Elaine Maslin takes a look at the Dan Bravo complex offshore Denmark, which, at 44 years old, has a new lease on life thanks to some serious steel reinforcement.
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Divers installing the conductor guide frame. Photos from Boskalis. |
When the Dan Bravo Complex came on stream in 1972, the facility – Denmark’s first producing offshore oil field – was expected to produce for only 25 years. Forty-four years later, it is still going.
Following a multi-year subsea structural reinforcement campaign, involving the installation of 136-tonne of new steel, over more than 200 diving days, the life complex’s wellhead platform, Dan A, has had its life expectancy extended to a total 70 years.
The structural overhaul of the Dan A platform’s jacket, undertaken by Dutch offshore contractor Boskalis was akin to a 3D, subsea puzzle, involving a large subsea photogrammetry campaign, site clean-up, preparation work and the installation of temporary cranes on vertical platform members to help maneuver the new steel into place at 10-41m water depth – all while the platform was in production.
Boskalis, which was contracted for the installation, as well as fabrication, procurement, equipment testing and structural examinations, was also tasked with installing a new boat landing ladder on Dan B, and removing obsolete equipment from the Dan A and B platforms as part of the Dan Bravo Rationalization (DABRAT) program.
“This had not been done before – fortifying underwater structures in this way on this scale,” says Bert Kamsteeg, contract manager, Maersk DABRAT, Boskalis. Had nothing been done, the platform’s life would have been shortened, says Jakob Knudsen, DABRAT project manager, Maersk Oil.
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The DSV Constructor working on site. |
The Dan field was discovered in 1971, about 125mi west of Esbjerg, Denmark. Dan A and C are wellhead platforms and Dan B is the process platform of the Dan Bravo Complex. The Dan Bravo complex had in fact been ordered from the US for the Kraka field, but it was installed on Dan when Dan was found to be larger.
Boskalis’ main scope was to reinforce Dan A’s jacket, using 10 clamps, called K-node clamps, and to install a new conductor guide level – both of which would require maneuvering 15 pieces of <4-5-tonne sections of fabricated steel through slots in the platform’s structure, guided by divers.
First, the existing structure was assessed using existing inspection data and new data acquired during July-September 2013, through a photogrammetry exercise – Boskalis believes the largest of its type – with some 20,000 high-definition, overlapping photographs taken. These were then converted, using computer software, into a geometric 3D model, from which the conductor level guide could be designed.
Inspection, surface cleaning and preparation followed, including underwater grit-blasting and measuring member wall thickness, prior to installation work. Steel fabrication started in winter 2013-14 in Denmark, alongside planning work.
In order to perform the offshore operations, including transferring the new steel from the ship to the position it needed to be in, beneath the water, larger capacity cranes needed to be installed on the Dan A platform.
“The platform had a small 2.3-tonne capacity crane, but the steel weighed in subsea conditions in some cases 4-5-tonnes,” Kamsteeg says. “One of the solutions, and actually the only sensible solution, was to build up two temporary cranes, 12m above sea level, on to the vertical members of the platform, something which has not been done before.”
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Divers working on a clamp. |
First, a clamp with a pedestal was installed on to the vertical member, and a conventional knuckle boom crane was built on to it. Two cranes Boskalis already had on its vessels were used and modified so they could be remote controlled from the dive support vessel (DSV).
Installing the temporary cranes meant steel could be lowered from one of the two DSVs being used, then connected to the temporary platform crane’s hoist wire, before being disconnected from the DSV’s hook by divers and maneuvered into place, assisted by divers and rope access personnel at the platform.
The K-clamps were created to reinforce areas of the jacket structure where a horizontal member is intersected by cross members. Each clamp comprised of two sections to create a steel-to-steel friction clamp, which is held fast using steel bolts, weighing 10 kilos a piece. To install the 10 clamps, some 2000 bolts were used, each having to be handled and set by the divers using specially-built hydraulic tensioning gear and a specially designed tool basket was used for handling the bolts and nuts subsea.
Next, the new conductor guide level, measuring 3m-wide and 8m-long, had to be lowered, tilted, and slotted through the platform members before being brought back on to the horizontal and lifted into position, between the six existing conductors, then bolted in place using further sections of steel.
With the new conductor guide level installed, the old levels were removed. As part of its scope, Boskalis also replaced a number of anode bracelets, which also involved significant inspection, cleaning and preparation work, including removing marine growth, grit-blasting, and taking measurements.
The final part of the 2014 offshore campaign was removing the temporary cranes before the winter season.
“The impact on Dan A, achieving a lifetime extension to 70 years in total, is quite an achievement,” Knudsen says. “The planning and preparation was key. We did as much as we could onshore. Once you go offshore there’s a high cost involved, every minute is a lot of dollars. We have had more than 200 diving days over the last two years.”
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