Model, wave effects

Hello everyone,

In the linearized model of the turbine, how can I find the relation between wave and other states of the model? I want to investigate the effects of wave on for example generator speed, platform pitch angle, …

Thank you

Dear Omid,

The linearization functionality of FAST does not currently allow one to linearize an offshore wind turbine model that has incidedent waves. That is, you can only linearize an offshore model in still water currently with FAST, with WaveMod = 0. If you want to determine the effect of waves on response in FAST currently, you must perform an analysis in the time domain.

Best regards,

Hi Jason,
Thanks for the response.

Hi,
Does FAST have the capability of finding the moment due to the hydrostatic or hydrodynamic force such as wave?
thank you

Dear Omid,

I’m not exactly sure what you are asking, but certainly the hydrostatic moments and hydrodynamic moments are computed by FAST’s HydroDyn module throughout a simulation.

Best regards,

Hi Jason,
Thanks for your reply, Where can I get those moments while I am doing simulation? among the parameters which are listed in Outlist, I can see for example “TwrBsMxt” , “PtfmMxt”,… Are they specifying hydrostatic and hydrodynamic moments?
Thank you

Dear Omid,

The tower-base load outputs from FAST (TwrBsFxt through TwrBsMzt) include all of the loads applied to the platform that are transmitted from the wind turbine and tower.

The platform load outputs (PtfmFxt through PtfmMzt or PtfmFxi through PtfmMzi) include all of the loads applied to the platform apart from the loads transmitted from the wind turbine and tower. So, they include the combined contribution from mooring loads, wave-excitation loads, added mass, radiation damping, hydrostatics, and viscous drag. At this time, one cannot output the individual contributions to the load (e.g., a hydrostatic-only load). This is something that we are currently working to add. Until then, you can access the individual load contributions through the source code.

Best regards,

Dear Jonkman,
For modelling a floating wind turbine with both waves and wind we need WAMIT files (.1,.3,.hst). WAMIT is a 3D radiation/diffraction analysis program,now suppose our floater model is Morison then how can we do this type of analysis. WAMIT don’t have the capability to model Morison model.And when we are using other programs like ANSYS-AQWA or SESAM we are not getting the -1 and 0 period added mass which is required for the coupled analysis .Suggest some thing.

Thank you.

Dear Anuj,

Presumably you’d like to model your floater with Morison’s equation because your floater is composed of slender members, where viscous effects dominate over potential-flow-based radiation/diffraction. In the current version of FAST’s HydroDyn module, viscous drag is only calculated for a single vertical constant-diameter cylinder. That said, we are working on a new release of HydroDyn that includes the ability to apply Morison’s equation to any number of members of a multi-member substructure (including inclined and tapered members). The new hydrodynamic calculations will apply to the viscous loads in slender members of a floater (e.g., the braces of a semisubmersible or arms of a tension-leg platform) and also to fixed-bottom multi-member support structures (e.g., jackets and tripods). This new release is planned for March.

In WAMIT, a period of zero represents infinite frequency and a period less than zero (e.g., -1) represents zero frequency. If the 3D radiation/diffraction program you are using cannot compute the zero- and infinite-frequency limits, you should estimate what these are from low and high frequency values, respectively, before using the data in HydroDyn. Please note that the hydrodynamic radiation damping is always zero at zero and infinite frequency.

Best regards,

Dear Jonkman,
I am trying to model a Hydrodyn model referring your technical report “Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine-2007” .
But i am confused about some points can u clarify them.
Actually my platform is a Tension leg platform (TLP) which is a unstable structure and the stability is provided by tethers. Now, the doubt is that the total draft at the given stable condition is used or the draft at which the platform alone is stable is used in Hyrodyn model.
In WAMIT also which draft i used to model and get the required files (.1,.3 and .hst).
The stable draft for the complete system is 17 m and the stable draft only for the platform is 4m.
Or it would be help full then we get some guidelines to make the Hydrodyn input files .

Thanking you

Dear Anuj,

A model in FAST should be set up such that combined weight of the rotor + drivetrain + nacelle + tower + platform balance with the hydrostatic buoyancy and mooring (tether) pretension in the vertical direction when the platform is undisplaced (where platform surge, sway, heave, roll, pitch and yaw = zero).

