Crack2NDMates

GYRO compass and its principle

GYRO COMPASS

The Free Gyroscope

This consists of a rotor, supported in such a way that it is free to have its spin axis pointing in any direction. (the free gyroscope is also referred to as the ‘sensitive element’.)

The method of support for the gyroscope must therefore provide three degrees of freedom:

freedom to spin about its own axis;
freedom to tilt about a horizontal axis;
freedom to turn in azimuth about a vertical axis.

A free gyroscope has 2 properties which are used in the construction of the gyrocompass:

Gyroscopic Inertia,
Precession.

Gyroscopic Inertia (sometimes referred to as ‘rigidity in space’:

A freely spinning gyroscope will maintain its axis of spin in the same direction with respect to space irrespective of how its supporting base is turned.

It resists any attempt to change its direction of spin.
Thus a free gyroscope has high directional stability.
This property is called GYROSCOPIC INERTIA or RIGIDITY IN SPACE or DIRECTIONAL STABILITY.
This is based on Newton's First Law of Motion which states: Everybody continues in its state of rest or of uniform motion in a straight line unless it is compelled by external forces to change that state.

Precession:

This phenomenon is found only in spinning bodies.
It is the movement of the spin axis when the force is applied to Gyroscope.
Precession always acts at 90° to the direction of applied force.
To determine the direction of precession, rotate the applied force 90° in the direction of the rotor spin.

When a couple is applied about its horizontal axis, the spin axis will turn at right angles tothe applied force in the direction of the spin of the wheel.
Similarly, couple applied about the vertical axis will make the spin axis turn about the horizontal axis in the direction of the spin axis of the wheel.

The Effects Of The Earth’s Motion On A Free Gyroscope

Due to the gyroscopic inertia of a free gyroscope, the spin axis will remain to point in a fixed direction in space, i.e. towards an imaginary gyro star on the celestial sphere.
As the earth rotates on its axis fixed stars in space appear to move.
This movement is noticed as a change in altitude and azimuth, resulting in the star appearing to trace out a circular path.
Depending on the observer’s latitude and the declination of the star, the circular path traced out by the star may either wholly be above the horizon or maybe carried below the horizon at some stage.
If a star appears to show this movement then the spin axis of a free gyroscope will also trace out a circular path as it remains pointing at the star.
The movement of a star can be described in terms of ‘altitude’ and ‘azimuth’. The terms used to describe the same movement of the spin axis of a free gyroscope are ‘tilt’ and ‘drift’.

Tilt

It is the angle of elevation or depression (upward or downward motion) of the spin axis above or below the Horizontal.
Equivalent to a true altitude of the Gyro star.

Tilting

This is the rate of change of Tilt of the spin axis. It is given by the formula
Tilting = 15° Sin (Azimuth) Cos (Latitude) / Hour
Az will always be in quadrantal form.
It is +ve or upwards when spin axis points east of the meridian &
-ve or downwards when spin axis points west of the meridian

Drift(Azimuth)

The direction in which the spin axis points w.r.t. the true North.
In respect to the Gyro, this is also known as Drift.

Drifting

Drifting is the rate of change of Azimuth of the spin axis.
It is given by the formula
Drifting = 15° Sin (Latitude) per hour
The formula is only applicable if the spin axis is almost horizontal or the Tilt is close to zero.
Drifting is +ve or easterly when spinning axis points below the pole & -ve or westerly when spinning axis points above the pole.

Motion of Gyro at Poles

Initially if the spin axis is kept horizontal. The axis maintains constant tilt and drifts around the horizon @ 15 deg/hour.
This rate is same as the earth’s rate of rotation (360 deg /24 hours).
At N pole the drift is in clockwise direction and at S pole it is in ACW direction.
At a pole latitude is 90; therefore, maximum rate of Dg occurs at poles. (Dg = 15 deg Sin lat/Hour )
If initially spin axis pointing at zenith i.e. at a tilt of 90.
It will continue pointing in the same direction with no tilt and drift.

Motion of Gyro at Equator

Spin axis is initially pointing E. (like a body at E on rational horizon and zero declination).
There will be no drift and tilting will be maximum; changing at the rate of 15deg per hour.
The azimuth will remain 090 and after meridian passage it will be 270, with tilting now changing @ -15/hour (negative sign to show downward tilt)
Thus Tilting is maximum at equator i.e. zero latitude and minimum at poles (90 deg lat) Also when pointing East azimuth is maximum = 90.
Hence the tilting formula Tilting = 15° Sin (Azimuth) Cos (Latitude) / Hour
If the Spin axis is initially pointing N.
It will remain pointing in North direction, with drift and tilt both zero.
Here azimuth is zero and Tilting is nil.

Motion of Gyro at intermediate latitude

As discussed above the Spin Axis will point towards the Imaginary gyro star
As the gyro star crosses the horizon it will be changing its azimuth as well as altitude and tracing a path in the sky centred at the pole.
Hence the spin axis will also keep on Drifting & Tilting.

Controlling The Gyro

The spin axis of a freely suspended gyro traces out a circular path as it remains pointing in a fixed direction in space, i.e. the apparent motion due to the earth’s rotation. The requirements of a gyrocompass are that the spin axis should point in a fixed direction, True North, 000°T.

In order for the gyroscope to do this it must be made to:

seek North,
settle and remain pointing North.

North Seeking

Only with free gyro cannot be used for direction determination.
Thus a system is required, which can not only sense this movement but also apply a force to
control the movement due to Dg and Tg.
The force of gravity is used for making free gyro North Seeking.
This method of making the gyroscope North seeking is termed ‘Gravity Control’.
The principle may be shown by suspending a weight on the spin axis.
This is done in two ways, known as top-heavy effect and bottom-heavy effect.
Top-heavy effect requires the rotor to rotate in ACW direction and bottom-heavy effect requires CW spin, when viewed from the south end.
It has the effect of converting the circular path traced out by the spin axis into an elliptical path.

The result is that the spin axis oscillates backwards and forwards across the meridian but does not settle and point in a fixed direction.

