Annulus Friction Pressure
Circulating pressure loss inherent in the annulus between the drill string and casing or open hole.
Source: API RP 59, Recommended Practice for Well Control Operations, Second Edition, May 2006. Global Standards
Annulus Friction Pressure (AFP)
Difference between bottomhole pressure and choke pressure due to friction; a function of flow rate, hole geometry, surface roughness, fluid properties.
Source: IADC UBO / MPD Glossary, December 2011. Global Standards