I have a donut-shaped floater that has a ring heave plate attached to the bottom both inside and outside the donut. I am wondering if OpenFAST can model this kind of floater without modifying source code. I tried to use hybrid approach to model it, with linear radiation/diffraction/hydrostatics imported from AQWA and viscous drag on members, amplitude of heave motion is around 5 meters for a normal working condition. It seems the heave plate was not modelled properly.
You mentioned in this link (heave plate) that physics of heave plate cannot be modelled by FAST v8.
May I know if the new version of OpenFAST has the capability? If yes, what are the parameters that I should change to account for the additional damping the heave plate provides?
How different the six DOF motion would be for the floater with and without the heave plate in HydroDyn input, assuming plate structure is included in AQWA?
Thanks in advance for any suggestions.
The link you refer to was discussing limitations of FAST v7, not FAST v8 or OpenFAST. FAST v8 and OpenFAST can both model heave plates, including potential-flow and viscous effects.
Relative to a floater without heave plates, heave plates account both for a large added mass and viscous drag, which should (if properly designed) lower the natural period and minimize the motion of the floater in heave, pitch, and roll.
You can import into HydroDyn the potential-flow solution for a floater of any shape, including a donut shape. The viscous-load model of HydroDyn assumes cylindrical members with end effects. To model the viscous loads on heave plates in the horizontal plane, you should place two strip theory joints at the same (X,Y) location and similar Z locations to ensure that the cylinder is very thin–with large end effects and small transverse effects. Donut shapes are not directly supported by the strip-theory solution of HydoDyn, but you should be to approximate the equivalent viscous effect by adding a number of circular heave plates distributed around the donut (with the same total equivalent heave plate area).
thanks for the suggestion, splitting the heave plate into small strips around the main column sounds like a good idea. Let me give it a try.
I’m also modelling the heave plate in Openfast because I want to include the viscous effects of damping plate on the heave motion. I saw the OC4-deepCwind also uses a similar approach as you discussed above.
I have a few questions, I hope you can help me.
1、If I want to include the viscous effects of the heave plate as a lumped force in Openfast, should I define the axial damping coefficient here(hydrodynamic input file). Am I right? What if I want to consider the damping force as a distributed force for heave plate. What can I do in Openfast.
- If the heave plate has a large thickness and the heave plate model is not modeled in the PF analysis. The definition of Axca can be used to consider the added mass in heave on the heave plate in openfast, is it right?
Thanks very much
Regarding (1), correct; the axial drag coefficient (
AxCd) will be used in the calculation of the viscous drag force at member ends, which can be used to model the viscous drag on a heave plate. HydroDyn only considers a lumped force (not distributed) at the end joint of the member. If you want to distribute the hydrodynamic force across the heave plate, you must define multiple joints/members, ensuring that the total area summed across all end joints is correct.
Regarding (2), correct; the axial added-mass (
AxCa) and axial dynamic-pressure (
AxCp) coefficient are used in the calculation of fluid-inertia and hydrodynamic added mass forces if the member is not modeled in potential-flow theory (
PropPot = False).
Thanks very much for your reply!