Jubilee deploys flexible subsea system

April 1, 2011
INTECSEA has completed owner’s engineer services for the subsea system of the Jubilee deepwater development offshore the Republic of Ghana. The Jubilee project achieved first oil in late November 2010 within three-and-one-half years of discovery.
Fasttrack project calls for robust subsea systems and cohesive team management from conceptual engineering through first oil

Cody Moffitt
Mac McKee

INTECSEA, a WorleyParsons Group company

INTECSEA has completed owner’s engineer services for the subsea system of the Jubilee deepwater development offshore the Republic of Ghana. The Jubilee project achieved first oil in late November 2010 within three-and-one-half years of discovery.

The landmark project is Ghana’s first major offshore development and was one of the industry’s largest deepwater discoveries in 2007. Water depths in the development area range from 3,600 ft (1,100 m) to 5,600 ft (1,710 m).

Located in the West Cape Three Points and the Deepwater Tano blocks, the field development is 37 mi (60 km) offshore Ghana. Kosmos Energy, technical operator, led an integrated project team (IPT) for the development. The IPT also included key project personnel from partners Anadarko Petroleum Corp., Tullow Oil, and Ghana National Petroleum Corp. Tullow, the unit operator, has built in-country infrastructure and a production operations organization. The IPT handed over the facility to Tullow just prior to first oil in fall 2010.

Compressed schedule

The fasttrack project called for robust subsea systems, a lot of anticipatory planning, and cohesive team management from conceptual engineering through first oil. First, as front-end subsea engineering/design (FEED) contractor and subsequently as owner’s engineer, INTECSEA integrated its personnel and interface management system into the Jubilee project.

The Jubilee integrated project team selected the turret-mooredKwame Nkrumah FPSO to support field development offshore Ghana. INTECSEA optimized the subsea system field layout based on the turret-moored vessel. Nine risers are hung from the FPSO turret with riser anchors on either side of channel banks.

The project’s subsea conceptual and front-end design launched in February 2008, and was completed in July. During this time, the engineering firm developed a synergistic team for the project’s technical challenges and compressed schedule. The firm subsequently transitioned to owner’s engineer, per its customary role, to become an extension of the IPT. In this role, it represented the interest of the owner throughout detailed design, construction, and installation of the subsea system.

Work scope

The scope of work in the conceptual phase included field layouts, a design basis document, technical specifications, and bid packages. The firm also participated in technical reviews of bids for supply of valves, connectors, and flexible risers, as well as installation of the subsea infrastructure. In the execution phase – when major supply contracts were awarded – INTECSEA seconded engineers to the IPT and a senior manager served as facilities engineering manager on the IPT.

Following contract awards to equipment suppliers, the firm provided package engineering for the risers and other major equipment delivery packages to facilitate the fasttrack schedule. The firm also completed detailed design engineering and, working with the IPT, optimized overall field layouts and flow assurance; conducted operability analysis; and supported the IPT installation and commissioning staff. Engineering services continued to be implemented in this manner throughout 2009, leading up to first oil.

Subsea systems

The Jubilee subsea design philosophy called for robust, flexible systems that would incorporate and adapt to evolving parameters as new data became available.

The integrated project team installs the production drill center’s two manifolds offshore Ghana. The Jubilee subsea system includes eight manifolds: five production manifolds; two water injection manifolds; and one gas injection manifold.

The fasttrack project, therefore, commenced design and layout of the subsea development using minimal field data, prior to any site-specific detailed survey, bathymetry, or metocean results.

The subsea architecture for the current project phase includes:

  • Nine risers: four production, two gas injection/lift, two water injection and one gas export
  • Two riser bases: each with four 12-in. diameter production headers and one 10-in. diameter gas injection/lift header
  • Five production manifolds
  • Two water injection manifolds
  • One gas injection manifold
  • Nine production trees
  • Six water injection trees
  • Two gas injection trees.

Infield flowlines are rigid, with wet insulation on the production lines. The risers are flexible pipe with external flotation and are anchored for stability. The riser configuration of a pliant wave eliminates most, if not all, issues associated with crossing a very active channel, which traverses the field north to south.

