Thank you so much for you time and you support.
- Yes,the platform-pitch motion is very large (over 24 degrees).NO I am not expecting such large platform-pitch motion from my floating system design.Because NTNU thesis report which I am using(same 10MW spar-buoy) right now,In that platform-pitch motion mean value is 2.85 deg. and maximum value is 11.32 deg.So as per my point of view problem is in the platform-pitch restoring.I have calculated Hydrostatic restoring in pitch and roll as per given equation in the HydroDyn manual((c44/c55 = -8159587654 N/m(ρgA/4R^2+ρgvzb))
where,ρ(Density of water)=1025 kg/m^3, g(Gravity)=9.80655, A(Water plan area)= (π)/4*8.3^2(where 8.3 is the diameter of spar at the water plan), R(Radius at the water plan)=)8.3/2, V( Submerged Volume )=13085.41480(As per HydroDyn output file), Zb(Center of buoyancy)=62.05271(I have manually calculated for my system).
But in the NTNU thesis report they have used another equation to define the hydrostatic restoring in roll and pitch. I have attached the picture of that equation as below.The value of that non zero term as per NTNU report is c44/c55=1690150000 N/m.
If I am using platform Hydrostatic restoring in pitch value(C55) = 1690150000 N/m (AS per NTNU report) or C55=0 N/m in Additional linear stiffness matrix then my Simulation running completely but starting of that simulation I am getting error of “Small angle assumption”(Only getting this error equal or grater than 17 m/s wind speed ). Also I have tried to solve by using very small time steps in OpenFAST but it does not work.I have also attached the picture of that error as below.
I would like to mention that for the (C55) = 1690150000 N/m (AS per NTNU report) and C55=0 N/m my system is in the hydrostatic equilibrium condition wihtout wind and waves because from the platform motion chart.
Question: I have seen in many other floating system HydroDYn file that they have not used pitch and roll hydrostatics stiffness value so It could be possible to run the simulation without those values??? Can you please explain me a bit why It should be possible without those values ??
What could be the reason for the error “small angle assumption” at the starting of the simulation for only 17 to 25 m/s wind speed?
- I can understand that the mooring stiffness or hydrostatic stiffness are too low. Also I have tried to solved by increasing the value of the mooring stiffness but It could not work. Currently I have used the equivalent mooring line extensional stiffness = 384243000.I have attached MoorDyn file as below.
The platform-pitch natural frequency predicted by FAST is higher(It is around 60 s) than I am expecting while using the c44/c55 = -8159587654 N/m. But The platform-pitch natural frequency predicted by FAST is lower (It is around 25 s) than I am expecting while using the c44/c55=1690150000 N/m (AS per NTNU report) and C55=0 N/m.Because as per NTNU thesis report it should be around 35 s.
Question:Is should be okey to use the platform Hydrostatic restoring in pitch value(C55) = 0.?
From the error " small angle assumption " at the starting of the simulation, does it really effect my output data?
I hope you can clearly understand my explanation and will help me out as soon as possible.
I really appreciate for your valuable time and your support.