Modelling of Hydrodynamic Loads on Aquaculture Net Cages by a Modified Morison Model

Abstract

A modified Morison model is presented for calculation of hydrodynamic forces on aquaculture net cages. The model is based on a simple method for conversion of \textit{"screen model force coefficients"} to approximate equivalent directional dependent \textit{Morison coefficients}. The motivation for this is that experimentally obtained force coefficients for net panels are generally presented as screen model force coefficients, while commercial analysis software are often restricted to a Morison model. Based on the screen model force coefficients defined by \textit{Løland's formulas}, the method is implemented in the nonlinear finite element program USFOS. Analyses with the modified Morison model are performed, investigating the viscous forces on a net panel for different inflow angles. The numerical results are benchmarked against Løland's original screen model, showing good agreement for all inflow angles. Comparison with the classical Morison model is made to illustrate the advantage of the proposed method. The model is also applied for calculation of hydrodynamic forces on the net of a real aquaculture structure exposed to a steady current. Again, the results are compared to Løland's original screen model, showing an almost exact reproduction of the global drag force.