I’m working on the simulation of DLCs 6.1 and 6.2 in IEC 61400-3-1:2019, using OpenFAST and the NREL 5MW baseline wind turbine (5MW_OC3Mnpl_DLL_WTurb_WavesIrr). I’m confused about the following problems:
The IEC standard requires constrained wave simulations for DLCs 6.1 and 6.2. This functionality does not exist in the current release version of OpenFAST, but I noticed that the dev branch of OpenFAST (commit e6052f28744366ed19b89c9b81c7c0f495cffdd1) implements this feature and I compiled the binary myself. However, When I change the MSL2SWL in SeaState.dat and MSL2SWL in 5MW_OC3Mnpl_DLL_WTurb_WavesIrr.fst from 0 to other value, the result of ReactFZss seems weird: It fluctuates drastically and deviates from the regular Fz (about 8.4e6 N) which is controlled by gravity. I further noticed that the HydroDyn.dat file says HydroFzi has been removed due to numerical instability in regression tests.
- Is this numerical instability causes the weird ReactFZss?
- If MSL2SWL == 0, the results of ReactFZss look reasonable. Can I trust the results in this case (ReactFZss and other forces/moments on the foundation)?
- If MSL2SWL != 0, can I trust the outputs of other channels?
I’m also confused about the constrained wave height I should embed into the simulation. The standards says I should use “N-year return wave height, H_N”. If the metocean data is lacking, may I use H_N = 1.86 H_s,N (i.e., for 50 year return wave height, H_50=1.86H_s,50, in which H_s,50 is the significant wave height with a return period of 50 years assuming a 3-hour reference period) for CrestHmax in SeaState.dat with ConstWaveMod == 2? Or I should use other value and setting?