Vibration Measurement

Describe how vibration measurement can be used with a main engine Turbo-charger.

Misalignment, unbalance and derived dynamic effects such as gyroscopic action & whirl force the component of a machine to vibrate.

Vibration may increase wear and other forces influences and lead to failure.

Hence signal sensing for vibration monitoring is carried out by accelerometer, velocity transducer, or inductive proximity probes.

The analysis is then carried out of either the displacement, velocity or acceleration of vibration. this produces peaks of high vibration at discrete frequencies.

(i) For Fault Analysis - Vibration monitoring is carried for the bearings and the turbine blade. Different signatory discrete frequency shall tell where the fault lies.

(ii)For Condition Monitoring - Roller Bearing generates shock pulse waves in operation. Measurement of these would provide an effective monitoring of such bearing. A special sensor & accelerometer are attached at loaded zone by using snap on coupling.

The signal received is passed through band filter that selects high frequency which are converted into pulses & displayed.

The fluctuation in curve are caused by variation in extent of damage and bearing rolling out of regions of existing damage. The normal maximum shock pulse limits can be calculated for very bearings.

When normal limits is passed, the shock pulses increases rapidly to approach the maximum allowed, before breakdown becomes likely. 

(iii)As a substitute to opening up machinery for survey

Analysis report obtained gives proper condition of bearings and motor condition.Surveys can be passed on basis of these reports without opening up the machinery. they can be used as a substitute for class survey where agreement has been reached with classification society.

Machinery which is operated with condition monitoring need only be opened for examaination & repair when readings indicate a deterioration.

Multi-tubed Heat Exchanger

With reference to sea water cooled multi-tubed heat exchangers state

a) The materials used for the construction of tubes, tube plates & water boxes.

Tubes - Cupro nickel(70% Cu-30%Ni) or 90%Cu-10%Ni

or Aluminium Brass (76% Cu-22% Zn -2% Al)

Tube Plate - Admiralty Brass (70% Cu - 29%Zn - 1% Sn)

Naval Brass ( 69% Cu-39%Zn - 1% Sn)

Water Boxes - Cast Iron or steel

90-10 Cupro-Nickel, Cast Ni-Al Bronze

b) Various types of corrosion that the parts in (a) are subjected to

General Corrosion - The static corrosion rate is of about 0.1 mm/year. This would increase with increase in velocity of 3 m/sec to 1 mm/ year for steel pipes. For stainless pipes, greater supply of oxygen improves the protective effect of surface films and above 1m/sec the dangers of pitting also decreases.

Impingement attack - Copper base alloys are not affected by increase in velocity but eventually a critical velocity is exceeded and an intensive localised corrosion called impingement attack occurs.

The metals and alloys have a well established surface water speed above which their protective film is removed and corrosion accelerated because a galvanic couple is set up between the exposed and unexposed metal.

Cavitation Attack - These remove oxide skin and also removes the metal until a hole is worn in metal. This is due to water hammering effect of steam of vapour bubbles caused by turbulence. The bubbles burst over small area of metal surface and chip away the metal.

Selective Corrosion - 

a)De-zincification of Brass - Removal of Zinc from brass, leaving behind porous & weak copper. These can be inhibited by addition of small amount of arsenic but duplex alloys as 60-40 brass cannot be inhibited.

b)Graphitisation - In sea-water, the iron in matrix form can be selectively corroded leaving behind a fragile shell largely constituting graphite. This is soft and easily get fractured. 

c)Measures employed to reduce or prevent above corrosion

Chlorine treatment - Chlorine or hypochlorite is sometimes added to sea-water systems to prevent marine fouling

Cathodic protection - 

Sacrificial anodes

When dissimilar metals are connected together and immersed in an electrolyte, a galvanic cell is set up. Hence corrosion would take place at anode. Sacrificial anodes which are more electro-positive than the other material(to be protected) are placed on water boxes making good electrical contact with it.

Zinc is used to protect Iron in sea water.

Ferrous Solution - Presence of iron or iron oxides has a beneficial effect on the tubes. Breakdown of protective layer may occur due to turbulence. To aid the formation of protective layer. FeSO4 solution is injected in sea water system.

