Forecasting Fractures in Pipes

Monday, August 15, 2011 @ 06:08 PM gHale

A computer model that tests automobile components for crashworthiness could also work in the oil and gas industry to predict how pipes may fracture in offshore drilling accidents.

To prove the model is effective, a team from MIT’s Impact and Crashworthiness Laboratory simulated the forces involved in the 2010 Deepwater Horizon explosion in the Gulf of Mexico, finding their model accurately predicted the location and propagation of cracks in the oil rig’s drill riser — the portion of pipe connecting the surface drilling platform to the seafloor.

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In a side-by-side comparison, the researchers found their model’s reconstruction closely resembled an image of the actual fractured pipe taken by a remotely operated vehicle shortly after the accident occurred.

This could help oil and gas companies identify stronger or more flexible pipe materials that could help minimize the impact of a future large-scale accident, said Tomasz Wierzbicki, professor of applied mechanics at MIT.

“We are looking at what would happen during a severe accident, and we’re trying to determine what should be the material that would not fail under those conditions,” Wierzbicki said. “For that, you need technology to predict the limits of a material’s behavior.”

MIT researchers found a near-perfect match between their pipeline fracture simulation (bottom) and an image of the Deepwater Horizon's ruptured pipeline taken by an underwater robot.

MIT researchers found a near-perfect match between their pipeline fracture simulation (bottom) and an image of the Deepwater Horizon's ruptured pipeline taken by an underwater robot.

Wierzbicki has already laid much of the foundation for what he calls “Fracture Predictive Technology” through his work in car-crash safety testing. Over the years, he’s fine-tuned a testing method that combines physical experiments with computer simulations to predict the strength and behavior of materials under severe impacts.

To safety-test materials used in automobile bodies, Wierzbicki first cuts small samples from a candidate such as steel, using a high-pressure water jet. He then sprays the sample with a fine pattern of speckles, covering the surface with tiny dots. After the spray dries, Wierzbicki clamps the cutout into a machine, which subjects specimens to different types of loading. A motion-capture camera, set up in front of the sample, takes images as it crumples, sending the images to a computer, which plots the image’s dots along a grid to show exactly when and where deformations occur.

By testing different shapes and sizes of materials under various pressures, Wierzbicki can determine a material’s overall mechanical properties, such as its strength and ductility. Knowing this, he said, it’s possible to create a simulation to predict a material’s behavior in any configuration, under any conditions. Determining the exact limits for materials is especially important for offshore drilling where pipes are continually facing tremendous pressures at great depths, he said.

Using those same principles, Wierzbicki and his team were able to predict the strength and breaking points of the Deepwater Horizon’s drill riser.

Since the researchers were unable to obtain a sample from the actual collapsed riser, they consulted an offshore-drilling handbook, finding the riser likely consisted of X70, a grade of steel commonly used in such risers. The material’s mechanical properties closely matched those of TRIP 690, a grade of steel the team previously tested in the lab.

The researchers drew up a computer model of the drill riser — a large-diameter pipe attached at one end to a large rectangle, representing the surface drilling platform. The team then ran a simulation that partially reconstructed the Deepwater Horizon accident: After methane gas erupted and shot to the surface, setting the entire platform on fire, the oil rig began to list and sink. The researchers simulated the sinking by slowly angling the rectangular platform downward.

As a result, the attached drill riser began to bend. A color-coded simulation showed points along the pipe where it was likely to crack: Green and blue meant the material was intact; yellow and red indicated it was at its breaking point. The group found four red areas where cracks — and oil leaks — were especially likely to occur.

The group had one point of comparison: an image, taken by an underwater robot shortly after the accident, of the ruined pipe. When the researchers compared their model with the real-life image, they found an almost perfect match.

Wierzbicki sees the results as an encouraging first step in applying the model to materials for offshore drilling.

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