Embracing towed pipeline production systems

Meg Chesshyre

November 1, 2013

Subsea 7 demonstrates the effectiveness of pipeline bundle solutions and a subsea depressor device in the North Sea.

By Meg Chesshyre

A cross-section of a typical bundle fabricated at Subsea 7’s Wick facility. Photo: Subsea 7.“Subsea 7 is currently experiencing the busiest period of bundle design/installation activity ever,” the company’s Bundle Design Manager, Martin Goodlad, revealed while presenting a paper at the Society of Petroleum Engineers’ (SPE) Offshore Europe Oil and Gas Conference and Exhibition in Aberdeen last month. A record eight bundles were installed throughout 2011-2012, with eight more confirmed installations to be completed by mid-2015, bringing the total number of bundles installed over the last 35 years to more than 70. He added that a number of the installed or currently indesign bundles are firsts for Subsea 7.

Using a number of North Sea case studies as examples, Goodlad discussed the technical and commercial benefits of bundle technology for new subsea structures, as well as for rejuvenating and extending existing ones. In all cases, coating the bundle’s interior with BuBi, a mechanically-bonded bimetal pipe developed by Butting, reduced overall cost compared with solid corrosion resistant alloy or clad pipe.

Apache’s 5.5km-long Bacchus bundle ties the Bacchus drill center back to the Alpha platform on the Forties oil field, the North Sea’s largest, which lies 170km east of Aberdeen. This was achieved by utilizing produced water to heat the production fluid, meeting challenging flow assurance requirements. Reusing the produced water as a heating medium reduces the CAPEX and operating costs of the field, as no additional heat-generating plant is required to be installed on existing facilities and very little power generation is required to support the heating.

Subsea 7’s autonomous inspection vehicle (AIV) designed in collaboration with SeeByte. Photo: Subsea 7.The 27.8km-combined length of the four bundles in the Andrew oil field, located 230km northeast of Aberdeen, is currently the longest subsea tie-back using towed pipeline bundles. These bundles connect BP’s new drill center at Kinnoull back to Andrew’s lone platform. The advantage of the bundle was that it could be installed underneath the floatel at the Andrew platform while on site, reducing development timescale. The increased flexibility of the bundle method also lessened the risk of impacting the other scheduled on-site operations.

At 155ºC, the Conoco Phillips Jasmine oil field bundles were the highest-temperature bundles designed at the time of installation in 2012. Two 4.1km-bundles tie the Jasmine wellhead platform to the Judy riser platform. The combination of the bundle’s expansion and resistance to lateral buckling made the bundle an ideal solution for this high-temperature application; both potential design issues of high axial force and lateral buckling are accommodated without need for expansion loops or buckle initiators.

At 160ºC, the Jasmine bundles are now being overtaken by Total’s West Franklin bundle, which also consists of the largest diameter carrier pipe at 56.64-in. in the fortified sections. The 6.7km single bundle connects the West Franklin wellhead platform to the Elgin well head platform. The use of a bundle meant that the respective pressures of the main and fortified zones could be fully strengthtested onshore. This could not have been done with a single flowline laid in-field. Bundle expansion reduces the build-up of axial forces, and allows field development without a cooling spool. System weight and tensile forces on the sleeve and carrier prevent the bundle from global lateral buckling.

The 5.1km bundle for BG Knarr project will be the deepest bundle installed to date at 410m. It will tie back the Knarr production manifold to the floating production, storage and offloading (FPSO) unit. The integrated design of the bundle flowlines and structures (cooling spool length) allows the system to be tuned to BG’s specific flow assurance requirements. The bundle will be installed under the drill rig while on site, reducing the development timescale, and the increased flexibility of bundle installation method again de-risked the schedule impact of other on-site operations.

A vessel towing out a bundle, with the towhead in the foreground, at Wick, Scotland. Photo: Subsea 7.The bundle for Shell Fram was the first bundle designed with two-midline structures, tying two drill centers back to a single FPSO. Unfortunately, due to poor drilling results, the Fram development in its current configuration has been cancelled. Using bundles would have reduced the number of risers.

Goodlad’s Subsea 7 colleagues Renaud Gueret and Subhajit Lahiri presented a paper in the same session on last summer’s installation of the 127km umbilical between the landfall at the Shetland Islands and the Laggan manifold. It consisted of a bundle arrangement, including two-way fiber optic communication, high voltage electricity supply, chemical supply, and hydraulic power.

The firing line in fabrication hall’s at the Wick facility. Photo: Subsea 7.The installation was particularly challenging as it consisted of laying a low density product (127mm diameter, 10.6 kg/m submerged weight) along a route with particularly high currents and a large range of water depths (20 to 600m). As a result, large umbilical deflections were expected, and a subsea depressor device (SDD) was devised to aid installation and act as a ballast to weigh down the catenary and oppose current.

The SDD was made of a weight element with a bellmouth funnel opening on each side through which the umbilical ran and was suspended from a deck winch. The original unit weighed 2.5 tons, but was ballasted with steel blocks to reach a weight of 4.6 tons to handle the unusually high currents. The wire length was tuned to control the SDD height above the seabed.

Gueret and Lahiri concluded that the use of the SDD has been successful in stabilizing the umbilical motion in the touchdown region and preventing it from being dragged from the vessel under the harsh current conditions met offshore the Shetlands. It enabled longer lengths of umbilical to be laid in higher sea-states and stronger currents for longer durations. As a result, the SDD is now the primary option for this kind of installation.

In the event of a storm, a through contingency procedure was planned to secure the umbilical in a lazy S-wave configuration; however, due to clement meteorological conditions, it was never required to be deployed.

Wick bundle fabrication site upgrade

Subsea 7 is currently investing over £5.5 million in refurbishing its pipeline bundle fabrication site at Caithness, Scotland. Established in 1978, the fabrication site is located six miles north of Wick, Caithness in the North of Scotland. The site runs 7.8km inland, covering a total area in excess of 300,000sq m and has a sheltered bay in which to launch the pipeline bundles. The refurbishment program will be carried out in two phases. It includes new, state-of-the-art equipment for five firing lines, a 230m extension to the main fabrication shop, and a new office and welfare complex at the landward end of the facility. The first phase, which began in 2012 is now complete, with the second phase of refurbishment is scheduled for completion in 2014.

AIV will be commercially available later this year

Subsea 7, in collaboration with SeeByte, software solutions provider for unmanned underwater vehicles, has developed an autonomous inspection vehicle (AIV), which will be commercially available later this year from the company’s life-of-field division. With its ability to recognise and respond to its surroundings, instinctively correcting its trajectory in real time based on information it gathers from its on-board sensors, the AIV is set to become a costeffective, low-risk inspection tool to aid field survey and integrity management and intervention activities. Subsea 7 says that the AIV, which can be operated directly from a host facility such as an FPSO or platform, or from infield support vessels or mobile rigs, will transform life-of-field projects. OE

Image Caption (top): A cross-section of a typical bundle fabricated at Subsea 7’s Wick facility.
Photo: Subsea 7

Image Caption (2nd from top): Subsea 7’s autonomous inspection vehicle (AIV) designed in collaboration with SeeByte.
Photo: Subsea 7

Image Caption (3rd from top): A vessel towing out a bundle, with the towhead in the foreground, at Wick, Scotland 
Photo: Subsea 7

Image Caption (bottom): The firing line in fabrication hall’s at the Wick facility.
Photo: Subsea 7