Hi

I am planning to compute RAOs for OC3-Hywind by coupling Sesam and FAST following the same method introduced in the paper “Investigation of Response Amplitude Operators for Floating Offshore Wind Turbines” by Ramachandran and Jonkman.

I used Sesam to build a freely floating panel model to compute the hydrodynamic quantities(added mass,damping and wave excitation forces) and hydrostatic restoring forces by assuming the center of gravity position is at the intersection of the still water level and the centerline of the platform which is also the reference point. Moreover, I compared the hydrodynamic quantities from Sesam with that from the paper “Definition of the Floating System for Phase IV of OC3” and agreed very well.

I then added some additional matrices according to the paper “Definition of the Floating System for Phase IV of OC3”,including the additional linear damping matrix for surge and sway, heave and yaw motions, the mooring stiffness matrix, the additional yaw stiffness matrix.

Mass matrix is the last I need, and I used FAST linearization to get it,the followings are my steps:

(1)Download the FAST OC3-Hywind model from “Index of /public/jjonkman/NRELOffshrBsline5MW/NRELOffshrBsline5MW_OC3Hywind.zip”

(2)Redefine the added mass and damping as zero in the file named “spar.1”

(3)Disable all DOFs except the six platform DOFs

(4)Set WaveMod,CurrMod and PtfmCD and all the initial platform displacement to zero in the platform file

(5)Set a wind file with zero wind speed and activate the feature flag-CompAero

(6)Set CalcStdy to false in the linearization file

(7)Shut all controllers down and set all the initial conditions(including Azimuth and RotSpeed) to zero, and set AnalMode to 2

(8)linearize it and got the mass matrix

I got my RAOs for surge,heave and pitch(rigid wind turbine with no wind) :

And they differed in some way from that RAOs in the paper by Ramachandran and Jonkman under the same condition(rigid wind turbine with no wind):My RAOs are less in amplitude but have similar shape.

Why they are different?I would really appreciate it if anyone could offer me some suggestions!