Q-Explain the influence of following external factors in higher consumption of fuel oil and how at best they could be controlled?
(i) Ship's hull condition (ii) Weather condition (iii) Maintenance of different equipment’s in a fuel oil system (iv) Damage to propeller blades.
ANSWER :-
As there is very close business competition in the market each and every company keeps a very good eye on their quality of service. This has to be done at minimum and optimum expenses on the part of the company as the venture must also be commercially viable for them.
As a part of daily expenses of ship running cost it takes about 40% cost of overall expenses for bunker and related operations.
Hence savings in fuel is as very important part for shipping companies and also part of machinery genuine malfunction. Few savings in bunker expenses becomes surprisingly a significant amount in a life time period of the ship.
1.SHIPS HULL CONDITION
Ship’s hull condition definitely influences the fuel consumption. Ship’s movement is restricted by resistance experienced by the ship, which is comprised by two types called frictional resistance and residual resistance.
Frictional resistance is a function of density of water, hull roughness and length of ship. Residual resistance is due to wake forming tendency caused due to movement in water and shape of ship.
Frictional resistance can be up to 70% of total resistance of the ship and hull condition is a major contributory factor in frictional resistance.
Thus it is very important to keep hull clean. Hull can be cleaned by several methods while ship is afloat, but majority of these are not effective in long run. So, regular dry docking is the best solution in this regard.
So any increase in underwater hull roughness will increase hull frictional resistance or vessel drag resulting in additional power requirement with increased fuel consumption and cost to maintain vessel speed.
Factor responsible for hull fouling are:- a) Use of improper techniques in applying paint. b) Poor quality of applied paint. c) Long port stays or anchorage. d) Damaged hull surface. e) Poor maintenance of hull protective system.
Appropriate measures should be taken regarding the above points to reduce the extent of hull fouling of the ship. Antifouling paint of approved type and well maintained antifouling system plays an important role in ship’s regular operating period between dry docks.
It is found that ship fouling pattern is not very regular. As shown in (speed/power) graph, it may be very slow in initial stages or may be very standardized in the initial years but in over a period of time it becomes very ―stiff are very fast.
Speed and power graph also indicates that the engine may be thermally overloaded with a badly fouled hull resulting in a decrease of the operating life of machinery parts causing frequent breakdown and coating very heavily on shipping companies.
Antifouling paints of approved type and a well maintained antifouling system plays an important role in ships regular operating period between dry docks.
2)Weather Condition
1. Ships are designed and constructed to withstand the forces of nature up to a certain extent for a certain time. Depending upon the area of trading weather conditions keep changing and also the condition of the sea.
2. Seasons such as summer, winter or monsoons of extreme nature are very common in the trade of shipping. If climatic conditions/weather conditions are favorable it may result in a +ve slip i.e. the ship travels more than the distance given by the engine and vice versa with a bad or heavy weather condition it may result in a –ve slip resulting in a extra fuel consumption due to higher power demands and overloading of engines.
3. Good judgement and regular updates regarding weather conditions help the master in closing a route to avoid adverse weather condition. This may result in less full consumption in long run.
4. Engine manufacturer guidelines should be strictly followed in severe weather conditions. Governor load index, hunting, R.P.M, scavenging air limit torque limits must be taken into account to avoid thermal and mechanical overloading of the engine. Thus these guidelines can be kept in mind for keeping fuel consumption within limit
5. Effect of humidity plays a important role and is also important as it reduces the NOx limit. Humidity effects the density of charge air, development of power heat release more temperature in a unit and exhaust temperature.
Weather routing :-Use of weather routing services to avoid rough seas and head currents, to optimize voyage efficiency.
3)Maintenance Of Different Elements In Fuel Oil System
1. Although the condition of the hull and weather play a significant role in regulating fuel consumption, elements which directly control fuel have a proportional relation to the consumption of fuel.
It is meant that if parts or equipments used are in good condition then lot of fuel can be saved for example fuel injectors, fuel pipes, fuel pump and VIT RACK shock absorber if maintained in a good way in regular inspection and overhaul problems of fuel leakages can be minimized.
The different elements of fuel oil system are:-
Fuel oil injector:- It should be regularly changed after fixed running hours as per maker’s guidelines. Overhauled and tested valves to be used.
Fuel pumps:- Injector timings to be checked regularly and adjusted as required.
VIT:- Adjusted as per requirement. V.I.T. mechanism and fuel control rack and its connection with governor must be lubricated periodically to eliminate sluggishness and wear and tear.
Fuel oil filter:- Regular cleaning.
Viscotherm:- Regular maintenance and maintaining proper fuel oil temperature to achieve desired viscosity as per fuel oil analysis report.
Service tanks:- Proper temperatures to be maintained.
Greasing of fuel linkage
Operation and maintenance of purifiers should be always carried out for proper operation.
