- Computer Model May Be Able to Predict Pipeline Fractures
Wednesday, September 07, 2011
by Karen Boman
A computer model designed by the Massachusetts Institute of Technology's (MIT) Impact and Crashworthiness Lab to test automobile components could also be utilized to predict how pipelines may fracture in offshore drilling accidents.
As a case study, a team at the lab simulated the forces involved in the 2010 Deepwater Horizon explosion in the Gulf of Mexico, finding that their model accurately predicted the located and propagation of cracks in the oil rig's drill riser – the portion of pipe connecting the surface drilling platform to the seafloor. In a side-by-side comparison, the researchers found that their model's reconstruction closely resembled an image of the actual fractured pipe taken by a remotely operated vehicle shortly after the accident occurred.
The group presented their results at the International Offshore and Polar Engineering Conference in June. The lab received a small grant from Shell and invested some of its own resources to prepare the paper for the conference. Three major oil companies have expressed interest in using the computer model, said Tomasz Wierbzbicki, professor of applied mechanics at MIT.
Wierbzbicki said such a simulation 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. "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."
A decade of intense research took place before the original MIT fracture technology found industrial applications, said Wierzbicki. The steel and automotive industry were first to recognize the value of the technology. "We have developed a substantial research program with the applications to these industries. This program is supported in my lab by 14 major domestic and overseas companies." MIT also is working with an aerospace company to utilize the methods and procedures of this technology.
Wierzbicki over the years has 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 says, 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, he says, where pipes are continually subjected to tremendous pressures at great depths.
Since the researchers were unable to obtain a sample from the actual collapsed riser, they consulted an offshore-drilling handbook, finding that the riser was likely made from 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 had 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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