Challenges of long-distance tiebacks
Ormen Lange subsea pipe installation obstacles overcome
Per Kristian Forbord, Lars Myklebost, Arne Skeie, Bård Owe Bakken, Gisle Morisbak Lund, Thore Grønvik - Acergy
Acergy was awarded the contract to install the MEG pipelines on Ormen Lange. The scope of work was laying and commissioning two 120-km (74.5-mi) long, 6-in. (15.25-cm) pipelines from the gas processing plant onshore at Nyhamna to the Ormen Lange manifold covering water depths to 850 m (2,789 ft). In addition, a 3.6-km (2.2-mi), 6-in. infield pipeline between the two production manifolds was included.
The Ormen Lange gas field is 120 km (74.5 mi) west-northwest of Kristiansund on Norway’s west coast. The field area includes the prehistoric Storegga Slide and water depths from 250 m (820 ft) to 1,200 m (3,837 ft) in the development area. The seabed in the Storegga Slide is irregular with soil varying from very stiff clay with boulders to soft clay.
Installation
The pipelines were installed by the Acergy Falcon, a dynamically positioned pipelay vessel which can install rigid lines up to 14 in. (35.5 cm) by J-lay in water depths to 1,500 m (4,921 ft). The vessel welds the pipe joints in a firing line onboard and can carry up to 2,000 metric tons (2,205 tons) of pipe joints. The joints also can be resupplied at sea. The vessel also has a Flexible Lay System for flexible flowlines. Flex lines can be stored on deck reels or below decks in a 1,600-metric ton (1,764 ton) capacity powers carousel. The vessel has two work-class ROVs for touch down monitoring and survey.
The 50-year design life of the pipeline required field joint coating. The requirement was a three-layer polypropylene system including an inner layer of fusion bonded epoxy.
Ormen Lange is characterized by uneven seabed with steep slopes in both longitudinal and transverse directions combined with strong currents throughout the water column. The current can increase without warning so the installation vessel must change the system settings at short notice.
The rough seabed bathymetry necessitated an intricate routing consisting of numerous tight lateral curves (down to 500 m [1,640 ft]) connected by short sections of straight pipelay in between. In addition, a very accurate lay tolerance was required to hit pre-laid rock berms and pre-cut trenches. In combination, this required low bottom tension so as not to pull the pipe out of its dedicated lay corridor.
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Volume 68 Issue 9
September 2008
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