Arrangement and method for regulating bottom hole pressures when drilling deepwater offshore wells

Abstract

An arrangement and a method to control and regulate the bottom hole pressure in a well during subsea drilling at deep waters: The method involves adjustment of a liquid/gas interface level in a drilling riser up or down. The arrangement comprises a high pressure drilling riser and a surface BOP at the upper end of the drilling riser. The surface BOP havs a gas bleeding outlet. The riser also comprises a BOP, with a by-pass line. The drilling riser has an outlet at a depth below the water surface, and the outlet is connected to a pumping system with a flow return conduit running back to a drilling vessel/platform.

Description

RELATED APPLICATIONS[0001]This application is a U.S. National Phase Application of PCT Application No. PCT / NO02 / 00317, filed Sep. 10, 2002, which claims the benefit of U.S. Provisional Application No. 60 / 318,391, filed Sep. 10, 2001. Each of these applications is herein incorporated in its entirety by reference.FIELD OF THE INVENTION[0002]The present invention relates to a particular arrangement for use when drilling oil and gas wells from offshore structures that float on the surface of the water in depths typically greater than 500 m above seabed. More particularly, it describes a drilling riser system so arranged that the pressure in the bottom of an underwater borehole can be controlled in a completely novel way, and that the hydrocarbon pressure from the drilled formation can be handled in an equally new and safe fashion in the riser system itself.BACKGROUND OF THE INVENTION[0003]This invention defines a particular novel arrangement, which can reduce drilling costs in deep ocea...

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Benefits of technology

[0044]The riser of the arrangements of the present invention preferably has no kill or chokes line, which is contrary to what is normal for most marine risers. Instead the annulus between the drill pipe and the riser becomes the choke line and the drill pipe becomes the kill line when needed when the subsea BOP is closed. This will greatly increase the operator's ability to handle unexpected pressures or other well control situations.

[0045]The arrangements and methods of the present invention, will in a specific new way make it possible to control and regulate the hydrostatic pressure exerted by the drilling fluid on the subsurface formations. It will be poss

Problems solved by technology

Particularly for smaller diameter hole sizes these additional dynamic forces become significant.

As the drilling fluid must have a specific gravity such that the fluid remaining in the well is still heavy enough to control the formation when the drilling marine riser is disconnected, this creates a problem when drilling in deep waters.

This is due to the fact that the marine riser will be full of heavy mud when connected to the sub sea blowout preventer, causing a higher bottom-hole pressure than required for formation control.

This results in the need to set frequent casings in the upper part of the hole since the formation cannot support the higher mudweight from the surface.

This also means that slimhole or slender wells are difficult to co

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