Hydrodynamic performance of a semi-submersible offshore fish farm with a single point mooring system in pure waves and current

Highlights

• A safe and reliable mooring form of the offshore fish farm is proposed.

• The mooring force and motion response of the fish farm in waves and current are investigated.

• Raising the fish farm by adjusting its drought is beneficial for reducing the mooring force to further secure the fish farm in severe sea conditions.

• The offshore farm is proved to have a good adaptability for cultured fish through field test of fish farming.

Abstract

A physical model of a semi-submersible offshore fish farm with a scale of 1:30 has been tested in a wave-current tank. The mooring force of the fish farm in waves was measured and compared for different single-point mooring arrangements, and henceforth, a safe and reliable mooring form is chosen as the mooring system for the fish farm. On this basis, the tested results for the mooring force, heave, pitch and roll of the fish farm in waves and current are presented, in which the effect of drought was discussed. The performance of blocking water flow for the fish farm in pure current was also evaluated. It has been found that increasing the anchor chain length and adding a sinker suspending from the chain could reduce the mooring force significantly despite the fluctuations in the mooring force. During the experiments, the tested value for the heave and pitch as well as the roll has been small, which indicates that the fish farm has a good stability. The mooring force has become larger, while the heave became smaller as the drought increased. Furthermore, the current velocity inside the fish farm is significantly lower than that of its outside. According to the field test results obtained in the fish farm, the farm exhibited good adaptability and it formed a suitable environment for growing cultured fish.

Introduction

Mariculture has proliferated around the world to meet the growing demand of proteins from the increasing world population. Its development leads to acute concerns of the environmental and possible ecological impacts, besides the increasing risk of competition between fish farmers and multiple users of coastal space and marine resources (Pérez et al., 2005). Limited possibilities for expansion in the inshore coastal areas, and technological improvements in farming structures, have pressurized widespread efforts in offshore aquaculture (Ferreira et al., 2012). Offshore fish farming, which indeed avoids possible conflicts between the development of commercial aquaculture operations and the environmental impact in coastal areas, can represent the greatest potential for the global expansion of the aquaculture industry (Benetti et al., 2010).

The gravity net cage, as main type of cultured facilities under the sea conditions, made of high-density polyethylene (HDPE) has been extensively used in the past few decades in China, Norway, and many other countries (Fisheries Bureau and China Ministry of Agriculture, 2018; Gansel et al., 2018). In general, a gravity net cage mainly comprises the floating collar, fishing net, sinker weights, and mooring system (Hao, 2008). Investigating the hydrodynamic behaviors of net cages under various sea states is a prerequisite to enhance the safety performance of the cage structure when exposed to heavy sea loads, besides providing the theory and data support for a better design and optimization of the cage system. Therefore, several studies have been conducted globally for the hydrodynamic properties of the components of the cage through numerical simulation, model tests, and field measurements. A modeling of the floating collar utilizing the lumped mass method and the principle of rigid body kinematics has been performed by Zhao et al. (2007); Tang et al. (2011), and Lee et al. (2015). Practically, a floating fish cage collar made of high-density polyethylene may have large deformations resulting from strong winds and waves. The elastic deformation of the floating collar under wave loading has been studied by Dong et al. (2010); Huang et al. (2016, 2018), and Zhao et al. (2019). Several scholars have studied the net system providing a cultured space for fish, the deformation of the fishing net in response to the current velocity, and the influence of the key factors for the deformation has been investigated with numerical or experimental methods (Lader and Enerhaug, 2005; Ding et al., 2007; Moe et al., 2010; Kristiansen and Faltinsen, 2015). Ding et al. (2007) have studied the relationship between the drag force and volume loss coefficient of the fishing net in the current, and the results have shown that the factors affecting the volume loss coefficient, listed in order of importance, are the flow velocity, weight, ratio of depth to width of the cage, and ratio of the twine diameter to the bar length of the fishing net. In the net cage systems, the mooring system is a critical component owing to its connectivity to the floating collar by rope to ensure the relative fixing of the cage at the offshore farming sites. Nevertheless it also has a significant role in resisting the sea loads together with the cage structure. The types of moorings employed comprise the multiple point mooring system (MPM) and single point mooring system (SPM). The mooring line tension of the fish cages under various conditions have been investigated in several studies (Huang et al., 2008, 2009; DeCew et al., 2010; Cifuentes and Kim, 2017; He et al., 2018; Hou et al., 2019). Huang et al. (2008) have analyzed the effects of the waves with a uniform current on the mooring line tension and net deformation, for marine aquaculture gravity-type cages equipped with the MPM system, by using a numerical model that was validated with physical tests. Hou et al. (2019) have investigated the short-term extreme value distribution of the tension on the mooring lines and the reliability of grid mooring system for fish cage under ultimate limit state. Compared with the MPM system, one of the advantages of the SPM system is the substantially reduced benthic accumulation of waste products because the fish waste is distributed over a larger area. Another advantage is a possible reduction in mooring costs, which can be halved or even lower (Huang and Pan, 2010). In addition, the cage system responds to the changes in waves and currents rather than resisting them, which helps to minimize the environmental loading on the mooring components. Shainee et al. (2014) investigated the submergence characteristics of a self-submersible SPM cage system in random waves with following current in numerical simulations and experimental model tests, and the results showed that this SPM cage concept has the potential to be used widely as an offshore cage system. Based on the above developments, the offshore cage aquaculture has been developing fast by combining the significant advantages with low construction cost and convenient operation, especially in China (Fisheries Bureau and China Ministry of Agriculture, 2018).

