U Blowout Preventer.

http://www.c-a-m.com/content/products/product_detail.cfm?pid=2797
The Cameron U BOP is the most widely used ram-type BOP for land, platform and subsea applications worldwide and offers the widest range of sizes of any Cameron ram-type BOP. Like all other Cameron preventers, the rams in the U BOP are pressure- energized. Wellbore pressure acts on the rams to increase the sealing force and maintain the seal in case of hydraulic pressure loss. Seal integrity is actually improved by increased well bore pressure. Other features of the U BOP include:

■Hydraulic stud tensioning available on larger sizes to ensure that stud loading is consistently accurate and even.
■Bonnet seal carrier is available to eliminate the need for high makeup torque on bonnet studs and nuts.
■Hydraulically operated locking mechanisms, wedgelocks, lock the ram hydraulically and hold the rams mechanically closed even when actuating pressure is released. The operating system can be interlocked using sequence caps to ensure that the wedgelock is retracted before pressure is applied to open the BOP.
■For subsea applications, a pressure balance chamber is used with the wedgelocks to eliminate the possibility of the wedgelock becoming unlocked due to hydrostatic pressure.
Other features include hydraulically opening bonnets, forged body and a wide selection of rams to meet all applications.

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