Angola deepwater FPSO tailored to shipyard work processes

Pre-2000, Doris Engineering was best known for its fixed platform concepts and designs. But since the turn of the century, many of the man-hours at headquarters in Paris have been taken up with floating production projects. The recent reference list includes:

• Design and supply of the world's first deepwater riser towers (Girassol, Ang-ola) as AMG-nominated detailed engineering contractor
• Engineering, procurement, and construction of the topsides of CPTL's Farwah FPSO offshore Libya, plus design and supply of associated turret-mooring system (which Doris would like to develop and sell more widely to the offshore sector) and marine operations for the Farwah installation on final site.
• Generic FPSO hull studies for Angola, as part of the Angola Deepwater Consortium (ADC)
• Basic engineering of full Akpo field development for Total Upstream Nigeria, including FPSO and all associated flowlines, risers, subsea production systems, and associated loading buoy. A feature of the Akpo studies included close scrutiny of steel catenary risers.

Doris is now putting the finishing touches to another new concept, the Octoplus FPSO, which is under review by several operators offshore West Africa and Brazil. This project, sponsored by the European Commission, and Doris' work on ADC's behalf, were both highlighted at last year's Deep Offshore Technology Conference in Marseille.

Aside from Doris, the partners in ADC are Angola's national oil company, Sonangol, and drilling contractor Pride-Foramer. The consortium's long-term aims include technology transfer between the partners to further future Angolan offshore projects; improving the country's fab- rication infrastructure and techniques; raising production efficiency in the country's deepwater fields, and optimizing extraction in general, including from deepwater marginal deposits.

The generic FPSO hull studies were conducted between 2001 and February 2003. In light of the large numbers of FPSOs being lined up for Angolan field developments, Sonangol wanted new joint-industry efforts to identify areas where construction cost reductions could be achieved, and to determine the characteristics of a new-build floater based on these recommendations.

The first study had five main thrusts:

• A review of experience from representative FPSO projects elsewhere, based on interviews with the main operators and contractors
• A review of costs incurred from selected projects, taking into account the views of contractors
• Building schedules to illustrate the key phases of Angolan FPSO development, with a review of cycle times and the influence of different contracting strategies on the overall schedule
• Clarifying which shipping standards and practices could be applicable to ship and barge-shaped FPSOs, and recommending a consistent regulatory and technical framework for FPSOs in Angolan waters
• Selecting concept options for Angolan deepwater fields, taking into account supplier interface issues.

FPSO hull forms, cargo management systems, and utilities share similar characteristics to equipment on VLCCs. But while many shipyards have the skills and organization to build VLCCs economically, they are less geared to the requirements of FPSOs such as special coatings, materials, and pipe sections. These are needed to keep the vessel on station for many years without dry-docking, to avoid financial penalties caused by the associated loss of production.

According to the ADC study group, designs for most FPSOs with long service lives have nevertheless been derived from the VLCC (very large crude carrier) style of design, with systems and components being modified to account for the impossibility of dry-docking. Concepts must therefore also address the need for inspection, repair, and maintenance in situ. Based on discussions with shipyards, the study group came to the following conclusions:

• There are not many ways to reduce steel quantities in an FPSO hull; steel forms could be made simpler, but this would not benefit the shipyards, which have more experience building double curvature shapes
• Modifying VLCC-type ballast and cargo systems for offshore requirements makes them more complex and more expensive; hull systems are also hard to design to a lower cost
• On the plus side, VLCC and FPSO hulls are similar; there are organized production lines in place to build many of these vessels each year, and cost and schedule overruns on these vessels are limited
• Again on the negative side, shipyards typically employ a systemized organization and production line technology, with a "no change" philosophy endemic among management and the workforce
• Also, most shipyards are not as familiar as specialist offshore yards with fabrication of one-off decks and production modules, and often have difficulties accessing information from other contractors involved in the same project.

Among the study group's main recommendations, the hull should always be ordered separately from the topsides. If topsides design changes after award of the hull contract, the shipyard should be kept free from alterations that would disrupt its production routines. Interfaces setting out clearly which parties are responsible for what should be drawn up at all stages of the project; mooring system design, which is not a shipyard's area of expertise, should be left to the designer of the other surface-seabed links. Robust design assumptions should be made for the hull, which could accommodate the changes as the topsides design evolves.

Additionally, items should only be ordered from the shipyard that can be defined clearly at contract award, or that can be guaranteed for delivery at the appropriate time. In general, the project team should aim for maximum clarity of contract scope with the shipbuilder and design the vessel with dimensions that would permit construction in dry-docks of the maximum number possible of competing shipyards.

Based on these findings, Doris drew up a design for an optimum generic FPSO hull in the second ADC study. Although tankers in the Angolan region pick up typically 1 MMbbl parcels, most also have the potential to load two parcels in one session. The generic FPSO's onboard storage volume has therefore been sized at 2.2 MMbbl. The attendant deck area is sufficient for the 200,000-250,000-b/d production rates normally realized on larger fields in this area.

Offshore Angola, weather conditions are generally benign and directional, but there is a long period swell from southern Atlantic storms. A spread mooring array (at each corner of the vessel) is recommended to hold the FPSO in a fixed orientation in line with the swell path, thereby avoiding the risk of severe rolling.

The double-sided, single bottom vessel's dimensions are 323 m long, 60 m wide, and 33.6 m high, suitable for construction in a variety of shipyards. Hull ends feature flat sections that give a barge-like appearance. All crude storage tanks are in the central 250-m section of the vessel's parallel-sided part. Tanks for water ballast and utilities are positioned at the vessel's ends.