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Boskalis jaarverslagen 2012

Innovative wreck removal Baltic Ace

The removal of 456,000 liters of heavy oil in 2014 marked the completion of the first stage of the salvage of the car carrier Baltic Ace by SMIT salvage. stage 2 followed in 2015 and comprised the removal of the wreck and the controlled dismantling of both the wreck and its cargo. This comprehensive project was completed successfully, partly thanks to the introduction of new working methods and the collaboration between various Boskalis business units.

The car carrier Baltic Ace set out from Zeebrugge, Belgium in December 2012 in poor weather with a cargo of around 1,400 new cars destined for Finland. 65 kilometers off the coast of the Western Dutch island of Goeree-Overflakkee the Baltic Ace collided with a container ship and sank on one of the world’s busiest shipping routes. “Where the ship went down the water depth is 37 meters; the ship was lying on its side and had a width of 25 meters, which meant that the highest point was only 12 meters below the water line,” said Pieter van Vuuren, Operations Manager at SMIT Salvage. “The wreck posed a hazard to the 16,000 ships that come in and out of the Port of Rotterdam every year. In addition, the fuel oil and other hazardous substances posed a threat to the environment.” The Dutch Department of Public Works awarded the salvage contract to a consortium of Boskalis Nederland and Mammoet Salvage, with SMIT Salvage executing the salvage operation on behalf of Boskalis Nederland. “Surveys showed that the planned approach for raising the ship in one piece was unfeasible. It was then decided to saw the wreck into sections,” explained Van Vuuren.

Removing the fuel oil

Following intensive preparations a 25-strong salvage team started work on removing the fuel oil in May 2014. They used an innovative method, partly developed in-house, based on the experiences gained while removing oil from the Kyung Shin and Costa Concordia vessels. Hot taps were applied to the largest tanks, after which steam was used to heat the fuel oil and liquefy it. Once the oil had been pumped out the tanks were repeatedly rinsed with hot water in order to remove any oily remnants. In less than two weeks 456,000 liters of heavy oil was removed and brought to a recycling company. Smaller tanks containing various types of oil could not be reached. This was taken into account during the subsequent stages, allowing these materials to be safely disposed of at a later date.

Eight sections

In late 2014 preparations started for the wreck to be sawn into six large sections which would then be lifted by a floating sheerleg crane. In order to attach the lifting cables divers drilled dozens of holes in the hull, partly using the cold cutting method, whereby water is mixed with an abrasive mineral sand under very high pressure (2,500 bar). “Because the divers could only work when the tide was turning – for about two hours, only four times a day – work was carried out both day and night in shifts of four,” said Van Vuuren. As the operation proceeded, it became clear that the condition of the wreck was deteriorating rapidly. It was therefore decided that during stage 2 the ship would be cut into eight smaller sections, that these would be lifted using a floating sheerleg crane and that the remaining pieces of the wreck would be raised using a grab crane.”

Mobilization

The follow-up operation got underway in the spring of 2015 when a longitudinal cut was made down the full length of the ship. “SMIT Salvage gained a great deal of knowledge of cutting ship wrecks during the salvage of the submarine Kursk in 2001 and the car carrier Tricolor in 2003/2004. Using that experience we developed new methods for cutting the Baltic Ace,” explained Van Vuuren. “For example, the saw wire was fitted with a new kind of ‘cutting bushes’ made of a higher grade of tungsten carbide. In addition, we used special heave compensators, which allowed the cutting process to be carried out from floating barges. This maintained the tension on the cutting wire. We were able to finish each cut in under 30 hours – a record speed. With support from various Boskalis departments we were also able to greatly improve the software control of the cutting process. Boskalis’ survey vessels and advanced survey technology also played an important role.”

Huge job

The next stage involved removing the eight sections and the remaining pieces of the wreck, for which the floating sheerleg crane Taklift 4 and a 200-ton hydraulic grab crane were deployed. The grab crane underwent drastic modification for this project. “The ship and its cargo had a combined weight of 13,000 tons,” explained Van Vuuren. “8,000 tons was lifted in eight sections using the floating sheerleg crane Taklift 4, while the remaining 5,000 tons was lifted from the seabed using the grab crane. Some sections were 25 meters long, so it was a huge job. During the cutting and drilling over 100 people and a vast amount of equipment were deployed on the job.” After the pieces of the wreck had been put on barges, tugs towed them to a specialized recycling facility, where they could be dismantled and recycled safely and under fully controlled conditions. In October 2015 the seabed underwent intensive checks and the last pieces of the wreck were removed. The shipping channel is now once again fully accessible.

Click here to watch the project video of the Baltic Ace project.

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