Design of multi-point mooring system for offshore aquaculture platform based on dynamic analysis

Abstract: As one of deep water aquaculture management facilities, the breeding barge needs to regulate the location on sea surface for daily use. The mooring system is one of the most important factors which affect both the location and the security of the breeding barge. Taking a offshore aquaculture platform with small size and shallow draft that is moored in around 10 m water depth as an example, a multi-point mooring system is designed under 3 limitations. The coupled numerical simulation of the offshore aquaculture platform and mooring system is performed by using the three-dimensional potential flow theory and the nonlinear time-domain analysis method based on full quadratic transfer functions (QTF) method. By solving the QTF of the coupled barge and mooring system, the 2nd order wave forces are obtained with consideration of the shallow water effect. Two mooring systems, a single chain catenary mooring system and a taut mooring system with single fiber rope, are used to carry out the further numerical simulation and the investigation is performed using the commercial software OrcaFlex. The results show that neither of the 2 systems can meet the mooring requirement for the offshore aquaculture platform in shallow water near reefs. For the catenary mooring system with 55 m mooring radius and 60 m anchor leg made by steel chain, the maximum tension of the anchor leg is 233.5 kN, and the displacement of the breeding barge is 6.856 m. Whereas, for the taut mooring system with 95 m mooring radius and 91 m anchor leg made by fiber rope, the calculated pretention is 86 kN, which is too large to meet the requirement, and the uplift force of the anchor that is not eliminated is 11.4 kN. The optimization space of the 2 kinds of mooring systems is very small due to the design limitations. Therefore, this paper proposes a new mooring mode by using the chain connected with fiber rope as the mooring legs, instead of single material. With the fixed mooring radius of length of 95 m, the varied pretension of the mooring system will be achieved by changing the length of the fiber rope from 76 to 80 m. The redesigned multi-point mooring system with 1×6 arrangement mode can uniformly distribute the environment loads. It finds out that the mooring system with a 95 m anchor leg that is composed of 15 m chain and 80 m fiber rope can meet the mooring requirement of the offshore aquaculture platform well in the case of 95 m mooring radius, and this kind of case is also investigated by using the commercial software OrcaFlex. The maximum displacement of the breeding barge is 1.55 m, and the maximum tension of the mooring leg is 53.914 kN, resulting in a safety factor of 3.227. And the pretension is 15.4 kN, which meets the requirement. Finally, 7 schemes, in which different masses of concrete blocks are added at different positions of the redesigned multi-point mooring system, are calculated to test the effect of the traditional approach of hanging concrete blocks with small mass (about 15 kg per block) on the mooring stiffness. The results show that the closer to the anchor chain the clump weights, the longer the chain length and the larger the maximum tension. However, it has little effect on the mooring stiffness, whereas leading to the increased pretension which may cause fatigue damage of the mooring legs and barge structure. Therefore, it is not recommended to add clump masses on the mooring system. The effect of shallow water on the mooring safety and the 6 degrees of freedom motion responses of the breeding barge still need to be further studied.