Delicate operation carried out on explosives-packed WWII shipwreck

Sheerness, England — A delicate operation aimed at preventing an 82-year-old U.S. shipwreck packed with tons of World War II explosives from unleashing a potentially catastrophic tidal wave on London has been completed. But while the most imminent danger has been addressed, the explosives remain inside the rusting wreck of the SS Richard Montgomery on the riverbed, and experts aren't taking their eyes off it.

The Montgomery, one of about 2,700 Liberty ships built in the U.S. to support the Allied war effort, was grounded on a sandbar off the town of Sheerness in the Thames Estuary, just 40 miles from the British capital, in August 1944. It was carrying munitions intended to support the Allies following the D-Day landings.

The vessel broke apart during salvage efforts and sank, with an estimated 1,500 tons of explosives in its cargo holds.

The ship's three rusting masts protruding from the water had been a familiar sight to residents and passing vessels for decades. But recent surveys revealed increasing deterioration in the structure, raising fears the masts could collapse and trigger an explosion.

"The vessel was starting to lean and deteriorate, so they could see some of the structures around the mast starting to collapse," Nolan Conway, a naval architect with Florida-based marine salvage firm Resolve Marine Group, which was contracted to remove the masts, told CBS News.

"The fear is the masts could have fallen onto the cargo itself, causing it to detonate," he said. "Some of the munitions are believed to be fused munitions with pressure fuses, so something like an impact could potentially set them off."

U.K. maritime minister Keir Mather told lawmakers in the House of Commons this week that the "complex project requiring specialist program, engineering and risk management expertise" had been completed ahead of schedule and on budget, with all three of the masts removed at a cost of just under $13 million, according to The Guardian.

Mather said in the U.K. Parliament that the masts had put "significant load on the structure and their removal has reduced that risk and supports the long-term safe management of the site."

The government has long maintained an exclusion zone around the wreck, which is about 1.5 miles off the Sheerness coast. 

Mather told lawmakers the wreck would "continue to be subject to continuous monitoring and control via the established exclusion zone."

Removing the remaining explosives would carry considerable risks, and the consequences of an explosion remain the subject of debate among experts.

"The impact of this wreck detonating is unpredictable," Conway said. "Estimates range from impacting the shoreline, to creating a tidal wave that could travel up the Thames, to a rather minor event that could potentially just close a nearby shipping channel. There's a large range of impacts, but none of them are really good."

Officials continue to assess the overall risk of an explosion as low, but the deteriorating condition of the masts prompted authorities to act.

Conway said the challenge for the firm was to devise a way to remove the masts without disturbing the sunken ship's hull and the perilous cargo inside, or putting the people doing the work at risk.

"How can you cut off that mast without vibrating or impacting the wreck?" he said. "We designed a custom lifting frame to safely pick it up without the mast swinging or swaying as we lifted it free of the wreck."

Engineers used a remotely operated underwater diamond-wire saw that clamps "around the mast with hydraulic arms and passes the saw through without getting divers in the water," Conway explained.

The removal of the masts marks the end of a distinctive chapter in the wreck's history. The masts, visible above the estuary since World War II, are expected to be preserved for public display.

Mather, the U.K. maritime minister, said the masts had been moved to a local museum, the Historic Dockyard Chatham, and would be displayed "in recognition of the historic significance of the SS Richard Montgomery and its role in supporting the Allied forces' efforts during World War II."

More than eight decades after the ship failed to reach the battlefields of Europe, the Montgomery and its dangerous cargo will remain where it came to rest, now entirely hidden from view, but far from forgotten.