fin stabilization

) why  Bilge keels are considered as an effective method of roll stabilization in ships 02

      b)   what are the types of tank stabilization                                                                    03

   c)    With reference to fin stabilisers:

      Sketch a block diagram showing an automated control system;                                 (4)

d)With reference to activated fin stabilisers give reasons why:

            i)for large vessels fin stabilisers are preferred to passive tanks;                          (4)

            ii) fin stabilisers are preferred for passenger and fast cargo ships;                    (3)

           

a)      bilge keels

l  up to 30% roll reduction

l  cheap and easy to maintain

l  effective whether making headway or not

l  simple in construction

b)      Pure Passive

            water movement controlled by baffle plates or manually operated valves

Controlled Passive

            water movement controlled by air or water valves

Active

            water moved by an impeller or pump

d)

         i)Fin stabilisers require much less internal volume than tank stabilisers and the internal space taken up by fins is not usually required for cargo.  Typically, the space taken by a passive tank stabilising system is approximately 900m3 which equates to approximately 20 containers.

         The mass of the fin stabiliser system is also very small compared to the deadweight, whereas passive tank stabilisers take up approximately 1.5% of the displacement.

         Since fin stabilisers are also much more effective than passive tanks, there is less chance of cargo movement/damage and crews are more likely to work at optimum efficiency.

(ii)     In passenger ships, comfort is of prime importance and this necessitates the best roll reduction system available.  Activated fins are the most effective method of roll reduction throughout all periods of wave encounter, mainly due to their rapid response time.  There are also considerations of financial income with regard consumption of food/drink and other purchases.  It has been shown that excessive ship motions have a marked effect on income.

bulbous bow

a) “bulbous bow (bulb) used as a important part of the hull “ discuss the reasons in support of this statement                                                                            04

b) explain the reasons that in modern ship construction sometimes , bulbous bow is completely eliminated or existing ones are being modified                        04

c)        

a)      the properly designed bulbous bow reduces wave-making resistance by producing its own wave system that is out of phase with the bow wave from the hull, creating a canceling effect and overall reduction in wavemaking resistance. as it can potentially reduce the hull resistance considerably this lead to better fuel economy for the vessel that are moving at speed thus it help in reducing co2 emission.   In addition bulbous bow gives additional buoyancy to the vessel at the forward end. Strong bulb gives addition protection to the vessel in the event of the damage in under water area of the forward

b)     it has been found that bulbs are not efficient at all service speeds (relate it to Froude numbers). In very low ship speeds, bulbous bows have found to increase the drag. Because a bulb is only effective when it makes its own wave, along with the bow wave. But at very low Froude numbers, wave making hardly occurs. But the bulb still being below the waterline, increases the total wetted surface area of the ship, therefore contributing to increase in its skin friction resistance.

Currently large ships are being built that are moving at super slow speed slow steaming is becoming a preferred option. It reduces consumption and emissions. Container ships and bulkers now are so large that they need not go fast because their attractiveness is the amount of cargo they can shift, reducing shipping costs.

So these large ships are , timetabled to deliver their goods at, more economical rate. The bulbous bow then becomes superfluous and can actually hamper efficiency as the ratios no longer mean it does its job. Due to above  on those ship bulbous bow is modified or eliminated

in refrigeration under cooling supercharging

Q5 With reference to a vapour compression refrigeration plant

a)Explain why Each of following is desirable

i) undercooling of the refrigerant at the condenser outlet    02 marks

Undercoolingor sub cooling  is defined as the reduction in a refregerent below its saturation temperature.  undercooling is measured in °C. undercooling of the refrigerant is necessary to avoid vapour bubbles in the refrigerant ahead of the expansion valve. Vapour bubbles in the refrigerant reduce capacity in the expansion valve and thereby reduce liquid supply to the evaporator. undercolingcooling of 4-5K is adequate in most cases. that will increase the net refrigeration effect. 

ii).supercharging at the compressor suction        02 marks

 Expansion valve ensure that the refrigerant gas leaving the evaporator is superheated by 5 to 7 ^OC to ensure that maximum utilization of the latent  heat of evaporation at the cooling compartment also the super heat will ensure the gas entering the compressor will behave as a gas. Any condensation of the refrigerant at the compressor inlet will lead to excessive pressure build up inside the compressor. It will lead to lifting up of the safety head without compressing the refrigerant. Due to liquid entering the compressor cylinder Consequently it may cause damage to the compressor body. 

b) Describe with the aid of a sketch how a   heat exchanger could be incoperated in the circuit to enhance undercooling.

