Post Processing of Wavy Output

Dear Jason,

I have a question regarding the Post Processing of wavy output of a TLP subjected to a regular wave. Why does the output response has a trend-fluctuate nature ? In this case, what response should i take?

How do i mitigate this, i.e. to get on a straight x axis rather than a curve x axis?

The example of my output is shown below. Could you please give suggestions on how to analyze.? Should i subtract the carrier wave from this to get my actual response or is it 0.1-(-0.2)=0.3m in this case?

what i was taking outputs till now is abs(max(range)-min(range)). range-> last 20mins for a 1 hr duration sea state.

Your help is highly appreciated.

Thanks
Jay

Dear Jay,

It looks like your system has two or my natural frequencies that are quite close to each other e.g. platform-heave, tower-bending, and mooring-axial modes (or the like), with little damping.

I’m not sure I understand your question regarding “what response to take”? What are you trying to do? I would normally take a PSD of the time-series in order to identify the frequencies being excited.

Best regards,

Dear Jason,

Thanks for your reply. What is the response value for this system that I should take as my output? I’ll explain the scenario in detail.

As i said earlier, i have run my TLP system with tethers. For example lets take ,my TLP have 3 pontoons. first case i ran simulation with 1 tether/pontoon then for second case with 2 tethers/pontoon.

The net pretension per pontoon is taken as same.

what i basically expect is… to have controlled the heave motions and pitch motions of the whole system setup. I’m using a Regular Wave of Height 2 m and period 5 s, what i basically observe in both the cases is the heave is higher for second case than first case(not expected) and pitch is controlled as expected. I see a wavy output as shown before in both the outputs. I’m little confused what output should i take. i.e. maximum response value as 0.1-(-0.2)=0.3 m for the heave time series figure??

Could you please tell why is there a wavy nature and how to get a proper steady response i.e. without a carrier wave in the output?
What changes do I have to make and what could be the possible reason and solution?

Thanks
Vijay

Dear Vijay,

As I said in my prior post, the wavy nature of the response is likely because the system has two natural frequencies that are quite close to each other e.g. platform-heave, tower-bending, and mooring-axial modes (or the like), with little damping. To change this, you could shift the natural frequency (by a change to the structural design) or add damping. Again, I suggest that you take the PSD of the time series to identify the frequencies getting excited.

I don’t understand the question, “What is the response value for this system that I should take as my output?”. What type of output do you want? Are you trying to calculate a Response Amplitude Operator (ROA) or the like?

Best regards,

Dear Jason,

Thanks for your reply. Sorry to have messed up with writing. Yes, RAO is what I’m looking for. I was expecting something like a reduced response (but the results are not reasonable) when the number of tethers are increased (keeping same pretension per pontoon). TLP2_T is TLP with tendons, rest are with tethers. As you say, when PSD is taken i could see the frequencies being excited, but I would like to discuss about the response magnitude for regular wave H=2 m, T=1 s to 30 s. I have taken my magnitude as abs(max(range)-min(range)). range-> response output of last 20mins for a 1 hr simulation.

Thanks
1.PNG

Dear Jay,

The FAST model inherently includes many nonlinear terms and one cannot expect that a simple sinusoidal input to a nonlinear model will result in a simple sinusoidal response. Instead of computing an ROA by looking at the ranges of a response to various regular-wave (sinusoidal) inputs, I suggest instead to compute RAOs using white-noise wave input and post-processing via frequency-response functions. The process has been discussed in the following forum topic: http://forums.nrel.gov/t/rao-calculation/1008/4 (especially our ISOPE 2013 paper referenced in that topic: nrel.gov/docs/fy13osti/58098.pdf).

Best regards,

Dear Jason,

Thanks a lot for your replies. I have a doubt regarding how to do linearization process mentioned as block (6) in the nrel.gov/docs/fy13osti/58098.pdf) to derive mass (M), stiffness (K), damping (C). The procedure I followed till now is, model the submerged volume of floater in a modelling software, get the co-ordinates and feed this to WAMIT (to account only for wave) and get hydrodynamic parameters (.1,.3,.hst). Feed these files to FAST (to account for aerodynamics and structural dynamics along with moorings) and post-process to compute RAOs.

Could you please share how to get M, K, C from FAST and linearize it, that I can feed back to WAMIT and get the responses, to compare against FAST/WAMIT that is done in paper?

I ran a case with still conditions, with only 6 DOF of platform enabled and rest are disabled as mentioned in paper, but how do i get M,C,K to linearize? I’m using FAST v8.15.00a-bjj

Please correct me if I’m wrong anywhere in the understanding and my approach in using FAST.

Thanks

Dear Jay,

Linearization functionality was added to FAST v8 with the release of FAST v8.16, but unfortunately, we have not yet been budgeted to implement linearization functionality of the offshore features (hydrodynamic, mooring) within FAST v8. You can, however, still use the linearization functionality of the old FAST v7 tool, which is what was used for the ISOPE paper you reference.

Best regards,