Norway’s deepest yet oilfield development is bringing a string of firsts to the Norwegian Continental Shelf, not least when it comes to the field’s flowlines. Elaine Maslin reports.
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BuBi pipe fabrication at Vigra, Norway. Image from Subsea 7. |
Statoil’s Aasta Hansteen spar development is moving Norwegian offshore operations into its deepest ever environment, at 1300m water depth in the Norwegian Sea.
The move has meant a few firsts for operator Statoil and for Norway. The Aasta Hansteen facility will be Norway’s first spar project (as well as being the world’s largest spar), the country’s first use of steel catenary risers (SCRs), the first synthetic rope mooring spread offshore Norway and the first use of mechanically lined pipe installed using reel-lay in the country.
For the latter, and for Subsea 7, the project is significant, both in terms of the scope – from engineering, through procurement and commissioning – and new technology deployment.
A rising spar
Aasta Hansteen was discovered in the Norwegian Sea in 1997, some 300km offshore, far from existing infrastructure. The development will produce the Luva, Snefrid and Haklang gas and condensate reservoirs, jointly known as Aasta Hansteen.
Under an engineering, procurement, installation and commissioning contract, Subsea 7 was contracted to procure, fabricate and install 18km of 12in mechanically lined pipe flowlines and four SCR systems, as well as overseeing the spool connections, mooring system, and umbilical and manifold installation, and performing the tow-out, hook-up and pre-commissioning of the spar.
Butting in
After a decade of development, it is the second reel-lay installation of Subsea 7’s mechanically lined pipe, BuBi, produced by Germany’s Butting. The product is used when production fluids are corrosive, which means the production lines need corrosion-resistant lining, and is an alternative to clad pipe. Clad pipe uses corrosion-resistant alloy (CRA), which is bonded to the inside of a steel pipe (a 100% CRA pipe would be too costly). Mechanically lined pipe uses carbon steel outer pipe with a CRA liner, which is hydraulically assembled inside the carbon steel pipe.
Butting has been producing such a pipe since 1994, but it wasn’t until 2013 that it was installed using reel-lay on the Saphinoá-Lula NE (formerly Guará-Lula NE) project in 2013 offshore Brazil. The challenge for reel-lay installation was to resolve issues around internal buckling of the liner during the laying process, but also during its operating life.
The main benefit of using BuBi pipe is the lower cost, says Stian Sande, Aasta Hansteen project manager at Subsea 7. “BuBi pipe is a very cost efficient way of achieving corrosion resistance. It has the right properties at a lower cost [than clad pipe].” This is largely due to lower procurement costs, he says, and a reduced lead time. “We can get BuBi pipe delivered a lot quicker than clad pipe.”
Lessons learned from Saphinoá-Lula, which saw 85km of 8in BuBi pipe installed in 2100m water depth, were used to help increase installation efficiency during the 2016 Aasta Hansteen installation campaign, Sande says. “On Aasta Hansteen, some of the challenges related to installation are the water depth and environment. It is deepwater with strong currents and severe weather conditions,” Sande says. “To optimize installation and reduce weather risk was a big focus area.”
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BuBi pipe being installed with buoyancy modules. Photo from Subsea 7. |
Subsea 7 worked with STATS Group to provide positive isolation and internal pressurization of the BuBi pipe to facilitate its reeling onto the Seven Oceans pipelay vessel at Subsea 7’s spoolbase in Norway, ready for installation offshore. In order to allow the 12in BuBi pipe to be reeled onto the vessel, the pipe was water filled and pressurized. The internal pressure is required to ensure no wrinkling of the internal liner during the reeling process.
But, as Subsea 7 uses this type of pipe in more projects, it sees greater potential for its use, Sande says. This is particularly related to place behavior of the pipe under different – and more severe – loading conditions on the pipe during production, i.e. pressure and temperature of the production fluids, which impacts its fatigue life.
“Building on our experience from Aasta Hansteen, we have done more work to investigate the behavior and capacity of the pipe when the pipe expands and increases in length and develops lateral buckling. Our work related to the buckle zones has increased our understanding of the capacity of the pipe, which means BuBi can be used in more projects,” Sande says. “We have broadened the possibilities [for its application] and this opens up more uses, including where clad pipe would have been used.”
An example of this is the product’s next application, on Wintershall’s Maria subsea tieback development. “Two years ago, we wouldn’t have used this on Maria,” says Sande, demonstrating how the improved understanding is expanding the scope of BuBi pipe. For Maria, which is in 300m water depth, in the Norwegian Sea, the first reel-lay deployment of mechanically lined pipe with a piggyback direct electrical heating (DEH) system will be seen – and also Subsea 7’s largest diameter BuBi pipe deployment to date (at 14in). Some 26km of the pipe will be laid starting in May.
SCR
Aasta Hansteen will also see Norway’s first use of SCRs and one of the first uses of spars outside the Gulf of Mexico, where conditions are not as harsh. Three, 2km-long SCRs will connect three subsea templates to the spar, with another 2km-long SCR to be used for export to the export pipeline system.
SCRs can be used with spars thanks to the reduced motion of the platform, compared to a floating, production, storage, and offloading unit, Sande says. But, the risers have to be produced to withstand the stresses they’ll still be under, which means paying close attention to welding, especially at the sag bend, near to the touchdown point, and their connection with the spar. “Which is where we came in,” Sande says, “in terms of making sure there is high quality on the welding in terms of engineering and fabrication. As a result of the quality of welding during pipe fabrication, our client could extend the design life of the risers.”
To make sure the risers could withstand the environmental conditions at Aasta Hansteen, the mooring system, with 17 lines, spread in two clusters of six and one of five, had to be designed as a taut system, to limit the motion of the spar. In another first, the mooring system was installed and the lines, made from Gama 98 polyester, by Lankhorst Ropes, have been wet stored on the seabed – a move which required qualification because this hadn’t been done before.
Ready and waiting
Over last summer, the subsea infrastructure installation campaign was completed and now just awaits the arrival of the spar for hook-up and commissioning.
The Seven Oceans was mobilized to Vigra in April 2016, with flowline and SCR installation carried out in May and June. The Normand Oceanic then installed the spool pieces, connecting the flowline system to subsea structures. The Seven Viking then handled the tie-in and commissioning with flowlines preserved ready for tie-in in 2018. Finally, the Skandi Skansen mobilized for pre-installation of mooring system, ready for hook-up in 2018.
The Aasta Hansteen hull substructure will be floated horizontally onto the Dockwise Vanguard heavy transport vessel and then transported to Norway. In Norway, it will be floated off in the Stord area, on the country’s west coast, and upended. There, the topsides – weighing 25,000-tonne, offering accommodation for 108 people – will be mated with the hull before being towed out to the field. The facility is expected to come onstream in 2018.
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