Akpo Project: The Successful Implementation of Steel Catenary Risers
Authors: Gueveneux H. 
Published: July 2010

After Bonga and Erha, Akpo is the third SCR network installed and the largest in term of numbers of risers in deep offshore Nigeria. The paper explains why SCR concept was selected and addresses all the imposed challenges in order to achieve the simplicity of the SCR solution in particular with respect to improved fatigue design, qualification of critical equipments etc. The paper depicts the strategy and measures implemented by Total from conceptual studies to start up (with their contractors) to make the SCR solution a viable one, which can be used on future deep offshore developments.

The construction of the Akpo field sub-sea infrastructures has been one of the largest installation operation ever undertaken involving more than 1,000 installation vessel days, which had to be carefully coordinated with up to 3 development drilling vessels and works on the FPSO. Start up and ramp up of production of Akpo field occurred on 03rd March 2009.

AKPO Development

TOTAL, is the Operator on OML 130 with a 24% interest. The partners are Petrobras of Brazil, Sapetro of Nigeria, CNOOC of China and NNPC of Nigeria.

Akpo is a standalone development based on a spread moored Floating Production Storage and Offloading (FPSO) moored in 1314m water depth with a storage capacity of 2 million barrels of condensate. The stabilized condensate is exported via offloading lines and a deep water buoy located 2 km away from the FPSO.

The Akpo development involves five reservoirs, deposited in a complex channels and lobe arrangement, situated at depth ranging from 2950mSS to 3750mSS. The reservoir fluid is a critical fluid, which implies that the fluid type is dependent on pressure, temperature and depth. The liquid produced is light oil/condensate (42° to 53° API), with a high gas-liquid ratio (GLR = 1600 to 7300 scf/bbl). Pressure maintenance in the reservoirs is necessary, with water injection in 4 of the reservoirs and gas injection in one.

The development includes 44 subsea wells (22 production, 20 water injection and 2 gas injection) drilled in clustered subsea architecture. For the initial production start-up, around half of these wells are required to be online for production and reservoir pressure maintenance purposes.

The subsea network includes 4 production loops (with 9 manifolds), 1 gas injection line (with 1 manifold), 4 water injection lines (using in-line tees), 1 gas export line connected to the FPSO by steel catenary risers and the associated static and dynamic umbilicals (refer to Figure 1 - Akpo development scheme).

The Akpo FPSO topsides processing facilities are typical of a high pressure / high temperature gas development but are still rare on FPSOs. They include a single process train of four stage separation (with the first one operating at 80 bars and 100 deg C) and an electrostatic dehydrator to meet the BS&W specification. The unstabilized condensate is heated (using hot water produced from waste heat recovery units installed on the power generation units) to achieve the RVP of 10 psia. The produced water is treated to achieve oil-in-water content of less than 30 ppm. The associated gas are recovered using electrically driven centrifugal compressors, dehydrated in a glycol (TEG) contactor, metered and further compressed to the gas export pressure of 200 bar and part to the gas injection pressure of 400 bar.