered sites in nature, are not well adapted to living
in high energy wave conditions as they prefer calm and peaceful environment. Although fish
can dive in deeper water where it is relatively calm, they still prefer to be near water surface
for sunlight, oxygen saturation, lower static sea pressure, nutrients/plankton and surface air that
is necessary for fish with swim bladders. Moreover, excessive wave action in offshore sites not
only harms fish’s well-being, but it can damage pen structures and moorings, interrupt a
worker’s routine operation or even place the worker in a hazardous situation.
Recently, several approaches for fish farming have been proposed to control the risk
associated with wave actions. These approaches include having a flexible structure that moves
with the waves, submerging parts or the whole structure, altering the environment condition by
using floating barriers/breakwaters or strengthening and enlarging the structure to withstand
the wave action. For example, submerging fish pens take an evasive action to reduce the effect
of wave load on the structure during bad weather. Liu et al. (2019) carried out physical
experiments and showed that the tension of the mooring rope and the movement of the floating
collar were also significantly reduced as the diving depth increases. However, when the fish
pen reached a certain depth, the attenuation trend tends to stabilize. Based on the results, it was
established that about one third of the depth of the water is the optimal submergence depth for
the fish pen.
A floating breakwater with a sufficient draught can attenuate wave transmission through
mechanisms of either reflection or destruction of water particle orbital motions so that
operational weather windows are lengthened (Beveridge, 2008; Chu and Wang, 2020; Dai et
al., 2018; Kato et al., 1979; Matsunaga et al., 2002; McCartney, 1985). When the farm site is
located at a certain distance from the shoreline and there is a prevailing wave direction, the
farm can be placed in the breakwater’s lee side. If the fish farm is in a sea space with multi-
directional waves, a floating closed breakwater (e.g., circular or octagonal shaped barriers) may
be used to attenuate wave forces in order to create a calm internal water basin for fish farming.
Floating breakwaters are excellent for sheltering offshore fish farms from waves as they are
relatively inexpensive when compared to bottom founded breakwaters. These floating
breakwaters may be moored by using catenary chains in relatively deep water and such
breakwaters do not interfere with currents. They can be readily reconfigured as farms expand
or pens are removed (Kato et al., 1979).
Figure 5 shows the use of the Bridgestone floating breakwater system to protect fish farms
in Japan. Figure 6 shows a porous collar barrier for COSPAR design which was introduced in
a numerical study done by Chu and Wang (2020). According to the study, the porous collar
barrier can be a part of semi-submersible floating fish pens so as it can reduce wave
transmission by at least 60% inside the fish pen.
Figure 5: Bridgestone breakwater systems,
Japan (Kato et al., 1979)
Figure 6: Porous collar barrier (Chu and Wang,
2020)
2.1.4. Seabed condition
The seabed condition in particular affects the mooring system and selection of the anchorage
method. A good anchor provides reliable holding power. Therefore, it is important to know the
sea bottom conditions in order to select the correct type of anchoring system. The continuous
action of seawater dynamic load on the seabed causes the accumulation of pore pressure in the
submarine soil layer. When the accumulated pore pressure exceeds the initial stress, the seabed
will be liquefied and eventually instability and destruction of the soil layer will occur (Jeng,
2018; Wang et al., 2014). Not only might shells and seagrass on upper layers prevent an anchor
from taking hold, but bottom layers with sand, mud, peat or clay require different anchoring
characteristics (Cardia and Lovatelli, 2016). Apart from these, the seabed might include
submarine fibre optic cables, telephone lines or pipelines, explosive areas, or historical
shipwreck sites (Cardia and Lovatelli, 2016). These limitations should be indicated and
considered for pen designs and mooring systems.
Detailed seabed analysis is needed for determining what kind of anchorage method would
be suitable for the site. For example, traditional anchorage (gravity method, see Figure 7) is
suitable at sites where there is adequate deep sediment layer (Kankainen and Mikalsen, 2014).
If the sea bottom is rocky, drilling would be a better method to keep the mooring system at the
site. Echo sounding and sea bottom samples are methods used to evaluate anchorage
(Kankainen and Mikalsen, 2014). Seismic survey will also be required for seabed soil
investigation.
Figure 7: Components of anchoring and mooring system (Cardia and Lovatelli, 2016)
2.1.5. Adverse weather and storms
Infrastructure and equipment failures caused by extreme environmental conditions not only
bring huge losses to investors, but they also bring a high-risk working environment for workers
(Jensen et al., 2010; Mapes, 2017). Storms (hurricanes, or cyclones or typhoons) are
meteorological phenomena that pose a risk to offshore fish farms due to associated strong
winds, resultant waves and currents generated (Beveridge, 2008; Kankainen and Mikalsen,
2014; Kapetsky et al., 2013; Tidwell, 2012). They mostly occur in the tropical-equatorial zones,
i.e. in the region between the Tropic of Cancer and the Tropic of Capricorn, but their incidence
can extend to the North Atlantic and North Pacific (Cardia and Lovatelli, 2016).
A storm surge is a long-term "wave" that can maintain a water level above normal levels
for hours or even days. A storm surge can combine with the astronomical tide to create a storm
tide. The magnitude of the surge is affected by several factors, such as storm intensity,
magnitude, wind speed, approach to the coast and coastal bathymetry (Wamsley et al., 2010).
Although there are few studies on fish pens under extreme sea conditions, we can still draw
some information about storms from other offshore structures.
The occurrence of storms should be rigorously analysed so as to detect an appropriate
offshore fish farming site and to predict the environmental forces for pen designs (Cardia and
Lovatelli, 2016; Huguenin, 1997). Submerged pens are more suitable for areas where there is
a high incidence of storms and extreme weather conditions since wave forces and associating
pitching and surging motion during storms reduce significantly with increasing water depth.
Therefore, the submergibility of offshore pens provides an excellent protection for pens and
fishes against the destructive storm incidence.
2.2. Operational challenges
2.2.1. Conducive environment for fish welfare
Fish demands the best environmental condition for growth. Shainee et al. (2013) listed the key
parameters for the survival needs of fish (see Table 3) under five factors (water quality,
stocking density, feed conversion, less motion and smaller net deflection). These factors and
parameters are very important for the design of fish pens.
Table 3: Design parameters for fish living (Shainee et al., 2013)
Factors
Parameters
1
Water quality
Dissolved oxygen, salinity, temperature, pH, turbidity,
pollution, infestation, biofouling
2
Stocking density
Net volume, dissolved oxygen
3
Feed conversion
Motion, stocking density, feeding frequency, feed type
4
Less motion
Waves, current, wind, pen design
5
Smaller net deflection
Waves, current, pen design
The best quality of water for fish farming is species-dependent, since each type of fish
thrives in a particular water temperature, salinity, dissolved oxygen, pH and turbidity (Pillay,
2004; Stien et al., 2013). However, the optimum design parameters for fish pen designs are
unknown for many fish species (Shainee et al., 2013).
The next important parameter is the stocking density which is dependent on the net water
volume. Sufficient living space is essential for the life of fish. A good pen design should
provide sufficient net volume under a strong current. Therefore, the minimum pen net
deformation due to current loads should be considered in the design to provide sufficient fish
pen volume (Faltinsen, 2015). Huang et al. (2008) found that the current-induced effects on the
net-pen system were more important than those due to waves only. So, they concluded that
farming sites should not be situated in areas where the current velocity exceeds 1 m/s, unless
engineering solutions are found to overcome serious net-pen volume deformation.
Feeding of fish is done daily from dawn to dusk by using a feed distributor that is remotely
controlled. Fish pens must be designed to securely hold the feed distributors so that they do not
get dislodged in the event of storms. In salmon farms, fish gather at the water surface to
consume dry pellets quickly which would otherwise become moist and sink fast and out of
reach from the fish. Note that dry pellets normally contain a high level of fish meal with
enriched nutrients and must be kept less than 10% moisture level and supplied at the water
surface (Lovell, 1989; Pandey, 2018). Therefore, it is important for a fish pen to have calm
surface water in order to reduce feed wastes and keep fish growth at an acceptable level. A new
fish pen design for deployment in energetic offshore sites may thus require an engineering
solution to reduce wave transmission inside the pen.
The environmental forces of the selected site can influence the fish welfare and the integrity
of the pen system. Hence, pen designs must not only be robust enough to survive the strong
environmental forces, but they should also have the means to avoid or dissipate the excess
energy in order to provide a stable and relatively quiet environment for fish to grow well.
Therefore, the challenge is to design a system that copes best with the environmental forces by
means of advanced technology and economically affordable methods (Shainee et al., 2013).
Table 4 presents the optimal growing conditions for various commonly farmed marine fish
species, from the species factsheets given in the Appendix of Le François et al. (2010).
