Abstract

A new riser system consisting of a lift pump located a distance down in a conventional marine drilling riser with a separate mud return line from the lift pump and up is used to reduce the hydrostatic head of the drilling fluid in the bore hole. The upper section of the riser above the pump outlet is vented to the atmosphere. A lower mud level allows a higher mud weight to be used. However, determined by the position of the fluid level it will result in a lower pressure at the casing shoe. The hydrostatic head of the drilling fluid and the seawater will be sufficient to control the bottom hole pressure after an emergency disconnection of the marine-drilling riser at the seabed. The riser joint which houses the pump can be located anywhere in the riser section depending on water depth, formation pore pressure etc., typically 200 - 300 m below the sea level. The system also allows the bottom hole pressure to be quickly adjusted by simply adjusting the air/mud level above the pump outlet in the marine-drilling riser.

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