Notes on marine engineering

8/12/2019 Notes on marine engineering

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For seafarers, IMO has issued A Pocket Guide to Recovery Techniques (IMO I947E).

Reducing on-board noise

Also entering into force on 1 July 2014 is the new SOLAS regulation II-1/3-12, which requires new ships to be constructed to reduce on-board noise and to protect

personnel from noise, in accordance with the revised Code on noise levels on board ships, which sets out mandatory maximum noise level limits for machinery spaces,

control rooms, workshops, accommodation and other spaces on board ships.

Fire-fighter communication on-board

Amendments to SOLAS regulation II-2/10 on fire fighting enter into force on 1 July 2014, to require a minimum of two two-way portable radiotelephone apparatus for each

fire party for fire fighters communication to be carried . The apparatus shall be of an explosion-proof type or intrinsically safe. Ships constructed before 1 July 2014 shall

comply with the above requirements not later than the first survey after 1 July 2018.

Instructions, on-board training and drills

Further amendments to regulation II-2/15 on instructions, on-board training and drills require an on-board means of recharging breathing apparatus cylinders used during

drills, or a suitable number of spare cylinders.

Protection of vehicle, special category and ro-ro spaces

Another amendment to regulation II-2/20 on protection of vehicle, special category and ro-ro spaces related to fixed fire-extinguishing systems, updates the requirements.

The amendments apply to ships constructed on or after 1 July 2014. Ships constructed before 1 July 2014 shall comply with the previously applicable requirements.

Forms of certificates and records of equipment

Other amendments to the appendix to the annex to the SOLAS Convention replace all forms of cer tificates and records of equipment, including its 1988 Protocol, and

further amendments relate to the forms of the Cargo Ship Safety Construction Certificate and Cargo Ship Safety Equipment Certificate of its 1978 Protocol.

Container convention amendments

Also entering into force on 1 July 2014 are amendments to the International Convention for Safe Containers (CSC), 1972, which were adopted in 2013 by resolution

MSC.355(92), to incorporate and facilitate the entry into force of amendments to the CSC Convention adopted in 1993 by resolution A.737(18), including amendments

relating to the form of the safety approval plate and to the approval of existing and new containers. The amendments also introduce a transitional period for marking

containers with restricted stacking capacity and include a list of deficiencies which do not require an immediate out-of-service decision by control officers, but require

additional safety measures to enable safe ongoing transport.

Oil in boiler water.

Sources of oil contamination

The most common sources of oil contamination observed on boilers originate from leaking heating coils fitted in fuel oil tanks, fuel/lub oil heaters, cylinder lub oil from

reciprocating steam engines for pumps and heating coils in DB tanks dedicated for sludge/waste oil tanks. It is also not rare for cargo tank heating coils and tank cleaning heaters

fitted on the cargo side to contribute to the contamination in some cases.

However, main propulsion boiler plants with a segregated saturated steam system as the main source of heating medium are least likely to be contaminated by oil.

Immediate effects

The immediate effects of oil contamination range from foaming and carry over in oil-fired boilers due to increased tension at the water surface to the malfunction of boiler water


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level controls and even protective shutdown devices. In the worst cases, the carry-over of water and moisture with the steam may even reach the intensity of priming, causing

havoc to consumers, eg: turbines, super heaters, steam piping and associated gaskets.

Consequential effects

Severe oil contamination leads to a collapse of the heat transfer rate through the boiler steel, contributing to a higher metal temperature than the design value.

With reference to the illustrations below, even an oil film or deposit as thin as 0.5 mm on the water side can easily increase the metal temperature on the furnace side (T2) from

the design value of 250 deg C to well above 600 deg C under normal operating conditions on an auxiliary boiler rated at 7 bar (g). This has a domino effect of exponentially

reducing the yield strength of the material until the pressure parts subjected to active heat transfer fail.

Temperature gradient across the material thickness

In cases where the reduction in the strength does not lead to immediate failure, the boiler steel can still be subjected to a time-dependent creep zone that is hard to evaluate (if

above 380 deg C) unless catered to at the design stage by alloying.