For a tension-leg platform (TLP) – as well as for other floating platform types – the draft (both in HydroDyn and WAMIT) should be chosen such that the above condition applies. More likely than not, the draft of a TLP will be set such that system would be hydrostically unstable without the tethers.

I hope that helps.

Best regards,

Dear Jonkman ,
Thanks for your reply but i check this link “http://wind.nrel.gov/public/jjonkman/” here you are given a example of a 5MW wind turbine on different floating platform.In that TLP is also there i downloaded that and check the WAMIT files but i want to know how are you getting that low value of hydrostatics stiffness’s.you are getting a range about 100 in heave heave motion . with a diameter of 18 . and i am getting a range of 100000 with a diameter of 10 only . and your TLP height is more than mine . That’s why the doubt came in my mind that up to what draft we have to model in WAMIT .
And the forces values are also very less in .3 file .because i match everything i make the platform file correct but not getting the results . and i am not sure about WAMIT model.

Dear Anuj,

The heave-heave component of the hydrostatic stiffness matrix should equal densitygravityarea, where area is the cross-sectional area of the undisplaced floater at the still water level. (The heave-heave component of the hydrostatic stiffness matrix is independent of draft.) WAMIT will nondimensionalize this value before writing it to the .hst file (all of the WAMIT output is nondimensional). For the heave-heave component of the hydrostatic stiffness matrix, the normalizing factor is densitygravity*ULEN^2, where ULEN is the length scale specified in the WAMIT input file. When the WAMIT data is read in by HydroDyn, HydroDyn will redimensionalize the data; in this process, HydroDyn assumes that ULEN is unity. (We plan to add ULEN as an input in a future release of HydroDyn.) Have you used a ULEN of unity in WAMIT?

Best regards,

Dear Jonkman,
Actually i am not using WAMIT , i am using ANSYS-AQWA and preparing the same files , that’s why i am getting the high values .which as you told are dimensionless in fast getting more bigger so i think the output is coming absurd.
Do you have any idea how to deal with hydrodyn model when we are using another program instead of WAMIT.

Sir ,
also want to know whether WAMIT will dimensionlise all the outputs or not .
And why are you giving 0 in VCG and Radius of gyration in .frc file of WAMIT.whether 0 have any significance.
The files i refer is from “Dynamics Modeling and Loads Analysis of an Offshore Floating Wind Turbine”.

Dear Anuj,

HydroDyn expects hydrodynamic input data derived from WAMIT. If you are not using WAMIT, I suggest you reformat your data according to the WAMIT format (including nondimensionalization) before inputting them to HydroDyn. Information on the WAMIT format is available from Chapter 4 of the WAMIT User’s Guide, available publicly from here: wamit.com/manual6.4/Chap4.pdf. HydroDyn needs data from the *.1, *.3, and *.hst WAMIT output files.

The reason why the vertical center of gravity (VCG) and radii of gyration are set to zero in WAMIT is described in the forum topic found here: http://forums.nrel.gov/t/about-wamit-model-for-5mw-oc3-hywind/482/1.

Best regards,

Dear jason
Do you ever use HydroDyn module to simulate the dynamic response of gravity-base foundation?
If yes, does the HydroDyn module consider MacCamy-Fuchs approximation?
Thanks for your help.

Best Regards
Jason. Lai

Dear Jason,

HydroDyn does not use the MacCamy-Fuchs approximation for large-diameter members.

I have not used HydroDyn myself to model gravity-base foundations, but other than the absence of MacCamy-Fuchs, it should be possible to model gravity-based foundations.

Best regards,

Dear jason
Thanks for your reply.
According to your reply, other than the absence of MacCamy-Fuchs, it should be possible to model gravity-based foundations.
Do you describe it in more detail?
Is the gravity-based foundation simulation the same as monopile?

Best Regards
Jason. Lai

Dear Jason,

The strip-theory solution of HydroDyn accounts for fluid-inertia, added-mass, viscous drag, and buoyancy loads, as well as axial loads on tapered members etc., which are appropriate for modeling gravity-based structures. Section 6.7 of the draft HydroDyn User’s Guide and Theory Manual provides some general guide on modeling fixed-bottom substructures, including gravity-based foundations: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.

Best regards.