According to project director Michel Mugnier-Pollet, Subsea 7 implemented some improvements to the riser tower fabrication process at Lobito for this project. These included maximizing construction onshore to avoid use of cofferdams as on previous projects.
"The railway on which the bundle sections are assembled onshore plunges into the bays through a wet dock. The dimensions of the top and bottom structures of the riser towers designed for CLOV meant these structures were too wide to pass through the initial wet dock inherited from previous projects such as Greater Plutonio. There were therefore two possible options:
- Either to perform both tie-ins of the HRT bundle section respectively to the top and bottom structures after the wet dock, using a cofferdam to remove the water around the tie-in joints and conduct the welding, NDT [non-destructive testing], and coating operations in a dry environment. This is what was done on previous riser tower projects.
- Undertake significant civil engineering work to widen the wet dock as required for the HRT top and bottom structures to pass through the wet dock. This was the option selected for the CLOV project. It was a significant investment, but it enabled all tie-in activities to be performed onshore, with major gains in terms of safety and the schedule."
The CLOV HRTs incorporated other new features such as polyurethane guide frames to secure the risers to the central core pipe, reducing the amount of steel required for the towers by around 300-400 metric tons (330-441 tons), with similar savings in the weight of the buoyancy foam blocks distributed along the HRT bundle. As a result, offshore installation was faster and safer, Mugnier-Pollet says.
The riser tower bundle is manufactured in sections of roughly 220 m (722 ft), the assembly of each section being as follows. Trolleys designed to support the bundle section are positioned on the assembly railway at each guide frame. Guide frame bottom clamps are installed on the trolleys. The string of the bottom WI riser is landed onto the clamps. Buoyancy foam bottom modules and the bottom-half guide frames are then installed on the top of the WI string.
Next, the core pipe and PIP risers are landed onto the buoyancy foam bottom modules and the bottom-half guide frames. Then, the buoyancy foam block top modules and the top-half guide frames are placed on the strings of core pipe and production risers. Finally, the string of top WI riser is landed onto the top-half guide frames. The top clamps are attached above the top WI riser string on the top-half guide frames. Bolting of the buoyancy foam blocks and guide frames is then applied and tensioned.
After each 220-m section is completed, it is tied to the previous section (or to the top structure for the first bundle section), and pulled in the bay at Lobito to free the bundle assembly area for the next 220-m long section. This process is repeated for each of the five bundle sections. Finally, the bottom structure is positioned on the launching railway and tied-in to the last bundle section, ready for the final pull and launch into the bay of the complete bundle assembly.
Subsea 7's construction vesselSeven Borealis installed the rigid pipe spools at the HRT bases (four spools per HRT) in two campaigns. Installation of the HRTs was performed by two Maersk tugs (one lead and one stern tug) and the Subsea 7 construction vessels Seven Eagle and Simar Esperança. Both were towed horizontally from Lobito Bay and upended on arrival at the field. "The first operation once the HRT has reached the site is to connect it to the foundation," Mugnier-Pollet says. "Then it is upended and connected to the HRT top structure, followed by connection of the flexible pipes between the top structure and the FPSO.
"Early on in the design phase we had planned a new diverless, automated clamp connection method for the connections between the flexible jumpers and the riser extremities on the HRT top structure; however, we ended up reverting to the conventional, field-proven method using divers, as with Girassol. We realized it would be too difficult to develop a new process in the time available, and in this case the water depth for the installation of 120 m [393 ft] was within diver range."
Subsea 7 has equipped each of the HRTs on CLOV with an automated monitoring system that records the following data:
- Temperature and pressure on the production spools at the HRT base
- Tension on the buoyancy tank core pipe
- Motions of the HRT top structure.
Both HRTs are designed for the life of the project, with ROVs performing inspection, repair, and maintenance on site.
The company will continue development of the concept, Mugnier-Pollet said, with interest growing for the next wave of Angolan projects in water depths down to 2,000 m (6,562 ft). "In ultra-deepwater, where stresses on the FPSO hull are greater, the concept works even better, because unlike free-hanging flexible and steel catenary risers, the tower is a self-standing structure."