Despite the difficult circumstances due to COVID-19, Boskalis successfully completed two major North Sea platform decommissioning projects to both clients’ full satisfaction. “The L10 and Viking Vulcan/Bravo campaigns were very successful because we were able to build on the experiences and achievements gained in previous campaigns and make use of the unique capabilities of our crane vessel Bokalift 1,” say project managers Frodo Leenhouts and Bas Mabelus.

Both projects involved the decommissioning (removal, dismantling and recycling) of oil and gas platforms in the North Sea. The first project consisted of the removal of three platforms from the L10 field, 50 nautical miles offshore the island of Terschelling in the Netherlands, which was followed by the second and last decommissioning phase of the Viking Vulcan/Bravo project.

Hundreds of aspects

The work in the Viking Vulcan/Bravo field off the east coast of the UK was executed for Chrysaor in a 50/50 joint venture with Scaldis. The project involved the removal of 18 platform topsides, including wellhead, processing and accommodation modules, as well as 21 steel foundation jackets, three bridge structures and two undersea templates. The largest of the structures lifted by the Bokalift 1 weighed in excess of 1,500 tons. “A project of this size requires a lot of engineering, calculations and CAD drawing,” explains Leenhouts. “It takes a lot of ingenuity and teamwork to work out how to remove the topsides from the jackets safely, which cutting and lifting methods to use, which temporary structures are needed to work in hard-to-reach places without risk, the amount and type of lifting points needed for the lifting work, how the stability and integrity of both the structures to be lifted and the crane vessel can be safeguarded, and how to optimize the workability in different weather conditions and sea states. We also needed to decide where to locate the grillages (support structures) on board the Bokalift 1 to transport the platform parts safely while keeping them balanced. Overall, there were hundreds of aspects to be kept in mind to guarantee a safe and successful operation.”

Platform removal

“We worked at different locations on the removal of the topsides, jackets and other elements in the Viking Vulcan/Bravo oil field over the course of six years,” says Leenhouts. “For the last campaign we used internal cutting tools (ICTs) to cut through the steel foundation piles. The ICT is a cutting device that makes a horizontal or inclined cut in the piles with a combination of ultra-high-pressure water jets and sand crystals. In that way, we were able to cut the piles three meters below the seabed from the inside.” Before the ICTs could be used, several meters of sand had to be removed from the piles after the topsides had been removed from the jackets. “We mostly used the airlift tool for that work. That’s a modular pipe structure which can be more than sixty meters long. It pumps air to the bottom of the pile to loosen the compact sand column,” explains Mabelus. “After we had cut through the jacket piles, we were able to lift the jackets from the seabed with the Bokalift 1 crane. For several topsides and jackets our internal lifting tools were used. These are hydraulic plugs that are clamped into the tubular elements of the platform structures. The parts of the platforms that we removed were lifted onto the deck of the Bokalift 1 with the 3,000-ton crane and sea-fastened. They were then taken to the specialized yard of our subcontractor Veolia Peterson in Great Yarmouth in the UK for dismantling and recycling.”

Bokalift 1

The heavy lift vessel Bokalift 1 played an important role in both decommissioning projects. “With an enormous deck of more than 7,000 square meters, this unique vessel can accommodate huge installations and large amounts of equipment, and the centrally located crane with a lifting capacity of 3,000 tons is incredibly strong and can reach the entire surface of the deck,” Leenhouts says. “By virtue of the well-positioned crane and the vessel’s DP2 dynamic positioning capability, we were able to perform the heaviest lifts where there was extremely little room to maneuver, even in challenging weather conditions. In addition to the performance of the Bokalift 1, the team spirit during both campaigns was excellent.”

All hands on deck

“We had a large team of more than 120 people on the Bokalift 1. It was very much a question of all hands on deck”. In addition to our own colleagues making up the marine crew and project team, there were a lot of subcontractors on board: ROV and crane operators, specialists for the internal dredging, cutting and lifting tools, welders, cutters, fitters and riggers, as well as scaffolders and rope access technicians. Everyone was dedicated and flexible, particularly when it came to crew changes during the COVID-19 outbreak, for which Boskalis had set up a special test facility and quarantine program. This was acknowledged by the clients of both projects, who expressed their satisfaction and appreciation regarding the successful execution. All the Boskalis colleagues, subcontractors and support organizations can be very proud of the end result,” says Leenhouts.