Dynamic Analysis of Semi-submersible Offshore Fish Farm Operated in China East Sea

• Main Author: Shi, Luhao
• Other Authors: Fu, Shixiao, Lader, Pal
• Format: Master Thesis
• Language: English
• Published: NTNU 2019
• Subjects: Norwegian Sea; Simo
• Online Access: http://hdl.handle.net/11250/2622910

In recent years, the aquaculture industry in China has achieved rapid development, which covers more than half of the world aquaculture production. For further development, Chinese aquaculture companies are looking for advanced farm technology available for exposed ocean condition. The semi-submersible offshore fish farm is one of the most foreseeable equipment, which is being tested in the Norwegian Sea. This thesis aims to simulate the dynamic performance of a semi-submersible offshore fish farm operated in Chinese ocean. The farm model is constructed based on the offshore farm "Ocean Farm 1", operated by SalMar ASA. The target location of this farm is in China East Sea with a depth of around 100m. Some simplification is thus carried out at the design stage considering the actual situation in China. There are some components of the model: a main body including central pontoon, steel net frame and connectors; fish nets; mooring system. A dynamic analysis is conducted to study the response of farm structure in both frequency domain and time domain. In order to calculate hydrodynamic properties of the farm structure, a composite model with both Morison beam and panel surface is constructed in GeniE. And simulating the composite model in WADAM, RAOs of response motion in six degrees of freedom are obtained. On the other hand, time domain analysis is executed in WASIM program, and compared with the results of frequency domain analysis. The kinetic results of the farm body acquired in WADAM are used as the input data of SIMO program to implement coupled analysis of mooring system in time domain. A catenary mooring system is modeled in SIMA to check its capability of keeping position in ultimate sea states. For all of the simulations carried out in SIMO program, the mooring lines presented to be capable of withstanding the hydrodynamic loads from waves and currents. This thesis compares the results of motion under various environment conditions. It indicates that current dominates drift motion of the farm compared with wave drift force. However, when considering the oscillation motion of structure, wave force comes to be the main factor. Due to the symmetry of farm structure, wave from different directions only result in slight difference, and the effect of misalignment of wave and current also shows to be insignificant. Furthermore, mooring systems with various pre-tension are tested in the same environment. The consequences of simulation verify that the pre-tension of mooring line has a significant effect on the drift of floating farm body. With the increasing of mooring line pre-tension, the performance of mooring system can be enhanced. Finally, the simplified quasi-static mooring system in WASIM is compared with the catenary mooring system in SIMO. And it illustrates the influence of nonlinear effects: the results from SIMO are less in accordance with wave motion.