Seattle was paying unusually close attention when Bertha, the world's largest tunnel-boring machine, floated into town this past April.

News choppers hovered and amateur videographers took aim as fire-rescue boats blasted their water cannons to greet the precious cargo. The riggers—who unloaded, moved and set the 7,000-ton TBM's massive parts into an 80-ft-deep launch pit so she could dig the new Alaskan Way Viaduct—were rarely accustomed to such fanfare.

"It was crazy," recalls Jeff Latture, senior vice president at Barnhart Crane & Rigging, contractor for the heavy lifting. "So much of what we do is inside the plant—a nuclear plant, refinery, petrochemical plant. Nobody ever sees it except people who are in the industry. It's odd for us to be in a job so public."

Risks and rewards are climbing for the world's biggest lift firms, including the challenge of managing the public. Construction fans have a growing menu of social-media platforms on which to share photos, videos and opinions about spectacular feats of engineering—Bertha even has her own Twitter account.

When things go wrong, the exposure is a double-edged sword. Seeing a crane accident on the evening news "is like watching a bunch of train wrecks," says Jim Yates, Barnhart's senior vice president of engineering. "There are thousands of lifts done every day by good companies, but, unfortunately, you have a high-profile accident, and it can paint our business in a negative light." Latture, who leads Barnhart's sales and marketing, echoes this idea, saying heavy lifting is "not inherently dangerous but extremely unforgiving."

In an industry sector in which the U.S. Occupational Safety & Health Administration estimates the cost of a fatality at $8.7 million and a non-fatal injury at $62,500, risk-conscious lifters place a strong emphasis on safety, and firms in heavy lifting have a resource advantage: Renting out bread-and-butter machines, such as crawler and rough-terrain cranes, earns companies an average profit margin of 6% or less, according to OSHA, while engineering and performing heavy lifts, in areas such as power and pipeline construction, averages more than 10%. According to firms that spoke to ENR, heavy-lift contractors target annual returns on capital of 15% or more with utilization rates of at least 75%.

The risks are high, but the potential payback is huge. Bulky components can be worth millions of dollars to owners, so jobs often must be carefully engineered weeks or months in advance (Barnhart has 50 full-time professional engineers on staff). The most common slipup on jobsites requiring critical lifts is a sudden change in the lift plan, according to Jim Wiethorn, a forensic specialist at Haag Engineering, Sugar Land, Texas.

"It is almost exclusively a change in site conditions or plans," Wiethorn explains. "If you are involved in a lift and see a change in plans, you need to sit back and stop it. The good companies plan, and they charge more for it."

According to industry observers, heavy-lift firms are on a growth track. Trade publication American Cranes & Transport ranks Barnhart as the largest specialized lifting company in North America, with 850 employees, 24 yards and a fleet of thousands of machines that have a combined lifting capacity approaching 300,000 tons (see table).

Last year, the company ranked at No. 52 on ENR's Top 600 Specialty Contractors list, up 10 points over the prior year with $249 million in 2011 annual revenue and $210 million in new contracts. Revenue was estimated to hit $300 million in 2012, and although the top line is expected to flatten out through 2014, Latture is upbeat about future work in downstream natural-gas processing, liquid-gas export terminals and newer markets such as civil construction, which is increasingly relying on modular assemblies to cut the time it takes to stick-build bridges, powerplants and other structures. All this activity should offset a slowdown in the nuclear sector, Latture adds.

Handling bulky objects is just part of the job these firms perform. "The biggest thing we move is time," Mammoet's leaders like to say. "The value [comes] whenever we can engage early on with the owner and the design firm in order to optimize the constructibility and strategy," explains Guus Stigter, vice president of sales and marketing for Mammoet's U.S. arm in Rosharon, Texas. "We can save time and get plans quicker to market."

As modular construction matures, the requirements rise for crane capacities. The largest heavy-lift crane in Mammoet's fleet, a ring-mounted rig called the PTC-140/200, is capable of handling up to 3,500 tons at a 180-ft radius. Mammoet has built three units, and they all have been busy in North America assembling an offshore oil spar in Ingleside, Texas; upgrading a petrochemical plant near Houston and supporting Canadian oil-sands operations in Ft. McMurray. Netherlands-based Mammoet ranks as the world's largest heavy-lift contractor, with $2 billion in annual revenue, according to trade magazine International Cranes and Specialized Transport (see table).

The world's two biggest land-based construction cranes, each with 7,500 tons of capacity, can be found building the V.C. Summer and Vogtle nuclear expansions in the southeastern U.S. Valued at more than $50 million apiece, the heavy-lift derricks (HLD) were designed by Bigge Crane & Rigging Co. CB&I, which owns and operates the units, has equipped them with a 560-ft-long boom good for 4,000 tons at a 240-ft radius. At 465 ft out, the boom can lift more than 1,500 tons.

Shipped in about 160 truckloads, each HLD is rigged with 22 parts of line to the block, which itself weighs 90 tons. In all, the machines contain eight miles of 2¼-in. wire rope weighing 200 tons. Their advantage is the ability to perform all the site's major lifts without moving the derrick. The downside is that they require constant monitoring and maintenance, though Bigge has designed redundancies into the power and electronic systems to prevent a total shutdown. Bigge is looking to build more. "We view this machine as a real tool that is being looked at throughout the world," says Gedge Knopf, U.S. sales manager. The design can be scaled up and down, he adds.

Does the rise of the supercrane make jobsites safer? "Yes and no," Stigter says. "What we historically have seen is the accident frequency on these large lifts is actually less than on the small lifts, because the small lifts aren't being scrutinized as much. The severity outcome is higher with such a major lift, but the frequency that something goes wrong goes down significantly." Last month, Entergy Arkansas filed a negligence lawsuit against Bigge for allegedly failing to perform a load test after a gantry collapsed and dropped a 550-ton stator, killing one ironworker. What is unclear is whether Bigge was obligated to perform such a test, which experts say is not always feasible in the field.