Direction of Mean Drift in OC4 Semisub

Wind & Water Structural Analysis
1

Hi,

I am running simulations of the OC4 SemiSub with different wave directions, and noticed that the platform drift that occurs near a wave period of 7 seconds is always in the positive X direction (pure surge, no sway), regardless of the what the input for WaveDir is (Figures 1 and 2). I would expect the mean drift of the platform to be in the direction of the waves. This seems to be the case for 0 degree wave (Figure 1, Line 2 tension higher), but not for 60 degree wave (Figure 2, would have expected Lines 2 and 3 to have equal higher tensions).

I understand that this mean drift does not occur when DiffQTF/SumQTF (2nd-order forces) are turned off (Figure 3).

Furthermore, when the platform & mooring system are rotated 60 degrees, the platform drifts in the direction of the incident waves (Figure 4) and the results are as expected (Lines 2 and 3 have higher equal tensions).

Figures attached. Can you explain why this behavior would occur in the model?

Thank you so much.

Casey Fontana
Direction Comparison.docx (483 KB)

2

Dear Casey,

I’m assuming you are running some variation of Test25 from the FAST CertTest, which is a model of the NREL 5-MW turbine atop the OC4-DeepCwind semisubmersible. In this model, the second-order hydrodynamic effects, including mean drift, are solved using DiffQTF = 12 i.e. using the full difference-frequency quadratic transfer function (QTF). This QTF contains the second-order wave-excitation loads per unit wave amplitude squared at pairs of first-order wave frequencies and directions, across a range of frequency and direction pairs, as derived from WAMIT. However, the *.12d file provided with Test25 only contains data for the zero-degree wave direction. It was the intent for HydroDyn to abort if the specified wave direction (WaveDir) is outside the range of the available WAMIT data, but there is a known bug in the upper bound on WaveDir in the WAMIT2 submodule of HydroDyn (unfortunately, we have not yet fixed the bug). See the following forum topic for details: Second order file WaveDir boundary check issue. (Please note that the second problem reported in that forum topic is not a bug; the direction pairs must be fully populated (not sparse).)

Regardless, unless you’ve changed the *.12d file to expand the QTF to other wave directions, this HydroDyn model should only be run with WaveDir = 0.

I hope that helps.

Best regards,