8.3 BOP Stack Configurations & Drilling Spools

A blowout preventer (BOP) stack is an assembly of preventers, drilling spools, and high-pressure valving mounted atop the wellhead. Governed by API Standard 53 (Well Control Equipment Systems for Drilling Wells) and API Specification 16A, stack arrangements are engineered to provide multiple independent pressure barrier envelopes. Proper stack design ensures crews can shut in on any tubular size, hang off and shear pipe in emergencies, and circulate kicks while isolating primary BOP bodies from erosive wear.

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API Standard 53 Stack Designation Codes

API Standard 53 establishes standardized shorthand codes to describe stack components from the casing head housing upward:

Stack designations combine rated working pressure (M = 1,000 psi), bore size, and component shorthand from bottom to top (e.g., $10\text{M} - 13\frac{5}{8}" - \text{S-R-R-Rd-A}$: 10,000 psi, $13\frac{5}{8}\text{-inch}$ stack with Drilling Spool, two Single Rams, one Double Ram, and Annular).

Identifying the Stack's Rated Working Pressure

From given data, Level 2 candidates must be able to identify the rated working pressure (RWP) of a BOP stack. In practice a stack is described by the rating of its ram preventers (a "10M" stack, for example), but every pressure-containing component has its own limit: the stack can never be used above the lowest rating of its rams, spools, connections, side-outlet valves and the wellhead below it. The annular preventer is often rated lower than the rams, so its own limit must be known separately.

Example: a stack with 10,000 psi rams, drilling spool and side-outlet valves on a 10,000 psi wellhead, topped by a 5,000 psi annular, is a 10M ram stack. The annular may only be used up to 5,000 psi, and under API Standard 53 it is tested to 70% of that (3,500 psi).

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Surface Stack Arrangements & Component Sequencing

Surface BOP stacks (land rigs, jack-ups, and platforms) ensure operational flexibility during well control circulation, pipe hang-off, and emergency pipe shearing:

One common surface arrangement is shown below; stack designs vary between rigs.

1. Lower Pipe Ram Positioned Below the Drilling Spool: Placing a fixed pipe ram below the drilling spool (and thus below choke and kill outlets) allows the well to be killed by pumping down the kill line into the annulus below the upper rams, or circulating an influx out through the choke line. If upper stack components sustain damage, the lower pipe ram isolates the wellhead as a master barrier.
2. Blind Shear Ram Positioned Between Pipe Rams: Placing the blind shear ram (BSR) between pipe rams allows the crew to hang off drill pipe on the lower pipe ram, adjust tool joint spacing, and cleanly sever the pipe body. The upper pipe ram (often a variable bore ram) provides secondary pipe containment above the shear point.
3. Annular Preventer at the Top: Occupies the topmost position to permit immediate well shut-in upon kick detection, regardless of what tubular geometry spans the stack.

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Drilling Spools vs. Integral Ram Body Outlets

To connect choke and kill lines to the annulus, operators use either a separate drilling spool or integral ram body side outlets.

Functional Superiority of Drilling Spools

A drilling spool is a flanged or clamped spacer spool equipped with two opposed side outlets. While ram bodies can feature integral side outlets beneath the ram cavity, industry standards strongly favor dedicated drilling spools:

• Erosion Isolation: High-velocity gas kicks carrying formation sand, barite, and sharp rock cuttings flow out through the choke outlet. Sacrificing a relatively low-cost drilling spool protects expensive forged ram preventer bodies from erosive wall thinning and structural condemnation.
• Vertical Spacing for Hang-Off and Shearing: Drilling spools provide vertical clearance between ram cavities to accommodate tool joint lengths, ensuring that when the drill string hangs off on a lower pipe ram, a tool joint is not positioned across shear blades.
• Maintenance Flexibility: A damaged spool can be refurbished locally without recertifying or replacing primary BOP bodies.

When Integral Outlets Are Used

On compact land rigs and older jack-ups with strictly limited substructure cellar height, integral ram body side outlets eliminate drilling spools to minimize overall stack height.

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Choke and Kill Line Outlet Specifications

Outlet sizes and valving are specified in API Standard 53 and the rig's equipment standards:

Dimensional Requirements

• Choke Line Outlets: Commonly 3-inch nominal or larger ($3\frac{1}{16}\text{ in}$ or $4\frac{1}{16}\text{ in}$ connections). A large bore minimizes circulating friction backpressure and fluid velocity, preventing erosive washouts during high-rate gas circulation.
• Kill Line Outlets: Commonly 2-inch nominal or larger ($2\frac{1}{16}\text{ in}$ or $3\frac{1}{16}\text{ in}$), providing a dedicated conduit for pumping high-density kill mud into the annulus.

Valving Redundancy

Both choke and kill outlets must incorporate at least two high-pressure isolation valves in series:

• Inner Valve (Master Valve): A manual gate valve bolted directly to the outlet flange, kept normally open and closed only to isolate the outer valve.
• Outer Valve: A hydraulically operated remote-controlled gate valve (HCR valve) on the choke line, opened instantly from the console. On the kill line, the outer valve is an HCR valve or high-pressure check valve.

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Flanged Connections and Hub Clamps

BOP stack components are joined via high-pressure bolted flanges or clamp hub connectors conforming to API Specification 6A and 16A:

• Bolted Flanges: Connected with high-tensile alloy steel studs (ASTM A193 Grade B7) and heavy hex nuts, torqued in a cross pattern for uniform gasket preload.
• Clamp Hub Connections (e.g., Grayloc / Cameron): Tapered hubs encircled by external clamp segments pulled tight with two to four bolts. Clamp hubs reduce make-up time, save weight, and fit compact rig cellars.

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API Ring Joint Gaskets: Types R, RX, and BX

BOP connections utilize precision-machined solid metallic ring joint gaskets seated in matching trapezoidal flange grooves. Forged from soft iron, low-carbon steel, or 316 stainless steel, gaskets are softer than flange steel so that deformation occurs in the disposable gasket, preserving the expensive flange face:

• Type R Ring Gaskets: Oval or octagonal cross-section rated for API 6B flanges up to 5,000 psi. Non-pressure-energized; relies entirely on bolt torque to coin gasket angles against groove walls. Flange faces remain separated. Single-use only.
• Type RX Ring Gaskets: Asymmetric, extended octagonal cross-section rated for API 6B flanges up to 5,000 psi. Pressure-energized; wellbore pressure forces outer sealing lips against groove walls. Fully interchangeable with Type R grooves.
• Type BX Ring Gaskets: Solid beveled square cross-section rated exclusively for API 6BX flanges and 16A clamp hubs from 5,000 psi to 20,000 psi. Pressure-energized; flange faces make full metal-to-metal (face-to-face) contact. Strictly non-interchangeable with Type R or RX grooves.