Selection depends on the application, required features, weight and size, and maintenance and monitoring system available.
by David Jones, Siemens Industry Inc.
February 28, 2014

Image 1 (above): Cutaway shows the heat-transfer path to the motor’s outside diameter where the water jacket is located. In the U.S., the land-rig market typically has around 1,700 silicon-controlled rectifier/electric rigs in operation. About 170 rigs are in operation offshore. Canada operates 300 onshore rigs. Rigs are equipped with the following major components:

  • Drawworks
  • Auxiliary brake
  • Travelling equipment
  • Top drive
  • Rotary table
  • Mud pumps
  • Well control equipment
  • Accumulator
  • Alternating current (AC) power (generators)
  • Drill string (drill pipe & collars)

Considerations for mud pump motors include new designs and new suppliers in the market. Potential problems associated with these environments should also be considered.

Mud Service

A series of mud pumps, often triplex pumps (three piston plunger type), are common to each rig. Mud pumps circulate drilling fluid (under high pressure) down the drill string and up the annulus—which is any void between the piping, tubing or casing and the piping, tubing or casing surrounding it. The number of mud pumps used per rig varies depending on the rig’s size. Some require two mud pumps. Others (especially offshore), use six or more. Drilling fluid, also called mud, is pumped through the hollow drill pipe to the drill bit, where it exits the pipe and is then flushed up the borehole to the surface. The drilling fluid is usually cleaned and recirculated for reuse. During this cleaning and recirculation process, centrifuges and shakers remove cuttings. Drilling fluid services of some kind are required at every well. They encompass a broad spectrum of systems, products, software, personnel specializations and logistical support. As wells become more complex, total drilling costs increase dramatically. Because the drilling fluid contacts almost every aspect of the drilling operation, proper drilling fluid selection can help the operator minimize costs throughout the well construction process.1

Mud Pump Motor Selection

Modern rigs use mud pumps that are usually driven by an electric motor. The market for mud pump motors has been dominated by a single source for many decades. The original configuration, modeled after a direct current (DC) train/traction motor, had a commutator and was fed by a variable DC source. Mud pump motors have since evolved into an AC design, controlled by an AC variable speed drive (VSD). New suppliers of AC drilling motors include Asian imports, specialty companies that served different vertical markets in the past and small company suppliers that started their business by rewinding/refurbishing used motors. However, new players are entering the market by capitalizing on demand fueled by the unconventional oil and gas market and horizontal drilling, specifically. Long lead times by major suppliers have allowed new suppliers to tap into this market. The correct mud pump motor must be selected based on several criteria, which are detailed in this section.

Ratings and Features

A mud pump motor typically has the following specifications:

  • 1,150 horsepower (HP) to 1,500 HP (the entire market range is 800 HP to 3,000 HP)
  • AC induction
  • 3 phases
  • 60 hertz
  • 600 volts AC
  • 6 poles (1,200 rpm)
  • Air-cooled, by a blower mounted on top of the motor
  • Suitable for use with VSDs
  • Resistance temperature detectors (RTDs) imbedded in the winding and bearings (for monitoring)
  • Field-movable conduit box (F1 or F2 assembly)
  • Suitable for use in Class 1, Division 2 environments (NEC code)
  • Encoders are typically offered as an option to the user
  • Used in multiple rig applications (in addition to the mud pump)
    • Drawworks
    • Rotary Table

Size & Weight

Typical onshore rigs use blower air-cooled mud pump motors, where the size (footprint) is not as critical as on offshore rigs. For offshore rigs, on which equipment size and weight is an economic consideration, the use of water-cooled (also known as water-jacketed) motors is prevalent. The size and weight considerations of offshore rigs has also led to the use of permanent magnet motors. Offshore mud pump motors often have additional certifications—such as ABS, ATEX, CE and EExe. Since footprint is king on offshore rigs/platforms, the market trend has been to incorporate smaller mud pump motors. These specially designed motors are completely sealed and liquid (water) cooled. In this case, more current is used to produce torque with fewer parts, making them torque-dense.2 The term “pancake motor” is often used to describe these motors because they are flat and compact. A water-cooled explosion and flame-proof induction motor provides the following reliability and rig safety improvements:

