Ice cover in the far north isn’t static, but varies in extent with the seasons and from year to year. Thanks to the Gulf Stream, the southern Barents Sea is usually ice-free all year round, but icebergs and sea ice can pose a real threat to installations if they collide.
“In purely statistical terms, sea ice and bergs can be found south of Bear Island. But the probability is low”, says PSA’s ice expert Arne Kvitrud.
“If they’re seen today, the rule is to disconnect and move away. That hasn’t been necessary since exploration began in the far north”.
According to Kvitrud, ice could theoretically occur on Goliat, for example, but the chance is so small that disconnection hasn’t been a requirement. A bit further north, however, Johan Castberg will need a way to disconnect if ice gets too close.
"The best solution is to avoid ice hitting the platform at all. It could damage risers or other structures. So the aim is to keep well clear. The most recent drilling jobs have had to be 50 kilometres from the marginal ice zone. Should that get any closer, the rig must disconnect. No operations have been so close to the ice edge".
Icing occurs when sea spray, super-cooled rain or wet snow freezes to vessels or facilities. The extreme Narve storm in 2006 caused heavy icing at the Melkøya gas liquefaction plant outside Hammerfest.
"Two types of icing occur. One comes from sea spray driven by wind blowing over the waves. The other is atmospheric icing, caused by rain or hoar frost", Kvitrud explains.
Spray is the dominant source of icing on units close to the sea surface, such as supply ships or lifeboats. On a rig or platform 20-30 metres above the surface, atmospheric icing will be the main contributor. As I say, hoar frost or precipitation causes freezing. Super-cooled rain or other types of precipitation can eventually build up layers of ice on the structure.