An important milestone in Apache's UK North Sea Bacchus field development was reached recently with installation of a 6.7km pipeline bundle. The job was overseen by Flexlife, whose operations director John Marsden here describes how it was planned and executed.
In 2009, Apache awarded Flexlife a subsea engineering and pipeline operational support contract to oversee the management of Forties subsea projects and routine maintenance of subsea pipelines and subsea infrastructure.
Via Apache's main Bacchus contractor Subsea 7, Flexlife managed the engineering, procurement, fabrication and installation of the pipeline bundle and a riser caisson. Integrated with towhead fabrications at each end (weighing 190t and 106t), the 6.7km, 40.5in diameter pipeline is one of the longest pieces of North Sea infrastructure installed in recent years.
The Subsea 7 contracted bundle comprises two insulated 6in piggable production lines, two insulated 4in heating/produced water reinjection lines, a gas lift line, scale inhibitor and control system lines. The latter include electricalpower and instrument cables, high and low pressure hydraulic lines, a chemical injection line and a methanol line.
Subsea 7 fabricated the bundle at its Wester site facility in northeast Scotland. After the towheads were integrated to the pipeline bundle, there was a fourweek programme of onshore tie-ins and testing followed by a site integration test programme to prove control system functionality.
A specialist fleet of vessels mobilised to Sinclair's Bay near Wick to undertake the bundle launch, overseen by Andrew Hoggarth, Flexlife's Bacchus project manager. Under control of site based Launchmasters, the launch began with two leading tugs being connected to the large 190t Bacchus towhead via preinstalled wire rope pennants. When weather and tide conditions were suitable, the 6.7km long bundle assembly was pulled into the sea via a launchway specifically installed on the beach.
With the bundle floating several metres above the seabed, a trail tug was connected, the site holdback winch released and control passed to an ROV support and command vessel. The move to the field began using the Controlled Depth Tow Method, with the tugs applying sufficient power to place the bundle into a suspended catenary.
The command vessel towmaster was provided with real time information on bundle position, depth, shape and catenary via an acoustic link with the bundle's in-built ‘data highway'. This allowed tow parameters to be adjusted to maintain full control of the bundle to compensate for effects of movement, weather and tide.
Arriving in the field, the bundle was laid down in a pre-surveyed parking area where final checks and weight control adjustments were made before the towfleet was reconfigured by placing one of the two large tugs at each end for the final ‘off bottom' tow to the installation area. In off-bottom tow the bundle floats 4-5m above the seabed and is moved at 1-21/2 knots to its final destination, being a pre-surveyed area with a target box at each end. The acoustic monitoring system was again deployed to ensure accurate positioning of both bundle ends.
Later this year, over a number of diver interventions, the bundle will be tied in to the platform and the three new wells and then tested. Tie in to the platform is achieved via the 149t, 48in diameter riser caisson installed last September.
Compared with an individual pipeline concept, the bundle solution is a cost effective means of controlling pipeline temperature to yield field-life corrosion, hydrates and wax management benefits. It yields significant vessel time cost savings by avoiding multiple individual pipelay installations. The piggable lines enable production rates to be optimised throughout field-life and allow individual wells to be routed to the test separator when necessary. Additional subsea connections are provided to facilitate installation of potential future tiebacks to maximise hydrocarbon recovery.
In support of the Bacchus development, Apache awarded Petrofac a contract for brownfield engineering, procurement, fabrication and offshore construction of the topsides modifications. The scope includes tie-ins to process and utilities systems, new subsea control, emergency shutdown, fire and gas and safety systems.
Weatherford Production Optimisation was awarded the contract to provide the subsea control system, comprising tree and manifold mounted control modules, hydraulic power unit and a spare tree set. GE VetcoGray is overseeing themodification and testing of the subsea horizontal trees.
Bacchus, a May 2005 discovery in Central North Sea block 22/6a, is operated by Apache (50%), partnered by Endeavour (30%) and First Oil (20%). The field has an anticipated 15-year life and peak production is estimated to be 18mbop/d and 5.7mmscf/d gas. The jackup rig Rowan Gorilla VII is expected to arrive at location in June to start drilling the three producing wells. OE
About the Author
John Marsden has 14 years' experience in the oil & gas industry, mainly in the subsea sector. Having previously worked for Amec on rig construction, before moving to Wellstream then Subsea 7, he founded Flexlife with two business partners in Aberdeen in 2007 and serves as its operations durector. Marsden has a BEng (Hons) in marine technology.