The annular blowout preventer (BOP) is the most widely used blowout preventer in the oilwell drilling industry. The purpose of BOP's in general is to close the well bore in the event that the well starts a kick. Ram preventers historically are limited to either blind rams which are closed to control the well (provided no tools are in the well bore) and to a specific pipe size, if pipe is in the hole. This normally precludes closing on tool joints, kellys, etc. If vertical movement of the pipe is required, much effort and duplication of ram sizes is needed to accomplish this operation.
Annular preventers were designed for the express purpose of providing well bore closure under any conditions, regardless of the size or shape of objects in the well bore, or an open hole if the need should arise.
Annular preventers in one form or another have been used in the industry for more than 35 years. These can, for purposes of historical discussion, be divided into two types: 1. Stripper-Type Units-—Preventers whose packoff units were without steel reinforcement.
a. Hydril Type "CP" (1937). This was the first annular pre venter to incorporate into one unit the ability to pass tools and drill bits and to close on any shape present in the well bore. It used both externally applied hydraulic pressure and the well bore pressure to effect a closure.
Well diverters are a special class of large bore preventers with nominal working pressures of less than 1,000 psi. Drilling regulations in many areas today require that adequate blowout protection be provided from the time the well is first spudded until it is completed. In the past, it was standard practice to reduce large conductor pipe to mate with 20 inch blowout preventers. In many instances, the planned casing program required either under-reaming the surface pipe hole or removing the preventers in order to run large-diameter surface casing, or both, thus possibly leaving the hole unprotected. The use of well diverters eliminates the necessity of under-reaming since the well may be drilled to full bore the first time in the hole. This also makes it possible to run large size casing without removing the preventers.
1. Hydri] MSP 29-112-500
This diverter is capable of full closure from an open bore of 29V2 inches. Currently being used on land rigs and platform operations.
This diverter is similar to the "KFL" except that it is adapted for use on marine riser systems. The unit is attached to or built into the rig floor structure on the top of the riser. Misalignment is accommodated by a bail joint below the diverter in the "KFD" version, where the "KFDG" is a gim-bal mounted unit. Packoff is achieved down to about AVz inches on pipe depending on the insert used. The insert must be removed to pass tools.
4. Vetco Type "HY" Diverter
A newcomer to the diverter market, this unit is similar to the Regan "KFD" line in installation, operation, and limitations.
Operational And Design Considerations
1, Past Usage
When one looks back over the history of blowout preventers it becomes apparent that the preventers in use today, both ram and annular, are not too different from those used 25 years ago or more. The principal difference lies in the operating mechanism and not in the concept of the sealing element.
The basis for most annular preventers on the market today was in part disclosed in a patent application (Knox) filed in 1947. In many respects, all annulars in use today are an offspring of this cluster of ideas. Cameron, in their Type "A," has attempted to make the only real departure from what has been used for many years.
This is not as bad as it may sound. However, the real need for a different approach has never presented itself. The industry requirements simply have not made such a quest necessary until now.
Up until about 1960, the prime consideration in annular preventer design was the ability of the preventer to establish well bore closure and to maintain this closure against well bore pressure, even in event of the loss of operating fluid pressure. It was this consideration that required the strong self-energizing closing features that characterized all preventers, both annular and ram, up to that time. This feature was not without a penalty; the penalty was reflected in the force required to move pipe through the packing unit with pressure in the well bore. The relatively high contact pressure of the packing unit rubber on the pipe during such movement, in many cases, shortened the life of the packing units.
In the early 1960's, floating drilling operations, as well as a change of operational problems on land rigs, required the movement of pipe through closed packing units as a matter of routine operations. On offshore rigs, the heave of the vessel moved the pipe up and down through the preventers, while for onshore operations the desire to strip pipe in and out of the well under pressure presented the same problems. Subsequently, the self-closing feature has been diminished almost to the zero point to accommodate the change in operational procedures.
The advent of subsea drilling stacks, especially those used by floating vessels, has pointed up some other considerations. One is the overbalance effect of the weight of the mud column in the riser on annular preventers when they are closed, with the net effect being to attempt to open the packing units. Another problem is the extension of the life of the packing unit; this is being approached by the use of new elastomers and better control of the force between the packing unit and the tools in the hole.
Modern day drilling has required the development of new sizes and pressure ratings i.e., 18% inch, 5,000 psi; 2\xk inch, 5,000 psi.
Principal High Pressure Annular Blowout
Preventers Available Today
1. Cameron Type "A"
The Type "A" represents anew approach to annular field.
The packing unit is radically compressed by a series of radically acting pistons. This radical layout gives the Type "A" the lowest overall height oftheannulars for a given size. To date, this preventer has been built in the 10 inch and 13% inch, 5,000 psi sizes.
2. Hydril "GK"
This preventer has been on the market since 1947 and has become the standard in the industry for both onshore and offshore operations. The preventer was designed so that the well pressure will assist the closing action of the packing unit.
3. Hydril Type"GL"
Designed for subsea usage, this unit has retained the best features of the "GK" while adding a secondary closing chamber called "balancing chamber" which functions as an independent and fully redundant closing chamber. It can be connected to a continuous pressure supply, or accumulator, to bias the piston to produce fail-safe closure mud-weight compensation in subsea risers, or to decrease the fluid requirements for normal operations of the preventer. The use of this chamber permits close control of packing units to permit safer stripping of tool joints as well as rotation of drill pipes by allowing the closing pressure to be set at optimum packoff tightness for longer packing unit life at any well pressure.
4. Reagan Type "KFL"
The KFL utilizes three packer elements to seal off on the drill string: insert packer, main packer, and packeT sleeve. Three hydraulic lines are required to operate the KFL, two lines to actuate the insert packer locking dogs, and one line to close or open the packing elements.
The KFL preventer, equipped with surge dampeners to facilitate stripping tool joints for subsea operation, is not a self energizing preventer. When closing the KFL, it is essential that pressure be maintained at 500 psi greater than casing pressure.
5. Shatfer "Spherical"
The Spherical preventer is a relatively new entry into the annular preventer market. It utilizes a spherical head cover ing the top of the packing unit to translate axial loading on the packing unit into radial movement. The overall height of the preventer is somewhat shorter than either the Hydril or Regan but is higher than the Type "A". Shaffer achieved this reduction in height through the use of a short piston and the spherical geometry of the head.
Hydraulic Operation Of The Annular Blowout Preventer
The preventer should be connected to an appropriately sized hydraulic accumulator system that will have 1,500 psi minimum hydraulic pressure available at all times. A manifold with a pressure reducing and regulating valve along with appropriate four-way valving should be used. The normal hydraulic operating pressure range of the Hydril preventer varies from a minimum of 350 psi up to 1,500 psi maximum. Check valves must never be installed in the piping system between the regulating valve and the preventer.
When stripping tool joints through the preventer packing element, the regulating valve must be able to relieve pressure for the packing element to cycle properly. For underwater use of the annular type preventer, it is recommended that 10 gallon accumulators be installed on both the closing and opening chamber hydraulic lines directly at the preventer in order to absorb the pressure surges created by the preventer packing element while stripping drill pipe.
The annular blowout preventer is one of the most important and useful items of blowout prevention equipment. Proper sizing, operation, care, and maintenance are necessary for optimum use and maximum life. The properly used and maintained annular blowout preventer may be the difference between the satisfactory completion of an oilwell or disaster during blowout conditions.
Was this article helpful?
What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.