## Modeling of linear foundation for onshore turbine

Discuss theory and modeling of wind-turbine structures.

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Meriem.Mbarek
Posts: 7
Joined: Thu Jun 04, 2020 3:09 pm
Organization: ENSTAB
Location: TUNISIA

### Re: Modeling of linear foundation for onshore turbine

Hi Jason,
Yes, I'm using CompSub=2.
Sorry if I were not that much clear at first, but I would like to check the effect of the foundation on the tower loads by giving foundation's mass, inertia and stiffness. To start a simulation with OpenFAST I need to give different shape modes of the tower based on the natural frequencies, which are calculated with the help of Bmodes v1.03.01 (Elastodyn_tower.dat). So far I simulated wind turbines with a rigid foundation, therefore I didn't need to consider foundation effects on the tower natural frequency, which I need to do now! My first question is how can I consider this effect of the foundation on the tower's natural frequency?
The other point is I would like to compare the calculated loads based on a rigid foundation (CompSub=0) with a modelled foundation (CompSub=2). I don't need to calculate the loads on the foundation and as I know the platform degrees of freedom ( in Elastodyn.dat) only support the offshore foundations.
The question is, shall I turn on the degrees of freedom of the platform by setting them "True" or not? and does OpenFAST consider the given foundation parameters by the external file (External Platform MCKF) to calculate the tower's loads when these degree of freedoms are "false" or not?

False PtfmSgDOF - Platform horizontal surge translation DOF (flag)
False PtfmSwDOF - Platform horizontal sway translation DOF (flag)
False PtfmHvDOF - Platform vertical heave translation DOF (flag)
False PtfmRDOF - Platform roll tilt rotation DOF (flag)
False PtfmPDOF - Platform pitch tilt rotation DOF (flag)
False PtfmYDOF - Platform yaw rotation DOF (flag)

Kind regards,
Meriem Mbarek

Jason.Jonkman
Posts: 5738
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Modeling of linear foundation for onshore turbine

Dear Meriem,

• To model a flexible foundation, you can use CompSub = 2 or wait until the new SoilDyn/SubDyn capabilities are available. Regardless, if you intend to use CompSub = 2 and model the tower in ElastoDyn, then you should specify the mass/stiffness/damping of the foundation in ExtPtfm and modify the tower mode shapes in ElastoDyn based on the mass/stiffness of the foundation (tower-base boundary condition); BModes_JJ can be used to calculate the tower mode shapes. Any change to the mass/stiffness of the foundation in ExtPtfm or to the tower mode shapes in ElastoDyn will impact the tower natural frequency.
• CompSub = 2 and the ElastoDyn platform DOFs work for either onshore or offshore wind turbines.
• If all of the ElastoDyn platform DOFs are disabled, then ExtPtfm will not have any effect; ExtPtfm will only have an effect on the solution if at least one ElastoDyn platform DOF is enabled.
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

Meriem.Mbarek
Posts: 7
Joined: Thu Jun 04, 2020 3:09 pm
Organization: ENSTAB
Location: TUNISIA

### Re: Modeling of linear foundation for onshore turbine

Dear Jason,

best regards,

Meriem.Mbarek
Posts: 7
Joined: Thu Jun 04, 2020 3:09 pm
Organization: ENSTAB
Location: TUNISIA

### Re: Modeling of linear foundation for onshore turbine

Dear Jason ,
i tried first time without considering the foundation(hub_conn=1), and i have as a result for the natural frequency:
eigenvalue( 1) = 0.493762D+11 mode 1 frequency = 0.370346
When i consider the foundation i have as a resulat(hub_conn=3) :
eigenvalue( 1) = 0.436656D+05 mode 1 frequency = 0.000348
i would expect to find the natural frequency few percent lower than this value with the consideration of the interaction between foundation and the soil with the consideration of the rotation about horizontal axes wind turbine.
====================== BModes v3.00 Main Input File ==================
NREL 5MW Tower

--------- General parameters ---------------------------------------------------------------------
true Echo Echo input file contents to *.echo file if true.
2 beam_type 1: blade, 2: tower (-)
0. romg: rotor speed, automatically set to zero for tower modal analysis (rpm)
1. romg_mult: rotor speed muliplicative factor (-)
87.6 radius: rotor tip radius measured along coned blade axis, OR tower height above ground level [onshore] or MSL [offshore](m)
0. precone: built-in precone angle, automatically set to zero for a tower (deg)
0. bl_thp: blade pitch setting, automatically set to zero for a tower (deg)
3 hub_conn: hub-to-blade or tower-base boundary condition [1: cantilevered; 2: free-free; 3: only axial and torsion constraints] (-)
20 modepr: number of modes to be printed (-)
t TabDelim (true: tab-delimited output tables; false: space-delimited tables)
f mid_node_tw (true: output twist at mid-node of elements; false: no mid-node outputs)

