The Maximum Allowable Annular Surface Pressure (MAASP) equals the formation breakdown pressure at the point under consideration minus the hydrostatic head of the mud/or influx in the casing. During well control operations the critical point to consider is the casing shoe.
MAASP = Formation BreakDown Pressure - Head of mud in use
or
MAASP = (E.M.W - MWMUD) x 0.052 x Shoe Depth (TVD)
Where
E.M.W = Equivalent mud weight at which formation breaks at shoe
MWMUD = Mud Weight
During the process of controlling and circulating out an influx, several stages can be distinguished in calculating the MAASP. However, the MAASP is only significant while the casing is full of fluid. For pre-kick calculation purposes, the value of the MAASP shall be revised whenever the hydrostatic head of mud in the hole changes.