Session: 06-11-01 Ocean Engineering Technology I

Paper Number: 79186

79186 - A Comparison of Design Wave Methods for Flexible Fish Farms 

Conventional fish farms with cone-shaped or cylindrical nets suspended from flexible floater rings are being deployed in increasingly larger arrays and in exposed locations with severe environmental conditions. The huge number of coupled elastic elements in such structures makes hydrodynamic analysis with irregular wave conditions computationally impractical in most cases. Therefore, a simplified scheme is necessary for design and dimensioning purposes. A commonly applied method in the offshore industry is to replace the irregular wave condition with regular waves with wave height 1.9 times the significant wave height (Hs), which, according to linear theory corresponds to approximately the height of the most probable largest individual wave in a 3-hour sea state. Although this methodology is widely used also for fish farms, there are two major concerns with this scheme applied on such structures. First, a fish farm has a large horizontal extent, with the largest farms today being close to 1km in length. This implies that a fish farm will never experience only one single wave but rather multiple waves. Secondly, depending on several factors, a conventional fish farm is very deformable while at the same time being subjected to large drag forces. This implies that, when subjected to changes in the environmental conditions, the fish farms take a long time to deform and reach a quasi-steady state. In this paper we present an extensive numerical study comparing forces from regular and irregular waves on a range of conventional fish farms varying in size from very small to the largest farms used in the Norwegian industry today. The results clearly demonstrate issues with the 1.9*Hs-method. Finally, we investigate the possibilities of an alternative regular wave method, aimed to better predict the dimensioning loads in realistic (irregular) wave conditions. 

Presenting Author: Leiv Aspelund Multiconsult Norway ASA

Authors:

Leiv Aspelund Multiconsult Norway ASA
Christoffer Skjulstad Multiconsult Norway ASA
Markus Bjørkli Jansen Norwegian University of Science and Technology
Tim Fristedt Multiconsult Norway ASA