--------- Blade-tip or tower-top mass properties --------------------------------------------
3.500003109E+005 tip_mass blade-tip or tower-top mass (kg)
-0.4137754432 cm_loc tip-mass c.m. offset from the tower axis measured along x-tower axis (m)
1.9669893542 cm_axial tip-mass c.m. offset tower tip measures axially along the z axis (m)
4.370E7 ixx_tip blade lag mass moment of inertia about the tip-section x reference axis (kg-m^2)
2.353E7 iyy_tip blade flap mass moment of inertia about the tip-section y reference axis (kg-m^2)
2.542E7 izz_tip torsion mass moment of inertia about the tip-section z reference axis (kg-m^2)
0. ixy_tip cross product of inertia about x and y reference axes(kg-m^2)
1.169E6 izx_tip cross product of inertia about z and x reference axes(kg-m^2)
0. iyz_tip cross product of inertia about y and z reference axes(kg-m^2)

--------- Distributed-property identifiers --------------------------------------------------------
1 id_mat: material_type [1: isotropic; non-isotropic composites option not yet available]
'CS_monopile_tower_secs.dat' : sec_props_file name of beam section properties file (-)

Property scaling factors..............................
1.0 sec_mass_mult: mass density multiplier (-)
1.0 flp_iner_mult: blade flap or tower f-a inertia multiplier (-)
1.0 lag_iner_mult: blade lag or tower s-s inertia multiplier (-)
1.0 flp_stff_mult: blade flap or tower f-a bending stiffness multiplier (-)
1.0 edge_stff_mult: blade lag or tower s-s bending stiffness multiplier (-)
1.0 tor_stff_mult: torsion stiffness multiplier (-)
1.0 axial_stff_mult: axial stiffness multiplier (-)
1.0 cg_offst_mult: cg offset multiplier (-)
1.0 sc_offst_mult: shear center multiplier (-)
1.0 tc_offst_mult: tension center multiplier (-)

--------- Finite element discretization --------------------------------------------------
61 nselt: no of blade or tower elements (-)
Distance of element boundary nodes from blade or flexible-tower root (normalized wrt blade or tower length), el_loc()
0 0.003481894 0.010445682 0.017409471 0.024373259 0.031337047 0.038300836 0.045264624 0.052228412 0.059192201 0.066155989 0.073119777 0.080083565 0.087047354 0.094011142 0.10097493 0.107938719 0.114902507 0.121866295 0.128830084 0.135793872 0.13990 0.149721448 0.156685237 0.163649025 0.170612813 0.177576602 0.18454039 0.191504178 0.198467967 0.205431755 0.212395543 0.219359331 0.22632312 0.233286908 0.240250696 0.247214485 0.250696379 0.320334262 0.37971 0.424791072 0.45961 0.486635 0.51366 0.54068 0.5677 0.594715 0.62173 0.64875 0.67577 0.70279 0.72981 0.75683 0.78385 0.81087 0.83789 0.864905 0.89192 0.91894 0.94596 0.97298 1.0

--------- Properties of tower support subsystem (read only if beam_type is 2) ------------
0 tow_support: : aditional tower support [0: no additional support; 1: floating-platform or monopile with or without tension wires] (-)
0.0 draft : depth of tower base from the ground or the MSL (mean sea level) (m)
0.0 cm_pform : distance of platform c.m. below the MSL (m)
0 mass_pform : platform mass (kg)
Platform mass inertia 3X3 matrix (i_matrix_pform):
0. 0. 0.
0. 0. 0.
0. 0. 0.
0.0 ref_msl : distance of platform reference point below the MSL (m)
Platform-reference-point-referred hydrodynamic 6X6 matrix (hydro_M):
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 5.650000E+7 0. 0.
0. 0. 0. 0. 5.650000E+07 0.
0. 0. 0. 0. 0. 0.
Platform-reference-point-referred hydrodynamic 6X6 stiffness matrix (hydro_K):
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 4.356000E+10 0. 0.
0. 0. 0. 0. 4.356000E+10 0.
0. 0. 0. 0. 0. 0.
Mooring-system 6X6 stiffness matrix (mooring_K):
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.

