Once a vessel has grounded there are a number of considerations that need to be addressed:
Has the hull of the vessel been broached during grounding. Sounding all the various tanks that are situated behind the outer hull can check this. If the hull has been broached, then the vessel should not be refloated unless it is determined that sufficient buoyancy and stability remains once the vessel is floating freely.
Has the hull of the vessel been deformed in any way. This could lead to increased bearing loads on the main engine and/or main transmission shafting
Has the propeller or rudder become fouled or damaged. Ensuring both units are free to rotate without any excess loads could check this.
Is the vessel discharging any oil. A check on the tank levels may assist in locating possible damage, but any external pollution should be controlled as soon as practical.
Are the sea suction intakes blocked/covered by the seabed. Any reduction in the seawater pressure could require that the sea suctions be changed over.
Soundings around the vessel should take place to determine where the vessel has grounded, and the type of seabed on which the vessel is laid.
Once the vessel has been grounded there should be an estimate of when the vessel may be re-floated. This could depend on the local tides, vessel loading condition, and vessel ballast condition. If the vessel were fully loaded, then discharge of the cargo may take a number of days, then it may be prudent to reduce the services supplied by the engine room. This will reduce the fuel consumption whilst aground. The limit of services may be dictated by any possible damage to the vessel’s hull.
One area of concern may be that the if the engine were normally operating on residual fuel, then to keep the fuel injection system warm this has to be circulated at all times. Should a fuel injector be leaking, then this could cause a build-up of fuel on top of the piston, which will cause excessive pressure rises when the engine has started. Thus the main engine should be changed over to diesel oil. This change over must avoid high rates of temperature changes, and once fully flushed the fuel oil system can be shutdown until the main engine is required again.
Once the vessel has been re-floated, then the following inspections should take place:
An in-water survey should be carried out with the agreement of the Classification Society. This will require that the vessel be in clear water, so that divers can examine the underwater portion of the hull. They will examine all hull plating, looking for dents, holes, etc.
A function test of the rudder and transmission shafting
A full set of crankshaft deflections should be taken. The readings obtained should be compared with previous readings, and also be within the engine manufacturer’s recommendations for that engine. Whilst measuring the deflections, bridge gauge readings may also be taken to ensure that the crankshaft is sitting firmly on the lower bearing half.
Even though the divers are examining the external hull shell plate, it may also be prudent to examine the cofferdam in way of the main engine seating, to ensure that the full strength of this important seating area is intact before engine power is resumed.
In order to monitor the engine performance, we would need to measure the power output and fuel consumption of the engine. This will allow the performance of the engine to be measured against previous and even test bed readings.
The staff would be instructed to the followings:
Measure the power of the each cylinder by electronic pressure measurement. If the engine was driving an electrical load, then the electrical output power could be used. This will allow the total engine power to be calculated, and also for any power imbalance to be detected. This power measurement would be taken every month.
Measure the fuel consumption of the engine every day over a 8 and 24 hour period. This consumption would be measured in tonne/hour, and thermal and density conversions from a volumetric flow rate at the meter would be required.
Calculate the specific fuel consumption of the engine in terms of g/kWh, so that fuel consumption at various engine conditions could be compared.
Engine room staff would be instructed to the followings:
Closely monitor the pressure differential across the lube oil filters each day, and to report any increase in this pressure, or the frequency of automatic filter blowdown.
Take a representative sample from the lube oil inlet to the engine every week.
This sample would be tested on board for
Water content
Change in viscosity
BN reserve
In addition every month a representative sample would be taken and sent for shore analysis so that a wider range of variables could be analysed.
The general condition of the engine could be monitored by measuring the various parameters taken by the data logger or manual log readings.
