Dear @Jason.Jonkman ,

1- Ok but i was wondering about the reference. For example, in OpenFast v2.7, if the reference was at the center of gravity of the floater and i run decay test and obtain the natural frequencies. And for the same wind turbine, i use OpenFast v2.5 to run decay test to obtain the natural frequencies. I should obtain the same natural frequencies no ?

In other words, the choice of the reference for these two matrices depend on the reference chosen for kinematics of the tower ?

2- Here i was refering to data like tower mass density (kg/m), or blade stiffness in flapwise, …

3- I’m sorry, i was working on several topics and i don’t remember where i found the value

4- In OpenFast documentation section 4.2.8.4.8.3 you give the linear hydrostatic matrix. I think it’s given at the reference (0,0,0) i.e SWL, is it right ? I said this because i saw that the integrals should be done at the undisplaced waterplane area of the platform.

5- When we said “platform”, that means we are talking about the whole wind turbine not only the floater right ?

6- The parameters in the linear hydrostatic matrix corresponds to the floater only or the whole platform ? For example if we take C44, it contains zG. Here zG is the center of gravity of the whole turbine ?

7- Can you suggest a book or a method to be able to calculate the position of the center of buoyancy of a floater knowing the position of the center of buoyancy for the same floater but containing ballast ?

8- In the technical report “Definition of the floating system for phase IV of OC3” page 7, you give the linear hydrostatic matrix. For the C33, the formula : rho * 9.81 * pi * 6.5^2 * 0.25 works well. But, for the coefficients C44 and C55 the formula: rho * 9.81 * (volume_undisplaced * distance(COB,COG)+pi * D^4/64) doesn’t work and also you obtained negative numbers. Could you please explain why ?

9- I realized also that you add a damping matrix to account for radiation damping page 14 in the technical report “Definition of the floating system for phase IV of OC3”. I thought that the viscous drag term in Morison’s formula takes into account the radiation damping but my question is why did you have zero for roll and pitch and non zero diagonal terms for the other motion ?

Sorry for bothering you

Thank you in advance for your explanations and for your help,

Riad