Winter and Heavy Weather Presentation_Presentation_rev.0-Dd.mm.Yy

04/08/23

1

Philippine Center for Advanced MaritimeSimulation and Training, Inc.

04/08/23Philippine Center for Advanced Maritime

Simulation and Training, Inc. 2

Course Scope

04/08/23

3


Course Objective

At the end of the course the participants At the end of the course the participants will be able to demonstrate acquired on will be able to demonstrate acquired on Ship Handling in Heavy Weather Ship Handling in Heavy Weather Condition based on IMO guidance Condition based on IMO guidance through simulationthrough simulation

04/08/23

4


Course Content

04/08/23

5


Course Content

04/08/23

6


I. Introduction to Winter and Heavy Weather Maneuvering

The mariner that neglects the weather factor will undoubtedly pay the price, sooner or later. Whether in ship handling or in the protection of cargo meteorology has always been an essential consideration to the prosecution of the voyage. In the days of sail the wind was all important. Favorable winds and currents will remain basic elements of passage planning and the successful execution of the ships endeavors.

04/08/23

7



To know the subject is to avoid heavy weather and all that goes with it. Heavy rolling or heavy pitching is at the very least uncomfortable. If and when it causes damage to ship or cargo then it should have been avoided if at all possible. Such avoidance cannot take place unless seafarers know their enemy and respect it across the waters of the planet.

04/08/23

8



Ship in rough Sea

Click image to play

04/08/23

9



Winter

Winter is one of the four seasons of temperate zones.

From a meteorological perspective, winter is the season with the shortest days and the lowest average temperatures. It has colder weather and, especially in the higher latitudes or altitudes, snow and ice. The coldest average temperatures of the season are typically experienced in January in the Northern Hemisphere and in July in the Southern Hemisphere.

04/08/23

10


METEOROLOGICAL TERMS

Anabatic This term refers to the upward movement of air due to convection. An anabatic wind ascends a hillside or blows up a valley. Anemometer This is an instrument used to register and determine the velocity of the wind. Aneroid Barometer

A dry mechanical instrument for measuring changes of pressure in the atmosphere.

04/08/23

11



Anti-cyclone An area of high pressure, with clockwise calculation of air in the northern hemisphere, and anti-clockwise in the southern hemisphere, defuses an anti-cyclone Winds are generally light to moderate. Aurora This shimmering area of light is caused by an electrical discharge in the atmosphere over high northern and southern latitude...The Northern Lights are called the Aurora Borealis and the Southern Light the Aurora Australia Backing Thins means a change in the direction of the wind in an anti-clockwise sense, e.g. from north through west to south and then east. This is the opposite of veering, which occurs when the wind direction changes in a clockwise direction.

04/08/23

12



Bar An international unit of atmospheric pressure, at sea level a bar is equal to the pressure of a column of mercury 20.53in high at a temperature of 32°F at latitude 45' Barograph This instrument provides a permanent recant, in graphical form, of the continuous changes in atmospheric pressure. It may be described as a continuous recording aneroid barometer Barometer This is au instrument for measuring barometric pressure. Corrections are made to the readings for latitude, temperature, and height above sea level.

04/08/23

13



Doldrums Thu area of calm, variable winds Les between the NE and SE Trades. Occasional squalls and torrential rain may be encountered within the area. Etesian A northerly wind encountered among the Greek islands the Etesian is of katabatic organ 'Katabatic wind. Evaporation In this process water or ice air convened into an aqueous vapour.

04/08/23

14



Fog It is defined as visible vapour at the earth's surface Mists may he similarly defined except that may tends not to impede navigation to the same degree as fog. A state of fog clam when visibility n less than 1000 yd (914,4 m). Gale A strong wind in excess of 40 knots and represented by forces 8 and 9 on the Beaufort Wind Scale constitutes a gale. Cone-shaped signals exhibited by coastal stations give warning of the approach and direction of a gale.

Gulf Stream This warm water current flows front the Gulf of Mexico sip the cast coast of the United States" and then moves m an easterly direction as the North Manor Drift Current, towards the European continent

04/08/23

15



Hail A hard ice pellet, which generally falls from cumulonimbus cloud, had is usually associated with thunderstorm Hailstones vary in sire. They are built up by concentric lawn of ice forming on top of each other. One theory is that the nucleus is a particle of dust which attracts moisture, and the moisture subsequently freezes. Halo A circle of light caused by retraction which forms about the sun or moon Haze A reduction of visibility caused by dust or smoke in the atmosphere, limiting the range to about 1.25 miles (2km), and haze is not to be confused with mist, which is brought about by condensed water particles.

04/08/23

16



Horse Latitudes This term is given to the area of calm and light, variable winds between the 30th and 4.0th parallels. In general, they lie between the trade vs. mils and the prevailing westerly winds. Mirage Abnormal refraction and reflection of light rays may cause a false horizon in the burr Lavers of the atmosphere because of the differing densities of the layers. When a mirage is seen over water, distant ships may appear, sometimes upside down. Monsoon This seasonal wind blows over much of SE Asia, sometimes from the land and sometimes from the sea. In fact, it may be compared to the definition for land and sea breezes above, except that the occurrence is seasonal rather than daily, and over a much larger area

04/08/23

17



Phosphorescence This luminous effect on the surface of the water, showing bluish points of light, has never been explained satisfactorily. Polar Front This is the line of demarcation between a cold polar air mass and warmer air from more temperate latitudes. Precipitation The conversion of water vapour into visible rain, snow, sleet, hail, dew etc. is called precipitation.

04/08/23

18



Radiation This is the process of heat being transferred by wave energy. Rain This comprises water droplets, formed by the condensation of water Vapour. The maximum size of each droplet will not exceed 5.5 mm, and its maximum velocity, depending on sire, when falling will nut exceed 17.9 mph (29 kmph) Rainbow An arc formed by refracted and reflected light from water droplet in the atmosphere, it can only be seen when the observer is looking into a rain cloud or shower of rain with the sun at his back.

04/08/23

19



Recurvature of Storm Often referred to as the vertex of the path of the storm, the recurvature represents that point which is as far west as the centre of the tropical storm will reach. Also known as the 'cod' Refraction This is the bending of a ray of light when passing from one medium to another of different density. Ridge The term may be applied to a 'ridge of high pressure', indicating a bulge or extension of a high pressure area between two lows.

04/08/23

20



Sleet A mixture of rain and snow or partially melted snow becomes sleet. Snow Light ice crystals fall as snow. Squall This is a sudden change in wind velocity.., often increasing considerably over a short period of time, with little warning. It can consequently cause serious damage, especially to small craft.

04/08/23

21



Stratosphere This is the region of the atmosphere above the troposphere in which the lapse rate is about tern and in which the phenomena comprising 'weather' do not occur. The stratosphere begins at a height of sonic 11 mile at the equator.

