In the following excerpt taken from OCIMF latest report on Offshore Vessel Operations in Ice and/or Severe Sub-Zero Temperatures in Arctic and Sub-Arctic Regions, OCIMF explains the importance of these properties in more detail.
Multi-year ice is normally harder than first-year ice owing to the amount of salt that will have leached out during warmer periods. There can also be differences in the hardness of first-year ice, depending on the formation process and source.
The amount and type of snow cover on the ice adversely impacts the friction against the hull of a vessel.
Another impediment to navigation is ice deformation.
Drifting sea ice is continually in motion, under the influence of wind, current and internal stresses, and where it becomes subjected to pressure the surface often becomes deformed.
Pressure can be caused by a number of factors but can be expected where drifting sea ice comes into contact with stationary fast ice, coastline features, cyclonic weather events and converging currents.
Where the ice is thinner, it may raft with ice sheets or floes riding over each other. Where the ice is thicker, pressure is likely to cause ridging or hummocks.
Normally, the visible top of a ridge, known as the sail, is significantly smaller than the downward extension below, known as the keel.
As ridges form, more ice is forced downwards than upwards in order to support the weight of the ice above the surface.
In deep water a typical ratio of keel to sail may be four or five to one.
The consolidated ice in a ridge is not as strong as level ice because it is made up of chunks refrozen together.
However the dimensions can be considerable and may present a significant navigational obstacle to vessels, including ice-capable vessels.
A less obvious but significant impediment to ice navigation is pressure itself. As described above, ice pressure features in the development of deformed ice but not all deformed ice is under pressure.
When a vessel encounters pressure, its ability to break ice and clear ridges is diminished, as is its ability to manoeuvre.
The hulls, rudders and propellers of vessels beset in ice may be subject to pressure forces that can become significant. When land fast ice forms over shallow waters and there are defined port approach channels, continual breaking of the ice can lead to the eventual blocking of the ice channel by refreezing of broken submerged ice chunks.
Keeping these channels clear can be enhanced by the careful selection of suitable icebreakers and propulsion systems.
Key to safe and efficient navigation in ice, even for ice capable ships, is avoiding as far as possible difficult ice conditions and maintaining freedom to manoeuvre.
In ice the shortest route is often not the safest and quickest route. Real time knowledge of the ice conditions through ice reports, observations and satellite imagery aided by ice forecasting, route modelling, weather, current and tidal informational play an important role in aiding the selection of the safest and most efficient routes.
In determining the ability of a vessel to safely operate in a particular area, the specific ice conditions should be carefully studied, with particular regard to the ice regime characteristics and anticipated weather conditions for the period of operation.
It is recommended that Masters avoid or navigate with extreme caution when working in open water close to the leading edge of pack ice, particularly during rough weather. Sections of the ice edge are liable to break off, creating a danger to ships as they have the potential to strike a vessel above an ice strengthened belt, causing significant damage.
Anchoring in ice conditions can be hazardous due to the high loads that may be imposed on the anchoring system by drifting ice, which may lead to the anchor dragging and/or damage to the anchor, cable or windlass.
Sea ice accumulation on the hull
Sea ice accumulation on the hull of the vessel, or sticking, can typically occur when the ambient air temperatures fall just below zero. However, in rare cases, it can be observed in severe sub-zero conditions.
The most typical ice conditions for sticking are young ice (grey-white ice) or first-year thin ice.
Sticking represents a particular hazard as icebreaking vessels may be capable of steaming at speeds of 10-14 knots
in light ice conditions.
The sudden onset of sticking quickly reduces their speed and manoeuvring capabilities which, when the vessel is leading a convoy or undertaking ice management operations, introduces danger of collision from any following vessels.
The intensity of the ice accumulation on the hull depends on the air temperature and the roughness of the vessel’s shell plating. Ice accumulation can be reduced by applying special coatings to the shell plating, thereby reducing its roughness.
Compressed air bubbling systems or deluge systems are installed on some vessels with the purpose of reducing friction between the vessel’s hull and the ice and may be effective in preventing ice accumulation on the vessel’s hull.
The air-bubbling system works by blowing compressed air under the water in the forward part of the vessel and from the sides, creating an ascending water flow and water turbulence around the vessel’s sides, which mechanically prevents ice formation.
Deluge systems work on a similar principle, using water flow instead of compressed air.