Autodriller, cylindrical mud tanks, generate breakthrough developments in drilling technologies

    Dec. 14, 1998
    Drilling Times Combined [32,530 bytes] Block Control and Brake System [24,292 bytes] New developments in braking, block control, and autodrilling technologies are changing the ways drillers "apply the brake handle." This console utilizes two primary modes for drilling operations, WOB and delta-P, both backed up by an ROP setting. (Fig. 3 [33,165 bytes]). Control Parameters [14,670 bytes]

    LAND RIG DESIGN-Conclusion

    Dean E. Gaddy
    Drilling Editor
    Breakthrough developments in autodriller and mud system technologies have helped reduce cycle times in the Lobo trend of South Texas.

    These technologies not only provide drillers and operators with the tools and means to drill wells faster and safer, but may ultimately alter drilling work processes and procedures. This conclusion of a two-part series describes the integration of brake, block control, and autodrilling technologies along with new developments in mud-system tank design, effluent control, and pulsation dampeners.

    Integrated drilling technologies

    Helmerich & Payne International Drilling Co. (H&P) has integrated three state-of-the-art pipe movement and drilling control technologies-brake system, block control system (BCS), and autodriller (AD)-into its IRI 1500 series of rigs, allowing for safer and more-efficient control of the block during drilling, tripping, running casing, geosteering, and other drillstring movement activities.

    Alan Orr, vice-president and chief engineer for H&P, says the BCS-AD system has already produced significant results both in extending bit life and increasing penetration rates (Fig. 1). Comparing 16 BCS-AD wells drilled in Zapata County, Tex., with 60 conventionally drilled offset wells, H&P has reduced its average rotating hours per well by 37%. In addition, the number of bits run per well fell by 34% since the system was first introduced in January (Table 1).

    These performance improvements are a direct result of the near steady-state weight applied to the drill bit using the BCS-AD system. As compared to a conventional pneumatic autodrilling system, often producing bit-weight fluctuations of 2,000-3,000 lb, the new AD system has reduced fluctuations to 1,000 lb, and "sometimes as low as 300-400 lb," said Mike Porche, senior field engineer for MD Totco (a Varco Co.).

    Ed Nordquist, Lobo drilling manager for Conoco Inc., said "Operational improvements from the BCS-AD system became immediately apparent." Rig 164 was the first rig to install the system, involving a high degree of teamwork between H&P, MD Totco, IRI International Corp., and operator personnel. The technology has since been installed on H&P's new builds including Rigs 165, 166, 167, 168, and 169 and will eventually replace many conventional systems across its rig fleet.

    Brakes

    and block control IRI caliper disc brakes aided by a Baylor electrodynamic auxiliary brake system (Fig. 2) have replaced the more traditional mechanical band and auxiliary hydromatic braking system.

    Jim Schlieper, rig superintendent for H&P, said the braking system virtually eliminates the physical effort required to control or stop the hook load, allowing the driller to focus his attention on more important activities such as stuck pipe avoidance. The new system controls the travel rate of the block utilizing the eddy current brake in a proportional manner.

    In addition, the company installed an electronic BCS used to control the up-and-downward movement of the block during trips. Porche said, "The driller allows the system to learn by setting the high-and-low set points through the push of a button." It is important to understand that these "high-and-low set points are the operating limits, not the absolute limits of travel."

    Thus, the BCS provides some play in pipe movement, allowing the drillstring to travel 0.5 ft below the low set point and 4 ft above the high set point. This provides enough working room to fully set the slips into the rotary table. It also serves as a backup safety device for the crown-o-matic, especially if the block is accidentally hoisted above the higher setpoint.

    Autodriller

    Working in concert with the braking system and BCS, a new state-of-the-art, steady-state autodriller (AD) system has been incorporated into the drilling package. Benefits include: * More-efficient energy transfer from the bit to the formation, resulting in increased penetration rates. * Reduced undesirable drillstring dynamics, resulting in increased bit and drillstring life.

    Whereas the BCS essentially controls the electric brake proportionally and the friction brake as an on-off control, the AD operates the electric brake on one setting, proportionately controlling the disc brakes to release the drum.

    The BCS and AD systems utilize the same sensors and controls. The AD controls the descent of the block while "watching" the attendant hookload. If either the rate of descent, which while drilling is referred to as ROP (rate of penetration), or the desired WOB (weight on bit), seen as the difference in the hookload, is exceeded, the block motion is retarded.

    A new way of drilling

    The BCS-AD technology may revolutionize drilling procedures as we know it today. According to Schlieper, "Drilling with the AD and BCS requires a new touch that in some instances may be unnerving for more experienced drillers."

