Stability Talkie Talkie (1)

Explain why it is beneficial to have small stern trim when entering dry dock

List stability and stress data required to be supplied to ship under the current Load line Regulations, stating for each how such information might be used.

The load line regulations require the master of the ship is to be provided with information relating to the stability of the ship. This usually takes the form of Stability Information Booklet which contains all that is needed to safely manage the vessels stability.

The required information is as outlined as below:-

1) General Particulars

This includes the ships name, official number, and port of registry, tonnage, dimensions,displacement, deadweight and draught to the Summer Load line. Useful as a reference in supplying information to various official organizations such as Port Authorities, canal authorities etc

2) General arrangement Plan

This usually consists of a profile and plan views of the ship showing the location of all compartments, tanks, store rooms and accommodation. Used to locate and identify individual compartments.

3) Capacities and Centre of Gravity of cargo, fuel, water, stores etc:

This will show the capacity and the longitudinal and vertical centre of gravity of every compartment available for the carriage of cargo, fuel, stores, fresh water and water ballast.

This information is required for

Transverse stability calculations (to calculate ships KG) and

Longitudinal Stability calculations (to calculate ships LCG).

Also used to calculate the space available for items of deadweight such as fuel, water, cargo etc.

4) Estimated weight and disposition of passengers and crew:

Of particular relevance to the passenger ships. For use in transverse and longitudinal stability.

5)Estimated weight and disposition of deck cargo including 15% allowance for timber deck cargo)

For use in transverse stability calculations involving calculation of the ships KG and GM.Used effectively so as to ensure vessel complies with the load line regulations throughout the voyage.10) Deadweight scaleA diagram showing the load line mark and load line corresponding to the various freeboards, together with a scale showing displacement, TPC and deadweight for a range of draughts between Light and Load condition.

Particularly useful when loading cargo (eg, comparing draught to estimate cargo loaded)

11) Hydrostatic particulars (Displacement, TPC, MCTC, LCB, LCF, KM)

A diagram or table showing the hydrostatic particulars of the ship such as Displacement, TPC, MCTC, LCB, LCF, KM et.

Particularly useful for a variety of stability calculations including transverse stability and longitudinal stability (eg., worksheets for the calculation of GM, trim and draughts forward and aft)

12) Free Surface Information (including an example)

Usually in the form of Free Surface Moments (FSM) for each tank in which liquids can be carried. The FSM given will be for a stated relative density of liquid (often 1.00) which will need to be adjusted if the liquid is of another density.

Used in transverse stability calculations in order to find the ships fluid KG and fluid GM.

There should also be a worked example.

13)KN tables / Cross curves (including an example)

This will take the form of a diagram or table showing the righting levers for an assumed KG (the KN is the GZ of the vessel assuming the KG is zero). There should also be a worked example showing how a GZ curve can be obtained using the tables / cross curves.

KN tables are used to obtain the value of GZ (as GZ = KN=-KGsin)

Cross curves are used to find the GZ of the vessel for any angle of heel.

14)Pre-worked Ship conditions (Light ships, Ballast Arr/Dep, Service loaded Arr/Dep, homogenous loaded Arr/Dep, Dry docking etc)

To include for each condition:a profile diagram indicating disposition of weighs.Statement of light weight plus disposition of weight on board.Metacentric height (GM)Curve of statical stability (GZ curve)Warning of unsafe condition.

Very useful in cargo planning since it is easier to use a ship condition similar to the proposed load condition. Also useful where the ships tables are presented in a form unfamiliar to the ships officer who can now follow the method of calculation normally used on that vessel.

Dry dock: Enables officer to plan the stability condition for entering dry dock.

Loaded: Provides officer an example to establish stability condition of the vessel when loaded with relation to draught, trim, displacement, stress (SF & BM) and also compliance with the loadline criteria.

Ballast: Provides officer an example to establish stability condition of the vessel when in ballast condition with relation to draught, trim, displacement, stress (SF & BM) and also compliance with the loadline criteria..

Homogeneous loaded: Provides an example on cargo distribution for a given cargo to achieve a required stability criteria so as to enable the officer to plan for loading of various cargoes.

15)Special Procedures (Cautionary Notes)

Sometimes known as Cautionary notes.These may take the form of procedures to maintain stability such as the partial or complete filling of spaces designated for cargo, fuel, fresh water etc.Examples of this are:

Sequence of ballasting during the voyage to maintain adequate stability, particularly to compensate for fuel/water consumption

Ballasting to compensate for strong winds when carrying containers or other deck cargo.

Measures to compensate icing in Arctic waters

Any special features regarding the stowage behaviour of a particular cargo.

16)Inclining experiment report:

This will take the form of a report on the details of the inclining test showing the calculation and other Light Ship information.

Useful in assessing the accuracy of the Lightship KG given in the stability booklet, (which may change over time)17)Information as to Longitudinal Stresses for vessels over 150m in length

This applies to vessels over 150 mtrs in length and contains information on the determination of the longitudinal stresses such as Shear Force, bending moment and torsion.

This will usually be in the form of comparison with maximum stress levels for the Seagoing condition and the Harbour condition.

In this way the ships officer can assess the magnitude of the stresses before, during and after any loading, discharging or ballasting operations whether in harbour or in a seaway.

List the surveys required by the current Loadline Regulations for a vessel to maintain a valid Load line Certificate.

1) Initial Survey Load line Assignment

2) Periodic Surveys:

Annual Survey within 3 months either way of the anniversary date of the load line certificate.

The surveyor will endorse the load line certificate on satisfactory completion of annual survey.to be carried out every year

Renewal Survey at interval not exceeding 5 years

The period of validity of the load line certificate may be extended for a period not exceeding 3 months for the purpose of allowing the ship to complete its voyage to the port in which it is to be surveyed.

List the items surveyed at a periodic Load line survey, describing the nature of the survey for EACH item.

The preparation for a load line survey will involve ensuring that the hull is watertight below the freeboard deck and weather tight above it (cargo tank lids on tankers must be watertight).

The following are checked for condition and / or weather tightness (hose test as necessary):

1)Superstructure / deck house weather tight doors effective means of closure and of securing weather tightness (dogs, clamps, hinges, weather tight seal)

2)Hatch covers effective means of closure and securing weather tight (cleats, clamps, wedges, rubber sealing)

3)Side scuttles (portholes) effective means of closure and of securing weather tight (clamps, sealing, hinges, deadlight operation).

4)Side cargo doors effective means of closure and of securing weather tight (clamps, sealing arrangements)

5)Other deck openings such as sounding pipe covers ullage pipe covers, tank lids, sighting ports, manholes (deck scuttles) effective means of closure and of securing water tight (hinges, clamps, sealing arrangements)

6)Air pipes permanently attached means of closure. Gauze to fuel tanks.

7)Ventilators effective means of closure and securing weather tight (unless over a specified height).

8)Freeing ports in bulwark free movement of flaps.

9)Scuppers, inlets and discharges effectiveness of non-return / storm valves.

10)Access walkways, ladders, safety rails, bulwarks in good condition.

11)Deck fittings and appliances for timber loadlines.

12)Loadline and draught marks measurements, correctly positioned and clearly visibility (clarity)

13)Any changes to hull or super structure which may materially affect stability (eg significant increase in Lightweight of ship).

14)Any departure from recorded Condition of Assignment (as detailed in Record of Particulars)

15)Presence of stability information Booklet and / or Loading Computer.

A vessel assigned Timer load lines is to fully load with timber on deck and in holds in a port in a Tropical Zone, for a destination in the Winter North Atlantic zone, during the winter months.

(a) State the minimum statutory requirements for the ships stability throughout the voyage.

1)

Initial GM

not less than 0.10 m (2011 TDC Code)

The maximum righting lever (GZ)atleast 0.20 mtrs

Angles of Maximum GZnot be less than 30 degs

Area under the curve

0 to 30 degsnot less than 0.055 mr

0 to 40 degs or f whichever is lessernot less than 0.09 mr

Between 30 degs and 40 degs or fnot less than 0.03 mr

2)Stability calculations to assess a vessels compliance with minimum stability criteria should include a 15% increase in the weight of the timber deck cargo due to water absorption.

3) Alternative KN tables taking into account the increased freeboard due to timber deck cargo of a specified height may be used. However such tables must assume a reserve buoyancy is only 75% of the deck timber because of the permeability of the timber deck cargo (assumed permeability 25%).

(b) Describe the various causes of any deterioration in the ships stability during the voyage.

1) The vessel is loading timber in tropical zone and in most cases the cargo will be in a dry state condition.

2) As the vessel progresses towards the destination in the loaded passage, she proceeds to the WNA area.