Table 4: Optimal farming condition for farming marine fish species (Le François et al., 2010)
Farming marine
fish species
Rainbow
trout
European
whitefish
Atlantic cod
Barramun
di
Atlantic
salmon
Atlantic,
Southern,
Pacific
Bluefin tuna
Commercial size
0.6 to 0.9
kg
0.8 to 1 kg
3 to 5 kg
0.4 to 3 kg
3 to 7 kg
80 kg
Years to reach
commercial size
10 to 13
months
2 to 3
years from
hatching
2 to 3 years
from hatching
1.5 to 2
years from
hatching
2 to 3 years
from 50 to
100 g smelt
1.52.5 years
from 15 kg
fish
Open pen culture
Yes
(Seawater
pen)
Yes
Yes
Yes
(2x2x2m to
16x16x8m
size of pen)
Yes
(5 to 20 m
deep sea
water)
Yes
(40 m
diameter
floating ring)
Close
containment
culture
Yes
(Indoor/
outdoor
tanks)
Yes
(Pond,
plastic
tanks)
Yes
(land-based
tanks)
Yes
(Pond <2 m
deep)
N.A
N.A
Rearing density
20 to 40
kg/m3
20 to 30
kg/m3
20 to 25 kg/m3
(open pen)
40 to 50 kg/m3
(closed
containment)
15 to 25
kg/m3
< 25 kg/m3
Norway: 5
to 15 kg/m3,
Australia
and Chile: 8
to 10 kg/m3
2 kg/m3
to 5 kg/m3
Optimal
temperature
15 °C
17 to
19 °C
< 15 °C
27 to 36°C
15°C
N/A
2.2.2. Vessel collision with fish pens
Large fish farming facilities in offshore sites are exposed to collision with non-aquaculture
vessels, or aquaculture support vessels such as well-boats and feed-barge (see Figure 8). The
aquaculture support vessels for offshore sites should be much larger than the nearshore fish
farms for their required fish capacity. Therefore, the collision accidents in the offshore sites
can be more catastrophic than those happened in the nearshore sites. The main consequence
from the collision is not only failure of the aquaculture system, but massive fish escapes that
can threaten biosecurity and profitability of the farming business. A strengthening method such
as berthing reinforcement for ships and/or a back-up system are required to protect the fish
farming facilities from such vessel collision accidents. Alternatively, a dynamic positioning
system may be applied in order to prevent accidents associated with interactive motions
between vessels and pens during harvesting or bathing operations.
Figure 8: Steel pen damaged due to ship collision
Photo from: <https://www.salmonexpert.cl/article/sernapesca-y-transmarko-activan-protocolos-de-
accin-por-accidente-en-centro/>
2.2.3. Marine animal invasion
Containing a large number of fish in offshore sites, net pens can attract wild predators such
as whales and sharks which normally do not venture into nearshore sites. To gain access, such
massive predators can damage parts or entire fish pens, thereby causing fish escapees. Figure
9 shows an invasion of whale (9m in length) in a Norwegian fish farm. At about 2m below the
water surface, a hole in net was observed that is damaged by the whale. The hole was covered
immediately to prevent salmons from escaping and the whale was towed out of the pen by
lowering part of the pen wall.
Figure 9: Whales enter the net pen (https://www.fiskeridir.no/Akvakultur/Erfaringsbase/Knoelhval-i-
merd)
Offshore fish pens must therefore be designed to keep out predators by introducing methods
such as acoustic deterrent systems (Croix, 2008) and employing more durable net (e.g. EcoNet)
or double net pen (e.g. Huon’s Fortress Pen). For example, EcoNet (see Figure 10(a)) is
developed by AKVA Group and used it for Ocean Farm 1 project for prevention of net’s wear
and tear. The EcoNet is made from very strong but light weight PET (Polyethylene
Terephthalate) and it has been certified under the Norwegian fish farming standard, NS 9415
(2009), to have a lifetime in the water for up to 14 years. Figure 10(b) shows Huon
Aquaculture’s patented Fortress Pen. This fish pen was developed in response to a need to keep
out predators (e.g. seals) by employing a durable double net system. The net material is made
from ultra-high-molecular-weight polyethylene (UHMWPE), the same material that is also
used in bullet-proof vests, that can withstand extremely high current flows.
(a) EcoNet
(b) Fortress pen
Figure 10: (a) AKVA Group’s EcoNet, (b) Huon Aquaculture's Fortress Pen
Photo from: (a) https://www.akvagroup.com/pen-based-aquaculture/pens-nets/nets-/econet, (b)
https://www.huonaqua.com.au/wp-content/uploads/2017/08/Huon-Fortress-Brochure.pdf
2.2.4. Infrastructure for offshore fish farming (e.g. utility vessels, power supply)
Fish farming has to cater for all stages of production from spawning, rearing fries and
fingerlings, producing mature fish, harvesting and packing. The distance between the farm site
and required land supporting facilities directly affects running costs (Cardia and Lovatelli,
2016). Therefore, Kapetsky et al. (2013) considered 25 nautical miles (46.3 km) as the limit
for economical offshore site development. Aquaculture Forum Bremerhaven reported the
urgent need to plan for a more comprehensive development of water-based infrastructure for
offshore fish farming (Kapetsky et al., 2013; Rosenthal et al., 2012).
Although there are many novel fish pen designs that aim to operate in offshore sites, the
global vessel fleets for the offshore fish farming is not yet sufficient to be fully viable for the
offshore fish farming operation so that there is a delay of industry maturity. Moreover, there is
no international code of practice for aquaculture vessels operating in offshore sites.
Power supply is needed for electrical devices and equipment for monitoring and automated
processes that have become essential for offshore fish farming. However, power supply is not
cheap and easily accessible at offshore sites. The fish farming industry currently relies heavily
on diesel to power operations such as venturation, feeding, lighting, net cleaning, fish bathing
and harvesting. Moreover, some submerged fish pen designs require a substantial ballast mass
to keep the fish pens in deep water. It will consume a lot of power to fill and empty the ballast
water for draught control or water exchange.
2.2.5. Economic sustainability for operation (including material selection)
California Environmental Associates presented the global review of offshore fish aquaculture
in 2018 (CEA, 2018). In the report, it is highlighted that small-scale offshore farming projects
will have a challenging time to become a profitable operation. As it is, these small-scale
offshore projects have high capital costs, have to contend with intense oceanographic
conditions, and have an unclear path to economies of scale. Although massive industrialization
and automation could provide a more profitable business model, the current offshore farming
projects have yet to prove their economic sustainability (CEA, 2018).
Ellingsen and Aanondsen (2006) emphasize that it is not only the quality of fish that is
important to consumers, but also the environmental impact of farming, processing and
transportation are becoming important issues. The performance of fish pen products is an
important part of the design for environmental sustainability. The reliability of product quality
is directly related to the safety of life and property. Poor-quality materials and facilities can
cause potential safety hazards to equipment, resulting in hazards such as broken fish nets. On
the other hand, construction cost is also a consideration for engineering design. From the
perspective of the structure of circular full-floating HDPE (High-density polyethylene) pen
facilities, the main factors affecting the price of pen facilities are the materials and prices of
pen frames and nets, as well as the mooring systems. Balancing safety and engineering cost are
a key part of sustainable development.
2.3. Design challenges
2.3.1. Lack of experience in designing mega/submerged offshore fish pens
Unlike nearshore fish pen designs which have matured over decades, there is very few offshore
fish pen designs for reference. Most offshore fish pen designs are recently developed and
remained a conceptual design stage. Only a few of them (such as Ocean Farm 1 and Havfarm
1) are in the trialling phase. There is little data on their long-term survivability, durability,
maintenance and repair methods, and guarantee of fish’s well-being. Therefore, there are still
many unknown problems associated with the new offshore fish pen designs in particular with
mega/submerged designs. As the development and operation of these offshore fish pen designs
are still in their infancy, offshore and marine engineers face unknown challenges which can
only be revealed in a long-term operation of these fish pens.
2.3.2. Lack of standardized and comprehensive design guidelines/codes
There is lack of design guidelines/codes which can help to create a path forward for building
offshore fish farms that are storm proof, ensure fish well-being and growth, and safe for
workers performing the farming activities.
Within the aquaculture industry, approval of the new fish pen design is necessary before
clients have the confidence to invest in building the fish pens and also for commercialization.
The design approval may be done by maritime classification societies in accordance with their
certifying rules for offshore fish farming. In recent years, some classification societies such as
ABS and DNV officially published certifying rules for offshore fish farming installations in
the following documents:
ABS, 2018, Guide for building and classing: Offshore fish farming installations
DNV, 2017, Rules for classification: Offshore fish farming units and installations
(DNVGL-RU-OI-0503)
The aforementioned certifying rules, however, only cover hull structures, onboard machinery,
and equipment that are not part of the aquaculture systems as is done for certifying the oil and
gas offshore units. The guidelines do not cover primary aquaculture elements such as floating
collars and net pens (made of polymers, concrete or equivalent) and their associated equipment,
feeding and production facilities, feedstock facilities and fish escape prevention devices. They
indicate that aquaculture systems shall be assessed under the jurisdiction of local authorities
which may not exist in some countries.
Owing to lack of standardized and comprehensive guidance for the offshore fish farming
industry, a wide range of fish pen designs has appeared; thereby making it difficult to establish
a single strategy for achieving more cost-effective business model. A design guidance/code for
offshore fish pen designs shall include many considerations such as maintenance methods,
manned or unmanned facilities, risks associated with fish farm sites, marine warranty, special
requirements from operators and legislative issues for utilisation of common ocean space.
3. Recent developments in offshore fish pen designs
3.1. Modified flexible collar pens
Flexible collar pens have been widely used for fish farming in Japan, Western Europe, North
America, South America, New Zealand, Australia. High-density polyethylene (HDPE) is the
most commonly used material in modern industrial fish farming. The main structural elements
of these pens are floatable pipes, which can be assembled in various ways to produce the
floating collar. The pipes are held together by a series of brackets with stanchions and
distributed throughout the entire boundaries to suspend the fish net (Cardia and Lovatelli, 2016).
Table 5 summarizes the flexible fish pens in terms of pros, cons and their suitability for
application in offshore sites.
Table 5: Flexible collar pens - pros and cons and suitability for application in offshore sites
Pros
Cons
Application for offshore sites
high resilience to wave
forces with a long service
life (>10 years),
high resistance to rotting,
weathering and
biofouling,
easily formed into various
configurations and
relatively cheap when
ordered in large volumes,
easily constructed in-land
and towed by boats to
install.
problems with
deformation of the net
due to strong waves and
currents,
twisting and turning
problems of stanchions,
limited walkway access
putting workers in
danger during bad
weather,
difficulty in placing feed
systems due to space
constraint,
needs large service
vessels.
some have shown to survive
storms with significant wave
height (Hs) of 10m,
Possible alterations for
offshore use by featuring
submergibility,
little empirical or theoretical
evidence to offer complete
confirmation of the extreme
sea state and survivability on
a long-term basis.