Temperature A condition which determines heat transfer from a hot to a colder body. Temperature may be expressed in degrees Fahrenheit (oF Celsius (°C), Kelvin (°K) or Absolute (°A). Thunder This is a violent report caused by the expansion of air as it becomes heated along the path of a lightning flash.Rumbling thunder is experienced at a distance from the lightning, and may be accentuated by echoes. As sound travels through the air at 1100 ft per second and light travels at the rate of 186.000 miles per second, there is always a delay after a lightning flash before the observer hears the sound of thunder.

04/08/23

22



Tornado

A violent whirlwind about an area of low pressure, the tornado is most common in the United States, where they have been known to create considerable damage. The diameter of the whirlwind area is small, usually 5O-200m, but wind speed, may be in excess of 200 knots about the centre. Actual wind speed in the centre is zero, but updraft may lift objects into the air.

Trade Winds Permanent winds which blow toward the equator, trade winds usually measure between 3 and 5 on the Beaufort Scale. They are generally referred to as NE Trades when they blow over the North Atlantic and North Pacific from below latitude 30°N towards the equator, and SE Trades when they Wedge A ridge of relatively high pressure, sutured between two low pressure areas, it is often toughly wedge-shaped,

04/08/23

23



Tornado A violent whirlwind about an area of low pressure, the tornado is most common in the United States, where they have been known to create considerable damage. The diameter of the whirlwind area is small, usually 5O-200m, but wind speed, may be in excess of 200 knots about the centre. Actual wind speed in the centre is zero, but updraft may lift objects into the air. Trade Winds Permanent winds which blow toward the equator, trade winds usually measure between 3 and 5 on the Beaufort Scale. They are generally referred to as NE Trades when they blow over the North Atlantic and North Pacific from below latitude 30°N towards the equator, and SE Trades when they Wedge A ridge of relatively high pressure, sutured between two low pressure areas, it is often toughly wedge-shaped,

04/08/23

24



Wind The movement of air parallel or nearly parallel to the surface of the earth, the wind is named after the direction from which it comes.

04/08/23

25


FORECAST AREAS

Fig. 12.1 UK coastal forecast areas

04/08/23

26


WEATHER SCALES

04/08/23

27


WEATHER SCALES

Table 12.2 Beaufort Weather NotationSymbols Meaning

b

C

bc

deffeghkqlmopqrnsttluvwz

Blue sky with clear or hazy atmosphere, with less than one quarter of the sky area clouded

Cloudy with detached opening cloud, where more than three-quarters of the sky area is

clouded Sky area clouded over between one-quarter and

three-quarters of the total area Drizzle or fine rain Wet air with no rain falling Fog Wet fog Gloomy Hail Line Squall Lightening Mist Overcast Sky Passing Showers Squalls Rain Sleet Snow Thunder Thunderstorm Ugly threatening sky Unusual Visibility Dew Dust haze

04/08/23

28


WEATHER SCALES

Table 12.3 Wave Scale

State of Sea Height in meters

Calm – glassyCalm – rippledSmooth waveletsSlightModerateRoughVery roughHighVery HighPhenomenal

00-0.10.1-0.50.5-1.251.25-2.52.5-4.04.0-6.06.0-9.09.0-14.0Over 14.0

04/08/23

29


WEATHER SCALES

Length of swell Height in meters

ShortAverageLong

0-100100-200Over 200

Height of Swell Height in meters

LowModerateHeavy

0-2.02.0-4.0Over 4.0

04/08/23

30


WEATHER SCALES

04/08/23

31


WEATHER SCALES

04/08/23

32


WEATHER SCALES

04/08/23

33


WAVY WEATHER PRECAUTIONS (GENERAL CARGO VESSEL)OPEN WATER CONDITIONS

Stability Improve the 'GM' of the scud (if appropriate). Remove tire surface elements if possible.Ballast the vessel downPump out any swimming poolInspect and check the freeboard deck seal.Close all water tight doors.Clear decks of all surplus gear.Slack off whistle and signal halyards.Warn all heads of departments of impending heavy weather.

04/08/23

34


EFFECTS OF HEAVY WEATHER ON VESSEL AT SEA

To describe the behavior of any vessel in a heavy sea the mariner should first be aware that every vessel, depending on her build, GM, state of loading etc. will perform differently

Stiff and link

A large GM will render a vessel stiff. i.e. give her a short period of roll and subsequent damage may be sustained by rapid rolling. A small GM will render the vessel tender, i.e. she will have a long slow roll motion. The two conditions, usually brought about by incorrect loading or ballasting, should be avoided, so that unnecessary stress in the structure of the vessel when in a seaway is avoided.

04/08/23

35



Periods of Roll and Encounter Period of roll may be defined as that time taken by a ship to roll from port to starboard, or vice-versa, and back again. The 'period of roll' will be to a great extent controlled by the GM of the vessel and by the disposition of weights away from the fore and aft line. Period of encounter may be defined as that time between the passages of two successive wave crests under the ship.

Figure 12.6 Vessel with short period of roll compared to period encounter

Figure 12.7 Vessel with long period of roll compared to period of encounter

04/08/23

36



Synchronism

This is most dangerous and a highly undesirable condition for a vessel to experience and occurs when the period of roll is equal, or nearly equal, to the half period of the waves.

Synchronized pitching - when the period of encounter is similar to the vessel's period of pitch - may also occur.

04/08/23

37


GENERAL BEHAVIOUR OF VESSEL IN HEAVY 'WEATHER

The options available to a vessels running into heavy weather can be restricted to five main categories:

1.Head to sea, or with wind and sea fine on the bow, running at reduced speed.

1.Stern to sea. at reduced speed, running before the wind.

1.Heaving to, preferably in the lee of a land nun. to allow the weather to pass

04/08/23

38


GENERAL BEHAVIOUR OF VESSEL IN HEAVY 'WEATHER

1. Heaving to

2. Use of Anchors

3. Use of Sea Anchor

4. Abnormal Waves

5. Tropical Revolving Storm

04/08/23

39


ICE TERMINOLOGY

Anchor Lee Submerged ice attached or secured to the bottom is known as anchor ice. Bar Ice Ice without any snow covering. Bergy Bit A large piece of floating ice, this is between 1 in and 5 m above the surface of the water.

04/08/23

40


ICE TERMINOLOGY

Brash Ice An accumulation of broken, floating ice, this contains pieces up to approximately 2 m across.

Compact Pack Ice A heavy concentration of pack ice, where no water is visible Computed Ice Edge A clear cut ice edge this is generally found on the windward side of an area of pack ice, compacted by the action of wind or current.

04/08/23

41


ICE TERMINOLOGY

Concentration A ratio expressing the density of ice accumulation, concentration is expressed in tenths of the total area. Consolidated Pack Ice A concentration of 10/10, where the ice flows are frozen together Crack This is a splint or fracture in the ice surface, which has not parted.

04/08/23

42


ICE TERMINOLOGY

Difficult Area A general term used to describe the area as difficult for purpose of navigation. Easy Area A general term used to describe the area as not too difficult tin the purpose of navigation.

Fast Ice This is sea ice which has become 'fast' to the shore, ice wall or other similar surface. It may be formed by the freezing of sea water close inshore or by pack ice freezing to the shore or other surfaces. Should its height extend more than 2 m. it would be referred to as an ice shelf.

