## hydrodynamic model for semisubmersible FOWT

Discuss theory and modeling of wind-turbine structures.

Moderator: Bonnie.Jonkman

Lixian.Zhang
Posts: 18
Joined: Wed Dec 05, 2018 11:04 pm
Organization: Dalian University of technology
Location: China

### hydrodynamic model for semisubmersible FOWT

Dear Jason
Sorry to bother you again!
I'm confused to some problems about hydrodynamic model.
As it is referred to hydrodyn user guide. the Vertical center of gravity(VCG) is used to determine the pitch and roll restoring associated with platform weight. In order to neglect the effect of these terms in WAMIT, VCG can be set to zero when solving the first-order problem. There are my questiones:
1) The cooridinate of COG of semisubmersible FOWT is (-32,0,-17)m. When I calculate the hydrodynamic coeficients by using WAFDUT or AQWA. Should the coordinate of COG be set to (0,0,0) or (-32,0,0)m？ And should the center of rotation be set to (0,0,0) or (-32,0,0)?
2)IF the COG is set to (-32,0,0), the restoring coeficient, the value of added mass and radiation damping et al are refered to the COG in AQWA. Should I convert these datas to the original coordinate systems by using the transmat martix?

Hope you can help me
Thank you!

best regards
Lixian Zhang

Jason.Jonkman
Posts: 4952
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: hydrodynamic model for semisubmersible FOWT

Dear Lixian,

HydroDyn expects the hydrodynamic coefficients for the potential-flow solution to be defined about the WAMIT reference point, which is (0,0,0). And the pitch and roll restoring associated with the center of mass of the floating wind turbine are intrinsically accounted for in the ElastoDyn module (and so, should not be accounted for in HydroDyn). Thus, the COG and the hydrodynamic reference point should both be set to (0,0,0) in AQWA (or WAMIT) for the first-order problem.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Lixian.Zhang
Posts: 18
Joined: Wed Dec 05, 2018 11:04 pm
Organization: Dalian University of technology
Location: China

### Re: hydrodynamic model for semisubmersible FOWT

Dear Jason

Best regards
Lixian Zhang

Saishuai.Dai
Posts: 6
Joined: Wed Mar 25, 2020 4:17 pm
Organization: University of Strathclyde
Location: UK-Glasgow

### Re: hydrodynamic model for semisubmersible FOWT

Jason.Jonkman wrote:Dear Lixian,

HydroDyn expects the hydrodynamic coefficients for the potential-flow solution to be defined about the WAMIT reference point, which is (0,0,0). And the pitch and roll restoring associated with the center of mass of the floating wind turbine are intrinsically accounted for in the ElastoDyn module (and so, should not be accounted for in HydroDyn). Thus, the COG and the hydrodynamic reference point should both be set to (0,0,0) in AQWA (or WAMIT) for the first-order problem.

Best regards,

Dear Jason

I am working on a similar 3 column V-shaped platform. Unlike OC3 spar and OC4 semi whose system LCG align with the centreline of the turbine system. The LCG of the platform sits at the middle of the V shape pontoon while the turbine is installed on top of one of the column. When preparing the hydrodynamic model (e.g. WAMIT), it is convenient to set the WRP at one of the column.

I can understand that by setting the VCG of the platform to zero, the restoring associated with the centre of mass of the floating wind turbine is ignored in the hydrodynamic calculation, so that this part will not be double calculated by ElasDy. I assume the hydrodynamic reference point you mentioned above is the rotation point, I can not understand why the rotation point should be set to (0,0,0) as well. The hydrodynamic coefficients, for example: added inertia, will be different depends on where the rotation axis is defined. Would be indeed apricated if you can spare some time to explain the reason why the hydrodynamic reference point should be set to (0,0,0).

Best Regards
David

Jason.Jonkman
Posts: 4952
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: hydrodynamic model for semisubmersible FOWT

Dear David,

The HydroDyn module currently assumes that the WAMIT reference point--i.e., the location where the WAMIT-derived potential-flow solution is specified in HydroDyn--is at (0,0,0). This was a natural choice when HydroDyn was first developed, but is problematic if you have WAMIT data defined about a difference reference point.

That said, we are working on an update to HydroDyn that will allow the user to specify multiple WAMIT bodies (the so-called "N-Body" option in WAMIT), and for each WAMIT body, will allow the user to specify the origin and orientation of that WAMIT body with respect to coordinate system at (0,0,0). While this capability is not yet released, it will provide some powerful functionality in the future.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Saishuai.Dai
Posts: 6
Joined: Wed Mar 25, 2020 4:17 pm
Organization: University of Strathclyde
Location: UK-Glasgow

### Re: hydrodynamic model for semisubmersible FOWT

Jason.Jonkman wrote:Dear David,

The HydroDyn module currently assumes that the WAMIT reference point--i.e., the location where the WAMIT-derived potential-flow solution is specified in HydroDyn--is at (0,0,0). This was a natural choice when HydroDyn was first developed, but is problematic if you have WAMIT data defined about a difference reference point.

