Loads on the Gravity-net-cage from Waves and Currents

Abstract

Due to limited space close to shore and environmental concerns, the trend has become to move the fish farms to more exposed areas. This motivates the study of loads on the fish farm in more energetic currents and waves.In this thesis the conventional gravity net cage is analysed from an experimental and numerical point of view. The focus is on mooring line loads, which represent the global loads acting on the system. The numerical program FhSim, which is SINTEF Fisheries and Aquaculture inhouse software, is subject for a comparison study in terms of mooring line loads. Two systems of the gravity net cage are analysed. Experiments conducted by MARINTEK in May 2013 are analysed to investigate the mean load features in large current and waves. The results are compared to mean load predictions from the numerical code FhSim. A comparative study between FhSim and the numerical code developed by Kristiansen and Faltinsen (2014) is also performed.Experimental analysis in irregular waves and currents show that the mean mooring line load is dominated by current. The mean load is found to increase with increased wave height and wave period. These trends are captured by the numerical programs. The mean load in currents and waves is decomposed into a static load which is the current only contribution, and the remaining dynamic force part. Alternative scalings of the mean dynamic force included the squared ratio of wave velocity to current velocity or the wave amplitude squared. From the comparative study, it is found that FhSim predicts the mean loads with a reasonable agreement in a combination of the current velocity U=0.5m/s and the wave steepness 1/30. In a combination of larger currents and steeper waves, FhSim overestimates the mean load for the studied model. This can be explained by the implemented Morison load model and the increased water flow velocity across the net. In the analysis of a complete mooring line system, a prediction error was detected in the numerical program FhSim. Together with the possible overestimation of mean mooring line loads in large currents and waves this must be accounted for when assessing mooring line loads by use of FhSim.In a comparison study, a set of high accuracy data from both numerical predictions and experimental measurements should be ensured. A reduction of systematic errors and detection of precision errors improves the quality of the data. In order to confirm the observed features regarding mean load predictions from FhSim, further investigations with a wider range of experimental test conditions in large currents and waves are required.