04/08/23

43


ICE TERMINOLOGY

First Year Ice A term derived from young ice, being sea ice of not mote than one winter's growth, this ice is between 30cm and 2m thick. Flaw A narrow dividing section between the pack ice and fast ice, a flaw is formed by the shearing of the former from the latter Floating Ice This general term is also used with regard to grounded or stranded ice.

04/08/23

44


ICE TERMINOLOGY

Floe This is a flat piece of ice more than 20m across. Floes are sub-divided according to size as giant, vast, big, medium and small.

Floeberg A massive piece of sea ice, a floeberg made up of one or more hummocks frozen together, the whole being separated from any other surrounding ice.

04/08/23

45


ICE TERMINOLOGY

Fracture

This general term is used to describe any fracture/break of unspecified length. The width of the Freak a called:

large when over 500m.medium when 200-500m, small when 50-200 m. andvery small when less than 50m.

04/08/23

46


ICE TERMINOLOGY

Growler This piece of ice shows less dun I m above the surface of the water. Its volume is less than that of a bergy “bit", and it usually has an area of approximately 20 sq. m. As a growler makes a very poor radar target, it is often very dangerous to navigation. Hummock A build-up of Ice forced up by pressure is called a hummock, and a similar build-up of broken ice forced downwards by pressure is referred to as a 'hummock'. Ice Belt A long pack ice feature, an ice belt is longer than it’s wide. Length will vary from about half a mile (1 km approx.) to more than 62 miles (100km).

04/08/23

47


ICE TERMINOLOGY

Iceberg An enormous piece of ice more than 5 mint height above the surface of the water, an iceberg originate, from a glacier and may be afloat or aground. When afloat, the greatest volume of the iceberg the beneath the surface. Ice Bound When navigation in or out of a harbour is restricted by an accumulation of ice, the harbour is said to be 'ice bound'. lce Cake A flat piece or cake of sea ice, Ins than 20 m across.

04/08/23

48


ICE TERMINOLOGY

Ice Edge This may be described as the dividing line between the open sea and the limit of sea ice (ice boundary). Pancake Ice Circular pieces of ice up to 3m in diameter and about 10 cm in thickness, pancake ice curls up at the edges when pieces crash into each other. Rafted Ice This is deformed ice caused by layer riding on top of each other. Pressure changes cause the overriding, which is more often found in young ice.

04/08/23

49


ICE TERMINOLOGY

Rotten Ice This is ice in an advanced state of decomposition, usually consisting of light small pieces breaking up continuously. Sea Ice Ice formed from freezing sea water, found at sea, is called sea ice. Stranded Ice This is ice left ashore by a falling tide.

04/08/23

50


ICE TERMINOLOGY

Tabular Berg A flat-topped iceberg in the southern hemisphere. Very Clue Park Ice A concentration of pack ice between tune- and ten-tenths coverage is described by this term.

04/08/23

51


ICE NAVIGATION

In general, when a vessel has to advance through ice areas, the progress of the ship will be dependent on:

a.The nature of the ice.

b.The qualities of the vessel, scantlings, ice breaker bow

c.construction, and motive power of machinery

d.Expertise and experience of the Master.

e.Operational qualities of navigational instruments.

f.Assistance of tugs or ice breaker vessels.

g.Ice convoy facilities.

04/08/23

52


ICE NAVIGATION

Operating in Ice

04/08/23

53


SINGLE-LETTERS SIGNALS BETWEEN ICE-BREAKER ASSISTED VESSELS

The following single-letter signals, when nude between an ice-breaker And assisted vessels, have only the modifications given in this table and arc only to be made by sound visual or radiotelephony signals.

WM Ice-breaker support is now commencing. Use special ice-breaker support signals and keep continuous watch for sound, visual or radiotelephony WO Ice-breaker support is finished. Proceed to your destination.

04/08/23

54



04/08/23

55


Ice Damage

The extent of any damage will depend on the condition the Ice the vessel is passing through. The mariner should be prepared to accept some damage to the vessel, while limiting the amount as much as possible.

04/08/23

56



The following single-letter signals, when nude between an ice-breaker And assisted vessels, have only the modifications given in this table and arc only to be made by sound visual or radiotelephony signals.

WM Ice-breaker support is now commencing. Use special ice-breaker support signals and keep continuous watch for sound, visual or radiotelephony WO Ice-breaker support is finished. Proceed to your destination.

04/08/23

57


II. Accident Case Studies Involving Heavy Weather

04/08/23

58


Heavy Weather

Heavy weather is any weather condition that results in high winds, extreme sea states, and heavy rain, snow and/or hail.

Click image to play

04/08/23

59


Maneuvering To Avoid The Storm Center

The safest procedure with respect to tropical cyclones is to avoid them. If action is taken sufficiently early, this is simply a matter of setting a course that will take the vessel well to one side of the probable track of the storm, and then continuing to plot the positions of the storm center as given in the weather bulletins, revising the course as needed.

04/08/23

60



In the Northern Hemisphere, that part to the right of the storm track (facing in the direction toward which the storm is moving) is called the dangerous semicircle. It is considered dangerous because; •the actual wind speed is greater than that due to the pressure gradient alone, since it is augmented by the forward motion of the storm, and

• the direction of the wind and sea is such as to carry a vessel into the path of the storm (in the forward part of the semicircle).

04/08/23

61



Northern Hemisphere

Right or dangerous semicircle Bring the wind on the starboard bow (045° relative), hold course and make as much way as possible. If necessary, heave to with head to the sea.

Left or Less Dangerous Semicircle Bring the wind on the starboard quarter (135° relative), hold course and make as much way as possible. If necessary, heave to with stern to the sea.On Storm Track, Ahead of Center Bring the wind 2 points on the starboard quarter (135° relative), hold course and make as much way as possible. When well within the less dangerous semicircle, maneuver as indicated above.

04/08/23

62



On Storm Track, Behind Center Avoid the center by the best practicable course, keeping in mind the tendency of tropical cyclones to curve northward and eastward.

Southern Hemisphere Left or Dangerous Semicircle Bring the wind on the port bow (315° relative), hold course and make as much way as possible. If necessary, heave to with head to the sea. Right or Less Dangerous Semicircle Bring the wind on the port quarter (225° relative), hold and make as much way as possible. if necessary, heave to with stern to the sea.

04/08/23

63



On Storm Track, of Center Bring the wind about 2000 relative, hold course and make as much way as possible. When well within the less dangerous semicircle, maneuver as indicated above.

On Storm Track, Behind Center Avoid the center by the best practicable course, keeping in mind the tendency of tropical cyclones to curve southward and eastward.

04/08/23

64


NAVIGATING IN OR NEAR ICE

Experience has shown that ships that are not ice strengthened and with a speed of 12 knots often become firmly beset in light ice conditions

Whereas an adequately powered ice strengthened, ship should be able to make progress through 6/10 to 7/10 first year ice.

The engines and steering gear of any ship intending to operate in ice must be reliable and capable of quick response to maneuvering orders

Navigational and communications equipment must be equally reliable and particular attention should be paid to maintaining radar at peak performance.