Tubenet
The most recent development of the flexible collar pen by the AKVA Group ASA is the
Tubenet as shown in Figure 11. The Tubenet system uses a net to keep salmon below the sea
lice layer (top 10m of water layer) and protect salmon from strong waves. A large cylindrical
and tarpaulin-walled passageway, called the “snorkel”, in the centre of the pen protects salmon
from sea lice when they swim to the surface to fill their swim bladders. Feed is delivered by
way of subsurface feeding tubes. Only the centre section, where the salmon surface to refill
their swim bladders, requires bird netting. In the outer ring the salmon are kept 14m below the
surface so that fish can be protected from strong waves. The tarpaulin “tube” extends to a depth
of 14m and the feeders are placed at 13m. The inner cylinder is 60m in circumference. Mowi
ASA (Norway) and AKVA have successfully trialled the Tubenet system and it is expected to
be commercially adopted by fish farmers in Scotland and Norway.
Figure 11: Tubenet pen
Photo from: <https://www.fishfarmingexpert.com/article/mowi-goes-deep-to-beat-lice-problem/>
3.2. Submerged fish pens
In order to avoid strong surface waves, submerged fish pens are proposed. The pens are
submerged to a suitable water depth below from the hazardous upper water column. A
hypothesis of the submerged fish pens is that fish welfare and production efficiency will be as
good or even better in the submerged position as in the surface position where high energetic
waves are present. These submerged pens may be raised temporarily to the surface for
necessary maintenance requirements and for fish harvesting. Table 6 summarizes the pros and
cons of submerged pens as well as their suitability for deployment in offshore sites.
Table 6: Submerged pens pros and cons and suitability for application in offshore sites
Advantages
Disadvantages
Application for offshore
sites
either be unattended by
surface units, accessed
only when needed, or
remotely controlled,
best features to avoid
surface debris and
effects of storms,
structural strength does
not need to be as great
as surface structures.
a lack of visibility in normal
operation,
relatively complex to
operate due to its submerged
mode and maintenance and
operating services are
difficult,
operating costs may be
relatively higher than
surface mode structures.
unknown submerged pens
being deployed in offshore
sites,
submerged operation is in
question to compete surface
operation.
Below, present various submerged fish pen designs that have been proposed or built.
Atlantis
Atlantis Subsea Farming (see Figure 12), the cooperative AKVA group, has developed a
submerged offshore fish pen with flexible collars and a compressed air chamber for large-scale
salmon production. This submerged pen has circumference of up to 160 m. Air and fish feed
can be added through hoses from supply vessel.
Figure 12: Atlantis subsea farming
Photo from: https://www.atlantisfarming.no/nyheter/atlantis-subsea-farming-a-farming-concept-
designed-for-the-future
Giant Offshore
Giant Offshore pens are designed for exposed localities with the aim of minimizing the risk of
escape, better protection against salmon lice and reducing point discharges of nutrients.
Operations and monitoring are performed in full from integrated base vessels. The pen is
designed in a flexible material, with the strength to withstand being more exposed than today’s
conventional nearshore fish farms.
Giant Offshore is a 500 m long cylindrical construction with pointed ends, where the middle
section is 300 meters long and 40 meters in diameter. The middle section has five large net
bags which together make up the farming volume of 290,000 m3, where it is possible to produce
2.2 million salmon. The construction is designed with a large distance and securing between
the net bags and mooring as well as foreign bodies, with the intention of reducing the risk of
damage to the net and minimizing the risk of escape.
Figure 13: Giant Offshore
Photo from: http://www.giganteoffshore.no/
AquaPod
AquaPod (see Figure 14) was developed by Ocean Farm Technologies in the United States. It
has a two-point anchor for mooring and some operational advances such as net cleaning and
removal of dead fish. It is located 8 miles offshore in a water depth of 45m, strong currents and
waves. It has already been proven to withstand waves up to 10m high. The structure is made
from recycled polyethylene plastic with fibre glass reinforcement. The reason for its geodesic
shape is that it has the least surface area possible for the volume it contains, and it makes the
pen completely predator proof. Future pods are equipped with a propeller mechanism and GPS
so that they can be used as transporting vessels that carry juvenile fish and arrive at the desired
location with fish that are ready to harvest.
Figure 14: Submerged AquaPod pen from Ocean Farm Technologies
Photo from: < https://www.wired.co.uk/article/aquapod-sustainable-fish-farm >
NSENGI fish pen
NSENGI (Nippon Steel & Sumikin Engineering Co., Ltd) has investigated the co-location of
an existing offshore platform and a fish farm. It has carried out offshore verification testing of
large-scale seabed resting pens at a salmon farm which is 3 km from shoreline of Sakaiminato,
Tottori Prefecture, Japan. Each pen has a volume of 50,000 m3. The fish pen is designed for
the following environmental conditions: significant wave height of 7 to 9m corresponding to
the wave period of 10 to 16s, a current velocity of 2 knots and a water depth of 60m. The pens
are suitable for farming fish species such as Seriola (Japanese amberjack) and Coho salmon.
The pens are serviced by a jack-up platform that houses the equipment and feedstock storage
facility for automated feeding of the fish (Figure 15).
Figure 15: Sinking fish pens
Photo from: < https://www.eng.nipponsteel.com/english/news/2016/20161003.html >
3.3. Novel offshore fish pen designs
In recent years, a few global fish farming companies have developed novel designs of offshore
fish pens and built them for trial tests at selected offshore sites. Norwegian fish farm operators
are leading the way to offshore in order to resolve problems related to parasites (e.g., sea lice)
occurred by water eutrophication, as well as expand salmon production that is expected to be
five times more by 2050. In order to induce innovation and development of next generation
fish pen designs for offshore use, the Norwegian government released a development scheme
that is financially lucrative for fish farm operators. Chinese fish farms also seek offshore sites
because of the need to expand seafood production in order to meet their exponentially
increasing domestic seafood consumption. China has the advantage of possessing a relatively
good construction infrastructure that allows to fabricate mega-size fish pen projects at a
relatively low cost. According to China’s Ministry of Agriculture “National Marine Ranch
Demonstration Zone Construction Planning (2017-2025)”, China plans to develop 2500 km2
of national fish farming waters by 2025. Several hundreds of offshore fish farming facilities
are expected to be deployed in the Chinese offshore sites. Construction of these offshore
facilities will reduce the scale of commercial fishing and restore ocean resources.
Offshore fish pen designs may be divided into two main streams; open net pen system and
the closed containment tank system based on fish containment methods (Chu et al., 2020). Both
systems will be discussed in detail in the following sections.
3.3.1. Open net pen system
The open net pen system is the most widely used for marine fish farming. These net-based pens
are generally slender structures, with a low mass when compared to the size of the structure.
They have a large damping-to-mass ratio, and this can effectively eliminate resonance
problems. Depending on net holding methods, open net pen system can be either deformable
(flexible type) or robust (rigid type). However recent offshore fish pen designs mostly adopted
the rigid type so as to withstand strong wave and current with providing a sufficient farming
volume. Table 7 presents features of the rigid open net pen system in views of its pros, cons
and application for offshore sites.
Table 7: Rigid open net system pros and cons and suitability for application in offshore sites
Pros
Cons
Application for offshore
sites
stable working platform
for all husbandry and
management operations,
potential for integral
feeding and harvesting
systems,
construction and repair
facilities may be done in
conventional shipyards,
If rigid frames are used,
they maintain farming
volumes and keep fish in
place,
need for large and heavy
structures,
requires good port
facilities and/or expensive
towing to install,
susceptibility to structural
failure in extreme
conditions,
large masses require
heavier mooring systems,
involves relatively high
capital costs,
rigorous engineering
deployed at some exposed
offshore sites where an
occurrence of extreme
storms is rare,
proto-type testing
performed in deployed
cage of Ocean Farm1 and
Shenlan1,
Despite not enough
operating records, it is the
most progressed type for
farming fish in offshore
sites,
Relatively small vertical
motions due to large mass,
Low natural frequency
avoiding wave resonance.
analyses, design and high-
quality control in
construction is essential to
ensure safety in offshore
operation.
Reported some failures
related to human error and
adverse weather.
Some examples of the open net pen system for offshore use are described and shown in
Figures 16 to 23.
Ocean Farm 1
SalMar Group, a Norwegian fish farm operator, developed Ocean Farm 1 (see Figure 16).
Ocean Farm 1 is a result of robust technology and principles used in submersible offshore units.
It is a full-scale pilot offshore fish pen deployed about 5km off the coast of central Norway.
With a diameter of 110m and volume of 250,000 m3, the pen is able to accommodate 1.5 million
salmons (Zhao et al., 2019). It is intended for offshore installation in water 100m to 300m in
depth with a 25-year lifespan. It has more than 20,000 sensors and over 100 monitors and
control units. It can be immersed in deep water by filling the ballast tanks and is moored by
eight lines tied at fairleads placed at the lower part of the side vertical columns. It uses Eco-
net, developed by AKVA Group by using PET (polyethylene terephthalate), which has a hard
surface that resists marine fouling and makes it easy to clean in the water, as well as improving
durability and preventing fish escape.