Recently, the increased needs of farther and deeper sea area, farming automation, and management integration for the offshore aquaculture, have put forward higher demands from aquaculture facilities. In general, utilizing large fish cages is considered an important method for greatly increasing the economic benefits of aquaculture in more exposed sea environments by reducing the high costs of risk prevention, control and culture management. In addition, in order to satisfy the multifunctional demands of fish farming, feed storage, and equipment fixation, which are development trends, an offshore fish farm with a rigid support structure rather than a flexible floating collar is more suitable for exposed locations with harsher environmental conditions. Therefore, novel and robust offshore fish farms have recently been proposed and used in practice, such as Ocean Farm 1 with a volume of 250,000 m³, which has been developed in Norway and built in China (SalMar, 2018). This rigid semi-submersible fish farm comprises fence-like braces, floating elements, and a slack mooring system. The nets are tensioned and attached to the braces to avoid deformation in currents. This type of fish farm may represent the first step toward a new era in offshore aquaculture. Nordlaks AS proposed a giant ship-shaped fish farm with a multiple cage system, called Havfarm 1, which has the capacity to accommodate up to 10,000 tonnes of salmon at one time (Ship Technology, 2018). A SPM system was designed to allow the whole fish farm to rotate about the bow, thereby reducing the environmental loads encountered. This mooring system also increases the distribution area for fish waste to reduce the probability of fish infections (Li et al., 2018). In China, the fisheries departments have recently begun to encourage aquaculture enterprises by active and effective policies, to develop offshore fish farming with high-performance facilities in open sea water to accelerate the transformation of mariculture industry (He and Zhang, 2018). Futhermore, the construction of Ocean Farm 1 and Havfarm 1 in China is recognized as a milestone that could accelerate the development of China’s offshore aquaculture industry. In this context, we have developed a semi-submersible offshore fish farm named Dehai farm 1 (see Fig. 1), and positioned it in Zhuhai waters of South China sea, which is approximately 30 km far away from the nearest land coastline. This is a preliminary work for a better understanding of the hydrodynamic performances of the semi-submersible fish farm at the initial stages of its designing. The objective of this paper is to analyze the mooring forces and motion responses of the fish farm in waves and current through a series of physical model tests with the scale of 1:30. Here the effects of mooring style, wave-current, and floating status on the dynamic response of the farm are also considered.

The remainder of this paper is organized as follows. Section 2 introduces the laboratory experiments including the physical model, model test setup, experimental conditions, and data collection methods. The comparison among the three kinds of mooring arrangements and the experimental results of the mooring force and motion response of the fish farm under various wave-current conditions are presented in Section 3. Furthermore, the effect of drought is analyzed and the adaptability of the farm to cultured species is verified. Conclusions are presented in Section 4.