Mickey Harp is a vibration application engineer for Ludeca, Inc. He can be reached at 305-591-8935, or www.ludeca.com.
A mud pump is a reciprocating piston/plunger device designed to circulate drilling fluid under high pressure down the drill string and back up the annulus (See Figure 1).
Figure 1. Mud pump
Mud pumps come in a variety of sizes and configurations, but for the typical petroleum drilling rig, the triplex (three piston/plunger) mud pump is the pump of choice. Duplex mud pumps (two piston/plungers) have generally been replaced by the triplex pump, but are still common in developing countries. A later development is the hex pump with six pistons/plungers.
The normal mud pump consists of two main sub-assemblies—the fluid end and the power end. The fluid end produces the pumping process with valves, pistons and liners. Because these components are high-wear items, modern pumps are designed to allow for their quick replacement.
Vibration Reduction on a Six-Pump Rig
To reduce severe vibration caused by the pumping process, mud pumps incorporate both suction and discharge pulsation dampeners. These are connected to the inlet and outlet of the fluid end.
The number of mud pumps varies per drilling rig depending on the size of the drilling rig. The larger the rig the more mud pumps that will be needed. The mud pumps are considered vital to the operation of the drilling rig. If the mud pumps fail it affects production and can be very costly to repair due to the downtime in production.
Six mud pumps were running on a drilling ocean rig.
The Monitoring System
To avoid any failures of the pumps, an online monitoring system was selected to collect and transmit vibration data back to a software system for analysis. This online monitoring and diagnostic system can also be expanded by a series of program modules (MUXs) that are specific to the application:
- Band analysis module for the automatic evaluation of complex vibration processes in rolling element bearings, gears or special machines
- Cepstrum analysis
- Orbit analysis
- Data server for the automatic or event-controlled readout of data and for transferring this data to higher systems
The online monitoring system was better suited for this customer than a normal walk around data collection system because of the hazardous environment. The mud pumps were in an explosive (EX) environment due to the different fluids being processed by the pumps. MUXs were mounted within the EX area, and the vibration data was transferred to the monitoring system outside the EX area as shown in Figure 2.
Figure 2. Online monitoring system setup
Since the monitoring system operates as a web server, the standard data interface is the TCP/IP Internet protocol. Therefore, the system can be directly integrated as another computer within an existing network. Data can then be accessed from any point in the network or even from outside the network via a router. Data exchange on the site is possible via an Ethernet patch cable (TCP/IP) or a serial cable connection via the RS232 interface (PPP protocol).
Further possibilities for communication include:
- GPRS router
- Satellite communication
- Fieldbus connection
- DDE connection to the PLC
Figure 3. Before and after vibration analysis
Vibration Indicates a Problem
The mud pumps operated at 619 rpm with an approximate 50 percent load. Vibration data collected using the system showed a BPFO bearing fault. Figure 3 shows a decrease in vibration levels after the faulty bearings were replaced.
By using the online monitoring system, the customer was alerted to the bearing faults before the pumps failed. This allowed the customer to save both production time and money from any unexpected downtime.
Figure 4. Evidence of current passing through the bearings
Cause of the Problem
A root cause analysis was performed on the bad bearings. Once the bearings were cut apart, it was easily seen that current was passing through the bearing during operation. See Figure 4. This is commonly known as fluting and can be easily corrected once the origin of the current has been detected.
All the pumps were examined, and the following occurred:
- The online condition monitoring system detected the same problem in 5 mud pump motors out of six.
- The same problem was also detected on draw work motors.
- Ceramic bearings will be installed to resolve the fluting issue.
Upstream Pumping Solutions, Spring 2011