Drill pipe float valves are normally installed directly above the drill bit. In the simplest terms they are NRVs (non-return valves). They enable circulation down the drill string only and provide instantaneous shut-off from the annulus whenever the pumps are turned off. While drilling their main function is to prevent backflow while making connections. They also provide fluid flow control at the bottom of the drill string during tripping or when shut-in. Two common types are the spring loaded float valves and the flapper type float valve. The flapper type valve incorporates a built-in latch which allows tripping the drill string into the hole with the valve in the open position, thus eliminating the need to fill the pipe. This also has the effect of reducing surge pressure. The latch is automatically released by initial circulation of mud. As soon as circulation is stopped the valve closes. Some flapper valves are vented to permit the reading of pressures during shut in conditions.
The SSTT assembly consists of a fluted hanger, slick joint, valve section and latch section. The fluted hanger is designed to land on the wear bushing in the wellhead. Above it is the slick joint spaced out so the lower pipe rams can close around it to seal off the annulus. The valve section contains two fail-safe valves, either a ball and flapper valve, or two ball valves. On top of the SSTT is the hydraulic latch section. This contains both the operating mandrels to open the valves and also the latching mechanism that releases this part of the tree from the valve section, in the event that disconnection is necessary.
This operates on the same principle, but instead of a spring there are three or four small wedges that slide in their own groove. Both of these catchers work well in all formations including unconsolidated formations, although care must be taken on the tip out of hole. Also offered are dog catchers, finger baskets and flapper catchers. These are aimed at soft formations but can create more problems than they solve. Soft formation coring is a special problem which requires special care but can be successfully conducted using virtually standard equipment.
Both manually and remotely operated valves are located along the flow line to the rig. These valves are usually the gate or ball type. These valves cannot be operated in a partially open position. The abrasive nature of the compressed air flow in the flow line would erode the gate or ball of the valve and render the valve ineffective in the closed position. At strategic locations along the flow line are check valves. These special mechanical valves allow compressed air flow in only one direction (toward the standpipe). These check valves are spring loaded and the force of the flow allows the mechanism (flapper-gate or ball) to open in correct direction of flow. If the flow is reversed, the mechanism is forced closed by the spring and the force of the reverse flow.
Figure 3-29 shows a typical drill string float valve. This is a safety valve device and is usually placed in the bit sub at the bottom of the drill string. These valves are used in nearly all deep rotary air and gas drilling operations. The valve prevents the back flow of compressed air (or other gas) and entrained rock cuttings from entering the annulus space into the inside of the drill string. The valve is fitted with a flapper mechanism. If circulation is stopped the compressed air and rock cuttings in the annulus will reverse flow and actuate the flapper which in turn stops the back-flow. The fire stop valves are placed just above the drill bit and along the drill string at several positions. These valves have a zinc ring that holds back a spring-loaded flapper mechanism (like the float valve above) allowing air circulation from the surface. Wireline equipment can be run through these valves when the fire stop is in the normal open position. Figure 3-30 shows the schematic of the...
Figure 3-29 Schematic of a typical flapper type float valve for direct circulation operations. 1) Cage, 2) side seal, 3) seal retainer, 4) seal, 5) pin, 6) flapper valve, 7) shock absorber, and 8) location of bevel guide (courtesy of Baker Oil Tools). Figure 3-29 Schematic of a typical flapper type float valve for direct circulation operations. 1) Cage, 2) side seal, 3) seal retainer, 4) seal, 5) pin, 6) flapper valve, 7) shock absorber, and 8) location of bevel guide (courtesy of Baker Oil Tools).
The drill pipe float valve and the flapper type of back pressure valve, serve essentially the same purpose, but differ in design. These valves provide instantaneous shut-off against high or low back pressure and allow full fluid flow through the drill string. Another advantage is that it prevents cuttings from entering the drill string, thus reducing the likelihood of pulling a wet string. Abnormal pressures and anticipated subnormal pressure zones should be the deciding factor regarding what type of valve to run or the possibility of not running any valve at all. Expectations of abnormal pressures have shown the vented type of flapper valve to be the most popular because of the ease involved in recording shut-in drill pipe pressures. The disadvantages are that the pipe must be filled while tripping in, and reverse circulation is not possible.
Kelly cocks are generally placed at the top and bottom of the kelly. These valves may be installed as a permanent part of the drillstring or just when a kick occurs. They can be automatic or manually controlled and they consist of subs with valves that may be of a spring loaded ball type, a flapper valve type, or dart type. The dart acts as a one way valve, with pressure from below closing the valve, and pressure from above, opening it. The drawback of these valves is that while preventing a blowout up the string they prevent the shut-in drillpipe pressure from being monitored.
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