In the case of exhaust gas water tube boilers with an extended surface area that forms part of the steam generation system by forced circulation, this may in the worst cases

lead to soot fires due to a lack of heat transfer from the gas side and rise in the metal temperatures due to the uncooled boundaries. Smoke tube exhaust gas boilers are prone to

cracks on the tube terminations (see image below) due to differential expansion of the overheated tubes with respect to the shell.


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It is also prudent to be aware that other long-term effects are local corrosion of the area subjected to exposure to the acidic nature of oil deposits. Hydrocarbon deposits have a

tendency to turn acidic when exposed to high temperatures in the presence of water.

Classification follow up

Observations of any degree of oil contamination on boilers lead to a requirement of the permanent restoration of the heat-transfer surfaces on the water and steam side prior to

the boiler being put back into service.

In exceptional cases and in order to support main functions, the short-term use of the boilers may be conditionally considered by DNV after careful assessment of the heat-

transfer surfaces on the water and steam side and satisfactory isolation of the source of the contamination. Compensatory and precautionary measures include derating the

boilers steam generating capacity by reducing the firing rate/heat input in conjunction with the design working pressure.

Boiling out the water side of the boiler using recommended chemicals and/or mechanical cleaning are normal procedures undertaken to facilitate satisfactory cleaning. This may

be additionally supported by hardness checks and a hydrostatic pressure test at 1.5 times the design working pressure to ensure the expected safety factor at the design

temperature.

In view of the oil deposits on the water side, it is also imperative that the impulse piping to the level transmitters is blown through and safety functions verified for satisfactory

operation.

One of the first preventive actions to be taken when oil leakage into the feedwater system is suspected is to take a look in the cascade or hotwell tanks. If oil is found in

the last compartment just before the water enters the feedwater piping, it is recommended to check whether a dark oily film contaminates the boiler water level glasses

inside. If so, the boiler should not be blown down from the bottom, only surface blow for several times. If the boiler is bottom blown, it will become totally covered in oil.

The boiler should be shut down and released from pressure and the venting valve opened. Afterwards, the boiler can be drained slowly until water stops flowing from the

loosened, upper manhole door before opening up the manhole for inspection. The oil will now only cover the boiler in the normal water level range and can be manually

removed. Hot or cold water high-pressure jet equipment together with oil dispersive additives would be efficient for removal of the oil. In case the boiler is completely

oily inside, the cleaning work could be turned over to a cleaning company specialised in such work.

Chemical cleaning of boilers

Boiling out of the boiler and the steam/feed water system shall be done:

1. before initial start up2. in case of oil in the boilers water side3. in case of heavy corrosion.There are two methods of cleaning:- alkaline boiling- acid cleaning.

Boiling out before the boiler is placed in service to remove any foreign products

The unit should be filled through the feed connections to the normal working level with water of the same quality as that of the feed water to be used for regular

service.Care should be taken to use only properly treated and deaerated water, so that optimum conditions will be in force from the start in limiting the formation ofscale, corrosion and other difficulties caused by improperly conditioned water.The chemicals used for boiling out are completely dissolved in water before they enter the unit. The dissolved chemicals are pumped into the boiler through thechemical feed connection or, as a second choice, the chemical solution may be poured through the manhole after bringing the water level up to about half way

between the bottom of the drum and the manhole.With all the water in the unit at the concentration noted below, a good cleaning job can be done with any one of the four following combinations of chemicals:

Trisodium phosphate (Na3PO412H2O)Caustic soda (NaOH)

2500 ppm2500 ppm


5000 ppm500 ppm


Soda ash (Na2CO3)

1500 ppm1500 ppm1500 ppm

Caustic soda (NaOH)

Soda ash (Na2CO3)

3000 ppm3000 ppm


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BOILING OUT BY USE OF BURNERS

Start firing slowly. The steam pressure for boiling out is 3.5-4 bar minimum. Once vapour is formed, the pressure should be reached slowly and at a minimumand consistent rate of firing.The boiler and each piece of the system, should be blown down at least once every 8 hours during boiling out. The total amount of water blown down should beequivalent of a difference in level of one half gage glass, divided equally between all main blow downs. If necessary, water should be added between the blowdowns of individual valves to hold the level within safe limits.