Top Heavy Control:

The gyroscope is made North Seeking by attaching a weight to the rotor casing above the COG of the rotor.
When the spin axis is horizontal the COG of the weight passes through the centre of the rotor producing no torque.
The earth’s rotation will, however, tilt the spin axis.
When the gyro axis tilts the COG of the weight does not act through the centre of the rotor and this weight produces a torque about the horizontal axis (or in the vertical plane).
This torque will result in Precession in the horizontal plane that tends to take the spin axis towards the meridian.
This precession is called control precession (Pc).
The direction of spin of the rotor must be in such as to produce a westerly precession of the North end of the spin axis when that end is tilted upwards.
And this direction turns out to be ACW in top-heavy type gyros

Bottom heavy Control:

This direction turns out to be clockwise in bottom-heavy type gyros
The path traced by the N end of the Spin Axis is now elliptical.
Because in gravity control gyroscope there are three vectors interacting with each other (Dg, Tg & Pc), instead of just two (Dg and Tg).
While the two vectors resulted in a circular path traced by the spin axis, centred about pole;
The introduction of the third vector results in an elliptical path.

Understanding the Ellipse:

Control precession (Pc), acts westwards, when Spin Axis is tilted upwards and eastwards
when the axis is tilted downwards.
Tilting acts upwards when east of meridian and downwards when west of the meridian.
Drifting is always Eastward as this ellipse is formed below the pole.
In this elliptical path, it is to be seen that, while the Dg vector remains same in size (15xSin lat),
but the Tg vector changes because Tg also varies with Sin Az and azimuth is continuously
changing. (Tilting = 15° Sin (Azimuth) Cos (Latitude) / Hour)
Pc vector also changes in magnitude because Pc is proportional to tilt.
In commercial gyros, the time period to complete one revolution of the ellipse is usually about 84-85 minutes.

North Settling:

Controlling Gyro by Liquid Ballistic
Practically the gyro is controlled using a liquid ballistic, mercury.
Mercury flows between pots in the N-S axis under the influence of gravity when the
gyro axis tilts out of the horizontal.
COG of the ballistic system coincides with that of the rotor.
This is similar to a top-heavy arrangement. The spin of the gyro axis is anticlockwise viewed from the south.
In order to make the gyro settle and point in a fixed direction, i.e. 000°T, it is necessary to impose a further precession which will damp out the gravity controlled elliptical path traced out by the spin axis.
This method of making the gyroscope North settling is termed ‘Damping’.

Damping the Ellipse:

We know that gravity controlled gyroscope also cannot be used as a compass because the axis does not point along the meridian, but oscillates along the ellipse repeatedly.
Thus some form of damping is needed to damp these oscillations and make the axis settle in equilibrium along the meridian.
In damping, the controlled ellipse becomes a spirally inward, towards the equilibrium position, where the axis will settle and if disturbed from that position will return to it.

There are 2 ways of achieving damping:

Damping in tilt (in case of top heavy type gyro) - when the spin axis moves out of the horizontal the damping precession opposes this movement, bringing the spin axis back to the horizontal.
Damping in azimuth (in case of bottom heavy type gyro) - when the spin axis moves out of the meridian the damping precession opposes this movement, bringing the spin axis back to the meridian.

Damping in Tilt: (in case of top-heavy effect)

In this method of damping, the damping precession Pd opposes the movement of the spin axis
when the spin axis is moving away from the horizon and assists it when moving towards the horizon. The torque about the vertical axis causes damping precession in tilt, i.e. up or down.
Damping precession depends on the angle of tilt, the greater the tilt, the greater the damping precession.

Effect of damping in tilt on the ellipse

As the controlled gyro follows the first part of the ellipse, the damping precession will oppose the tilting. This means the gyro’s angle of tilt when reaching the meridian, is not as great as for the undamped gyro.
Thus the control precession is less and the eastward drift is greater, therefore, the gyro spin axis will not travel as far west.
As the gyro spin axis returns to the horizon the damping precession will assist its return.
As the axis tilts below the horizon the damping precession will oppose it, reducing the maximum angle of tilt downward and thus reducing the eastward drift and control precession.
The gyro therefore does not travel as far East. Next time around the ellipse, the damping precession will again oppose movement away from the horizon, so again the maximum angle of tilt will be reduced making the ellipse smaller.
Eventually, the gyro will settle where the control precession cancels the drift and the damping precession cancels the movement of tilt.

Applying damping in tilt:

Damping in tilt is achieved in the Sperry MK 20 gyrocompass by adding a small weight (17 gr) on the top of the rotor case.
The weight is offset to the west of the vertical axis.

With the spin axle horizontal, weight is directly above the tilt bearings and hence causes no precession.
When the axle tilts, the weight has a tipping effect on the gyro. Since the weight is offset, the tipping will have a vertical and horizontal component.
The vertical component generates a torque around a horizontal axis which causes a precession around the vertical axis at the same direction of Pc.
This component is seen/calculated within the control force. The horizontal component generates a torque about a vertical axis which causes a precession (Pd) around a horizontal axis.
This opposes the tilt and brings the spin axis towards the horizon.

How to compensate for the damping error?

The damping error in gyro compasses which utilize damping in tilt is to be removed. (Like Sperry gyro compasses)
The First method is by a mechanical means in which the latitude is set. The whole phantom
ring turns according to the set latitude therefore the compass card turns to eliminate
the damping error.
By using a torque motor which produces a precession to cancel the drift at settling point
and hence causing the spin axis to point north. This is the same motor used for
correcting the speed error.
In digital gyro compasses, this error is simply corrected by feeding (inputting) the
latitude to the microcomputer unit.

Damping in azimuth:

In this method of damping, the damping precession opposes the movement of the gyro spin axis when it is moving away from the meridian and assists the movement when moving towards the meridian.
The torque about the horizontal axis will cause a damping precession in azimuth. It depends on the rate of tilting, the greater the rate of tilt, the greater the damping precession.

Flammability diagram - "Explained"

Please read the below definitions before proceeding to understand the flammability diagram.

Definition:

LEL (Lower Explosive Limit) / LFL (Lower Flammable Limit)

The concentration of a Hydrocarbon gas in air below which there is insufficient Hydrocarbon (fuel) to support and develop combustion.

UEL (Upper Explosive Limit) / UFL (Upper Flammable Limit)

The concentration of a Hydrocarbon gas in air above which there is insufficient oxygen to support and develop combustion.

Flammable range

Range of Hydrocarbon gas concentrations in oxygen between the LEL and UEL (LFL and UFL).

