lecture - ice accretion

8/12/2019 Lecture - Ice Accretion

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Ice Accretion


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DefinitionIce accretion refers to the accumulation of

ice on the ships superstructure due to the

joining of solids or liquid.

Sometimes referred to as:Freezing rain/fog;

Icing;Rime;

Black ice.


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Rime is a white or milkyand opaque granular

deposit of ice formed bythe rapid freezing ofsupercooled water dropsas they impinge upon ancold exposed object.

Black iceis a thin sheetof clear ice or glaze,which is rather dark in

appearance. Black ice isoften mentioned in roadweather forecasts


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Heat supplied to sample

Temp

eratureC

335 kJ

420 kJ

2260 kJ

Changes of state of water

0

100

SOLID LIQUID VAPOUR

SOLID LIQUID VAPOURSolidification Condensation

Liquefaction Vaporisation


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Source Air Temperature

Sea spray -2CRain/drizzle 0C

Fog/black ice 0C

Conditions for Freezing


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Rate and Causes

The rate of accretion is categorised as:

Light: less than 0.7cm/hour

Moderate: between 0.7 and 2cm/hour

Heavy: greater than 2cm/hour

Small amounts are caused by:

Precipitation

Fog

Sea spray - Wind less than force 7

Large amounts are caused by:

Sea Spray - Wind greater than force 7


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Conditions

In order for ice accretion to occur:

the ships structure must be at the same

temperature as the air;

the water must be cooled to its freezing point

through conduction with the ships structure;

the latent heat released through freezing to be

dispersed through the ships structure; once ice has formed on the surface then the

latent heat is dispersed by the air.


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Amount of Accretion

The lower the air temperature the greater the accretion:

Due to the ability of the structure disperse energy.

The lower the sea temperature the greater the

accretion: Due to the spray requiring less cooling to reach freezing point.

The higher the wind speed the greater the accretion:

Due to the greater amount of spray and the wind chill factor.


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Probable Areas

High wind speeds are likely to be

encountered near middle latitude

depressions.

Lower the sea temperatures will be

encountered in high latitudes.

Low air temperatures with Arctic/Antarctic

winds or wind blowing off frozen landmasses.


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Case study


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Forecast areas


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High Seas Forecast

HIGH SEAS BULLETIN FOR METAREA 1 ISSUED AT 2000 ON MONDAY 09

JANUARY BY THE MET OFFICE, ABERDEEN, UNITED KINGDOM STORM

WARNING AT 09/1200 UTC

LOW 47N 39W 1010 EXPECTED 58N 18W 963 BY 10/1200 UTC. WINDS WILL

REACH STORM FORCE 10 TO HURRICANE FORCE 12 IN THE SOUTHERN

QUADRANT WITHIN 300 MILES OF CENTRE FROM 10/0900 UTC.

AREA FORECASTS FOR THE NEXT 24 HOURS

DENMARK STRAIT CYCLONIC 5 TO 7, BUT NORTH GALE 8 AT FIRST IN

NORTHEAST. SNOW OR SNOW SHOWERS. GOOD FALLING POOR IN

SNOW. LIGHT TO MODERATE ICING AT FIRST IN NORTHEAST.

TEMPERATURES MS02 TO MS05

NORTH ICELAND CYCLONIC BECOMING WEST OR NORTHWEST 7 TO

SEVERE GALE 9, THEN BACKING EAST 4 OR 5, INCREASING 6 OR 7 IN

SOUTH LATER. RAIN AND SNOW. MODERATE OR GOOD, OCCASIONALLY

POOR. LIGHT TO MODERATE ICING AT FIRST IN NORTH. TEMPERATURE

MS01 TO MS04
http://www.metoffice.gov.uk/weather/marine/highseas_printable.htmlhttp://www.metoffice.gov.uk/weather/marine/highseas_printable.html


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Sources of data

http://polar.ncep.noaa.gov/mmab/oper.html

Ship Structure Ice accretion model

http://weather.gmdss.org/VI.html

The Mariners Handbook NP100
http://polar.ncep.noaa.gov/mmab/oper.htmlhttp://polar.ncep.noaa.gov/marine.meteorology/vessel.icing/http://weather.gmdss.org/VI.htmlhttp://weather.gmdss.org/VI.htmlhttp://polar.ncep.noaa.gov/marine.meteorology/vessel.icing/http://polar.ncep.noaa.gov/mmab/oper.html


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Ice Accretion Graphs

lecture - ice accretion

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