In recent years, due to various environmental problems and resource conflicts in coastal areas, nearshore mariculture in China has been increasingly confronted with the severe challenge of space compression. However, in more exposed sea areas with the depth exceeds 20 m, the water quality is much better and the water utilization rate is much lower, less than 1%. With the development of aquaculture technology and equipment engineering, it is an inevitable trend for fish farms to transfer to offshore areas. To reduce the risk of fish farming in open sea areas with strong waves and current, offshore fish farm that having the distinct advantages with moderlization of culture and management were encouraged to develop in the past two years in China. Generally, offshore farm was considered as an integrated farming system composed of cultured fish, floating structure, automatic equipments, various facilities, which was expected to play a positive role in leading and promoting the offshore aquaculture industry. In this study, we conducted a series of physical model tests for a semi-submersible offshore fish farm called Dehai No. 1 in waves, in which the model scale was set as 1:30 and the scale of 1:1 was set for the net mesh size. The Dehai No. 1 offshore fish farm with truss structure was co-designed by South China Sea Fisheries Research Institute and Tianjin De-Sai Environmental Protection Technology Co., Ltd in the year of 2018. The main experimental parameters of the offshore farm were as follows: length 3. 04 m, width 0.92 m, height 0.34 m, designed draft of 0.22 m, no-load weight of 15.33 kg. Considering the actual production requirements, three kinds of single point mooring systems (L=2.67, L=5.34 or L=5.34 plus GW=200 g) and three kinds of ballast status (no-load, half-load or full-load) as well as different regular waves conditions (H=16. 7-23.3 cm, T=1. 64-2.01 s) were set as the test conditions for the fish farm. Through physical model experiments, the dynamic behaviour of the fish farm under different conditions were studied, and the results of mooring force and motion response were given and compared. Owing to the characteristics of the single point mooring system, the fish farm in practice inevitably drift within the scope of sea area in the center of mooring point on seabed with the variation of low-to-high water surface, studying the sway and surge as well as yaw for the fish farm could not testify its good stability. Therefore, we chose the heave, pitch and roll for analyzing the motion characteristics of the fish farm. The experimental results showed that the single-point mooring system with anchor chain length of 5.34 m and a weight of 200 g could better reduce the mooring force of the fish farm in waves. Among all of the wave conditions, the peak value of each movement component of the fish farm was relatively small. When the fish farm was subjected to the sea loads with wave height of 23.3 cm (7 m in full-scale) and wave period of 2.01 s (11 s in full-scale), the peak value for the heave, pitch and roll was 4.21 cm, 6.55° and 1.19°, respectively, which indicated that the fish farm had good performance for the stability of floating state. Comparing the maximum motion and mooring force of the fish farm under various floating conditions, including no-load and half-load as well as full-load, we found that, in the full-load floating state, the peak value for the mooring force, pitch and roll was biggest, but the peak value of the heave was biggest under the no-load floating state for the fish farm. With increasing wave height, both of the mooring force and motion of the fish farm became larger. The results provide a basis for further understanding the dynamic characteristics of fish farm, and help to provide theoretical and data reference for the structure design and installation of fish farm. © 2019, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.
