3.1. Ice conditionsThe hazards of m

3.1. Ice conditions

The hazards of maritime winter ice navigation typically induce loads on a vessel due to contact with sea ice (Riska et al., 2007). Finnish maritime areas may experience several forms of floating and fasted ice each winter, with the sea ice cover extending to the south from the Bay of Bothnia and to the west from the eastern Gulf of Finland. In the Bay of Bothnia, sea ice is present every year and the ice period may be longer than the open water season (Leppäranta, 1998). On the other hand, ice may form in the Gulf of Finland and the Sea of Bothnia for shorter periods, which depend on the severity of the winter (BIM, 2010).

The formation of sea ice starts with the appearance of the first small ice spicules above few centimeters of the water. The falling of snow over those ice spicules results in the creation of a layer of shuga. Waves and winds move and regroup the shuga, transforming it into what is called “new ice”. As the cooling process continues, ice nilas start to build up. These ice nilas may be from five to ten centimetres thick, and they appear in several individual fields which are later grouped into very open and open packed ice by wind and water current stresses. The very open and open packed ice continue to freeze, so that the space between packed ice diminishes, creating close packed ice.

Once sea water is no longer visible between the packed ice, the ice is regarded as level ice or consolidated and/or very compact packed ice. Sea ice is generally classified into two categories based on its mobility: drift and fast ice. Drift ice is continually in motion, affected by wind and current forces, and fast ice is firmly attached to coasts or islands. The pressure between the encountering fields of drifting ice creates a deformation of the ice surface. These deformations in thicker ice create ridges and hummocks, and the draught of these ridges can be three to five times greater than their height (Jalonen et al., 2005 and Frederking et al., 2005). This results in a situation in which the ice thickness of a ridge is several times higher than the level ice.