LAUSR

Abstract Semi-submersible offshore fish farm has been considered a step forward to the new aquaculture era. To make it more durable to the excitations caused by waves and current, this paper examines the nonlinear vertical accelerations and mooring loads of a semi-submersible offshore fish farm model under extreme conditions. The steady-state non-dimensional first- and second-order harmonic accelerations at five different positions along the frame and the mooring tensions are obtained via experimental measurements and numerical simulations. The results show that both the first- and second-order harmonic accelerations are independent of the positions on the model but dependent on the wave period. Under wave-only conditions, when H ≤0.15m , there is little difference between the experimental measurements and the numerical predictions on the accelerations. When H ≥0.20m , the increasement of acceleration amplitude is 0.5 %–4.1 % for experiment, while the value is 3.3 %–19.0 % for numerical simulation. Under combined waves and current conditions, compared with H ≤0.15m , the second-order harmonic acceleration increases exponentially under larger wave height, and most of the experimental results are larger than the numerical predictions when the current velocity V 2 =0.2m/s . The damping from the net enhances the nonlinearity of the second-order acceleration but it has little effect on the first-order. The current has a pronounced effect on the mooring tensions. When T =1. 39 s , the mean values of front mooring load at V 2 =0.2m/s are 6.4 times of that at V 1 =0.1m/s in maximum, but when the wave height reaches H =0.25 m , the increase rate of mooring force gradually decreases. The free surface effect is also assessed in this paper.