At the end of blow down, the water should be brought up to the normal working level and replenished with chemicals to the proper concentration.

Experience indicates that 36 to 48 hours should be sufficient. The procedure should be continued until examination of water samples from blow down lines

indicate that no foreign products are present.

ACID CLEANING

This procedure is to be used when rust or mill scale is known to exist to a high degree in the boiler. Normally, acid cleaning will not be required on new boilers.

The procedures given below outlines one method which can be used on marine boilers. Other methods are possible, such as cleaning with citric acid. Acidcleaning by any method should always be performed under the guidance of an acid cleaning firm or specialist.

PREPARATION

Chemical cleaning temporary piping should be completed.Temporary thermocouples should be installed so that temperatures can be monitored at the following locations:1. Steam drum (if not permanently installed).2. Near the centre of the furnace wall.3. One loser furnace wall header.

Fill the boiler with clear, filtered, preferably softened water.

Fire the boiler at a very low rate until boiler metal temperatures (as indicated by thermocouples) are in the range of 60-90C.

Acid dosing:The chemical solvent used in this step is composed of the following:5 % hydrochloric acid (HCI)

0.5% ammonium biflouride (NH4F .HF)

0.1-0.2%inhibitor

Adjust water temperature to 65C with steam while running to waste.

Begin injection of inhibited concentrated acid solvent (5 % HC1) to boiler.

Adjust steam supply to maintain a dilute acid solution of 65C at a thermowell downstream of the steam addition point.Continue to fill boiler until the level of acid solution is at the top of the glass. Stop fill.Wait a few minutes and check to see if level is stable.Let boiler soak for six (6) hours with acid solution. During this period, flush chemical cleaning header to waste.At the completion of the soak period, close drum vents and drain boiler and economizer under 0.2 - 0.3 bar nitrogen pressure to waste. During the drain, collect acomposite sample of the spent acid. At the completion of the drain, check for complete drainage of the acid solution.

RINSINGOpen drum vents and fill the boiler and economizer with clear, filtered, preferably softened water with a pH of less than 8.0 until a level is indicated in thetemporary gage glass.Close drum vents and drain the boiler and economizer under nitrogen pressure as above to waste.

Open drum vents and operate the boiler feed pump to flush the boiler.Continue to fill the boiler through chemical cleaning header with water as indicated above until water is discharged at the drum vents.Close drum vents and drain the steam drum under nitrogen to its midpoint.

Close drum vents and drain boiler under nitrogen to waste. Check the pH of the rinse water. If the pH is less than 5. 0, another rinse (fill and drain) is required. If

pH is greater than 5.0proceed to neutralize.Remove the temporary gage glass and prepare the unit for operation.

NEUTRALIZATION BOIL-OUTOpen drum vents and through the chemical cleaning header, pump into the boiler the dissolved concentration sodium carbonate which should be equivalent to

0.5-1.0% sodium carbonatewhen boiler is filled to normal firing level.Operate boiler feed pump and continue to fill to normal firing level.Fire the boiler until a minimum of 7 bar is reached.

Hold the 7 bar minimum pressure with intermittent firing for four hours.At the completion of the boil-out period, cool the unit to 1 bar and drain (nitrogen pressure not required).

PASSIVATION

While the unit is still hot, blend the concentrated sodium nitritesolution with condensate to obtain 0.5percent sodium nitrite and fill the boiler and economizerto the of steam drum.After the unit has soaked for approximately one (1) hour, drain the boiler and economizer to waste (nitrogen pressure not required).

