Fluid pumping and mixing is an integral part of the well drilling process. Different types of fluids are mixed and pumped down the well hole while drilling in order to cool the drill bit and circulate cuttings back to the surface. The characteristics of the fluid, such as density, pressure, and temperature, need to be monitored closely and controlled precisely during operations.
Because they excel at fluid handling and are adaptable to changing environmental conditions, general-purpose electrohydraulic motion controllers that drive the pumps are some of the most useful and versatile pieces of equipment on the oilfield. So says Frank Braswell, principal at Systems of Merritt Inc., Upland, Ind., an expert in control system design.
Braswell has been working with Impact Technologies LLC, Tulsa, Okla., to design hydraulic motor and pump control systems for a wide range of fluid handling roles in the oil and gas industry. Recently, Impact called on Braswell to upgrade one of its hydraulic control systems and design several new controllers for pumps and hydraulic motors at Impact’s development facility.
“We wanted to be able to use the same basic control system and human interface with different applications,” said Braswell. “That drove us to select a motion controller with high performance and easy programmability to give us the flexibility. Moreover, we needed a motion controller that would interface easily with our LabView front end.”
One controller for many applications
Impact chose LabView software by National Instruments to implement easy-to-use visual human interfaces for the drilling process. The LabView program is used to acquire data from the drilling operation, such as current pressures and flow rates at the well head, which can be graphed continuously so the operator can review progress and to control elements of the drilling process, such as the flow of fluids down the well.
The need for flexible, easy-to-use controls led Braswell to validate the earlier choice by Impact to use the RMC75E general-purpose closed-loop electrohydraulic motion controller, Figure 2, manufactured by Delta Computer Systems Inc., Battle Ground, Wash.
Braswell started his work by upgrading the control system that was provided by Impact. He retained the Delta controller, but replaced a lot of point-to-point wiring with an Ethernet network and standardized RJ45 connectors in order to make the updated control system easier to install and service. “Using Ethernet makes a huge difference in cutting installation and maintenance costs,” said Braswell. “The RMC75E interfaces directly to the network, making these tasks easy.”
Three systems in one
Braswell actually implemented three different control systems into one oilfield installation, each one using a dedicated Delta RMC controller, Figure 3. The first RMC (Delta #1 in the diagram) was used to control a diesel engine that powers the drilling fluid pump.
Because high power electric lines are not always present at the well head, diesel engines often provide the power to drive hydraulic systems. In the oilfield control system that he designed for Impact, Braswell attached a linear actuator to the diesel engine to control the throttle, Figure 4.
At start-up, the diesel engine is disengaged from the pump, and the throttle is operated under open-loop control. The Delta controller is commanded to set the throttle position, via the linear actuator, to a predetermined idle position for startup.
After the diesel engine is running smoothly, the Delta RMC is put into closed-loop control of the pump speed (speed is monitored by a rotary encoder that is attached to the pump). The controller’s software also checks to ensure that the diesel engine is running. Being able to program and watch the different transducers is critical, especially for determining emergency situations. If the pump’s pressure should drop off rapidly toward zero (for example, if a hose ruptures), the system is switched back into open-loop control so that the engine doesn’t exceed its operating parameters in trying to maintain pressure. During pumping operation, controlling the diesel throttle with a closed-loop motion controller is critical to achieving desired results because the engine’s response to control inputs is non-linear, and its operation is heavily influenced by temperature and other environmental conditions. The controller’s ability to respond quickly to feedback compensates for these factors. When the driller wants to shut the pump down, he or she can switch back to open-loop control before disengaging the pump and shutting down the engine.
The second Delta RMC (Delta #2 in Figure 3) helps control the mixing of drilling fluids. The proprietary mixtures of high-tech fluids that this system handles shouldn’t be confused with what the industry has traditionally called drilling “mud.” The Delta controller receives pressure, temperature, and pump shaft velocity and position information from the fluid mixing unit and sends controlling outputs to an electro-mechanical valve that governs how fluid is mixed in the unit. Because the valve is capable of proportional control, it can vary the fluid flow smoothly from full open to full closed. Therefore, the fluid mixture can be controlled more precisely than with systems using two-position on/off valves and schemes that measure pressure and flow rate by counting strokes of a mud pump. Besides the hydraulic valve, the Delta RMC also controls an electric motor in another part of the mixer (see Figure 3).