04/08/23

65


NAVIGATING IN OR NEAR ICE

Ship's operating in ice should be ballasted and trimmed so that the propeller is completely submerged and as deep as possible, but without excessive stern trim which reduces maneuverability.

If the tips of the propeller are exposed above the surface or just under the surface, the risk of damage due to the propeller striking ice is greatly increased.

Ballast and fresh water tanks should be kept not more than 90% full to avoid risk of damage to them from expansion if the water freezes.

Good searchlights should be available for night navigation, with or without ice breaker report.

04/08/23

66


PASSAGE THROUGH ICE

The ice should be entered from leeward, if possible. The windward edge of an ice field is more compact than the leeward edge, and wave action is less on the leeward edge.

The ice edge often has bights separated by projecting tongues. By entering at one of the bights, the surge will be found to be least.

Ice should be entered at very low speed and at right angles to the ice edge to receive the initial impact, and once into the ice speed should be increased to maintain headway and control of the ship.

04/08/23

67


USE OF ENGINES AND RUDDER IN ICE

Engines must be prepared to go full astern at any time.

Propellers are the most vulnerable part of the ship.

Propellers are the most vulnerable part of the ship.

Ships should go astern in ice with extreme care, and always with the rudder amidships.

If a ship is stopped by a heavy concentration of ice, the rudder should be put amidships and the engines kept turning slowly ahead. This will wash the ice astern clear.

04/08/23

68


USE OF ENGINES AND RUDDER IN ICE

Violent rudder movements should only be used in emergency. They may swing the stern into the ice.

Frequent use of the rudder, especially in the hard over position, has the effect of slowing down the vessel's passage through ice.

Too much rudder when pushing through the ice or following an ice breaker may bring the vessel to a complete stop.

04/08/23

69


ANCHORING NEAR ICE

In a heavy concentration of ice anchoring should be avoided.

If ice is moving, its tremendous force may break the cable.

When conditions permit anchoring, the windlass and main engine should be kept at immediate notice, and the anchor weighed as soon as wind threatens to move ice on to the ship.

04/08/23

70


BEAUFORT SCALE

04/08/23

71


BEAUFORT SCALE

04/08/23

72


Accidents at Sea Involving Heavy Weather

1992 - Aegean Sea

On 3 December 1992, the 114,000 tonne Greek-flagged OBO carrier Aegean Sea, carrying 80,000 tonnes of crude oil, grounded in bad weather while entering La Coruna, Spain.

The pilot was just about to board the ship when she grounded. The impact fractured the hull spilling about 74,000 tonnes which subsequently caught fire and the ship exploded. Being an OBO ship Aegean Sea had a double hull. The cause of the accident was again human error caused by faulty navigation in bad weather conditions.

04/08/23

73



1993 – Braer

Following engine failure, Braer ran aground in severe weather conditions on Garth's Ness, Shetland on 5 January 1993. Over a period of 12 days the entire cargo of 84,700 tonnes of Norwegian Gullfaks crude oil, plus up to 1,500 tonnes of heavy bunker oil, were lost as almost constant storm force winds and heavy seas broke the ship apart.

Weather conditions prevented the use of mechanical recovery equipment at sea, although about 130 tonnes of chemical dispersant was applied from aircraft during periods when the wind abated slightly and some oil remained on the surface. Oiling of shorelines was minimal relative to the size of the spill and cleanup involved the collection of oily debris and seaweed by a small workforce.

04/08/23

74



MV Princess of the Stars

The MV Princess of the Stars, flagship of the Sulpicio Lines fleet, left the port of Manila on June 20, 2008 on its way to Cebu City. The number of passengers is variously reported between 700and 800. The ferry sent a distress signal at midday on June 21 when its engines allegedly stalled in rough seas near Sibuyan Island. San Fernando mayor Nanette Tansingco sent a speedboat and confirmed that the ferry had a hole in the hull, was partially submerged and that several bodies had been found nearby. Later reports revealed that the hole in the hull was actually the ship's bow thruster.

04/08/23

75



Click image to play

Accident at Sea involving Heavy Weather

04/08/23

76


III. Drifting Pattern of Disabled Ship in Heavy Weather

04/08/23

77


Drifting Pattern of Disabled Ship in Heavy Weather

Prediction technology of the drift speed and direction of disabled ships which include extraordinary ship forms due to shipwrecked, broken, capsized situations has to be established. The hydrodynamic force has been estimated by the experimental results using the tanker model divided into 10 parts. The drifting experiment using broken ship model was carried out to estimate the drift speed and direction.

04/08/23

78



04/08/23

79



Figure 2. shows an example of steady drift simulation results on the certain sea condition. It can be found that we have 5 solutions in this case. This image is the graphic expression of Optimum Towing Support System (OTSS). Theoretical calculation method of wave drifting speed was derived from the analysis of measured drifting speed of a buoy

04/08/23

80



Drift motion simulation

Figure 1 shows the drift motion of the tanker model in regular waves. This shows the trajectory of the ship with different initial wave incident angle. It is found that the plural drift condition can be confirmed experimentally in certain wave condition.

04/08/23

81



04/08/23

82



Figure 3 shows the relation between wave slope and drifting speed and Figure 4 shows the linear coefficient of wave drifting speed of the buoy. In long wave range, drifting speed is decided by wave induced current speed, and it is proportional to the square of wave slope. Taking this wave drifting mechanism, the estimation method covering entire wave range was proposed.

04/08/23

83



04/08/23

84



Figure 5 shows the flow of the estimation method of drift resistance. This method can treat every shaped bodies. Starting from the estimation of 2-D case with different dimension and flow direction, 3-D effect of depth can be considered. It can also include the round corner effect.

04/08/23

85



04/08/23

86



04/08/23

87



04/08/23

88


IV. Freak Waves

04/08/23

89


Freak Waves

It is well known that extreme waves often occur in areas were waves propagate into a strong opposing current.

A well known example where many large ships have encountered difficulties is the Agulhas current outside South Africa. The strong current going south meets strong swell from storms in the Antarctic Ocean.

04/08/23

90


Freak Waves

04/08/23

91


Freak Waves

In areas where waves from storms in the open ocean approach shallower waters (e.g. several locations along the Norwegian coast), the waves will be refracted and diffracted as shown in the picture below (Aerial photo of an area near Kiberg on the coast of Finnmark, taken 12 June 1976 by Fjellanger Widerøe A.S.)

04/08/23

92


Freak Waves

On January 1st 1995 an extreme wave was measured under the Draupner platform (16/11-E) in the North Sea providing indisputable evidence that such waves do indeed exist.

This wave has been known in the international scientific community as the "new year wave". The maximal amplitude of 18.5 m is more than three times the significant amplitude for the wave train! The maximal wave height of 25.6 m is much more than twice the significant wave height of about 10.8 m. The time series is reproduced below with the surface elevation in meters as a function of the time in seconds.

04/08/23

93


Freak Waves

04/08/23

94


Freak Waves

Analysis of the ocean state around this waves shows that the wave train as a whole is weakly nonlinear and has relatively small bandwidth. This justifies the use of nonlinear Schrödinger equations as simplified mathematical models for wave description.