Distributed (hydrodynamic) added-mass per unit length along a flexible portion of the tower length:
0 n_secs_m_distr: number of sections at which added mass per unit length is specified (-)
0. 0. : z_distr_m [row array of size n_added_m_pts; section locations wrt the flexible tower base over which distributed mass is specified] (m)
0. 0. : distr_m [row array of size n_added_m_pts; added distributed masses per unit length] (kg/m)

Distributed elastic stiffness per unit length along a flexible portion of the tower length:
0 n_secs_k_distr: number of points at which distributed stiffness per unit length is specified (-)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 : z_distr_k [row array of size n_added_m_pts; section locations wrt the flexible tower base over which distributed stiffness is specified] (m)
595318000.0 1165208000 1129400000 1095553000 1059931000 1024493000 989209000 953643000 918718000 883287000 847803000 812541000 777187000 741870000 706616000 671440000 636229000 600957000 565919000 530470000 495081000 459574000 385327000 305479000 280059000 254125000 227500000 200112000 171927000 143115000 114173000 80184000 52237000 35561000 20912000 9000000 1156000 : distr_k [row array of size n_added_m_pts; distributed stiffness per unit length] (N/m^2)

Tension wires data
0 n_attachments: no of wire-attachment locations on tower [0: no tension wires] (-)
3 3 n_wires: no of wires attached at each location (must be 3 or higher) (-)
6 9 node_attach: node numbers of attacments location (node number must be more than 1 and less than nselt+2) (-)
0.e0 0.e0 wire_stfness: wire spring constant in each set (see users' manual) (N/m)
0. 0. th_wire: angle of tension wires (wrt the horizontal ground plane) at each attachment point (deg)

END of Main Input File Data *********************************************************************
*************************************************************************************************

Best regards

Jason.Jonkman
Posts: 5738
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Modeling of linear foundation for onshore turbine

Dear Meriem,

With hub_conn = 3, the tower-base (platform) point has 4 DOFs--surge, sway, roll, and pitch. It looks like you've specified the lumped inertia and stiffness in roll/pitch, but you have not specified any stiffness in surge and sway. This will result in two rigid-body modes (in surge and sway), each with zero frequency (or near-zero frequency due to numerical round off in the solution).

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

Meriem.Mbarek
Posts: 7
Joined: Thu Jun 04, 2020 3:09 pm
Organization: ENSTAB
Location: TUNISIA

### Re: Modeling of linear foundation for onshore turbine

Dear Jason,
thank you for your fast reply , i would like to tell you although I have changed the values of the matrix but the result of the natural frequency it is the same :
eigenvalue( 1) = 0.436656D+05 mode 1 frequency = 0.000348

====================== BModes v3.00 Main Input File ==================
NREL 5MW Tower

--------- General parameters ---------------------------------------------------------------------
true Echo Echo input file contents to *.echo file if true.
2 beam_type 1: blade, 2: tower (-)
0. romg: rotor speed, automatically set to zero for tower modal analysis (rpm)
1. romg_mult: rotor speed muliplicative factor (-)
87.6 radius: rotor tip radius measured along coned blade axis, OR tower height above ground level [onshore] or MSL [offshore](m)
0. precone: built-in precone angle, automatically set to zero for a tower (deg)
0. bl_thp: blade pitch setting, automatically set to zero for a tower (deg)
3 hub_conn: hub-to-blade or tower-base boundary condition [1: cantilevered; 2: free-free; 3: only axial and torsion constraints] (-)
20 modepr: number of modes to be printed (-)
t TabDelim (true: tab-delimited output tables; false: space-delimited tables)
f mid_node_tw (true: output twist at mid-node of elements; false: no mid-node outputs)

--------- Blade-tip or tower-top mass properties --------------------------------------------
3.500003109E+005 tip_mass blade-tip or tower-top mass (kg)
-0.4137754432 cm_loc tip-mass c.m. offset from the tower axis measured along x-tower axis (m)
1.9669893542 cm_axial tip-mass c.m. offset tower tip measures axially along the z axis (m)
4.370E7 ixx_tip blade lag mass moment of inertia about the tip-section x reference axis (kg-m^2)
2.353E7 iyy_tip blade flap mass moment of inertia about the tip-section y reference axis (kg-m^2)
2.542E7 izz_tip torsion mass moment of inertia about the tip-section z reference axis (kg-m^2)
0. ixy_tip cross product of inertia about x and y reference axes(kg-m^2)
1.169E6 izx_tip cross product of inertia about z and x reference axes(kg-m^2)
0. iyz_tip cross product of inertia about y and z reference axes(kg-m^2)

--------- Distributed-property identifiers --------------------------------------------------------
1 id_mat: material_type [1: isotropic; non-isotropic composites option not yet available]
'CS_monopile_tower_secs.dat' : sec_props_file name of beam section properties file (-)

Property scaling factors..............................
1.0 sec_mass_mult: mass density multiplier (-)
1.0 flp_iner_mult: blade flap or tower f-a inertia multiplier (-)
1.0 lag_iner_mult: blade lag or tower s-s inertia multiplier (-)
1.0 flp_stff_mult: blade flap or tower f-a bending stiffness multiplier (-)
1.0 edge_stff_mult: blade lag or tower s-s bending stiffness multiplier (-)
1.0 tor_stff_mult: torsion stiffness multiplier (-)
1.0 axial_stff_mult: axial stiffness multiplier (-)
1.0 cg_offst_mult: cg offset multiplier (-)
1.0 sc_offst_mult: shear center multiplier (-)
1.0 tc_offst_mult: tension center multiplier (-)

