Ram Preventer

Key Takeaways

• The dimensional compatibility between the pipe rams and the drill string in the hole is a safety-critical design requirement: pipe rams are fitted with elastomeric packing elements (also called rams or ram blocks) with an internal bore precisely machined to match the outer diameter of the tubular it must seal against; a set of 5-inch pipe rams will seal against 5-inch drill pipe but will not seal against a 6.625-inch drill collar or a 3.5-inch drill pipe, and attempting to close 5-inch rams against the wrong tubular size leaves a gap that allows wellbore pressure to bypass the preventer; this dimensional specificity requires that the BOP configuration include ram sizes matched to every distinct OD of tubular in the drill string, and that the rig crew maintain a current record of the tubular sizes in the hole at any moment so that the correct rams can be selected for closure in a well control emergency; the consequences of closing the wrong ram size during a kick are severe, as the leaking preventer may fail to contain wellbore pressure long enough to initiate kill operations.
• The blind shear ram is the well control system's ultimate backstop, designed to cut through the drill string and seal the wellbore even in the worst-case scenario where all other barriers have failed and the wellbore must be immediately isolated to prevent a surface blowout: the cutting blades of the blind shear ram are hydraulically forced together with sufficient force (typically several hundred thousand pounds) to shear through the drill pipe body — though not through the tool joint couplings, which are too thick and hard for standard shear rams to cut; the design of the drill string in critical wells therefore ensures that no tool joint is within the shear ram closure window when the rams are closed, which requires the driller to know the drill string tubular geometry and maintain awareness of tool joint positions relative to the BOP; after shearing, the upper section of the drill string falls into the wellbore, the blind seal elements of the shear ram close against each other to seal the open wellbore, and the kill and choke lines on the BOP stack are used to pump heavy kill mud into the wellbore to overbalance the formation pressure and bring the well under control; on subsea deepwater wells, the blind shear ram is one of the critical elements of the emergency disconnect sequence that allows the drilling vessel to leave a blowing well safely.
• The hydraulic operating pressure and closure time specifications for ram preventers are governed by API Specification 16A (Specification for Drill-Through Equipment) and are regularly tested to verify that the preventer will close within the specified time (typically 30 seconds for a surface preventer and 45-60 seconds for a subsea preventer) using the available hydraulic pressure from the BOP accumulator system; the accumulator system must contain sufficient pre-charged hydraulic fluid volume (typically nitrogen-charged bladder accumulators) to close all preventers in the stack and perform other hydraulic functions without the need to run the high-pressure hydraulic pumps, so that a BOP operation can be performed even if electrical power to the pumps is lost; on the Deepwater Horizon platform, the investigation found that the blind shear ram failed to cut through the drill pipe and seal the well because of problems with the ram design, the condition of the blade, and the position of the tool joint in the shear ram window, allowing the blowout to continue; the investigation contributed to significant revision of regulatory requirements for BOP design and testing under the US Bureau of Safety and Environmental Enforcement (BSEE).
• Variable bore rams (VBRs), also called variable pipe rams, are a design variant that accommodates a range of pipe outer diameters within a single ram, using an elastomeric packing element that can seal against multiple tubular sizes without requiring a ram change; a VBR rated for 3.5-5.0 inches OD can seal against 3.5-inch, 4-inch, 4.5-inch, or 5-inch drill pipe, providing flexibility in wells where multiple drill string sizes are used in a single hole section; the sealing mechanism relies on the elastomeric element flowing under hydraulic closure force to conform to whatever tubular size is in the bore, and VBRs generally provide adequate sealing performance across their rated range although the sealing load distribution is less optimal than a fixed-size pipe ram at the exact tubular OD; the operational benefit of VBRs is reduced risk of closing the wrong ram during an emergency (a VBR configured for the current drill string range is correct for all tubulars in that range), at the cost of slightly less robust sealing performance than a properly matched fixed-size ram.
• Ram preventer maintenance and testing requirements specify that function tests (opening and closing the rams against wellbore pressure) be performed at regular intervals throughout the well (typically every 14 days under BSEE regulations for offshore US operations and at similar intervals under other national regulations) to verify that the preventer seals correctly against wellbore pressure and that the hydraulic actuation system responds within specification; pressure test frequency and acceptance criteria differ between initial testing (full rated pressure, to API 16A or API 16A PR2 standards), periodic function tests (typically 70% of rated working pressure), and ram-to-ram tests (testing seal between closed rams with no wellbore pressure, to verify that the preventer can isolate the wellbore above and below the rams independently); test records are required regulatory documentation in most jurisdictions and are reviewed during regulatory audits and accident investigations to establish whether the preventer was in proper working condition at the time of a well control incident.