about HydroDyn input parameter “PtfmVol0”

[cjwan12589]

Dear Professor jjonkman, I have some doubts about HydroDyn parameters：

1. I learned from reading HydroDyn's manual that the input parameter "PtfmVol0" is the same as the value in WAMIT output, but at the same time, I also learned from the manual that: This value needs to satisfy the stillwater balance equation in 6.8.1. According to the description, the V0 in the stillwater balance equation takes into account the drainage volume of the entire system (including the weight of the rotor-cabin assembly, tower and substructure), but the WAMIT modeling does not model the entire system (only the individual floating platform), is this correct? Isn't the input parameter "PtfmVol0" the same value as V0 in section 6.8.1? So how should the parameter "PtfmVol0" be set?

2. In addition, regarding the pretension of the mooring system in the still water balance equation, I did not find this value in the relevant input file. Could you please tell me where or how to set this value?

I would appreciate it if you could give me some answers and suggestions. Thank you very much!

[jjonkman]

Dear @cjwan12589,

Regarding (1), I would expect that HydroDyn input PtfmVol0 would both match the value of the WAMIT output and satisfy the vertical equilibrium equation. I'm not sure I understand your concern with the WAMIT model, which for the first-order solution is based only on the surface geometry of the floater, and not any body mass.

Regarding (2), the vertical mooring pretension is not really a standard output from OpenFAST or the mooring modules. But you can infer it by, e.g., outputting the tension along each mooring line at the fairleads, computing the vertical contribution (based on the inclination angle of the line), and summing. You could also get it by reviewing operating point values from an OpenFAST linearization analysis. Regardless, I’m not sure you need to know the value exactly so much as to set up the OpenFAST model such that it does not heave in the absence of wind and wave excitation.

Best regards,

[cjwan12589]

Dear Jason, Thank you very much for your answer, I still have some questions: 1.If I want to design a new floating platform, when the mass and geometry of the platform is determined, then according to the principle of buoyancy, the drainage volume of the floating platform in the static water state has been determined. Is this value "PtfmVol0" in the HydroDyn input file? Or is this value calculated by the static equilibrium equation? Both methods, the value should not be the same (the vertical balance equation takes into account the weight of the entire fan structure). I am more concerned about how "PtfmVol0" should be determined when a new platform is designed.

2.When a new floating platform is designed, how can it satisfy the vertical balance equation without calculation, and how should the mooring preload be set? Is it an estimate of a value? Or is it set to 0?

Best regards,

[jjonkman]

Dear @cjwan12589,

Regarding (1), PtfmVol0 should be the displaced volume of the floater in its undisplaced condition. If the floating offshore wind turbine (FOWT) is designed properly, this volume should be based on the total weight of the FOWT (including turbine and ballast), as well as on the vertical mooring pretension such that the vertical equilibrium equation is satisfied. If you removed the turbine from the floater, I would expect the platform would heave upward and the displaced volume would go down, but that would not be the undisplaced condition.

Regarding (2), you can run a simulation in still water (no waves) and without wind and ensure that the platform heave displacement is near zero. I would not expect the mooring preload to be zero, unless the moorings are absent or neutrally buoyant.

Best regards,