If we suppose that the wave above is long crested, we can simulate it numerically forward and backward in space. Below we show how the time series develops upstream (left figure) and downstream (right figure) in intervals of 50 meters. One characteristic wave length is about 260 meters.

04/08/23

95


Freak Waves

04/08/23

96


Freak Waves

The following important observations can be made from the numerical simulation above:

A group of a couple of large waves is visible up to several wavelengths upstream.

Close to the extreme wave crest there is an almost equally dramatic wave trough.

An observer on the platform would have seen a wall of water, twice as high as all other waves, approaching during about one minute.

04/08/23

97


Rogue Waves

Rogue waves (also known as freak waves, monster waves, killer waves, and extreme waves) are relatively large and spontaneous ocean surface waves that are a threat even to large ships and ocean liners.

Rogue’ is a generic term given to an unusually large wave appearing in a smaller set of waves. Trip reports often talk about the biggest wave (or set of waves) seen that day arriving during a beach launch or exit, damaging boats and threatening life and limb.

04/08/23

98


Rogue Waves

Click image to play

04/08/23

99


Rogue Waves

Some of the characteristics of rogue waves are: they are greater than twice the size of the ‘significant wave heights’ of surrounding waves,they are often deep water waves,they may be associated with a very deep trough and other uncommonly large waves moving in a set or ‘train’,

04/08/23

100


Rogue Waves

they often come unexpectedly from directions other than prevailing wind and waves,

they probably last only a short time or distance (minutes or a few hundred metres), and

they are unpredictable - though they do occur more frequently in some places in the world.

04/08/23

101


Rogue Waves

SO HOW DO THESE MONSTER WAVES FORM?

There are a number of factors that generate waves. Underwater seismic movements and other natural phenomena can generate huge waves (called tsunamis), but most waves are generated by wind.

04/08/23

102


Rogue Waves

Causes

The phenomenon of rogue waves is still a matter of active research, so it is too early to say clearly what the most common causes are or whether they vary from place to place. The areas of highest predictable risk appear to be where a strong current runs counter to the primary direction of travel of the waves

04/08/23

103


Rogue Waves

Suggested mechanisms for freak waves include the following:

Diffractive focusing — According to this hypothesis, coast shape or seabed shape directs several small waves to meet in phase. Their crest heights combine to create a freak wave.

Focusing by currents — Storm forced waves are driven into an opposing current. This results in shortening of wavelength, causing shoaling (i.e., increase in wave height), and oncoming wave trains to compress together into a rogue wave.

04/08/23

104


Rogue Waves

Nonlinear effects — It seems possible to have a rogue wave occur by natural, nonlinear processes from a random background of smaller waves.

Normal part of the wave spectrum — Rogue waves are not freaks at all but are part of normal wave generation process, albeit a rare extremity

04/08/23

105


Rogue Waves

Wind waves — While it is unlikely that wind alone can generate a rogue wave, its effect combined with other mechanisms may provide a fuller explanation of freak wave phenomena.

04/08/23

106


Rogue Waves

There are three categories of freak waves: "Walls of water" travelling up to 10 km (6.2 mi) through the ocean

"Three Sisters", groups of three waves

Single, giant storm waves, building up to fourfold the storm's waves height and collapsing after some seconds.

04/08/23

107


IV. Intact Stability and Wind/Weather Criteria

04/08/23

108


Intact Stability and Wind/Weather Criteria

The aim of this topic is to introduce dynamic criteria for the intact stability of ships which cover the phenomena of parametric roll and pure loss of stability on the wave crest. These phenomena can be related to alterations of the righting levers in wave crest and wave trough condition.

The dynamic criteria proposed in this report shall be seen as additional criteria to the existing intact stability criteria and they should be applied for all types of ships covered by IMO- instruments.

04/08/23

109



Principle of the Criteria

Figure 1: Righting levers of a (RoRo) ship in still water, crest and trough conditions at the actual limiting GM according to the Intact Code (left) and for the same ship according to the Damage Stability Limit(right).

04/08/23

110



The dynamic criteria reflect on the following two phenomena which may occur in rough weather: Pure loss of stability on the wave crest

Excessive roll angles due to parametric excitation

04/08/23

111



Figure 2: Areas below still water righting levers (left) and the area difference trough- crest (right) for the same ship as above.

04/08/23

112



Structure of the new Criteria

Dynamic Criteria for Pure Loss and Parametric Rolling

The areas under the still water righting lever from 0 Degree to 15 Degree (A15Still) and from 0 Degree to 40 Degree (A40Still) shall take at least the following value:

A15Still = [0.5](A15Trough − A15Crest) (1)A40Still = [0.75](A40Trough − A40Crest) (2)

04/08/23

113



where: • A15Still means the area under the still water righting lever up to an angle of 15 Degree• A15Trough means the area under the wave trough righting lever up to an angle of 15 Degree• 40Still means the area under the still water righting lever up to an angle of 40 Degree• 40Trough means the area under the wave trough righting lever up to an angle of 40 Degree• 40Crest means the area under the wave crest righting lever up to an angle of 40 Degree

04/08/23

114



Determination of the righting levers

Figure 3: Determination of the critical wave for crest and trough condition. Note that the critical wave has to be determined for the ship in full equilibrium with respect to draft, trim and heel which has been omitted in this figure for simplification reasons.

04/08/23

115



Calculation procedure, summarized

The following procedure should be applied for each draft which is relevant for the limiting GM- required or KGMAX- Curve:

04/08/23

116



The picture below shows a calculation example for the A40Crest (left) and the final righting levers of the same ship as in the figures above which fulfill the dynamic criteria suggested (right)

Figure 4: Calculation example for the determination of A40Crest (left). The resulting area is a1+a2=-0.004 mrad. The area must be calculated to 40 Degree and not be limited by the angle of down flooding.

04/08/23

117



Severe wind and rolling criterion (weather criterion)

Weather criterion

Assumptions he ability of a ship to withstand the combined effects of beam wind and rolling is to be demonstrated for each standard condition of loading, with reference to Fig 1 as follows:

Figure 1 : Severe wind and rolling

04/08/23

118



04/08/23

119



Heeling levers The wind heeling levers lw1 and lw2, in m, are constant values at all angles of inclination and are to be calculated as follows:

04/08/23

120



04/08/23

121



Angles of heel

04/08/23

122



Angles of heel

04/08/23

123



Note 1 : The angle of roll q1 for ships with anti-rolling devices is to be determined without taking into account the operations of these devices.Note 2 : The angle of roll q1 may be obtained, in lieu of the above formula, from model tests or full scale measurements.The rolling period TR, in s, is calculated as follows:

where:

04/08/23

124



04/08/23

125



04/08/23

126


1.6 Basic Ship’s Motion in a Seaway and Wave Theory

04/08/23

127


Basic Ship’s Motion in a Seaway and Wave Theory

The response of a ship to waves is very complex. Having a certain velocity of advance, a ship experiences the wave excitation at an encounter frequency. As we will see, this frequency is not related linearly to the wave frequency, as seen from a fixed point, but varies with ship speed and predominant direction between waves in a nonlinear fashion.

