Estimation of the second order hydrodynamic effects is one of major design and analysis concerns for performance and safety in marine structures. Aqwa provides a numerical simulation to investigate these effects by means of both time domain and frequency domain analysis approaches.

In this chapter, a general description of the first and second order motions and forces is provided, and the formula of the far field solution of the second order mean wave drift force and moment coefficients in the horizontal plane under unidirectional waves are presented. Then the general forms of the second order forces (the quadratic transfer function (QTF) matrices) due to a pair of regular waves with different frequencies and directions are deduced based on the near field integration over the mean wetted hull surface. As a special case, the expressions of mean wave drift forces and moments (near field solution) are given for both unidirectional waves and bi-directional waves cases. Some simplified approaches applied in Aqwa, such as the extended Newman's approximation of the difference frequency second order forces and the Pinkster's approximation of the contribution of the second order potential on the second order forces, are also discussed.

By employing the generated QTF matrix database, the equilibrium position prediction, time domain response simulation and frequency domain response statistical analysis of marine structure systems with the second order force effects under irregular waves can be implemented. These topics are discussed in Equilibrium Estimation and Stability Analysis of Structure System, Frequency Domain Dynamic Simulation, and Time Domain Dynamic Simulation.