Figure 16: Ocean Farm 1 (Photo courtesy of Charles Lim)
Smart Fish Farm
SalMar group is planning to build an upgrade version of Ocean Farm 1 which is even a larger
fish pen with a diameter of 160m and can accommodate three million salmon and produce
23,000 tonnes round weight. It has been named as Smart Fish Farm (see Figure 17) that will
have five to ten units in the first phase. The first unit is expected to have an investment level
of EUR 225 million, falling to EUR 147-200 million for additional units. The product cost is
expected at EUR 3.6/kilo of salmon for the first unit, falling to EUR 3.3 at several units. SalMar
expects production start-up of Smart Fish Farm in the second quarter of 2024, based on
approval of sites and volumes by summer 2021 and enters into a construction contract during
the fourth quarter of 2021.
Figure 17: Smart Fish Farm
Photo from: <https://salmonbusiness.com/gustav-witzoe-our-biggest-challenger-is-land-based/>
Shenlan
Shenlan 1 and Shenlan 2 were developed for salmon and trout farming about 130 nautical miles
off the shore of Rizhao in east China’s Shandong province. Shenlan 1 has already been
deployed at the site and it has a diameter of 60m and is 35m in height; it is able to culture
300,000 salmons. A noticeable feature, that differentiates it from Ocean Farm 1, is the presence
of a centre oscillating buoy to generate power for fish farm operations (Figure 18). Shenlan 2
is in the planning stage. It will have a 60m diameter and a height of 80m and it will be able to
accommodate about 1 million salmons.
Figure 18: Shenlan 1
Photo from: <https://www.swissre.com/reinsurance/property-and-
casualty/reinsurance/marine/offshore-fish-farming-facilities-challenges-marine-insurers.html>
Havfarm 1
Havfarm 1 is the world’s longest fish farm, 385m long and 59.5m wide and has a capacity for
10,000 tonnes of salmon (over 2 million fish). Constructed in the Yantai Shipyard, China for
Norwegian farmer Nordlaks, Havfarm 1 comprises a steel frame for six 47m x 47m pens made
of HDPE mixed with copper (from Garware, India), with open nets at 60 m depth (Figure 19).
The facilities are designed to withstand 10m significant wave height. Havfarm 1 is moored
with 11 anchors, each weighing 22 tonnes and each anchor has a holding power between 300
and 450 tonne. It is sited 5 km south-west of Hadseloya in Hadsel municipality in Vesteralen,
Norway. A 7 km long subsea power cable supplies power to the Havfarm 1 from the shore.
Havefarm 1 is a vessel-shaped fish farm concept which intends to minimize the wave loads
coming from the bow in open sea sites. A single-point mooring system is employed for the
vessel-shaped fish farm concept to allow the entire fish farm to rotate freely in order to reduce
the environmental loads on the structure and improve fish wastes dispersion. In Li et al. (2017)
study, the hydrodynamic properties of a basic geometry of the vessel hull were obtained from
the frequency-domain analysis. Time-domain simulations were performed by coupling the hull
with the mooring system. Mobron et al. (2020) developed a method to estimate the fatigue
utilization by using Dynamic Amplification Factor for a fatigue assessment of Havfarm 1
design.
Figure 19: Havfarm 1
Photo from: <https://www.fishfarmingexpert.com/article/in-the-shadow-of-the-giant-watch-a-video-
of-the-havfarm/>
Zhenyu 1
Zhenyu 1 aquaculture platform (Figure 20) was constructed in Lianjiang County, Fuzhou City,
Fujian Province and launched in 2019. It is located in the sea area of Dinghai Bay, Fujian
Province. The platform is olive-shaped, with a total length of 60m and a width of 30m. The
aquaculture water body reaches 130 million cubic meters. It is expected to produce 120 tonnes
of high-quality commercial marine fish annually. It is mainly composed of a floating body
structure, an aquaculture frame, and a rotating mechanism. Zhengyu 1 was designed based on
a concept for preventing biofouling. The pen can be rotated about the horizontal axis so that
the underwater bio-fouled part of the pen can be brought above the water surface for cleaning.
There is a wind turbine to supply power.
Figure 20: Zhenyu 1 aquaculture platform
Photo from: <http://cn.zpmc.com/news/cont.aspx?id=1296>
Viewpoint Seafarm and Spider cage
Nova Sea AS, a Norwegian company, designed two innovative concepts: the Viewpoint
Seafarm (see Figure 21) and the Spider cage (see Figure 22) for offshore fish farming based on
the semi-submersible technology. Viewpoint Seafarm comprises a hub on a semi-submersible
platform and four floating net pens connected to the semi-submersible by a hinge system. Each
floater has a projected area of 50m x 35m. Scale testing has been done with 11m significant
wave height and the system showed a stable motion response (Lindeboom, 2018).
The Spider Pen has a dedicated 100m diameter circular barrier, with an outer steel ring and
another ring inside for heave compensation. It is designed to shield the actual fish pen from
heavy sea conditions and sea lice. The design has been tested up to a significant wave height
of 11m with and without current, where general motions, accelerations, loads and sloshing have
been accessed (Lindeboom, 2018).
Figure 21: Viewpoint Seafarm
Photo from: <https://www.viewpointaqua.no/seafarm/>
Figure 22: Spider cage
Photo from: <https://www.viewpointaqua.no/spidercage/>
De Mass SSFF150 pen
De Maas SMC, a firm operating in the offshore oil and gas services industry, is partnering the
Chinese government to build a $151million deep-water aquaculture farm off the coast of China.
De Maas will design and build five SSFF150 (Semi-submersible Spar Fish Farm) pens; each
is 139m in diameter and 12m high (see Figure 23). The central tower will house machinery
spaces, feed storage and provides accommodation for operators. By submerging underwater,
the pen is able to be protected from storms.
Figure 23: De Maas SSFF150 pen
Photo from: <http://www.demaas-smc.com/>
Impact-9 submersible salmon pen
The Irish company Impact-9 invented a submersible fish pen design which can survive storms
based on an innovative flexible design that allows salmon production at offshore sites (see
Figure 24(a)). The fish pen, of 90 m in diameter and 125,000 m3 in volume, can produce 3,000
tonnes of salmon in every 12 month grow-out cycle. The design considers the environmental
conditions of Scottish marine sites that are over 60 m deep. Impact-9’s salmon pen design is
based around a central, strong backbone structure, which is centrally moored with a single-
point mooring. It will provide a safe working platform and a home for feed and farming
equipment. Each net is suspended from the backbone and includes an inflatable collar near the
base. The pen would alleviate sealice and algal bloom problems by keeping salmon six metres
below the surface or more if required. Adoption of inflatable beam collars (made of HDPE)
into the structure allows operational innovations and flexibility (see Figure 24(b)) so that the
design will achieve inexpensive access to offshore zones, a lot less operational cost on divers,
less dependence on well boats, and resistance to sealice and algal blooms.
(a)
(b)
Figure 24: (a) Impact-9 submersible salmon pen, (b) diagram showing deployment of Impact-9 pen
system
Photo from : < https://www.fishfarmingexpert.com/article/taking-the-plunge-a-submersible-cage-for-
scotland/>, <https://thefishsite.com/articles/novel-offshore-fish-farm-edges-closer-to-commercial-
reality>
3.3.2. Closed containment tank system
In order to control the water quality and the production process, closed containment fish tanks
have been introduced in the 1990s (Beveridge, 2008). These closed containment fish tanks,
also known as recirculating aquaculture systems (RAS), are generally sited on land and involve
a recirculation system for the water where internal water is constantly filtered (Tidwell, 2012).
Such a system is efficient with freshwater, but when dealing with saltwater, there is production
of sulfates at the tank bottom that kill fish if the tank is not properly cleaned.
Floating closed containment tanks for offshore fish farming are recent developments
prompted by the need to protect the internal environment from negative external factors. These
tanks contain water that is constantly refreshed using a flow-through system. The water that is
pumped into the containment is controlled up to the desired requirements of the internal
environment. This may include, extracting water from greater depth, filtering, oxygenation or
other water treatments. Water that flows out of the system may also be required to be treated
before discharging. So far, floating closed containment tanks for fish farming have been
deployed in benign waters. For use in exposed sites where there are stronger waves and currents,
the closed containment tanks have to be designed to mitigate sloshing of the water in the tank
to ensure the well-being of the fish. Current developments are presented below regardless their
use for nearshore or offshore fish farming.
Table 8 summarizes the floating closed containment tank systems in terms of pros and cons
and their applications in offshore sites.
Table 8: Floating closed containment tanks - pros and cons and suitability for application in offshore
sites
Advantages
Disadvantages
Application for
offshore sites
Has control over water
replacement so that water can be
constantly disinfected to remove
pathogenic organisms,
external environmental events like
algae bloom is no longer a
problem,
organic wastes can be removed by
biofiltration system before
discharging the water back to the
sea,
threat of predators (such as sharks
and seals) is completely
eliminated,
achieve a higher production rate
when it compared to the open pen
system.
Requires a power supply
system to deploy in offshore
sites,
maybe too expensive to
bring power from land for
offshore fish farming,
requires significant
construction and equipment
costs, more management
demands for monitoring and
intervention,
detrimental sloshing effect
to both structure and fish by
the contained water.
It is still
unknown as to
whether closed
containment
tank can be
deployed in
offshore sites,
several
challenging
issues are raised
such as sloshing,
swirling and
power resource.
Figures 25 to 34 show some recent developments of the floating closed containment tanks for
fish farming.
Fish farm egg
The fish farm egg concept (see Figure 25), developed by “Hauge Aqua” uses a fully enclosed
egg-shaped structure. The water flow enables the system to draw inlet water segregated from
where outlet water is released. Water enters by the use of two main pumps that suck water from
20m below the bottom of the structure. The water quality and volume can be controlled,
ensuring steady oxygen levels. It is estimated to cost about NOK 600 million (about USD 60
million).