INSPECTION AND FLUSHINGWhen the boiler has cooled down, inspect the steam drum and remove scale or piles of iron oxide present. Be sure that none of the foreign material is allowed toenter downcomers or connecting tubes.Open the lower wall header and flush.Renew all boiler hand hole fittings installed.It might be well to point out the following precautions:1. Under no circumstances should the boiler be fired while the acid is in the unit.2. Vents should be opened when the inhibited acid is in the unit. Hydrogen gas is evolved through the reaction of the acid and steel. Avoid open flames or usingelectrical equipment that might produce a spark until it is certain that the hydrogen gas has been dispersed .

3. Neutralizing boil-out chemicais should be stored in a dry place prior to use.4. Have adequate protective clothing for men who are to handle and mix chemicals. The protective clothing should include goggles, rubber gloves, aprons etc.


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1.Using feeler gauge- the thrust bearing is opened up and by using jacks or wedges thrust collar is pressed hard on set of the thrust pads, long feelers are inserted on the

side which is open. The feelers should be long enough to extend from one corner of the pad to the corner diagonally opposite. they must be inserted on one corner and

diagonally eased up through the other, so clearances can be taken without tilting the pads.( if the collar cannot be hardened to any side, we can take clearances on both

side and add.

2. Using dial gauges- without opening up, the collar is pressed hard using jacks or wedges, then the axial shift of shaft (clearance) is measured on the dial gauge.

normal value of clearance- .35mm (value low because no wear since no metal to metal contact. the clearance can be corrected by adding shims between back-plate and

housing).

Main bearing clearance methods

1) Bridge with Depth Gauge

This method is used in SULZER 2 stroke marine engines where the bearings shell is removed along with the keep (the bearing shell is lined with

the keep). After that a bridge is fitted over the top of journal pin, from port to starboard, making a bridge over the crankshaft with two ends

supported on the cross girder.

A simple vernier type depth gauge is then inserted in the hole provided on the bridge and the scale of depth gauge is rested on the crankshaft pin.

The total depth on the scale is measured and compared with the previous reading and the reading in the manual for calculating the wear down of

bearing.

In old model SULZER engines, a collar is provided in the bearing shell along with a small hole. Thus without removing the keep, the bridge is

fitted adjacent to the keep and the depth gauge is used from the hole provided in the shell to measure the shell wear down.
http://www.marineinsight.com/wp-content/uploads/2012/03/14076_74_1.jpg


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2) Bridge With Feeler Gauge

In some engines, after removing the shell and the keep, the bridge is installed as explained in the above point. Also, in place of depth gauge, a

feeler gauge is used to measure the clearance between the journal pin top and the bridge bottom. The bridge used here is different in terms of

height and the gap between the pin and the bridge is very less as compared to that of the bridge used in the above mentioned method.

3) Telescopic or Swedish Feeler Gauge

In engines like MAN B&W, this is the most common method used to measure the bearing clearance of the top shell. In this method there is no

need to remove any connection or keep for measuring the clearance.

The telescopic gauge is inserted between the gap of the crank web and the bearing keep. When the tip reaches the shell top, the feeler is

inserted between the shell and the pin to check the clearance.
http://www.marineinsight.com/wp-content/uploads/2012/03/22.jpghttp://www.marineinsight.com/wp-content/uploads/2012/03/12.jpghttp://www.marineinsight.com/wp-content/uploads/2012/03/22.jpghttp://www.marineinsight.com/wp-content/uploads/2012/03/12.jpg


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4) Dial type Depth Gauge

This method is used in new MAN B&W engines (SMC-C) which does not require the top keep to be removed. The lube oil pipe connection screw

hole is in the bearing keep which can be accessed from the hole on the bearing shell.