04/08/23

128



Ship Motions in a Seaway

Encounter frequency / period as far as ship motions are concerned, it is the period of encounter with the waves that is important rather than the absolute period of the wave

the ship is moving relative to the waves and it will meet successive peaks and troughs in a shorter or longer time interval depending on whether it advances into the waves or is travelling in the same direction as the waves

04/08/23

129



the situation can be generalized by considering the ship at an angle to the wave crest line as shown:

04/08/23

130



measured at a fixed point, the wave period is:

T = Lw / Vw

if the ship travels at Vs at a to the direction of wave advance, in time TE (encounter time), the ship will have travelled distance TEVs cos a in the wave direction and the waves will have traveled TEVw

04/08/23

131



if TE is the period of encounter, then:

if the ship travels in the same direction as the waves, the period of encounter is greater than the wave period, if it is running into the waves, the period of encounter is less

this is the frequency / period that would be seen in the spectrum of the ship motions, not the actual frequency of the wave as it would appear in an inertial reference frame

04/08/23

132



encounter spectra in head seas

04/08/23

133



Synchronous roll

when the encounter period is the same or nearly the same as the natural period of the ship, a superposition of inclining energies exists, and the result is very heavy roll

this is analogous to an elastically mounted rigid mass being forced at its natural frequency

04/08/23

134



such heavy rolling is not uncommon and it can be clearly distinguished from rolling due to a lack of stability

synchronous rolling is NOT due to a lack of stability

ship of large GM or large static righting moments are those that are more apt to encounter synchronous roll

ship of low GM are much less frequently subject to such rolling

04/08/23

135



follows from, rolling in a seaway:

WHERE:

C is an empirical constant (0.38- 0.55, depending upon ship and loading)

B is extreme beam (ft)

04/08/23

136



the roll period varies inversely as the root of the metacentric height

therefore, the greater the GM for the same ship beam, the shorter is the natural roll period

at the same time, for larger vessels, the shorter the period of roll (12 seconds and lower), the greater the probability for synchronizing with the wave period e.g. large Atlantic storm waves are 500 – 600 ft in wavelength and have a period of 10 – 11 seconds

under such conditions, a large ship of low GM would have a period in excess of the period of these waves and would be safe from synchronous roll

on the other hand, a similar ship of large GM with a period of about 10 – 11 seconds would be susceptible to synchronous roll

04/08/23

137



Coupled pitching and heaving pitch considered analogous to roll except that the axis of rotation is 90 degrees to the roll axis in the same plane

undamped natural pitch is typically between 1/3 and 2/3 of the natural period of roll

with pitch, yaw, and heave, more difficult to describe ship motion as an isolated phenomenon as you can in roll

04/08/23

138



pitch and heave are inter-related and affected by roll, yaw, sway and surge

pitch and heave motion in a real sea are coupled and produces undesirable ship operation conditions, namely: speed reduction, slamming, and wet decks and their interference with human and machinery functions

more convexity in the forward and after sections of a ship can reduce these undesirable effects

these requirements often conflict with those for high cruising speeds

04/08/23

139



Yawing

ship yaw is the result of three possible mechanisms:

inequality of static pressures on the hull orbital motions of the water in a seaway gyroscopic action

04/08/23

140



in general, the wave profile on the port and starboard sides of the ship are not the same therefore, the longitudinal position of the center of pressure on one side of the submerged portion of the ship is offset longitudinally and vertically from that on the other side

Yawing is motion around the ship's vertical axis.

04/08/23

141



this creates a rotating couple about the vertical axis – this manifests as a yawing and heeling moment

as the wave profiles change with the seas, the yawing couple changes in magnitude and direction, producing an oscillation

this oscillation occurs at the apparent period of the waves passing the ship could correct by anticipating the motion and then

compensating with appropriate rudder action

04/08/23

142



dynamic yawing action also produced by the orbital rotation of the water in a wave

as shown in the diagram, a ship moving in quartering sea or the sea at an angle to the bow is subjected to a yawing couple

as the wave passes the ship, changing form the crest to the trough at the bow and from the trough to the crest in the after portions of the ship, the couple direction is reversed

net result is a yawing oscillation with the same period as the period of encounter of the waves

rudder compensation for dynamic yaw and orbital motions is difficult – every half wavelength, the water in the vicinity of the rudder will be moving in the same direction as the ship and a reduced turning couple is the result

04/08/23

143



Why roll mitigation

small waves of frequency equal to the ship's natural frequency cause the ship to roll heavily

04/08/23

144



Motion-damping devices

all stabilization systems depend on the motion of mass and can be classified as follows: 1.type of force used

a. counterweight – gravitational forceb. acceleration – inertial force

2.location of system

a. internalb. external

3.type of mass

a. solidb. liquid

04/08/23

145



only those devices that are frequently used are discussed next for anti-roll:

bilge keels

controllable fins

anti-rolling tanks

active gyrostabilitizers

04/08/23

146



Bilge keels

model of design with twin bilge keels

04/08/23

147



long fin-like projects attached to ships along the turn of the bilge and extending from ½to 2/3 of the length

simple, well tested, economical, successful for anti-roll

continuous attachment of a single, heavy steel plate structure that projects 2 – 4 ft form the hull and roughly perpendicular to the hull surface

on large ships may be a v-shape cross-section and fitted solidly to prevent damage when docking or grounding

04/08/23

148



regardless of shape or fitting, bilge keels operate according to

a simple theory, recall:

where kx = radius of mass gyration

with bilge keels projecting from the sides of the ship, have an increased mass of water to roll with the ship, value of kx in above equation is increased

period of roll is increased

04/08/23

149



under forcing by waves, with the increased natural period the amplitude of roll is decreased overall

major effect of bilge keels is the increased resistant to roll

bilge keels more effective when moving ahead through waves than when stopped (i.e. sitting in water)

there is hydrodynamic lift created on the forward section of the bilge keels which resists the lateral forces of roll and adds stability to the ship – i.e. a special case of fixed stabilizing fins

04/08/23

150



will not get complete elimination of roll

disadvantage: added drag in forward motion

if dynamically suppressed roll is desired should use active stabilizing fins

04/08/23

151



Active stabilizing fins

04/08/23

152



used on some large ships and pleasure craft

consists of a projecting fin – one on each side at the bilge line and forward of amidships

some fins are retractable (axially or radially) and when fully extended can rotate within a limited arc in a similar manner to a stabilizing fin on an aircraft of the dive planes on a sub

fin angle-of-attack is controlled

04/08/23

153



a gyroscopic sensing device actuates the motors, which creates a response to, and anticipates, the wave roll force

transmission of motion to the fins produces, at the right time, the desired angle and results in a force at the fins that opposes the heeling or rolling wave forces

port and starboard fins operate simultaneously with

a 180 degree phase relationship to produce a correcting roll moment (i.e. one that is opposite to that created by the waves)

04/08/23

154



Effect of employing active stabilizing fins

04/08/23

155



Anti-rolling tanks

04/08/23

156



the Frahm anti-rolling tank consists of a U-shaped tank system transversely arranged from side to side (e.g. port to starboard)

when the system is half-filled with water, it is designed so that the natural period of oscillation of the water (the sloshing) is approximately equal to that of the ship (or slightly less)

motion of ship is transferred to the water which then dissipates it

located above the ship CG

effectiveness of anti-rolling tanks

04/08/23

157



success of the anti-roll tank is that the motion of the water should always be in harmony with the wave excitation

only happens if frequency of the exciting waves is equal to the natural frequency of the tank

at other frequencies motion of water can even cause an increase in roll motion; in the following graph it is evident that the roll motion has in fact doubled at 0.4 rad/s.

