BeamDyn usage question

[nedisy]

Is your feature request related to a problem? Please describe.
Currently, I cannot get OpenFast to work without BeamDyn input. I want to simulate barge/pontoon only.

Describe the solution you'd like
I need to get pontoon displacements given hydrodynamic inputs. From what I understand, HydroDyn cannot calculate displacements, because it relies on the PRP displacement. When I run, I always get zeros surge, heave, etc (only 3 options to input PRP, and the only file independent input are all zeros input: 0), but the forces are non-zero. I envision that the OpenFast, HydroDyn, or any implementation that could work, can calculate free or tied pontoon-only (no wind turbine) displacements and forces. This project is modular so I hope if you can simulate a wind turbine on a floating platform, you can simulate a wind turbine-less floating platform too.

Describe alternatives you've considered
An alternative would be linearizing the pontoon support (mooring/pile) and putting the stiffness matrix into hydrodynamic solvers like nemoh. Then use the displacement RAO to calculate the probability of the load on the support. Another one would make DFT of displacement RAO (that already has the stiffness matrix) to get displacement vs time for a given spectrum. That would limit the simulation to be linear and frankly doesn't need OpenFast/HydroDyn at all in that case. But it would be great if something like MoorDyn be used to calculate the nonlinearities of the displacements and forces of the pontoon (that would require OpenFast).

Additional context
Pontoon-only solver would help even more people beyond the many wind turbine baseline files this project.

[jjonkman]

Dear @nedisy,

OpenFAST can already model a floating platform (including barge or pontoon) without a wind turbine atop. To model wave-body interaction for a freely floating or moored body without a wind turbine, you can run OpenFAST with ElastoDyn and HydroDyn (without BeamDyn enabled, but optionally with a mooring module such as MoorDyn enabled or structural flexibility of the floater with SubDyn enabled, if desired). To "remove" the rotor, nacelle, and tower from an ElastoDyn model, disable the relevant degrees of freedom (blades, drivetrain/generator, nacelle, tower) and set the relevant geometry and mass properties to zero (or epsilon for the variables that do not permit a value of exactly zero).

FYI: The standalone HydroDyn driver requires the user to prescribe the 6-DOF motion of the floater and cannot compute this motion itself. But OpenFAST can calculate the 6-DOF motion of the floater through ElastoDyn when HydroDyn is coupled within OpenFAST.

Best regards,

[nedisy]

Thankyou @jjonkman , I successfully simulated the pontoon, although with a dummy massless turbine of size 1 meter in diameter. I think it is still inappropriate to simulate a pontoon in this hacky way. I believe a proper way to disable wind turbines is needed. It can be very beneficial in modeling floating solar panels or similar infrastructures without wind turbines.

[jjonkman]

Dear @nedisy,

I agree that the workaround to model floating platform without a wind turbine atop is a bit "hacky", though it does work.

In my opinion, the preferred way to model other floating systems (such as floating solar panels) would not be to modify OpenFAST, which is quite wind energy system centric, but to develop a new driver code, perhaps with a new structural module that is tailored to the floating technology of interest, of which HydroDyn, MoorDyn, and perhaps other modules of interest that are not wind-specific could be coupled too.

Best regards,