Dear Jason,
I am still struggling with meeting the expected natural frequencies of our TLP with BModes.
From the tank tests we got natural frequencies for our symmetric TLP for Surge and Sway at about 0.0215 Hz.
When running a respective decay test in FAST, with strip theory applied in HD, and MoorDyn used, I meet quite well with the
above mentioned Surge and Sway frequencies for the TLP. The FAST results are 0.0196 Hz However, I am not able to meet the expected values in BModesJJ.
Thus, I would like to ask you kindly once again to have a look, in order to rule out possible mistakes in my inputs.
- The top of our transition piece, therefore the bottom of our tower is at 30 m above MSL. Thus, draft = - 30 (negatively valued)
- The center of mass is 10 m below MSL. Thus, cm_pform = 10 (positively valued)
- hub_rad would be 0 for our setup, as there is no “unflexible” part of the tower but the tower is flexible from its very beginning at draft height.
I obtained the orientation (negatively or positively valued) from your post Tue Nov 06, 2012 6:00 pm BModes : Input parameters about tower support subsystem - #2 by Jason.Jonkman - I had a mistake before, and therefore I did not use the correctly formated hydro_k and hydro_m matrices. However, by help from Frank Lemmer (from SWE) able to now correctly format the matrices, to meet with the WAMIT definitions as expected by BModes/HydroDyn.
I have attached the .hst as well as the .1 file.
Thus, I wrote the hydro_m matrix in the BModes inputs:
5.858172e+03 0.00000E+00 0.00000E+00 0.00000E+00 -1.075658e+05 0.00000E+00
0.00000E+00 5.858562e+03 0.00000E+00 1.075658e+05 0.00000E+00 0.00000E+00
0.00000E+00 0.00000E+00 5.105279e+03 0.00000E+00 0.00000E+00 0.00000E+00
0.00000E+00 1.074390e+05 0.00000E+00 3.170353e+06 0.00000E+00 0.00000E+00
-1.074390e+05 0.00000E+00 0.00000E+00 0.00000E+00 3.170353e+06 0.00000E+00
0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 0.00000E+00 2.487648e+06
as well as to the hydro_k matrix:
0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00
0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00 0.000000e+00
0.000000e+00 0.000000e+000 2.713241e+01 4.937715e-02 -4.780529e-02 0.000000e+00
0.000000e+00 0.000000e+00 4.937715e-02 -1.739686e+05 -1.147753e-02 0.000000e+00
0.000000e+00 0.000000e+00 -4.780529e-02 -1.147753e-02 -1.739686e+05 0.000000e+00
0.000000e+00 0.000000e+00 0.000000e+00 -1.228136e-02 -2.445528e-03 0.000000e+00
Herein, (4,4) and (5,5) have to be augmented with the contribution from the body weight-related restoring which I calculated by:
- m * g * z = -25.05e + 06
So the new values for (4,4) and (5,5) are: -1.739686e+05 + (-)25.05e + 06 = - 25.223969 *10^6 (negatively valued)
What is interesting, is that the contribution did not change anything in the resulting frequencies but the 5th or 6th decimal digit in some of the nat. frequencies calculated by BModes.
Probably this is due to the higher yaw stiffness or/and the shallow draft of the floater.
-
The mooring stiffness matrix (mooring_k) I received from the standalone python wrapper for MAP++.
I used the same values for the MAP++ inputs as I have used for MoorDyn, which I use together with my FAST simulation. To keep
in mind, the results of those FAST simulations do match very well with the tank tests result with regard to the SS/FA substructure
frequencies. So I think the inputs are plausible. Nevertheless, I tried to play around with the unstretched lengths of the tendons.
And this - as expected - has a high influence on the floater frequencies.
So probably something is wrong with my application of MAP++? -
The frequencies I got from BModes are
-------- Mode No. 1 (freq = 0.402640E-01 Hz)
-------- Mode No. 2 (freq = 0.803631E-01 Hz)
-------- Mode No. 3 (freq = 0.892912E-01 Hz)
-------- Mode No. 4 (freq = 0.124583E+00 Hz)
-------- Mode No. 5 (freq = 0.128184E+00 Hz)
-------- Mode No. 6 (freq = 0.368710E+00 Hz)
-------- Mode No. 7 (freq = 0.380267E+00 Hz)
-------- Mode No. 8 (freq = 0.390270E+00 Hz)
-------- Mode No. 9 (freq = 0.701860E+00 Hz)
-------- Mode No. 10 (freq = 0.805302E+00 Hz)
.
.
.
I would be helpfully for any hint.