Questions about the phase angle caused by large displacement

Wind & Water Computer-Aided Engineering Software Tools
1

Dear @Jason.Jonkman

I’m checking OpenFAST for the calculation of wave excitation loads in the case of large amplitude motions. If I understand correctly, HydroDyn seems to use interpolation directly to calculate wave excitation loads when ExctnMod = 1 (DFT method).

  ! Compute the load contribution from incident waves (i.e., the diffraction problem):
      if ( p%ExctnMod == 0 ) then
         
         m%F_Waves1 = 0.0_ReKi
         
      else if ( p%ExctnMod == 1 ) then
                  
            ! Abort if the wave excitation loads have not been computed yet:
         IF ( .NOT. ALLOCATED ( p%WaveExctn ) )  THEN
            ErrMsg  = ' Routine WAMIT_Init() must be called before routine WAMIT_CalcOutput().'
            ErrStat = ErrID_Fatal
            RETURN
         END IF

         DO I = 1,6*p%NBody     ! Loop through all wave excitation forces and moments
            m%F_Waves1(I) = InterpWrappedStpReal ( REAL(Time, SiKi), WaveTime(:), p%WaveExctn(:,I), &
                                                     m%LastIndWave, p%NStepWave + 1       )
         END DO          ! I - All wave excitation forces and moments
      else if ( p%ExctnMod == 2 ) then
         
         call SS_Exc_CalcOutput( Time, m%SS_Exctn_u, p%SS_Exctn, x%SS_Exctn, xd%SS_Exctn,  &
                                z%SS_Exctn, OtherState%SS_Exctn, m%SS_Exctn_y, m%SS_Exctn, ErrStat, ErrMsg )
         m%F_Waves1  (:) = m%SS_Exctn_y%y
         
      end if

However, when the platform displacements are large, the displacement phase angle must be considered. The displacement phase angle is defined as the phase of wave propagation between the platform current position and the global coordinate origin. Because the wave excitation coefficients from WAMIT are obtained based on the fixed structure.

Has HydroDyn considered this elsewhere and I missed it?

Best regards,

Tianyuan

2

Dear @TianYuan.Wang,

Presuming you are using OpenFAST v3.5.4 or older. In these versions, the wave-excitation is assumed to be calculated at the undisplaced position of the body.

However, in OpenFAST v4, consideration of large body displacement has been added, including considering of the local wave kinematics with the proper phase. For first-order wave-excitation from the potential-flow solution, this is done by generating the solution across a 2D X-Y grid of possible displacements, and interpolating not only in time, but within this 2D domain based on the instantaneous body displacement. See our IOWTC 2022 paper for more information: https://www.nrel.gov/docs/fy23osti/83679.pdf. OpenFAST v4 is due out very soon, but if you don’t want to wait, you can access this capability through the Dev branch of OpenFAST.

Best regards,

3

Hi,

That makes sense. Thank you for your quick reply.

Best regards,

Tianyuan