The BOP system was formed from two basic structural assemblies. The lower assembly, referred to as the BOP stack, rests on the wellhead connector. The upper assembly, referred to as the lower marine riser package (LMRP), was placed through a remotely detachable connection on top of the BOP stack and had roughly the same gross dimensions as the BOP stack. These assemblies, and basic functional components discussed below, are shown schematically in Figure 3-1. The LMRP had two annular preventers, and the BOP stack had four principal sealing elements: one blind shear ram (BSR) and three variable bore rams (VBRs). It also had a casing shear ram (CSR) that could shear drill pipe and casing but was not designed to seal the well. In addition, various control systems were located on the BOP system. In the event of an emergency disconnect, the LMRP was supposed to separate from the BOP stack, and the rig, riser, and LMRP were to move away from the well, which was to have been sealed by that point by the BSR in the BOP stack.
The LMRP contained two well-sealing components: the upper annular preventer and the lower annular preventer. The preventers were, as the name implies, annular in shape, and they were essentially flexible, elastomeric “doughnut” seals backed by steel elements that could accommodate a range of diameters of pipe and seal the annular space between the drill pipe and the LMRP. The annular seals were used so that the well could be tested, for example, for the so-called “negative test” discussed in Chapter 2, or potentially to stop any unwanted flow up or down the annulus.
In a blowout-prevention situation, the annular seals (if intact) could be activated and seal off the annular space between the pipe and the LMRP, although a blowout could still occur as a result of flow through the drill pipe itself if the drill pipe was not sealed.
A limiting factor was the maximum allowable differential pressure across the annular preventers. Reportedly, the upper annular preventer was designed for up to 10,000-psi differential pressure for sealing against a drill pipe or 5,000 psi when sealing the entire hole. The lower annular preventer was apparently designed for a 5,000-psi differential pressure for sealing around a drill pipe (BP 2010; Transocean 2011a).
Blind Shear Ram
The BSR was the uppermost of the five rams of the BOP stack and is shown for nominal operation in Figure 3-2. A BSR is like a massive metal scissors with two opposing blades that are designed to slice through the drill pipe as the blades pass by each other, as shown in Figure 3-3, and seal the well. The design intent was that, when the two blades of the “scissors” passed by each other and fully penetrated into the “side packers” on the other side, the seal