3) It is possible that the timber cargo may absorb more moisture which may increase the weight more than 15%. This reduces the GM and therefore GZ curve.

4) Free surface effect when fuel and water is consumed from the full tanks which reduced GM and therefore GZ curve.

5) Consumption of fuel, stores, FW during the passage will cause G to rise thereby reducing the GM and therefore GZ curve

6) During winter seasons, as the vessel moves towards higher latitude, will encounter series of depression resulting in bad weather.

7) Seas on deck will cause raise in G due to added weight and also cause FSE which reduces GM and GZ curve

8) Whilst experiencing heavy seas, if any of the lashing gives way and cargo break loose, it can result in catastrophic result due to deterioration of the stability of the vessel.

9) If the vessel is experiencing severe wind and spray on one side, it can result in unsymmetrical icing on deck and superstructure

10) As a result of this the vessel may list or loll over to due to increase in weight on one side.

11) This list or loss will reduce the vessels stability by way

a)reduction in GMi

b)produces heeling arm

c)reduction in Area under the curve or the Dynamical stability

d)Reduces the range of positive stability of the righting lever curve.

e)Reduces the maximum righting lever.

12) If the vessel is lolled over, then the situation is further worsened.

13) This is because, if the vessel is experiencing severe weather and is lolled over then wind and wave motion will further heel the vessel.

An unstable vessel lying at an angle of loll to starboard has an empty double bottom tank subdivided into four watertight compartment of equal width. The tank must be ballasted to return the vessel to a safe condition.

Describe the sequence of action to be taken and the possible effects throughout each stage.

An angle of loll is caused due to the vessel being in an unstable condition with negative GM when upright and the vessel may heel to port or starboard.

1. Since the angle of loll is caused by G being too high, efforts is to be directed towards lowering it

2. As a first means of correcting measure, one should look towards lowering weight and reducing the free surface effect where possible.

3. Since the vessel has an empty double bottom tank subdivided into four water tight compartment of equal width following ballasting sequence must be carried out to return the vessel to a safe condition:

SEQ. 1: Ballast the inner low side completely marked A on the following figure.

SEQ. 2: Ballast the inner high side completely marked B

SEQ. 3: Ballast the outer low side completely marked C

SEQ. 4: Ballast the outer high side completely marked D

SEQ 1:1. The first sequence is to ballast the inner low side tank marked A.

2. While filling up the tank, due to the introduction of more free surfaces the situation will initially worsen.

3. Moreover an increase in the initial list will happen due to the off centre weight.

4. However as the tank starts to fill further, the G will start lowering down and the list will start to reduce.

SEQ 2:

1. The second sequence is to fill the inner high side tank B.

2. The condition of the vessel while filling up this tank is some what similar to SEQ 1.

3. In this sequence although there is free surface effect initially, the KG of the vessel will decrease as the tank is filled up due to concentration of weight at the lower part of the ship.

4. As the tank is finally filled, the free surface effect is eliminated and the KG will reduce even further thereby improving the vessels stability.

SEQ 3:

1. Fill up the outer low side tank marked C .

2. The purpose is to further reduce the KG and improve the stability of the vessel.

3. One of the main reason the lower KG is to have positive GM so as to eliminate the angle of loll.

4. As the tank is filled up it will have free surfaces initially.

5. However by now tank A and B are filled fully which has reduced the KG considerably.

6. By filling this tank the starboard list moment created by filling tank A and B initially will be counter set by the port moment produced by filling this tank.

7. At the same time the G will be further concentrated down improving the GM of the vessel.

SEQ 4:

1. This will be the final sequence of ballasting which will be the outer high side tank marked D.

2. By filling up this tank the GM is further improved and the port moment produced by this tank will offset the starboard moment produced by filling tank A and B.

3. The G of the vessel will be lowered sufficiently and the ship should be completely upright condition when this tank if completely filled.

DONTs:

1. Do not fill the outer high side tank first because the added weight may cause the vessel to suddenly and violently roll over to the other side with a possibility of the moment of the roll carrying the ship over past the angle of vanishing stability and therefore capsizing the vessel.

2. This is because generally at loll the port list moments is equal to the starboard list moments and there is no list. It is only with the case of list it is prudent to fill the high side tank.

3. Even if the vessel does not capsize, such a sudden roll may result in injury to personnel or shift of cargo with its implications on ships stability.

Describe how a vessel lying at an angle of loll may be returned to a safe condition.

An angle of loll is caused due to the vessel being in an unstable condition with negative GM when upright and the vessel may heel to port or starboard.

1) Ensure that the heel is due to the negative GM rather than off centre weight.

2) That is to ensure that the port listing moment is equal to the starboard listing moment.

3) Since the angle of loll is caused by G being too high, effort is to be directed towards lowering it.

a)This can be done by shifting weight onboard.

b)If the vessel has high ballast tanks then these may be emptied by discharging the ballast from high side tank first. Once the high side tank is emptied then empty the lower side tank.

4) One should look towards lowering the weights and reducing free surface effect where possible i.e., by pressing up tanks.

5) Should it be necessary to fill the double bottom, it is important to choose a divided tank first so as to minimize the free surface effect

6) One tank should be filled at a time and always fill the lower side first. This will probably cause an initial increase in the list because of the off centre weight and generated free surface effect, but after that the list will start to reduce as G is lowered.

7) Where a double bottom is subdivided into three equal water tight compartments, then

a)it is logical to fill the centre tank first since the added weight will cause the G to move

vertically downwards and the heel will therefore reduce as the tank fills.

b)Neither it will cause the vessel to roll over to the high side since the added weight is not

off centre.

c)Fill the low side tank completely

d)Finally fill the high side tank. By the time this tank is completely full the vessel will be in

upright condition as the vessels stability is improved by this time and GM being

positive.

8) Where there are four athwartship tank the order recommended is:

a)Ballast the inner low side first.

b)Ballast the inner high side completely

c)Ballast the outer low side completely

d)Ballast the outer high side completely

9) Prior considering any of the above, if the vessel is at sea where the ship is lolled over then following shall be carefully observed.

a)Alter course to put the ships head into the predominant waves.

b)It is essential that the ship stays in lolled to the same side.

Explain why the information provided by a curve of statical stability, derived from KN values should be treated with caution

1) GZ curves are the best way of assessing a ships stability but they do have limitations as they are based upon theoretical values.

2) This is because no account is taken of what may happen in practice at large angle of heel e.g., flooding through ventilators, shifting of cargo, etc.

3) The KN values are tabulated for various angles of heel for a range of displacements. These values are derived based on the fact that it would be convenient to consider the GZ that would exist if G were at Keel, termed KN.

4) The KG of the vessel is assumed to be zero, therefore all KN valued need to be corrected in order to take into account the actual KG of the vessel.

5) The GZ value is predominantly dependant upon the KG.

6) Hence in order to obtain the actual GZ for a given value of KG, a correction need to be made for the actual height of G above the keel.

7) GZ = KN-KGsin. KN value must be interpolated between two sets of displacement to arrive at a desired displacement. KG values dependant upon displacement and the displacement is dependant upon accuracy of weights onboard including the lightship displacement and KG.

8) The lightship KG and displacement is no longer the same that was calculated when the ship was built.

9) GZ values are based upon an assumed trim condition which may not be the vessels actual trim, although some vessels have different KN tables for different trim conditions.

10) A further complication is that of Free trim where the vessel changes its trim as it heels.

11) This condition is very much obvious in case of smaller vessels like offshore supply vessels. Trimming by stern on such vessels will reduce the water plane area especially when vessels low stern goes into the water and the aft deck floods.

12) Reduction in water plane area reduces the vessels stability and therefore the KN values for that angle of heel.

13) Thus the GZ curve obtained using KN values of fixed trim, then the curve obtained will be incorrect one and will tend to show that the vessel has better stability.

14) Water shipped on deck will not be accounted for. Such water will change the vessels KG creating free surface moment as the vessel rolls in seaway.

15) Also dynamic factors such as synchronous rolling, parametric rolling and loss of stability cannot be appreciated by inspection of a curve of statical stability such as righting lever or righting moment curve.

Describe the effect of a heavy list on a vessels stability.

1) When a vessel is listed the G lies off the centre line to port or starboard.

2) GZ is actually capsizing lever with a negative GZ when the vessel is upright.

3) GZ is negative until the angle of list.

4) At angle of list GZ is zero.

5) If the ship is heels beyond angle of list, positive GZ is produced and it is now a righting moment.

6) Maximum residual GZ is reduced. The loss of GZ due to list = GGH x Cos

7) As Cos = 1, the loss of GZ is maximum when the ship is upright.

8) Area under the curve (dynamical stability) is decreased due to losing the area under the heeling arm curve.

