ACCURACY OF SIMPLIFIED METHODS FOR FATIGUE DAMAGE ESTIMATION OF EXPOSED FISH FARMS

Abstract

Fish farms are being placed in more exposed locations than earlier, encouraged by the “development licenses” that the Norwegian Directorate of Fisheries have introduced. Traditional design methods for estimating fatigue damage for fish farms are based on formulations given in the code NS - 9415 Marine fish farms. These methods are initially developed for sheltered areas and may not give an adequate level of safety in more exposed locations, where the dynamic response from waves is of greater importance. Accurate calculations using state of the art methods are very time consuming both with respect to man hours, but especially with regard to CPU consumption. Hence, for practical design, simplified procedures, such as combination of the design wave method with an assumed Weibull distribution, are often used to limit the complexity of the analysis as well as the costs. To the authors knowledge, the accuracy of such simplified methods is not well documented for exposed fish farms and with limited full-scale experience it is difficult to conclude that the desired safety level is achieved. This paper addresses this problem by investigating the accuracy of simplified methods to estimate the fatigue damage. A case study of a modern fish farm concept for exposed waters is performed where the simplified methods are compared to more complex time-domain analyses using state of the art modeling techniques.