Dear Jason,
Thanks for the reply.
I tried to make the changes you suggested, however these have not yet solved the problem.
Nonlinear inertial terms (Figure 1), i.e., omega x (omega x r_cm * mass) and omega x I dot omega have little effect (around 5 N or 5Nm for each force / moment except Fz where there are peaks of 800N). Can you confirm that omega = [rxd, ryd, rzd]?
I tried to multiply the accelerations obtained by FAST by the mass matrix M, obtaining the resulting forces acting in the 6dof (Figure 2): as you can see it takes moments on Mx and Mz of order 10 ^ 4, I can’t understand how they are caused since non-linear terms have such a low incidence.
I modified the mass matrix, translating it for each timestep in relation to the movements of the platform:
I obtained the mass matrix relative to the center of mass through the following inverse transformation: Mcg = (TransMat^T)^-1 * M_swl * TransMat^-1 (where TransMat is the one indicated in this topic "OC3-Hywind RAOs where the inputs of the rotation matrix are those in still water). Then the mass matrix (in 0 0 0) relative to the position of the system is obtained by applying the direct transformation M_swl = TransMat^T * Mcg * TransMat, using the current positions of Cg as the input of TransMat.
Even with this change there are no improvements, Mx and Mz of my model still remain too low.
Best regards,
Riccardo