The dial gauge is inserted in this screw hole and the reading is taken as the clearance for upper shell. For complete bearing overhaul procedure

pleased read-Procedure for Removing Main Bearing of MAN B&W
http://www.marineinsight.com/tech/proceduresmaintenance/procedure-for-removing-main-bearing-of-man-bw-mc-c-engine/http://www.marineinsight.com/tech/proceduresmaintenance/procedure-for-removing-main-bearing-of-man-bw-mc-c-engine/http://www.marineinsight.com/wp-content/uploads/2012/03/untitled11.jpghttp://www.marineinsight.com/wp-content/uploads/2012/03/2y.jpghttp://www.marineinsight.com/wp-content/uploads/2012/03/untitled11.jpghttp://www.marineinsight.com/wp-content/uploads/2012/03/2y.jpghttp://www.marineinsight.com/tech/proceduresmaintenance/procedure-for-removing-main-bearing-of-man-bw-mc-c-engine/


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Test and Overhaul of Fuel Injection Valves

written by: Chief Engineer Mohit Sanguri edited by: Lamar Stonecypher updated:

10/18/2013

This article discusses the testing and the overhaul of fuel injectors of marine engines,the

testing of the needle and guide condition of the fuel valve, and the procedure to overhaul andinspect the injectors taken out of the marine diesel engines.

Fuel Valve Checks

The fuel valves taken out from the engine must be checked for function and performance.

Even in engines which are stopped on heavy fuel oil in ports the fuel injector taken out must

be immediately tested with diesel oil before they get cold as this will flush and clean the

components. It must be noted that if the fuel valves taken out are tested after they have

cooled, will show bad performance even if they were performing satisfactorily in service.

In the majority of cases the fuel injectors have a good spray profile but they open up at a less

pressure. The pressure adjustment can be done without opening up the valve and should be

done so. The engine manufacturers also instruct that unless the fuel injector valve has a major

problem like holes choked or valve dripping, they should not be opened up. The valve should

be cleaned from the outside, pressure checked, pressure adjusted and tagged.

Inspection and RepairsIn the case where the fuel injector valve is not performing as required and has some defect,

then it needs to be opened up and overhauled. The assembly and the disassembly have to be

done as per the instructions given by the engine manufacturer. However, below is a general

guide about what you will most likely have to do.

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Effective removal of Carbon soot For trucks, dumpers, excavators

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After the fuel valve has been disassembled then the following checks have to be done:

1. The needle guide should be immersed in clean diesel oil and the needle taken out and

checked for free movement. In the case of any resistance which may be due to the presence of

carbon or fuel sludge the needle may be put in and pulled out in succession many times while

keeping it submerged in diesel oil. It is important to do this in a container full of clean diesel

oil so the contaminants can be flushed away.2.After the needle guide has been cleaned, the needle should be taken almost out and then let

it fall in with its own weight. A free and smooth movement with small jerks as the clearance

is making way for the oil to come out is an indication that the clearances are all right and the

needle guide is in good condition. It must be noted that the needle should fall fully into the

seat.

3. On the other hand if the needle falls fully in one go, then the clearances have increased and

the fuel will leak past the spindle and less fuel will go in the cylinder. The needle must be

inspected for any wear marks if this happens. The needle guide can be used but must be

changed soon.

4. If the needle does not go down and gets struck then it must be thoroughly cleaned again. If

still there is no improvement then the needle might have become bent. Check the needle for

any signs of overheating.

5. The push rod end should be checked for any abnormal wear.

6. The seating between the nozzle body and the valve body if damaged can be repaired by

lapping with fine lapping paste. It must be noted that the lapping paste should be thoroughly

flushed away with clean diesel oil and thereafter blown dry with compressed air.

7. Check the nozzle spring for breakage, poor seating and other defects. Change if required.

8. Check the leak off pipes, shims, packing etc for the condition. If the fuel valve is water

cooled, the cooling pockets should be cleaned with compressed air.

Tests and Adjustments1. After the parts are cleaned and inspected the fuel valve is assembled as per the

manufacturers instructions and thereafter tested for function and performance.

2. The assembled fuel valve is installed on the test stand and after purging the pipe line the

manual handle is operated in quick succession. The nozzle should start discharging with a

sharp crackling noise at the set pressure. The pressure at which the injector is supposed to firedepends upon the manufacturers engine design but normally is between 250 to 350 kg/cm2

with an allowance of plus or minus 10 kg/cm2.