04/08/23

158


1.7 Accident Case Study on Parametric Rolling

04/08/23

159


Accident Case Study on Parametric Rolling

Parametric Roll - vessel motion

The head sea parametric roll is a recently identified phenomenon and seems especially likely to affect large container ships. Its occurrence depends on the wave conditions in relation to the dimensions of the ship, and leads to especially pronounced variations in stability as the ship sails through critical head or following seas.

04/08/23

160



Critical vessels

Parametric rolling occurs in large vessels characterized by the following: large flare in the fore and aftship,

flat aftership,

slim fore and aft body,

righting arm varies significant with draft

Critical conditions

04/08/23

161



In general parametric rolling occurs only under the following critical conditions: head/stern sea conditions

when the natural period of roll is nearly twice the wave encounter period, resulting in two pitch cycles per roll cycle

wavelength in relation to ship’s length

wave height exceeds critical values

the roll damping is low

04/08/23

162



Possibilities to increase the safety

A solution to increase the safety of cargo could be the use of open-top container vessels.

These have a higher freeboard and the containers are secured in cell guides.

More realistic provisions to increase the safety of cargo and vessel in practice could be: the application of higher lashing levels and optimized lashing angles the prevention of parametric rolling in service

04/08/23

163



Parametric rolling--the why and wherefore: parametric rolling has become a major problem for...

Excitation of roll motion caused by wave slope is well known and comparatively easily explained, especially when the ship is under way or drifting in beam seas. This excitation can lead to considerably greater roll angles in case of resonance, ie when the wave period is approximately equal to the ship's period of roll. However for the latest container ships and passenger vessels, with their typical hull shape, a physically different cause of roll motion becomes more and more relevant to ship operation. This occurs in longitudnal seas where the wave slope is of negligible influence--it is parametric excitation of roll.

04/08/23

164



The meaning of "parametric“

The stability moment of a ship is the product of the righting lever and its total weight. Both are values which have an influence on the stability moment. Such values are called "parameters".

In longitudinal waves both parameters oscillate, which causes a periodic variation of their product, namely the stability moment. This can trigger roll if it occurs with an appropriate period. Weight varies only within a limited range but the righting lever can undergo periodic variations to a large extent. This is the predominant cause of possible roll excitation, hence the terms "parametric excitation" and "parametric roll".

04/08/23

165



Parametric roll is a serious, but rare phenomenon that can cause major property damage, up to millions of dollars per incident.

04/08/23

166



Parametric Roll

Click image to play

04/08/23

167



Parametric resonance

- occurs when long waves hit the bow or stern of the ship, with a frequency of about twice the natural roll frequency.

APL China after parametric roll

04/08/23

168



Head-sea parametric rolling of container ships

04/08/23

169


1.8 Revised Guidance to Master for Avoiding Dangerous Situation in Adverse Weather Conditions

04/08/23

170


Revised Guidance to Master for Avoiding Dangerous Situation in Adverse Weather Conditions

1. The Maritime Safety Committee, at its eighty-second session (29 November to 8 December 2006), approved the Revised Guidance to the master for avoiding dangerous situations in adverse weather and sea conditions, set out in the annex, with a view to providing masters with a basis for decision making on ship handling in adverse weather and sea conditions, thus assisting them to avoid dangerous phenomena that they may encounter in such circumstances.

04/08/23

171



2. Member Governments are invited to bring the annexed Revised Guidance to the attention of interested parties as they deem appropriate.

3. This Revised Guidance supersedes the Guidance to the master for avoiding dangerous situations in following and quartering seas (MSC/Circ.707).

04/08/23

172


ANNEX

REVISED GUIDANCE TO THE MASTER FOR AVOIDING

DANGEROUSSITUATIONS IN ADVERSE WEATHER AND SEA

CONDITIONS


04/08/23

173


1. GENERAL

1.1 Adverse weather conditions, for the purpose of the following guidelines, include wind induced waves or heavy swell. Some combinations of wave length and wave height under certain operation conditions may lead to dangerous situations for ships complying with the IS Code. However, description of adverse weather conditions below shall not preclude a ship master from taking reasonable action in less severe conditions if it appears necessary.


04/08/23

174


1.2 When sailing in adverse weather conditions, a ship is likely to encounter various kinds of dangerous phenomena, which may lead to capsizing or severe roll motions causing damage to cargo, equipment and persons on board. The sensitivity of a ship to dangerous phenomena will depend on the actual stability parameters, hull geometry, ship size and ship speed. This implies that the vulnerability to dangerous responses, including capsizing, and its probability of occurrence in a particular sea state may differ for each ship.


04/08/23

175


1.3 On ships which are equipped with an on-board computer for stability evaluations, and which use specially developed software which takes into account the main particulars, actual stability and dynamic characteristics of the individual ship in the real voyage conditions, such software should be approved by the Administration. Results derived from such calculations should only be regarded as a supporting tool during the decision making process.


04/08/23

176


1.4 Waves should be observed regularly. In particular, the wave period TW should be measured by means of a stop watch as the time span between the generation of a foam patch by a breaking wave and its reappearance after passing the wave trough. The wave length λ is determined either by visual observation in comparison with the ship length or by reading the mean distance between successive wave crests on the radar images of waves.


04/08/23

177


1.4 Waves should be observed regularly. In particular, the wave period TW should be measured by means of a stop watch as the time span between the generation of a foam patch by a breaking wave and its reappearance after passing the wave trough. The wave length λ is determined either by visual observation in comparison with the ship length or by reading the mean distance between successive wave crests on the radar images of waves.


04/08/23

178


1.5 The wave period and the wave length λ are related as follows:

1.6 The period of encounter TE could be either measured as the period of pitching by using stop watch or calculated by the formula:

Where V = ship’s speed [knots]; andα = angle between keel direction and wave direction (α = 0° means head sea)


04/08/23

179


1.7 The diagram in figure 1 may as well be used for the determination of the period of encounter.

1.8 The height of significant waves should also be estimated.


04/08/23

180


2. CAUTIONS

2.1 It should be noted that this guidance to the master has been designed to accommodate for all types of merchant ships. Therefore, being of a general nature, the guidance may be too restrictive for certain ships with more favourable dynamic properties, or too generous for certain other ships. A ship could be unsafe even outside the dangerous zones defined in this guidance if the stability of the ship is insufficient. Masters are requested to use this guidance with fair observation of the particular features of the ship and her behaviour in heavy weather.


