Practical Applications of Numerical Modeling using Aqua-FE: A Case Study

A numerical model of an American Soybean Association (ASA) cage system was constructed using a finite element program developed at the University of New Hampshire (UNH) called Aqua-FE. The small volume, high density aquaculture system was modeled to determine how the system will operate in normal and extreme environmental conditions. The goals of the study were to determine the maximum loads in the system during tropical storm conditions and determine a similar cage system´s response under specified environmental criteria. The cage is currently deployed in Weitou Bay, China. The system consists of a 100 m³ cage (2 m times 4.5 m times 7 m) secured in a single point mooring. The rigid HDPE cage is held to the mooring by two sets of bridle lines, attached to the upper and lower cage framework. Chain ballast hangs below the lower cage rim providing a restoring force. A deadweight anchor secures the system to the seafloor. A 90 kg float suspends the single point mooring and serves as a tie-up location for servicing vessels. Aqua-FE can apply wave and current loading on truss and buoy elements by utilizing the Morrison equation adopted for analysis of aquaculture net pen systems. The algorithm employs a nonlinear Lagrangian formulation to account for large displacements of structural elements. In addition, the unconditionally stable Newmark direct integration scheme is adopted to solve the nonlinear equations of motion. Hydrodynamic forces on the structural elements are calculated using the Morison equation modified to account for relative motion between the structural element and the surrounding fluid. Maximum loads in the mooring gear approached 56 kN during the storm events. When various current velocities were applied, the cage submerged to a maximum depth of 16.4 meters