3. In case the lifting pressure is not correct, it can be adjusted by the adjusting screw.

4. The spray characteristics should be satisfactory and as per the manufacturers advice.

5. All the holes of the injector should be firing and can be checked by a torch light or a filter

paper can be folded as a cone and then the injector tested. The holes on the filter paper will

show the number of holes firing. In this procedure you must be careful as the high pressure

spray can enter the skin and is toxic for us.

6. The spray angle should be as stated by the manufacturer. The atomization of the fuelshould take place and solid spray should not come out.


11/19

7. Clean diesel oil should be used for the testing purpose.

8. In the case that the fuel valve is dripping the needle guide should be taken out and repaired.

CautionThe needle and the guide is always a pair and should not be interchanged with another one.

Cleanliness is the most important factor in making fuel valves. A clean fuel valve lasts a

longer time. The fuel under pressure can enter the skin and the blood stream and is toxic forhumans. Take care that you stay away from the spray. The fine mist can catch fire and in

inflammable. Do not smoke or use naked lights where the fuel injectors are being tested.

Bunkering Procedure

Before Bunkering

1. The chief engineer should calculate and check which bunker/fuel oil tanks are to be filled after he receives confirmation from the shore office

about the amount of fuel to be received.

2. It might be required to empty some tanks and transfer the oil from one tank to other. This is required so as to prevent mixing of two oils and

prevent incompatibility between the previous oil and the new oil.

3.A meeting should be held between the members that will take part in the bunkering process and they should be explained about the

following:-

a. Which tanks are to be filled.

b. Sequence order of tanks to be filled.

c. How much bunker is to be taken.

e. Emergency procedure in case oil spill occurs.

f. Responsibilities of each officer are explained.

4.Sounding is taken before bunkering and record is made.

5. A checklist is to be filled so that nothing is missed on.

6. All deck scuppers and save all trays are plugged.

7.Overflow tank is checked to be empty.

8. Adequate lighting at bunker and sounding position is to be provided.

9. No smoking notice should be positioned.

10.On board communication between the people involved in bunkering is made.

11. Red flag/light is presented on masthead.


12/19


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6. The temperature of bunker is also to be checked; generally the barge or supplier will provide the bunker temperature. Temperature above this

may lead to shortfall in bunker.

7. A continuous sample is taken during bunkering with the help of sampling cock at the manifold.

After Bunkering

1. Draught and trim of the ship is checked.

2. Take sounding of all the tanks bunkered.

3. The volume bunkered should be corrected for trim, heel and temperature correction.

4. In general for each degree of increase in temperature the density should be reduced by 0.64 kg/m3.

5. Four samples are taken during bunkering. One is kept onboard, one for barge, one for analysis, one for port state or IMO. One sample is

given to barge.

6. The chief engineer will sign the bunker receipt and the amount of bunker received.

7. If there is any shortfall of bunker received the chief engineer can issue a note of protest against the barge/supplier.

8.After everything is settled the hose connection is removed.

9.The sample is sent for laboratory analysis.

10. The new bunker should not be used until the report from the lab

mpressed Current Cathodic Protection System

Impressed Current Cathodic Protection System

Corrosion:

The ships hull is corroding in sea water. Generally this is electro-chemical reaction in which the metal combines with an oxygen, to form a metal oxide or other compound. This depends

upon the nature of the environment. Different metals have different tendencies to corrode, activity or potential.

Some metals and alloys have two positions in the series, marked Active and Passive.
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The active position is when the corrosion is occurring and approaches the electro-chemical series position for the material. The passive position relates to a non-corroding situation

where the material is protected by a self forming surface film.

If two metals are placed in an electrolyte (e.g. sea water or damp soil) and are in direct electrical contact, a current will pass through the electrolyte from the more active metal onto the

least active metal.

The least active metal does not corrode and is termed the cathode. The more active metal, the anode, passes into solution and the flow of electrical current increases. This is a metal ion

and electron transfer process i.e., it corrodes

Cathodic Protections:

The anodic and cathodic areas in a corrosion cell may be due to the electrical contact of two dissimilar metals, galvanic corrosion. Anodic and cathodic areas may be formed on a single

metal surface as micro-cells for instance by rain drops on uncoated steel. Alternatively, they may be close but discrete cells found when accelerated corrosion occurs at uncoated anodic

areas on a generally coated cathodic structure.

