Page 176 - Well Control for Completions and Interventions

168 Well Control for Completions and Interventions

Figure 4.22 Blind/shear ram.

designed to close in the well whether or not pipe is present. If pipe is
across the stack it will be cut by the closing blades in the ram block.
Elastomer packings then seal off the well. Although blind/shear rams are
mainly used in subsea applications, they should also be considered for sur-
face applications on high-pressure wells, wells where H 2 S is present, or
wells in urban and environmentally sensitive areas.
Depending on pipe size, wall thickness, and material yield, high pres-
sure may be needed to shear the pipe and effect a seal (Fig. 4.22).

4.9 BLOW OUT PREVENTER CONTROL SYSTEM

It is vital that the well is closed in quickly following a kick. The con-
trol system should be able to close any of the BOP rams within the response
time recommended by theAPI.Asalready stated,thisis30 s for ram preven-
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ters. Annular preventers smaller than 18/4 in. also need to close within 30 s,
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whilst those sized 18/4 in. or larger must close in less than 45 s. Annular pre-
venters require a large volume of hydraulic fluid to close. For example, the
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Hydril 13/8 in. 10,000 psi GK preventer requires over 37 US gallons to
close. A pump that can supply enough pressurized fluid within the required
closure time would be impractical in terms of size and cost. Moreover,
pumps cannot operate if there is a loss of power at the wellsite. For these rea-
sons, accumulator bottles are used to store pressurized hydraulic fluid for
quick release. The volume of fluid stored in the accumulator is set out in
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API RP 53, specification 16D, and recommended practices 16E. They
require a BOP system to have enough usable hydraulic fluid volume (with
pumps inoperative) to satisfy the greater of the two following requirements:
• Close from a full open position at zero wellbore pressure all of the
BOPs in the BOP stack plus 50% reserve.