9) Angle of maximum GZ value is increased by a small amount.

10) Range of stability is reduced.

11) No change in the angle of deck edge immersion but it is easily reached on the listed side when acted upon by the external forces.

12) Since the ship is already listed, external forces can easily heel the ship to more dangerous angle of heel on the listed side.

(If this question forms part of a question where they have asked to show the GZ curve with list condition, only the above answer will suffice. If asked as a stand alone question then curve need to be drawn)

Discuss the use, limitation and relative accuracy of EACH of the following means of stability assessment.

Simplified Stability tables (e.g., Max KG)

Use:

(a) These are incorporated in the ships stability booklet either as a diagram or a table.

(b) A quick assessment of the ships stability as to whether or not all statutory criteria are complied with is achieved by means of a single diagram or table

(c) Eliminates the need to use cross curves or GZ curves for different loading conditions.

(d) Three methods of presentation are:

- Maximum deadweight moment or table

- Maximum permissible KG diagram or table.

- Minimum permissible GM diagram or table.

Initial Metacentric Height (GM)

Use:

(a) Used to determine the initial stability of the vessel i.e., the stability of the vessel at small angles of heel.

(b) IMO Load Line Regulations stipulates the minimum value of Initial GM for different type of vessel.

(c) Hence at a glance of initial GM for that type of vessel, once can ascertain the stability condition of the vessel.

However in order to comply fully with the regulations there are other criteria which needs to be complied with.

Explain the meaning of Free Trim and its particular reference to offshore supply vessels.

Free trim is the sudden and significant moment suffered by the offshore supply vessels after a certain angle of heel due to the shift of LCB and LCF forward.

The bow is up and the stern in trimmed down. This effect is explained as follows:

1. Free trim effect is observed in offshore supply vessels with high forecastle (normally forward superstructure) and a low working after deck.

2. When ship is heeled over to immerse the after deck line, the forecastle remains well over the water line.

3. The water plane area aft on the low side has been lost causing the F to move forward. The ship starts to trim by the stern.

4. As the ship progressively heels further the reserve buoyancy of the forward superstructure takes effect, volume of buoyancy being transferred from the high side aft where it is not being used to the low side on the heeled side.

5. This causes the LCB to move forward.

6. This accompanied by the continuing forward movement of the LCF causes the ship to trim significantly further by the stern as it continues to heel.

7. This situation leads to a danger of after deck being flooded.

8. The stability of the vessel is greatly reduced due to the reduction in the water plane area and hence reduction in the KN value.

9. If the ships KN value has been calculated for fixed trim they will result in an incorrect GZ curve and will tend to show that the vessel has better stability than it actually has at large angles of heel.

10. Fixed trim KN data will give greater GZ values than what the ship will actually have when heeled beyond the angle of deck edge immersion - stability will be overestimated.

11. Therefore it is preferable that the KN values of the ship be derived on a free to trim basis and the KN tables should have the statement Corrected for Free trim.

A vessel with a high deck cargo will experience adverse affects due to strong beam winds on the lateral windage areas.

Explain how the effects of steady and gusting winds can be determined and state the minimum stability requirements with respect to wind heeling under the current regulations 1. A vessel with high deck cargo may have their stability considerably reduced when subjected to strong beam winds.

2. A heel angle will be produced by the strong beam winds upon large lateral areas of the ship.

3. This lateral area may be a combination of high freeboard and tiers of containers on deck.

4. The wind heeling moments are the moments produced by this force, multiplied by a heeling lever, tending to incline the vessel.

5. The components of wind heeling moments are:

a)Wind Force (F) Force per unit area (kgs/m2).

b)Windage area (A) Area (m2).

c)Lever (d)Distance of centriod of windage area fromthe centriod of buoyancy (B)

6. Heeling moments = Force x distance = FAd 1000 tonnes. Metres.

7. The vessel will continue to heel until an equal and opposite force is produced i.e., righting moments of equal value to the heeling moments, resulting in a steady angle of heel.

Righting moments

=Wind Heeling moments

x GZ

= FAd

1000

8. Therefore GZ loss at angle of heel=Heeling Moment

=FAd 1000 x

9. The GZ loss due to wind heeling produces a heeling arm

10. The wind heeling moments are usually represented by a straight horizontal line on the curve of statical stability.

11. This is due to the presumption that the wind heeling moments do not change as the vessel heels.

12. In practice the wind heeling moments will tend to reduce as the vessel heels due to the inclination of the windage area reducing the heel force. However for the purpose of stability it is assumed that the wind heeling force remains constant throughout, resulting in the horizontal heeling arm across the curve.

MIMINUM STABILITY REQUIREMENTS:

1. Applies to container ships.

2. Where the height of the lateral windage area from the load water line to the top of the containers is greater than 30% of the beam, the regulations require that the ship builder produces a curve of righting moments for the worst possible service conditions together with the total windage area, the position of its centroid and the lever to half draught.

3. Steady wind Heeling Moment () = F.A.d 1000 (t.m.), where F = 48.5 kgs/m2.

4. Wind force is dynamic which is equal to Gusting wind + 50%.

5. Gusting wind heeling moment = Steady wind Heeling Moment () x 1.5

6. Therefore Heel arm maximum = GZ loss x 1.5.

7. From the following curve , it is required that

a)Steady wind heel 1 is not more than 65% of the Angle of deck edge immersion (de).

b)Angle of dynamic Heel (dy) not more than Angle of progressive flooding (f)

c)Area S2 is equal to or more than Area S1 up to f

With regard to the modern shipboard stability and stress finding instrument:

(a) State the hydrostatic and stability data already pre-programmed into the instrument.

1) Ships dimensions and general particulars.

2) Capacity of all internal spaces.

3) VCG / LCG / FSM of all internal spaces (cargo spaces, ballast tanks fuel , FW etc)

4) Hydrostatic particulars Displacement, draught, TPC, MCTC, LCB, LCF, KM

5) Light ship data Light ship displacement and KG.

6) KN data

7) Stability limits (Loadline, Grain, Timber etc)

8) Simplifies Stability Data (e.g., MAX KG)

9) Structural Stress Limits

10) Grain Loading data (as in grain loading booklet)

11) Wind Heeling Data

12) Ice Allowance Data

(b) Describe the information to be entered into the instrument by the ships officer.

1) Location and weight of individual items of deadweight cargo, fuel, ballast, stores, fresh water, passengers etc.

2) Loadline zone

3) R.D. of seawater / dock water

4) R.D. of liquids fuel, ballast, liquid cargo etc.

5) S.F. of bulk cargoes (e.g. grain)

(c) Describe the output information

1) Deadweight summary.

2) Trim and draught (forward, aft, midships, freeboard)

3) Heel

4) Stability Assessment Gm, GZ curve, dynamical stability etc.

5) Simplifies Stability diagram and assessment.

6) Stress Assessment Shear force, Bending moment, Torsion.

7) Grain loading assessment.

8) Load line assessment e.g., container stack weight

The stress data is usually given as a percentage of the maximum allowable at that particular point along the length of the vessel. Hence two variables are the actual stress encountered and the corresponding strength of the vessel at that point which resists that particular stress.

State the purpose of the inclining experiment.

The purpose of performing inclining experiment on vessel is to determine the value of the KG in the lightship condition.

The determination of light ship KG is required because the light KG changes over a period of time. Moreover, the lightship KG and displacement value are the basis from which the KG is determined for every other condition. An error in the KG calculated for any condition of loading will result inaccuracy in all stability parameters dependant on this value GM, GZ values and dynamical stability.

Also during the experiment, the LCG for light condition will also be determined.

Describe the precautions to be taken by the SHIPs OFFICER before and during the inclining experiment.

1) The ship must be moored in quiet sheltered waters free from the effects of passing vessels.

2) There must be adequate depth of water under the keel so that the bottom of the ship does not touch the sea bed on inclination.

3) There should little or no wind. If there is any wind the ship should be head on or stern to it.

4) The ship should be floating free. There should be no barges alongside.

5) Moorings should be slackened right down.

6) Shore side gangway if any must be landed to allow unrestricted heeling.

7) All loose weights must be removed or secured.

8) All fittings and equipments such as accommodation ladder , derricks/cranes should be stowed in their normal sea going positions.

9) Free surface should be minimized. All tanks should be verified as being completely empty or full. Bilges should be dry.

10) Deck should be free of water. Any water trapped on deck will move during the test and reduce the accuracy of the result.

11) The ship should be upright at the commencement of the experiment.

12) All personnel not directly concerned with the experiment should be sent ashore.

13) In tidal conditions, conduct experiment at slack water.

14) Efficient two way communication must be established between a person in charge of the operation and the central control station, the weight handlers and each pendulum station.

