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No. 1122:
Offshore Rig Disposal

Today, we ask what happens when a machine grows old. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

We've built so much in the last century. Bridges, dams, office buildings, power plants, automobiles, computers. How do we go about decommissioning all that stuff? We've become pretty good at reclaiming material from old bridges, buildings, and cars.

But computers, which grow obsolete in a few years, pose disposal problems we've barely considered. We think we know how to decommission nuclear plants, but we don't have much experience doing it yet. The first hydroelectric dams are just growing old. They pose terrible dangers. Then there are offshore oil platforms:

Some 3800 platforms now line the Gulf of Mexico -- 6500 world-wide. Only a fraction sit in less than 30 feet of water. Some are in water over a half-mile deep. One in four is over 25 years old. Most production equipment sits on the deck of an underwater tower until the oil below is gone. Then what becomes of that huge structure?

A few platforms could possibly be used by marine scientists. Several have been toppled onto the ocean floor where they continue to shelter marine life -- growing coral and forming artificial reefs. That, by the way, is what became of a huge part of the Japanese Navy, long after it was sunk in Truk Lagoon during WW-II.

But most towers have to be cut loose, turned on their side, and floated off to land for recycling. It's in that removal process that trouble arises. A neutral panel of experts recently convened to improve the federal regulation of those removals.

The worst problem is cutting the tower away from the ocean floor. The cuts must be made well below the mudline -- below water -- so currents that scour the bottom won't expose the stumps. Protruding steel can threaten fishing -- especially shrimping.

The policy has been to cut the legs off 15 feet below the mudline. That can be done by a diver inside the legs of the tower, or by digging a great pit around the footings -- then cutting the legs off from outside. The cleanest way to sever the metal is with oxyarc cutting torches. But that's also the most dangerous. Divers have died from oxygen explosions and they work in fear of shifting mud.

It's far safer to lower explosive charges down the hollow legs and fire them from above. It's safer, but it's also far messier. On the wrong day, thousands of fish might be killed by the blast -- the very fish that make their habitat in the tower structure.

So engineers, out of the political limelight, work to save people and fish alike. They ask how far into the mud should we really cut the legs? Can we shape explosive charges so they won't send killing shock waves through the water? What's possible? What's not?

It's one more problem spawned by our ravenous appetite for goods and energy. It'll be followed by some new problem. All the while we wonder if we'll ever learn to consider the death of a machine -- as well as its life -- when we first design it.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

The Committee on Techniques for Removing Fixed Offshore Structures and the Marine Board Commission on Engineering and Technical Systems, National Research Council, An Assessment of Techniques for Removing Offshore Structures. Washington, DC: National Academy Press, 1996.

I am grateful to Alan Powell, UH Mechanical Engineering Department, for suggesting this episode and providing the National Research Council report along with other materials. I am also grateful to him and to Era N. Ford, Twachtman Snyder & Thornton, Inc. Consultants, for their critical help with this episode. Ms. Ford also notes that Allan G. Pulsipher, Center for Energy Studies, LSU, is a primary expert on this problem.

For more on offshore oil, see Episode 1394.


From the cited NRC document


Offshore rigs photographed from Galveston with a
600 mm lens. The left-hand one is in place. The
right-hand one is being moved.