Sea ice forms from sea water, at a temperature of about -1.9C. Initially, it consists of crystals of ice within the water surface, but its evolution depends on the state of the ocean. If conditions are calm, then the ice grows into sheet ice, called nilas, which thickens through the vertical growth of crystals beneath the ice. In rough water conditions, the ice develops into "pancakes" (as in the photograph) which have raised edges.

Sea ice eventually compacts into floes of first-year ice, with thicknesses of up to about 2 metres. If sea ice survives one summer it becomes second-year ice, and after further summers it becomes multi-year ice, with typical thicknesses between 3 and 4 metres. Although during summer the ice becomes weakened by melt, sea ice which survives the summer will become stronger through the process of desalination as well as thicker, with the result that the second and multi-year forms of sea ice represent a significantly greater hazard to most vessels and offshore platforms than first-year ice. Under pressure of wind and currents, and with natural (coastline) and man-made obstacles, ice will deform to create ridges and rubble fields. Pressure ridges can be 20 metres or more in thickness. In shallow water areas deformed ice can become grounded to form stamukha.

Deformed sea ice represents a significant risk to navigation and fixed operations, and to pipelines buried in the sea bed (which can become damaged by ice scouring). Key sea ice parameters of interest for oil and gas operations include total concentration (cover in tenths), concentration of multi-year ice (and other ice types),snow cover (which adds to sea ice resistance), floe sizes, coverage, thickness and type of deformed ice, distribution of stamukha, landfast ice edge, drift velocity and ice melt state (if applicable).

Nilas formed under calm conditions in a newly frozen lead surrounded by first-year ice floes.

Pancake ice formed under the influence of waves along the ice edge (courtesy, Ted Scambos, US National Snow and Ice Data Center).

Multi-year ice with pressure ridge.