Pressures and motions due to the waves, current, and forward speed at a given phase are extracted from a hydrodynamic data file. The hydrodynamic data file information is translated either into surface element pressures or into line element pressures based on Morison's equation.
The hydrodynamic data (.ahd) file must be prepared in a specific format for it to be read during the ocean loading procedure.
To apply the ocean loads, an ocean environment and loading data need to be defined using the ocean family of commands (specifically, OCTYPE, OCZONE, OCDATA, and OCTABLE).
For line elements (BEAM188, BEAM189, PIPE288, PIPE289, and LINK180), the ocean loading is applied automatically if it is defined.
For shells or solids, the hydrodynamic loading is applied via
SURF154 elements with KEYOPT(8) =
KEY, where KEY activates or
deactivates ocean effects. If KEYOPT(8) = 0, the ocean load is not applied to
SURF154 elements even if the ocean environment
exists.
For loads applied to shells or solids only, the hydrodynamic load components are included. Hydrostatic loading can be included on SURF154 using face 4 with PL = -(gravity acceleration) * (water density) and by setting KEYOPT(6) = 1. For loads applied to pipe elements, hydrostatic loading is included automatically.
It is not necessary for the hydrodynamic and structural meshes to be identical, and the loading is mapped automatically from one to the other; however, it is assumed that the hydrodynamic axis system is identical to that used for the structural analysis. Small displacements are assumed in the load-mapping process, and mapping may not be accurate if this condition is not met.
The .ahd file contains data indicating which side of a panel is the wetted side. The program uses the data to adjust which direction to apply the hydrodynamic pressure on the SURF154 elements.
Each structural integration point is located in its nearest hydrodynamic panel. The program then calculates the corner pressures of the panel based on a weighted averaging of that panel and its neighboring panels. Finally, using a bilinear fit across the panel, the program calculates the hydrodynamic pressure at the structural integration point.