Formula
      d = [ (log ( R / (60 × N) ) / log ( (12 × W) / (1000 × D) ) ] × ( P_n / P_m )^c
Where,
d = d-exponent value
R = Penetration Rate
N = Rotary Speed
W = Weight on Bit
D = Drill Bit Diameter
P_n = Mud Weight Equivalent
P_m = Mud Weight Used
C = Shale Compactibility Coefficient
Use this simple online D Exponent calculator to detect transition from normal pressure to abnormal pressure.