Workers decommissioning the 25,000 tonne Brent Bravo oil platform. Photo: Shell
The Afterlife of Old Offshore Oil Rigs
Oct 31, 2019
by Lina Zeldovich
Miles away from the shore in the open ocean stands an oil platform towering above the water like a skyscraper. There is little oil left in the well beneath it, and the platform is nearing the end of its working lifespan. Although it remains firmly moored to the ocean floor, something is about to happen.
A small, submarine-like vehicle descends along the tower that anchors the top deck to the seabed, a diamond saw clipped onto its sturdy metal leg. After some radio exchanges, the saw comes to life and it begins to eat steadily through the metal. The small fish, disturbed by the unusual activity, scatter. The platform takedown begins.
Offshore oil and gas platforms are among the largest structures humanity has ever built. There are more than 7,500 of them towering up from seas all over the world, according to some recent estimates. As wells dry up and pumping equipment wears down, many of those structures are growing obsolete.
But producers cannot just walk away from their creations. Those oil wells will have to be decommissioned and capped off and the platforms taken down. It will be a vast effort. In Europe’s North Sea, where production has declined to 1.5 million barrels per day from 6 million barrels per day in its prime, decommissioning could cost producers $150 billion, according to a recent report by Boston Consulting Group (BCG).
BCG has identified several other emerging decommissioning hotspots around the world. Southeast Asia is the largest of them, with more than 1,500 platforms and 7,000 wells. Over the next 20 years, however, the Persian Gulf will likely have more shutdowns, since it has 1,000 structures and 3,000 wells that will be more than 30 years old by 2038.
In the Gulf of Mexico, where the United States Bureau of Safety and Environmental Enforcement (BSEE) oversees decommissioning, operators and contractors have been removing about 150 to 250 structures a year since the 1980s. This makes the Gulf—where offshore production began—the region most experienced with decommissioning.
More challenges lie ahead, and the Gulf shows why. Its shallow continental shelf waters have been thoroughly exploited since 1937, when the predecessors of Chevron and ExxonMobil teamed up to build the first true offshore platform one mile off the coast of Louisiana in 14 feet of water. Within 10 years, production had moved 20 miles from the shore, though water depth was still only 18 feet.
Gulf platforms continued to move further from shore and into deeper water. BCG estimates that operators will need to remove more than 2,000 structures. They are an order of magnitude heavier than previously decommissioned platforms—think 12,000-ton structures in waters more than 330 feet deep.
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The North Sea is gearing up for an even steeper challenge. It has about 600 platforms and more than 7,000 wells to decommission. Some of those structures are an order of magnitude larger than those slated for decommissioning on Gulf of Mexico, and they sit in deeper water as well.
For example, certain structures in Shell’s Brent field, located in the East Shetland Basin of the North Sea, weigh 330,000 tons. That is the equivalent of the Empire State Building, according to Duncan Manning, decommissioning manager for Royal Dutch Shell’s Brent field.
Taking down something that large is a massive engineering project that requires state-of-the-art equipment.
A science and an Art
Oil platforms come in many types and sizes. Those designed for the deepest waters float on the surface, stabilized by ballast and held in place by tension cables anchored to the sea floor.
More common, however, are structures designed for shallow waters. These consist of a deck resting on top of a so-called jacket, usually a four-legged metal tower with cross-bars to hold it steady that sits on foundations driven into the seabed. Deck-tower structures account for the vast majority of aging platforms.
Unlike floating platforms, which can be towed anywhere once their moorings are loosened, deck-tower systems are permanent structures. Each platform is unique, and cutting and disassembling them is as much an art as it is a science. The job often requires remarkable feats of engineering, combining new types of saws with deep-diving subsea robots.
A typical takedown begins with dismantling the top platform, which usually hosts production equipment, living quarters, and a helicopter pad for bringing in crew and supplies, said Chris Mattingly, a project manager at Bluegrass, a company that specializes in “nonexplosive demolition.” This usually means cutting concrete and metal structures with diamond-studded saws.
“It starts out with a crew of welders and they take down as much of the top side as they can,” Mattingly said. “Then we get involved with our diamond wire saws, cutting steel members and piping, to get the top sides off—which is everything you see above water.”
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Bluegrass is best known for pioneering the diamond saws it uses, and it invented the first practical underwater diamond saw. Expectedly, these instruments do not look anything like garage saws or chain saws.
Instead, they resemble calipers outfit with pulleys. The cutting edge consists of long wires studded with diamond beads and pulled around the pulleys by a hydraulic motor. The beads steadily rub against the material, applying the force of friction to the surface to efficiently cut any material, from reinforced concrete to titanium.
The placement of the saw depends on the depth of the water. In shallow waters, a human diver mounts the saw on the metal or concrete struts with a hydraulic clamp to keep it in place. An operator controls the cut by regulating hydraulic power from a workboat.
“In the Gulf of Mexico, often the water is shallow enough that you can use a diver,” Mattingly said. “Anything deeper than 300 feet requires a remotely operated vehicle, or ROV, essentially a small submarine.”
Exactly how and where the jacket is cut depends on what will be done with it. In some cases, these robust long-lasting structures can be reused in another oil field. Companies from Vietnam, Thailand, or China sometimes buy the old structures and re-install them over their own oil wells, Mattingly said.
In that case, the team preserves the entire tower structure. The four legs, usually 4-inch diameter each, are cut at the very bottom and the jacket is lifted onto a barge—or a superstructure supported by two barges—in its entirety.
