Screw Conveyors ( Augers ) are commonly used to move drilled cuttings and associated fluid. They can be arranged to collect the cuttings (usually relatively dry oil-based cuttings) from the individual pieces of solids control equipment and convey them to another area of the drilling rig where they are used to load cuttings boxes (skips). The standard screw conveyor is composed of an auger or screw housed in a flanged, U-shaped trough with bolt-on covers. It is powered by an electric motor and equipped with an appropriate gearbox. The motor must be sized to provide enough horsepower and torque to permit the transport of cuttings at a rate at least equal to the maximum rate at which they are delivered to the screw. The feed and discharge ends are fitted with flanges and ports to allow the cuttings to flow into and out of the conveyor without plugging. For multiple screw sections, hanger bearings are used to support the ends of the screw sections where they are joined. Operating parameters such as loading, housing enclosures, and length of the section are all considered in determining the required bearing type.
Screw Conveyors ( Augers ) are used to convey relatively dry cuttings, such as those generated while drilling with oil-based fluid. They are generally not used for conveying liquid discharges other than the associated liquid with the cuttings. The conveyors are usually sloped at a shallow angle so that liquid can drain into a catch tank located under the lower end. The cuttings should flow directly from the feed inlets into the screw in a manner that prevents their building up and plugging the inlet. The most common arrangement has the conveyor running perpendicular to the flow down the shaker screens, so that each shaker has its own inlet to the screw. The inlet opening should be as wide as required to maintain a steady flow without plugging.
Because of the variety of rig designs and operating conditions, particular care must be taken to ensure that the sizes of the conveyor, motor, and gearbox are adequate for the application.
Screw Conveyors ( Augers ) are available in different diameters and lengths. Common diameters are 9 inches to 18 inches. Augers are typically supplied in 10- or 12-foot lengths that can be linked together to form up to 50- or 60-foot sections. It is imperative that individual lengths align precisely within a section. A motor is supplied for each section. Auger runs greater than 50 or 60 feet require multiple sections. Angles can be formed where one section meets another.
Utilization of correctly sized Screw Conveyors ( Augers ) for the drilling conditions is critical. The auger must provide the capacity to transport cuttings at the maximum rate at which they will reach the surface. Attempted use of Screw Conveyors ( Augers ) that are too small for the drilling conditions can lead to bottlenecks, breakdowns, and the need to interrupt, or shut down, the drilling process.
The estimated peak volume of cuttings and associated fluid to be handled by an auger, where all of the cuttings are being collected by one Screw Conveyors ( Augers ) and the solids-control efficiency is high, can be estimated as double the gage hole volume at the maximum instantaneous penetration rate. A typical maximum instantaneous penetration rate could be 300 ft/hr. Table 1 indicates several examples of volume of cuttings that need to be handled by a Screw Conveyors ( Augers ).
|Hole Size ( in.)||Handling Volume (ft³/hr)||Handling Volume (m ³/hr)|
Table 1. Handling Volume for 300 ft/hr Drilling Rate
Screw Conveyors ( Augers ) sizing is dependent on the required handling volume, Screw Conveyors ( Augers ) speed, and trough loading. For cuttings handling, it is recommended to use relatively low auger speeds and minimize trough loading. Higher speeds may increase capacity, but higher wear may also occur. Higher loading may increase likelihood of plugging. Table 2 gives an example of auger sizes based on speed and trough loading.
|Volume (m³/hr)||Screw conveyor 30% loading high speed (in.)||Screw conveyor size,30% loading (in.)||Screw conveyor size, 15% loading (in.)||Maximum RPM|
Table 2. Auger Size Based on Volumetric Rate and Loading Conditions
Typically, 18-inch-diameter augers are installed and used when all of the cuttings are discharged into a single run of augers and when 17 1⁄2-inch hole cuttings are collected. By running a 14-inch-diameter auger with higher loading rates, the same capacity could be achieved, but with higher likelihood of plugging. A 12-inch-diameter auger would require both higher loading and higher speed and would pose greater danger of plugging incidents.
Occasionally, a single auger run cannot collect all of the cuttings from the solids-control equipment. In this case, a smaller branch auger can be used to collect isolated cuttings. For instance, a 9-inch auger might be used to bring centrifuge cuttings to the main auger run, where the centrifuge is isolated from the shale shakers. Also, if smaller hole size and slower rates of penetration are anticipated before the Screw Conveyors ( Augers ) is required, then smaller augers can be used. This condition is typical where oil-based drilling fluid is used in lower hole sections and the oily cuttings are collected for disposal.
Screw Conveyors ( Augers ) can represent significant safety hazards. The bolt-on covers must be kept in place when the auger is turning. However, when they are being used to convey drilled cuttings, this is not always possible. Grates can be used to cover open sections. If neither grate nor cover is possible, then a barrier fence with a warning sign posted should enclose the exposed section. A remote cutoff switch should be located within reach.