Tombua Landana subsea program showcases Angolan construction
Graham Sharland - Subsea 7
Last year, Subsea 7 completed subsea installation activities for Chevron’s Tombua Landana EPIC project in block 14 offshore Angola. The contract was awarded in September 2006, shortly after the completion of work for Chevron’s Lobito Tomboco development in the same region.
This was Subsea 7’s third major pipeline project off Angola; the duties in this case encompassing design, procurement, engineering, and installation of a complete “drill center to platform tie-in” system comprising five pipelines, a steel tube umbilical, all applicable connectors, and flexible jumpers.
Although Tombua Landana was technically similar to Lobito Tomboco, a key difference was the increased level of local content. This resulted in manufacturing of the steel tube umbilical, coating of 45 km (28 mi) of multiple pipe sizes, and fabrication of pipelines and pipeline end terminations all being executed in Angola.
Engineering and project support was managed from Subsea 7’s Houston office, assisted by in-country fabrication and operations support in Luanda and the offshore vessel teams. The project team overcame the complexities of dealing with supply chain partners based in multiple geographic locations to deliver all components on time, despite a climate of high market demand.
Pipe coating procedure
The execution method drew from experience and the capabilities available within Angola. With only 2½ years from award to mechanical completion, the schedule left little room for delays, especially in an overheated supplier market with rapidly increasing lead times for key components.
To accommodate the schedule, the pipe procurement subcontract was placed simultaneously with contract award. In tandem, the decision was taken, from both the project and the local content perspective, to undertake coating at a newly built plant adjacent to the Subsea 7 spoolbase in Luanda.
Subsea 7 and Chevron worked towards mutually agreeable solutions, and fusion bonded epoxy (FBE) application started in late 2007. Once completed on all 45 km (28 mi) of the pipe joints, five layer polypropylene (5LPP) wet insulation was applied. By this point, subcontracts were being placed in-country for the steel tube umbilical and in Europe for the diverless vertical connectors and flexible jumpers.
With so many packages under assembly in different countries, and with the need to ensure prompt and timely resolution of any technical or commercial issues, regular progress meetings were established, with the procurement package leads traveling monthly to each contractor.
The Tombua Landana project team used the newbuild deepwater reeled pipelay vesselSeven Oceans for pipeline installation and Subsea 7’s diver-less construction vessel Toisa Perseus for the drill center and platform tiein.
The team resolved to constantly challenge the status quo and to consider new ideas. One proposal involved dispensing with a separate survey vessel by transferring that part of the work to theSeven Oceans. Although initially seen as prohibitively expensive, it proved technically feasible and having access to a vessel capable of a broader range of activities during pre-commissioning and survey work helped reduce the overall number of vessel days on the project. To ensure any issues were identified ahead of the offshore campaign, a program was instigated onboard the vessel to trial and test the ROV and applicable systems prior to departure for Angola.
Local manufacture
Early in the bid process, Subsea 7 committed to fabricating the pipeline end terminations (PLETs) in Angola. As the design phase progressed, use of a local fabricator became the preferred option and a subcontract was placed with a company in the Sonils Base in Luanda. The 10 PLETs ranging in weight from 16 to 22 metric tons (17.6 to 24 tons) were assembled using Angolan and South African fitters and welders. All factory acceptance testing and site integration testing was completed prior to arrival of the pipelay vessel.
Pipeline fabrication was undertaken at Subsea 7’s spoolbase in Luanda, developed in 2006 to support the Lobito Tomboco project. The site underwent a multi-million dollar upgrade in 2007 which included additional firing line and tie-in stations, and improved plant, equipment, and environmental conditions.
To facilitate the drilling schedule, execution of offshore works was performed in two phases. Phase 1 took place at the Tombua South drill center in mid-2008, with Phase 2 occurring at the platform in early 2009 following installation of the compliant tower.
Mooring spread
Pipelay for Lobito Tomboco used the pipelay vesselSeven Navica. However, with the arrival of the much larger Seven Oceans at the Luanda spoolbase, an enhanced seabed mooring arrangement was designed and installed to provide an all-weather mooring capability for the vessel during pipe spooling. This worked well, with no weather downtime on the vessel during spooling works and no stalk tie-in welds requiring repair.
Rigid flowline installation was successful, with accurate placement of the pipelines within their required lay corridors and the respective end terminations with their target boxes. Due to technical issues in using a standard wet insulation field joint at the start and end terminations, a novel cast polypropylene (PP) half-shell and injected polyurethane (PU) joint arrangement was deployed.
Tie-ins for the Tombua South locations were undertaken immediately following pipelay, using theToisa Perseus.
Phase 2 was undertaken early in 2009, resulting in the installation of the five platform jumpers from the seabed PLETs to the platform risers at 90 ft (27 m) off seabed, and pull-in to the platform I-tube of the steel tube umbilical installed by theToisa Perseus during Phase 1. A successful system leak test concluded both the Phase 2 activities and Subsea 7’s offshore scope in March 2009.