The third controller (Delta #3 in Figure 3), a larger RMC150E, is used in the fluid recovery unit. This multi-axis controller can control up to eight electromechanical valves. In this application, the controller’s function is to both monitor and control. It monitors fluid temperature and pressure as fluid returns from the well hole and controls the recovery of the fluid by operating multiple proportional valves in the fluid recovery unit to regulate fluid and gas flow.
In the case of the Delta #2 and Delta #3 controller systems, the valve positions are updated by the controller 1000 times per second in order to ensure that the pumps respond very quickly to changing conditions at the well head.
Figure 5 shows the actual control hardware as it was developed and tested at Impact’s facility. Each control box (in the background of the photo) contains the RMC and a local operator display.
Standard and custom designs
Each Delta RMC controller is programmed and its operations monitored by interacting with dedicated locations in the controller’s memory. Although the RMC controllers are provided with a full repertoire of pre-programmed motion functions, Delta Computer Systems also allows system designers to implement their own functions using what Delta calls user programs. In the Impact system, Braswell applied user programs extensively for a variety of tasks including safety limit checks for speed, human-machine interface (HMI) communication, synchronizing valves, and to illuminate LEDs for visual problem indication and diagnostic purposes.
The central computer uses LabView to implement an HMI for the drilling operator. The system runs LabView under Windows Vista through the use of VMWare Fusion virtual machine software. The Delta controllers connect to the master computer via Ethernet using National Instruments’ OPC server to allow LabView to access the memory maps of the Delta motion controllers. OPC is a standardized communication specification that defines how software applications share data under Microsoft operating systems.
For example, the LabView program lets the machine operator change the hydraulic pump’s speed variable, and the Delta #1 controller will respond by moving the throttle to adjust the speed of the engine that drives the pump. Likewise, the operator can change the pressure variable in the memory of the Delta #2 controller and monitor the mixture flow results on the screen. The program uses tabs to display individual controller information and overall system data for the drilling operator.
In addition to the visual display making it easy to tell at a glance what’s going on, it also makes it easier to train new operators on how to use the system. In this regard, the PC and the Delta motion controller make an ideal pair: the PC for human interface and data logging tasks and the controller for performing precise high-speed control.
As this development shows, a general-purpose electrohydraulic controller works well with oilfield equipment. The key, says Braswell, is for that controller to be very versatile. “In all the systems that I built for Impact, I designed the controllers to be as similar as possible,” said Braswell. “Even though the controllers are doing different things, they could be substituted for one another. This was possible because of the flexible Delta controller software architecture and hardware design options.”
Mechanical simulation proves controls work safely
Systems of Merritt engineer Frank Braswell developed a system simulator to aid him in programming and tuning the hydraulic controls. The system, at left, includes the same sensors that are used in the real hydraulic control system in the field. “You want to prove out the controls in a friendlier environment than on the oilfield,” says Braswell. “And it would be very expensive and dangerous to run the real fluid handling machinery while you’re debugging.”
The mechanical simulator was invaluable in not only troubleshooting proportional, integral and differential (PID) control loop issues, but also allowed debugging of the human-machine interface (HMI). While PID gain values are obviously different between the target system and simulator, testing open-loop to closed-loop control transitions, failure mode responses and startup/shutdown sequences were possible without the risk of damaging the real target system. Safety was a key consideration in the design and use of the mechanical simulator.
To help tune the motion of the hydraulics, Braswell used Delta’s RMCTools software. RMCTools contains a plot manager which allows the control system designer to view plots of actual vs. target motion. The goal is to get the actual plot lines to overlap the target plots. When that happens, the system is optimized perfectly. After making tweaks to control loop gains, the system is re-run and the plots re-examined to see how the behavior of the system is improved.
“The Delta controller allows the control engineer to bring together both hardware and software engineering principles yielding a flexible design and efficient operation,” says Braswell.
Free book details motion control applications
Delta Computer Systems, Inc., Battle Ground, Wash., has released the second edition of its popular practical design guide authored by Delta president and chief motion control engineer Peter Nachtwey.
This 64-page guide covers 16 technical topics in fluid power motion control systems by combining theoretical and practical insights. Designers, mechanical and control systems engineers, fluid power technicians and others will benefit from this guide. The guide’s purpose is to be a resource when applying fluid closed-loop motion control systems in a wide variety of industrial and testing applications.
Topics covered include applications in different industries along with valuable tips related to open and closed loop motion control as they apply to component selection of accumulators, valves, and cylinders. Sensor resolution hints and tuning secrets round out the added information in the guide.
Click here to request a free copy, then enter your contact information and state “Design Guide” in the comments section. You may also call Delta at (360) 254-8688 or e-mail [email protected] to request a copy.