Large currents can occur at small anodic areas and lead to rapid corrosion of marine structures such as ship's internal tanks, external hull plates, sheet steel piling in harbours and

tubular structures common in jetties and petrochemical drilling and production platforms.

Cathodic Protection is a system of preventing corrosion by forcing all surfaces of a structure to be cathodes by providing external anodes.

Sacrificial anode cathodic protection achieves corrosion prevention on a particular structure or component by forming galvanic cell where an additional anode of zinc, magnesium or

aluminium corrodes in preference to the structure. The galvanic corrosion current (see simple cell before) available from this anode / electrolyte / structure combination should be

sufficient to overcome the local surface corrosion currents on the structure until no current flows from anodic areas of the structure i.e the structure is entirely cathodic or under complete

cathodic protection As indicated previously, a metal can be made cathodic by electrically connecting it to a more anodic metal within the electrolyte. The most commonly used anodic

metals are alloys of aluminium, zinc and magnesium. Anodes of these metals corrode preferentialy, the corrosion current of the anode achieving cathodic protection of the structure to

which they are connected.

The anodes deteriorate as an essential part of their function and they are therefore termed sacrificial

Introduction of ICCP

A metal also can be made cathodic by electrically connecting it to another metallic component in the same electrolyte through a source of direct electric current. The current flow from this

metallic component must be sufficient to overcome the natural corrosion current. Thus we will direct the current flow to occur off the surface of added metalli c component (anode), into
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the electrolyte and onto the metal (cathode).

All we need is to measure what the natural corrosion current is. So we add one more electrodereference cellcompletely passive metal. The potential difference between the hull and

reference cell will form the natural corrosion current. So another electrodeanode - with a power source is introduced so that the current flow from this electrode is sufficient to

overcome the natural corrosion current.

Because an external current source is employed, this type of protection is termed 'IMPRESSED CURRENT CATHODIC PROTECTION'.

Cathodic Protection

A source of direct current is required, this is generally obtained from mains power units that contain a transformer and rectifier.

The magnitude of this current may be automatically controlled in response to a continuous monitor of the cathode / electrolyte potential or may be manually controlled after intermittent

measurement.

The impressed current anode material is ideally non-consumed by the passage of current from it into the electrolyte, in practice the materials used are a compromise between this ideal

and the cost and physical properties of available materials. Impressed current anodes are made from graphite, silicon iron, lead alloys some with platinum dielectrodes, platinised

titanium or more exotic combinations such as platinum clad niobium. The selection of the correct anode material is critical in the formulation of an effective and economic cathodic

protection scheme

Generally, for a given current demand, less impressed current anodes than sacrificial anodes are required for protection, as high anode currents are feasible.

Impressed current systems of cathodic protection are more sophisticated in design than sacrificial systems.

The tipycal elements of ICCP
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- As the crane is operated or the power is supplied, the spring gets pulled inward or compressed due to the electromagnetic effect of thecurrent. This allows the crane to be operated normally.

2) Emergency stop is provided in the remote so that the operator can stop the crane at any time.

3) The motor is fitted with distance limit switch in both transverse and longitudinal direction so that the travel of the trolley and hence craneshould not overshoot the racks end.

4) Mechanical stoppers are provided for both directions in case the electrical distance limit trips fail.

5) The up and down travel of the hook is also attaches with automatic stopper to avoid overloading of the motor.

6) The motor is fitted with thermal protection trip. When the motor windings get overheated, trip will activate saving the motor winding fromburning.

7) Load limit switch is also fitted which will trip the motor if the load to be lifted is above the crane capacity.

8) Its the responsibility of senior officers to operate the crane and to make sure all the personnel involve in any lifting operation are at a safedistance during operation of the crane.

9) Additional tools like i-bolts, shackle, wire sling, belts etc. used for lifting must be checked before use.

10) It should be noted that no one walks or stand below the crane when it is in the loaded condition.

