How Blowout Preventers Work

Abstract

Technology Today Series articles provide useful summary informationon both classic and emerging concepts in petroleum engineering. Purpose:To provide the general reader with a basic understanding of a significantconcept, technique, or development within a specific area of technology. Well Control In normal drilling operations, the primary well control is the hydrostaticpressure exerted by the drilling fluid in the well. This pressure can beadjusted to the specific conditions by a variation in the fluid density. Properwell planning requires the hydrostatic head of Proper well planning requiresthe hydrostatic head of the drilling fluid to overbalance the formationpressure by a certain safety margin. pressure by a certain safety margin. Incase of primary control loss resulting from a sudden increase of formationpressure or lost circulation, it becomes necessary to seat off the well by someother means to prevent an uncontrollable flow, or blowout, of formation fluids. The equipment that performs this secondary control function is the blowoutpreventer (BOP). BOP's are mounted directly to the wellhead in combinationscalled the BOP stack. Such a stack will normally contain several of the twobasic BOP types: ram and annular. In special situations, a third BOP typetherotating BOPcan also be used in combination with rams and annulars. Ram BOP's Sealing elements, or rams. are located in the BOP body on opposite sides ofthe wellbore (Fig. 1). Opening and closing is performed with hydrauliccylinders attached to both sides of the BOP body. When open, the rams willleave an unobstructed passage through the wellbore. When closeddepending on thetype of ram selectedthey will seal around the drillpipe, seal off the openhole, or in emergencies shear the drillpipe and seal off the hole. The basic concept of the ram BOP has not changed in 50 years. Suchimprovements as automatic locks, shear rams, and load-carrying pipe rams haveincreased the efficiency but left the technical concept the same. Manyimprovements were developed as subsea drilling activity increased. The majorone was the development of multibore or variable-bore rams, which solved theinherent problem of the ram BOP sealing on only one specific size of tubulargoods. Multirams can adapt to different sizes within a limited range e.g., between all tubulars with diameters between 3 1/2 and 5 in. [8.9 and 12.7 cm]. Ram BOP's are still considered the main pressure-control tools in oilwelldrilling, primarily pressure-control tools in oilwell drilling, primarilybecause of their high degree of reliability. Annular BOP's The main feature of the annular preventer (Fig. 2), sometimes called"universal" or "bag"-type BOP, is the capability to close and seal on almostany size tools in the boreholedrillpipe, tooljoints, drill collars, kellys, casing, etc. within most of its range. It also has the capability to seal offthe open hole. The heart of the annular preventer is the sealing element (Fig.3). When the closing mechanism is actuated, hydraulic pressure is applied tothe piston, causing it to move upward and force the sealing element to extendinto the wellbore around the drillstring. Steel segments molded into theelement partially close over the rubber to prevent excessive partially closeover the rubber to prevent excessive extrusion when sealing under highpressure. P. 935