04/08/23

181


2.2 It should further be noted that this guidance is restricted to hazards in adverse weather conditions that may cause capsizing of the vessel or heavy rolling with a risk of damage. Other hazards and risks in adverse weather conditions, like damage through slamming, longitudinal or torsional stresses, special effects of waves in shallow water or current, risk of collision or stranding, are not addressed in this guidance and must be additionally considered when deciding on an appropriate course and speed in adverse weather conditions.


04/08/23

182


2.3 The master should ascertain that his ship complies with the stability criteria specified in the IS Code or an equivalent thereto. Appropriate measures should be taken to assure the ship’s watertight integrity. Securing of cargo and equipment should be re-checked. The ship’s natural period of roll TR should be estimated by observing roll motions in calm sea.


04/08/23

183


3. DANGEROUS PHENOMENA

3.1 Phenomena occurring in following and quartering seas A ship sailing in following or stern quartering seas encounters the waves with a longer period than in beam, head or bow waves, and principal dangers caused in such situation are as follows:

3.1.1 Surf-riding and broaching-to When a ship is situated on the steep forefront of a high wave in

following or quartering sea conditions, the ship can be accelerated to ride on the wave. This is known as surf-riding. In this situation the so-called broaching-to phenomenon may occur, which endangers the ship to capsizing as a result of a sudden change of the ship’s heading and unexpected large heeling.


04/08/23

184


3.1.2 Reduction of intact stability when riding a wave crest amidships

When a ship is riding on the wave crest, the intact stability can be decreased substantially according to changes of the submerged hull form. This stability reduction may become critical for wave lengths within the range of 0.6 L up to 2.3 L, where L is the ship’s length in metres. Within this range the amount of stability reduction is nearly proportional to the wave height. This situation is particularly dangerous in following and quartering seas, because the duration of riding on the wave crest, which corresponds to the time interval of reduced stability, becomes longer.


04/08/23

185


3.2 Synchronous rolling motion Large rolling motions may be excited when the natural rolling period of a ship coincides with the encounter wave period. In case of navigation in following and quartering seas this may happen when the transverse stability of the ship is marginal and therefore the natural roll period becomes longer.

3.3 Parametric roll motions 3.3.1 Parametric roll motions with large and dangerous roll amplitudes in waves are due to the variation of stability between the position on the wave crest and the position in the wave trough.


04/08/23

186


Parametric rolling may occur in two different situations:

.1 The stability varies with an encounter period TE that is about equal to the roll period TR of the ship (encounter ratio 1:1). The stability attains a minimum once during each roll period. This situation is characterized by asymmetric rolling, i.e. the amplitude with the wave crest amidships is much greater than the amplitude to the other side. Due to the tendency of retarded up-righting from the large amplitude, the roll period TR may adapt to the encounter period to a certain extent, so that this kind of parametric rolling may occur with a wide bandwidth of encounter periods. In quartering seas a transition to harmonic resonance may become noticeable.


04/08/23

187


.2 The stability varies with an encounter period TE that is approximately equal to half the roll period TR of the ship (encounter ratio 1:0.5). The stability attains a minimum twice during each roll period. In following or quartering seas, where the encounter period becomes larger than the wave period, this may only occur with very large roll periods TR, indicating a marginal intact stability. The result is symmetric rolling with large amplitudes, again with the tendency of adapting the ship response to the period of encounter due to reduction of stability on the wave crest. Parametric rolling with encounter ratio 1:0.5 may also occur in head and bow seas.

3.3.2 Other than in following or quartering seas, where the variation of stability is solely effected by the waves passing along the vessel, the frequently heavy heaving and/or pitching in head or bow seas may contribute to the magnitude of the stability variation, in particular due to the periodical immersion and emersion of the flared stern frames and bow flare of modern ships.


04/08/23

188


This may lead to severe parametric roll motions even with small wave induced stability variations.

3.3.3 The ship’s pitching and heaving periods usually equals the encounter period with the waves. How much the pitching motion contributes to the parametric roll motion depends on the timing (coupling) between the pitching and rolling motion.

3.4 Combination of various dangerous phenomena The dynamic behaviour of a ship in following and quartering seas is very complex. Ship motion is three-dimensional and various detrimental factors or dangerous phenomena like additional heeling moments due to deck-edge submerging, water shipping and trapping on deck or cargo shift due to large roll motions may occur in combination with the above mentioned phenomena, simultaneously or consecutively. This may create extremely dangerous combinations, which may cause ship capsize.


04/08/23

189


4. OPERATIONAL GUIDANCE

The shipmaster is recommended to take the following procedures of ship handling to avoid the dangerous situations when navigating in severe weather conditions.

4.1 Ship condition

This guidance is applicable to all types of conventional ships navigating in rough seas, provided the stability criteria specified in resolution A.749(18), as amended by resolution MSC.75(69), are satisfied.


04/08/23

190


4.2 How to avoid dangerous conditions

4.2.1 For surf-riding and broaching-to

Surf-riding and broaching-to may occur when the angle of encounter is in the range 135°<α<225° and the ship speed is higher than (1.8 √L) cos (180 −α ) (knots). To avoid surf riding, and possible broaching the ship speed, the course or both should be taken outside the dangerous region reported in figure 2.


04/08/23

191



04/08/23

192


For successive high-wave attack

4.2.2.1 When the average wave length is larger than 0.8 L and the significant wave height is larger than 0.04 L, and at the same time some indices of dangerous behaviour of the ship can be clearly seen, the master should pay attention not to enter in the dangerous zone as indicated in figure 3. When the ship is situated in this dangerous zone, the ship speed should be reduced or the ship course should be changed to prevent successive attack of high waves, which could induce the danger due to the reduction of intact stability, synchronous rolling motions, parametric rolling motions or combination of various phenomena.


04/08/23

193


4.2.2.2 The dangerous zone indicated in figure 3 corresponds to such conditions for which the encounter wave period (TE) is nearly equal to double (i.e., about 1.8-3.0 times) of the wave period (TW) (according to figure 1 or paragraph 1.4). 4.2.3 For synchronous rolling and parametric rolling motions 4.2.3.1 The master should prevent a synchronous rolling motion which will occur when the encounter wave period TE is nearly equal to the natural rolling period of ship TR.


04/08/23

194


4.2.3.2 For avoiding parametric rolling in following, quartering,

head, bow or beam seas the course and speed of the ship should be selected in a way to avoid conditions for which the encounter period is close to the ship roll period (TE ≈ TR ) or the encounter period is close to one half of the ship roll period (T E ≈ 0.5 ⋅T R ).

4.2.3.3 The period of encounter TE may be determined from figure

1 by entering with the ship’s speed in knots, the encounter angle α and the wave period TW.


04/08/23

195



04/08/23

196



Date post:	18-Nov-2014
Category:	Documents
Upload:	mckhoy10
View:	317 times
Download:	2 times

Winter and Heavy Weather Presentation_Presentation_rev.0-Dd.mm.Yy

Documents