Patent slip dry dock

The process ofslippinga vessel is an inexpensive and straightforward way to take a large vessel out of water for inspection or repair. In tidal harbors and ports, it is

normally necessary to wait for high tide. In many cases, it is possible to take the vessel out of the water on one tide, and to make repairs and return it to the water on the

next tide.

The first step in use of a paten t slipor marine railway system involves a "cradle" being lowered to the bottom of the inclined plane (the slip/slipway), at which point the

vessel is moved into position directly above the cradle. The vessel is then be moored to the cradle with a number of ropes fore and aft to prevent the vessel from moving

in any direction. Large marine railways can handle vessels of 6,000 tons.[4]

Once the vessel is secured to the cradle, the process of hoisting the cradle out of the water and up the slipway begins. Care is taken when the cradle starts to bear the

weight of the vessel; if the vessel is not sitting correctly in the cradle, it may damage the cradle or fall from it when fully out of the water. The vessel will usually sit on large

wooden wedges when the lift out of the water begins; in the case of larger vessels, they may be temporarily welded to the cradle bydivers.When slipway engineers and

staff are satisfied that the vessel is correctly cradled, it is hoisted to the top of the slipway, normally beyond the high tide mark.

Originally, men or horses andblock and tacklewere used to to providemechanical advantageto drag the cradle and vessel up the slipway. With the advent of the steam

engine, most marine railways were converted to steam powered operation. For remaining railways in existence, electric or electro-hydraulic winches are the norm.
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Dry Dock Methods

The type of Dry docking method selected depends on the type of the s hip and the severity of maintenance and repair required. The four main types of Dry docking Methods are as follows:

Floating Dock Excavated Dock Patent Slip Ship lift

Floating Dock

A Floating Dock is a kind of a pontoon with sponsons on both the sides. The pontoon is divided into number of tanks. When the ship has to be docked, the tanks are filled with water due to which the

dock submerges into the water. The ship is then navigated into the dock and the tanks are emptied so that the dock rises above the water.

The pumping of water is done with the help of ballast pumps which are located in sponsons. The sponsons are also equipped with cranes and bollards. Both the sponsons are connected with a bridge.

Apart from the ballast pumps, the water is also filled manually with the help of manual valves.

To prevent any movement of the ship, keel blocks are provided. These keel blocks are placed on the tanktops. For increased stability side blocks are also provided.The side blocks are provided in such a

way that the forces they exert don't damage the hull. Special reinforcement material is provided to prevent this.

1)Keel Blocks

2)Side Blocks

3)Side Sponson

4)Rails for crains

Excavated DockThis method of docking is almost similar to that of Floating dock. The Only difference is that it has a closing door at the opening. Also, the dock is sloped slightly towards the opening and the ballast

pumps are located near the door itself. Excavated dock can be a floating structure or a dock at the harbour.

Patent SlipThis system is generally used for small ships of length 140 meters or less. It works some thing like the "jack up" system. The system consists of cradles that moves with the help of rails. The cradles roll

into the water till they reach exactly beneath the ship. Once there, they are pulled in a perpendicular direction which brings the ship above the water surface.
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Ship liftThis system is somewhat similar to the Patent Ship method. It also consists of cradles which are used to slid a platform into the water, below the ship. Once the platform reaches below the ship, the ship

is properly navigated on the platform.After that, Winches are used to lift the ship and to place it in the desired position. This system can lift ships only of length up to 125 meters. Approximately 14

cradles are used to move the p latform.

Types of threads

There are a different types of threads used in machines and fasteners around the world, generally many countries use their own threads, however now it has been standardized. The

different threads are as follows:

1. British standard whit-worth or B.S.W threads

2. British association or B.A threads

3. Buttress threads

4. Sellers thread or AmericanNational threads

5. Square threads

6. Acme threads

7. Knuckle threads8. Unified threads

9. I.S.O threads or International Organization for Standardization threads or the Metric threads. Generally metric threads are u sed on board ships.
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Documents

Notes on marine engineering