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Bad Vibrations PUBLIC ACCESS

Currents Pluck the Strings, and Drillers Try to Stop the Music.

[+] Author Notes

Associate Editor

Mechanical Engineering 126(07), 30-31 (Jul 01, 2004) (2 pages) doi:10.1115/1.2004-JUL-2

This article focuses on offshore drilling that is thought to be impossible. The depths are so great, and the pipes are so relatively insubstantial, that it seems a miracle anyone can get it to work. Strakes used in riser applications typically are made out of a high-strength plastic that is resistant, though not immune, to marine growth. Since such growth changes the hydrodynamic properties of the strakes, the devices are designed to ensure that they work even when encrusted with life. One company went so far as to test its wares with squares of shag carpet glued on. Fairings have the advantage of being cheaper than strakes. In addition, fairings have much lower drag than helical strakes and are much less sensitive to marine growth on their surface, which is a common problem. To accommodate shifting currents, fairings are only loosely attached to the risers; they swing around like weathervanes. Near the surface, where currents are most likely to change direction, strakes, which are onmidirectional, are often better bets. Hybrid systems—strakes on top, fairings below—are increasingly common.

If you stop to think about it, offshore drilling ought to be impossible. The depths are so great, and the pipes are so relatively insubstantial, that it seems a miracle anyone can get it to work. Imagine poking holes in the ground with a 50-foot length of doweling. It's something like that.

Except that it's worse. Ocean currents impart a much greater force on the pipes, or risers, as they are called, descending from an oil platform than would any atmospheric wind. The risers catch these currents and vibrate ever so slightly. The vibrations can, over time, create stress failures in the risers, leading to costly replacements.

"These vortex-induced vibrations create fatigue in the risers," said Subrata K. Chakrabarti, an engineer with Offshore Structure Analysis Inc. of Plainfield, III. "This fatigue cuts the lifetime of a typical riser to just about one year."

Increasingly, engineers have to tackle these vibrations by fitting their risers with suppression mechanisms. The fittings have been adapted from designs of proven aerodynamic structures. The challenge is to keep down costs-both material and labor costs.

It's a truism, but one that bears repeating: Ali the easy places to drill have been drilled. The first oil wells were drilled in the most obvious sites-patches of land where oil was close to the surface. Edwin Drake's 1859 well in Titusville, Pa., the world's fir st, was less than 70 feet deep, and was drilled near the site of a natural seep, where oil-saturated soil could be found.

In the late 1800s, California oilmen discovered fields on the Pacific Coast that grew more profitable the closer you got to the waterline. Wells sprouted up on the beach. And, in 1887, workers in Summerville, Calif., built a wharf extending 300 feet into the ocean and placed a drilling rig at the end-the world's first offshore platform. More drilling wharves followed, with one extending nearly a quarter- mile into the ocean.

It wasn't just the United States that was edging offshore. Vast fields were discovered in the Caspian Sea; the shallows off Baku in Soviet Azerbaij an were famous for a forest of drill rigs.

Still, for a variety of reasons, drilling stayed close to the water's edge until the end of World War 11. Kerr-McGee Corp. drilled a well at Ship Shoal, La., in 10 feet of water some nine miles from the shore using an innovative barge and platform combination and within two years, 11 fields were discovered in the Gulf of Mexico. The postwar explosion in the demand for oil and gas combined with new technology to fuel an offshore exploration boom.

There are now some 4,000 offshore platforms in American waters, the vast majority lying in the Gulf of Mexico. And drilling goes on into ever-deeper water: In May, an offshore rig operated by Transocean Inc. of Houston set a world water depth record for a moored rig by drilling in 8,951 feet of water in the Gulf of Mexico. A Trans ocean ship broke the 10,000-foot depth mark in November 2003. The push is on for ever deeper exploration, since the shallowest sites, of course, have been drilled.

But this push into deeper water comes with a need for greater ingenuity. One major problem is hydrodynamic drag on the risers. As water currents pass by the risers, small vortexes form in the wake; these vortexes rattle the risers, setting up vibrations in the pipe. Failure of welds is one problem, according to Don Alien, a research engineer at Shell Global Solutions in Houston. Another potential risk is that risers could be displaced enough that they bash into each other.

"Even though a deepwater riser is a steel pipe, it's like spaghetti," Alien said. " It's very flexible."

