Diameter of the OC3 platform


I am now trying to get more details of the floating platform of OC3.
In the file “NRELOffshrBsline5MW_Tower_OC3Hywind.dat”, I found the term “PtfmDiam” (effective platform diameter in calculation of viscous drag term from Morison’s equation) is set as 6.5 meters.
However, according to the report “Definition of the Floating System for Phase IV of OC3”, the platform diameter above taper is 6.5 meters, while platform diameter below taper is 9.4 meters.
Does this mean we only use the platform diameter above taper in the calculation?


Dear Yulin,

The current version of FAST/HydroDyn only allows for a single “effective” diameter for the viscous-drag calculation on floating platforms. We are working on an uprade to HydroDyn that will support tapered members (and many other features).

Until then, we’ve compiled a custom version of FAST that includes the depth-varying diameter (tapered region) for the viscous drag calculation of the OC3-Hywind system. So, in effect, the PtfmDiam = 6.5 m specified in the OC3-Hywind input file is not needed when you use this custom version of FAST. We’ve also included in this customized version of FAST the additional linear damping" documented in the OC3-Hywind specifications report. We’ve included this customized compilation of FAST in the OC3-Hywind model archive available from here: wind.nrel.gov/public/jjonkman/NR … Hywind.zip.

If you need to recompile FAST yourself for some reason, please let me know and I can tell you the source code changes that are required to accomodate these two customizations.

I hope that helps.

Best regards,

Hi Jason,

Thank you for the detailed reply. Actually I am trying to add more structural control features into FAST, so I need to modify and recompile the code to create the executable file.

Yes, I found the simulation results are different between the version “FAST_v7.01.00a-bjj_AeroDyn_v13.00.01a-bjj_BladedDLLInterface_OC3Hywind.exe” and my compiled “FAST.exe” based on the code from “http://wind.nrel.gov/designcodes/simulators/fast/”. Probably the difference is caused by the new features in FAST, especially “linear damping”.

It would be great if I can update the code under your instruction. Thanks in advance.

BTW, does the “OC3-Hywind specifications report” mean “Jonkman_DefinitionOfTheFloatingSystemForPhaseIVOfOC3_NREL-47535_2010.pdf” ?

Kind regards,

Dear Yulin,

Yes, by “OC3-Hywind specifications report,” I meant report #47535.

Two model-specific changes (customizations) must be made to the current version of FAST/HydroDyn for the OC3-Hywind spar buoy.

In order to have the viscous drag coefficient depend on the depth-varying platform diameter (taper region), the following logic must be added before variable ViscousForce is computed in SUBROUTINE HydroCalc.f90/FltngPtfmLd():

IF ( WaveKinzi0(JNode) > - 4.0 ) THEN
PtfmDiam = 6.5
ELSEIF ( WaveKinzi0(JNode) < -12.0 ) THEN
PtfmDiam = 9.4
PtfmDiam = 9.4 + ( 6.5 - 9.4 )*( WaveKinzi0(JNode) + 12.0 )/( -4.0 + 12.0 )

In order to augment the platform damping with additional linear damping in surge, sway, heave, and yaw, the following logic must be added after variable PtfmFt is computed in SUBROUTINE HydroCalc.f90/FltngPtfmLd():

PtfmFt(1) = PtfmFt(1) - 100000.0XD(1)
PtfmFt(2) = PtfmFt(2) - 100000.0
PtfmFt(3) = PtfmFt(3) - 130000.0XD(3)
PtfmFt(6) = PtfmFt(6) - 13000000.0

Also, the torque and pitch control system for this turbine is implemented as a DLL in the style of GH Bladed. So, to run FAST with this controller, the BladedDLLInterface.f90 source file must be used according to the instructions documented in the FAST User’s Guide.

I hope that helps.

Best regards,

Sorry to dig up an old thread, but I’ve just noticed this diameter input myself and I have a few quick questions about it:

  1. The two diameters in question here, 6.5m and 9.4m, differ quite a bit. What effect will choosing the smaller diameter have on the overall hydrodynamic response, especially in surge?

  2. The platform is predominantly 9.4m in diameter and only has a relatively small section of 6.5m. What prompted the choice of the 6.5m for the ‘official’ input?

Thanks in advance for any help! I’m looking for numbers and methods to compare my CFD results to.

Sean Q

Dear Sean,

Wave loads descrease exponentially with depth, so reducing the diameter near the free surface would lead to smaller wave forces. Of course, it is always necessary to ensure that the structure is sufficiently designed to support the internal structural loads. As such, it is unclear whether the diameter of the OC3-Hywind system could be reduced substantially.

The external geometry of the OC3-Hywind spar (including the 6.5-m diameter at the water line with a taper to 9.4-m diameter) was chosen to match the conceptual version of the Hywind platform as described in the OC3-Hywind speciciations document: nrel.gov/docs/fy10osti/47535.pdf.

I hope that helps.

Best regards,