Controlled Mud Level
A variant of Dual Gradient Drilling (DGD) used only on floating rigs by placing a special pump in/on the drilling riser to vary the level of fluid in the drilling riser. The fluid level is adjusted up or down to control bottom-hole pressure and to compensate for variations in equivalent circulating density due to frictional pressure losses.
Source: ABS Guide for Classification and Certification of Managed Pressure Drilling Systems, September 2017. Global Standards