The composition of this mixture has to lie within a range of proportions, and this range is called the FLAMMABLE RANGE.

Too lean

A tank atmosphere made incapable of burning by the deliberate reduction of the hydrocarbon content (fuel content) below the LFL (i.e < 1%)

Too rich

A tank atmosphere made incapable of burning by the deliberately maintaining the hydrocarbon content (fuel content) over the UFL (i.e > 10%)

Limits of flammability:

Flammable mixture (HC+O2) mixtures will ignite and burn only over a well-specified range of compositions.
The mixture will not burn when the composition is lower than the lower flammable limit (LFL); the mixture is too lean for combustion.
The mixture is also not combustible when the composition is too rich; i.e, when it is above the upper flammable limit (UFL).
In this diagram, the following figures are used for HC gas 'C' LFL as 1% of vol and point 'D' UFL as 10% vol (LFL 1% and UFL 10% is only for understanding the purpose of the diagram, for each cargo LFL and UFL varies, for that refer MSDS of that cargo.

LFL (Lower flammable limit): 1% gas to 99% air.
UFL (Upper flammable limit): 10% gas to 90% air.

Therefore the FLAMMABLE RANGE IS 9%
In inerted condition:

The Oxygen level in the tank if below 11% will not support combustion.
Maximum permissible allowance is 8%.

Easy way to enumerate "Boxing of compass"

Remembering Boxing of Compass will be very helpful during the examination as well as in day to day life for sailors.

32 points of Compass is mostly used by sailors.

4 "CARDINAL" Points: N, E, S, W

4 "INTER CARDINAL Points" 1/2 way between these are the combination of the two major points: NE, SE, SW, NW

Till here, anyone can remember easily.😄

Further how to enumerate the compass points?

To enumerate compass points there is a technique called "Rule of 8"

The below pictures will show you how to enumerate compass points for

1st quadrant or NE quadrant.

In same way you can enumerate other quadrants as well😊

Exposure limit of toxic substance

The toxic hazards to which personnel are exposed in tanker operations arise almost entirely from exposure to gases of various kinds such as H2S, Benzene, Mercaptan, etc.,

A number of indicators are used to measure the concentrations of toxic vapours and many substances have been assigned Threshold Limit Values (TLVs), sometimes referred to as Permissible Exposure Limits (PELs).

Exposure limits may be set by international organisations, national administrations or by local regulatory standards. Any limits established by regulation should not be exceeded.

Where a PEL is not available for an airborne contaminant, other Occupational Exposure limits such as the Threshold Limit Values (TLVs) published by the American Conference of Governmental Industrial Hygienists (ACGIH) may be used. The values quoted are expressed as Threshold Limit Values (TLVs) in parts per million (ppm) by volume of gas in the air.

Threshold limit value (TLV)

Is referred to airborne concentrations of substances under which it is believed that nearly all workers may be exposed day after day with no adverse effect.

There are three different types of TLV's:

Time Weighted Average (TLV-TWA)

The airborne concentrations of a toxic substance averaged over an 8 hour period, usually expressed in parts per million (ppm).

Short Term Exposure Limit (TLV-STEL)

The airborne concentration of a toxic substance averaged over any 15 minute period, usually expressed in parts per million (ppm).

Ceiling (TLV-C)

The concentration that should not be exceeded during any part of the working exposure.

Extracted from MSDS

Important Tanker definitions

-Reference ISGOTT:

Anti-static additive:
A substance added to a petroleum product to raise its electrical conductivity to a safe level above 50 picoSiemens/metre (pS/m) to prevent the accumulation of static electricity.

Auto-ignition:
The ignition of a combustible material without initiation by a spark or flame, when the material has been raised to a temperature at which self-sustaining combustion occurs.

Bonding:

The connecting together of metal parts to ensure electrical continuity.

Cathodic protection:

The prevention of corrosion by electrochemical techniques. On tankers, it may be applied either externally to the hull or internally to the surfaces of tanks. At terminals, it is frequently applied to steel piles and fender panels.

Clingage:

Oil remaining on the walls of a pipe or on the internal surfaces of tanks after the bulk of the oil has been removed.

Cold Work:

Work that cannot create a source of ignition.

Corona:

A diffuse discharge from a single sharp conductor (less than 5 mm in diameter) that slowly releases some of the available energy. Generally, the corona is incapable if igniting a gas like propane or vapours like this given-off by gasoline. Corona may ignite vapours like hydrogen or acetylene, which require much lower energies for ignition.

Earthing /Grounding:

The electrical connection of equipment to the main body of the earth to ensure that it is at earth potential. Onboard ship, the connection is made to the main metallic structure of the ship which is at earth potential because of the conductivity of the sea.

Enclosed space:

A space that has limited openings for entry and exit, unfavourable natural ventilation, and that is not designed for continuous worker occupancy. This includes cargo spaces, double bottoms, fuel tanks, ballast tanks, pump rooms, cofferdams, void spaces, duct keels, inter-barrier spaces, engine crankcases and sewage tanks.

Entry permit:

A document issued by a Responsible Person allowing entry into space or compartment during a specific time interval.

Explosimeter:

An instrument for measuring the composition of hydrocarbon gas/air mixtures, usually giving the result as a percentage of the lower flammable limit (LFL).

Explosion-proof /Flame-proof:

Electrical equipment is defined and certified as explosion-proof when it is enclosed in a case which is capable of withstanding the explosion within it of a hydrocarbon gas/air mixture or other specified flammable gas mixture. It must also prevent the ignition of such a mixture outside the case either by spark or flame from the internal explosion or as a result of the temperature rise of the case following the internal explosion. The equipment must operate at such an external temperature that a surrounding flammable atmosphere will not be ignited.

Explosive range:

The range of hydrocarbon gas concentrations in air between the lower and upper flammable (explosive) limits. Mixtures within this range are capable of being ignited and of burning.

Flame arrester:

A permeable matrix of metal, ceramic or other heat-resisting materials which can cool a deflagration flame, and any following combustion products, below the temperature required for the ignition of the flammable gas on the other side of the arrester.

Flame screen:

A portable or fitted device incorporating one or more corrosion resistant wire woven fabrics of very small mesh which is used for preventing sparks from entering a tank or vent opening or, for a short time, preventing the passage of flame. (Not to be confused with ‘Flame arrester’). Capable of being ignited and of burning. For the purposes of this guide, the terms ‘flammable’ and ‘combustible’ are synonymous.