Production risers, Alien said, are typically under tension like guitar strings to reduce the amplitude of the vibrations and scale back displacement and drag. They have vibrations in the 20th to 50th mode, meaning there are more than 20 peaks and valleys running along the thousands of feet of pipe. The amount of displacement is small-on the order of a few inches-but over time it can weaken the material significantly. And should the vortexes induce drag on the risers, they can drift by several feet.

By the late 1990s, vortex-induced vibrations were recognized as a problem that would have to be dealt with through means other than tension. Alien and his colleagues at Shell, as well as other companies, sought to find a simple and cheap method to reduce them.

The team tried a number of different systems, including wrapping a test pipe with a spiral of wire, or even mounting beads to the pipe. But two approaches stood head and shoulders above the rest.

The first involved wrapping the risers with a plastic strake: a helical lip that spirals down the pipes like a stretched-out Slinky. Strakes have been used on industrial chimneys for years. As air flows past the chimney, the strake chops up the airflow and creates vortexes at various places along the cylinder. These vortexes are out of phase with one another and produce destructive intetference; the net result is a significant reduction in the amplitude of vortex-induced vibrations.

Strakes used in riser applications typically are made out of a high-strength plastic that is resistant, though not immune, to marine growth. Since such growth changes the hydrodynamic properties of the strakes, the devices are designed to ensure that they work even when encrusted with life. One company went so far as to test its wares with squares of shag carpet glued on.

Another approach involved encasing the risers in a plastic sleeve, known as a fairing. The cross-section of the fairing resembles that of an airplane wing, and water slips past the surface much more cleanly, generating fewer vortexes.

Fairings have the advantage of being cheaper than strakes. And they are often easier to mount, Alien said, no small consideration when a day's worth of labor on an offshore platform costs upward of half a million dollars. In addition, fairings have much lower drag than helical strakes and are much less sensitive to marine growth on their surface, which is a common problem.

To accommodate shifting currents, fairings are only loosely attached to the risers; they swing around like weathervanes. Near the surface, where currents are most likely to change direction, strakes, which are onmidirectional, are often better bets. Hybrid systems-strakes on top, fairings below-are increasingly common.

Engineers are seeking ways to further reduce vortex induced vibrations. Cutting the amount of labor needed to deploy the devices or extending the working lives of the suppression technology will lead to significant reductions in cost. Surprisingly, though, the cost of the systems themselves is much less a factor.

Fairings or strakes are usually applied to just the top part of the riser, which means the material cost runs only in a range around $100,000-less than 2 percent of the cost of a typical production riser and much less than the lab or needed for mounting one.

As oil production moves to greater depths, the pipes running to the ocean floor are more susceptible to damage from strong ocean currents.

Grahic Jump LocationAs oil production moves to greater depths, the pipes running to the ocean floor are more susceptible to damage from strong ocean currents.

In the near term, the most promising innovation might just be doing the same with less. Partial fairings- essentially the tail of the device-have been shown to reduce vortexes by almost as much as full scale fairings. What's more, these "tails" can be mounted with just a fraction of the labor needed for conventional fairings. Allen said that one of these can be attached in just 30 seconds, versus five to 10 minutes apiece for present-day models.

Another possible new approach was discovered by accident, Alien said. In advance of a test, as part of research into interference between risers in 1997, a smooth cylinder was placed into a tank of a Naval Surface Warfare Center facility in Carderock, Md.

"It was smoother than any cylinder we had ever tested," Alien said. "We didn't think much about that until we placed it in the water and the pipe wouldn't vibrate. Or it would vibrate until you got to a critical Reynolds number, and then it would stop and wouldn't vibrate again."

Engineers had speculated for years about whether vibrations would occur at critical Reynolds numbers, Alien said. But every test to that point had shown plenty of vortex-induced vibrations, or VIV. No test, however, had ever used as smooth a cylinder as this one, and that seemed to make all the difference.

"We could kill VIV by having a smooth surface," Allen said.

Later that year off the coast of Trinidad, 30 glossy fiberglass sleeves were attached to risers. Even though the site was subject to very swift currents, hardly any deflection in the riser could be detected.

Still, it will be a while before smooth sleeves start to displace conventional suppression technology. Fairings are so cheap to make and easy to install that they are more economical in all but the fastest currents.

Copyright © 2004 by ASME
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