Jason:
I am making a query about the direction of the force in the mooring line, as it was not clear to me from the MoorDyn or MAP+ theory manuals.
The tension at the fairlead given in the OpenFAST output results, is it the vertical tension or the diagonal tension from which the vertical and horizonal forces would then be determined?
Thank you.
Regards,
AOAW
Dear Andre,
The tension outputs from MAP++ and MoorDyn are the tension force directed along the line.
Best regards,
Thanks Jason.
Regards,
AOAW
Hi Jason:
I am just checking if OpenFAST allows for modelling other types of mooring lines besides the steel chain type.
For example, can I model a wire type or rope?
Thank you.
Regards,
AOAW
Dear Andre,
Yes you can. The properties of any mooring line are specified in terms of the their general properties like unstretched length, mass per unit length, cross sectional stiffness, etc.
Best regards,
Thanks Jason.
Regards,
AOAW
Hi Jason:
Just double checking something. When you get this message:
" Fairlead tensions did not converge within TMaxIC=60 seconds."
Does that mean something is wrong with model?
I got this message but OpenFAST terminated successfully and forces were still computed at anchors. Please advise, as mooring arrangement that gave this message resulted in average heave of zero and other mooring arrangements that did not have that message resulted in average heave either above or below zero.
Thank you.
Regards,
AOAW
Hi Andrew,
That message just means that the initial condition solver did not find equilibrium during the allowed 60 seconds of pre-simulation in MoorDyn. In other words, the mooring system will be out of equilibrium when the FAST simulation starts. That usually doesn’t make any important difference because there are always start-up transients at the beginning of a simulation. But if you want ensure the mooring system starts in equilibrium you can increase the TmaxIC option in MoorDyn to give it more time to settle.
Either way, this process shouldn’t affect the mean heave of the system over the course of the simulation.
Best,
Matt
Noted. Many thanks Matt.
Regards,
AOAW
Hi Jason/Matt:
I have a query regarding the average heave of FOWT system being zero.
Should the average heave of FOWT be zero for all sea states?
I am looking at a FOWT in 100 m deep water and have adjusted the plan radius of the original 837.6 m mooring system with 850 m unstretched length to a mooring system 418.5 m plan radius and 392 m unstretched length.
I am looking at the steady wind case from 3 m/s to 25 m/s (2 m/s time steps); for each wind speed there is a different sea state. For some of the sea states, average heave of zero obtained but for others heave is either above or below zero. Please clarify if for a particular system all average have values should be zero and if so, could you provide any ideas on how this could be achieved?
Thank you.
Regards,
AOAW
Dear Andre,
We typically set up a floating wind turbine model in OpenFAST so that the platform heave is zero in the absence of wind or wave excitation. The mean heave may not be zero for all simulations, especially cases with a high mean thrust and subsequently large mean surge offset.
Best regards,
Okay. Thanks Jason.
Regards,
AOAW
Jason:
In the absence of wind and wave I imagine that a perturbation would have to be given to one of the DOFs of the FOWT system, so for example an initial value would be set to surge DOF. Please let me know if this understanding is correct.
Regards,
AOAW
Dear Andre,
I’m sorry, but I’m not sure I understand your question.
In the absence of wind and wave excitation, then the heave motion will remain stationary if the vertical buoyancy balances with the vertical mooring pretension and full-system weight of the FOWT (floater + tower + RNA). This is generally how we recommend that you set up an FOWT model in OpenFAST. The floater will heave in the absence of wind or wave excitation if this condition is not yet.
Best regards,
Hi Jason:
I am sorry that I was not clear. Let me attempt to make it clearer. When checking that buoyancy force is balanced by full system weight:
1) In the absence of wind and wave, when a simulation is run which of the flags in the .fst file should be on?
I suspect ElastoDyn, ServoDyn and MoorDyn will be on
HydroDyn will also be on but for a still water condition.
2) I was also asking if the initial conditions in ElastoDyn for surge, sway, heave etc. would all be set to zero?
Hope I am clearer now.
Thanks.
Regards,
AOAW
Hi Andre,
The simplest simulation to verify the heave equilibrium would be to disable all structural DOFs in ElastoDyn except for PtfmHvDOF, with all initial conditions in ElastoDyn zeroed. Then in the FAST / OpenFAST primary (*.fst) input file, only ElastoDyn, HydroDyn, and MoorDyn (or MAP++) would need to be enabled. Of course if the blades were modeled in BeamDyn instead of ElastoDyn, you’d have to enable BeamDyn as well.
Best regards,
Many thanks Jason.
Regards,
AOAW
Hi Jason:
I have attached the .fst and ElastoDyn files. I believe I have made changes as you have indicated but would like to make sure, so would be grateful if you could check it for me.
Thank you.
Regards,
AOAW
Dear Andre,
Looks correct, although you can also disable the blade DOFs in ElastoDy (FlapDOF1 = FlapDOF2 = EdgeDOF = False).
Best regards,
Okay thanks Jason. Will do.
Regards,
AOAW
