Subsea pumping systems improve the safety and efficiency of drilling operations in tophole in water depths down to 525m (1720ft). The system collects mud at the seabed and pumps it back to the rig before the rig BOP / riser has been run.

This equipment can be reconfigured to a ‘pumped riser’ mode for use after the riser is run. Drilling mud is pumped from an outlet in the marine riser with a blanket fluid, usually seawater above. Varying the level of heavy mud in the riser compensates for the Annular Pressure Loss. By dropping the level as mud circulation starts, there in no increase in the Bottom Hole Pressure (BHP). The Equivalent Circulating Density (ECD) is the same as the mud weight. There is a constant BHP when drilling, circulating, making connections, tripping, logging etc.

The system is simple to understand and doers not rely on surface pressure or pressurising the marine riser. Connections are likely to be slower than the alternative methods but this is not considered a significant problem. In some ways is reassuring to be able to change conditions slowly and steadily.

Pumped riser systems may also be used to drill surface holes, without any gas, at bottom hole pressures below seawater hydrostatic. This may be done to avoid losses into vugular formations or to improve drill rates in hard formations.

Subsea pumping has also been used in a deepwater 5000ft (1500m) field trial. This proved up deepwater RMR but is a step towards the goal of full Dual Gradient Drilling in deep water.

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