SNEPCo's Bonga North West (BNW) Project has successfully installed and commissioned the Gulf of Guinea's first brownfield deepwater triple manifold daisy chain system, integrating two different subsea Vendor's production and water injection systems in a unique achievement. Two new-build 90Te 4-slot subsea manifolds were connected to an existing end-of-line Bonga Main 4-slot production manifold via a retrofit Cross Over Module (COM), using a series of pipe-in-pipe flowlines and sleds. This paper will describe the technical and organizational enablers that were identified, developed and implemented to successfully achieve this complex task in over 1000 m water depth offshore Nigeria.
Integration of the two new subsea production manifolds to the existing subsea system was achieved via the COM. In principle, the design of the COM is a dual header fixed hub system that bridges the new subsea infrastructure to the existing manifold, originally proposed to enable concurrent operations and new field tie-in. The COM is equipped with an isolation arrangement giving double isolation to each of the dual headers, to enable continued Bonga Main production after COM installation and prior to completion of other BNW facilities. This decoupled and de-risked the short COM installation operation from the lengthy BNW facilities installation phase, giving schedule flexibility, and minimum production deferment.
Removal of the selected Bonga Main Pigging Loop Module (PLM), located at all end manifolds, was required without hydrocarbon leakage to the environment. Various options for displacing stabilized flowline hydrocarbons present within existing flowlines, necessary for replacement of the PLM by the COM, were considered. Treated produced water available on the Bonga Main FPSO to flush the flow loop was selected for the displacement process, and was readily achieved by re-directing produced water into a dedicated clean slop tank on the FPSO. Displacement (de-oiling) of the production flowlines using this method enabled the regulatory water quality requirement of 30ppm required for discharge into the environment to be met. Good collaboration between various specialized teams in the replacement of the PLM with the COM, covering Operations, Intervention, Subsea Surveillance, Flow Assurance and Subsurface, was critical to the overall delivery success of the project.
Integration of BNW subsea hardware and flowlines with existing Bonga Main subsea infrastructure was completed safely and seamlessly to give first oil on time and with minimal disruption to existing production. New flow assurance challenges associated with the triple manifold daisy chain system were observed during early operation and will be discussed further herein.
Significant insights and learnings were gained regarding in-country fabrication, specifically concerning onsite dimensional control and achievement of fabrication tolerances.
