Resonance in OpenFAST due to hydrodynamic effects

Dear Jason, and others who may have experienced similar,

we have conducted some tank tests for our TLP a while ago. The 1:50 scaled model of the platform was equipped with a representation of the NREL 5MW reference wind turbine.

We ran some OpenFAST simulations in order to compare simulation results against tank tests for different load cases. We’ve tuned the mooring (we had some issues with our moorings for the TLP) and hydrodynamic parameters in the simulation in a way that the natural frequencies meet well with those gained from free decay tests for the model.

For simulating wave only and combined wind and wave tests, we’ve used the wave elevations captured during the model tests as inputs for wave force generation in HydroDyn (WaveMod5). As a matter of fact the wave gauge signal from the tank tests shows some gritty noise before running the actual waves (leftovers after calm down time, from the wave and reflections before). This is also visible for the waves computed by HydroDyn.

During this time (i.e. before the actual waves start), interestingly the platform shows a significant surge response in the simulation, which however cannot be observed in the results for the tank tests.

Attached are plots for wave elevation and surge deflection for 2m and 9.5m waves. What can be seen is a considerable surge deflection for the simulation which is quite high, compared to the deflection for the model. An fft showed that the main frequency of the wave noise is near the 3rd harmonic of the floaters surge 1st natural frequency.

We´d be curious if you have any explanation or thoughts on this, or if you have experienced similar effects before.

We have used OpenFAST V2.3.0 precompiled, x64 on Win10. HydroDyn: PotMod: 1 (WAMIT files available), 2nd order waves and 2nd order platform forces disabled.

Kind Regards,


Dear @Daniel.Walia,

Just guessing, but I suspect your OpenFAST model has too little damping of the surge mode. When the TLP gets excited by waves, the response does not decay.

Best regards,