Coating - Bituminous or epoxy coatings, rubber linings etc. may be used. They are electrically insulated from sea water which are in contact with other parts.

Air Receiver Capacity, Copal Varnish

Define the considerations governing main air receiver capacity.

The rule used by government bodies and classification societies which states that the total capacity of reservoirs or receivers must be sufficient to make 12 or more starts without refilling the reservoirs.

If the ship has twin screws, the size of reservoirs must allow not less than 12 starts on each engine.

When ship has CPP (Controllable Pitch Propellers) which are used to for supplying astern power, it is usual to have the air reservoir capacity such that six starts can be made.

Ships are usually supplied with air reservoir capacity well in excess of these requirements. Actual number of starts that can be made without refilling the air reservoirs is dependant on engine temperature, lubricating oil temperature, the skill of engineer manoeuvring the engine and the minimum air pressure that will start the engine.

Usually most engines will start when air pressure has fallen to half or even less than half of designed max starting maximum pressure.

Explain why internal surfaces of air receivers are coated with clean copal varnish

The internal surfaces of an air receiver are subject to pitting and corrosion caused by the water and oil (which becomes oxidised and turns acidic) carried over in the compressed air. If this becomes excessive, the strength of air receiver is compromised and may necessitate expensive replacement.

To prevent this from happening the internal surfaces are given a protective coating which may be derived from organic products or synthetic

The properties required are:- 

1. The coating is clear, so that the surface of the receiver can be inspected.

2. Coating is flexible, so that it can expand and contract with the receiver.

3. Coating has good adhesive properties so that it adheres to the surface of eceiver.

4. coating is impervious to oil and water.

Explain why air receiver drain valve should be overhauled at every available oppurtunity.

Air receiver drain valaves must be operated reliably to ensure that water and oil are drained from air reservoir. If they become blocked then oil and water accumulation at the bottom of receiver is likely to cause corrosion and there is a chance that accumulated oil carried over by air compressors could be carried over with compressed air into the air start lines, where it poses an explosion risk.

If the drain valve do not seat correctly after operation, then this will lead to excessive compressor running hours, with resulting increased maintenance and cost.

Mill Scale, Application of Paints to Hull & Coating Scheme

Explain the causes of formation of millscale on steel plate.

Millscale is a thin layer of iron oxides which forms on the steel surface during hot rolling of plates and sections.

This scale is insoluble and, if maintained over the whole surface, would reduce corrosion.

It is however, very brittle and does not expand either mechanically or thermally at the same rate as the steel plate.

Hence, this millscale has to be removed before painting.

Describe the preparation necessary before the application of conventional paints to the underwater surface of hull.

1. Wipe off all the dirt, grease and oil with cotton rags.

2. Use a solvent de-greaser to remove any oily stains.

3. Wash the solvent off with a mild steel detergent and rinse with fresh water; Taking care not to let the washing go overboard.

4. Remove all rust and scale.

If the plate is exposed to atmosphere rust forms under the mill-scale. On wire brushing majority of mill-scale is removed. this is known as weathering.

If the plates are immersed in a weak solution of sulphuric acid or hydro-chloric acid for a few hours, the majority of the mill scale is removed. This system, known as pickling, has been used.

The pickled plate must be hosed down with fresh water on removal from tank, allowed to dry before painting.

During drying a light coat of rust is formed and must be removed before painting.

Flame cleaning, oxy-acetylene torch having several jets is used to brush the surface, burns the dirt and grease loosens the surface rust, loosens the mill scale. The surface is immediately wire brushed and the priming coat applied while still warm.

Mill scale is removed by shot blasting. This removes dirt, rust and grease as well. This removes 95% to 100% mill scale and results in slightly rough surface which allows adequate adhesion of the paint.

Describe a coating scheme for the underwater hull using a conventional paints.

The ships bottom has priming coats of corrosion inhibiting paint applied which are followed by an anti-fouling paint. Paints used are required resist alkaline conditions.

The paint should have a good electrical resistance so that the flow of corrosion currents between the sea-water and steel is limited.

Suitable corrosion inhibiting paints for ships bottom are pitch or bitumen types, chlorinated rubber, coal tar/ epoxy resin or vinyl resin paints. 

The anti-fouling paints may be applied after the corrosion inhibiting coatings and should not come into direct contact with the steel hull.