Flammable range (also referred to as ‘Explosive range’):

The range of hydrocarbon gas concentrations in air between the lower and upper flammable (explosive) limits. Mixtures within this range are capable of being ignited and of burning.

Flashpoint:

The lowest temperature at which a liquid gives off sufficient gas to form a flammable gas mixture near the surface of the liquid. It is measured in a laboratory in standard apparatus using a prescribed procedure.

Flow rate:

The linear velocity of flow of liquid in a pipeline, measured in metres per second (m/s). The determination of the Flow Rates at locations within cargo pipeline systems is essential when handling static accumulator cargoes. (Also see ‘Loading rate’).

Gas free:

A tank, compartment or container is gas free when sufficient fresh air has been introduced into it to lower the level of any flammable, toxic, or inert gas to that required for a specific purpose, e.g. hot work, entry, etc.

Hazardous area:

An area onshore which for the purposes of the installation and use of electrical equipment is regarded as dangerous. Such hazardous areas are graded into hazardous zones depending upon the probability of the presence of a flammable gas mixture.

Hot work:

Work involving sources of ignition or temperatures sufficiently high to cause the ignition of a flammable gas mixture. This includes any work requiring the use of welding, burning or soldering equipment, blow torches, some power driven tools, portable electrical equipment which is not intrinsically safe or contained within an approved explosion-proof housing, and internal combustion engines.

Hot work permit:

A document issued by a responsible person permitting specific hot work to be done during a specific time interval in a defined area.

Hydrocarbon gas:

A gas composed entirely of hydrocarbons.

Inert condition:

A condition in which the oxygen content throughout the atmosphere of a tank has been reduced to 8 per cent or less by volume by the addition of inert gas.

Inert gas:

A gas or a mixture of gases, such as flue gas, containing insufficient oxygen to support the combustion of hydrocarbons.

Inert gas plant:

All equipment fitted to supply, cool, clean, pressurise, monitor and control the delivery of inert gas to the cargo tank systems.

Inert gas system (IGS):

An inert gas plant and inert gas distribution system together with means for preventing backflow of cargo gases to the machinery spaces, fixed and portable measuring instruments and control devices.

Inerting:

The introduction of inert gas into a tank with the object of attaining the inert condition.

Insulating flange:

A flanged joint incorporating an insulating gasket, sleeves and washers to prevent electrical continuity between ship and shore.

Interface detector:

An electrical instrument for detecting the boundary between oil and water.

Intrinsically safe:

An electrical circuit or part of a circuit is intrinsically safe if any spark or thermal effect produced normally (i.e. by breaking or closing the circuit) or accidentally (e.g. by a short circuit or earth fault) is incapable, under prescribed test conditions, of igniting a prescribed gas mixture.

Loading over the top (also known as ‘Loading overall’):

The loading of cargo or ballast through an open-ended pipe or using an open-ended hose entering a tank through a hatch or other deck opening, resulting in the free fall of liquid.

Loading rate:

The volumetric measure of liquid loaded within a given period, usually expressed as cubic metres per hour (m3/hr) or barrels per hour (bbls/hr).

Lower flammable limit (LFL):

The concentration of hydrocarbon gas in air, below which there is insufficient hydrocarbon to support and propagate combustion. Sometimes referred to as a lower explosive limit (LEL).

Manifold:

The flanged pipe assembly mounted onboard ship to which the presentation flange of the marine loading arm or spool piece connects.

Material Safety Data Sheet (MSDS):

A document identifying the substance and all its constituents, providing the recipient with all necessary information to safely manage the substance.

The format and content of an MSDS for MARPOL Annex I cargoes and Marine Fuel Oils is prescribed in IMO Resolution MSC.150 (77).

Mercaptans:

A group of naturally occurring sulphur containing organic chemicals. They are present in some crude oils and in pentane plus cargoes. They have a strong odour.

Naked lights:

Open flames or fires, lighted cigarettes, cigars, pipes or similar smoking materials, any other unconfined sources of ignition, electrical and other equipment liable to cause sparking while in use, unprotected light bulbs or any surface with a temperature that is equal to or higher than the minimum ignition temperature of the products handled in the operation.

Non-volatile petroleum:

Petroleum having a flashpoint of 60ºC or above, as determined by the closed cup method of test.

Odour threshold:

The lowest concentration of vapour in the air which can be detected by smell.

Oxygen analyser/meter:

An instrument for determining the percentage of oxygen in a sample of the atmosphere drawn from a tank, pipe or compartment.

Pellister:

An electrical sensor unit fitted in a flammable gas detector for measuring hydrocarbon vapours and air mixtures within the flammable range.

Permit to work:

A document issued by a responsible person which allows work to be performed in compliance with the vessel’s SMS

Permit to work system:

A document system for controlling activities that expose the ship, personnel and the environment to hazard. The system will provide risk assessment techniques and apply them to the varying levels of risk that may be experienced. The system should conform to a recognised industry guideline.

Phase of oil:

Oil is considered to have three phases in which it can exist depending on the grade of oil and its temperature. The three phases are the solid phase, the liquid phase and the vapour phase. The

phases do not exist in isolation and operators must manage the carriage of oil with an understanding of the combinations of the phases of oil in the cargo being carried.

Pour point:

The lowest temperature at which a petroleum oil will remain fluid.

Pressure surge:

A sudden increase in the pressure of the liquid in a pipeline brought about by an abrupt change in flow rate.

Pressure/vacuum relief valve (P/V valve):

A device which provides for the flow of the small volumes of vapour, air or inert gas mixtures caused by thermal variations in a cargo tank.

Purging:

The introduction of inert gas into a tank already in the inert condition with the object of:

-further reducing the existing oxygen content; and/or

-reducing the existing hydrocarbon gas content to a level below which combustion cannot be supported if the air is subsequently introduced into the tank.

Reid vapour pressure (RVP):

The vapour pressure of a liquid determined in a standard manner in the Reid apparatus at a temperature of 37.8ºC and with a ratio of gas to the liquid volume of 4:1. Used for comparison purposes only.

Relaxation time:

The time is taken for a static charge to relax or dissipate from a liquid. This time is typically one-half minute for static accumulator liquids.

