During the coupled time domain dynamic simulation, it's been considered more accurate to use filtered slow drift motion rather than the total motion of each time step for wave phase shift calculation. Butterworth filter is one of the most widely used low pass filter due to its "smoothest" frequency responses and simplest transfer function. Based on a deepwater semisubmersible production platform, sensitivity studies, including with and without filtering and different cut-off frequencies, are conducted to investigate the effects of using filtered slow drift motion for wave phase shift calculation in the dynamic simulation on the platform global performance.
For offshore deepwater platform, its wave force database is commonly calculated based on small wave amplitude assumption. Wave phase angle is a function of both time and position. In time domain simulations, the platform's position is affected by large waves, and in turn the wave forces on the platform are also affected. This leads to nonlinear responses in waves. However, the quadratic part of the nonlinear behavior is already accounted for in the quadratic transfer function (QTF) data. Therefore, there are risks of double counting the quadratic loading on platform - one is the application of the linear diffraction analysis loads using the instantaneous platform position, and the other is the use of QTF data.
The instantaneous motion of moored floating platforms in deep water typically comprises two categories: wave frequency motion which is response to wave loads; and low frequency motion such as slow drift motion due to wind, wave and current. Slow drift motion is relatively steady with oscillating period much longer than the dominant period in the wave-spectrum of interest, due to the relatively large mass of the platform and the weak restoring forces of the mooring system (Emmerhoflf, 1996).
In the time domain simulation, if wave phase shift calculation is based on the slow drift motion (low frequency position), instead of the total motion (instantaneous position), then double counting of quadratic loads can be minimized or avoided.