Rudder Repair

A rudder of a vessel requires an extensive welding repairs and as a chief engineer you are requested to supervise.

Suggest a suitable type of welding process.

The welding process most suitable for rudder repair is MANUAL METAL ARC WELDING.

The reasons for using the basic process are:-

It will probably be in-situ repair in Dry-dock requiring multi-position welding (down hand, overhead, horizontal, vertical).

MMA is versatile in this respect.

There will be no special steel involved, thus basic electrodes techniques are to be used. (although coatings may be in rudder structure taken care by pre-heating.)

Welding in an exposed environment means the semi-automatic process such as MIG would be difficult(shielding gas blown away) unless protected by an enclosure.

State with reasons 4 common welding defects

Undercut - This is a groove or channel along the toe of the weld caused wastage of the parent metal which could be due to too high a welding current or low welding speed.

Inclusions- Any slag or other entrapped material is an inclusion defect. Surfaces to be welded must be free from foreign matter. 

E.g.:- grease oil, millscale, metal chipping etc. 

During welding the slag must not be allowed to get front of the molten metal or it may get entrapped.

Also when welding is interrupted, or another run is to be laid, the already deposited metal should be allowed to cool, slag should be chipped and brushed off.

Incomplete root penetration - It is a gap caused by failure of the weld metal to fill the root. This may due to fast welding speed or too low current.

Lack of Fusion- This would occur between weld metal and the parent metal, between different layers of weld metal or between contact surfaces of parent metal. It could be caused by incorrect current or voltage, dirt or grease etc. 

State what tests may be carried out before returning the rudder in service

Rudder should be hydraulically pressure tested upto 2.45 m of water head for confirming water-tight integrity from the top surface of rudder.

Welding work first should be tested by Non-Destructive Testing means X-rays, Ultrasonic testing. If it is satisfactory then hydraulic test should be carried out. 

Rudder drop to be checked to confirm load on the pintle bearings, condition of the pintle bearings, free movement of rudder 35 deg. on either side, external rudder stoppers before rudder is put back into service.

Proper fitment of sacrificial anodes. rudder closing plates and bottom drain to be checked for proper fitment. Rudder stock palm nuts to be covered with cementing.

LNG Membrane Tank Insulation

With Reference to membrane tanks for the carriage of liquified gas at very low temperatures

Describe with a sketch one method of building up of insulation
Membrane tanks are not self supporting. They consist of very thin tank material supported by insulation which is in turn supported by ships hull. The system is not competitive for the carriage of LPG and is primarily for LNG.





The GAZ Transport System uses a 36 % Nickel Iron alloy called "Invar" for both the primary and secondary barriers. Invar has a vary low coefficient of thermal expansion which makes any corrugations in the tank structure unnecessary.

The invar sheet membrane used is only 0.5 mm to 0.7 mm thick which makes for a very light structure.


Insulation consists of plywood boxes filled with perlite.


State which alloy is used and the reason for the membrane.
The GAZ Transport membrane systems uses a 36 % Nickel-Iron Alloy called "INVAR" for both the primary and secondary barriers.
INVAR has a very low coefficient of thermal expansion. This makes any corrugations in the tank structure unnecssary.
The INVAR sheet membrane used is only 0.5mm to 0.7 mm thick which makes for a very light structure


Explain why a secondary barrier is installed.
Many gases are often liquified and carried in bulk. In order that the gases may remain liquid, they must be kept under high pressure or at low temperature.
This liquid must not normally be allowed to come into direct contact with the ship's hull or to take up heat from the surrounding air.
Hence a secondary containment system must be incorporated to protect the ships hull from the effects of low temperature in the event of cargo leakage.
This is known as "SECONDARY BARRIER".
LNG is carried at its boiling temerature of -164 deg. cel. at slightly higher than atmospheric pressure.
The 'secondary barrier' must be provided to contain any leakage from the tanks if the liquid is at temperatures of below -10 deg. cel.
For liquids between -10 deg. cel. to -55 deg. cel. the outer hull of the ship may serve as secondary barrier.
For liquids whose temperature is below -55 deg. cel., hull is not acceptable for this purpose and specially designed insulation, separated from tank by a space which is often filled with nitrogen gas.

