Influence of platform mass to natural frequencies

I am running some simulations with on Hywind OC3 in OpenFAST where only ElastoDyn, HydroDyn and Moorings are the only modules enabled. The initial position of only one rigid body DOF is changed from equilibrium and I am observing the decaying motion.

I have perfect match of the natural frequencies with previous calculations of the six rigid body modes, but I see no difference to the natural frequency of heave when increasing PtfmMass in the Elastodyn.dat file. What is strange is that I see a immediate drop in the position of the platform when increasing the mass and which is expected and corresponding to the mass added. I have tried increasing at least 1 000 000 kg, about 1/7 of the original mass and I have the same natural frequency as initially. I have not checked whether this is happening with the other two translational DOFs or the three rotational when changing moment of inertia.

Can someone advice why the static behaviour of the platform is changing, but not the dynamic when adding mass to the platform? I have probably missed something obvious.

Dear Even,

I would expect that increasing the platform mass would change the natural frequency of the rigid-body modes of the floating platform. Perhaps the change in natural frequency is small for the change in you mass you’ve introduced?

Keep in mind that if you change the system mass and the system reaches a new heave equilbrium position then the mooring restoring/stiffness will also be effected by that change (due to the geometric nonlinearity of the mooring system).

Best regards,

I figured this out:

You are correct that a greater mass would have changed the eigenfrequency, but with such large masses we are talking about that would have changed the whole dynamics of the platform.

The simple reason for not being able to seeing the change in eigenfrequency was due to poor frequency resolution. I reduced this by increasing the simulation time (Tmax), but could also have reduced the time resolution (DT).

I am hijacking my own post as the subject is the same, Hywind OC3 and responses from setting the initial position different to zero:

I am trying to link the responses I get for the platform and tower with the known natural frequencies for the structure. According to the NREL document Model Development and Loads Analysis of an Offshore Wind Turbine on a Tension Leg Platform, with Comparison to Other Floating Turbine Concepts the natural frequencies should be as follows:

Platform surge and sway: 8e-3 Hz
Platform heave: 3.24e-2 Hz
Platform roll and pitch: 3.4e-2 Hz
Platform yaw: 1.2e-1 Hz
1st Tower SS: 4.6e-1 Hz
1st Tower FA: 4.7e-1 Hz

I get the same results in FAST with and without disabling DOFs from the elastodyn dat-file when reusing the original Hywind OC3 model - all good.

What I find strange is a fuzzy frequency response curve at about 0.1 Hz I get with both continuous forcing or when the the structure is pulled out of the stable position. The attached images show the decaying response when initial positions for surge and sway is set to 5 m + rotation is set to 5 degrees for roll and pitch. As can be seen for the surge time response the amplitude after the transient is over 1 m!

I first suspected that it was due to yaw and a singular value decomposition analysis of the frequency response indicated that the following five outputs are highest correlated to this response: PtfmSway, NacYaw, TwrClrnc1, YawBrFyp and TwHt1MLxt. What is contradicting this is that the yaw response for the platform is low and I get the same response even when the NacYaw and PtfmYaw dofs are disabled.

Next I suspected the natural frequencies of the mooring to be the reason, and this would explain the fuzzy response curve, but I do not see any changes in the response at 0.1 Hz when changing the drag force of the mooring or increasing the bottom damping.

Can anyone tell me why I get this fuzzy response at about 0.1 Hz?

Dear Even,

You mention in your post that the results you are obtaining are associated with “continuous forcing”, but you don’t indicate what kind of forcing that is. Are you applying wind or wave excitation on the structure? Does the 0.1-Hz correspond with an excitation frequency e.g. a 10-s peak-spectral wave period?

Best regards,

Dear Jason,

These cases seems to solve themselves: I realized while answering this post that had not disabled the wave field when running the cases. The 0.1 Hz was due to 6 m significant wave height. I apologize for the confusion!

Best Regards,