how to define the buoyancy of a beam under wave by user defined function?

[Cathy039]

Abaqus standard cannot predict the buoyancy correctly, it can either give the full submerged buoyancy or 0, depending on the relative location of the mid point in cross section and still water level. Now I want to write the correct buoyancy for submerged beam under wave condition. As I know, I need to define the changing submerged area, and then calculate the buoyancy through equation pgv, and put the buoyancy at this time to get the submerged area at the next time.
The problem is how to define the changing cross section area? Can someone help me out of this? Thank you very much!!