Dredging and marine contractor Van Oord is midway through a major subsea rock installation contract for the Ormen Lange project off mid-Norway. The contract, covering 2004-2007, comprises the installation of rock for support and protection of Ormen Lange’s pipelines, umbilicals, and MEG lines from shore at Nyhamna on the west coast to the field development area.
Van Oord’s scope also extends to procurement of around 2.8 million tonnes of rock, transport thereof from quarries along the coast of Norway and installation on the seabed.
Ormen Lange, 120-km west of Kristiansund and with an estimated 400 bcm of natural gas, is the second-largest gas discovery on the Norwegian shelf. The water depth is 850-1,100 m, making this the first truly deepwater project on the Norwegian shelf. Hydro is operator during the development phase, including construction of the gas processing plant at Nyhamna as well as the export pipeline project.
Van Oord's flexible fallpipe vessel Tertnes has been contracted for rockdumping on the Ormen Lange gas export pipelines.
The field’s untreated wellstream will be transported onshore in two 120-km long, 30-in. flowlines. These pipelines have to pass over the Storegga slide edge that rises 200-300 m up toward the continental shelf in very steep slopes and causes a very hilly seabed, with peaks that jut up 30 to 60 m.
At Nyhamna, the gas will be processed before being exported in a 1,200-km long pipeline to Easington, UK.
For Van Oord, the Ormen Lange rock placement contract combines several challenging factors, including:
• Water depths from 20-850 m
• Strong sea currents of 2.5 knots
• Fierce weather conditions, which especially during the winter season, may cause wave heights of over 20 m
• Very uneven seabed
• Wide variety of soil conditions along the pipeline route, from rock to soft clay
• Small tolerances and strict survey specifications.
The company chose the flexible fall pipe technique for the rock placement. The main focus of this program last year involved the construction of pre-lay sleepers to provide support for the pipeline, thereby ensuring that the acceptable freespans are not exceeded.
By the end of 2004, nearly 80 sleepers had been constructed, the smallest involving the installation of 350 tonnes of rock, while the largest called for installation of 90,000 tonnes of rock. Water depths varied from 50-270 m.
Early this year the second phase started. Another 40,000 tonnes of gravel were installed to finish off the 2004 installed sleepers. The final required elevation then was accurately placed, and this time corrections for settlement of the sleepers into the soft soils were applied (the so called `pre-lay topping-up’ phase). The nearshore area is now ready to receive the pipelines, installation of which got under way in June.
Engineering work is well advanced to define further pre-lay support work that will be carried out in the offshore deepwater section, where water depths range from 400 to 850 m. The first post-lay protection and upheaval buckling mitigation works will be carried out for pipeline sections scheduled to be laid this summer.•
