It was one of the most spectacular and complex missions conducted by BOURBON in recent months. Initiated in 2013, with the first installation engineering requested of BOURBON Subsea Services experts by the US Company, Exxon, this job was completed in July 2015 from the MPSV Bourbon Vissolela, off the coast of Angola, in the Gulf of Guinea.
The operation consisted of placing two cylindrical steel tendons, each weighing 8 tons and measuring 20 meters long by 200 mm in diameter, onto a Single Hybrid Riser (SHR) intended to bring the gas produced on the neighboring platforms to the shore. "The SHR is in the form of a 900 m long tube that goes from the sea bottom to the surface and is suspended by a chain under a big submerged buoy," explains Julien Boucard, one of the two engineers (with Paul Gaspard de Bovis) dispatched by BOURBON Subsea Services for this operation.
"The problem," he adds, "was that the chain looked like it was about to break and let the SHR fall to the bottom of the water. Therefore, the idea was to install two tendons on each side, so that they could take on some of the tension supported by the defective chain and that they would be able to take on the entire load in the event the chain breaks."
"Everything that was used, or almost everything, didn't exist before this job and was used only this once."Julien boucardSUBSEA ENGINEER
To prevent running into any unknowns during the final step at sea, all tools, equipment, and procedures were tested beforehand with Exxon in Houston, TX, before being shipped to Luanda for a final preliminary test phase at the jobsite. No particular problems occurred to complicate or delay the work. "Yet these are very complex operations," insists Julien Boucard. "Everything that was used, or almost everything, didn't exist before this job and was used only this once."
For this specific mission, one of the main difficulties was having to maneuver parts weighing several tons underwater, and to maneuver the two Work ROVs in an unconventional manner with regard to the tendon assembly site. "The danger for the vessel," Julien Boucard continues, "was if the chain broke during the operation, resulting in the sudden rise of the SHR buoy. The Vissolela couldn't be positioned overhead, and risk getting hit by the buoy under the hull." Finally, a real success for all BOURBON Subsea Services team which illustrates its ability to provide custom solutions.