Modeling multi-member floating wind turbine

Wind & Water Computer-Aided Engineering Software Tools
13

Dear @Shengtao.Zhou,

OpenFAST uses its internal spatial mesh-to-mesh mapping functionality to connect the hydrodynamic nodes from HydroDyn to the structural nodes from SubDyn: 4. User Documentation — OpenFAST v3.4.1 documentation. Because HydroDyn and SubDyn use point-element meshes, effectively a nearest-neighbor mapping is used. The HydroDyn potential-flow mesh is based on HydroDyn inputs PtfmRefxt/PtfmRefyt/PtfmRefzt.

Please note, however, that the structure you show seems to have members that are quite small in volume, so, perhaps a strip-theory solution is satisfactory? (We typically recommend that large-volume members be represented in the potential-flow solution and small-volume members be represented in the strip-theory solution.) Large-volume members requiring potential flow are likely quite stiff and may need not be split out into multiple potential-flow bodies in HydroDyn and beam elements in SubDyn. Rather, it may be satisfactory to treat each large-volume body as a separate potential-flow body in HydroDyn with rigid-link elements in SubDyn.

Dear @Bart.Klootwijk,

Here are my general recommendations for setting the FEA and Craig-Bampton parameters in SubDyn for floating wind applications:

  • FEAMod = 3
  • NDiv = 1, unless the members aren’t manually discretized by adding joints, in which case NDiv > 1. That said, NDiv only supports a uniform division of all members, so, unless all members are roughly equal length, it may be better to manually discretize by adding joints.
  • CBMod = True
  • Nmodes = large enough to capture the lowest modes of the substructure, say up to 5-10 Hz, as calculated by SubDyn. This is typically less than 10-20 for most floating substructures. Because you likely wont know the frequencies until you run SubDyn, this will likely take some trial and error (by running SubDyn with different Nmodes). You can use the SubDyn mode visualizer (Mode shape visualization) to visualize each mode.
  • JDampings = 1 (if you know nothing) or tuned to match known full-system damping ratios.
  • GuyanDampMod = 0

Best regards,