A rising spar

Elaine Maslin

October 1, 2016

Elaine Maslin reports on the progress of Aasta Hansteen, Norway’s first spar development.

Aasta Hansteen topside under construction at HHI. Living quarters in place. Images from Statoil.

Statoil’s Aasta Hansteen spar development will move Norwegian operations into a new deepwater environment. The spar will be moored in 1300m water depth in the Norwegian Sea – the deepest previous project is Shell’s Ormen Lange, at 900m. It will produce from the Luva, Snefrid and Haklang gas and condensate reservoirs, jointly known as Aasta Hansteen.

While the topsides are conventional, this will be Norway’s first spar project – also the world’s largest spar – and the first use of steel catenary risers (SCR) in the country. Three SCRs will connect three subsea templates to the spar, with another SCR to be used for export to the export pipeline system.

In another first for Norway, the spar’s 17 mooring lines, spread into two clusters of six and one of five, are made from Gama 98 polyester, by Lankhorst Ropes, and are some 2.5km-long, each. When awarded the contract, Lankhorst said that it was the largest deepwater mooring rope contract.

The spar hull is also unique. Unlike others, it is fitted with storage space for condensate from the Aasta Hansteen fields, which will be exported by shuttle tanker. Gas will be piped through the Polarled pipeline to the Shell-operated Nyhamna gas plant. Statoil is also using mechanically lined pipe, another first for the North Sea (Reel lay gets real attention, OE: January 2015). Some 19km of 12in BuBi-Pipe has already been installed, using reel-lay.

Construction of the spar is nearing completion at Hyundai Heavy Industries (HHI) in Ulsan, Korea, under a consortium deal with Technip. The topsides are also under construction at HHI. Transport from Korea, and mating of the topsides and spar will take place during 2017. Meanwhile, subsea construction is complete, with just platform tow-out and hook up on the field remaining in 2018.

Torolf Christensen, project director, Aasta Hansteen, Statoil, says that for this project, new technology solutions were required on the Norwegian Continental Shelf (NCS).

Initially, a number of options were assessed, including ship-shaped and circular FPSOs. But, a moored spar with SCRs came out as the best solution. “This is a different development solution from what had been used before,” Christensen says. “On the NCS, flexible risers are usually used. [In the North Sea] a spar in combination with SCR and this mooring system gave the best solution, for the forces that will be put on the risers. The wave and current pattern in the [Aasta Hansteen] area exceed what we have been exposed to before on the NCS. We have experience from high waves and high currents, but the combination of the two and the deepwater is unique on Aasta Hansteen.

“The fact that we are now also in 1300m depth means the normal size flexible risers normally used on the NCS would be far too heavy and wouldn’t stand the forces and movements. We would have to have a small size flexible, which would increase the number of flexibles needed to something not very smart. Using SCRs was an elegant solution.”

Aasta Hansteen, as it will look. Artists' impression.

The SCRs and flowlines were fabricated at Subsea 7’s spoolbase in Vigra, Norway. Marine operations to install the subsea infrastructure ran through 2015 and this summer. Christensen says that the marine operations were taken to the next level. “We have been through two seasons of marine operations with really good success,” he says. “Subsea 7, the primary contractor, delivered excellent results.”

Instead of guidewires to place equipment on to the templates in the fields, so-called toast rack guides are an integrated part of the template structures.

Christensen has been most impressed by the 45,000-tonne, 200m-long, 50m-diameter spar substructure construction. It is being constructed laying on its side on the ground, with sections weighing 2.5-6000-tonne then lifted in. Some five mega-lifts were required to complete the construction, all with the hull laid on its side. The final block installed was the top deck weighing 4500-tonne using a 10,000-tonne barge crane. It has been like a massive puzzle, Christensen says.

“There has been some fantastic precision work on the structure,” he says. It contains 52 different compartments, all needing welding, painting, etc. “The steel tolerance is 4-5mm in 3D, but everything fit every time it was lifted [into place].” The tolerances were so tight that one mega-lift was done in the morning, before the steel could get warm and potentially expand.

The hull substructure will be floated and then transported to Norway on the Dockwise Vanguard horizontally. In Norway, it will be floated off in the Stord area west coast of Norway and upended. There, the topsides – weighing 25,000-tonne, offering accommodation for 108 people – will be mated with the hull in Digernessundet, Norway, at Stord before being towed out to the field. The living quarters were fabricated in the Netherlands.

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Aasta Hansteen Spar construction