How to model a rope-towed floating ocean-current generator at 2 kn in OpenFAST

[tptgrg]

I want to simulate a floating ocean-current power generator that is towed by a rope/bridle at a constant 2-kn speed, while the whole device remains free to respond in 6 DOF under waves/currents.

[jjonkman]

Dear @tptgrg,

When we have used OpenFAST to model towing conditions in the past, we have changed the frame of reference and modeled the towing as an incident current rather than as a forward speed. In that way, the rope/bridle can be attached to the floater and a fixed point (or points) upstream) and current is added to consider the forward-speed effect.

For wave-loads, the wave spectrum is influenced by the forward speed / current (Doppler effect). In the past, SeaState would simply vectorally sum the wave and current kinematics, and so, we'd modify the incident wave spectrum to account for the encounter spectrum. However, SeaState was recently improved to directly consider different options for wave-current interaction, which is scheduled for release in OpenFAST v5.0 due out soon: #2978,

Best regards,

[tptgrg]

Thanks

[Shahram-73]

for modelling a rope-towed floating generator at 2 kn, here’s a way that tends to work:

• Define the floating body properly in the .fst file — set mass, buoyancy, and stiffness for the tow line.
• Set the tow speed using the environmental wind/wave conditions you want — this should propagate to the hydro coefficients.
• Enable motion outputs so you can verify the response — use OutList to record motions and forces.
• Run a test with simple waves first and see if it stays stable. Then add your 2 kn conditions.