Because the static pressure is being measured against the operating pressure, a negative value means that it is simply less than this value. Example static pressure -10 kPa with operating pressure of 100 kPa, then the absolute pressure is 90 kPa.

Again, this pressure does not include the hydrostatic component. For that, you will have to likely construct a new variable using either the CCF or a Named Expression. The best approach would be to incorporate the depth by using the VOF to determine how much liquid is "above" the bottom surface.