Arctic Meteorology and Oceanography

Vessel IcingEffects and Causes

Click to play animation.

Vessel icing from sea spray can result in loss of stability and even capsizing of ships, particularly smaller vessels. Ships with large superstructures and low freeboard are particularly vulnerable.

Sea spray icing is caused by a combination of several environmental factors, including

• Air Temperature
• Water Temperature
• Freezing Temperature of Water
• Wind Speed
• Wind Direction, Relative to the Ship
• Wave Characteristics
  • Wave Size
  • Wave Length
  • Wave Propagation Direction

Icing occurs when sea spray becomes supercooled and freezes on contact with the ship. It can start to occur when the air temperature falls below the freezing point of water, -1.7°C (29°F) for sea water. Icing potential increases as air and water temperatures drop and wind speed increases. Icing rarely presents a problem if the water temperature is above 6°C.

The most intense icing conditions occur within 200 km of a shore or ice edge with strong, cold offshore flow. As air moves offshore, it gradually warms in response to heating from the sea. Once the air temperature rises above freezing, icing does not present a problem.

Wind and wave characteristics, relative to the direction the ship is heading, determine how much sea spray strikes the vessel superstructure. Travelling faster and into the wind and waves will increase sea spray reaching the deck and superstructure, and thus increase icing. Smaller boats will be affected more strongly. Ships that have been "cold soaked" in very cold temperatures for several days may experience more severe icing than otherwise expected.

Vessel IcingPrediction

Overland (1990) developed a method that predicts icing for vessels 20 to 75 meters in length that are steaming into the wind. Icing rates can be classified as follows:

This nomogram was developed from the Overland icing algorithm. It shows results for a water temperature of 3°C, typical of the Bering Sea. In this case, a windspeed of 20 knots at a temperature of -10°C results in moderate icing.

Icing nomograms are available at: http://www.weather.nps.navy.mil/~psguest/polarmet/vessel/predict.html#nomogram

Overland's icing prediction method also provides the basis of forecast products produced by the National Weather Service. They use weather parameters forecast by the GFS to create icing predictions like that shown here.

NOAA Vessel Icing Prediction Website: http://polar.ncep.noaa.gov/marine.meteorology/vessel.icing/.

Vessel IcingAvoidance and Mitigation

Conditions that lead to icing are associated with cold air advection within 200 km of shore and cold water temperatures. If you can avoid these conditions, you will avoid most icing. If you cannot, the best thing to do is seek immediate shelter. If shelter is not available, steer the vessel downwind to minimize sea spray.

Once ice has formed, it can be removed with a variety of tools, the best of which are baseball bats and large wooden mallets. The wooden tools cause less damage to the ship and equipment than metal tools, but having extras is good as they often break. Remove ice as soon as possible, before the buildup becomes dangerous.

Chemicals (like rock salt) can help prevent the buildup of ice or aid in its removal. Heaters may be used to remove ice from small, critical areas, but are not effective for removing large amounts of ice.