One of the key offshore installation contractors on the Aasta Hansteen deepwater spar development offshore Norway outlined why Steel Catenary Risers (SCRs) were chosen for the pioneering project.

Stian Sande, Subsea 7’s project director for Statoil’s project, said at the MCE Deepwater Development event in Madrid that the SCRs were chosen after considering metocean conditions in the Norwegian Sea. “Traditionally in Norway we have used a lot of flexible risers,” Sande explained. “That does not work in a conventional way when the water is as deep as it is on Aasta Hansteen – it is 1,300 m (4,264 ft).”

Flexible risers were ruled out because the top tension on the risers would be too high, and with buoyancy, they would allow too much movement for a spar platform. “So basically you need to look for other solutions and that can be Steel Catenary Risers, which is very common, it is well used, a known technology, and it is well proven,” Sande added.

“One of the challenges in the Norwegian Sea as you move north along the Norwegian coast is that you have got a fairly hostile environment, so you need a robust solution. Basically you need a very stable platform so, as Statoil progressed through their concept design, they ended up with a spar. This particular one is the world’s largest and it is 196 m (642 ft) high so it has a very deep draft, which makes it very stable. Then still [using] SCRs is a challenge in these conditions mainly due to fatigue, and to overcome that this particular spar has a very taut mooring system, which makes its footprint very small. In that way you can make the SCR work – basically by limiting the motions of the spar you limit the movement, and you limit the strain in to the risers, which is very good for fatigue.”

In other words, less movement means less fatigue, extending the service life of the SCRs.

Sande pointed out that although in the Gulf of Mexico there are generally good metocean conditions, it can of course experience severe hurricanes, although these are of short duration. “But in northern parts of Norway you have consistently bad weather for six, seven or eight months of the year. You don’t have the same extremes as you have in the Gulf of Mexico but you have reasonably bad weather more than six months of the year,” he said.

This has meant that in order to complete its contract scope Subsea 7 has a limited weather window in which to complete its offshore construction assignments at Aasta Hansteen.

Subsea 7 is basing its calculations for offshore installation work using a measure of 3 m (10 ft) significant wave height, a fairly common value for designing installation operations. The contractor is also using a 10-year extreme wave height scenario, and a one-year extreme current value or vice versa.

Nevertheless these values would still make installation at Aasta Hansteen difficult because it would produce very low sea state criteria. “But instead we have looked at weather statistics and we are using a value for current which is a 99% exceedence value, which is lower than the extreme values, and this again brings the design current down, and it increases the installation criteria. Obviously then we are removing some of the contingency or conservatism in the analysis but to compensate for that we are doing a lot of real time monitoring through the water column so that we will at any time know what the current actually is. By controlling the current [values] we can use less conservative input values,” Sande adds.

Subsea 7 is due to fabricate and install 19 km (12 miles) of rigid flowlines, along with the 12 SCRs at Aasta Hansteen under a US $380 million contract commencing in 2015. First production is targeted for the third quarter of 2017.

Offshore