Drilling Spools

While choke and kill lines may be connected to side outlets of the blowout preventers, many operators prefer that these lines be connected to a drilling spool installed below at least one preventer capable of closing on pipe. Utilisation of the blowout preventer side outlet reduces the number of stack connections by eliminating the drilling spool and shortens the overall preventer stack height. The reasons for using a drilling spool are to localise possible erosion in the less expensive spool and to allow additional space between rams to facilitate stripping operations.

Drilling spools for blowout preventer stacks should meet the following minimum specifications:

  1. Have side outlets no smaller than 2" nominal diameter and be flanged, studded, or clamped for API Class 2M, 3M, and 5M. API Class 10M and 15M installations should have a minimum of two side outlets, one 3" and one 2" nominal diameter.
  2. Have a vertical bore diameter at least equal to the maximum bore of the uppermost casinghead.
  3. Have a working pressure rating equal to the rated working pressure of the attached blowout preventer.


For drilling operations, wellhead outlets should not be employed for choke or kill lines Such outlets may be employed for auxiliary or back-up connections to be used only if a failure of the primary control system is experienced.

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Drilling Spools

FIG C.5 ARRANGEMENT S*RRA Double Ram Type Preventers, Rd Optional.


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  • mirabella
    What is a drilling spool in the bop stack?
    2 years ago

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