Heave difference between Openfast and simpack for semi-submersibles

Dear all,

I did heave free decay in Simpack and compared it with Openfast, both of them give an initial displacement of 8m, but the result of simpack has a negative offset in the Z direction compared to Openfast, I want to know Why is that.

Best regards,

Dear @Monge.Low,

Do both models have the same total mass, undisplaced buoyancy, and vertical mooring pretension?

Best regards,