Figure 25: Closed fish farm concept “fish farm egg”
Photo from: < http://sysla.no/fisk/skal-bruke-600-mill-pa-lukkedeoppdrettsegg/ >
Neptun
The Neptun was developed by Aquafarm Equipment (Figure 26(a)). The tank has an internal
diameter of 40m, a circumference of 126m, a depth of 22m and a gross volume of 21,000m3.
The tank is designed against a wind speed of 30m/s, and a current velocity of 1.0 m/s and its
design life is 25 years. Figure 26(b) shows an internal view of the tank with inlet and outlet
holes for water circulation. The tank is made from Glass Fibre Reinforced Polymer (GFRP)
elements and reinforced with steel in areas that bear the most stress. The design also includes
a pump system to extract large volumes of water from a depth of 25m or more. As the concept
of the containment tank is to collect dead fish, fish waste and uneaten fish feed from the sloped
bottom, there is a flexible pipeline that connects the lowest point to the waste separator.
(a)
(b)
Figure 26: (a) Neptun closed containment fish tank, (b) Internal view of Neptun semi-closed
containment fish tank
Photo from: < http://aquafarm.no/closed-pen/ >
Salmon Home 1
Dr. Techn. Olav Olsen, a Norway based marine technology consulting company, proposed a
closed containment concrete tank, named Salmon Home 1, for offshore farming (see Figure
27). The cylindrical concrete tank has a 14.8m inner diameter, 16.5m outer diameter and 6m
height providing 1000 m3 for salmon farming. It uses an existing mooring system such as a
system of 2x4 or 2x8 nets in a rectangular array. It has a sloping bottom for easy collection of
organic wastes (Olsen, 2020). For a larger tank, about 16,000 m3, in water depth of 80m and
significant wave height of 2.5m, it is proposed that the mooring system comprises eight 48mm
chains.
(a)
(b)
Figure 27: (a) Salmon Home 1, (b) Salmon Home 1 on site
(Photo courtesy of Tor Ole Olsen)
Eco-Ark
AME2 Pte Ltd, a Singapore based company, has developed a closed-containment flow-through
floating fish farm called the Eco-Ark as shown in Figure 28(a). It has four large tanks of 500m3
and is about the size of an Olympic swimming pool. It can produce 166 tonnes of fish annually,
and enables sustainable farming out at sea in volumes that are 20 times more than average
minimum production levels at traditional coastal farms. It has a flow-through water supply
system. It has a roof equipped with solar panels to supply electricity for the fish farm (Leow
and Tan, 2019).
The following environmental conditions were used for designing the Eco-Ark: wind speed
15m/s, significant wave height 0.5m, current velocity 1.2m/s, and water depth 10m. The
mooring system adopted is a spud housing on the port and starboard of the hull. The spud
housing allows for self-installation and removal for quick mobilisation and demobilisation
when required. A total of six spuds with a diameter of 762mm and length of 25m are installed
at the site in order to keep the Eco-Ark in position. The total weight of Eco-Ark is 5300 tonnes.
The Eco-Ark was constructed in Batam Island, Indonesia and was deployed in the northern
coast of Singapore near Pulau Ubin in September 2019. Eco-Ark was designed, constructed
and surveyed to the rule of classification society Bureau Veritas as a special service floating
fish farm and is fully insured for H&M and also has Third Party Liabilities insurance for up to
$500 million.
The Eco-Ark allows augmentation and integration by forming a fleet connected to a lift-
dock facility that enables one to cultivate and process fish on site (see Figure 28 (b)).
(a)
(b)
Figure 28: (a) Eco-Ark closed containment system, (b) Eco-Ark fleet connected to lift-dock
(Photo courtesy of Mr Ban Tat Leow)
Marine Donut
The Norwegian salmon farmer, Marine Harvest ASA now known as Mowi ASA, developed a
closed containment design named the Marine Donut. The concept is owned by ODP Donut
Solutions (see Figure 29). The Marine Donut is a tube made out of HDPE with a volume of
20,000m3 and has a capacity of approximately 1000 tonnes of biomass. The design should be
able to withstand a significant wave height of 3m. Water is taken in via six inlet pipes that
reach below the sea lice barrier and water is continuously circulated inside the donut. In 2019,
Norway’s Directorate of Fisheries granted permission for 1,100 tonnes of biomass to be used
to test the design.
Figure 29: Marine donut Close containment concept design of Marine harvest
Photo from: < http://marin.bergen-chamber.no/en/teknologi/Growth-through-innovation/ >
Stadion Laks
Stadion Laks, is a bathtub-shaped floating aquaculture system made out of reinforced concrete
that holds 34.000m3 of water. It is designed for a stock density of 50-75 kg/m3 and should be
able to accommodate smolts and post-smolts up to harvest size 4-6kg fish. Water is pumped
from below the sea lice barrier, at least 20m depth, and is circulated. Various installations and
management systems are in place to secure water quality to have top health of the fish (see
Figure 30). Construction starts in 2020 and first fish in the pool is planned in 2022.
Figure 30: Stadion Laks concept
Photo from: <https://stadionlaks.no/en/home/>
Preline
Preline Fishfarming Systems AS has developed a concept that is closely related to a raceway
system as described by Tidwell (2012). Water is not circulated but runs though the containment.
It is a 40m long oval HDPE tube with a volume of 2000m3 that is suspended at the water surface.
It is designed to hold 100k-200k salmon of up to 1 kg in weight. Slots in the tube that extend
above the free surface allow the fish to reach for air. At the bottom, waste collection traps are
installed. On the two far ends, it holds two tubes that reach down below the sea lice barrier.
Figure 31 shows the artist impression of the Preline. The right-hand tube is the inlet and the
other the outlet. Pumps ensure a water velocity of roughly 0.15m/s through the system. Fish
are kept in the horizontal part of the system and internal net boundaries are put in place to
ensure fish do not escape via the inlet or outlet.
Figure 31: Artist impression of the Preline
Photo from: <http://www.preline.no/>
FishGLOBE
The FishGLOBE fish pen has been developed in order to reduce the required sea space,
minimize production cost and have minimal negative impact on the environment. When viewed
from the water surface, the FishGLOBE appears as a floating iceberg (see Figure 32 ). The
latest model is the FiskGLOBE K10 with a total volume of 30,000 m3 and a biomass capacity
of 2000tonnes. The structure has a total height of 35m with a cylindrical shape. The inlet and
outlet pipes together with the central pipe are a part of load bearing structure by increasing its
stiffness. The FishGLOBE can withstand waves with a significant wave height of up to 2.5m
and currents up to 1.0m/s. Oxygen and large quantities of water are supplied by pumping from
below the structure where it is free from sea lice. The interior can be airtight. By increasing
internal air pressure, water can be pressed out to gently transport the fish out of the tank. An
internal current can be introduced to converge the waste as shown in a study by Gorle et al.
(2018) through CFD simulations.
Figure 32: Fish globe deployed in Norway
Photo from: <https://www.fishglobe.no/>
Eco Cage
Serge Ferrari, a France base company, has developed a composite fabric that is suitable for
flexible bag-type closed containment aquaculture systems and it is called Biobrane Aqua. They
have a range available to suit different purposes and environmental loads. The Biobrane Aqua
2050 is applied in the Eco Cage, which is produced by EcoMerden AS in Norway (see Figure
33). The construction consists of three main parts: (1) a steel circular shaped collar ring which
holds the heavy-duty flexible wall; (2) the flexible wall creating an enclosed internal
environment; and (3) an internal net inside the fabric where the fish reside.
Figure 33: Eco Cage - a flexible type closed containment aqua culture systems
Photo from: < https://www.ecomerden.com/>
FiiZK
Another flexible closed-containment aquaculture system that uses the Biobrane Aqua 2050 is
a concept developed by the Norwegian-based company FiiZK AS. They deliver a flexible
closed-containment solution for the aqua culture industry. Designs in different sizes are
possible (see Figure 34). Water is drawn from greater depths by extended pipes reaching below
sea lice barrier. Before the water is pumped into the containment it is oxygenated. Waste is
collected at the bottom of the pen, from where it is removed and processed externally.
Figure 34: Flexible closed containment aqua culture systems designed by FiiZK
Photo: <https://fiizk.com/en/product/closed-cage/>
3.4. Key observations with regard to recent offshore fish pen developments
Most offshore fish pen designs are at the conceptual stage. There are, however, a few real
scale proto-type designs which have been built (e.g. Ocean Farm 1, Shenlan 1 and Havfarm
1).
To date, operation of offshore fish farms is still in its infancy. For example, Ocean Farm 1
and Shenlan 1 started operation only a few years ago!
Instead of an array of small pens in nearshore fish farms, offshore fish farms tend to have a
single “mega” fish pen that can accommodate more than a million fish.
Instead of flexible pens and fabric nets, offshore fish pens involve the use of rigid frames
and stiffer nets (e.g. PET or metal) for robustness.
Submerged pen is a good solution for offshore fish farms as it avoids strong surface waves
and its structural strength does not need to be as great as surface structures.
Offshore fish pens are commonly equipped with remote and autonomous devices for
operation, maintenance, monitoring and surveillance (feeding, venturation, lighting,
cleaning, and removal of wastes).
Offshore fish pens that are sited at a considerable distance from the shoreline will have to
tap on wind, wave and solar energies for power supply.
The existing submerged pens and closed containment tank designs are small in size.
Closed containment tank designs have only been deployed in nearshore sites.
4. Integration or co-location of offshore fish farm with other marine sectors
Offshore fish farming would have to generate high economic value in a short period for quick
return of investment. Therefore, additional functionalities to the fish pens are necessary. These
include integrating or co-locating the offshore fish pens with renewable energy production, or
involving cruise tourism, maritime transport and leisure and entertainment activities that can
bring added income streams.
