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Flawless float-over

With a spectacular topside float-over installation in a Norwegian fjord, Boskalis completed one of the most impressive transport and installation projects ever late last year. ‘The winter conditions were a huge challenge but everything went according to plan. We managed to complete the entire project – more than 100,000 man-hours – safely and without a single lost time injury,’ says project director Bas Polkamp. ‘We didn’t use a single plaster!’

Bas, can you explain how this topside installation was carried out?

“Once the White Marlin arrived in Norway, the topside was transferred onto two S-class vessels, the smallest in the Dockwise fleet. After accurately positioning these two vessels on either side of the White Marlin, they started de-ballasting whilst the White Marlin ballasted down and maneuvered out from underneath. The result was a ‘catamaran barge structure’ with the topside linking the two hulls.”

“Then the catamaran barge configuration had to go to the ‘mating location’ where the topside is installed on the substructure. At this stage there were four tugs alongside and one deployed for contingency purposes. With this marine spread, we travelled to the mating location 16 nautical miles away. This was all taking place inshore, within the fjord. The catamaran barge with the topside was safely towed out to the vertical SPAR hull. The float-over process started with the ‘entry operation’, where the topside was positioned above the substructure. We used the tugs followed by using mooring-mating wires for further fine-tuning the positioning. Once the topside was in the right position the mating operation commenced: the substructure was de-ballasted and the catamaran barge was ballasted down. In other words, another load transfer, yet this time from the catamaran barge to the substructure.”

Being prepared is key?

“Preparing for this final operation alone represents more than two years of detailed engineering. In the last four weeks of the project, we established an office on-site with more than 50 people based there, including representatives from the owner Statoil and our client HHI. We mobilized the core team on 22 November and successfully completed the dual float-over on 14 December – in just three weeks!”

Given the nature of this extraordinary project, what would you say was the biggest challenge?

“Undoubtedly the weather! Executing this operation in the Norwegian wintertime was for sure the biggest challenge. The project was extremely weather sensitive, with very strict limits on the maximum current and wind speed that could be handled. And at this time of the year, there was a low pressure front coming every few days. We had to have many go/no-go meetings to decide whether it was safe to carry out the operations in the next time window.”

The Norwegian wintertime was for sure the biggest challenge

“There are big storms at that time of the year, with gusts of up to 40 knots. In fact, we had to wait four days on one occasion for a suitable weather window. We employed a dedicated meteorologist who was providing a presentation to the team every four hours and outlining whether we could proceed to the next stage. The probability of three days of good weather was actually once every 50 days, so we were very lucky.”

Watch the video of the Aasta Hansteen Topside Transport and Installation.