Calculation on horizontal wave force using two wave theories for a square sea-cage

Sea-cages are mechanically influenced by both waves and currents.An effective operation requires hydrodynamic research in the perspective of durability,security and productiveness of the cage.Current research is hindered by insufficient knowledge of wave force.It must be considered the intrinsic factors that the cage frame was rigid body but the nets and ropes were flexible system,of which the former dynamic behavior is used as reference of ocean engineering research and the later hydrodynamics is in view of fishing gear characteristic.Intrinsic factors of the cage,such as framework pattern (shape and size) and materials (rigid or flexible net and rope),must be considered in any simulation of the sort.Generally speaking,water depth in the zones of cages moored is very deep,and the waves in some zones tend to fragmentate.In the other hand,the bottom in some zones is made of mud and sand by sediment which is easy to float by waves.The waves containing mud and sand have much greater effect on cages than those clear.Furthermore the wave force caused by the former had nonlinear character.Therefore the calculation method of sine wave had some difference from reality,and other wave theory should be used to calculate the wave force for a square cage. By means of calculation methods of Morison's and Kuznetsov's,the characters of horizontal wave force for a square sea-cage and its components were analyzed,and two iterative calculation methods of horizontal wave force among frame,rope and net were put forward based on sine wave theory and Stokes second-order wave theory.Compared the two wave theories calculated values with the lab flume test results of the two kinds of cages respectively,cage A's dimension was 0.67m long,0.67m wide and 0.34m high;the frame and rope diameter was 32mm and 5mm;the net mesh was 16.8mm;the net cutting coefficient was 0.707 etc.Cage B's dimension as the above was 0.53m,0.53m and 0.27m,and 16mm,5mm,16.6mm,and 0.707,respectively.The results showed that the horizontal wave forces on the cage changed periodically with two peaks per period.On the other hand,the results also showed the two average error ratios between calculation and experiment for cage A were 11.19% and 13.67%,but for cage B were 11.03% and 13.87%.It also indicated that the calculated results by using the above wave theories had a little difference; the calculation accuracy is positive; the methods of calculation and analysis were effective.The horizontal wave force of the actual cage which cage A was simulated was discussed.It is concluded that the calculated force had some great difference between the second-order and the sine wave to some extent,and the difference ratio of the two wave theories calculated was 8.70% under the applied condition of wavelength 150m,period 10.2s,wave height 10m and water depth 40m,and the difference ratio has positive relation with wave height and wavelength,and negative with water depth.Thereby the way to select the correct wave theory to make the calculation under different wave situations was presented.