The renewable energy sector is actively seeking offshore sites for capturing strong and
sustainable natural energy resources and expecting less societal impact from inconspicuous
offshore locations than nearshore locations. In general, offshore renewable energy refers to the
generation of electricity from ocean-based resources including winds, waves, tides, and salinity
and thermal properties as well as conversion of generated electricity to hydrogen (Wiersma and
Devine-Wright, 2014). In order to succeed in the offshore renewable energy business, it is
necessary to suggest a stable but cost-effective floating substructure and mooring system that
should be robust enough to withstand the highly energetic environments, and save capital &
operating costs including power delivery cost from the long-distance sites to end users.
There is a great potential for collaboration between the aquaculture and renewable energy
sectors as they share a common challenge to explore more energetic and exposed localities to
sustain production growth (Weiss et al., 2018). By having an integrated and/or co-located
solution for offshore fish farming and renewable energy harvest, not only the fish farms can be
remotely and autonomously operated via clean energy, but the renewable energy partners can
also save capital costs by sharing of the substructure and mooring system, and reduce power
delivery cost through direct power supply to the end users (i.e., fish farms). Both industry
partners can leverage their profit significantly and synergetic benefits are expected to attract
other marine sectors by suggesting better utilization of sea space, and reduction of service and
maintenance costs by sharing labour, transportation, monitoring and operational control
(Sakellariadou and Kostopoulou, 2015).
CAPEX (Capital Expenditures) and OPEX (Operational Expenditure) for offshore fish
farms are certainly much higher than for nearshore fish farms. Co-location or integration of
offshore fish farm with other marine sectors will allow sharing maintenance and support ships
(e.g. utility vessels, work boats, general cargo ships and barges) that are required to operate
multiple activities (Holm et al., 2017; Kaiser et al., 2011). Moreover, offshore renewable
energy facilities can provide freshwater via a desalination process, and hydrogen power and
oxygen for fish pens via water splitting (Papandroulakis et al., 2017; Wang et al., 2019). The
floating platforms will be able to accommodate fish feed silos and equipment so that
transportation costs and service vessels for fish feed delivery can be reduced.
Below are some examples of an integrated offshore fish farm and renewable energy
production facility.
PLOCAN
The Oceanic Platform of the Canary Islands (PLOCAN) as shown in Figure 35 is sited in water
depths ranging from 40m to 200m and 1.5 km from the coast of the island of Gran Canaria,
Spain. The PLOCAN comprises multidisciplinary laboratories for analysing bio-geo-chemical
variables in the water column. It has a main deck area with an office building and a helideck.
It carries equipment for loading and unloading the material for experimental testing, and other
general-purpose facilities such as workshops, cranes and other equipment providing basic
operational support. This facility also aims to encourage development of offshore aquaculture,
offshore wind turbines and other marine structures.
Figure 35: Oceanic Platform of the Canary Islands (PLOCAN)
Photo from: < https://steemit.com/steemstem/@geronimo14/plocan-a-boost-to-the-blue-economy>
Blue Growth Farm
The Blue Growth Farm project is EU’s ambitious project to produce advanced industrial
knowledge in a fully integrated and efficient offshore multipurpose floating platform. This
platform provides a central protected pool to farm fish, as well as a large storage and deck areas
to host a commercial 10MW wind turbine and a number of wave energy converters (WEC).
Figure 36 shows a 1/15 scale of the Blue Growth Farm deployed at a site near the port of Reggio
Calabris in Italy.
Figure 36: Blue Growth Farm Project’s Multipurpose floating platform
Photo courtesy from Prof Maurizio Collu of Strathclude University
GIEC’s semi-submersible wave powered aquaculture pen
An integrated offshore renewable energy facility, water desalination plant and fish farms has
been implemented by the Guangzhou Institute of Energy Conversion (GIEC). Figure 37 shows
GIEC’s semi-submersible wave powered aquaculture pen with seawater desalination plant on
board and solar panel roof. This multipurpose open sea aquaculture platform is named the
Penghu platform.
Figure 37: Penghu open sea aquaculture platform (Photo courtesy of Mr Ban Tat Leow)
Hex Box
Ocean Aquafarms developed a new concept of offshore salmon farm named the Hex Box as
shown in Figure 38. The concept is able to operate at sites with a significant wave height of up
to 10m, a water depth greater than 100m and a wind speed of up to 100 knots; thereby allowing
salmon farming in areas that are inaccessible with today’s technology. The Hex Box uses a
ballasting system to be able to change the draft from 4m to 30m for inspection and replacement.
It has a 275m circumference providing a net submerged volume of 430,000 m3. The net bag
can be suspended in winches further 20m below the frame structure. For Australia and New
Zeeland, the pen can be armed with a full double net against predators. It is equipped with two
or three deck cranes for operation. In addition to two diesel generators for power supply, the
Hex Box carries three wind turbines (3x100kW) with batteries to reduce usage of hydrocarbon
fuel. The mooring system comprises 6 to 9 mooring ropes with fixed anchor points in the
seabed. The overall mooring system, including ropes, has been demonstrated to meet
Norwegian requirements by over three times. A scaled model has been tested and showed
promising results (SalmonBusiness, 2020). The construction cost of the Hex Box is
approximately USD70 to 90 million.
Figure 38: Hex Box offshore salmon farm by Ocean Aquafarms
Photo from: <http://www.oceanaquafarms.com/product/hex-box-norway-2/>
FOWT-SFFC
Zheng and Lei (2018) presented an integrated design of a Floating Offshore Wind Turbine and
a conical Steel Fish-Farming Pen (FOWT-SFFC) as shown in Figure 39. The integrated pen
design is able to generate multi-megawatt power and encloses a 200,000 m3 volume of seawater
for farming fish. The inner space of the pen can be subdivided into eight sectors to raise a
variety of fishes. In order to balance the gravity with buoyancy, high density concrete is placed
in the radial and ring pontoons for ballast. The bottom net is attached to lifting devices inside
the pen so that can be moved vertically from the bottom to water surface for harvesting. Nets
are made of copper alloy to resist seawater corrosion and biofouling.
Figure 39: Illustration of concept of FOWT-SFFC (Zheng and Lei, 2018)
COSPAR
Chu and Wang (2019) proposed a combined design of a spar platform and a fish pen with a
partially porous collar barrier, named COSPAR fish pen (see Figure 40). The pen design
features an octagonal shape with a partially porous collar barrier to attenuate wave energy for
a calmer water environment inside the pen. The pen has a diameter of 80m, a height of 39m
and encloses a water volume of about 180,000m3. The deep draught spar is 82 m in height and
is made from concrete for its bottom half and from steel for its top half. The spar carries a wind
turbine and a control unit. The pen is connected by four truss girders (above water) and 16
girders at the base to the spar so that both pen and spar work as a monolithic rigid body. The
four top girders form walkways to access the control unit and the wind turbine. For mooring,
four catenary chains are attached to the spar 38m under the water surface (outside the fish pen)
so as to mitigate the tension force in the mooring lines and to reduce the benthic footprint.
Figure 40: COSPAR fish pen design (Chu and Wang, 2020)
Genghai No.1
Another interesting design combines aquaculture production with leisure/entertainment
activities. Swimming pools, scuba diving facilities and hotels can be placed onto offshore fish
farming platforms. Figure 41 shows Genghai No.1” which is an aquaculture farm, ocean
monitoring centre, as well as a leisure centre. It has aquaculture volume of 27,000 m3 equivalent
to 14 standard swimming pools, and it can accommodate 300 visitors at any given time.
Figure 41: Genghai No.1
Photo from: < https://www.swissre.com/reinsurance/property-and-
casualty/reinsurance/marine/offshore-fish-farming-facilities-challenges-marine-insurers.html>
5. Concluding remarks
In summary, there is an increasing interest in the fish farming industry to move offshore for
sustainable farming, larger sea space and higher fish production. However, offshore operation
generally requires high capital and production costs (Jansen et al., 2016) and therefore rigorous
researches and developments must be carried out urgently to seek cost effective solutions. Also,
fish pen designs should consider the health of the fish, fish diseases, exposure to toxicity, fish
growth, harvesting of fish, transportation to the market and environmental issues. Feasibility
of offshore fish farming may be achieved through adoption of new development of multi-
functional, modularity for ease of scaling farm size and autonomous infrastructure that has
been validated in oil and offshore industry (Dalton et al., 2019; Grinham et al., 2020). By co-
locating offshore renewable energy systems (wind turbines, wave energy converters) and
floating platforms (that can accommodate fish feed silos, feeding equipment, harvesting cranes
and nets, fish processing and packaging plants, waste treatment plant, desalination plant) with
offshore fish farms is possible to leverage the benefits of collocation, vertical integration and
shared services and to reduce operating time and cost (Chen et al., 2020). Also, the use of
offshore renewable energy helps to decarbonize the fish farming industry.
Nevertheless, combination of fish farming with other marine activities are desirable from
an economic viewpoint. The overall infrastructure and operational procedure will no doubt be
more complex, and the increased functionalities will bring more risk and require more rigorous
assessments for warrants and insurance coverage than solely fish farming activity. More
research and developments are needed in this space.
6. Acknowledgement
The authors acknowledge the financial support of the Blue Economy Cooperative Research
Centre, established and supported under the Australian Government’s Cooperative Research
Centres Programme.
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... Examples of this group include large circular pens developed by AKVA group [17] and the Fortress pen developed by Huon [23]. Some tests showed that the modified pen is able to survive storm with 10 m significant wave heights [24]. Advantages of using the modified pens include cost-effectiveness, and simplicity for construction, installations and operations as existing nearshore farming technologies may be adopted with minimal modifications. ...
