Structural modelling of aquaculture nets with square meshes by quadrilateral membrane finite elements

Coupled fluid-structure interaction analyses of aquaculture nets are predominantly based on structural models consisting of 1D, truss or spring, elements that model a large number of phys-ical twines. Although this allows for the use of existing structural analysis programs, preparing input data for these models is cumbersome and an improper scaling of the forces involved in the problem can lead to accuracy and convergence issues. In this paper, a quadrilateral 4-node membrane FE for aquaculture nets with square meshes is proposed, featuring simple input data and a good convergence behaviour demonstrated by the proposed patch test. Rigid-body and de-formation modes of the element were investigated by eigenvalue analysis showing an agreement with the presented benchmark numerical model for netting. Additionally, a simple non-linear material model was used to account for the lack of compressive stiffness in the netting twines. Finally, the differences in applying the Morison’s hydrodynamic load model on the quadrilateral vs. triangular element are discussed and the proposed approach was validated against two ex-periments with nets of increasing complexity.