Similarly pipe joints, V/V glands booster pump section or any fuel oil leakage in fuel oil system if attended immediately also improve the safety of the ship.
Proper temperature of fuel supplied to the engine should be maintained for optimum efficiency.
4) Damage to the propeller blades
Propeller should be maintained in top condition at all times. The main factors that detract from optimum condition are fouling, cavitations and physical damage. But, any distortion from their true shape can cause an imbalance and hence vibration, which in turn causes increased cavitations, loss of thrust, drive shaft damage, wear on numerous bearing and increased fuel use due to decreased efficiency.
1. Propeller blades can be damaged/ rough for a number of reasons. They invariably become rougher during service as a result of cavitation damage to the metal surface itself, calcium deposits, mechanical damage and marine fouling, including slime, algae, barnacles, tube worms and other marine organisms as with the ship’s hull in general.
2. Although the surface area of the propeller is minuscule when compared to that of the entire hull, the effect of a rough propeller or Damaged propeller on the vessel’s fuel consumption is comparatively large.
On the other hand, the cost of remedying a rough propeller compared to that of remedying a rough hull is very slight. Thus remedies for a rough propeller are not only simple and quick to execute, they also represent a fast, high return on investment.
4. Propellers can be cleaned or polished in the water or in dry dock.
5. Economically, the fuel saving from the more frequent cleaning of a propeller before it has become seriously fouled and rough greatly outweighs the cost of the cleaning itself. This propeller cleaning can be combined with a general hull inspection by divers making it even more economically viable.
Physical damage mostly causes vibration. The solution in this case is to trim the blades equally to remove the damage and achieve proper balance, and reduce excessive cavitation. But this should be done carefully as bad trimming can result in even worse problem.
Blade edge damage usually takes the form of cracks, bends, or breaks and may include the loss of a small section of the blade.
Minor bends or tears can sometimes be repaired without removing the propeller from the shaft.
Very slight distortions along the blade edge can usually be straightened cold by hammering carefully. After straightening however, the area worked should be dye-penetrant examined for cracks which may have resulted from the hammering.
DEFORMATION
Minor edge deformation can be corrected in-situ, using hand operated straightening levers and ‘soft’ flame heating torch (not oxy-fuel). Major straightening and pitch checking would be carried out in a shore
ROUGHENING
Roughening may lead to rapid degradation. Very small pits (less than lmm) can be
ground out and polished. Deeper pits can be repaired by welding, grinding and polishing.
Temporary repair can be carried out using resin fillers.
EDGE CRACKS
Minor edge cracks can quickly become more serious, especially at the leading edge. If
the cracks are less than 10mm in length, they can be ground out and the edge ‘faired’.
Larger cracks may need to be repaired by drilling a small hole at the root of the crack,
gouging out, welding, grinding and polishing (not in the inner third of the blade).
Temporary arresting of propagation has been carried out by drilling a small hole at the
root of the crack, plugging the hole, grinding and polishing.
SERIOUS EDGE DAMAGE
Repaired by welding on a new cast piece of the blade. The blade is cropped and the new
portion clamped in position, welded on one side, ground flush, clamps reversed, other
side gouged, welded and ground. A final polish gives the required finish to the blade
surface.
The best welding techniques are the semi-automatic, inert gas shielded processes such as
MIG (MAGS) and TIG (TAGS). The repair should be in the outer 55% of the propeller
radius (outer two thirds of the blade) followed by stress relieving. The major repairs must
be earned out at a shore based facility with a stress reliving oven.
APPLICATION OF HEAT
This can be used to assist the fiting /removal of some propellers, but great care must be
taken to avoid high residual stresses being introduced when cooling. The consequences of
residual stresses could be stress corrosion cracking’ of the boss which may not occur
until some weeks or months after the application of the heat. Stress corrosion cracking in
the boss due to the incorrect use of heat for fiting /removal usually results in a scrapped
propeller. The heating process has been successfully carried out using steam, electric
blanket or soft flame such as produced by paraffin, or propane and air.
A force flame such as oxygen and fuel should not be used.
Under no circumstances should heat be used with shrink fit bosses since the resin bond to
the insert is destroyed.
There are 3 types of modification:-
Diameter reduction:- Easily and inexpensively performed underwater, this is the usual method for increasing RPM and balancing the ratio. The blade tips are cropped and faired.
Pitch reduction:- This involves twisting of blades and can only be accurately done in a workshop as blades need to be heated to prevent cracking. Although more expensive this is most effective modification as there is no loss of blade material. It is ideally suited to blades smaller than 4,000 mm diameter.
Trailing edge modification:- This is achieved by either bending the trailing edges or by cutting them. Both operation can be performed in water and can achieve an effect on the RPM of approximately 5%.
