The Morison's equation approach is widely used for slender body components when the characteristic diameter of a structural component is less than 1/5th of the shortest wavelength. In this equation, the drag load component is induced by viscosity and proportional to the relative velocity between fluid particles and the structure surface, and becomes important when the structural members are slender and wave amplitude is large.
Morison element forces relate to all non-panel elements that can attract wave and current loading. In Aqwa, cylindrical tube (TUBE), slender tube with general cross section form (STUB) and disc (DISC) elements can be defined.
Aqwa employs a hybrid method to model the large-volume components of structure by diffracting panels and the rest of the small cross sectional components by Morison elements. The diffracting panel method is discussed in Hydrodynamic Radiation and Diffraction Analysis by Source Distribution Method and Second Order Wave Excitation Forces.