The staff would be instructed to the followings:
Take a full set of readings twice a day on all the major parameters of the engine, such as
Exhaust gas temperatures
Lube oil pressure and temperature
Fresh water cooling pressure and temperature
Scavenge air pressure and temperature
T/C rev/min
Exhaust gas smoke levels
These readings would form the basis of the datum readings for that engine, and any major changes to the measurements should be reported to the Chief Engineers without delay.
In addition, should electronic power measurement not be available on-board, then peak pressure readings of each cylinder would be taken monthly to ensure even loading of the cylinders
One unit may need to be disabled when any of these conditions exist:
Damage to a main component of the combustion chamber
Damage to a crosshead or bottom end bearing on that cylinder
The following sequence would be carried out for our vessels which use the MAN B&W slow speed engine:
Stop engine, isolate systems and allow to cool
Ensure a procedure is written that minimises the risk to personnel during the operation. Discuss the task and written procedure with the engine room personnel to ensure they are familiar with the risks, and the methods to be used to minimise these risks.
Ensure the fuel pump is de-activated by lifting roller and locking.
Lift exhaust valve actuators so exhaust valve remains closed during running (Note: the air spring supply to be left open)
Dismantle air start supply line, and blank with suitable steel plates, the main and control air pipes
Dismantle bottom end bearing, and turn engine to suspend piston, crosshead and connecting rod from supplied crosshead supports. Secure big end of connecting rod in crankcase.
Blank off main oil inlet to crosshead within the crankcase with a blanking plate
Isolate the cylinder lubricator for that cylinder by placing all lubricators on no stroke.
When one engine cylinder is isolated, then one problem that may occur is a “dead spot” during manoeuvring.
This is due to the air start valve being isolated for that unit, and is more likely when a smaller number of cylinders are present. The Master must be informed that this could occur, and the remedy would be to kick the engine in the opposite direction, and then restart in the required direction.
As the name implies, Purifier purifies fuel oil used on board. It is a machine that separates impurities from fuel oil. Impurities can be both solid and liquid/water.
Why there are impurities in fuel oil?
Crude oil is refined in the refinery and produces various fine products. Marine residual fuel is the last product in terms of purity in the refining process.
Besides to make the process more economic silica-alumina is used to break the heavier molecules lighter. This silica-alumina often is not possible to remove completely.
The amount of impurities depends upon the source of crude oil and quality of refining process.
Principle of operation:
Oil, water and other impurities are of different density. Whenever the oil is subjected to centrifugal force heavier particles (water and solid) will be thrown outside and lighter particles will remain close to the center.
Difference between Purifiers and Clarifiers:
Functional difference: Purifiers can separate two liquids of different densities due to centrifugal force. i.e. water and solid particles from fuel. Purifiers can also remove small amount of solids.
Clarifiers remove solid particles from the fuel oil. Clarifiers will also remove small quantities of water.
Purifiers may be arranged to used as clarifier.
Constructional difference: Purifiers have gravity disc whereas clarifier has no gravity disc.
Clarifier has no sealing water as handles only one liquid i.e: fuel oil. So there is no interface inside clarifier.
Purifiers have two liquid out: one for liquid oil and another for water.
Clarifiers have only one liquid outlet.
Main components of purifier:
Bowl body: Bowl body is connected with the vertical shaft by a lock nut. It houses the main bowl. Main bowl slides within the bowl body to facilitate de-sludge operation and to make seal with bowl hood.
Main bowl:Unpurified oil accumulates in the main bowl. It moves up and down by water pressure to facilitate sludge discharge mechanism.
Bowl hood: The cylindrical shaped hood makes seal with main bowl so that oil cannot leak out. It houses the main seal ring for the sealing purpose.
Main bowl and Disc
At the top it houses dam ring/gravity disc, impeller.
Discs:
These discs provide a way for the cleaned purified oils to go upward in a separate path through the holes cut into them.
Gravity disc:
It maintains the interface between oil and water. It is done by the inner diameter of the gravity disc.