Settling time:

The time it takes for tank contents to stop moving once the filling has stopped. The movement can be because of thermal currents, solids and/or water settling or of gas bubbles rising. Typically this time is 30 minutes.

Sounding pipe:

A pipe extending from the top of the tank to the bottom through which the contents of the tank can be measured. The pipe is usually perforated to ensure the level of liquid in the pipe is the same as the level of liquid in the body of the tank and to prevent the possibility of spillages. The pipe should be electrically bonded to the ship’s structure at the deck and at its lower end.

Sour crude oil:

Crude oil containing appreciable amounts of hydrogen sulphide and/or mercaptans.

Spontaneous combustion:

The ignition of material brought about by a heat producing (exothermic) chemical reaction within the material itself without exposure to an external source of ignition.

Spread loading:

The practice of loading a number of tanks simultaneously to reduce the velocity of the cargo in the pipelines serving individual tanks to avoid static electricity generation when loading static accumulator cargoes.

Static accumulator oil:

An oil with an electrical conductivity less than 50 picoSiemens/metre (pS/m), so that it is capable of retaining a significant electrostatic charge.

Static electricity:

The electricity produced by dissimilar materials through physical contact and separation.

Static non-accumulator oil:

An oil with an electrical conductivity greater than 50 picoSiemens/metre (pS/m), which renders it incapable of retaining a significant electrostatic charge.

Stripping:

The final operation in draining liquid from a tank or pipeline.

Surge pressure:

A sudden increase in the pressure of the liquid in a pipeline brought about by an abrupt change in flow rate. eg. through starting or stopping of a pump, a rapid closure or opening of a value or a reduction of pipeline diameter. The pressure surge may cause a rupture of piping and an extensive oil spill.

Tank cleaning:

The process of removing hydrocarbon vapours, liquid or residue from tanks. Usually carried out so that tanks can be entered for inspection or hot work.

Threshold Limit Value (TLV):

Airborne concentrations of substances under which it is believed that nearly all workers may be exposed day after day with no adverse effect.

Topping off:

The operation of completing the loading of a tank to a required ullage.

Topping up:

The introduction of inert gas into a tank which is already in the inert condition with the object of raising the tank pressure to prevent any ingress of air.

Toxicity:

The degree to which a substance or mixture of substances can harm humans or animals.

‘Acute toxicity’ involves harmful effects to an organism through a single short term exposure.

‘Chronic toxicity’ is the ability of a substance or mixture of substances to cause harmful effects over an extended period, usually upon repeated or continuous exposure, sometimes lasting for the entire life of the exposed organism.

True vapour pressure (TVP):

The true vapour pressure of a liquid is the absolute pressure exerted by the gas produced by evaporation from a liquid when gas and liquid are in equilibrium at the prevailing temperature.

Ullage:

The space above the liquid in a tank, conventionally measured as the distance from the calibration point to the liquid surface.

Upper flammable limit (UFL):

The concentration of hydrocarbon gas in the air above which there is insufficient oxygen to support and propagate combustion. Sometimes referred to as the upper explosive limit (UEL).

Vapour:

A gas below its critical temperature.

Vapour emission control system (VECS):

An arrangement of piping and equipment used to control vapour emissions during tanker operations, including ship and shore vapour collection systems, monitoring and control devices and vapour processing arrangements.

Vapour lock system:

Equipment fitted to a tank to enable the measuring and sampling of cargoes without the release of vapour/inert gas pressure.

Volatile petroleum:

Petroleum, having a flashpoint below 60ºC as determined by the closed cup method of testing.

Water fog:

A suspension in the atmosphere of very fine droplets of water usually delivered at high pressure through a fog nozzle for use in fire fighting.

Water spray:

A suspension in the atmosphere of water divided into coarse drops by delivery through a special nozzle for use in fire fighting.

CAP-Condition assessment program

Condition Assessment Program(CAP) is a specialized survey program which offers owners a detailed assessment of a ship's actual condition.
CAP is a voluntary service.
CAP has been created voluntarily, for the ship owners to document the quality of vessels.
Class is aimed at ensuring a minimum standard for the vessel whereas the main purpose of CAP is to evaluate and report the vessel’s condition above minimum class standard.
CAP is designed for Tankers and Bulk carriers aged 15 years and above and may be used by other vessels as well and at any age.
Assess physical condition and maintenance of vessel above the minimum requirements for Class

Review of Class Records
Fatigue Analysis
Condition Assessment Survey
Verification of Gaugings
Structural Evaluation
CAP Report and rating

Most charterers require a CAP rating on all vessels aged 15 years and above.
The rating of each main structural element is based on the following input:

Visual inspection.
UTM (ultrasonic thickness measurement).
Paint coating (only for ballast tanks).

A CAP rating is assigned to the vessel.

CAP 1 (Very Good)
CAP 2 (Good)
CAP 3 (Satisfactory)
CAP 3 (Poor)

CAP 1 and CAP 2 rating becomes more attractive to charterers.
A ship participating in a classification society’s CAP is subject to a CAP survey which, if satisfactory, will issue a CAP Certificate.
The certificate of CAP indicating the ship’s overall rating (Overall Rating for CAP-HULL, CAP-MACHINERY / CARGO SYSTEM) is issued.

The benefits of CAP are:

Less time off-hire.
To have the vessel judged based on the actual condition on board rather than age.
To contribute to protecting life, property and the environment and to ensure the safest possible transportation of the cargo.
To establish a sound basis for decisions on repair or investments in order to extend the lifetime of the vessel.
To document a vessel's technical condition.
Higher resale value.

CAS-Condition asssessment scheme

Explain CAS (Condition assessment scheme):

The IMO initiated the Condition Assessment Scheme (CAS) for single-hull oil tankers which was developed in line with the Enhanced Survey Program (ESP).
CAS entered into force on 5th April 2005.
The purpose of CAS is to provide an international standard to meet the requirement of MARPOL annex 1.
CAS is one of the mandatory surveys that need to be completed by all the Oil tankers which fall in category 2 and category 3 of MARPOL annex 1.
To Category 2 and Category 3 oil tankers, In order for these tankers to operate beyond 2010 to their mandatory phase-out date under IMO rules, the CAS requires that they are satisfactorily surveyed and certified.
It has specific responsibilities with the deadlines to plan the survey. The entire process starts approx six to eight months in advance.
A CAS survey includes an overall survey, close-up survey, thickness measurements and pressure testing within the cargo area and ballast tanks, including fore and aft peak tanks, of an oil tanker.
The first CAS survey shall be carried out concurrent with the first intermediate or renewal survey after 5 April 2005, or after the date when the ship reaches 15 years of age, whichever occurs later.
On completion of the CAS survey, the attending surveyors will issue an Interim CAS Statement of Compliance, valid for 5 months, which will enable the completion of the CAS Final Report and verification by the flag State Administration, who will then issue the final CAS Statement of Compliance. Further CAS surveys will then be carried out at intervals of 5 years.