    Over the years, these drillers have acquired an inherent feeling for the brake handle. For example, they know by touch when a drill break happens or when the bit begins to torque up.

    This innate feeling is being replaced with "auto-pilot type" electronic control systems and indicators that monitor torque, pump pressure, bit weight, rotary speed, and other operational parameters. "In some ways, it is easier for younger drillers to learn the new system since they don't have to unlearn years of drilling with the [conventional] brake-handle," Schlieper said. In addition, "They grew up with computers," thus are more responsive to new technologies.

    Autodriller modes

    The two primary modes of operation are controlled by the WOB and delta-P (change in pressure) settings. The ROP setting serves as a secondary, or integrated backup function to the WOB or delta-P setting.

    The WOB setting works as described above to maintain a constant WOB whereas the delta-P mode uses standpipe pressure as its primary input, controlling the drill-line "payout" to maintain a constant standpipe pressure. This function is very advantageous for downhole motor applications.

    In both primary modes, if the penetration rate becomes more than what was set by the driller with the ROP limit adjustment, the AD will limit itself to the ROP setting. "This is particularly useful when drilling very soft or fractured formations," Porche said. When turned on in either mode-WOB or delta-P-the AD first sets the brake to about one-half maximum and then slowly lowers the braking force. The pressure continually decreases until it senses the drum turn.

    The system then notes this setting and increases pressure until the drum stops. "This is the range of operation used while drilling," Porche said. After "feeling out" the range, the block moves and stops one more time. When the AD has completed the "feeling-out" process, the drum releases itself to achieve the correct bit weight or standpipe pressure, depending on the drilling mode.

    Porche said, "The ROP control adjusts the maximum allowed penetration rate, not the rate of penetration that might be seen. In other words, it is a limit, not a command." As a general rule, the ROP knob should be set about 25% above the existing ROP. Schlieper pointed out that the system does not relieve the driller of responsibility for any block operations, but simply shifts the focus. For example, when lowering the drillstring, an attentive driller might notice the drillstring slowing its descent and stop the operation before sticking the pipe.

    The BCS on the other hand, will react to the pipe slowing by releasing the brake so as to maintain a constant speed. This seemingly poor characteristic is offset with the realization that "with the BCS smoothly controlling the brake, the driller's attention can be concentrated on the indicated hookload, providing an earlier indication of a stuck pipe situation than the drill string slowing," Porche said.

    A similar circumstance occurs when drilling with the WOB control. Drillers constantly are on the alert for "drilling breaks." If the AD's ROP limit adjustment is set just above the existing WOB, a drilling break will not be seen as an increase in ROP, but instead as a decrease in WOB.

    Autodriller operation

    The driller initiates the controls to effect control of the BCS and AD in the following manner.
    1. Proceed to bottom, engaging the top drive or rotary and mud pumps as normal (Fig. 3).
    2. Ensure the BCS is not in override.
    3. Set the ROP limit control to about 20% after drilling is initiated.
    4. Drilling with conventional hand operations, build up bit weight to 2,000 lb less than the maximum desired WOB.
    5. Apply friction brakes and bring the drum to a stop.
    6. Release the electric brake and turn on the AD.
    7. Immediately begin releasing the friction brake, watching the braking-force pressure gauge for indication that the AD is controlling the friction brake controller. Note that both the electric and friction brakes are fully released from the control of the driller at this point, keeping in mind that the system is designed to deliver control back to the driller as needed.
    8. During the next few seconds, the AD "feels out" the brake and hole conditions. The AD then slowly releases the brakes until it senses movement, then reapplies the brakes, noting both the forces required to release the drilling line and that needed to bring it to a stop. During this period, the AD light blinks.
    9. Once the light remains steady, the AD has found the "sweet spot" and begins limited feed off to the registered WOB setting or the ROP limit set on the control, whichever results in the least movement.
    The driller can then increase the ROP control knob to a higher position if he wishes to maintain the chosen WOB.

    Thus, "The DAC [data acquistion computer] method of controlling the disk brake will, to a large extent, mimic the action of an attentive driller," Porche said. When first alerted to the AD function, the system notes the present hookload and subtracts 2,000 lb from that value to serve as the desired hookload.

    This ensures operation of the unit in formations where there might be difficulty in building up and maintaining a specific hookload. During Step 8 above, the AD slowly releases pressure on the brake until movement is barely initiated, increasing pressure to arrest movement. At this point, the system evaluates the nominal values for the minimal output required to hold the drillstring, or the minimum holding torque (MHT).

