Hi All,

I wanna to linearize OC3-Hywind model with FAST.

According to linearization part of the FAST’s users guide, it seems that the linearization is done only for land based wind turbine and there are no talk about floating wind turbine linearization.Is it possible to linearize OC3-Hywind via FAST? if yes How?

For example does hydrodynamic forces effects the periodic steady state solution? How the hydrodynamic loads will effect the second order linearized reprezentation of equations? In this way what will be the states??? What is the orders of the states?

Is there any extara information(except FAST users Guide) about linearizing such sytems?

Many thanks beforehand,

regards,

Dear Mehdi,

As you may have noticed, the hydrodynamics module of FAST (called HydroDyn) is currently an undocumented feature of the code. That is, the input and output parameters are not presently explained in any user manual. That said, HydroDyn has been tested by many users, so while the code is currently undocumented, it is well tested. We are in the process of reworking HydroDyn so that it (1) is a stand-alone module that can be called by a variety of structural codes and (2) can be applied to multi-member fixed-bottom systems (e.g., jackets and tripods). We don’t plan to release a user’s guide until this version is complete.

Regardless, “yes,” you can linearize a FAST model of a floating wind turbine, but there are limitations. The features of the platform linearization are as follows (in addition to the linearization features of the turbine as documented in the FAST User’s Guide):

Included in the linearized model:

-platform DOFs

-platform mass and inertia

-coupling of platform motions with turbine motions

-hydrostatic restoring (buoyancy)

-linearization of the nonlinear hydrodynamic viscous drag, including possible effects from (steady) sea current

-impulsive hydrodynamic added mass (this modifies the mass matrix)

-linearization of the nonlinear mooring system

Not included in the linearized model:

-hydrodynamic wave-radiation damping and free-surface memory (i.e., RdtnTMax must be set to 0.0 during linearization)

-hydrodynamic wave-excitation forces (i.e., you can only linearize a model in still water, with WaveMod set to 0)

I hope that helps.

Best regards,

Hi Jason,

Don’t you(NREL) want to include the following effects in the linearized model of FAST?

-hydrodynamic wave-radiation damping and free-surface memory.

-hydrodynamic wave-excitation forces.

If Yes? When it will be done? In near future or …

What about Adams/linearization?

What extra parameters are capable to be perturbed to obtain the linearized model?

For example in terrestrial one the linearized models can be achieved for different wind speeds in different rotor azimuth angles. What about in floating one?

Best regards,

Dear Mehdi,

Hydrodynamic wave-radiation and free-surface memory is implemented in the HydroDyn module of FAST in two ways: (1) direct time-domain convolution and (2) linear state-space form. The former is not suitable for linearization, but the later is. The state-space radiation solution will enable linearization of the hydrodynamic wave-radiation and free-surface memory once linearization functionality has been added to FAST v8.

Hydrodynamic wave-excitation loads in the HydroDyn module of FAST are computed as time-dependent (periodic for regular waves or stochastic for irregular waves) parameters, which are independent of module states, inputs, or outputs. I’m not sure what it would mean to linearize such a term.

NREL is not currently funded to support further development of the ADAMS interface.

I’m not sure I understand your last question, but the same linearization features available for land-based turbines are available for floating wind turbines.

Best regards,

Hi Jason,

As you know, the offshore wind turbine system is affected by two kinds of disturbances, wind and wave loads. So by linearizing the model using FAST, we expect to see the both disturbances indicators in B_d and Delta_u_d matrices in state space model (I expect the Delta_u_d be 2*1 vector contains Wind and wave effective speeds or any other terms relating to wind and wave loads acting on machine), but the Delta_u_d appeared after linearizination is a vector only related to the wind disturbances according to the FAST user’s guide. I am amazed why there aren’t wave input disturbance settings, like Table 6 in FAST user’s guide for wind. Or I am misunderstood and there are some concepts that I must pay attention?!

In another words, which terms are related to wave disturbances in linearized offshore wind turbine state space model using FAST linearization module? (Please indicate frankly)

Regards,

Mehdi,

Dear Mehdi,

FAST v7 cannot linearize a model with wave excitation (WaveMod must equal zero during the linearization process) and the linearized model does not include wave disturbances. This is because of two features of wave excitation that are different from wind:

- Wave-excitation loads in the HydroDyn module of FAST are computed as time-dependent (periodic for regular waves or stochastic for irregular waves) parameters, which are independent of module states, inputs, or outputs (as stated in my Jul 27, 2015 post above).
- Wave-excitation loads are periodic with zero mean (assuming first-order regular waves here; FAST cannot consider stochastics during the linearization process, including irregular waves or turbulent wind).

