# Hydrostatic restoring forces and moments in Elastodyn

Hello,
I am currently working on Openfast and more precisely on hydrostatic restoring coefficients of the platfom , and I found that when considering the change of the inclination（like pitch) , nonlinear hydrostatic restoring hydrostatic restoring coefficients (calculated by my own method associated with the heeling angle) are quite different with the linear coefficients for large rotational motion. So next step is that I want to apply these results in time-domain simulations for floating wind turbines. I have an assumption ,if I want to show the effects of the nonlinear coefficients for different angles, I can change the hydrostatic forces or moments in simulations. And I have found that the hydrostatic forces or moments are included in FZhydro (It is shown in FAST kinetics and the code of Elastodyn). So what I should do is to find the hydrostatic part and change it using DLL.
Is this assumpution available?
If anyone could give me some advice , I would be very grateful.

Best regards,

Dear @Zhengyang.Pang,

It sounds like you want to modify the HydroDyn module of OpenFAST to include hydrostatic nonlinearities that are not currently accounted for; is that correct? Can you clarify what hydrostatic nonlinearities you are referring to? And how are you currently modeling the hydrodynamics/hydrostatics of the platform within HydroDyn, e.g., potential flow or strip theory?

Best regards,

Dear Jason,

Thank you for your quick reply. When a floating platform rotates in motion, the hydrostatic restoring changes, and we want to implement this effect in the time-domain calculation of FAST.

To do this, we have calculated the hydrostatic stiffness of a floating platform with several inclinations using a potential flow code like WAMIT, and obtain the hydrostatic stiffness lookup table. For example, for platform pitch of 0, 5, 10 deg, we have a set of 3 hydrostatic stiffness. Then we need to update the linear hydrostatic forces and moments in the time domain based on information of platform motions from FAST.

My initial plan is to modify HydroDyn and correct the hydrostatic stiffness at each time step. [ADD which line in which FORTRAN CODE]. Could you advise if my understanding is correct?

Dear @Zhengyang.Pang,

That sounds reasonable to me.

Best regards,