Hydrodynamic loading on a marine structure is mainly caused by the kinematics of water particles in waves, motions of the structure, and interactions between waves and the structure.

Offshore structure designers and engineers are normally concerned with three categories of hydrodynamic loading on marine structures: drag load, wave exciting load, and inertia load ([4]).

Drag loads are induced by viscosity and are proportional to the square of relative velocity between fluid particle and structure surface. They are important when structural members are slender and wave amplitude is large.

In small amplitude waves, the wave exciting load consists of the first order incident wave force (Froude-Krylov force) and the diffraction force which is induced by the disturbance wave due to the existence of a body. In larger seas, both the first order forces and the second order forces are important. In severe seas, bottom and flare slamming transient forces may also be included.

Wave inertia load or radiation load is caused by the disturbed waves induced by the body motions. Fluid potential theories are commonly used for solving the wave inertia load and wave exciting load.

Three dimensional panel methods are the most common numerical tools to analyze the hydrodynamic behavior of a large-volume structure in waves. These methods are based on the fluid potential theory and represent the structure surface by a series of diffraction panels. The Morison's equation approach is widely used for slender body components. Aqwa employs a hybrid method to model the large-volume components of a structure by diffracting panels and the small cross sectional components by Morison elements.

In this chapter, the numerical approach (the source distribution method) is described for estimation of the first order wave load. Methods for calculating the second order force are described in Second Order Wave Excitation Forces. The formula for slender body force in waves is presented in Morison Element Forces.