The following covers the use of the blowout preventer (BOP) stacks and equipment used when drilling from a floating vessel. Figure 3-14 shows typical BOP stack with the major components normally used in a typical BOP stack. Though the size and pressure ratings may vary, all major BOP stacks in use for subsea drilling today contain the major components shown. These major components are: (1) hydraulic wellhead connectors; (2) ram type BOP's; (3) annular type BOP's; (4) hydraulic or elec-trohydraulic control system for actuation of all the components on the BOP stack; (5) a four-post guide frame used to guide the BOP stack down the guidelines to the wellhead landing base and line up the BOP stack with the wellhead for proper sealing.
Ram BOP's contain pipe rams sized to fit the drill pipe being used. In some cases, rams are installed to fit the various size casing strings being landed through the BOP stack. At least one chamber in the ram BOP's will contain blind rams or, as in common practice now, a combination blind and shear ram assembly. The combination blind/shear rams operate under standard conditions as blind rams to close off the well bore at any time that drill pipe or tools are removed from the hole giving a complete closure of the hole.
However, in an emergency these rams can be closed effectively on the drill pipe severing the drill pipe in an emergency
(3 -5/8"- 5000 and 10000 16-3/4" - 5000 18-3/4" & 21-1/4" 10000
situation when it is not feasible to remove pipe from the hole. For instance, if anchor chains or lines break while drilling and the marine riser must be immediately removed to keep from breaking off from the BOP stack, or if the possibility of a blowout going uncontrolled through the drill pipe exists, the drill pipe must be severed to stop the blowout at the BOP stack rather than at the surface.
The annular BOP, however, is designed to close off any tools that may be in the hole. In most cases, it is drill pipe, but may include casing, tool joints on drill pipe, drill collars, etc. In an emergency the element of the annular BOP can be completely closed on the open hole in the same manner as the blind rams in the ram preventers are concerned. In subsea drilling, the annular BOP is really the heart of the BOP stack. This preventer is the most versatile and most used preventer in subsea drilling.
Unlike drilling on land where visibility allows the location of the kelly or the drill pipe and tool joints to be determined, drilling through a subsea BOP stack does not allow the location of the tool joints to be determined in relation to the BOP stack. It is not feasible to close the pipe rams and ram preventer without knowing the location of the tool joints. If an attempt is made to close the pipe rams and the rams close on a tool joint, the BOP could possibly be damaged. Damage could involve the rams and/or the tool joint to the extent that it may part, dropping the drill string to an emergency situation.
This leaves only one alternative. The annular preventer should be closed first in all cases when necessary to close the BOP on pipe. Since the annular BOP has the ability to close on both tool joints and drill pipe, there is no danger of damaging the BOP or the tool joint itself. Therefore, the annular preventer is used both to close off the hold and to locate tool joints. The forces required to strip a tool joint through the annular BOP are much greater than the forces required to strip drill pipe up or down through the element of the annular BOP.
The drill pipe can then be raised and, with a noticeable change in the weight of the drill string, will indicate a tool joint passing through the element of the annular BOP. As the tool joint is pulled up through the element of the annular BOP, it is posi tively located. Then it is possible to safely close the ram BOP on drill pipe, not on the tool joint.
Since a floating vessel is being used, the drill pipe will be in continual action at all times with the heave or movement up and down of the drilling vessel caused by the sea. When the BOP is closed, the pipe must still be free to move up and down in order to follow the heave of the vessel. In the past, the stripping life of the BOP has been very minimal. Due to its inherent design features, the annular BOP has been able to strip a greater footage of pipe than the ram preventers.
When feasible, a set of pipe rams should be closed and the drill string lowered until a tool joint is supported on top of the pipe rams. This is possible only when the drill pipe is being handled in elevators on the rig—not when the kelly is attached.
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