Further clarification for category 2 and 3 oil tankers:

A Category 2 tanker is an oil tanker of 20, 000 dwt and above carrying crude oil, fuel oil, heavy diesel oil or lubricating oil, or of 30, 000 dwt and above carrying other oils which complies with the MARPOL requirement for protectively located segregated ballast tank arrangements;
A Category 3 oil tanker is one of 5, 000 dwt and above but less than the tonnage specified for Category 2 tankers.
Depending on their delivery dates, Category 2 and Category 3 tankers must be phased out between 2005 and 2010.
A Category 2 or Category 3 tanker of 15 years and over after the date of delivery must comply with the CAS.
The flag State Administration may allow a Category 2 or Category 3 tanker to operate beyond 2010 subject to satisfactory CAS surveys, but they must be taken out of service by the anniversary in 2015 of the delivery date or when they reach 25 years of age, whichever is earlier.

Preparations for CAS survey:

***RO - Recognised organisation

***SOC - Statement of compliance

Gas measuring Instruments

SOLAS Requirement:

SOLAS Regulation XI/1-7 states that:

Every ship to which SOLAS Chapter 1(General provisions) applies shall carry an appropriate portable atmosphere testing instrument or instruments.
As a minimum, these shall be capable of measuring concentrations of 1)oxygen, 2)flammable gases or vapours, 3)hydrogen sulphide, 4)carbon monoxide.

The SOLAS Convention requires that ships carrying cargoes that are likely to emit a toxic or flammable gas or to cause oxygen depletion in a cargo space are provided with an appropriate instrument for measuring the concentration of gas or oxygen in the air, together with detailed instructions for its use.

Methods to Test "Weather tightness"

There are 3 main types for testing "Weather tightness" of hatch cover:

Hose test
Ultrasonic test
Chalk test

Hose test:

Procedure for Hose test:

Close and secure the hatch cover,
A powerful jet of water taken by hose from the fire main,
Jet should be directed towards every watertight joint in the hatch cover,
One person will be hosing on every joint of the hatch cover and inform the second person inside the hold, which location is being tested,
The second person will view the appropriate seam and makes a note of any leakage.

Limitations of Hose test:

Testing only be carried out when the hold is empty.
Cannot be carried out when the temperature is below freezing.
Testing requires at least 2 responsible crew.
Difficult to observer the leak in the unlit hold and if the18-20m above the observer. powerful light and binocular may be required.
Time-consuming process.
Scuppers should be open near the testing area and the risk of pollution.
The test cannot show the precise point of the leak.

Ultrasonic test:

Procedure for Ultrasonic test:

Ultrasonic equipment consists of a transmitter, where it emits ultrasonic waves when switched on,
The ultrasonic transmitter is placed inside the hold, then close and secures the hatch cover,
An operator on the hatch covers then takes a hand-held detector along the length of each joint in the covers,
In any position where the seal is not tight the receiver emits audible sound and an accurate reading in decibels (dB) can be taken from the luminous display,

Advantages of Ultrasonic test:

Can be used by one man.
Can also be used in a loaded condition.
Will not damage cargoes.
Can be used when the temperature is below freezing.
Reveals the exact location of any leak.
Is accurate.
Is simple to use and requires no special training.
Is compact and portable.

Disadvantages of Ultrasonic test:

Required class approved Ultrasonic equipment.
Additional expense for the shipowner.
The equipment requires regular calibration.
The equipment is not normally part of the ship’s equipment.

Chalk test:

Procedure for Chalk test:

Rub the chalk along the full length of the compression bar,
Close and secure the hatch cover,
Reopen the hatch cover and gasket must be inspected,
On any place, no chalk mark on the gasket means no contact with the compression bar and the joint cannot be watertight.

Disadvantages of Chalk test:

This method of testing shows where the compression bar has touched the gasket but doesn't show the compression is adequate.
Therefore this test is not effective to check weather tightness.

Dock Labour regulation (DLR), 1990

Dock labour = Dock worker

Understand, what is the Dock worker?

“Dock worker” means a person employed or to be employed in, or in the vicinity of, any port on work in connection with the loading, unloading, movement or storage of cargoes, or work in connection with the preparation of ships or other vessels for the receipt or discharge of cargoes or leaving port.

eg: Stevedores, foreman, Crane operators, a person coming onboard for ship repair, etc...

Dock Labour regulation:-

These regulations may be called the Dock Workers (Safety, Health, and Welfare) Regulations, 1990.
To safeguard the lives and interests of dock workers.
It applies to all major ports in India.

Definitions concerning Dock labour regulation:

"Authorised person" means a person authorized by the employer, the master of the ship, or a responsible person to undertake a specific task and possessing the necessary technical knowledge and experience for undertaking the task.

"Competent person" means a person belonging to a testing establishment in India who is approved by the Chief Inspector for the purpose of testing, examination or annealing and certification of lifting appliances, loose gears, or wire ropes; any other person who is recognized under the relevant regulations in force in other countries as competent for issuing certificates.

"Responsible person" means a person appointed by the employer, the master of the ship, the owner of the gear or port authorities, as the case may be, to be

responsible for the performance of a specific duty or duties and who has sufficient knowledge and experience and the requisite authority for the proper performance of the duties.

"safe working load" in relation to an article of loose gear or lifting appliance means the load which is the maximum load that may be imposed with safety in the normal working conditions and as assessed and certified by the competent person.

"lifting appliance" means all stationary or mobile cargo handling appliances including their permanent attachments, such as cranes, derricks shore-based power operated ramps used onshore or onboard ship for suspending, raising or lowering, cargo or moving them from one position to another while suspended or supported, in connection with the dock work and includes lifting machinery.

