Dear @ Jason.Jonkman,
Thank you for your reply, it was very helpful.
Refer to the method in OC3-Hywind RAOs ， I first defined a transformation matrix：
1 0 0 0 89.9155000000000 0
0 1 0 -89.9155000000000 0 0
0 0 1 0 0 0
0 0 0 1 0 0
0 0 0 0 1 0
0 0 0 0 0 1
K_Lines_cg = TransMat’K_Lines_swl TransMat
K_Lines0_cg = TransMat’*K_Lines0_swl
C_AdditionalDamping_cg=TransMat’C_AdditionalDamping_swl TransMat
K_Hydrostatic_cg :For the C33, the formula : rho * 9.81 * pi * 6.5^2 * 0.25 ,for the coefficients C44 and C55 the formula: rho * 9.81 * (volume_undisplaced * (ZCOG-ZCOB)+pi * D^4/64) (volume_undisplaced=8.0573e+03,ZCOG=89.9155,ZCOB=62)
0 0 0 0 0 0
0 0 0 0 0 0
0 0 333320 0 0 0
0 0 0 2260237325.20468 0 0
0 0 0 0 2260237325.20468 0
0 0 0 0 0 0
Using Strip theory, I computed the added mass matrix with its coordinate origin located at the platform’s center of mass.(Ca=1)
8442486.95734304 0 0 0 231124957.726698 0
0 8442486.95734304 0 -231124957.726698 0 0
0 0 0 0 0 0
0 -231124957.726698 0 16343912822.7079 0 0
231124957.726698 0 0 0 16343912822.7079 0
0 0 0 0 0 0
In the pitch-direction free decay analysis of the structure, apart from the vertical gravity and buoyancy forces that remain constant, the loads acting on the structure are as follows:
F=-K_Lines_cg* q-C_AdditionalDamping_cg* q_dot-K_Hydrostatic_cg*q+K_Lines0_cg;
Using this method, I performed the solution, but there is a significant discrepancy between the results and those obtained from OpenFast. Could you kindly help me identify where I might have gone wrong in this approach?
I hope you can clarify my confusion, point out mistakes in my calculations, or identify any logical or knowledge gaps in my understanding.
Thank you again for your reply and help.
Best regards,