Wave induced effects on floaters of aquaculture plants

Abstract

The main objective of this work is to study wave loads on floaters of fish farms. Several incidents of major fish escapes due to structural collapse of fish farms in harsh weather have motivated this study, as a step towards obtaining better load models for the wave loads on the floaters. Floaters of fish farms are typically small compared to dimensioning waves. The local two-dimensional problem of a floater subject to beam sea regular waves is considered. The problem is addressed by means of numerical simulations, model tests and simplified numerical models. A CFD numerical wave tank (NWT) for fully nonlinear wave body interaction problems is developed. The numerical model applies a one-fluid model, where a fractional step approach is used to solve the incompressible Navier-Stokes equations in time on a Cartesian staggered grid. Further, a combined Constrained Interpolation Profile (CIP-) and finite difference procedure is used. An immersed boundary technique is applied to impose boundary conditions on the floater.

Two sets of model tests have been conducted. In the first set, wave loads on fixed horizontal cylinders in beam sea waves were studied. The purpose was to obtain validation data for the numerical wave tank. Two models were tested. One with a circular cross-section and one with a rectangular cross-section. Model draft was varied. Other test parameters were wave period and wave steepness. The primary variables measured were the clamping forces of the model and the wave elevation at eight positions in the flume. Numerical simulations similar to the physical experiments were performed and results compared, showing good agreement. Wave over-topping on the models was observed, both in the experiments and in the simulations.

In the second set of experiments, a floating circular cylinder in beam sea regular waves was tested. The cylinder was moored using pre-tensioned mooring lines, yielding a natural frequency of the sway motion which is representative for fish farms. Primary variables measured here were the model accelerations used to obtain the body motion, surface elevation and mooring line forces. Results were compared with linear potential flow theory, semi-empirical theories and simulations with the CIP-based numerical wave tank. An instability phenomenon was observed in the experiments when the wave frequency was two times the natural frequency in sway, causing large amplitude sway motion which is not predicted by linear potential flow theory. The same instability behaviour was also observed in simulations with the CIP-based numerical wave tank and is believed to be due to nonlinear hydrodynamic effects. Experiments and numerical simulations also shows that linear potential flow theory largely over-predicts the sway motion near resonance. This is explained by effects of viscous flow separation. It is also shown that higher order harmonics of the wave loads can be significant and should be considered when fatigue analyses of fish farms are performed.