Blowout preventer

Semi-submersible rigs are often referred to as semis , and are a floating type of rig. Like the jack-up, a semi is self contained. The structure is supported by large pontoons which are ballasted with water to provide the required stability and height. The rig is held in position by anchors and mooring lines or dynamically positioned by thrusters. A large diameter steel pipe ( riser ) is connected to the sea-bed and serves as a conduit for the drill string. The blowout preventer (BOP) is also located at the sea-bed ( sub sea stack ). 

An important safety feature on every modern rig is the blowout preventer (BOP). As discussed earlier on, one of the purposes of the drilling mud is to provide a hydrostatic head of fluid to counterbalance the pore pressure of fluids in permeable formations. However, for a variety of reasons (see section 3.6 Drilling Problems ) the well may kick , i.e. formation fluids may enter the wellbore, upsetting the balance of the system, pushing mud out of the hole, and exposing the upper part of the hole and equipment to the higher pressures of the deep subsurface. If left uncontrolled, this can lead to a blowout, a situation where formation fluids flow to the surface in an uncontrolled manner. 

Blowout preventers are opened and closed by hydraulic fluid stored under a pressure of 3000 psi in an accumulator, often referred to as a Coomy unit. 

The casing will also carry the blowout preventers described earlier. 

In the event of a sudden loss of mud In an Interval containing overpressures the mud column in the annulus will drop, thereby reducing the hydrostatic head acting on the formation to the point where formation pressure exceeds mud pressure. Formation fluids (oil, gas or water) can now enter the borehole and travel upwards. In the process the gas will expand considerably but will maintain its initial pressure. The last line of defence leff is the blowout preventer. However, although the BOP will prevent fluid or gas escape to the surface, closing in the well may lead to two potentially disastrous situations ... 

It generally is recommended, and often required, that gas dcicciiuii systems be installed in a fail-safe manner. That is, if power is disconnected or otherwise interrupted, alarm and/or process equipment shutdown (or other corrective action) should occur. All specific systems should be carefully reviewed, however, to ensure that non-anticipated equipment shutdowns would not result in a more hazardous condition tlian the lack of shutdown of the equipment. If a more hazardous situation would occur with shutdown, only a warning should be provided. As an example, a more hazardous situation might occur if blowout preventers were automatically actuated during drilling operations upon detection of low levels of gas concentrations than if drilling personnel were only warned. 

After the surface casing has been tested and the blowout preventer installed, drilling operations are resumed. They will continue until the well has been drilled to the total depth decided upon. Usually the... 

Add weighting material to rise density. Thin mud with water and thinners. Use degasser to clear gas from mud. Continue to circulate and avoid use of blowout preventers if possible. 

High Pressure. Limestones. Blowout prevention. Maintain adequate mud density. Maintain hole full of mud to prevent reduced hydrostatic head resulting from short column of mud. 

The minimum requirements for well blowout prevention equipment for aerated mud drilling operations are basically the same as those for normal mud drilling operations. 

A formation fluid kick can be efficiently and safely controlled if the proper equipment is installed at the surface. One of several possible arrangements of pressure control equipment is shown in Figure 4-351. The blowout preventer (BOP) consists of a spherical preventer (Hydril), blind and pipe rams, and the drilling spool. 

The most fundamental difference found between onshore and offshore drilling occurs when the wellhead is located at the seafloor. This configuration makes communication with the well more complex. A marine riser provides communication and circulation capability between the surface and the seafloor, and is used at some point during most offshore drilling. The riser consists of large-diameter (17-20) in.) steel pipe joints of approximately 50-ft lengths, with quick-connect couplings. The riser can be connected at the seafloor to a wellhead or to a subsea blowout preventer stack. A diverter system is usually attached at the... 

Offshore, well control equipment and associated operations present some differences from that seen and used onshore. In some instances onshore equipment can be employed, but the offshore environment generally dictates a modification of equipment and procedures. There are several different well configurations used offshore, often on the same well at different drilling intervals, and each configuration has specific well control procedures that should be followed. A well may be equipped with a surface blowout preventer stack a subsea blowout preventer stack, riser and diverter system a riser and diverter system with no blowout preventer a diverter only or a riserless system with no well control equipment. 

In deep water, it is common to find kick fluids trapped above the blowout preventer stack when the well is shut in. These fluids must be vented through the diverter system. 

API Bulletin D13, First Edition Installation and Use of Blowout-Preventer Stacks and Accessory Equipment, API, February 1966. 

Goins, W. C., Jr., Blowout Prevention, Gulf Publishing Co., Houston, 1969. 

API Recommended Practice S3, Second Edition Recommended Practice for Blowout Prevention Equipment Systems for Drilling Wells, May 1984. 

Blowout Prevention Systems for Drilling Wells, Second Edition, American Petroleum Institute Recommended Practice 53, Section 2-a, p. 6. 

Blowout preventers (BOPs) function is to cut off the flow of potential blowout. In all wells being drilled there are normally three holes or pipes within pipes that are at the surface of the wellhead -conductor pipe, casing pipe, and drill pipe. The drill pipe is the actual hole while the outer two are annulus formed around the inner pipe. Any one of these under varying conditions can be a source of through which oil or gas can escape during drilling. The annular preventer is a valve that appears... 

The most common cause of a well to become uncontrolled and develop into a blowout is improper mud control operations and the inability of the blowout prevention system to contain it because of system failures, i.e., lack of testing and maintenance. 

French Oil and Gas Industry, Blowout Prevention and Well Control, French Oil and Gas Industry Association/Editions Technip, Paris, France, 1981. 

Blowout Preventer (BOP) - An assembly of heavy duty valves attached to the top of a well casing to control pressure and flow. 

Gas trapped in a subsea blowout preventer (BOP) can be a serious problem in a deepwater drilling. 

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