"loose gear" means hook, shackle, swivel, chain, sling, lifting beam, container spreader, tray, and any other such gear, using which the load can be attached to a lifting appliance and includes lifting device.

"Thorough examination" for the purpose of this regulation shall mean a visual examination, supplemented if necessary by other means such as hammer test, carried out as carefully as the conditions permit, to arrive at a reliable conclusion as to the safety of the parts examined, and if necessary for this purpose, parts of the lifting appliance and gear shall be dismantled.

"Inspector" means a person appointed by the Central Government under Sec. 3 of the Act and includes the Chief Inspector.

What are the "Powers of Dock safety Inspector" as per DLR?

An Inspector may at any port for which he is appointed,

Provided that no person shall be compelled to answer any question or, give any evidence tending to incriminate himself;

Enter, with such assistance, (if any), as he thinks fit, any ship, dock, warehouse or other premises, where any dock work, is being carried on, or where he has reason to believe that any dock work is being carried on
Make an examination of the ship, dock, lifting appliance, loose gear, lifting device, staging, transport equipment, warehouse, or other premises, used or to be used, for any dock work
Require the production of any testing muster roll or other document relating to the employment of dock workers and examine such document
Take on the spot or otherwise such evidence of any person which he may deem necessary:
Take copies of registers, records or other documents or portions thereof as he may consider relevant in respect of any offense which he has reason to believe has been committed or for the purpose of any inquiry;
Take photographs, sketch, sample, weight measure, or record as he may consider necessary for the purpose of any examination or inquiry;
Hold an inquiry into the cause of any accident or dangerous occurrence which he has reason to believe was the result of the collapse or failure of any lifting appliance loose gear, transport equipment, staging non-compliance with any of the provisions of the Act or the regulations;
Issue show-cause notice relating to the safety, health, and welfare provisions arising under the Act or the regulations;
Prosecute, conduct or defend before any court any complaint or other proceedings, arising under the Act or the regulations;
Direct the port authority, dock labour board, and other employers of dock workers for getting the dock workers medically examined if considered necessary.

A person having general management and control of the premises or the owner, master, officer-in-charge or agents of the ship, as the case may be, shall provide such means as may be required by the Inspector for entry, inspection, examination, inquiry, otherwise for the exercise of his powers under Act and the regulations in relation to that ship or premises which shall also include the provision of launch or other means of transport.

What are the Obligations of Dock workers?

No dock worker shall wilfully interfere with, or misuse, any appliance, convenience or other thing provided in connection with any dock work for the purpose of securing the health, safety, and welfare of dock workers;
No dock worker shall wilfully and without reasonable cause do anything likely to endanger himself or others; and
No dock worker shall wilfully neglect to make use of any appliance, convenience, or other thing provided in connection with any dock work for the purpose of securing the health, safety, and welfare of dock workers.
If any dock worker violates any of the provisions of sub-section (1), he shall be punishable with imprisonment for a term which may extend to three months, or with fine which may extend to one hundred rupees, or with both.

What are the guidelines for Protective equipment as per DLR?

Where other means of protection against harmful agents are impracticable or insufficient, dock workers shall be provided with adequate protective clothing and personal protective equipment to shield them from the effects of such agents.
Protective clothing personal protective equipment shall be of suitable quality and maintained in good condition and shall be cleaned and disinfected at suitable intervals.
Where protective equipment and clothing may be contaminated by poisonous or other dangerous goods, it shall be stored in a separate accommodation where it will not contaminate the dock workers clothing and other belongings.
Suitable protection equipment shall be issued to the dock workers employed in the reefer holds or chambers or reefer containers for affording complete body protection.

What are the general precautions during loading and unloading operations as per DLR?

No cargo shall be loaded or unloaded by a fall or sling at any intermediate deck unless either the hatch at that deck is securely covered or a secure landing platform of a width not less than that of one section of hatch coverings has been placed across it: Provided that this regulation shall not apply to any loading or unloading work the whole of which may be completed within a period of half an hour.
No loose gear or any other object shall be thrown in or out of the holds.
During the loading and unloading of bulk cargo, a record of all dock workers employed in the hold shall be maintained and produced on demand to the Inspector.
Where necessary cargo shall be secured or blocked to prevent its shifting or falling. In breaking down, precautions shall be taken where necessary to prevent the remaining cargo from falling.
(a) Dragging of cargo shall be done with the ship's winches only when the runner is led directly from the derrick heel block, and (b) Pulley blocks shall be used to provide a fair lead for the runner so as to prevent it from dragging against obstructions.
If the head room in the hold of a ship for the purpose of stacking and unstacking is less than 1.5 metres, the suitable measures shall be taken to guard against accident.
Loads shall be safely slung before being hoisted. Loose dunnage or debris hanging or protruding from loads shall be removed and suitable means shall be adopted to prevent cargo from falling out.
Cargo handling bridles, such as pallet bridles, which are to remain attached to the lifting appliance while hoisting successive sling loads, shall be attached by shackles, or other positive means shall be taken to prevent them from becoming accidentally disengaged from the hook of the lifting appliance.
When lifting appliance is operating without a load on the slings:

slings or chains shall be hooked securely to the appliances before the operators are given the signal to move;
sharp hooks, multiple hooks, and claws shall not hang at one end but shall be reeved through slings; and
the operators shall raise the appliance sufficiently to keep clear off dock workers and objects.

Effective measures shall be taken, by the use of suitable packing or otherwise, to prevent edges of any load from coming in contact with any rope or chain sling which is likely to cause any damage to the rope or chain sling.
Buckets, tubs, and similar appliances shall not be loaded above their rims.
No loads shall be lifted at strappings unless such strappings are approved as proper and safe slinging points. The movement of such loads is allowed only if everybody has left the danger zone.
When working with lifting appliances, slope-pulling, horizontal dragging, and intentional swing of loads and lifting devices are prohibited.

What are the restrictions of Loading and Unloading operations as per DLR?

No other work, for example, maintenance or repair work such as sandblasting or welding shall be performed at places where dock work is in progress if it is likely to endanger or obstruct the person carrying out dock work.
When more than one gang of dock workers are employed in dock work simultaneously in a hold, it shall be ensured that,

(a) the operation of loading or unloading is harmonised to avoid dangers to the working gangs from one another;

(b) the slung cargo of one gang will not endanger the dock workers of the other gang;

(c) where gangs are working at different levels, a net shall be rigged and securely fastened to prevent dock workers from falling down or cargo falling on to dock workers below;

(d) over crowding of gangs in a section of the hold is avoided.