“Imagine if you had a tree house in your backyard and you sold it to your neighbor,” Mattingly explained. “You would just cut the posts at the bottom where the grass is, and he would just pick up the whole tree house, take it into his yard and set it back up.”
Other times, it makes sense to scrap the jacket entirely. In that case, Bluegrass or a similar contractor will cut the tower into smaller pieces, usually 10 to 20 foot sections, which are then loaded onto barges and shipped to shore for recycling.
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The exact depth at which they cut the legs depends on how the ocean area will be used once the structure is gone. Clearly, the presence of oil and gas platforms restricts boat traffic and commercial fishing because of the risk of collision and pipeline damage.
With the platforms gone, however, planners can reopen that ocean area, but its intended use will determine the final stage of decommissioning. If they plan to use the area for fishing—and in the Gulf, that often means shrimping—the platform legs must be cut below the ocean sediment level, or “mud level.” Otherwise, metal scraps poking through the ocean bottom would catch and destroy shrimper nets, which scrape the bottom of the sea.
Since the Gulf seabed is prone to changes due to current and weather, engineers must cut the platform legs 15 feet below mud level to assure they will not get in the way of shrimp nets.
To do that, some demolition crews use special explosives called shaped charges, said professor Mark Benfield at the Louisiana State University’s Department of Oceanography and Coastal Sciences. These are plastic explosives molded into shapes that channel the blast’s force into a particular direction. As a result, the detonation force doesn’t radiate everywhere, but targets a specific spot and punches a hole through the metal.
Still, detonations, even when small and well directed, are more destructive to the marine life than cutting. “The concussion can usually kill all marine organisms that have grown on the metal over the platform’s 25 to 30 years of the life,” he said. They get blasted off, and you no longer have the community of invertebrates living there.”
That may not sound like a big deal for a pile of scrap metal, but it is actually critical if the platform is destined to become an artificial reef.
To Reef or Not to Reef
Turning towers into reefs might prove a less expensive and more environmentally friendly way to retire or reuse the aging oil platforms, according to a team of researchers from the Aarhus University in Denmark. Rather than removing the structure, these scientists—along with many of their colleagues around the world—suggest converting towers into artificial ocean reefs that would provide safe living and spawning grounds for ocean creatures.
In Denmark’s case, leaving the platforms in place could help remediate some of the country’s environmental problems, scientists say.
In the late-twentieth century, Denmark mined many of its underwater rocks for construction projects. This flattened the ocean floor and destroyed important marine habitats, including breeding grounds for fish and host surfaces for algae and mussels, said Jonas Teilmann, a senior Aarhus University researcher who took part in the study.
“If young fish don’t have a place to hide, they get eaten—and then you see a cascade of issues,” he said.
Along Denmark’s coast, destroying natural underwater structures led to an ecosystem collapse. The situation is so severe, that the country is trying to rebuild its underwater reefs with boulders bought and transported from other places.
The North Sea’s oil and gas platforms give them another option. When the team looked at the photos taken by underwater cameras installed at an old oil platform, they found thriving marine communities, teaming with cod, flatfish and other bottom sea creatures.
“We also see many more porpoises around these structures than in the surrounding sea,” Teilmann said. “It’s easy to understand why the porpoises enjoy the area. One can’t throw a fish hook without catching one of the many cod around the legs of the oil platform.”
These findings are not limited to the North Sea. A joint study of the University of California and Occidental College found that that oil platforms off the California coast are some of the most productive marine fish habitats in the world.
This is because oil platforms have a high ratio of structural surface area to seafloor surface area. This creates a secure habitat for a gamut of species over a relatively small area of seafloor. The jacket serves almost like an apartment building for fish, where various species inhabit different parts of that water column. Some, like clams and mollusks, attach to the metal piping, while smaller fish find protection inside the hollow tubes.
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Reefing a platform still requires demolition work and there are different ways to do it, Benfield said. One is cutting the jacket level at the mud line and burying it on the spot.
The second is cutting the jacket at the depth safe for boats to pass over and sinking the top part next to its bottom half.
The third is to move the severed platform or its top to a new and more favorable location for reef formation. In some cases, for example, cut jackets are aggregated together in one area to form a larger reef.
Reefing is not without controversy. Some environmental groups see it as a loophole for oil companies to avoid platform removals. Some scientists also point out that creating new reefs where they did not exist before opens up doors to invasive species.
“The potential damage is that they are clustering reefs in places where they shouldn’t be, providing a super highway for disease vectors,” Samantha Muka, a professor at Stevens Institute of Technology, said.
When these species float through the water, they cannot ordinarily settle, she added. But when they find new surfaces to colonize, they can drastically alter the balance of an existing ecosystem.
Also no one knows how well these artificial reefs will hold up over time.
“Will these rigs stand up for 100 years? Maybe. But could they fail somehow? Possibly,” Muka said.
The debates about reefing and full or partial platform removal will likely continue and perhaps intensify as more platforms become obsolete.
But no matter what companies choose to do, they will still need to employ engineers and contractors to decommission old wells and structures. With so many aging platforms approaching their end of lifespan, should one expect a boom in business? Mattingly thinks it remains to be seen.
“Yes, every year we hear that there will be a boom in business,” he chuckles. “We are just waiting for it to happen.”
Lina Zeldovich is a freelance writer based in Woodside, N.Y.