Ram type preventers have two opposing packing elements that are closed by moving them together. Rubber packing elements again, form the seal. A major difference between these and the annular preventer is that they are designed for specific applications. Rams are designed for a certain size of pipe and will only work on that type of pipe. Also, most ram type preventers are designed to seal in only one direction. They will only hold a pressure exerted from the lower side. Thus they will not function if installed upside down and will not pressure test from above. Ram pistons are universal in that they may accept any of the following types of ram elements.
Pipe rams: These have semi-circular openings that match the diameter of the pipe being used. A drillstring comprising different pipe sizes, such as 3-inch and 5-inch drill pipe, would require two sets of pipe rams to accommodate both sizes of pipe. These are also operated hydraulically, and close around the tubing portion of drill pipe when used.
Blind rams: These are designed to close off the hole when no pipe is in the hole. If they are shut on drill pipe, they will flatten the pipe, but not necessarily stem the flow.
Shear rams: These are a form of blind rams that are designed to cut drill pipe when closed. This will result in the dropping of the drillstring below the BOP stack unless the stack is designed in such a way as to have a set of pipe rams below the shear rams on which a tool joint can be supported. They will stop the flow from the well. Shear rams are usually only used as a last resort when all other rams and the annular preventer have failed.
The ram preventers will have a manual screw-type locking device that can be used in the event of a hydraulic failure.
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