Getting our bearings straight

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A while back, some of the nimble minds at Royal Caribbean were confronted with a first-of-its-kind puzzle. They had to solve an engineering problem that was bristling with thorns. It’s not giving away too much to say that they did.

Allure of the Seas, an Oasis-class ship, needed repairs. It was the kind of work that would normally be done in a dry dock, and the ship would be sent on its way.

But Allure is one of the largest cruise ships in the world, eclipsed only slightly by RCL’s newly launched Harmony of the Seas. And therein was the puzzle. Longer than three football fields and with a displacement weight of some 200 million pounds – about equal to that of a Nimitz-class aircraft carrier – Allure is too big to fit in any commercial dry dock in the Western Hemisphere, where it sails year round.

Bill Baumgartner, RCL’s senior vice president for Global Marine Operations, says a veritable army – or more aptly, navy – of RCL managers, crew, engineers and other specialists pondered, collaborated, studied, debated, discussed and analyzed the challenge. This is what they faced:

  • Two of Allure’s three Azipods, mounted under the stern and used to propel and steer the massive ship, had worn bearings that needed to be replaced.
  • For scheduling reasons, Allure couldn’t be out of service for more than seven days.
  • While there are dry docks in the Eastern Hemisphere that could handle the job, crossing the Atlantic one way would itself take 10 days. Then there would be the trip back. And the time in between to fix the problem.

So, Baumgartner said, “We jacked her up.”

This solution was the brainchild of Veli-Matti Junnila, a naval architect who often consults on RCL shipbuilding projects.

Using the dry dock at Grand Bahama Shipyard in Freeport, Allure was painstakingly backed in and the stern was raised slightly while the bow protruded from the front of the dry dock, still supported by the water. Had the entire ship been raised out of the water, about a third of it would be unsupported and the ship might well have broken in two.

With the stern raised only as much as necessary, purpose-built cofferdams – huge steel boxes open on top and with a door at one end – were slid under and around the two Azipods, their doors closed and sealed by divers, and their interiors pumped dry. Then the pods were dismantled in place, the bearings swapped out, the pods reassembled, the cofferdams refilled and backed off, and the ship leveled and sent on her way.

Despite only a virtual hair’s-breadth of clearance on either side of the ship, pod weight of 250 tons each, bearing weight of about eight tons, the possibility of bad weather and countless other things that could go wrong, none did.

The project succeeded beyond hope. Five days later Allure of the Seas was back to sailing in the Caribbean.