Fatigue of Engineering Components

With Reference to fatigue of engineering components explain the influence of stress level and cyclical frequency on expected operating life.

Fatigue is a property of metal which when be subjected to thousands or millions of cyclic load applications in which the maximum stress developed in each cycle is well within the elastic range of material.

The fatigue test determines the stress which a sample of material can safely endure for a given number of cycles.

Specimens are tested to failure using different loads and the number of cycles before failure is noted for each each load. the results are plotted as graphs of applied stress against the logarithm of the number of cycles of failure. The curve is known as S-N curve.

The number of cycles for failure reduces with increase in maximum stress and vice-versa.

Explain the influence of material defects on the safe operating life of an engineering component

Any material defect increases the stress in the material which drastically reduces the operating life.

Stress raisers like sharp corners decreases operating life and hence must be eliminated.

Sharp surface resulting from punching, stamping, shearing results in stress raiser which reduces operating life.

Surface irregularities during manufacture. Inherent residual stress during manufacture, cracks, fractures on surface increases  the maximum stress to which it is subjected which seriously affects the operating life.

State the factors which influence the possibility of fatigue cracking of a bed plate transverse girder and explain how the risk of such cracking can be minimized.

The transverse girder of a slow speed engine incorporates the main bearing pocket.

It is subjected to bending moment as the gas loads force the crankshaft down into the bearing pocket.

This bending is resisted by TIE BOLTS. In addition forces are trying to flex the transverse girder as the gas loads alternate on either side.

The most obvious place for cracking to occur in the transverse girder is under the bearing pocket. These cracks can be radial or circumferential. 

Cracking can also occur at the joining welds between transverse and longitudinal girders and at very very sharp change in section.

E.g. :- At root of stiffening webs

The older box section fabricated transverse girder is susceptible to cracking. This cracking will be due to fatigue i.e. action of a varying tensile stress.

RISK OF SUCH CRACKING CAN BE MINIMISED

1. Prevention of excessive vibrations

2. To check Tie bolts tightness at regular intervals. to Re-tighten at correct tightness if found slack. REFRAIN from operating the engine with slack tie-rods.

3. Do not overload the engine.

4. Reduce the power imbalance within the engine cylinders by taking performance curves at regular intervals.

5. Bed-plate inspection to check for cracks, welding, castings and corrosion etc.

Condition of Assignment

Explain the term "Condition of Assignment"

If the conditions listed below for the reasons for freeboard requirements are to be achieved which are:-

1. To ensure that ship cannot be loaded beyond her strength.

2. To provide her with adequate reserve buoyancy

3. To keep the decks high enough from the water to enable the crew to navigate and handle her in all weathers.

it is important to that all openings in the freeboard deck should be properly protected and closed and that provision should be made to allow the ship to free quickly of water which may come in upon the decks. 

To ensure that this done, the ship is required to comply with certain "Conditions of Assignment".

These are conditions which must be met before freeboards are assigned to a ship and a LOAD LINE CERTIFICATE issued, which enables the load lines and mark to be graved in and painted on the side of the ship.

The conditions are concerned with water tight integrity of the hull and state requirements for the strength, height of coamings, closing arrangements of items such as super-structure and bulkheads, hatchways, machinery space openings, openings in freeboard & superstructure decks ventilators, air pipes, cargo ports, freeing ports, side scuttles, scuppers, inlet and discharges.

Also means provided for protection of crew.

Explain how these are maintained for a ship?

Vessels are surveyed annually to ensure that they fulfill the "CONDITIONS OF ASSIGNMENT"

Conditions of Assignment need periodic inspection to ensure that the ships condition is such that those conditions of assignment are still being fulfilled.

Hatch-Covers, Cargo-ports, Watertight doors, scuttles, and other closing down appliances would get corroded, reduced resilience or damage of seals, locking arrangements might get damaged.

Coamings of hatches, ventilators, air pipes get corroded at weather deck level thereby reduction in strength.

Freeing ports or scuppers blocked or valves not working.

Gangways, Rails, bulwarks gets damaged or not secure.

Hence continuous maintenance, inspection & surveys ensure that they fulfill "CONDITIONS OF ASSIGNMENT"