Explain why a vessels Lightship KG may change over a period of time.1) Constant of the vessel keep changing due to accumulated sludge in fuel tanks, mud and rust in ballast tanks (unpumpables)

2) Various stores remaining unconsumed might add to the constant.

3) Any structural changes will affect the light ship KG and light ship displacement

4) Lightship KG for a passenger vessel will change considerably over a period of time mainly because of the left over baggage etc will accumulate over a period of time and add to the constant considerably.

List the circumstances when the inclining experiment is required to take place on passenger vessel.

1) When the vessel is built.

2) When any major modifications are made to the ship so as to materially affect the stability.

3) Every 5 years.

4) If any significant change is found Light displacement changed by 2% or Lightship LCG changed by 1% of ships length.

State the formula to determine the virtual loss of GM due to a free surface liquid within a rectangular tank, explaining each of the terms used

The formula to determine the virtual loss of GM due to free surface liquid is given by

Free surface correction (FSC) = Loss in GM = L x B3 x RD of liquid in tank

12 x x n2

Where

L Length of the rectangular tank. Loss of GM is directly proportional to the length of the tank so will be the value of free surface moments (loss of GM).

B Breadth of the rectangular tank. From the formula it can be seen that breadth of the tank is the most critical factor which determines the amount of loss in GM i.e., loss of GM is directly proportional to the cube of the breadth of the tank.

DensityRelative density of liquid filled in the tank.

Loss of GM is directly proportional to the RD of the liquid, greater the density of the liquid, greater the loss in GM.

Displacement of the vessel. Greater the displacement of the vessel lesser the loss of GM and vice versa.

12It is part of the formula. The free surface correction can also be given by

Free Surface Moments (FSM)

Displacement

FSM = Moments of Inertia of the free surface liquid x RD of the liquid

=L x B3 x RD of liquid

12

n number of longitudinal subdivision of the tank. The longitudinal subdivision of the tank greatly reduces the FSC as it is indirectly proportional to the square of number of subdivision.

Further it can be seen that if the tank is divided into two equal subdivision then the FSC will reduce by a quarter and 3 equal division will reduce the loss by one ninth and so on.Explain the effects on the virtual loss of transverse GM due to the free surface effects when the slack tank is subdivided

(a) Transversely:


12 x x n2

1. Although the tank is transversely sub divided yet the effective length and breadth of the tank still remains the same.

2. One should not mistake n in the formula for transverse sub-division as it refers to the longitudinal sub-division.

3. The area available for the free movement of the liquid still remains the same.

4. The free surface effect remains the same as it was before.

5. The following diagram shows an example of tank transversely subdivided into two equal parts.

(b) Longitudinally


12 x x n2

1. Longitudinal subdivision of the tank greatly reduces the free surface effect and hence the loss of GM.

2. AS it can be seen from the formula, that the Loss in GM is inverse proportional to the square of the number of subdivision (n2).

3. For example, if the tank is divided into two equal subdivision then the FSC will reduce by a quarter and 3 equal division will reduce the loss by one ninth and so on.

4. The following figure illustrates an example of tank longitudinally divided into two equal parts.

Explain why a vessel laden to the same draught on different voyages may have different natural rolling period.

Rolling period (T) in seconds is the time taken for the ship to complete one complete oscillation i.e., the time it takes for the ship to roll from one side back through the upright to the extent of its roll on the other side and back again. (port starboard port).

(1) The natural rolling period in still water is given by the formula:

T = 2 K

GM x g

Where

T = period of roll in seconds

g = acceleration due to gravity (9.81 mtrs / sec2)

K = Radius of Gyration.

GM = Metacentric height of the ship.

2)Radius of Gyration is the distance from the centre of gravity or the rolling axis at which the total weight (W) would have to be concentrated in order to give the ship same moment of inertia as it actually has.

3) For any particular ship the Radius of Gyration can be changed by altering the distribution of deadweight about the rolling axis.

4) If the weights are moved away from the rolling axis, the radius of gyration is increased resulting in the longer period of roll and the ship will roll slower (moving weight outwards towards the side of the ship is known as winging out weights)

5) Conversely, moving weights inwards towards the rolling axis will cause the ship to roll faster.

6) The roll period varies inversely as the GM. Hence larger the GM, shorter the rolling period (stiff ship) and smaller the GM, longer the rolling period (tender ships).

7) Also the roll period will change when weights are loaded, discharged or shifted, since both the GM and the moment of inertia (measure of distribution of weight about the rolling axis) will be affected.

From the above statement it can be seen that although the laden vessel has the same draught for different voyages, yet its rolling period will change because of the following reasons:

CHANGE IN GM FOR THE SAME DRAUGHT:

1) For the same draught, the GM of the vessel may not necessarily be the same. GM of the vessel varies with concentration of weight distributed on the ship with reference to the keel.

2) A vessel loaded with high density cargo (low SF) will have large GM (reduction in KG) compared to when loaded with low density cargo both resulting in same draught.

3) It is possible that the vessel may have loaded slightly less cargo but may have bunker tanks fully filled which cause the G to move down resulting in increase GM.

4) Also the KG of the cargo loaded has direct effect on the resultant GM of the vessel (For example a vessel with more deck cargo will have less GM)

5) So the change in GM for the same draught will result in change in rolling period as discussed above.

DISTRIBUTION OF WEIGHT WITH RESPECT TO ROLLING AXIS:

1) The Radius of Gyration may vary for every voyage (with same draught) as the distribution of weight with respect to rolling axis may vary.

2) Hence the rolling period will change for each voyage.

However it should be borne in mind that the period of roll is not affected by the amplitude or magnitude of the roll.

Describe the different rolling characteristics of a vessel in a stiff condition and a vessel in tender condition.

The natural rolling period for the vessel is given by

T = 2 K

GM x g

Where





Stiff Ship:

1) A stiff ship is one with a very large GM caused by the KG being too small.

2) This occurs if too much weight is placed low down within the ship.

3) The ship will be excessively stable, righting moments will be so large as to cause the ship to return to the upright very quickly when heeled.

Rolling characteristics:

a) It can be seen from the above formula that the rolling period is inversely proportional to the GM of the vessel.

b) Since the stiff ships have large GM, the rolling period will be short.

c) The ship will offer greater resistance to being rolled and will be rolled to lesser angles of heel.

d) Generally a ships natural rolling period is greater than the wave period. Since stiff ships have shorter rolling period they are more vulnerable in the beam sea.

Tender Ships:

1) A tender ship is one with a very small GM caused by KG being too large.

2) This occurs if too much weight is placed high up within the ship.

3) The ship will have less stability, righting moments as compared to the stiff ship.

4) This causes the ship to be sluggish and slow return to the upright.

Rolling characteristics:

a) Because of small righting moments the ship will only offer limited resistance to being rolled, causing the ship to be rolled to larger angles of heel.

b) Also from the Rolling period formula, Rolling period varies inversely as GM.

c) Since the tender ships have small GM, their rolling period will be long.

d) The ship will be slow to return to the upright and will tend to remain at the extent of the roll for a comparatively long time.

The Radius of Gyration also has effect on the ships rolling characteristics. However in both Stiff and Tender ship it varies with the circumstances as the distribution of weight with respect to rolling axis is not the same at all times.

Discuss how a vessels still water rolling period is affected by changes in the distribution of weight aboard the vessel.

The distribution of weight aboard the vessel can be discussed with respect to following factors:

(1) Distribution of weight with respect to the Keel of the vessel (KG of the weight)

(2) The relative density of the weight distributed.

(3) Distribution of weight with respect to the rolling axis.

Distribution of weight with respect to the Keel of the vessel (KG of the weight)

If the weight is distributed high up within the vessel, then the resultant GM if the vessel will be reduced due to increase in the resultant KG of the vessel (because the KG of the weight distributed will be more).The relative density of the weight distributed.

The relative density or the SF of the weight distributed will contribute a major factor in determining the GM of the vessel. For example, if a high density cargo is loaded in a ship then the GM of the vessel will increase as compared to loading a low density cargo in the same hold.

Thus it can be seen that both the above factors are affecting the GM of the vessel.Distribution of weight with respect to the rolling axis

The distribution of weight with respect to the rolling axis affects the Radius of Gyration. If weights are distributed inwards towards the rolling axis then the Radius of Gyration is reduced. Conversely if the weights are distributed away from the rolling axis the radius of gyration is increased.

The natural rolling period for the vessel is given by

T = 2 K

GM x g

Where





Using the above formula in conjunction with the explanation of above section it can be seen that:

1) The distribution of weight aboard the vessel can change the GM of the vessel.

2) The Rolling period varies inversely as the GM and hence change in GM changes the rolling period.