Using this configuration, the risers hang from the FPSO turret with riser anchors on either side of the channel banks.

Channel challenges

A major early challenge called for a development scheme incorporating data available in early 2008 while envisioning proposed architecture adaptable to either a spread-moored FPSO or a turret-moored FPSO. The channel that runs directly through the heart of the Jubilee development is the region’s largest: in some cases 500 ft. (152 m) deep and nearly 1 mi (1.6 km) wide.

As an extension of the Jubilee integrated project team, INTECSEA provided owner’s engineer services to deliver a robust subsea system. Viewed here is the Jubilee subsea architecture at the riser base east location. The field was developed in water depths from 3,600 ft to 5,600 ft.

Plateaus on both sides of the channel exhibit mega-ripples, near-surface faulting, and other features indicating a geohazard-rich environment. These issues, discovered during the later stages of initial FEED, affected FPSO mooring positions but had minimal impact on field architecture, other than minor flowline routing modifications.

Bathymetry showed these features on a rough scale, prompting INTECSEA to reconfirm the conceptual field layout following completion of survey work and final procurement of the FPSO.

Turret-moored FPSO

Notably, both survey and preliminary geotech and metocean data were received within the same time frame during which the IPT made its final decision on a turret-moored FPSO.

The team then optimized the field layout based on a turret-moored vessel located north of the drill centers and over the center of the channel.

Effective positioning of the FPSO averted placement of risers or flowlines through the channel and mitigated issues because of the channel.

Pliant configuration of the risers facilitates landfall on both sides of the channel and provides lateral stability. Production and gas injection risers terminate at riser bases, allowing for gas lift without additional subsea architecture or additional risers.

Subsea development scheme

Design of the riser bases allows for connection of additional production flowlines and risers in future development phases, and facilitates gas lift of the production lines if pressure and water cut require it.

Riser bases at the east and west sides of the channel include two production risers and one gas injection/lift riser, with provision for two more production risers at each riser base.

Two water injection risers terminate at a pipeline end termination (PLET) and can inject more than 230,000 b/d of treated water from the FPSO to maintain reservoir pressure. A water injection flowline connects the risers to two daisy-chained water injection manifolds on the west side of the channel.

The gas injection line on the east side sends gas into two injection wells and provides gas lift to the production risers as required. The west side of the development does not have gas injection wells, dedicating the west side gas injection riser to gas lift operations.

Fasttrack

Early start-up called for an aggressive project schedule and field-proven designs with standard equipment as much as possible. The IPT chose horizontal trees and standard four-slot manifolds, along with standard control packages for these components.

To mitigate the impact of long lead times for subsea trees on the project delivery schedule – just prior to commencing engineering in January 2008 – the partners purchased four FMC horizontal trees in a standard configuration that FMC previously furnished for one of the IPT partners. This decision facilitated final procurement of trees and controls. Nevertheless, delivery of the remaining subsea infrastructure proved challenging.

Major contractors with known reliability for on-time delivery supplied the remaining infrastructure. The IPT purchased major components, including valves, connectors, and piping for riser bases and manifolds, for free issue to selected contractors for fabrication of the subsea structures.

Mitigate flowline walking

Design reviews during the execution phase revealed a potential for flowline walking and expansion that might overstress some of the subsea equipment. Finite element analysis models indicated sufficient risk of this such that the IPT mitigated the concern late in the installation campaign. The IPT subsequently relied on INTECSEA, in joint effort with installation contractor Technip, to mitigate the risk of impairment to the installed subsea architecture. With the mitigation elements installed, Jubilee achieved first oil in November 2010.

Commissioning

During the later stages of the project INTECSEA personnel supported installation, hook up, and commissioning, as well as development of commissioning and start-up procedures and providing onsite assistance.

WorleyParsons Singapore joined the project in May 2010, to provide engineering services to the IPT for theKwame Nkrumah. WorleyParsons Singapore assisted with technical queries and commissioning procedures during the push to first oil and provided assistance with final documentation review to the IPT. 

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