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Keeping it steady

Extracting stabilizers underwater

When ships encounter rough seas, how do they stay level? The secret is stabilizers, the movable fins attached to the hull below the surface of the water. Removing a damaged stabilizer usually involves stopping at a dry dock. But in a world first, SKF performed this task while the vessel was still at sea.

Stabilizers have a huge effect on ship performance: They can reduce roll by more than 85 percent. Shaped like airplane wings, they are deployed from the side of the hull and pivot up or down to exert dive or lift, thereby counteracting the effect of waves on the ship.

When the cruise ship MS Prinsendam’s starboard stabilizer was damaged by a collision during high season, its operator, Holland America Line, contacted SKF to repair it. This task is usually performed in a dry dock, where the stabilizer and crux can be removed more easily. But cruise ships schedule their dry dock periods several years in advance, and with cruises ongoing, the company was determined to find a different solution. Would it be possible to quickly replace the stabilizer during a cruise, while Prinsendam was docked in a port?

Ship Facts

Name:
Owner:
Passenger capacity:
Crew capacity:
Gross tonnage:
Length:
Beam:
MS Prinsendam
Holland America Line
916
470
39,051 grt
172 meters
29 meters

Getting the experts on board

Such an underwater extraction had never been performed before. And when it comes to cruise ships, even the “relatively small” components are still huge. The stabilizer fin and crux that needed to be removed weighed about 25 metric tons.

To carry out the task, Holland America Line approached underwater repair specialists Subsea Global Solutions to develop a solution. The companies collaborated under the supervision of SKF technical consultant Jens Miesner. Subsea’s Rick Shilling explains, “We designed a detailed rigging procedure based on the stabilizer design, using fixed as well as buoyant rigging sufficient to manage the load. The process was consistent with what they do in dry dock. We just adapted and modified it so it can be done by divers underwater.”

This meant that metal rings known as “pad eyes” needed to be fixed onto the ship to support the rigging during extraction. Divers were sent down to weld the pad eyes onto the ship several port visits beforehand. “It was extremely difficult,” Miesner says. “The pad eyes in the fin box were not welded, as we normally need them to be. Another problem was the very narrow space we would have to maneuver the 25-ton unit.”

The MS Prinsendam at sea.

The MS Prinsendam at sea.

Overcoming the challenges of wet welding

Subsea Global Solutions’ coded welder/divers used a class-approved procedure to perform the underwater wet welding. They employed innovative solutions, such as pad eyes designed to fold like a hinge to help prevent them clashing against the fin box. Divers also removed bolts from the stabilizer to ensure it was ready for extraction when Prinsendam arrived at the port in Amsterdam.

However, removing a hull component presents an obvious problem: how to keep water from getting inside the vessel. From the inboard side of the hull, the crux mating surfaces were blanked off with a specially built internal cofferdam. For the exterior, the diving company had prepared covers for the two holes, with special sealing strips that would take advantage of the 7.8 metric tons of water pressure to seal them up.
“Basically, you’re opening a hole in the vessel, and the challenge is to keep water from getting in and oil from getting out,” Shilling says. “Part of the process involved balancing pressures in the systems and flushing fluids out of the unit. For any residual fluids that can’t be pumped into the vessel we have systems that can encapsulate them underwater and pump them to the surface.”

All bases covered

The team conducted a risk analysis to submit to the necessary authorities for approval. “We are a known entity in the industry,” Shilling says, “and we prepare the necessary repair plans, risk assessments and contingency plans, so approvals from ports and other authorities were not hard to get.”

Once the ship arrived in Amsterdam, additional dive teams were on site to execute the extraction. “Our biggest concern was with internal release of the crux assembly,” says Shilling. “With any component damage like this, there are some unknowns, and while the stabilizer was fully inspected after the initial damage had occurred, you don’t see everything. All the contingencies were in place, and if worse came to worst, we could just put the bolts back in and try again some other time.”

55 hours
to complete the entire extraction process in port.

Fortunately, weather and sea state conditions were favorable, so the extraction went ahead. All the careful preparation paid off: The divers applied the rigging solutions to the stabilizer, using heavy chains to tie the equipment to a floating pontoon. They removed the remaining bolts and used specialized rigging techniques and buoyancy control to carefully maneuver the stabilizer to a location where a crane could lift it from the water. In the end, the extraction process in port took around 55 hours.

Miesner says, “A lot of shipping companies observed the process because it means that a lot of time can be saved.” The stabilizer was sent to SKF for overhaul and repairs, before it was reinstalled on Prinsendam a few months later at a shipyard in Malta. Now the cruise ship can continue sailing steady.

How the stabilizer was removed

  • Several days prior to the ship’s arrival at the extraction port in Amsterdam, an onboard team consisting of SKF and SubSea Global Solutions experts prepared the vessel from the inside. A specially built cofferdam sealed off the stabilizer/hull connection points from the rest of the interior.
  • At the same time, the team followed the vessel at different ports to perform preparatory work on the exterior and interior of the hull. Divers welded innovative hinged pad eyes to be used as rigging connections to the hull. As many bolts as possible were removed from the stabilizer and joint attachments to save time during extraction.
  • In the final port, additional dive teams were on site to execute the extraction. Using heavy chains, divers latched the stabilizer fin to a floating pontoon.
  • Divers removed the remaining bolts, and the team extracted the equipment using advanced rigging (based on the stabilizer design) and buoyancy control.
  • The stabilizer and crux were moved to a location in the port where a crane could lift the equipment from the water.

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