*2.2. Net System*

The net is stretched over the sides and bottom of the fishing cage. The type of net used in Ocean Farm 1 is EcoNet, which is a non-fiber material with a hard surface that can resist ocean pollution [5]. It is made from very strong but light PET (Polyethylene Terephthalate), and has been certified according to the Norwegian fish farming standard NS 9415 [16], and its service lifetime in the water can reach up to 14 years.

The solidity ratio is the ratio of the projected area of screen threads to the total area of the net panel. For a square net, as illustrated in Figure 4, the solidity ratio *S*n is calculated by [5,17]:

$$S\_{\rm n} = \frac{2d\_{\rm w}}{l\_{\rm w}} - \left(\frac{d\_{\rm w}}{l\_{\rm w}}\right)^2\tag{1}$$

where *d*w is the diameter of twine, *l*w is the length of twine.

**Figure 4.** Illustration of net wine.

In the analysis, it is necessary to consider the wave forces applied to the net attached to the fishing cage and the total force for all net elements. Nevertheless, the number of all cells in a fish net is too large to be included in a model. In order to improve simulation efficiency, it is necessary to put forward a simplified approach, so that the total net force can be transferred to the rigid cage. Therefore, an equivalent net method, which has been studied by Duo [5] and Li and Ong [18], is used in this paper. The solidity ratio of the net on Ocean Farm 1, 0.16 [19], is applied in this work. An equivalent diameter *d*<sup>w</sup> = 0.4 m is introduced, with an assumed twine length *l*w= 4.8 m, to obtain the same solidity ratio based on Equation (1).
