When should I use diffraction theory to calculate wave forces on a structure?

Diffraction theory

When analysing hydrodynamics of offshore structures, the simple and appealing Morison equation has its limitations. It is applicable only to slender members where the wave length is longer than approximately five times the member diameter or member cross width. For more voluminous bodies there is no real alternative to using the diffraction theory.

In Sesam, both diffraction (waves reflected from a rigid body) and radiation (waves generated by a moving, i.e. floating, body) are available.