With I’m not sure what it would mean to linearize such a term.

Best regards,

Hi,

I linearized the OC3-Hywind based on the following input files for steady wind speed of 18m/s:

NRELOffshrBsline5MW_Linear.txt (2.1 KB)

NRELOffshrBsline5MW_AeroDyn.txt (2.71 KB)

NRELOffshrBsline5MW_Platform_OC3Hywind.txt (7.91 KB)

NRELOffshrBsline5MW_Platform_OC3Hywind.txt (7.91 KB)

and I get the AvgAMat as below:

0 0 1.00000000000000 0

0 0 0 1.00000000000000

-0.0141066666666667 3.06972222222221e-09 -0.0554466666666667 -0.000458908333333333

-0.0562255555555556 -7.80000000000001e-07 -2.28905555555556 -0.240502777777778

While according to manual computations in my next post I expected the AvgAMat be in the form of A.

Could anyone explain why they are not in similar form? Where is my fault?

Regards,

Mehdi

Dear Mehdi,

I have only looked briefly at your post, but see several errors in your “manual computation”. For example, the first equation in your state-space model should be:

phidot = phidot

= [ 0 0 1 0 ]*x

where [ 0 0 1 0] should be the first row of A, but you have several terms showing up in the first row of A. The 3rd and 4th rows of your manually computed A also look incorrect, not being consistent with Eq. (3-4).

Best regards,

Dear Jason,

For some control purposes I want to replace the whole hydrodynamic forces acting on platform during linearization process, to calculate the hydrodynamic moment about y axis as shown in below pic,

What is the best value for h?

Regards,

Mehdi

Dear Mehdi,

You could answer your own question by examining the total hydrodynamic force and pitching moment output by FAST. My guess is the answer depends on the sea state and floater geometry.

Best regards,

Dear Jason,

I am a little confused between PtfmMyi and PtfmFxi. It seems I must consider both of them for calculating total hydrodynamic moment about y axis???

Is it true?

Dear Mehdi,

PtfmMyi is the pitching moment and PtfmFxi is the surge force. So, the moment arm is PtfmMyi/PtfmFxi.

However, a problem I see with this is that these FAST v7 outputs include all of the loads applied to the platform, including wave-excitation, radiation, hydrostatics, viscous, and mooring loads. You really want to isolate the wave-excitation loads. However, the isolated wave-excitation loads are not standard output from FAST v7 without customization to the source code. However, the isolated wave-excitation loads are possible outputs from FAST v8. You may consider upgrading to FAST v8.

Best regards,

As i said in my previous post, I want to get h for liniearization condition, and you said in your first post of here that the following effects is not included in the linearized model:

-hydrodynamic wave-radiation damping and free-surface memory (i.e., RdtnTMax must be set to 0.0 during linearization)

-hydrodynamic wave-excitation forces (i.e., you can only linearize a model in still water, with WaveMod set to 0)

So I think no need to refer to FAST.v8, yet.

Dear Mehdi,

As discussed above, the FAST v7 linearization process cannot involve wave-excitation loads. However, you can derive “h” from time-domain simulations for use in a linearized model manually adjusted to accommodate wave-excitation loads.

Best regards,

Hi,

I do the following procedure to write the EOM of floating wind turbine and linearize it to get state space of the EOM to be usable in control issues.

some questions about the linearized model appeared for me, hope some one could answer them.

- I was hoped to get a wind input disturbance matrix like wave exciting force disturbance matrix but as you see there is not such term in my equation. I am wondering how FAST/Linearization can produce such wind input disturbance matrix but my EOM couldnt? What extra assumptions I have to consider to get such term in my linearized equation?
- As you can see, there are some derivatives in my A and B matrices, but I dont know how to determine the operating points in different wind speeds to calculate them, pleaase help if is possible.

IS not there anyone to have opinion about my question?

any comments will be appreciated.

Dear Mehdi,

I haven’t tried to follow all of your math (I don’t have the time now), but here are a couple comments:

- The linearization in FAST considers perturbations of the wind speed about the mean (operating-point value) wind speed, which would end up in the input disturbance (or input) matrix. The mean wind speed causes some states (e.g. platform-surge) to have a nonzero mean operating-point values; the linearized matrices represent the perturbations about this mean.
- The operating points should be based on the controller and determined such that the system is in a sort of equilibrium. Normally, I would expect the mean (operating-point values of the) rotor speed and blade-pitch angle to depend on the mean wind speed.

I hope that helps.

Best regards,