Aqwa can generate a time history of the simulated motions of floating structures, arbitrarily connected by articulations or mooring lines, under the action of wind, wave and current forces. The positions and velocities of the structures are determined at each time step by integrating the accelerations due to these forces in the time domain, using a two stage predictor-corrector numerical integration scheme (see Integration in Time of Motion Equation).
Aqwa can employ one of two main simulation approaches in the time domain:
Irregular wave responses with slow drift (Aqwa-Drift)
This analysis is used to simulate the real-time motion of a floating body or bodies while operating in irregular waves. Wave-frequency motions and low period oscillatory drift motions may be considered. Wind and current loading may also be applied.
The difference frequency and sum frequency second order forces are calculated at each time step in the simulation, together with the first order wave frequency forces and instantaneous values of all other forces. These are applied to the structures and the resulting accelerations are calculated, from which the structure positions and velocities are determined at the subsequent time step. The system properties at the end of one time step are then the starting conditions for the next, and so a time history of the motion of each structure is constructed.
As well as the instantaneous values of all other forces (i.e. wind and current drag, cable tension, etc.), one of four options can be selected to define forces due to the sea state: slow drift only, wave frequency response only, wave frequency response with slow drift, and wave frequency response with slow drift and sum frequency second order force excitation. The Aqwa-Drift analysis is normally applicable for low and moderate sea states.
Severe wave responses (Aqwa-Naut)
This analysis is used to simulate the real-time motion of a floating body or bodies while operating in regular or irregular waves. Nonlinear Froude-Krylov and hydrostatic forces are estimated under an instantaneous incident wave surface. Wind and current loads may also be considered.
The analysis involves meshing the total surface of a structure to create a hydrodynamic and hydrostatic model. Nonlinear hydrostatic and Froude-Krylov wave forces can then be calculated from this model at each time step in a simulation, along with instantaneous values of all other forces. These forces are then applied to structures via a mathematical model (i.e. a set of nonlinear equations of motion), and the resulting accelerations are determined.
As well as instantaneous values of all other forces (i.e. wind and current drag, cable tensions, etc.), either regular or irregular wave response may be selected to define forces due to the sea state. Airy wave theory or second order Stokes wave theory, in finite-depth or deep water, can be applied for the regular wave response analysis. The Aqwa-Naut analysis is normally applicable for severe sea states, or if the wetted surface of a structure changes significantly during the simulated time (for example, during the analysis of a jacket structure launching and installation).