2B) I

THE KEY STAGES IN THE FORMATION OF SEA ICE UNTIL IT IS ONE YEAR OLD :

SEA ICE:There

are two kinds of sea ice encountered at sea. Sea ice formed from seawater & icebergs, which break off from the
glaciers & form shelf ice. The cooling of surface water increases its density & it sinks, to be replaced by warmer, less dense
water from below. The process continues until the whole column of water from top to bottom has attained its maximum density
& all conventional descent ceases. Until this stage is reached ice cannot form at sea it forms more readily where the water
column is stratified into layers of different density. In this situation conventional sinking is confined to the topmost layer.
Before ice crystals begin to forming, sea water must attain a sub zero temperature. Fresh water freezes at a higher temperature
than sea water. Sea water with a salinity of 34 % for example must be chilled to 1.89C. the surface of the ocean is chilled by
the polar air, but is not just the air-sea contact that must be chilled to the freezing point. As surface ocean water cools and
increases in density, it sinks and mixes with the slightly warmer and deeper water lies immediately below. This requires that an
upper mixed layer of surface water must cool to slightly below the freezing point before sea ice formation begins. Fortunately
the surface waters of polar regions are characterized by low salinities. This low density surface water effectively creates a lid
on the heat that is contained within water below the mixed layer. Without the strong density gradient in the surface waters, the
mixed layer would progressively deepen as it is cooled. The fresh water seal, however, necessitates only a shallow mixed layer
be chilled to sub zero temperatures. Once the surface waters are chilled to the freezing point, the very first ice crystals begin to
form. Microscopic spheres of pure ice form first. Under calm conditions, they can grow into long needles of hexagonal dentric
stars. The ice crystals grow in size and begin adhereing to each other. When the ice aggregates become large enough to be
seen by the human eye, the the sea ice is called frazil ice.further freezing in calm conditions can produce thin (<10 cm) sheets,
called Nilas, formed from consolidations of frazil ice at the surface. Calm seas are rarely the case, however , and the frazil ice
is typically stirred through several meters of the upper ocean, giving it a greasy or grainy textural surface appearance called
grease ice. As the icy surface layer thickens further, ice growth eventually dampens significantly the wave action allowing ice

FRAZIL ICE

GREASE ICE

NILAS
SEA ICE
chunks to meld together. Aggregations of slush are compressed and compacted together and discoidal ice chunks form and
begin to harden, freezing still larger chunks together. As sea ice solidifies, dissolved constituents are for the most part expelled.
New ice salinity is strongly dependent upon the rate of ice formation. Typically, however, two thirds of the salt is initially
expelled and by summer, processes have produced first year ice with only 5-10% of its original seawater content.Therefore the
formation of sea ice can be a lengthy process, especially in deep water with high salinity. Sea ice first forms in shallow water
where the delay due to conventional sinking is least.

Stage 1:
Seawater freezes at about 1.89C depending on salinity. At certain stage of freezing brine is trapped in small pockets in ice. At
this stage this newly formed ice is relatively soft & pliable.
Stage2:
As the new ice is subjected to further freezing in winter, salt gradually drains out through vertical channel like openings linking
the pockets of brine. The rate at which salt is lost will depend on temperature.
Stage3:
After about one year or more depending on temperature all salt is lost & the ice becomes pure fresh water ice. This ice
gradually grows in thickness & after two more summers may grow into an average thickness of 3.5 or more. At this stage it is
extremely hard, & has the characteristic blue green colour.
New ice: A general term for recently formed ice which includes Frazil ice, Grease ice, Slush, Shuga, Ice rind, Nilas,
Pancake ice (Thickness to 10cm)
Young ice: a term which is used for grey ice and grey white ice. Usually 10 30 cm in thickness. Sea ice is the transition
between new ice & first year ice. Grey ice10-15 cm, Grey white ice15-30cm.
First year ice: Sea ice of not more than one winters growth developing from young ice 30cm-2m.Thin first year (White
ice) 30cm-70cm.
Medium first year 70cm-120cm.
Thick first year over 120cm.
2 B) II PACK ICE
Pack ice is the ice which is free to move under the action of wind and waves . Found in open water and when under 7/10ths
concentration often termed as drift ice. One tenth to three three tenths concentration is sometimes known as sailing ice or more
often very open pack ice. The pack can be described as very open pack ice (with an ice concentration of 1/10 to 3/10), open

pack ice (4/10 to 6/10), upto 7/10ths the pack ice still remains navigable, close (7/10 to 8/10, composed of floes mostly in
contact) very close (9/10 to less than 10/10) and consolidated (10/10) with no open water visible).
Pack ice is reasonably free to move under the action of wind and current. The concentration of the pack normally decreases in
the last few miles towards outer (oceanic) edge of the ice, but this decreases depending on the lately prevailing winds over a
few days.
PANCAKE ICE
Predominantly circular pieces of ice from 30 cm to 3m in diameter & upto about 10cm in thickness with raised rims due to the
pieces striking against one another. It may be formed on a slight swell from grease ice, Shuga or slush or as a result of breaking
of the ice rind, Nilas or, under severe conditions of swell or waves of Grey ice. It also sometimes forms at some depth, at an

interface between water bodies of different physical characteristics, from where it floats to the surface; its appearance may
rapidly cover wide areas of water. Pancake ice consists of flat pieces, roughly circular in shape, often with a rim round the edge
due to rubbing against adjacent pieces.
ICE RIND
A brittle shiny crust of ice formed on a quite surface by direct freezing or from grease ice, usually in water of low salinity.
Thickness to about 5cm.Easily broken up by
the wind or swell, commonly breaking into
rectangular pieces.