    To be conservative, the MHT is then increased by a fixed amount of about 20% to ensure drillstring control, although continual monitoring of the ability of the MHT to stop the drillstring is performed by the DAC.

    Next, the DAC formulates a plan of control for the drillstring according to the initial setup conditions. The control philosophy is understood easier by examination of Fig. 4. Note that a pulse is determined by two parameters:

      1. Pulse width defined in DAC cycles
      2. Pmin, the brake pressure that with certainty causes the brakes to release.
    As the pulse lengthens, the pressure (p) becomes the controlling means that dominates control, particularly at higher penetration rates. The time (t) is set by the overall controlling algorithm, with p being set by the ROP controlling algorithm. This results in smooth and continuous control from one parameter to another.

    Thus, the block is controlled through the release of the braking pressure. The DAC then makes a calculation to release the drill line, bringing the pressure from the maximum brake to the minimum brake setting. While releasing the line, the ROP portion of the AD then begins to modulate the pressure, upward from the minimum brake setting, so as to effect a smooth payoff of the drilling line, at the desired penetration rate.

    Once the desired WOB is obtained, the drum stops. Thus, the time periods of line release extend from fractions of a second to continuous payoff at the indicated ROP.

    "As experience is gained with the AD, the driller will be able to recognize its characteristics and advantages," Porche said. When encountering a harder formation, which brings the WOB to more than the initial setting, the ROP will slow so as to keep it from exceeding the allowed WOB.

    Future possible benefits of the block control and auto driller system include:

    • Rotary torque-up inhibitor-Senses drillstring torque
    • Surge-swab limitation-Monitors and controls tripping speed in and out of open hole
    • Drillstring over-pull and drag limitation-Monitors and controls over-pull on the drill pipe when tripping out of the hole.
    • Linkage to MWD and LWD applications.

    Cylindrical mud tanks

    The mud system, like many other components of H&P's next-generation land rigs, also incorporates new innovations. Most important is the installation of a 727-bbl system featuring cylindrical (patent pending), skid-mounted tanks with hemispherical bottoms (Fig. 5).

    Each cylindrical tank utilizes electric mixers that eliminate "dead spots" in the drilling fluid. The cylindrical shape enhances the mud-mixing path, maximizing the agitation-to-volume ratio of the entire mud-system volume. Thus, all of the drilling fluid volume becomes usable. Baer said the new system is "much easier to clean than conventional rectangular systems."

    According to H&P superintendent Vic Garcia, Rig 157 crew members, on location for 138 days, never once dumped the mud-system pits until TD. Furthermore, for the last 61 days of the well, oil-based muds with weights up to 18.6 ppg were used. In the end, the roughnecks simply rinsed the inside of the tanks with a water hose, requiring only 8 man-hours to wash through the mud lines and pumps. Baer said on Rigs 164, 165, 166, and 167, it only takes 1.5-3 hr to clean the entire mud system.

    All piping is located on the tanks' exteriors, visible to employees for understanding function and performing maintenance. Another feature includes covered tank tops that reduce employees' exposure to harmful drilling-fluid vapors (Fig. 6). For inspection, cleaning, and maintenance purposes, each tank contains a light-weight hatch.

    Dump valves, located at the base of each tank, permit the complete evacuation of the tank contents. The dump valves all tie in to a 6-in. dump header line that runs along the skid. To expedite the emptying process, it is possible to hook up a suction hose to the dump line with a vacuum truck.

    Mud tanks

    The drilling fluid system incorporates an active mud system-mud-processing and mud-mixing components-consisting of 10 tanks (Fig. 7). In addition, there are two trip tanks and a single slug tank. Headed from the flow line towards the shale shakers, the six-tank, skid-mounted, mud-processing system includes a sand-trap tank overlaid by two shale shakers, a degasser tank, a desander tank, a desilter tank, a back-flow tank, and a volume tank (Fig. 7, green tanks).

    Fluids in the mud-mixing system beginning at the hoppers, flow from the mix tank to the blend tank and finally into the suction tanks leading to the mud pumps (Fig. 7, blue tanks).

    The mix tank, blend tank, slug tank, and hoppers are skid mounted as one piece. Also, each of the two Gardner Denver 1,350-hp triplex mud pumps, individually attached to a suction tank, is skid mounted. Finally, the two trip tanks and gas buster are skid mounted together as one piece.

    "We can conceivably use four [discrete] fluid systems," Schlieper said. For example, the slug tank can be used to pump an LCM (lost circulation material) pill. The trip tank can be used to pump and recapture a specially formulated hole conditioner. The slug tank can be used to pump a sweep. Finally, the active tanks can be used to circulate the drilling fluid.

