Programmable motion controller adds flexibility to testing blowout preventers
A general-purpose test system can be viewed as a research and development tool and a production and reliability support tool. An advanced motion controller allows customers to go beyond design verification to study materials science and design integrity.
In a fast-moving industry such as the production of equipment for oil exploration, use of general-purpose test stands for quality assurance can deliver long-term benefits. With the inclusion of programmable functionality and test data logging and display, the usefulness of a particular test stand design can extend far beyond its initial design purpose.
For example, Tomball, Texas, system integrator WorxAmerica developed a test system to verify that a customer's oil well blowout preventer (BOP) product design met American Petroleum Institute (API) standards. The task of a blowout preventer is to center the drill pipe in the oil well, regulate the pressure of fluid in the well hole, and either clamp or cut off the drill pipe to minimize leaks should the pressure exceed a set limit. But beyond proving basic BOP device functionality, the customer is finding that the test setup can just as easily be programmed to do life testing, too. And WorxAmerica suggests that ultimately, the test system (see Figure 1) can be used to test subcomponents, such as the BOP's rubber seals.
Using an easily-programmable and configurable motion platform is fundamental to the versatility of the BOP test system that WorxAmerica built. The company's tester is based on an 8-axis motion controller manufactured according to the API test standard, so the system was designed to measure the amount of fluid leakage past the BOP collar as a simulated drill rod is cycled through multiple down and up strokes at 24 in./sec. for nearly 60 in. of total stroke. The rod is driven by a hydraulic cylinder that has a 6-in. bore with a 3-in diameter rod and is capable of handling 5,800 psi for a total force of 82 tons.
By programming the test system to rapidly move the simulated drill rod (which is crafted with dimensional variations to simulate the changes in rod diameter that the BOP will encounter as rod segments pass through it in the real world) into and out of the BOP, the test system can produce the amount of stress and wear on the BOP in a few hours that the device would normally sustain over a period of years. The motion controller supports the ability for an external PC running data acquisition software to examine parameters during a motion operation, such as the force required to move the drill rod. A change in a key force value may signal that the structures within the BOP device are wearing out. Likewise, if at the end of a "lifetime" of wear, the BOP is still performing to spec, then that is an indicator of the quality of the product. "Previously there was no way to dynamically look at problems that can signal BOP failure," said Chuck Camp, WorxAmerica chief engineer. "There were mathematical models, but there has not been any actual proofing that the model adheres to real life."
The WorxAmerica test system employs all 8 motion axes of the motion controller (Figure 2). One axis controls the hydraulic cylinder's positions, speeds, and forces exerted. 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 connected to a flowmeter to measure leak rate of the BOP. This illustrates the fact that motion axes can be used for parameter monitoring and decision-making, not just for motion axis control. For the WorxAmerica system, control loops are set up in the motion controller to compare external sensor data with setpoints.
The motion controller makes decisions based on the data and can vary how the simulated drill rod is operated as a result. Because the system needs to react very quickly as the rod passes through the BOP, the motion controller was programmed to close the control and monitoring loops of all axes faster than 1,000 times per second. To move the rod at the specified rate, the motion controller commands a proportional servo valve that can deliver varying amounts of hydraulic fluid to enable precise control of the rod motion.
PC-based control software
The motion controller was programmed and tuned by the WorxAmerica team, using motion control PC-based software. With the software tools "we can plot key parameters as we're tuning the system and save them for later review," said Chuck Camp. "The plots clearly show when the gains of the control loops are set correctly." The data interface between the motion controller and an attached PC was handled via the software. This tool enables an external PC to read and write data to and from the motion controller and acquire test data using popular PC-based control software packages.
"A general-purpose test system such as ours can be viewed as both an R&D tool and a production and reliability support tool," said Chuck Camp. The "motion controller will allow our customers to go beyond design verification to study materials science and design integrity." The cost of capable test machines isn't insignificant, and it makes sense to make them work in as many test roles as possible.
- Bill Savela, PE, Delta Computer Systems Inc.; edited by Mark T. Hoske, content manager, Control Engineering, email@example.com.
- System integrator provides motion control expertise for 8-axis simulation of a blowout preventer.
- A data interface connected the motion controller and PC.
- Testing provides design verification and ability to study materials science and design integrity.
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Delta Computer Systems motion controllers
See a related story by the author on oil-rig pipe testing below.