Gravity Disc
If the diameter is increased more water will get its way to escape the bowl. The amount of water will be reduced, so the force exerted by the water will also reduce. The force exerted by oil will push the oil-water interface towards periphery. If the diameter increases more, the interface will move towards a position where oil will escape through water line. i.e: overflow.
Centripetal impeller: Oil collected from purifiers will be drawn inside and delivered by this impeller.
Centripetal Impeller
Inlet pipe: As the name implies fuel oil gets inside the bowl by the inlet pipe.
Distributor: Feed oil is introduced through feed inlet pipe and the distributor to the main bowl chamber.
Vertical shaft: The revolution of the horizontal shaft transmitted and increased by vertical shaft gear teeth and the bowl rotates at the top. Bowls are supported by upper bearing housing arrangement.
Vertical shaft
Purifier rpm and motor rpm:
Motor runs at a rpm of 2000-2500 rpm and purifier bowl rotates around 15000-18000 rpm. This is done by increased gear mechanism of horizontal shaft.
Horizontal shaft: It is connected with motor via frictional block arrangement. Bearings in the bearing housings are provided at two ends of the shaft. Between them spiral gear is fitted.
Water supply device: Underneath the bowl body cylinder water supply device is fitted to supply low and high pressure operating water.
Water chamber
Low pressure operating water used to close the main cylinder.
High pressure operating water used to perform sludge discharge.
Frictional block, frictional pulley and frictional boss : Motor is not directly coupled with the purifier’s horizontal shaft. Friction blocks are used for soft starting and acceleration of the motor and hence avoid the motor being overloaded. There are arrangements to house frictional blocks which will rotate as the same rpm of motor.
During start up the motor will run in slow rpm and so do the friction block. These blocks due to centrifugal force will keep hitting the circular frictional block of the horizontal shaft. So the horizontal shaft will start to move.
Connecting the motor with the horizontal shaft directly would cause huge load on the motor at starting and motor to trip.
Brake:
Purifier Brake
To stop the purifiers in emergency this break is used to stop the rotation immediately. The brake linings are pressured against the outer surface of the pulley to stop purifier rotation.
External equipment in the purification system:
Gear pump:
To supply fuel oil to the purifier external feed is fitted. Either the pump is driven by an electrical motor or the pump is attached by a safety joint with the horizontal shaft.
Heaters:Purifiers are fitted with heaters to facilitate fuel oil heating.
Factors influence the interface:
Through put/ Feed rate
Temperature of the fuel
Diameter of the gravity disc
Density of the fuel
Through put:
If the feed rate is excessively increased
The increased feed rate will increase the mass of fuel.
The increased mass of fuel will exert more force to push the water out of bowl.
This will cause the water-oil interface to move towards periphery and cause overflow.
It is of vital importance to run the purifiers with optimum feed rate. High feed rate reduces purification efficiency. Feed rate depends upon fuel consumption of the ship.
Running with low feed rate will cause the fuel oil service tank level to decrease. If feed rate not adjusted for long time
Normally, feed rate should be 110 percent of the fuel consumption.
Temperature of the fuel:
Temperature plays an important role to maintain oil-water interface.
If temperature increases the density of oil reduces.
The force exerting by oil reduces.
Oil-water interface tend to move towards center.
Diameter of Gravity disc:
It is also known as dam ring.
Gravity disc inner diameter determines the amount of sealing water inside the bowl.
Thus it also determines the position of the oil-water interface.
If the diameter increases interface moves toward periphery of the bowl.
If the diameter reduces interface moves toward center.
Density of oil: An increase in density will also results in increase of mass and centrifugal force to push more water out of the bowl. i.e. interface will move towards periphery.
To handle with high density oil gravity disc diameter has to be decreased and temperature has to be maintained.
Operational sequence:
Gain rpm of the motor and purifier.
Bowl close: Low pressure operating water will push the main bowl up and make seal with top hood.