2B) III

FORMATION AND MOVEMENT OF A TYPICAL ANTARCTIC ICEBERG

In the antartic icebergs are produced from shelf ice, the seaward extention of the antartic ice cap. Of the many ice bergs
encountered in the southern hemisphere, the main type of is a tubular shape. These are defined as large flat topped ice bergs,
which have usually calved from an ice shelf. They will vary considerably in size upto 30 miles in length and having an average
height above sea level of 40 to 50 meters. The general appearance of antarctic bergs is white providing the observer with a
plaster of paris effect, given off by the white bubbly ice, common to this regions bergs. Occasionally bergs will be sighted and
described as black or of a greenish black appearance. The composition of these is often in a banded form or distinctive layers.
Black and white ice bergs are often encountered in the weddell sea.
TABULAR BERGS - This is the most common form and is the typical berg of the Antarctic of which there is no parallel in the
arctic. They are largely derived from ice shelves and show a characteristic horizontal banding, are flat topped and rectangular
in vertical cross section. Tabular icebergs may be of great size, stretching for miles in length. The average height of bergs tends
to be about 50 metres above the water. It is also probable that the calving of the larger tabular bergs is caused by the sea surface
disturbances due to underwater movement of the crust of the earth or directly by earths tremors originating within the
Antarctic lands mass.
BLACK AND WHITE BERGS - These are of two kinds, which are difficult to tell apart at a distance. The two types are morainic
in which the dark portion is black and opaque containing of mud and stones and the second type is bottle-green in which the
dark part is deep green and translucent with mud and stones appearing to be absent.

There are large concentrations of bergs close to their source region. Since the sea temperature is low they do not melt
significantly. Inside the Antarctic Circle the prevailing current carries the bergs W to WNW. This track is more northerly near
the Antarctic Circle. North of 63 S the bergs are carried by the Southern Ocean Current. There is thought to be very little
seasonal variation.

TYPES OF ICEBERGS:

1.
2.
3.
4.
5.
6.
7.
8.

Tabular: Long, flat-topped icebergs that are longer than they are high by a ratio of 5 to 1 or more.
Non-tabular: All other icebergs, which include those types listed below:
Blocky: Flat-topped, steep-sided icebergs with a length-to-height ratio of 2.5 to 1.
Drydock: Icebergs marked by a prominent U-shaped slot flanked by twin towers of ice.These are carved by wave erosion.
Dome: Smooth, evenly eroded icebergs that slope down to the water on all sides.
Pinnacle: The classic, craggy iceberg, marked by a prominent tower or pyramid.
Wedge: A flat-topped iceberg with steep vertical sides on one end and sloping on the other.
In addition to these forms, there are smaller chunks of ice called "growlers" or "bergy bits." They might range in size from
as small as a Mazda Miata to as large as a house and are often considered dangerous to shipping since they're very low in
the water, invisible to radar, and can easily be hidden by choppy seas.

2B) IV

THE MAIN HAZARDS TO A SHIPS OPERATIONAL EFFICIENCY AND SAFETY DUE TO ICE ACCRETION

The formation of ice on a vessels superstructure and deck causes a number of hazards
Ice increases ships KG thereby reducing GM of the vessel. Also increases displacement of the vessel reducing freeboard
and the angle of deck edge immersion. and an increased likelihood of capsizing if the vessel is heeled by the action of
wind and waves
Building up of ice due to head wind can result in extra weight forward which will cause bow to dive more heavily into seas
which only helps to add more weight.

 An a-symmetrical formation of ice on ship can cause vessel to list and causing reduction in all aspects of stability.
Radio and radar failures may result from icing of aerials. Ice accretion can cause mooring problems because of winches
being covered up with ice and hydraulic lines getting frozen up.
Can also cause damage to deck machinery.
Can also cause possible breakdown of communication due to icing up of aerials/antennae.
Dangerous for crew to work on deck and reduction in effective working time due to cold conditions.
Slippery decks can also be considered a hazard due to ice accretion
All these hazards affect the seaworthiness and maneuverability of a ship. If ice accretion continues and ice cannot be removed
their is a great danger of instability and possible capsizing
2B)V

ESTIMATE THE RATE AT WHICH ICE WOULD ACCUMULATE IN GALE FORCE WINDS IF THE SEA TEMPERATURE IS

AND AIR TEMPERATURE IS

-8C USING THE DIAGRAMS IN THE MARINERS HAND BOOK

A heavy rate of icing should be expected(>2cm/hour)

+1 C