    Lobo trend

    H&P's mud-tank system has provided a versatile system for handling tough hole conditions in the Lobo trend of South Texas. Nordquist said drilling the Queen City formation, located at a depth of 3,700-6,050 ft in Webb County, "is the biggest drilling problem we have. We need just the right mud weight. If it goes over 10.5 ppg, we lose returns. However, if it goes too much under, we have hole stability problems."

    Normally, the hole is short tripped twice to ensure a well-conditioned hole before running casing. Prior to tripping, walnut-hulls are run as sweeps, and the hole is continuously treated for swelling shales with conditioning fluids.

    "The problems we have to overcome in the Lobo trend require a mud system capable of quickly swapping out muds, dealing with unstable hole conditions, and handling gas zones," Nordquist said.

    After setting 7-in. intermediate casing at about 8,300 ft in the Wilcox formation, the hole is displaced with 15-16 ppg oil-based mud. "Because we encounter temperatures exceeding 280° F., the covered tank tops protect the hands from the strong vapors produced from the oil-based mud," said Brian Sharp, toolpusher for Rig 164.

    In some cases, "High-dollar hole sweeps, including a special blend called a 'black-magic mixture,' are circulated down hole and captured in the trip tanks for future use," Schlieper said. "The versatility of the mud system saves our customers a lot of money in both material costs and time."

    The tanks are also equipped with visual PVT (pit-volume totalizer) equipment for purposes of fluid management. "These gauges are most useful for well-control while drilling pay zones," Sharp said.

    Environmental aspects

    H&P's substructure is designed to contain 100% of the drill-floor runoff, minimizing the labor needed to keep the rig floor and substructure clean and diverting the fluid to containment points (Fig. 8).

    More importantly, this has reduced slipping hazards, since there are fewer wet spots. Baer said, "The rig-floor drainage system was built as an integral part of the substructure. It wasn't added on after the fact, but was part of the design."

    Since the drainage system reduces the time spent on cleaning and painting, "The roughnecks can apply their time more constructively, such as in maintaining equipment," Sharp said. An oil-and-lubrication system has also been added and includes "jiffy-lube" hoses that extend across the location. "Five-gal buckets are no longer used on location to haul motor oil or antifreeze," Sharp said.

    Each hose has a meter that allows for the exact measurement of oil. In addition, oil, lubricants, and antifreeze are contained and hauled in DOT certified tanks. "Because of this, we can haul the tanks on the road full or partially empty without pumping them out," Baer said. "Plus, it reduces the chance of contamination and accidental mixing."

    Finally, the rig itself sits on a small footprint (160 x 255 ft), saving the operator location-construction costs by enabling rig up on small, environmentally sensitive locations.

    Pulsation dampeners

    In 1992, H&P, with the assistance of Gardner Denver Inc. and White Rock Engineering Co., developed flow-through, maintenance-free, bladderless, pulsation-dampening equipment. These dampeners eliminate the requirement for precharge and periodic changing of bladders and have improved the fluid-discharge characteristic of the mud pumps.

    "You have a more constant fluid flow instead of cyclic fluctuations," Baer said. Thus, "Pump expendables [swabs, liners] last longer and the MWD tools send a cleaner signal to surface." The mud pumps are SCR (silicon control rectifier) controlled and utilize charging pumps on the suction ends. Mud line valves are air-actuated with "knife valves" that facilitate the opening, closing, and routing of fluids throughout the mud system.

    Other equipment

    Each of the IRI 1500 series of rigs utilize a National 271/2-in. rotary table, independently driven by a GE 1,000-hp dc motor. This rotary table system replaces the conventional mechanically powered, vertical-chain rotary drive. If needed, the masts have been modified to accept a 700-hp top drive.

    Two Caterpillar D3512 diesel engines, each rated at 1,435 hp, are unitized with Kato 1,050-kw generators to supply ac power to the SCR house for powering mud pumps, rotary table, and auxiliary equipment.

    Two Caterpillar 3412E electronic controlled diesel engines, rated at 750 hp each, power the 1,500-hp IRI 2550 draw works. Additional equipment includes a Varco 6800 hydraulic kelly spinner, a Varco SSW-30 hydraulic pipe spinner, a crown-a-matic, and a Mathey slickline unit with 16,000 ft of 0.105-in slickline. "Air hoists have been entirely replaced with four hydraulic hoists that are man-rider certified by OSHA," Baer said.

    Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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