Sealing water: From the top via the inlet line sealing water will be supplied to fill up the full chamber. At this stage full chamber is filled with water.
Sludge discharge: High pressure operating water will push the pilot valve or spring loaded valve plate to drain the water that used to make the seal with top hood. The seal will break and sludge will be thrown out of the main bowl.
Again main bowl closes by low pressure water.
Sealing water supplied to fill the chamber.
Fuel oil is supplied inlet via inlet pipe through distributor.
By centrifugal force impurities and water will be thrown outside and purified light oil will get its way through the drilled hole. The light oil will be guided by top disc to the centripetal pump to the service tank.
Sludge discharge/ De-sludge:
An important function of purifier is sludge discharge operation. It removes the accumulated sludge inside the bowl at either set time interval or manually.
Different makers have different methods of discharging. But basic principle is almost same.
Low pressure operating water/ bowl closing water will enter through solenoid valve for predetermined time and then stop.
This water will gain energy due to centrifugal movement of the bowl. The centrifugal force will push the main bowl up and make seal with the top hood.
Sealing liquid will enter inside the bowl through inlet pipe from the top for set time.
The sealing liquid will occupy the full bowl chamber.
High pressure water/ bowl opening water acts on the lower side of the pilot valve. It acts relatively on large surface area than the low pressure water that acts on the upper side of the pilot valve and causes the operating water to drain off through the drain orifice.
This drainage of water will cause the main bowl to fall down. Hence the seal will break. The sludge inside the bowl comes out of bowl with force. High pressure water will be injects for set period of time.
Pilot Valve
Instead of pilot valves in some designs, spring plates are used.
Safety features of purifiers:
Low back pressure alarm
High temperature alarm
Safety joint of feed pump
Properly tightening:
Some parts of the purifiers are secured by screws and nuts. It is of vital importance to tighten all lock nuts and crews are perfectly. The tally marks on the body of the parts must match perfectly.
Bowl nut
Disc nut
Cap nut of vertical shaft
Set bolts of frame
Lock nut of inlet pipe.
Regular checks:
Fuel temperature and feed rate
Any abnormal vibration
Purifier motor load
Crankcase oil level and color.
Any leakage.
Dealing with different grade of fuel:
If there is any change of fuel grade the gravity disc has to be changed according to the specific gravity of the fuel. The gravity disc inner diameter can be obtained from the monogram provided in the manual.
Selection of gravity disc inner diameter for fuel oil.
Suppose the specific gravity of fuel at 15 ⁰C is 0.93, operating temperature 80 ⁰C and feed rate is 1200 l/h. Diameter for the operating data will be 72mm.
Here, the blue line represents the fuel oil specific gravity which intersects operating temperature (80 ⁰C, marked yellow) at point C. Draw a parallel line to the X axis (Separating temperature axis) and extends up to 100 ⁰C. Connect the right end of the line with the 1200 L/h point. It will intersect the gravity disc inner diameter line at 72mm.
(a) Explain how lubricating oil can be tested onboard.
Answer:
L.O can be tested on board by using Maker’s supplied test kit , which can give the result whether it is safe for further use or not.
For L.O test , sample oil must drawn out from correct point , such as from test cock or from pump’s discharge pressure gauge connection. It should not be drawn out from , filter out let and purifier out let .
Sample oil must be taken with standard plastic tube with seal for both on board test and Lab. analysis.
Acidity Test
– Release a drop of sample oil onto the blotter paper. Then wait about 5 min. allow to spread itself.
– Add a drop of indicator solution ( Potassium Hydroxide ) onto the spread area of sample oil. Then wait about 5 min
– Acidity can be determined by color change of sample oil.
– Bluish Green means: – No acidity in sample oil , it is alkaline stage.
– Yellowish Orange means: – Neutral stage , but no longer satisfactory.
– Reddish Color means: – Acidity in sample oil , not fit for further use.
Another test is extracting the acid from sample oil , known as Ph. Test.
