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Question about wave elevation
Dear David,
I am working on floating offshore wind turbine e.g. semi-sub and I have some questions regarding the output data of wave elevation. I would like to know difference in between “HYDRO Wave Elev.” and “NP Wave Elevation” ? Also, is this elevation refer to the incident wave elevation or also include the radiation effect?
Thank you.
Chris
Hello Chris,
the difference between these two outputs is the location at which the wave elevation is recorded.
- HYDRO Wave Elev. records the elevation at the current “REF_HYDRO_POS”, the reference position at which the potential flow (WAMIT,etc.) forces are evaluated and applied.
- NP Wave Elevation records the elevation at the neutral point position of the floater, which is defined as the initial (0,0,0) position of the substructure.
In both cases the elevation is the “undisturbed” wave elevation of the linear wave field, not taking into account any contributions frm radiation.
BR,
David
Thank you very much David.
Best regards,
Chris
Dear David,
I would like to have a follow up question. I notice the HYDRO Wave Elev. Ref. Pos follow the wave elevation time series that is shown in the Wave generator while NP Wave Elevation is different. I wonder if this is the intend that the HYDRO Wave Elev. Ref. Pos (a moving position) directly matching the wave generator? If I understand correctly, the HYDRO Wave Elev. Ref. Pos is changing according to the displacement of the floater.
thank you very much,
Chris
Hi Chris,
what you are observing is the intentional behavior.
There are several options on how the wave kinematics are evaluated for potential flow and Morison element hydrodynamics. You can find more information if you search the documentation for the keywords: WAVEKINEVAL_POT and WAVEKINEVAL_MOR.
The kinematics for the potential flow hydrodynamics evaluations can be obtained in three different ways:
- local evaluation of wave kinematics
- evaluation at the fixed, undisplaced/unrotated initial reference position (this is the default value if not specified)
- evaluation at a lagged position (controlled by
WAVEKINTAU).
The main reason for this differentiation is the fact that the potential flow database is created under the assumption of only small displacements of the platform. Large displacements of the floater may violate this assumption and cause undesirable effects, especially when comparing/validation agains other software it is important to model this consistently across the different tools!
The data that is shown under “HYDRO Wave Elev. Ref. Pos” relates to the evaluation position for the potential flow database.
BR,
David
