FLOATING PRODUCTION Foinaven flowlines connected remotely in 600 meters water depths

Ashley Leng
SubSea Offshore

The world's first guidelineless, ROV-based flexible flowline pull-in and connection has been successfully completed in BP's Foinaven Field west of the Shetland Islands.

Aberdeen-based SubSea Offshore managed the work, in cooperation with the Foinaven Subsea Alliance comprising BP, McDermott, Coflexip Stena, Alcatel, Cooper Oil Tools, and GEC.

The first flowline was connected at the end of 1995 using SubSea's DMaC (diverless maintained cluster) flowline and umbilical connection ROV tooling system. It was the first of over 70 planned connections to be undertaken as part of the field development, which lies in the Faeroes/Shetland trough area in 400-600 meters of water.

Working from the MSV Iolair in harsh weather conditions, 12 flowline jumpers were deployed either for immediate pull-in and connection or for wet storage. The first jumper, a 30 meter long, 6 x 6x 2-in. multi-bore flowline, was connected between the tree and the manifold.

Other connections subsequently completed included a 160m long 10-in. flowline jumper between the manifold and rigid export line. With Iolair now in dry dock, equipment has subsequently been relocated to an alternate mobile drilling unit.

The DMaC system is designed to facilitate phased deepwater production and so allow early oil at low installed cost. Wells are clustered in close proximity around a manifold, enabling simultaneous ROV-based flexible flowline pull-in and connection as well as drilling operations from the same semisubmersible.

Pre-positioned flowlines are pulled in to porches on subsea equipment via the ROV pull-in tool. The pull-in tool skid is equipped with two hydraulically-driven rope winches. The ROV locks these ropes into the subsea porch and the vehicle then maneuvers over and latches to the flowline pullhead. Winches are then used to steer and pull the flowline over the seabed to the porch for final alignment and connection.

A complementary development is an umbilical connection tool (UCT) with which umbilical jumpers are deployed and interconnected using a dedicated connection tool attached to an interface skid.

Jumpers are first lowered to the seabed in a deployment basket, the umbilical being specially configured in the basket and secured during deployment by an ROV-operable clamp. The ends of the umbilical jumper are fitted with termination plates with electric and hydraulic couplers, then secured into two parking receptacles attached to the basket.

Once equipped with the tool, the ROV subsequently docks to the basket and unlocks the clamp with a variable buoyancy system enabling it to maintain trim and fly with the plate to the termination hub on the tree. The plate is finally connected to the tree using a Retlock clamp.

For Foinaven, SubSea Offshore also designed and built two change-out tools for common component retrieval and maintenance such as control pods, choke inserts and valve blocks. Components vary in size and weight up to a maximum of 2.5 tonnes.

Each tool is identical and is equipped with an interface kit of parts for alignment, pick-up and weight requirements particular to each component. Tools engage on a docking and weight exchange interface standardized for all trees, manifold and SUT structures. They are operated in similar fashion to the DMaC pull-in tool, i.e. they are deployed on the seabed in a basket with the replacement component.

Equipped with an interface skid, the ROV flies and locates the tool, latches and releases from the basket. The entire, neutrally buoyant system is then maneuvered to the tree and docked to the interface. The component is then unlocked, drawn up and latched into the tool while pre-determined weights are deployed at the tree interface, making the entire system once more neutrally buoyant.

This is followed by unlatching of the tool which is flown back to the basket where the exercise is repeated in exchange for a replacement component. Meanwhile, the replacement tool can also be simply converted to enable guidewire operations.

The ROV tooling systems are the key components in the overall DMaC system which was first tested last fall in Scapa Flow in a series of trial pull-ins of 12-in. flexible jumpers on a seabed dummy tree and manifold.

They have been under final development for BP by the FUEL SubSea Offshore Alliance formed in January 1994. SubSea Offshore has been responsible for design, manufacture and testing of the ROV tooling which comprises flexible pipeline pull-in, umbilical connection and pig launcher/receiver recovery tools.

FUEL itself has been involved with the DMaC concept since its inception in 1989, working initially with Esso and subsequently as part of a Joint Industries Project to enhance system capability with BHP (Americas), BP, Exxon and Statoil. The Alliance has undertaken conceptual and detail design of tools and subsea architecture and has also manufactured and supplied the flowline hub alignment and connection porches for Foinaven.

The Alliance was formed in response to BP's development plans for Foinaven which rely heavily on an ROV-based pull-in and connection system. BP recognized at the time that no suitable field-proven system existed and so decided to accelerate the building of a DMaC system for offshore trials.

Intensive design, manufacture and testing followed, including simulated dry pull-ins and wet mating of ROV and skids in SubSea Offshore's test tank. The system was mobilized aboard the semisubmersible Sovereign Explorer to the Foinaven Field, where flowline jumpers and umbilicals were successfully pulled in and connected to dummy structures. The Alliance produced a fully working offshore system in under 10 months.

Experience gained in the trials was subsequently reviewed and incorporated into the development of two new systems. The trials also provided BP with the necessary confidence to adopt the system for Foinaven as well as evaluating it for future field developments.

As result, the DMaC flowline connection system now forms part of a suite of tools designed to meet all diverless needs of future BP projects, including control pod/insert valve change-out, umbilical jumper installation/connection and pig launcher/receiver recovery.