3) Also distribution of weight with respect with the rolling axis affects the Radius of Gyration. Therefore the distribution of weight is such that if the Radius of Gyration is increased then the rolling period is increased as it is directly proportional and vice versa.

Explain the term Synchronous rolling and describe the dangers, if any associated with it.

Synchronism is the name given to the condition when the ships natural period of roll is the same as the apparent period of wave.

1) When this occurs the waves give the ship a push each time she rolls (like a swing) causing her to roll more and more heavily.

2) Theoretically this could cause the vessel to eventually capsize.

3) However Synchronism is less likely to happen as the rolling period of the ship increases with the angle of roll at large angles of heel.

4) Moreover the period of sea waves tends to vary over time.

5) The ships natural rolling period will be greater than the wave period.

6) Ships which has a long natural rolling period are less vulnerable in a beam swell than the stiff ships with their short periods of roll.

7) If the sea is forward of the beam the apparent period of waves will be reduced whilst the sea abaft the beam will increase the apparent period of waves.

8) Therefore the sea on the quarter will increase the likelihood of synchronism.

Dangers associated with Synchronous rolling:

a) Danger of capsizing the vessel.

b) Heavy roll may cause shift of cargo, especially deck cargo which is at greater distance from the rolling axis.

c) The vessel will then roll in a fashion dictated by righting moment, heeling the vessel excessively to the listed side and increasing the chances of subsequent shift of. Cargo.

d) The dynamical stability of the vessel will be greatly reduced under these circumstances and there is always a risk of capsizing.

e) Structural damage to the vessel (racking, surge of liquids).

f) Personal injury.

g) Down flooding.

State the action to the taken by the ships officer when it becomes apparent that the vessel is experiencing Synchronous rolling.

1) Alter course, ideally towards the wave since this shortens the apparent period of the waves.

2) Alter speed except when the wave is not on the beam.

3) Alter vertical distribution of the weight so as to change the GM.

4) Alter the vertical and transverse distribution of the weight aboard the vessel so as to change the ships radius of gyration. E.g., winging out weights.

5) The later two measures can be achieved by ballasting, deballasting or shifting other items of deadweight such as fuel or fresh water.

Describe the methods of improving the initial stability if the GM at the critical instant is found to be inadequate.

The major considerations that should be borne in mind during dry docking are

1) that the P force is kept to an acceptable level and

2) that the Ship maintains an acceptable positive GM during the critical period.

Loss of GM =P X KG(OR)P X KM

P

If it is found that the GM at critical instant is found to be inadequate the following measures to be taken to improve the initial stability.

1) The loss in GM is directly proportional to the KG of the vessel. Hence lower the effective KG of the vessel by lowering the weights within the vessel, discharging weights from the high up or taking on an acceptable amount of ballast in the double bottom tanks.

2) Empty the high wing tanks if possible.

3) Stow derricks, cranes and riggings in stowed position.

4) Eliminate or minimize free surface effects by topping up or emptying slack tanks where possible.

5) Keep minimum stern trim as recommended by the dry docking plan. Smaller the trim, smaller the P force and hence smaller the loss of GM.

Explain why the values of trim and metacentric height in the freely afloat conditions are important when considering the suitability of a vessel for drydocking.

Trim:

1) The trim of the vessel plays a very vital roll in vessels dry docking.

2) The vessel should enter the dry dock with a small stern trim as recommended by the dry docking plan available on the ship.

3) P force or the upthrust generated at the block when the vessels stern first touches the block continues to increase as the buoyancy force is reduced.

4) The formula for calculation of the P force is given by

P =Change of Trim X MCTC

LCF

5) From the formula it can be seen that greater the stern trim more the P force.

6) Although the stern frame is designed to take force exerted on it during dry-docking, there is a maximum limit that must not be exceeded.

7) If the P force is exceeded then it will lead to structural damage.

Metacentric Height (GM):

1) Loss of stability (Loss of GM) commences as soon as the ship touches the block aft and continue to worsen as the value of the P force increases.

2) The maximum loss of GM occurs at the instant immediately prior to the ship settling on the blocks forward and aft known as Critical Instant.

3) The vessel must have positive stability (positive GM) at this critical instant.

4) That is to say that it is essential that the righting moment afforded by the upward acting buoyancy force (remaining due to pumping out of dock water) remains greater than the capsizing moment afforded by the upthrust of P force acting at the keel at all times prior to the ship touching the blocks forward and aft.

5) If this is not so, then the ship will become unstable resulting in negative GM and would topple over in the dock.

6) Therefore the metacentric height of the vessel when she is in freely afloat condition is very important when considering the suitability of the vessel for dry-docking.

7) The formula for loss of GM at critical instant is given by


P

8) From the formula it can be seen that loss of GM is directly proportional to the P force and the KG of the vessel.

Hence the values of trim and metacentric height of the vessel in the freely afloat conditions are important for the purpose of dry docking the vessel.

Describe the two methods of determining the upthrust (P force) during the critical period.

The two methods of calculating the P force are

a) Calculation of P force at any stage during dry-docking process.

b) Calculation of P force during the critical period when dry-docking.

Calculation of P force at any stage during dry-docking process.

1) Throughout the dry-docking procedures the true mean draught of the vessel reduces.

2) This situation is similar to the vessel rising out of water due to weights being discharged.

3) Rise in cms is given by the formula w(t) TPC.

4) The P force may be considered to have the same effect on True mean draught as if weight had been actually discharged.

5) Therefore reduction in TMD (cms) = P force (t)

TPC

6) Transposing this formula we can find that

P force (t) = Reduction in TMD (cms) x TPC

7) This formula can be used at any draught before or after the critical instant since what is being found is the loss in buoyancy due to the reduction in the draught.

Calculation of P force during the critical period when dry-docking.

a) In the period between the ship touching the block aft (start of critical period) and touching the blocks forward and aft (critical instant) the ship undergoes a change of trim.

b) The change of trim at any stage during the critical period may be considered to be the same as the change of trim that would have occurred when a weight w has been discharged from a position at the aft perpendicular equivalent to the upthrust P in tones.

c) The formula to find change of trim is given by

COT (cms) = Trimming Moment = w x LCF MCTC MCTC

d) If the P force is considered to have the same effect as a weight discharged at the aft perpendicular, then

COT (cms) = P x LCF MCTC

e) Transposing the above formula we can find P as given under

P force at any instant during critical period = COT(cms) x MCTC LCF foap

Explain why it is beneficial to have small stern trim when entering dry dock.

1. The critical period during drydock is between when the ship touches the blocks aft and eventually comes to rest on the blocks along its entire length.

2. During the critical period prior to taking the block fully forward and aft, the P force will be acting at a single point on the stern frame of the ship.

3. The stern frame is strengthened to accept the force exerted on it during the dry-docking but there will be a maximum limit that must not be exceeded.

4. If the P force becomes too large, structural damage will occur.

5. It is usual to have acceptable near light conditions for dry-docking.

6. An obvious method to limit the P force during critical period is to keep the initial trim by stern small.

7. The formula for calculating the P force during the critical period is given by

P =Change of Trim X MCTC

LCF

8. It is clear from the above formula that P force is directly proportional to the change of trim that the ship will undergo.

9. Limiting the trim will therefore limit the maximum load that will be experienced by the stern frame.

10. The greater the ships displacement, more importance will be needed to limit the docking trim.

11. The formula to find the loss of GM is :


P

12. From the above formula, clearly greater the trim, greater the P force; greater the P force, greater the loss of GM.

13. If the loss of GM results in negative GM then the ship will fall over in the dock before the shores were properly set up.

14 Thus it is beneficial to have small stern trim on entering dry-dock so as to avoid structural damage due to excessive P force and to have effective control over the ships stability by having positive stability at all times till the vessel sits on the blocks fore and aft.

A ship is loading in a port in a tropical zone for one in the Winter North Atlantic zone during winter months.

Describe the various precautions and considerations which must be borne in mind at the loading port in order that the voyage is accomplished safely and in accord with the statutory requirements, for example the Load Line rules.

1) The primary consideration is to have the vessel complying with the load line regulations throughout the voyage for ensuring intact reserve buoyancy - Cargo hatches, ventilators, sounding pipes, air pipes, freeing port)

2) Since the vessel is going to another Load line zone, the vessel should be loaded in such a way she does not breach the load line requirements.

3) Although she is loading in Tropical zone, yet she cannot immerse the marks more than a lever i.e., Winter load line + due allowance for consumables + bunkers.

4) To calculate the bunker consumption and FW consumption up to a point on the vessels intended route where it enters the winter load line zone.

5) Also the loading should be in such a way that the vessel will have adequate stability throughout the voyage.

