Transient 'shut down' effect of the semi submersible

Wind & Water General
1

Hi everybody,

I’m running simulations with hydrodynamics only. My model is the OC5 semi submersible and I’m using a hybrid (potential flow + drag terms of morison equation approach.
My wave input is a file in the GHbladed format, containing the wave elevations of a bichromatic wave load case.

Wave Elevation:

Wave_Elev.jpg
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When I look at the platform motion (into translational x-direction), there is not only a transient start up, which I expected, but also a kind of a ‘shut down’ phenomenon. It is independent of the simulation time I choose, it always happens in the last 60 seconds.

Surge motion:

Surge_motion.jpg
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When I look at the loads, the Second-order wave-excitation loads from diffraction at the WRP are conspicous. The maximum force increases significantely.

Second order loads from potential theory

2ndOrderPotLoad.jpg
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I wonder where this comes from?

Did anyone notice a similar behaviour?

Thanks,

Manuela

2

Hi Manuela,

I’m not sure. Perhaps the FFT of the original wave-elevation signal looks strange e.g. because the wave-elevation input is not fully periodic? Have you tried windowing the wave-elevation input before using it as a user-defined wave-elevation time series (WaveMod = 5)? E.g. you could try ramping up the wave-elevation input so that it starts at zero, ramps up to full over some period of time, and ramps down to zero at the end of the time series. Does that reduce the spikes in the second-order loads at the beginning and end of the time series?

Best regards,

3

Hi Jason,

my first guess was that this is somehow related to the FFT, too. But it only occurs when the platform is floating. When I use the same model and the same wave elevation input in a constrained configuration, the second order wave signal look clean.

Maybe it’s somehow related to the motion computation?

It does’t happen if I use a wave signal which is significantly longer than my FAST simulation time.

Best regards,

Manuela

4

Dear Manuela,

What do you mean when you refer to the platform as floating versus constrained; are you referring to the enabling/disabling of the platform degrees of freedom in ElastoDyn? Actually, the second-order hydrodynamic loads computed by HydroDyn do not depend on the platform motion from ElastoDyn, so, I’m a bit confused by your response.

Best regards,

5

Hi Jason,

I’m sorry, I was confused. I checked it again this morning, and it is only related to the signal length (the signal needs to be fully periodic). I was looking at the wrong data last night.

Best regards,

Manuela