| 摘要 |
A technique is presented for providing a double-barrier to formation fluids during drilling operations with a concentric drill string having an inner bore and an annulus. A primary flow control system is used to provide a first barrier to formation fluids in the inner bore and annulus of the drill string. A backup flow control system is used to provide a second barrier to formation fluids. The backup flow control system comprises a backup inner bore shutoff valve and a backup annulus shutoff valve. The backup inner bore shutoff valve is dropped from the surface through the inner bore of the drill string. The backup inner bore shutoff valve has spring-biased tabs that are configured to extend outward to contact an inner surface profile of the inner bore of the drill string. The backup annulus shutoff valve may be provided with the profile to receive the spring-biased tabs from the backup inner bore shutoff valve. When the tabs of the backup inner bore shutoff valve are disposed opposite the profile, the tabs of the backup inner bore shutoff valve are extended outward into the profile by a spring assembly, securing the backup inner bore shutoff valve within the drill string.
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