Sea ice in the far north does not cover a fixed area – its extent varies with the seasons and from year to year. Nor is it uniform. Different types exist, each with its own properties.
Icebergs break off – calve – from glaciers into the sea and then drift under the influence of winds and currents. Since their ice contains no salt, it is extremely hard. The biggest concentration of bergs in Norway’s far north is found around Franz Joseph Land north-east of Svalbard, where about 50 glaciers regularly calve. Others come from Nordaustlandet east of Spitsbergen.
Bergs can weigh several million tonnes when they start their journeys, but usually go aground and break into smaller pieces as they float south. However, even these chunks are substantial. Those calving from Nordaustlandet often go aground on the Spitsbergen Bank. The pieces float south to Bear Island, where they meet currents which carry them northwards again. Icebergs from Kong Karls Land have been observed as far south as the Finnmark coast in 1881, 1929 and 1939. They were also seen off Russia’s Kola peninsula in 2002.
The Norsok N-003 industry standard defines approximate 100- and 10 000-year limits for how far south a collision between an iceberg and an installation is likely. While the Johan Castberg discovery under consideration for development lies within the 10 000-year boundary, for instance, the Snøhvit and Goliat fields are just outside it.
Sea ice is frozen seawater. Since it contains salt, it is less hard and also imposes a lower load than an iceberg when colliding with an installation. Drift ice floats freely in small floes.
Seawater turns to ice when its temperature falls below freezing, around -1.8°C, and when calm conditions prevail. A distinction is made between first-year and multi-year ice.
The first of these is the commonest type in the Barents Sea. A higher salt-water content than multi-year ice means it is less rigid. First-year ice also drifts further south than multi-year, which is largely confined to the northernmost parts of Kong Karls Land. Sea ice occasionally blocks the north-west Russian port of Murmansk.
The marginal ice zone was visible from land on the Finnmark coast in 1881. That is as far south as pack ice has extended in historical times. In some years, the ice margin in the newly opened parts of Barents Sea South East could affect its northernmost area. This happened in one year, 2003, in the 2001-11 period.
Icebergs on a collision course pose a genuine threat to offshore facilities. In areas where bergs and sea ice might be met, good strategies and tools are needed to manage the risk.
These can include: