Hi everyone,

I hope you can help me with the problem I’m currently facing.

As part of my masters’ thesis, I am simulating the response of a FOWT with a TLP foundation. I am interested in using response data, which has been generated by a directional wave spectrum ie. response in a multidirectional sea.

My problem arises when comparing the wave elevation time series obtained from FAST, with a theoretically calculated direction wave spectrum.

I use the MatLab toolbox WAFO, to analyse and obtain the directional wave spectrum, based on 5 measurement point described in the hydrodyn file, this data is then used as an input in the WAFO function dat2dspec, along with additional required information.

I also use the WAFO toolbox to generate a theoretical directional wave spectrum based on the coefficients used in the hydrodyn file. When comparing these spectra, I am not able to obtain the same spreading function and therefore not able to validate the directional wave spectrum used in FAST.

In the figure below I have compared to two resultant spectra with their spreading functions as well, as you can see, the spreading functions do not match.

I have tried to change the spreading coefficient, but this does not seem to make a difference in the result calculated by WAFO, neither does changing WaveNDir, which in the above case is set to 125.

I have also tried to change the distance between the wave gauges, but this didn’t seem to make a difference either.

I have tried to utilize some of the other methods for determining the directional wave spectrum from data, such as BDM, MLM, and MEM, but all gave similar results.

I have attached a compressed file containing the hydrodyn file used in FAST and the MatLab script used to plot the above figure.

If wanted, the data from FAST used in the MatLab script can be downloaded here.

Best Regards

Thomas Hansen

Aalborg University

Dir_spectrum.rar (5.54 KB)