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

[b]====================== 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
0. hub_rad: hub radius measured along coned blade axis OR tower rigid-base height (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 *********************************************************************
*************************************************************************************************[/b]

Best regards,

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,

Dear Jason,

Thank you for your helpful response! We appreciate.

Kind regards,

This technique from Jens seems to work quite well for the intended purpose. However, I was wondering if anyone knew how to output the overall turbine response if using this technique? Since the normal ED outputs use the xt coordinate systems, they won’t capture any “platform” response, and since you’re not using the standard SubDyn input file you can’t request outputs there. Would the solution involve the final two lines of the MCKF file (which as far, as I can tell, don’t seem to be doing anything)?

Dear Eric,

I’m not sure I really understand your question, but you can output the platorm (tower base) displacements from ElastoDyn in both the tower coordinate system (t), e.g., PtfmTDxt for surge, or in the inertial frame coordinate system (i), e.g., PtfmTDxi for surge.

Best regards,

Thanks as always for the help Dr. Jonkman. Those output parameters work perfectly. It turns out I was still using an older copy of the OutListParameter spreadsheet that didn’t list those options, or they were there under a different name and I didn’t spot them.