Properly functioning blowout preventers — devices that keep oil wells from leaking under pressure — are critical pieces of equipment in an oilfield. Multiple blowout preventer (BOP) devices are typically stacked on top of one another for redundancy surrounding the drill pipe at the wellhead, Figure 1. It is their task to center the drill pipe, regulate the pressure of fluid down the well hole, and either clamp or cut off the drill pipe to minimize leaks. With this in mind, it is critical to test the seal on blowout preventers (both annular and ram types) to ensure that they don’t fail in the field. The testing is typically done to standards that are specified by the American Petroleum Institute (API) and witnessed and certified by third-party testing firms.
A unique testing system was developed by HydrobotWorx LLC, Seabrook, Texas, to apply real-world stresses to units at the BOP facility of Worldwide Oilfield Machine Inc. (WOM), Houston. The system is designed to measure the amount of leakage past the BOP collar as a simulated drill rod is cycled through multiple down strokes and up strokes at 24 in./sec for nearly 60 in. of stroke. The rod is driven by a hydraulic cylinder that has a 6-in. bore with a 3-in. rod and is capable of handling 5800 psi for a total force exerted of 82 tons. The allowable leakage as well as the rate at which the drill pipe should be moved during a test are specified by the API.
Electrohydraulic controllers take action
It was natural for HydrobotWorx to use an electrohydraulic motion controller to control the cycling of the drill rod, but the company also wanted to take advantage of the fact that some motion controllers connect easily with PCs running data acquisition software in addition to controlling actuators that apply testing stresses. With these motion controllers, the same feedback that is used to close control loops can be logged to preserve a record of how key parameters varied during a test sequence.
Figure 2. The eight-axis RMC150E motion controller connects directly to position and pressure transducers and servo valves, and uses Ethernet and RMCTools software to connect to PCs for data logging.
One motion controller with this capability is the RMC150E, manufactured by Delta Computer Systems, Figure 2. All eight axes of the RMC150E were employed in this application. One axis controls the hydraulic cylinder’s position and force. The other seven axes are used for high speed analog inputs for data acquisition: four pressure inputs from the BOP, two temperature inputs from the hydraulic system, and one for a flow meter to measure leak rate on the BOP. Because the system reacts very quickly as the rod (with variations in diameter and segment joints) passes through the BOP, the motion controller was programmed to close the control loop faster than 500 times/sec by activating a Yuken servovalve to move the rod at the specified rate.
The whole system is controlled and monitored via a touchscreen interface inside a control room. The ram’s pressures, position, and force are displayed in real time in bar graph form. Figure 3 shows all the major components of the test system, which has been designed for portability. Figure 4 shows another view, including the BOP (below the orange structure) under test.
Software provides visual management
The motion controller was programmed and tuned by Chuck Camp, VP of Engineering at HydrobotWorx, using Delta Computer Systems’ RMCTools software, which includes a plot manager for plotting the motion versus time. The plots show the actual motion compared to the target for each operation. When the actual and target plot lines for each parameter overlap (as they do in Figure 5), the system is tuned perfectly. In the plot, the “jolts” in the force readings occur as the tool joint passes through the annular seal. The tool joint simulates the diameter change from pipe to threaded couplings. So, as the wider portion of the tool joint is thrust through the sealing element from above, the pressures spike momentarily on the ram, the annular, and the test flange. This causes the plot of that parameter to show a slight “wiggle” instead of a straight line. Then when the tool joint exits the bottom of the BOP, the diameter decreases, and another pressure jolt occurs. Due to the optimized closed-loop control of the RMC controller, even as the pressures fluctuate significantly, the position of the ram that moves the drill rod (the red line on the plot) changes very smoothly.
Figure 4. The Worldwide Oilfield Machine annular BOP being tested is mounted below the orange gantry which houses the hydraulic cylinder and the simulated drill rod.
HydrobotWorx also used Delta’s RMCLink software. This tool enables an external PC to read and write data to and from the motion controller and interact with popular software on the PC. For example, the HydrobotWorx engineers used RMCLink to capture the pressure in the well bore and the velocity of the drilling rod and insert them into an Excel spreadsheet for test verification.
“Delta’s software library includes example programs and instructions for doing this,” said Mike Zilai, president of HydrobotWorx. “We didn’t need an expensive PLC or data recorder — just a PC and the Delta RMC, and we can deliver plots and Excel files that do the same thing.”
The BOP test system is typical of many hydraulically-activated testers. The challenges posed by applications such as this are significant because the conditions are changing widely while tight control of speed and direction of motion must be maintained.
As the engineers at HydrobotWorx found out, by selecting a motion controller with built-in Ethernet that’s easily interfaced to a PC, the necessary data can be logged and archived by reading parameters from the motion controller. Doing so avoids the need for separate data recorder hardware.
More information is available at www.deltamotion.com, www.worxamerica.com, and www.womusa.com. Contact the author at [email protected]. Read more articles from Bill Savela here.