Structural Icing

StormGeo Shipping Insights, November 2011
By Mike O’Brien, StormGeo Shipping, Senior Operations Manager

The approach of winter in the northern hemisphere brings a variety of weather challenges to shipping. Mid latitude systems become more intense with increasingly heavy seas along with colder sea and air temperatures. The phenomenon we are concentrating on here, structural icing, is largely defined by the sea temperature, the air temperature and the wind speed. Structural icing is most common in the northern hemisphere shipping lanes during the months of January, February and early March over the NW Atlantic and the Gulf of St. Lawrence, the Baltic Sea, the Bering Sea and along the Kuril Islands of the western Pacific.

What is structural icing and what is the danger?
What is structural icing? Structural icing occurs when water spray, blown by the wind, freezes to the deck, deck equipment and superstructure of the vessel. The accumulation of ice on deck poses an immediate, obvious risk to the crew operating on deck. A more ominous risk, however, exists when icing continues to build on the deck resulting in a change in the stability and/or trim of the vessel due to the weight of the ice. If a vessel continues to experience the building weight of the ice on the forward portion of the vessel, she could quickly become trimmed down by the head or become “top heavy’. Once these changes to the vessel stability take place there is an increased risk for catastrophic vessel motions, especially if the sea conditions are rough.

How do we forecast where structural icing will develop?
Forecasting when there will be a risk for structural icing requires the input of predicted conditions for the air temperature, the sea temperature and the wind speed. Once these values are determined, the risk for structural icing can then be evaluated. With colder air temperatures, colder sea temperatures and heavier winds, the risk for structural icing will increase. At a minimum, there generally needs to be air temperatures below zero degrees centigrade, sea temperatures below five degrees and winds above 18kts before there is any significant risk for structural icing.

Structural icing rates are defined as:

Light = less than 20.32 mm per 3 hours
Moderate = less than 60.96 mm per 3 hours
Heavy = greater than 60.96 mm per 3 hours

In addition to these environmental factors that make structural icing possible, another key ingredient is the heading of the vessel, in relation to the wind. Given environmental conditions that are favorable for moderate structural icing, a vessel heading into the wind will accumulate ice more readily than a vessel with the wind on the stern. The breaking waves on the bow create more spray, freezing on the vessel deck and superstructure.

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BVS Displays the Risk for Structural Icing
StormGeo's BVS 6 software contains a variety of tools to aid in voyage planning while at the same time providing a wide range of forecast information. It contains a structural icing forecast which is available to the Master by selecting the appropriate icon in the easy-to-use BVS graphical display. The Master can quickly see if there will be a risk for light, moderate or heavy structural icing along the vessel track. The system uses a graduated color coding on the display map to clearly indicate where the risk for these conditions will exist.

StormGeo strives to continuously improve our systems and the addition of the structural icing feature in BVS 6 is an example of our commitment to vessel safety.