Direction of Mean Drift in OC4 Semisub


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)

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: (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,