Birger Enerhaug

Current Loads on Net Panels With Rhombic Meshes

In this paper, hydrodynamic load models applicable to rhombic net structures are discussed and compared to experimental results with net panels in steady currents. Net panels with mesh sizes and mesh opening angles as is commonly used in fishing gear, were tested in the SINTEF operated flume tank in Hirtshals, Denmark. Drag and lift forces acting on the net panels were measured in two different water currents with attack angles ranging from 0 to 90 degrees.These measurement data were used as a benchmark to compare the performance of several established mathematical models of hydrodynamic loads on panels. Subsequently, an element-based approach with summation of lift and drag forces acting on all the individual twines of the element was performed, and the output from the simulations was compared with experimental data. The correspondence between results from the numerical simulations and experiments were in general quite acceptable for higher angles of attack, while the lack of a wake model overestimated the drag at lower angles. Lift was predicted with high accuracy for all panels.Copyright

Numerical Modeling of Wake Effect on Aquaculture Nets

Accurate modeling of drag forces on net cages due to water current is important when designing floating fish farm systems. These drag forces give a major contribution to the total environmental forces on a fish farm, especially mooring line forces. When subjected to current, the net cage will deform. High current velocities can result in large deformations and lead to collapse of the net cage. For circular fish farms with a flexible floating collar, large deformations may induce contact between the weighting system and the net, resulting in abrasion that can cause tearing of the net material and consequently failure that will lead to fish escape.The motivation for this paper is to obtain a better understanding and more accurate model for drag forces and corresponding deformations of circular net cages due to water current. Calculation of drag forces on a net cage is complicated due to the porous nature of the net, geometry and flexibility of the system. Adding to the complexity is the wake effect, or reduced velocity, behind each individual twine which will have a significant effect on the forces and deformations of the net cage. This wake effect will result in reduced inflow velocity on parts of the net being downstream.A method for estimating wake effects acting within an aquaculture net structure was developed and implemented in a numerical code taking net deformation into account. Numerical simulations of a cylindrical net cage were compared with experimental results. Comparison between simulations with and without wake effect revealed a reduction in total drag up to 22% when wake effect was applied. Although the model consistently overestimated drag forces on the net cage (average deviation of 25%), simulation results compared well with measurement data, particularly for low current velocities where deviations were as low as 7%. This indicates a consistent wake effect and drag model that produces conservative estimates of drag forces on net cages.Copyright

Friction Forces Between Seabed and Fishing Gear Components

Bottom trawls are still the most common, and most energy consuming type of fishing gear. For this type of fishing gear, as well as other types with bottom contact, the contact forces between gear elements and seabed have a significant influence on both resistance and shape.This paper describes the work that has been done in research programs at SINTEF in Norway and Marine Scotland Science in Scotland. In both studies sections of rock hoppers were examined, full scale sections and 1:5 scaled model sections were tested at sea and in laboratory, respectively. Scaled models at SINTEF were tested with the submerged models in water along with the additional tests in order to establish associated hydrodynamic forces. The tests were undertaken at angle of attack 0–90 degrees. Similar procedure was followed for full scale models giving a scope for potential comparison. The coefficients of friction for each model are presented as a function of penetration depth, towing speed and angle of attack, and the relevance as friction load models for fishing and off-shore gear are discussed.© 2012 ASME

Experimental Study on Interaction Between Waves and Netting: Damping Effects and Forces

One of the possibilities to expand sea-based fish farming is to move the aquaculture installations away from the conflicts of the coastal zone, and into more open ocean locations. However, open ocean aquaculture puts other demands on the structures than aquaculture in sheltered locations, and in this context it is necessary to understand the behaviour of the aquaculture structures as they are exposed to large sea-loads from waves and current. Flexible netting is a main part of most sea-based aquaculture structures, and in this paper the interaction between waves and netting is studied. Experiments were conducted at the narrow wave flume facility at the University of Oslo, Norway, where several different regular wave cases were run through netting with different solidity. The wave energy was measured after the wave had passed through the net and compared with the energy of an undisturbed wave to assess the wave damping properties of the net. The vertical and horizontal forces were also measured. The findings show that the damping effects of the netting are not necessarily correlated with the wave forces, indicated complex nonlinear processes contributing to the fluid-net interaction. The amount of nonlinear energy in the wave and force waveforms is also investigated, and it is shown that the nonlinear energy in the incoming wave results in an even higher level of nonlinear components in the forces experienced by the net.Copyright