What are the precautions of Stacking and Unstacking of cargo as per DLR?

Where stacking, unstacking, stowing or unstowing, stuffing or de-stuffing of cargo or handling in connection therewith cannot be safely carried out unaided, reasonable measures to guard against accidents shall be taken by shoring or otherwise.
Stacking of cargo shall be made on a firm foundation not liable to settle and the weight of the cargo shall be such as not over-load the floors.
Cargo shall not be stacked against partition or walls of warehouses or store places unless it is known that the partition or the wall is of sufficient strength to withstand the pressure.
Cargo shall not be stacked to such a height and in such a manner as would render the pile unstable.
Where the dock workers are working on stacks exceeding 1.5 meters in height, safe means of access to the stack shall be provided.
Stacking and unstacking work shall be performed under the supervision of an authorised person.

Wire rope inspection and when to discard the wire rope?

No rope shall be used for dock work unless -

(a) it is of suitable quality and free from patent defect, and

(b) in the case of wire rope, it has been tested and examined by a competent person.

Every wire rope of lifting appliance or loose gear used in dock work shall be inspected by a responsible person once at least in every 3 months, provided that after any wire has broken in such rope, it shall be inspected once at least in every month.
No wire rope shall be used in dock work if in any length of eight diameters the total number of visible broken wires exceeds 10 percent. of the total number of wires or the rope shows sign of excessive wear, corrosion, or other defects which in the opinion of the person who inspects it or Inspector, renders it as unfit for use.Click this link for numericals

Means of access between Ship and Shore as per DLR?

When a ship is lying at a wharf or quay for the purpose of dock work, adequate and safe means of access to the ship, properly installed and secured, shall be provided.
Safe means, required by sub-regulation (1) shall be as follows:
Where practicable, the ship, accommodation ladder or a gangway or similar construction, shall be-

(i) not less than 55 cms, wide at the steps and of adequate depth with steps having permanent non-skid surface;

(ii) properly secured and securely fenced throughout its length on each side to a clear height of one metre by means of upper and lower rails, taut ropes or chains or by other equally safe means except that in the case of ship's accommodation ladder such fencing shall be necessary one side only provided that the other side is properly protected by the ship's side;

(iii) constructed of suitable material and maintained in good condition and suitable for the purpose;
(iv) maintained in a condition as to prevent slipping;
(v) fitted with a platform at either end of the ladder and the lower platform or the treads resting on the dock;
(vi) an angle maintained not exceeding 40 degrees to the horizontal irrespective of resting on the dock.

When a fixed tread ladder is used and the angle is low enough to require dock workers to walk on the edge of the treads, cleated duck boards shall be laid over and secured to the ladder.
Whenever there is a danger of dock workers falling between the ship and the shore, a safety knot or other suitable protection shall be rigged below the accommodation ladder in such a manner as to prevent dock workers from falling.
If it is not possible to rest the accommodation ladder or gangway and the foot of the same is more than 30 cms. away from the edge of the wharf or quay, the space between them shall be bridged by a firm walkway equipped with railing on both sides with a minimum height of one meter with the mid-rails.
Access to the ship shall not be within the swinging radius of the load.
When the upper end of the means of access rests on or is in flush with the top of the bulwark substantial steps properly secured and equipped with at least one substantial handrail one meter in height shall be provided between the top of the bulwark and the deck and the steps provided shall be, as far as practicable, in line with the gangway.
The use of swinging derricks or rope ladders for access shall be prohibited.

What is annealing and how is it carried out? What is the annealing intervals for various parts?

Annealing is also known as the heat treatment process while the gear is heated to high temperatures and cooled slowly. Annealing will soften the material, release any internal stress from the material, which if untreated may cause cracks or fractures.

The annealing must be carried out under the supervision of a competent person.

It is recommended though not required by the regulations, that annealing should be carried out in a suitably constructed furnace, heated to a temperature between 1100° and 1300° Fahrenheit (600° to 700° centigrade), for a period between 30 and 60 minutes. After being annealed, the gear is allowed to cool slowly and shall be carefully inspected.

Annealing intervals:

All chains (other than bridle chains attached to derricks or masts), rings, hooks, shackles, and swivels made of wrought iron which are used in hoisting or lowering shall be annealed at the following intervals:

12.5mm(0.5 inches) and smaller chains, rings, hooks, shackles, and swivels in general use at least once every six months.
All other chains, rings, hooks, shackles, and swivels in general use at least once every six months.
In the case of gear used solely on lifting machinery worked by hand, twelve-month shall be substituted for six months and two years for twelve months as stated above.
The period between annealing shall not exceed two years.
Gear made of steel or gear which contains (as in a ball bearing swivels) or is permanently attached to equipment made of materials that cannot be subjected to heat treatment shall be exempt from annealing. Such gear shall be thoroughly examined.

What is a Chain register and What are the contents of a Chain register?

The chain register is the register carrying the details concerning a ship's lifting gear. The lifting gear includes lifting gear used on deck and in the engine room, such as derricks, cranes, lifting machinery, lashing equipment, etc including chains, rings, hooks, shackles, etc.

Chain register is a Blue colored booklet, also referred to as form II, which is maintained up to date requirement of Dock safety regulations.

Contents of Chain register:

The chain register is divided into three parts:

Part 1 – Initial and periodical load test of lifting appliances and their annual thorough examination.
Part 2 – Initial and periodical load test of loose gear and their annual thorough examination.
Annealing of chains, rings, hooks, shackles, and swivels (other than those that are exempted).

Certificates shall be obtained from a competent person and attached to the register in Form II, in respect of the following, in the forms shown against each:

Initial and periodical test and examination for winches, derricks, and their accessory gear in Form III
Initial and periodical test and examination for cranes or hoists and their accessory gear in Form IV
Test, examination, and re-examination of loose gears under regulation 47 in Form V;
Test and examination of wire ropes under regulation 48 in Form VI;
Heat treatment and examination of loose gears under sub-regulation (1) of regulation 49, in Form VII;
An annual thorough examination of the loose gears under sub-regulation (2) of regulation 47 in Form VIII, unless required particulars have been entered in the register in Form II.