Raising the bar on oil-rig pipe testing

Subsea oil well pipe stands undergo tremendous stresses in operation. They must bear the weight of attached equipment, such as oil well blowout preventers, as well as stand up to the force applied by the constant motion of the oil rig. Then there are forces of nature, such as tidal waves and other storm-like conditions.

Knowing the environments in which they operate forces the companies that manufacture oil well pipes and pipe connectors to employ rigorous tests to make sure that their products will perform as designed in the field.

Historically, structural qualification testing of subsea pipe connectors has been performed by machines with limited capability of applied axial and bending loads, but one threaded connector manufacturer, Dril-Quip Inc. of Houston, believes that qualification testing of its equipment should not be limited by the capacity of currently available test machines. To verify the integrity of such assemblies, the company has developed a test system that is capable of producing millions of pounds of axial and bending force on an assembled pipe stand.

Applying such large forces is the domain of the largest of hydraulic rams, and controlling them requires an electro-hydraulic motion controller with special capabilities for closed-loop force control. Following an extensive search, company engineers found the right controller for this task. "We saw that using an 8-axis motion controller could save us a lot of time and effort in implementing our hydraulic controls," said Mike Lynch, PE, Dril-Quip senior scientist engineer.

The testing apparatus the company designed uses two large hydraulic rams, mounted on either side of a pipe stand assembly that can be in excess of 25 ft long (see Figure 1). The rams are operated in concert to exert bending and tension or compression forces on the pipe stand specimen of more than 6 million pounds.

"We need to repeatedly test the specimen to 95% of its yield force (the force required to bend the pipe assembly without failing to return to its original shape)," said David Trevas, Ph.D., Dril-Quip senior scientist engineer.

The test sequence

To set up a typical test sequence, the specimen pipe stand is filled with a fluid or gas test medium. The test cycle is initiated and the system moves as controlled by the 8-axis motion controller. During this operation, the system is monitored for leakage. The test limits (such as allowable leakage) are standardized by the American Petroleum Institute (API). Maintaining functional integrity of the connection at such close proximity of yield is a motivating factor behind developing a powerful and highly precise test apparatus.

Figure 2 is a block diagram showing how the main control components are connected. The setup includes a Microsoft Windows-based PC operating in a control room, providing a safe, noise-free environment for the test operator.

The test specimen and the hydraulic cylinders are located in a test pit for safety. The motion controller receives commands over an Ethernet link from a NI LabWindows/CVI program running on the PC. These commands provide instructions to the motion controller on how to move the hydraulic cylinders and at what pressure.

The motion controller’s manufacturer developed a special software package that allows other applications on the PC to communicate with the controller—both sending and receiving data—over an Ethernet connection.

The motion controller’s role

The motion controller (Figure 3) reads the differential hydraulic pressure across the piston of each cylinder to calculate and control the force being applied to the specimen. The controller uses position feedback from magnetostrictive displacement transducers (MDTs), mounted between the cylinder housing and the cylinder rod, to measure the deflection of the specimen. Because it’s a multi-axis controller, it can simultaneously control different motion profiles for each axis.

To complete the instrumentation of the system, strain gauges mount directly onto the specimen at specific places and send the strain signal to strain gauge module in the chassis. Pressure and temperature transducers mount on various parts of the test fixture and connect to another data acquisition chassis. The transducer signals are converted to engineering units inside of the software. Process status, parameter values, and limit warnings are updated on operator screens attached to the PC.

Test sequence, a step at a time

Before conducting a test on a specimen, the test engineer sets up a schedule of test points to load and/or bend the specimen. These test points are converted into a series of steps for the motion controller to carry out. These steps are organized in a Microsoft Excel spreadsheet residing on the PC. The development software program then transfers one test step at a time to the motion controller and the motion controller executes the step. The PC program monitors the status registers within the controller and determines if the controller is operating within specified parameters as it completes the step. The development software program also logs test data.

To gain optimal performance from the motion control system, the control loops need to be tuned, with the control loop coefficients set to minimize the error between target profiles specified by the PC test program and actual motion profiles observed by the motion controller. To assist with the tuning process, engineers used the plotting application that also came with the motion controller. Figure 4 shows a typical plot of the motion of the system.

With the use of the test system, the company can simulate field conditions with load, bending, and pressure that wouldn’t otherwise be possible in a lab setting. In doing so, the company has increased its understanding of the wellhead performance and gained knowledge to give its customers confidence in the subsea wellhead system.

– Bill Savela is a Professional Engineer with Delta Computer Systems Inc. Edited by Sidney Hill, Jr., a CFE Media contributing content specialist. This article is part of the oil and gas coverage from Control Engineering and Plant Engineering, CFE Media publications. 

Key concepts

• Verifying that pipes and connectors can withstand the forces applied to them in subsea drilling environments requires extremely rigorous testing.
• Conventional testing equipment has limited capabilities when it comes to applying force, which sometimes prevents equipment manufacturers from testing products to their maximum limits.
• Test system can produce millions of pounds of axial and bending force on an assembled pipe stand, comparable to the harshest subsea drilling conditions.

Consider this

Given the potential loss of dollars associated with the failure of pipe or connector in the field, doesn’t make sense to have those products undergo the most rigorous tests possible? 

ONLINE extra

This online version adds relevant links and additional product information.

www.api.org 

www.deltamotion.com 

www.dril-quip.com 

www.ni.com/lwcvi 

Data acquisition test system for oil drill rig testing

Dril-Quip Inc. of Houston, performs qualification testing of its oil rig equipment with instrumentation including NI LabWindows/CVI strain gauge module in the chassis, mounted onto the specimen at specific places and send the strain signal to a strain gauge module in the chassis. Pressure and temperature transducers mount on various parts of the test fixture and connect to another NI LabWindows/CVI data acquisition chassis. The transducer signals are converted to engineering units inside of the NI LabWindows/CVI program, which transfers one test step at a time to the motion controller and the motion controller executes the step. The LabWindows/CVI program, from National Instruments, also logs test data. 

Original content can be found at Oil and Gas Engineering.

Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our CFE Media editorial team and getting the recognition you and your company deserve. Click here to start this process.