That said, we are working on an update to HydroDyn that will allow the user to specify multiple WAMIT bodies (the so-called "N-Body" option in WAMIT), and for each WAMIT body, will allow the user to specify the origin and orientation of that WAMIT body with respect to coordinate system at (0,0,0). While this capability is not yet released, it will provide some powerful functionality in the future.

Best regards,

Dear Jason

Can I conclude that at the current stage, a conventional way of simulating this kind of V shaped platform is not accurate due to the difference in the hydrodynamic coefficients introduced by different reference point? That is the required hydrodynamic coefficients for HydroDyn should be specified at (0,0,0) while the actual correct hydrodynamic coefficients should be specified at the COG? The resulting hydrodynamic coefficients due to rotation motions about different reference points are different.

Since the whole purpose of setting the COG to (0,0,0) is to avoid double counting of the restoring due to mass in the ElsaDyn, do you think this can be achieved by only changing the hydrostatic output from hydrodynamic calculation, say wamit, and keep the hydrodynamic coefficients obtained at the cog? This would require run the WAMIT twice, once about (0,0,0) and the other one about cog. Use the hst file from the (0,0,0) and 1,3 files from the cog simulation as the input to FAST.

Alternatively, I guess I can set the WRP to the LCG of the V platform, however, this will require movement of the tower centreline to one of the platform column. Is there a way to achieve this?

Best Regards
David

Jason.Jonkman
Posts: 4952
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: hydrodynamic model for semisubmersible FOWT

Dear David,

I'm not understanding how you are arriving at your conclusions.

There should be no problem to simulate the V-shaped semi in OpenFAST / HydroDyn. But without changing the the source code, the hydrodynamic reference point for the hydrodynamic coefficients and hydrostatic restoring, must be (0,0,0), which is equivalent with the undisplaced / undeflected tower centerline at the mean sea level (which I assume would be the vertex of the V). And the hydrostatic restoring should be defined without consideration of center of mass offsets, i.e., with the center of mass also at (0,0,0), so as to not double count the body-mass-based restoring. The center of buoyancy and center of mass can be specified in OpenFAST's HydroDyn and ElastoDyn modules, respectively, with offsets relative to (0,0,0).

I hope that helps.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Saishuai.Dai
Posts: 6
Joined: Wed Mar 25, 2020 4:17 pm
Organization: University of Strathclyde
Location: UK-Glasgow

### Re: hydrodynamic model for semisubmersible FOWT

Dear Jason

First of all, as usual, thank you for your prompt reply. Indeed appricated.

Maybe I misunderstood something here. Attached figure is the roll added inertia of the platform, where the baseline denotes the simulation result of the case rotates about the COG, and the RotateZero refers to the case rotates about (0,0,0). The simulations were done in Open Source package Nemoh. There is quite a bit difference between the two as indicated. I originally thought that Elasdyn only corrects the restoring part by setting the cog and cob offsets as you mentioned above, this will add the restoring contribution due to the mass into the hydrostatic restoring matrix, and will not have any impact on the hydrodynamics, for example the added inertia. Does Elasdyn take care of the hydrodynamics coefficients due to different rotation point as well?

Best Regards
David
Capture.JPG (102.51 KiB) Viewed 499 times

Jason.Jonkman
Posts: 4952
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: hydrodynamic model for semisubmersible FOWT

Dear David,

The translation and rotation DOFs in ElastoDyn are about (0,0,0) (assuming that you've set PtfmRefzt = 0 in ElastoDyn), just like they are in HydroDyn. So, the rotational added mass should be defined about (0,0,0) as well.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Yajun.Ren
Posts: 37
Joined: Mon Jan 21, 2019 6:54 am
Organization: The University of Edinburgh
Location: UK

### Re: hydrodynamic model for semisubmersible FOWT

Jason.Jonkman wrote:Dear Lixian,

HydroDyn expects the hydrodynamic coefficients for the potential-flow solution to be defined about the WAMIT reference point, which is (0,0,0). And the pitch and roll restoring associated with the center of mass of the floating wind turbine are intrinsically accounted for in the ElastoDyn module (and so, should not be accounted for in HydroDyn). Thus, the COG and the hydrodynamic reference point should both be set to (0,0,0) in AQWA (or WAMIT) for the first-order problem.

Best regards,

Dear Jason,

Followed with your answer regarding to the COG problem, I would like to know when I calculate PtfmRIner and PtfmPIner in ElastoDyn, should I calculate about (0,0,0) as used in WAMIT or the real centre of mass of the platform as specified by PtfmCMzt.

Many thanks,
Yajun

Jason.Jonkman
Posts: 4952
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: hydrodynamic model for semisubmersible FOWT

Dear Yajun,

The platform mass moments of inertia in the ElastoDyn structural-dynamics module of FAST / OpenFAST (PtfmRIner, PtfmPIner, PtfmYIner) are all specified bout the platform center of mass location (PtfmCMxt, PtfmCMyt, PtfmCMzt).

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov