Movement characteristics of floating circular collar of net cages subjected to waves

Floating collar, an important component of deep-water net cage, plays a great important role in the performance of withstanding strong winds and waves for net cage, and studying the movement characteristics of floating collar exposed to waves is a key to deeply understand the properties of withstanding strong winds and waves about net cage. In this paper, based on the lumped mass method, floating circular collar is discreted with lots of micro-elements, and by using the linear wave theory and the rigid body kinematics principle, a numerical model for movement response of floating collar exposed to waves is set up. Reference to practical floating collar parameters: 40 m in circumference, 12.7 kN in total weight, 15 m in water depth, r=125 mm, t=15 mm,ρ=953 kg/m3. Under the conditions of different wave heights（H=4.2-7.0 m）and wave periods（T=7.2 s, 8.6 s）, the motion trajectories of the fore and rear points of the floating collar, the horizontal and vertical displacement of the central point of floating collar, pitch angles of floating collar are calculated in the way of computer numerical simulation method. By comparing and analyzing, the calculated results show that:（1）The vertical motion amplitude of the collar is greater than the horizontal one, and the influence of wave period on the vertical displacement of the collar is less than that of the horizontal one;（2）The horizontal displacement, the vertical displacement and the pitch angle of the collar all have direct proportional relation with wave height. Under the condition of wave height of 4.2-7.0 m, the increase of the horizontal displacement, the vertical displacement and the pitch angle of the collar are 1.84 m, 1.0 m and 7.18° when T=7.2 s, while they are 1.66 m, 1.05 m and 5.43° when T=8.6 s;（3）When wave period increases, the vertical displacement of the collar increases accordingly, while the horizontal displacement and the pitch angle of the collar decrease.