Equipment Issues

Anytime drilling waste is handled, equipment must be used. This section deals with some of the equipment issues in moving, storing, and handling drilling waste.

16.7.1 Augers

Augers (screw conveyors) 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.

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.

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 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 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 auger 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 16.6 indicates several examples of volume of cuttings that need to be handled by an auger.

Auger sizing is dependent on the required handling volume, auger 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 16.7 gives an example of auger sizes based on speed and trough loading.

Typically, 18-inch-diameter augers are installed and used when all of the cuttings are discharged into a single run of augers and when 1772-inch

Table 16.6

Handling Volume for 300 ft/hr Drilling Rate

Table 16.6

Handling Volume for 300 ft/hr Drilling Rate

Hole Size

(in.) Handling Volume (ft3/hr)

Handling Volume (m3/hr)

8 Vi

250

7

121/4

500

14

17 Vi

1000

28

Table 16.7

Auger Size Based on Volumetric Rate and Loading Conditions

Auger Size,

Auger Size,

Auger Size,

Volume

30% loading,

30% loading

15% loading, Maximum

(m3/hr)

high speed (in.)

(in.)

(in.) rpm

7

9

12 50

14

9

12

14 50/100

28

12

14

18 45/90

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 auger 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.

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.

16.7.2 Vacuums

Vacuum transfer systems provide an alternative to augers. The initial applications were on jackups, primarily in the Gulf of Mexico, where the cramped areas required multiple conveyors to get around the legs and other obstacles. It was also easy to close the discharge chute, making a sump to collect the cuttings that is suitable for the vacuum nozzle.

A major advantage of vacuum transfer systems is that they permit more flexibility in siting components on the rig. The individual components (collection troughs, conveyors, collection boxes, etc.) are connected via hoses. This facilitates the routing of flow through and around congested areas on the rig and makes it possible to place equipment at different elevations when this is advantageous. The use of these systems has grown in recent years as the demand for total containment of wastes (zero discharge) has increased.

Vacuum transfer systems come in various types. Typical components include one or more vacuum blowers, a filtration device to protect the blower, vacuum hoses, and cuttings boxes with vacuum hose ports or vacuum lids that seal on the box lip. The vacuum pump may be skid mounted and can be either electric or diesel powered. Noise reduction devices and sound insulation should be used to reduce noise levels to below 80 dB, if possible. Many of the units commonly in use exceed noise levels set by the Occupational Safety and Health Administration and require hearing protection to work around them.

A vacuum pump is connected via hoses and/or pipes to a filtration unit, which in turn is connected to a collection box (or hopper, although these are rarely used). A hose or pipe connects to either a single point or to a manifold with multiple suction points at the waste source. The vacuum must be continuous to permit transport of the cuttings. The cuttings box allows the cuttings to fill in from one of the openings while the vacuum is pulled through the other side. While cuttings and waste are traveling through the hoses, sufficient velocity must be maintained to prevent them from settling in the line. Once the waste reaches the cuttings box, the material velocity slows, allowing the transported material to drop out into the collection chamber. Once the box is full, the hoses are switched, or the valve positions on the manifold are changed, to fill other boxes. As boxes are filled, they are lifted away and replaced with empty boxes.

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