Dear Yu Ma,
I believe your question regarding platform loads was recently answered in the forum topic found here: Clarifications PtfmFxyz/mxyz in FAST. Please let me konw if additional clarification is needed.
It is possible to obtain the complete linear 6x6 stiffness matrix associated with the mooring system from FAST. Let’s call this matrix K_m. When linearizing a FAST model, one has to always consider the influence of “effective stiffness” – see the following forum topic for more information: FAST: Model linearization. To get around this issue, the following process can be used to calculate K_m directly:
- Set-up a FAST model of the foating wind system.
- Instead of using the correct WAMIT data files for your floating platform, use WAMIT data where all of the coefficients are zero-valued. You can find suitable zero-valued WAMIT data files attached. These were obtained by replacing the values in a normal WAMIT output with zeros at all periods.
- Include the 6 platform loads PtfmFxi through PtfmMzi in the FAST output list.
- Run a FAST linearization analysis with CalcStdy = False.
- Grab the 6x6 matrix relating the 6 platform load outputs to the 6 platform DOFs – that is DspCMat.
- Derive K_m = -1000*DspCMat.
The factor of -1000 in step 6 is needed to correct for units (kN → N) and sign.
When using FAST in conjunction with turbulent wind data files from TurbSim, the wind “grids” (“planes”) will propagate along the xi-axis regardless of the wind direction specified in TurbSim, which is why we always recommend setting the wind direction to zero when using TurbSim in conjunction with FAST. See the TurbSim User’s Guide for more information. To model a yaw error with Turbulent wind, set the nacelle-yaw angle to be nonzero in FAST instead of changing the wind direction. When using AeroDyn’s so-called “hub-height wind files,” the wind direction need not align with the xi-axis.
I hope that helps.
Best regards,
Zeros.zip (58.8 KB)