Fastv8 Heave Convergence Problem: TLP

Wind & Water Computer-Aided Engineering Software Tools
1

Hello to everyone on these forums.

Firstly thanks for providing such a great forum and code Fast v.8

Now onto my problem

I am attempting to edit the CertTest: Test23, with my own TLP model from WAMIT.

I have changed to my WAMITFiles, edited the relevant mass and inertia properties in ElastoDyn, I am running with 0 wind speed and Still Water.

The problem is the heave motion of the platform will not converge to a still water level. Instead it oscillates in heave alone at about 0.15m. This does not dampen out over time at all.

I’m wondering if anyone might have a solution as to why this might be happening?

2

Hi Chris,

If the platform/tower/nacelle/blade mass changes, then the mooring might need an adjustment to get sufficient pretension for static equilibrium. This can be one reason for the heave oscillation. The total total pretension+FOWT weight should balance the buoyant force at equilibrium (on a similar note, the line length is given minor adjustments in the coupled FAST+OrcaFlex tool (FAST v7) to remove TLP heave imbalances). The initial post doesn’t indicate this step was performed, and I want to make it wasn’t accidentally overlooked.

regards,
Marco

3

Hi Marco,

Thanks for the swift reply.

Sorry forgot to add to my intitial post - I have adjusted the un-stretched mooring length to the correct value. The floater starts at the correct position 0, and then oscillates up to 0.14m

4

Dear Chris,

I agree with Marco’s response. Section 6.8.1 of the draft HydroDyn User’s Guide and Theory manual provides guidance on this.

Best regards,