6) If the ship is less than 100 mtrs in length she cannot immerse more than winter north atlantic mark when in winter zone.

7) Vessel need to have sufficient bunker reserve to meet bad weather and contingencies.

8) All derricks and cranes must be stowed in position.

9) Eliminate free surface effects by emptying or pressing the tanks if possible.

10) Adequate lashing arrangements for deck cargoes particularly for heavy lifts.

11) Stow heavy cargo as low as possible to bring down G.

12) Vessels loading and stability condition throughout the voyage must take into account ice accretion.

13) Shearing force, bending moments and Torsional stresses must be well within limits.

Describe Type A vessel under the current Load line Regulations, including the flooding, Stability and assumed damage requirements for a newly built vessel.

A type A ships any ship designed to carry liquid cargoes in bulk such as tankers, chemical carriers, LPG and LNG carriers.

According Regulations 27 of Loadline Regulations a type A ship is defined as one which:

1) is designed to carry only liquid cargoes in bulk.

2) Has a high integrity of the exposed deck with only small access openings to cargo compartments, closed by watertight gasketed covers of steel or equivalent material3) Has a low permeability of loaded compartments.Flooding requirements:1) If the vessel is over 150 mtrs in length and has an empty compartment when fully loaded at the Summer loadline, the ship should be capable of remaining afloat after flooding of such a compartment with an assumed permeability of 0.95 and shall remain afloat in a satisfactory condition of equilibrium.2) If the vessel is over 150 mtrs in length then the machinery space shall be treated as a floodable compartment, with an assumed permeability of 0.85.Stability requirements: - Condition of Equilibrium

1) The final waterline after flooding, taking into account sinkage, heel and trim, is below the lower edge of any openings such as air pipes, top of a ventilator coaming, door sill and openings which are closed by means of weathertight doors or hatch covers through which progressive flooding may take place.

2) If pipes, ducts or tunnels are situated within the assumed extent of damage penetration, arrangements shall be made so that progressive flooding cannot thereby extend to compartments other than those assumed to be floodable in the calculation for each case of damage.

3) The angel of heel due to unsymmetrical flooding does not exceed 15 degs.

4) If no part of the deck is immersed, an angle of upto 17 degs may be accepted.

5) The metacentric height (GM) in the flooded condition must be positive and must be atleast 0.05m.

6) The vessel must have adequate residual stability after flooding

The right lever curve must have a minimum range of stability of 20 degs.

The maximum righting lever (GZ) must be at least 0.1 mtrs within this range of stability.

7) The area under the righting lever curve within this range shall not be less than 0.0175 mr.

Damage assumptions:

1) The vertical extent of damage in all cases is assumed to be from the base line upwards without limits. - Keel to deck

2) The transverse extent of damage is equal to 20% of beam or 11.5 mtrs which ever is lesser.

3) Longitudinally Between transverse bulkhead (B-100 to include one bulkhead other than machinery space bulkhead)

Describe the provisions of the current Load Line regulations governing the ability of some Type B vessels to withstand flooding due to damage and the stability in the final conditions.

1) A type B ship is one which is not a Type A ship not designed to carry liquid cargoes in Bulk.

2) Has a greater freeboard than type A vessel.

3) Has lesser degree of sub-division.

4) Has large deck openings which are only weather tight.

5) Access to under deck compartments in Type B vessels is through large hatches.

There are two classification of Type B vessels viz., Type B-60 and Type B-100

Type B-60:

1) Any type B ship which is over 100 mtrs long.

2) Provided with steel hatch covers which are weather tight.

3) Since provided with steel hatch covers, qualifies for a reduction in the tabular freeboard of 60% the difference between type A and type B freeboards, hence the term B-60.

4) Flooding requirementa) When loaded in accordance with the initial condition of loading, shall be able to withstand the flooding of any single compartment with an assumed permeability of 0.95 and shall remain afloat in a satisfactory condition of equilibrium..

b) If the vessel is over 150 mtrs in length then the machinery space is regarded as a floodable compartment with assumed permeability of 85%

Type B 100

(a) Any type of B 60 ship over 100 mtrs long.

(b) Provided with steel hatch covers which are weather tight.

(c) Access to the engine room from deck protected by house.

(d) Provided with open rails for 50% of the length of the vessel and not bulwark.

(e) Crew access by gangway or under deck passage.

(f) Qualifies for a reduction in the tabular freeboard of 100% the difference between type A and type B freeboards, hence the term B-100.

(g) Flooding requirement:a. When loaded in accordance with the initial condition of loading, shall be able to withstand the flooding of any two fore and aft adjacent compartment with an assumed permeability of 0.95 and shall remain afloat in a satisfactory condition of equilibrium..

b. If the vessel is over 150 mtrs in length then the machinery space is regarded as a floodable compartment with assumed permeability of 85%

Condition of Equilibrium Applicable all class of Type B vessels.

1) The final waterline after flooding, taking into account sinkage, heel and trim, is below the lower edge of any openings such as air pipes, top of a ventilator coaming, door sill and openings which are closed by means of weathertight doors or hatch covers through which progressive flooding may take place.

2) If pipes, ducts or tunnels are situated within the assumed extent of damage penetration, arrangements shall be made so that progressive flooding cannot thereby extend to compartments other than those assumed to be floodable in the calculation for each case of damage.

3) The angel of heel due to unsymmetrical flooding does not exceed 15 degs.

4) If no part of the deck is immersed, an angle of upto 17 degs may be accepted.

5) The metacentric height (GM) in the flooded condition must be positive and must be atleast 0.05m.

6) The vessel must have adequate residual stability after flooding

7) The right lever curve must have a minimum range of stability of 20 degs.

8) The maximum righting lever (GZ) must be atleast 0.1 mtrs within this range of stability.

9) The area under the righting lever curve within this range shall not be less than 0.0175 mr.

When converting tabular freeboard into assigned Freeboard as specified in the Load Line rules a number of corrections have to be applied. With the aid of simple sketches describe each of the corrections and indicate how each may be applied.A tabular freeboard is the freeboard that would be assigned to a standard ship built to the highest recognized standards having specific characteristics as laid down in the Load Line regulations.

The following corrections required to be applied in order to convert Tabular freeboard to assigned freeboard.

Type B-60 / B-100 correction For type B vessels only:

1) If the ship qualifies for the reduction in tabular freeboard, either 60% or 100% then this correction is applied.

2) Qualification requires provision of steel hatches, subdivision, improved water freeing arrangements, crew protection etc.

Wooden Hatch correction for type B vessels only:

The tabular freeboard is increased if the vessel has hatches other than those of the steel pontoon type on the exposed freeboard deck / raised quarter deck or the forward 25% of the super structure deck (i.e., Position 1)

Flush deck correction only Type B ships:

1) This correction is applicable if :

a) The length of the vessel is less than or equal to 100 mtrs and

b) The effective length of the superstructure is less than or equal to 35% of ships

length.

2) The tabular freeboard in this case is increased.

Block co-efficient correction:

1) The block co-efficient is measured at 85% of the summer draught.

2) Modified tabular freeboard is increased if the block co-efficient of the vessel exceeds 0.68.

3) Freeboard is multiplied by (0.68 + Cb) 1.36.

Depth correction:

1) The standard freeboard depth of a ship under the Rules = L 15

2) If the freeboard depth is more than L 15, then the freeboard is increased

3) If the freeboard depth is less than L 15, the freeboard may be decreased provided that the superstructure is at least 60% of length amidship position or trunk over entire length of the vessel.

Correction for position of deck line:

1) Freeboard must be capable of vertical measurement.

2) If the vessel is having a rounded gunwale, then the freeboard must be corrected by the vertical difference between the actual position of the deck line and the correct position.

Superstructure correction:

1) Freeboard will be reduced if:

(a) The ship is with sufficient standard height superstructure (OR)

(b) Has sufficient water tight trunking to a minimum height and width.

2) This reduction will vary according to the length of the superstructure / trunk as a percentage of the vessels length.

3) If the superstructure or trunk is of less than the standard height / breadth then the correction will be reduced proportionally.

4) If it is not of sufficient height or % length or width then no reduction in freeboard.

Sheer correction:

1) Load line regulations assume a standard sheer for the vessel.

2) If the vessel has a greater sheer than standard, the basic freeboard is decreased.

3) If the vessel has a lesser sheer than the standard, the basic freeboard is increased.

4) If the vessels amidships superstructure is less than 10% length, then there is reduction in freeboard.

Bow height correction:1) The load line rules contains a formula for calculating the minimum bow height based on the vessels length and block co-efficient.

2) If the bow height is les than the calculated height, freeboard is increased.

Summer Freeboard: - Assigned only upon Owners request only increase in freeboard.