– Mix unknown amount of distilled water to sample oil and shake well.
– Then mix with known strength of indicator solution
– Compare with a series of color standard each representing their Ph value .
– Determine Ph value of sample oil quite accuracy.
Fuel Oil Contamination Test ( Viscosity Test )
Fuel oil contamination with L.O may cause by dribbling fuel valve and leaking fuel pump due to excess wear of its components. When L.O is contaminated with fuel
– L.O color change to dark brown
– Flash point become low
– Viscosity decrease
If fuel oil contamination is occurred , viscosity of L.O to be tested for determination , whether fit or unfit for further use.
– The viscosity test can be done 3 tube rolling ball viscometer
– One tube is filled with used sample oil . ( Let say its viscosity grade is SAE 40 when fresh )
– Fill lower and higher viscosity fresh oil into another two tube each and individually ( Let say SAE 30 & SAE 50 oil )
– Then placed all 3 tube in hot water contained plastic bucket to allow same temperature for a moment.
– Mount all 3 tube in tilted board and inverted. Internal hollow ball in each tube will rise to surface.
– If the time taken by ball in sample oil tube is between the time taken by remaining two ball in their tubes , it means that , the viscosity of sample oil is fit for further use.
– If not , contaminated L.O must be changed.
Water Dilution test
Water may dilute in L.O from…
– Condensation from vapour in the crankcase
– Leakage from Cylinder cooling system ( i.e , JK seal from liner )
– Leakage from steam heating of sump tank.
– Sea water cooler tube..
Water dilution can be check with a Crackling test.
Take sample oil with test tube and heated up while shaking .
If there is no crackling sound , oil is dry.
If there is a slight crackling sound, the oil having trace of water.
If there is heavier crackling and frothing , the oil is heavily diluted with water.
Insoluble Content test
It is very simple test use with blotter paper.
Release each single drop of sample oil and similar fresh oil with known insoluble content . Wait for complete spread of both oil drops.
Compare the two oil spread condition and determine the content result of test oil.
For straight mineral oil , test result should below 1 ~ 1.5 % of upper limit.
For detergent/dispersant oil below 5 % of upper limit.
(b) How will you rectify in the case of lubricating oil which has been contaminated with a certain amount of sea water.
Answer:
Lubricating Oil can be contaminated with sea water due to leakage from air cooler tubes and L.O cooler tubes. It is mostly happen while engine is stopped. Contamination of sea water can be detected by abnormal rise of sump oil level abnormal water discharge from L.O purifier and colour change of L.O .
If certain amount of S.W contamination with L.O is detected , batch purification method is recommended . For the successful purification , the following process should be done.
– Transfer the whole quantity of dirty L.O from sump tank to a storage/renovating tank .
– In this tank , L.O to be heated up to 60 ‘C by provided heating arrangement and allow to settle at least 24 hour. While settling , water and sludge from L.O to be drained out frequently.
– At this time , open up M.E crankcase , both wet and dry sump tanks , properly ventilated , thoroughly wipe out and cleaned , make inspection and closed back . All L.O piping to be drained out.
– If condition of L.O in renovating tank is found satisfactorily free from water and sludge in drainage , start to purify with L.O separator , renovating tank to renovating tank circulation process.
– To achieve optimum purification effects
– Use correct size of gravity disc
– Minimum feed rate
– Heated up inlet oil to 77 ~ 82 ‘C by purifier heater
– Water washing by using warm water.
– Frequently de-sludge the purifier
– During the process , possible sources for Sea water ingression to be rectified and pressure test to be done , to
make sure no further sea water contamination.
– L.O acidity , viscosity and water content test to be done frequently and keep batch purification process until test results are satisfactory.
– Then release L.O from renovating tank to sump tank via purifier outlet for normal engine operation.
– Even though the test results are found satisfactory , Oil sample to be sent for laboratory analysis for further recommendations.