... The main advantage of submerged pens is that they need not be very large, strong and stiff to withstand the strong surface waves in exposed/offshore sites since they can be lowered to calmer waters beneath the water surface. However, their operations are more complex [24] due to the need to submerge and to refloat the pen, and feeding the fish in deeper waters. In a recent development, fish were put in a submerged Atlantis pen to study the fish behaviour, well-being and growth under submerged condition [28]. ...
... They may also be classified as floating rigid pens with frame structures. Examples of these fish pens are Havfarm 1- Fig. 5a (385 m long, 59.5 m wide, and has a capacity for 2 million fish) [29], Ocean Farm 1- Fig. 5b (a circular cage with diameter of 110 m, and able to accommodate 1.5 million salmons) [30], Shenlan 1 (a circular cage with diameter of 60 m and 35 m in height, and able to accommodate 300,000 salmons) [31], Zhenyu 1 (an olive-shaped, with a length of 60m and a width of 30 m) [24]. Advantages of floating rigid pens include: high stability that provides safety for workers and operations; and potential for integral harvesting and feeding systems [32]. ...
Chapter
This paper is concerned with advances in research and developments on offshore aquaculture and renewable energy production. We first discuss the motivation and challenges for moving offshore in these two blue industries. This is followed by a summary of recent advances and research needs in offshore fish farming, seaweed cultivation, and harvesting energy from offshore wind, solar, wave and tidal current.
... Two contrasting design philosophies for offshore fish pens have emerged due to the highly energetic offshore environment and deep water. One design philosophy is to make a fish pen with significant size, rigidity, and strength to withstand the strong waves, currents, and winds [10]. The pens with large net depths allow fish to swim to the pen bottom as the cultured water volume contracts under strong waves passing through the pen. ...
... The pens with large net depths allow fish to swim to the pen bottom as the cultured water volume contracts under strong waves passing through the pen. Examples of such fish pens are Ocean Farm 1 [11,12], Havfarm 1 [13], Shenlan 1 [14], Zhenyu 1 [10], and Dehai 1 [15]. These large offshore fish pens have been built in China with steel as the preferred structural material. ...
... For the SeaFisher, the net is attached to the HDPE frame structure. In order to strengthen and stiffen the frame structure and the Kikkonet, a diagrid rod system is installed as shown in Figure 1 (10). The diagrid is made of glass fibre-reinforced polymer (GFRP), which is a durable but lightweight material used for marine structures. ...
Article
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Moving offshore for fish farming poses challenges due to the more energetic sea environment. 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 × 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 v.2.17.3, 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.
... With diminishing appropriate fish farming locations nearshore, due to contested sea space utilizations and stressed ecosystems from massive fish faeces and uneaten feed, fish farm operators have started to install their fish farming infrastructure at more exposed/offshore sites [1][2][3][4]. These offshore sites offer large water columns because of deeper waters that can aid in waste dispersion, provide more pristine water and cooler water temperatures, as well as better return on investment from a larger scale of fish production [5][6][7]. On the other hand, offshore fish farming requires the use of large and complex fish pens in order to withstand the energetic environment. Therefore, designing of offshore fish pens will rely heavily on offshore engineering competences, including reasonable design condition selections and rigorous global performance analyses, including hydrostatic, hydrodynamic, and mooring system assessments [8]. ...
... They range from structurally advanced versions to submerged structures, and some have even integrated offshore fish pens with offshore renewable energy production devices. More information and pictures of the various offshore fish pen designs and recent developments can be found in a review paper written by Chu et al. [6], and a book chapter by Wang et al. [7]. However, a consensus has not been reached as to which of these new offshore design concepts will become globally accepted and give Mooring system analysis: quasi-static method, dynamic method, mooring assessment for intact and damage cases. ...
... When several net pens are arranged in a row, such as Havfarm 1 (see a review paper written by Wang et al. [7]), the rear net pen is less affected by the current force than the front net pen [50][51][52][53]. The current velocity drops rapidly after passing through nets one after the other in a row. ...
Article
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While moving fish farms to offshore sites can be a more sustainable way to expand farmed fish production, the fish pens have to contend with a harsher environment. Thus, it is necessary to draw on offshore engineering competences for designing and analysing the offshore fish farming infrastructure. This paper reviews existing design and analysis guidance from maritime classification and national/international authorities that can be applicable for offshore fish farms. Based on the existing design guidelines, a review of design criteria for offshore fish farms under the following subtopics is provided: design life, design environmental loads, combining environmental loads, and miscellaneous load conditions. This review on the global performance analysis procedures and methods is presented based on practices used for neighbouring industries, such as offshore oil and gas and wind energy production, under the following subtopics: hydrostatic analysis, hydrodynamic analysis, and mooring system analysis with introducing theoretical background and modelling techniques. This paper also highlights limitations and cautions when using these design and analysis methods. Providing this comprehensive information, as well as commentary on their applications, will help engineers and designers to develop offshore fish farming infrastructure with confidence.
... This is evident from the design and construction of large and robust fish cage systems to withstand highly energetic offshore environments. Examples are Ocean Farm 1 with a height of 69m, a diameter of 110m that can accommodate 1.5 million fish (Zhao et al., 2019), and HavFarm 1 with a length of 380m, width of 59m and houses 10,000 tons of salmon (Li et al., 2017;Wang et al., 2022). However, these cages come with a substantial price tag, exceeding USD100 million each. ...
... The hawser is attached to a connecting plate that spreads the mooring force to the SeaFisher structure via cables as shown in Figure 5. This SPM system allows the SeaFisher to weathervane, reducing environmental loads on the fish cage (Wang et al., 2022) and ...
Conference Paper
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Offshore aquaculture is rapidly gaining momentum, driven by the need to address environmental concerns associated with nearshore aquaculture and to mitigate conflicts over coastal water usage among local communities. This keynote paper introduces a novel offshore fish cage design called the SeaFisher. This fish cage is designed to overcome the challenges of fish farming in offshore environments, characterized by strong waves, deep waters, rapid currents, and high winds, especially in storm events. The SeaFisher consists of a 2 × n array of interlocking modular cubic fish cages. Each cubic cage frame is constructed from members formed by bundling four high-density polyethylene (HDPE) pipes, and stiffened by diagrid glass fibre reinforced (GFRP) rods. The HDPE pipes are secured by regularly spaced pipe bundling brackets and at the joints by connector pods. Supported by aluminium tubular frames, the pyramidal shaped top and bottom nets provide air space for salmon jumping and for easy removal of fish morts, respectively. Vertical aluminium ballast tubes, located at the top corners of each cage, allow the SeaFisher to submerge to avoid strong surface waves during storms and to resurface after the storm. The ballast tubes control filling ratio automatically to ensure hydrostability, including compensation for additional biofouling mass. Depth control buoys manage the cage's descent and maintain its submerged position. The SeaFisher is moored by a single mooring point (SPM) system, comprising a buoy, hawser, studlink chain and suction anchor. This system allows the SeaFisher to weathervane; reducing environmental loads and spreading fish waste over a wider water column. Presented herein are the design details of the SeaFisher, its modelling and hydroelastic analysis using the software package AquaSim. The SeaFisher is designed to operate in a significant wave height of 7.58m and current speed of 0.8m/s in the Storm Bay of Tasmania. With its resilient and cost-effective design, the SeaFisher is poised to revolutionize marine fish farming by facilitating the relocation of traditional nearshore farms to more expansive offshore enabling increased production of high-quality fish.
... Submersible net pens, which can be submerged to depths of 50 to 200 m [46], are particularly suitable in certain areas of the South China Sea and East China Sea where deeper water temperatures are relatively stable, helping to maintain the temperature range needed for cultivating Larimichthys crocea. Semi-submersible net pens offer higher flexibility and can be adjusted to different environmental conditions, making it possible to find suitable water layers for aquaculture even in seasons with significant temperature fluctuations [47]. ...
Article
Full-text available
This research evaluates the potential spaces of deep offshore waters for cultivating the Larimichthys crocea, analyzing ocean profile temperature data from 2000 to 2022 according to the species’ environmental temperature suitability. There are significant seasonal variations and differences in habitat distributions of different temperature ranges in China’s surrounding waters. The range of maximum living space obtained according to the tolerance temperature shows a trend of being larger in summer and smaller in winter; and the range of viable habitat space obtained based on the suitable and optimal temperature shows a trend of being smaller in summer and larger in winter. Broad areas meeting tolerance temperatures offer broad, yet impractical, site selection options. In contrast, areas with optimal temperatures are limited, which means the availability of ideal site locations is very restricted. Regions consistently within the 20–28 °C range are best for practical site selection. Year-round suitable areas are primarily found at depths of 30 to 90 m in the southern East China Sea and the South China Sea, particularly within the 40 to 50 m depth range. Water mass like the South China Sea Surface Water and the Kuroshio Surface Water consistently maintain suitable temperatures, making them ideal for aquaculture.
... For the latter, it is best to use at least two inlet valves and two outlet valves to minimize the likelihood of damage of the ring structure during the submergence process. fish farming [8]. Offshore sites offer several advantages: (i) the vast expanse of open sea permits large-scale seaweed aquaculture; (ii) seaweed platforms can be positioned at optimal depths to ensure sufficient sunlight and temperature for maximal growth; (iii) potential environmental risks can be minimized, and (iv) cooler temperatures [6]. ...