Freeboards may also be increased at the owners request or where there are no openings or cargo port holes below the freeboard deck

Corrections are then applied to the Assigned Summer Freeboard in order to determine the Tropical, Winter, Fresh Water and Tropical Fresh water freeboards.

When converting TABULAR FREEBOARD to BASIC FREEBOARD as specified in the Load line Rules a number of corrections have to be applied.

(a) List the geometric features of the ship which give rise to these corrections.

The ship which are required for these corrections are Type B vessels.

1) A type B ship is one which is not a Type A ship not designed to carry liquid cargoes in Bulk.

2) Has a greater freeboard than type A vessel.

3) Has lesser degree of sub-division.

4) Has large deck openings which are only weather tight.

5) Access to under deck compartments in Type B vessels is through large hatches.

There are two classification of Type B vessels viz., Type B-60 and Type B-100

Type B-60:

1) Any type B ship which is over 100 mtrs long.


3) Since provided with steel hatch covers, qualifies for a reduction in the tabular freeboard of 60% the difference between type A and type B freeboards, hence the term B-60.

Type B 100

1) Any type of B 60 ship over 100 mtrs long.


3) Access to the engine room from deck protected by house.

4) Provided with open rails for 50% of the length of the vessel and not bulwark.

5) Crew access by gangway or under deck passage.

6) Qualifies for a reduction in the tabular freeboard of 100% the difference between type A and type B freeboards, hence the term B-100.

(b) Explain the reason for each of these corrections and indicate how each correction should be applied to Tabular Freeboard (actual values not required)

Type B-60 / B-100 correction

B-60 : Since provided with steel hatch covers, qualifies for a reduction in the tabular freeboard of 60% the difference between type A and type B freeboards, hence the term B-60.B-100: Qualifies for a reduction in the tabular freeboard of 100% the difference between type A and type B freeboards, hence the term B-100.Wooden Hatch correction:

The tabular freeboard is increased if the vessel has hatches other than those of the steel pontoon type on the exposed freeboard deck / raised quarter deck or the forward 25% of the super structure deck (i.e., Position 1)

Flush deck correction:

1) This correction is applicable if :

a)The length of the vessel is less than or equal to 100 mtrs and

b)The effective length of the superstructure is less than or equal to 35% of ships length.

2) The tabular freeboard in this case is increased.

Block co-efficient correction:

1)The block co-efficient is measured at 85% of the summer draught.

2) Modified tabular freeboard is increased if the block co-efficient of the vessel exceeds 0.68.

3) Freeboard is multiplied by (0.68 + Cb) 1.36.

State with the aid of a labeled sketch, the minimum stability criteria required by the current Load line Rules.

Initial GM

not less than 0.15 mtrs

The maximum righting lever (GZ)atleast 0.20 mtrs

Angles of Maximum GZnot be less than 30 degs

Area under the curve

0 to 30 degsnot less than 0.055 mr

0 to 40 degs or f whichever is lessernot less than 0.09 mr

Between 30 degs and 40 degs or fnot less than 0.03 mr

The current Load line rules permit a reduction of the permissible minimum initial GM for some vessels with timber deck cargo and the inclusion of the volume of this cargo in the derivation of the cross curves.

Outline the circumstances under which this reduction is allowed and explain why this reduction is permitted.

1) The vessel must have timber certificate.

2) Must have Assigned Timber Freeboard.

3) Must have solid stow of deck cargo full length of deck.

4) The vessel must have positive stability at all times and should be calculated with regard to:

a) the increase of timber weight due to

absorption of water.

Ice accretion if applicable.

b) variations in consumables.

c) Free surface effects of the liquids in tanks.

d) Weight of water trapped in the broken spaces within the timber deck cargo especially logs.

5) The stability calculations should include 15% increase in weight due to water absorption during the voyage.

6) KN values may be increased for additional freeboard BUT only 75% of the deck cargo volume may be used for additional reserve buoyancy.

The reason for reduction in the minimum permissible GM is as follows:

1) The deck cargo secured stowed on full length of freeboard deck acts as additional reserve buoyancy.

2) The additional reserve buoyancy is applicable only when the deck cargo is well secured and covers the entire length of the ships cargo deck up to alteast standard superstructure height.

3) The timber cargo also provides a greater degree of protection for the hatches against the sea.

4) The KN values may be increased for additional freeboard however only 75% of the timber volume must be considered as reserve buoyancy.

5) The principle of inclusion of the timber as reserve buoyancy in the derivation of the alternative KN data is illustrated in the following figure.

From the above diagram

1) In figure A when the vessel is heeled beyond the angle of deck edge immersion, GZ values are small when reserve buoyancy of the timber is not included i.e., the GZ values are derived from ships ordinary KN values.

2) In figure B we can see that by using KN values which include 75% of the volume of the immersed timber as reserve buoyancy caused an outward movement of B which increased the GZ values.

3) This increase in GZ value increases the range of stability of the vessel and the dynamical stability.

With regard to Load Line rules distinguish a Type A vessel from a Type B vessel and explain why they have different TABULAR freeboards.

TYPE ATYPE B

1)Designed to carry liquid cargo in bulkOther than Type A vessels which are not designed to carry liquid cargo in bulk

2)Allows a small freeboard i.e., less reserve buoyancy.Has a greater freeboard than Type A

3)The longitudinal hull framing in Type A vessels results in a high degree of sub-divisionsHas less degree of sub-division.

4)Exposed weather deck has high degree of integrity.Exposed weather deck has low degree of integrity as compared to Type A vessel

Access to under deck compartment is through small deck openings which are watertight steel coversAccess to under deck compartment is through large hatch openings which are only weather tight.

5)High degree of safety against flooding because of low permeability of loaded cargo spaces.Vulnerable in heavy weather to flooding

6)Has high degree of sub-divisionLess degree of sub-division

Type A vessel and Type B vessel have different tabular freeboard because:

1) The structural layout of both vessels are different

2) Types of cargo carried are different.

3) Moreover the permeability of the cargo tanks in Type A ships are low as compared to the Type B ship.

4) Therefore in an event of flooding of a compartment, oil from cargo tank of Type A vessel will run out causing decrease in displacement and increase in freeboard, whereas in case of type B ship, the sea water will enter the cargo space resulting in increase in draught and reduction in freeboard.

State the general requirement for a TYPE B vessel to be given the same TABULAR freeboard as TYPE A vessel of the same length.

A type B vessel can be given the same TABULAR freeboard as Type A vessel of same length if the following criteria are satisfied:

Any Type B-60 ships of over 100 mtrs long (Type B-100) satisfying the following conditions at summer draught:

a)Provided with steel hatch covers which are weather tight.

b)Access to the engine room from deck protected by house.

c)Provided with open rails for 50% of the length of the vessel and not bulwark.

d)The weather deck must be fitted with a protected raised catwalk or under deck ways to allow safe access for the crew.

e)Shall remain afloat after flooding of any two fore and aft adjacent compartment with an assumed permeability of 95% at summer draught

Identify the additional corrections required when converting BASIC FREEBOARD to ASSIGNED FREEBOARD, explaining the reason for each correction.Depth correction:

The standard freeboard depth of a ship under the Rules = L 15

If the freeboard depth is more than L 15, then the freeboard is increased

If the freeboard depth is less than L 15, the freeboard may be decreased provided that the superstructure is at least 60% of length amidship position or trunk over entire length of the vessel.

Correction for position of deck line:

Freeboard must be capable of vertical measurement.

If the vessel is having a rounded gunwale, then the freeboard must be corrected by the vertical difference between the actual position of the deck line and the correct position.

Superstructure correction:

Freeboard will be reduced if:

a)The ship is with sufficient standard height superstructure (OR)

b) Has sufficient water tight trunking to a minimum height and width.

This reduction will vary according to the length of the superstructure / trunk as a percentage of the vessels length.

If the superstructure or trunk is of less than the standard height / breadth then the correction will be reduced proportionally.

If it is not of sufficient height or % length or width then no reduction in freeboard.

Sheer correction:

Load line regulations assume a standard sheer for the vessel.

If the vessel has a greater sheer than standard, the basic freeboard is decreased.

If the vessel has a lesser sheer than the standard, the basic freeboard is increased.

If the vessels amidships superstructure is less than 10% length, then there is reduction in freeboard.

Bowheight correction:The load line rules contains a formula for calculating the minimum bow height based on the vessels length and block co-efficient.

If the bow height is les than the calculated height, freeboard is increased.

Summer Freeboard: - Assigned only upon Owners request only increase in freeboard.