  • Extended motor life and increased reliability
  • Omni certification coverage for any type VSD
  • No motor losses into a room or ductwork
  • Elimination of lower-motor and blower noise
  • Higher level of safety for the protection of life and property
  • Minimized design liability and legal exposure

Motor Reliability & Maintenance

Since day-rates (daily rental rates for rigs) can be up to $25,000 dollars per day, the uptime of critical components and systems is vital to a profitable operation. Vibration is a key issue for electric motors that drive mud pumps or other similar equipment. If a motor problem occurs, the source of the problem should be promptly identified and corrected. With the proper knowledge of motor vibration sources, accurate vibration measurement and comprehensive diagnostic procedures, quickly identifying the root cause of motor vibration is possible. By using proper data collection and analysis techniques, the source of the vibration can be more accurately determined. This analysis includes, but is not limited to, the following vibration sources:

  • Electrical imbalance caused by the stator and/or rotor bar
  • Mechanical imbalance, which may emanate from the rotor, coupling or driven equipment
  • Resonance and critical speeds
  • Mechanical effects—such as looseness, rubbing or bearings
  • External effects—such as the base, the driven equipment or misalignment

When a vibration problem occurs, a systematic, analytical approach for resolving the problem must be used. The process starts by listing all the possible causes for the frequency of vibration and any variations under different operating conditions. Next, eliminate the incorrect causes one at a time until only the true source of the problem remains. Then the root cause can be efficiently eliminated.

Example of a CMS's architectureFigure 1. Example of a CMS’s architecture

Vibration problems can vary from a mere nuisance to an indication of imminent motor failure. With solid knowledge of motor fundamentals and vibration analysis, the root cause of the problem can be identified.3 Electric motors may have other areas of concern in addition to vibration. Trouble with a motor, like trouble with any rotating machinery, ranges from aggravation to crisis. Certain problems seem to occur more frequently than others. Some problems could be avoided if the application and environment were understood, while others may be caused by a changing environment in which the motor operates. Of course, some are caused by the motor. The latter includes, but is not limited to:

  • Improper voltage
  • Inadequate motor torque to drive the load
  • Unacceptable motor acceleration time (too long or too short)
  • Overload or instantaneous trip
  • Overload relays that turn the motor off during full-load operation
  • Unsatisfactory vibration

The key to successful troubleshooting is to identify and analyze all the possible causes and then eliminate each one, either by testing or calculations until the problem is properly identified. This method helps the troubleshooter avoid jumping to conclusions and making unnecessary modifications to components. Even if a component does not perform properly, it does not necessarily mean that the component is at fault. Other environmental factors may cause the failure, and the failure(s) will continue until these environmental factors are corrected.4 One way to detect problems with electric motors is with a condition monitoring system (CMS). These systems are implemented to minimize downtime and optimize the use of equipment and personnel. In particular, vibration monitoring is one of the most reliable methods for the early detection of mechanical damage. One CMS solution is a high-performance system that can record up to 180 vibration signals in parallel and synchronously. Analysis and diagnostics of this data are carried out using a wide range of standard function blocks, with which process signals from all automation components can be considered at the same time. The design of this CMS system includes these features and advantages:

  • Connection of up to 30 interface nodes for the detection of vibration acceleration and analog signals
  • Bus technology: Institute of Electrical and Electronics Engineers (IEEE) 1394a for transmitting measurements to a computer
  • A standard PC as the platform for system software
  • Open software design
  • Creation and protection of custom analyzing models based on off-the-shelf function blocks
  • Detailed analysis, diagnostics, visualization and archiving5

References

  1. World Oil, Drilling, Completion and Workover Fluids, June 2012.
  2. DRS Technologies, www.drs.com
  3. Finely, W.; Loutfi, M.; Sauer, B.J. “Motor Vibration Problems – Understanding and Identifying.” IEEE/Portland Cement Association, April 2013.
  4. Finley, W. “Troubleshooting Induction Motors.”
  5. Siemens Condition Monitoring Systems, flyer.
  6. Early detection of mechanical damage through permanent condition monitoring, flyer.