--------- Finite element discretization --------------------------------------------------
61 nselt: no of blade or tower elements (-)
Distance of element boundary nodes from blade or flexible-tower root (normalized wrt blade or tower length), el_loc()
0 0.003481894 0.010445682 0.017409471 0.024373259 0.031337047 0.038300836 0.045264624 0.052228412 0.059192201 0.066155989 0.073119777 0.080083565 0.087047354 0.094011142 0.10097493 0.107938719 0.114902507 0.121866295 0.128830084 0.135793872 0.13990 0.149721448 0.156685237 0.163649025 0.170612813 0.177576602 0.18454039 0.191504178 0.198467967 0.205431755 0.212395543 0.219359331 0.22632312 0.233286908 0.240250696 0.247214485 0.250696379 0.320334262 0.37971 0.424791072 0.45961 0.486635 0.51366 0.54068 0.5677 0.594715 0.62173 0.64875 0.67577 0.70279 0.72981 0.75683 0.78385 0.81087 0.83789 0.864905 0.89192 0.91894 0.94596 0.97298 1.0

--------- Properties of tower support subsystem (read only if beam_type is 2) ------------
0 tow_support: : aditional tower support [0: no additional support; 1: floating-platform or monopile with or without tension wires] (-)
0.0 draft : depth of tower base from the ground or the MSL (mean sea level) (m)
0.0 cm_pform : distance of platform c.m. below the MSL (m)
8.119E+10 mass_pform : platform mass (kg)
Platform mass inertia 3X3 matrix (i_matrix_pform):
0. 0. 0.
0. 0. 0.
0. 0. 0.
0.0 ref_msl : distance of platform reference point below the MSL (m)
Platform-reference-point-referred hydrodynamic 6X6 matrix (hydro_M):
8.119000E+10 0. 0. 0. 0. 0.
0. 8.119000E+10 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 5.650000E+7 0. 0.
0. 0. 0. 0. 5.650000E+07 0.
0. 0. 0. 0. 0. 0.
Platform-reference-point-referred hydrodynamic 6X6 stiffness matrix (hydro_K):
6.036395E+08 0. 0. 0. 0. 0.
0. 6.036395E+08 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 1.074849E+10 0. 0.
0. 0. 0. 0. 1.074849E+10 0.
0. 0. 0. 0. 0. 0.
Mooring-system 6X6 stiffness matrix (mooring_K):
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.
0. 0. 0. 0. 0. 0.

Distributed (hydrodynamic) added-mass per unit length along a flexible portion of the tower length:
0. n_secs_m_distr: number of sections at which added mass per unit length is specified (-)
0. 0. : z_distr_m [row array of size n_added_m_pts; section locations wrt the flexible tower base over which distributed mass is specified] (m)
0. 0. : distr_m [row array of size n_added_m_pts; added distributed masses per unit length] (kg/m)

Distributed elastic stiffness per unit length along a flexible portion of the tower length:
0 n_secs_k_distr: number of points at which distributed stiffness per unit length is specified (-)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 : z_distr_k [row array of size n_added_m_pts; section locations wrt the flexible tower base over which distributed stiffness is specified] (m)
595318000.0 1165208000 1129400000 1095553000 1059931000 1024493000 989209000 953643000 918718000 883287000 847803000 812541000 777187000 741870000 706616000 671440000 636229000 600957000 565919000 530470000 495081000 459574000 385327000 305479000 280059000 254125000 227500000 200112000 171927000 143115000 114173000 80184000 52237000 35561000 20912000 9000000 1156000 : distr_k [row array of size n_added_m_pts; distributed stiffness per unit length] (N/m^2)

Tension wires data
0 n_attachments: no of wire-attachment locations on tower [0: no tension wires] (-)
3 3 n_wires: no of wires attached at each location (must be 3 or higher) (-)
6 9 node_attach: node numbers of attacments location (node number must be more than 1 and less than nselt+2) (-)
0.e0 0.e0 wire_stfness: wire spring constant in each set (see users' manual) (N/m)
0. 0. th_wire: angle of tension wires (wrt the horizontal ground plane) at each attachment point (deg)

END of Main Input File Data *********************************************************************
*************************************************************************************************

Best regards,

Jason.Jonkman
Posts: 5738
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

### Re: Modeling of linear foundation for onshore turbine

Hi Meriem,

I see that you've also set tow_support = 0, which means nothing in the section "properties of the tower support subsystem" will be used. This means that actually you'll have 4 rigid body modes. Set tow_support = 1.

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

Meriem.Mbarek
Posts: 7
Joined: Thu Jun 04, 2020 3:09 pm
Organization: ENSTAB
Location: TUNISIA

Dear Jason,