Mud Cap Drilling
A drilling process that involves maintaining a mud level in the annulus below the surface for hole stability and well control purposes, and no surface back-pressure. In MCD, there are no flow returns to the surface while maintaining mud column on the annular side. There exists several MCD variations, such as PMCD, FMCD etc. MCD is an umbrella term used for FMCD and PMCD.
Source: ABS Guide for Classification and Certification of Managed Pressure Drilling Systems, September 2017. Global Standards