Article
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This paper investigates the hydroelastic response of a submersible circular ring structure, designed for offshore seaweed cultivation, under wave action and during the submergence process. The ring structure comprises two circular HDPE pipes connected to each other by equally spaced brackets. The structure carries seaweed grow-out lines, and is kept in position by a mooring-line system used for fish pens. The HDPE collar is equipped with multiple inlet and outlet valves, allowing it to be submerged to avoid strong waves and to be raised to the water surface when the strong waves die down. The software AquaSim was used for the hydroelastic analysis of the moored structure. It is found that we can significantly reduce the von Mises stresses in the ring structure as well as the mooring-line forces by submerging. However, the structure can experience significant increase in stress during the submergence process due to bending from combined wave action and non-uniform distribution of filled water in the ring structure. This stress increase may cause structural damage or even failure. Therefore, it is important to submerge the ring structure in calm waves ahead of predicted storms and to control the distribution of seawater into the ring structure. For the latter, it is best to use at least two inlet valves and two outlet valves to minimize the likelihood of damage of the ring structure during the submergence process.
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Aquaculture in exposed and/or distant ocean sites is an emerging industry and field of study that addresses the need to improve food security along with the challenges posed by expansion of urban and coastal stakeholders into nearshore and sheltered marine waters. This move necessitates innovative solutions for this industry to thrive in high-energy environments. Some innovative research has increased understanding of the physics, hydrodynamics, and structural requirements enabling the development of appropriate systems. The blue mussel ( Mytilus edulis ), the New Zealand green shell or green lipped mussel ( Perna canaliculus ), and the Pacific Oyster (Magallana gigas), are the primary targets for commercial exposed bivalve aquaculture. Researchers and industry members are actively advancing existing structures and developing new structures and methodologies for these and alternative high-value species suitable for such conditions. For macroalgae (seaweed) cultivation, such as sugar kelp ( Saccharina latissimi ), oar weed ( Laminaria digitata ), or kelp sp. ( Ecklonia sp.), longline systems are commonly used, but further development is needed to withstand fully exposed environments and improve productivity and efficiency. In marine finfish aquaculture, three primary design categories for open ocean net pens are identified: flexible gravity pens, rigid megastructures, closed pens, and submersible pens. As aquaculture ventures into more demanding environments, a concerted focus on operational efficiency is imperative. This publication considers the commercial and research progress relating to the requirements of aquaculture’s expansion into exposed seas, with a particular focus on the cultivation of bivalves, macroalgae, and marine finfish cultivation technologies and structural developments.
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Offshore aquaculture has gained momentum in recent years, and the production of an increasing number of marine fish species is being relocated offshore. Initially, predictions of the advantages that offshore aquaculture would present over nearshore farming were made without enough science-based evidence. Now, with more scientific knowledge, this review revisits past predictions and expectations of offshore aquaculture. We analysed and explained the oceanographic features that define offshore and nearshore sites. Using Atlantic salmon (Salmo salar) as a case study, we focussed on sea lice, amoebic gill disease, and the risk of harmful algal blooms, as well as the direct effects of the oceanography on the health and physiology of fish. The operational and licencing challenges and advantages of offshore aquaculture are also considered. The lack of space in increasingly saturated sheltered areas will push new farms out to offshore locations and, if appropriate steps are followed, offshore aquaculture can be successful. Firstly, the physical capabilities of the farmed fish species and infrastructure must be fully understood. Secondly, the oceanography of potential sites must be carefully studied to confirm that they are compatible with the species-specific capabilities. And, thirdly, an economic plan considering the operational costs and licencing limitations of the site must be developed. This review will serve as a guide and a compilation of information for researchers and stakeholders.
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This paper presents a design concept of a porous collar barrier for a novel floating open-net fish cage that is integrated with a floating spar wind turbine (referred to as COSPAR fish cage). The COSPAR fish cage has an octagonal shape with each side length of 30m. The collar barrier, having an array of rectangular cut-outs with round corners, is installed at the top portion of the cage to attenuate wave transmission inside the cage as well as to protect fish from external predators and debris. Single and double collar barrier designs corresponding to single net and double net cages are studied. The wave transmission, reflection and energy-loss coefficients of barriers are determined from numerical analysis based on the linear potential wave theory and the eigenfunction expansion method. Various underwater heights (2m ≤ h ≤ 8m) and porosity (0.25≤ ε≤ 0.75) of the collar barriers are examined with the view to obtaining the barrier design for minimal transmission coefficient and energy-loss coefficient. Without a collar barrier, the single net and double net cage can only provide average wave transmission coefficients of 0.9 and 0.8, respectively. This study finds that the transmission coefficient could be reduced below 0.4 by having a single collar barrier withh = 4m andε= 0.25. On the other hand, the transmission coefficient could be further reduced below 0.3 by a double collar barrier with the sameh and ε.In addition, the double collar barrier gives lower energy-loss coefficient and better proofing against fish escape, biosecurity and predator intrusion than the single collar barrier. A double collar barrier design with porosity combination ofε1=0.25,ε2=0.5 is recommended for the COSPAR fish cage as it yields competitive wave scattering performances and saves collar material by 25% when compared with the best performing porosity combination ofε1= ε2=0.25.
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Fish farm operators worldwide are planning to move offshore due to lack of available nearshore production sites in heavily utilized coastal zones, where there is increasing community opposition to coastal development and conflict with other usages such as shipping, fishing, tourism, conservation and recreation. Moreover, offshore sites provide more sea space and generally better water quality, which are needed to increase the production of healthy fish. This review paper begins with the definition of offshore for fish farming based on unified viewpoint and proceeds to highlight the challenges faced by going offshore. Next, the paper presents a review of designs of fish cages from conventional nearshore fish farms to next-generation offshore fish farms, which have to contend with a high energy environment. The fish cages may be divided into the open net cage system and the closed containment tank system. The open net cage system can be categorized further into 5 types. The advantages and disadvantages of the various fish cage designs will be discussed. Further, different types of cage designs are compared with the view to guide feasibility of offshore fish farming. Co-location with other synergetic industries is discussed as a possible example of future offshore fish farms.
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A series of physical model experiments was performed to investigate the hydrodynamic responses of a semi-submersible offshore fish farm in waves. The structural configuration of the fish farm primarily refers to that of the world’s first offshore fish farm, Ocean Farm 1, developed by SalMar in Norway. The mooring line tension and motion response of the fish farm were measured at three draughts. The study indicated that the tension on the windward mooring line is greater than that on the leeward mooring line. As the wave height increases, the mooring line tension and motion responses including the heave, surge, and pitch exhibit an upward trend. The windward mooring line tension decreased slightly with increasing draught. The existence of net resulted in approximately 42% reduction in mooring line tension and approximately 51% reduction in surge motion. However, the heave and pitch of the fish farm increased slightly with the existence of net. It was found that the wave parameters, draught, and net have noticeable effect on the hydrodynamic response. Thus, these factors are suggested to be considered in structural designs and optimization to guarantee the ability of the fish farm to resist destruction and ensure safety of workers during intense waves.
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In this study, swim‐tunnel respirometry was performed on Atlantic salmon Salmo salar post‐smolts in a 90 l respirometer on individuals and compared with groups or individuals of similar sizes tested in a 1905 l respirometer, to determine if differences between set‐ups and protocols exist. Standard metabolic rate (SMR) derived from the lowest oxygen uptake rate cycles over a 20 h period was statistically similar to SMR derived from back extrapolating to zero swim speed. However, maximum metabolic rate (MMR) estimates varied significantly between swimming at maximum speed, following an exhaustive chase protocol and during confinement stress. Most notably, the mean (±SE) MMR was 511 ± 15 mg O2 kg⁻¹ h⁻¹ in the swim test which was 52% higher compared with 337 ± 9 mg O2 kg⁻¹ in the chase protocol, showing that the latter approach causes a substantial underestimation. Performing group respirometry in the larger swim tunnel provided statistically similar estimates of SMR and MMR as for individual fish tested in the smaller tunnel. While we hypothesised a larger swim section and swimming in groups would improve swimming performance, Ucrit was statistically similar between both set‐ups and statistically similar between swimming alone v. swimming in groups in the larger set‐up, suggesting that this species does not benefit hydrodynamically from swimming in a school in these conditions. Different methods and set‐ups have their own respective limitations and advantages depending on the questions being addressed, the time available, the number of replicates required and if supplementary samplings such as blood or gill tissues are needed. Hence, method choice should be carefully considered when planning experiments and when comparing previous studies.
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Ballan wrasse (Labrus bergylta) are used extensively as cleaner fish to control salmon lice (Lepeophtheirus salmonis) infestations in the Atlantic salmon (Salmon salar) aquaculture industry. Fish are either cultured or caught in the wild before being transferred to salmon sea cages. Ballan wrasse are a poorly studied species, and fundamental knowledge of physiological performances and environmental limits are therefore needed for better deployment strategies and to predict when animal welfare may be at risk. We acclimated ballan wrasse for a minimum of 2 weeks to 5, 10, 15, 20 and 25 °C, representing the full range of temperatures wrasse may experience in salmon sea cages. Swim tunnel respirometry was performed at each temperature to measure standard and maximum metabolic rates, aerobic scope, and critical swimming speed (Ucrit). No mortalities occurred at any acclimation temperature. However, fish were generally inactive at lower temperatures, as evidenced by low metabolic rates. It was not possible to stimulate fish to swim continuously between 5 and 20 °C, and Ucrit was only obtained at 25 °C as 27 cm s−1 (1.1 body lengths s−1). The aerobic scope increased throughout the thermal interval tested from 129 ± 7 mg O2 kg−1 h−1 at 5 °C to 265 ± 18 mg O2 kg−1 h−1 at 25 °C. Owing to weak swimming capabilities, ballan wrasse deployment at locations with moderate to strong current speeds will likely result in poor welfare. Low metabolic rates and inactivity at 5–10 °C suggests that their efficiency as cleaner fish will be limited in winter and in higher latitude locations. Overall, ballan wrasse differs substantially from Atlantic salmon in physiology, behaviour and morphology, and may not thrive in some farm environments suitable for salmon.