Hydrodynamic Response Analysis of Combined Spar Wind Turbine and Fish Cage for Offshore Fish Farms

Abstract

This paper is concerned with the hydrodynamic response of a novel offshore fish farm that combines a floating spar wind turbine and a fish cage (named as COSPAR for brevity). The open net steel cage is octagonal in shape with a partially porous wave fence at its top end to attenuate wave energy for a calm fish farming environment as well as to keep predators out. The deep draught spar is made from concrete for its bottom half and from steel for its top half. The spar carries a control unit and a 1[Formula: see text]MW wind turbine that provides the required power to operate the offshore salmon fish farm. The COSPAR fish cage has four catenary chains as mooring lines attached to mid length of the spar (outside the fish cage) so as to mitigate tension force in the mooring lines and to reduce the benthic footprint. ANSYS Design Modeler and Aqwa are used to perform the hydrodynamic response analysis of free-floating condition of COSPAR in the frequency domain and coupled analysis involving COSPAR and the mooring lines in the frequency domain and time domain. Environmental conditions, representing 5-year, 20-year and 50-year wave return periods with a constant current flow at an exposed fish farming site in Storm Bay of Tasmania, Australia, are adopted for the analyses. A comparison study is made against having a floating fish cage only (i.e. without the bottom half concrete of the spar) with four catenary chains attached to side vertical columns of the cage so that the fish cage behaves like a semi-submersible cage. Based on the comparison study, the COSPAR fish cage shows enhanced hydrodynamic responses in the following respects: (1) more stable motion responses in heave and pitch against wave and current forces, (2) less susceptible to the viscous damping when it is assumed by a linearized drag force of Morison elements in the frequency domain and (3) reduction of tension forces in the mooring lines. Interestingly, the pitch motion response of COSPAR fish cage in the frequency domain is in close agreement with the time domain result due to its greater pitching stiffness that reduces nonlinear effects from viscous drag and mooring interaction.

Hydrodynamic Responses of a Barge-Type Floating Offshore Wind Turbine Integrated with an Aquaculture Cage

“…Based on the studies of these scholars it can be determined that the numerical modeling of floating wind turbines can get excellent results through the modeling theory of ANSYS-AQWA. For the new structures of floating wind turbine with a fish cage, Chu and Wang [28] modeled the Spar-type platform with a fish cage by means of ANSYS-AQWA and obtained reasonable results. The above references verify the validity of the theoretical methods implemented in ANSYS-AQWA in performing the hydrodynamic analyses of the floating wind turbine and the integrated structure.…”

“…The earlier research on floating offshore wind turbines with a fish farming cage in China was conducted at Tsinghua University [27]. In order to explore the motion performance, a floating offshore wind turbine with a steel fish-farming cage (FOWT-SFFC) was designed, and the random response and nonlinear dynamic performance of the structure were studied simultaneously; it is noted that the large motion of the pitch has little effect on the nonlinearity of the viscous drag and the mooring interaction [27,28]. Chu and Wang [28] were concerned with the hydrodynamic response of a novel offshore fish farm in the combined floating spar wind turbine and fish cage.…”

“…In order to explore the motion performance, a floating offshore wind turbine with a steel fish-farming cage (FOWT-SFFC) was designed, and the random response and nonlinear dynamic performance of the structure were studied simultaneously; it is noted that the large motion of the pitch has little effect on the nonlinearity of the viscous drag and the mooring interaction [27,28]. Chu and Wang [28] were concerned with the hydrodynamic response of a novel offshore fish farm in the combined floating spar wind turbine and fish cage. An offshore floating multi-purpose platform supporting a 10 MW wind turbine on one side of this barge platform with a fish cage was designed for Blue Growth Farm.…”

Offshore aquaculture of finfish: Big expectations at sea

“…Numerical modelling has been used to predict the dynamic response of the various components in a fish farm and their mutual interaction. 242 Structural response modelling of conventional farms was first performed by Tsukrov et al 239 using the finite element method to establish a baseline system design for a demonstration site. The publication identified the problem of accurately modelling the net cage that has been the focus of further research.…”

Wave diffraction of a hybrid wind turbine foundation with a double-layer aquaculture cage

“…Through comparative study, it was shown that the anti-overturning ability, surge, heave and pitch motions of the proposed structure outperformed those of OC3Hywind and OC4DeepCwind. Chu and Wang (2020); Chu et al (2022) proposed a novel offshore fish farm that combined a floating spar with a fish cage. A partially porous wave fence was installed on the structure to attenuate wave energy.…”