2.3. Coupling hydrodynamic loads computation and structural analysis
The coupling between nonlinear FEM structural solver and the hydrodynamic model based on the maneuvering model is made possible by the use of the user defined load subroutine (LOADUD). The user-defined load subroutine has found limited application in literature. A paper by Adoum and Lapoujade [28], which contains several basic examples demonstrating the application of LOADUD is useful to start with. The user-defined load subroutine in LS-DYNA provides the nodal information of displacements, velocities, accelerations and user-defined inputs and allows users to define nodal loads or pressure loads as a function of the information. This should be sufficient to implement the current model. However, a problem arises when applying the added mass forces as nodal accelerations in the user load subroutine are only available for deformable bodies, but not for rigid bodies. If a deformable body is to be used to represent the hull girder, the body will generate large structural vibrations and nodal acceleration oscillations that will yield wrong results..