Freeboards may also be increased at the owners request or where there are no openings or cargo port holes below the freeboard deck

Corrections are then applied to the Assigned Summer Freeboard in order to determine the Tropical, Winter, Fresh Water and Tropical Fresh water freeboards.

Describe the general provisions of the current Passenger Ship Construction Rules governing the ability of a Class I Passenger vessel to withstand flooding due to damage, and the stability in the final condition.

General Requirements:

1) Margin line is the water line and must be atleast 76mm below the upper surface of the bulkhead deck.

2) Floodable length depends upon the permeability of the compartment.

(a) Permeability for cargo and store spaces = 60%

(b) Machinery spaces = 85%

(c) Passenger spaces = 95%

3) The vessel should remain afloat in the event of damage to any compartment.

4) Factor of sub-division (to determine max spacing between transverse bulkhead) varies inversely with the ships length, the number of passenger and the proportion of under water space used for passenger / crew and machinery space.

5) Greater degree of subdivision (or small factor of subdivision) must be provided when

(a) The vessel is long

(b) The number of passengers is large

(c) Much space below the waterline is used for passenger / crew accommodation and/or machinery space.

6) Permitted length between bulkhead = Floodable length x Factor of sub-division.

Assumed damage:

1) Vertical extent is from keel to deck

2) Transverse extent must be 20% of the Beam of the vessel.

3) Longitudinal extent of damage must be:

11 mtrs between bulkhead (OR)

3m + 3% of the length of the vessel

Assumed Flooding:

The vessel must be able to withstand the flooding of the following number of compartments (final waterline at, or below margin line)

1)Factor of sub-division more than 0.5

THEN

any one compartment

2)Factor of sub-division between 0.5 and 0.33THEN Any 2 adjacent compartments

3) Factor of sub-division 0.33 or less

THEN

Any 3 adjacent compartments

Required Stability after Flooding:

In the final stage, after any equalization (CROSS FLOODING) measures, the vessel must comply with the following condition:

1) Residual GM atleast 50mm.

2) Final heel not to exceed:

7 degs with one compartment flooding (OR)

12 degs if two or more compartment is flooded.

3) Positive residual GZ curve with a range of at least 15 degs

4) Area under residual GZ curve to be at least 0.015 mr up to:

Either

22 degs for one compartment flooding (OR)

27 degs for two compartment flooding (OR)

Angle of progressive flooding f

WHICHEVER IS LEAST

5) Maximum residual righting lever to be at least either:

a) 10 cms (OR)

b) Heeling moment + 0.04 m WHICHEVER IS GREATER

Displacement

6) The heeling moments to be calculated from:

Crowding of all passengers towards one side (OR)

Launching of fully loaded davit launch survival craft, (OR)

Wind pressure

WHICHEVER IS BIGGEST

With reference to the current Passenger Ship Construction and Survey Regulations

(a) Explain the extent of hull flooding assumed when calculating the ships ability to survive hull damage.

In order to arrive at the minimum required stability for the Passenger vessel after suffering flooding of compartment, the following two factors are taken into consideration:

1) Assumed Flooding

2) Assumed damage.

Assumed Flooding

The number of compartments involved in the assumed flooding conditions are based upon the Factor of sub-division. Lesser the factor of sub-division, lesser the Permissible length of the compartment and hence more the number of compartments taken into consideration for assumed flooding. However at any instant not more than 3 compartments are assumed to be in flooded condition.

The vessel must be able to withstand the flooding of the following number of compartments (final waterline at, or below margin line)

1)Factor of sub-division more than 0.5

THEN

any one compartment

2)Factor of sub-division between 0.5 and 0.33THEN Any 2 adjacent compartments

3) Factor of sub-division 0.33 or less

THEN

Any 3 adjacent compartments

Assumed damage:

Assumed damage:

4) Vertical extent is from keel to deck

5) Transverse extent must be 20% of the Beam of the vessel.

6) Longitudinal extent of damage must be:

11 mtrs between bulkhead (OR)

3m + 3% of the length of the vessel

If the damage of lesser extent than indicated above would result in a more severe condition regarding heel and GM loss, such damage shall be assumed for the purpose of the calculation.

(c)Outline the additional factors taken into account to determine the permissible length of compartments in ships built after 1990.

Permissible length of the compartment having its centre at a point in the length if the ship means the product of the floodable length at that point and the factor of sub division of the ship.

Permissible length = Floodable length x Factor of Sub-division.

The features of the ship that are considered in determining the length for the purpose of subdivision calculation includes:

1) Block co-efficient of the vessel

2) Freeboard ratio

3) Sheer Ratio

4) Compartment permeability

5) Length of the vessel

6) Number of passengers.

7) The proportion of the underwater space used for passengers / crew and machinery space

The permissible length between the compartments is reduced (due to decrease in the Factor of sub-division) when

1) The length of the ship is more

2) More number of passengers are carried

3) Much of the space below the waterline is used for passenger/crew accommodation and or machinery space.

Describe Stockholm agreement with reference to the stability requirement of Passenger Ro-Ro vessels.

Purpose of Stockholm agreement:

1) Lays down the stability requirement for Passenger Ro-Ro vessel.

2) Agreement concluded after the disaster of Estonia

3) Signed between nine northern European states in 1996.

4) These upgrades SOLAS 90 standards.

5) Takes into account the effect of water accumulation on the vehicle deck after damage , making the ship safer in heavy seas.

6) Applies to all Passenger Ro-Ro vessels operating on scheduled international voyages between or from designated ports in northern Europe irrespective of Flag.

Requirements by the Agreement:

1) Demands that a vessel satisfies with the requirement of SOLAS 90 with a constant height of water on deck.

2) The height of water on vehicle deck is based on a 4.0 mtr significant wave.

3) The height of water should be

0.5mtrs if the residual freeboard at the damage opening is 0.3 mtrs or less

0.0 mtrs if the residual freeboard at the damage opening is 2.0 mtrs or more

4) Intermediate values can be determined by linear interpolation

Describe the stability problems associated with a conventional Ro-Ro ferry.

The stability of vehicle ferries poses particular problems due to the following:

Free Surface Effect:

a)Because the vehicle deck usually extends over the length and breadth of the vessel without restriction, this type of vessel is especially vulnerable to the effects of free surface

b)Such vessel may rapidly lose all stability and capsize if the vehicle deck becomes flooded.

c)Causes of such flooding include

Damage to bow or stern door at sea

Bow or stern door left open at sea

Bow or stern door open and unattended during loading / discharging operations.

Loss of watertight integrity due to collision with another vessel or rocks.

Loss of water tight integrity due to shift of a vehicle in heavy seas.

Use of water curtains (coupled with inadequate drainage)

Inadequate Stability Information due to:

a)Speed of turnaround in port.

b)Lack of detailed information about cargo units and disposition

Other factors:

a)High KG of cargo units on vehicle deck

b)Vulnerability of Ro-Ro units to shifting in bad weather.

c)High windage area of Ro-Ro vessels.

What precautionary measures must be adopted to improve stability of Ro-Ro ferries

a)Automatic draught gauges at the stem and stern with remote readout should ensure that flooding of the vehicle deck in port is avoided.

b)A loading computer must be available to the ships officer in port for rapid calculation of stability before the vessel sails.

c)Indicator lights must be provided on the bridge to show when shell/loading doors are open / closed.

d)Heavy Ro-Ro cargo units must be weighed ashore and the information provided to ships officers. Such units must be secured by chains to the deck before departure.

e)Increased drainage requirements for vehicle decks.

f)Stockholm agreement provides enhanced stability requirement for Ro-Ro passenger ferries with 50 cms of water on vehicle deck.

c)Provision of some form of sub-division on the vehicle deck.

Discuss the stability problem associated with the design and operation of a conventional Oil Rig supply vessel.

The stability of the Offshore supply vessel poses particular problem due to the following:

Loading and/or Discharging cargo at sea:

a)Affects the vertical, transverse and longitudinal position of the G of the vessel.

b)This is of particular relevance since cargo operations may be taking place as the vessel is rolling and pitching in a seaway.

c)The cargo is often in liquid form (water, fuel, mud etc) which will result in virtual loss of stability due to FSE during the cargo handling operation.

Excessive Stern Trim

a)Occurs through longitudinal distribution of loaded weight.

b)It may occur during an ill advised discharge / load, or when working with cables/anchors.

c)Considerable stern trim develops during these stages.

d)This may cause the working deck to become awash thereby reducing the water plane area and critically reducing the vessels stability.

Water entrapment

The working deck is often used to carry drill supplies, machinery, and pipes etc., some of which have been found to retail large amount of water due to seas on the after deck. An allowance for such volume of water entrapped must be made in the stability calculation.

Free Trim

Free trim affects the GZ curve of the ve

Stability Talkie Talkie (1)

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