As shown in the diagram below refrigerant liquid leaving the condenser is further cooled by removing sensible heat, using an internal heat exchanger here the cooling medium is the superheated refrigerant gas returning from the evaporator.

internal heat exchanger

c) Explain the possible consequences of the refrigerant having a dryness fraction at the compression suction    02 marks

Wet compression cycle is not preferred since liquid droplets of refrigerant will damage the valves and moving parts of the compressor, liquid refrigerant carrying lubricating oil from compressor would adversely affect the heat transfer rates, has low volumetric and mechanical efficiency Effect of operating variables on performance of vapour compression cycle are super heating of suction vapour increase refrigerating effect,.

hollow type coupling bolt and the hydraulic head/nut

Q4. With regards to main transmission shaft flange coupling arrangement :

a) Sketch a hollow type coupling bolt and the hydraulic head/nut and loading rod which are

 used to fit it; [4 Marks]

b) Describe how the bolt is fitted  [4 Marks ]

High pressure oil pumped into the head pushing down seal piston and rod this acton stretches the bolt (within its elastic limit)and reduces it diametersufficently to make a running fit into the hole. When the fluid pressure is released bolt will expand tightly into the hole with a radial grip simalutaniously  longitudinal contraction with the hand tight nutmaking a complressive load on the face of the nut hydralic head and loading rod is remved and  protective cap and seal is fitted to complete the assembly

c) state the advantage of the hollow coupling bolt as compared to the traditional type of

 coupling bolt [2 Marks ]        

Reduction in fitting and dismantling time, bolts can be used repeatedly.  During installation of traditional bolts, tight tolerance fit into the bolt hole will sometimes cause damage to both the bolt and hole During repeated fitting and removal. Also during the tensioning process the bolt is stretched and reduced in cross section which leads to reduction in the friction between the surfaces of the bolt and the bolt hole Dismantling in.

increasing its deadweight capacity without altering the ships length.

CE20175

Q1       You have been appointed as Chief Engineer to a dry cargo vessel recently purchased by your  shipping company.  The Company requests that you examine the vessel with a view to increasing its deadweight capacity without altering the ships length.  Outline the suggestions that you would make, justifying your proposals

To increase the deadweight through increasing the size of the ship requires an increase of either length, breadth, draught or block coefficient.

                The question rules out an increase in length.

                Of the remaining three parameters, it would seem to be totally impractical to convert the ship by increasing breadth or block coefficient.

                To increase the draught requires a reduction in freeboard, thus by considering the Load Line Regulations, there may be scope for a freeboard reduction.

                The vessel is a dry cargo ship, hence Type B, therefore attracts the maximum freeboard.

                If the vessel had wooden hatch covers fitted, then the replacement with steel, gasketed covers would reduce the freeboard. 

                In the design/build stage the Assigned freeboard is increased if the design is deficient in sheer, extent of superstructures and bow height.  Thus, if any of these deficiencies existed, there is capacity for structural alteration - adding superstructure - increasing deck sheer - adding a forecastle, (although the first two of these options would be very demanding). 

                The ship could also be deficient in depth and this could be increased (there have been conversions of this type carried out) which would raise the freeboard deck and, for the same freeboard, increase the draught.

                The ship may be an open shelter deck type which could be converted by ensuring all the openings had permanent means of watertight closing, thus raising the freeboard deck, effectively increasing draught.

                It may be possible to modify the structure to a bulk carrier and have Type B-60 assigned which would allow a reduced freeboard.

                Although an increased draught by decreasing freeboard would seem to be the only option, there are some modifications that can be made.  Sponsons have been fitted to some vessels (notably RO-RO's) to improve stability.  However, the extra buoyancy provided could be used to increase deadweight.  Other hull protuberances could be fitted for some other reason than extra buoyancy and allow an increase in deadweight.  For instance, a bulbous bow may be fitted to reduce wave making resistance, but it does add extra buoyancy.

6. With reference to the metallurgy of plan carbon steel

a) Sketch an iron carbon equilibrium diagram , labelling the salient point ;    [ 6 Marks]

b) Explain EACH of the following terms

         I.            Austenite                   [ 2 Marks ]

When steel heated, above the critical temperature of 723°Ca  non-magnetic solid solution of carbon and iron that exists in steel. Its face-centred cubic (FCC) structure allows it to hold a high proportion of carbon in solution. As it cools, this structure breaks down into a mixture of ferrite and cementite usually in the structural forms pearlite, or undergoes a slight lattice distortion known as martensitic transformation. The rate of cooling determines the relative proportions of these materials and therefore the mechanical properties (e.g. hardness, tensile strength) of the steel. Quenching (to induce martensitic transformation), followed by tempering (to break down some martensite and retained austenite).

The addition of  manganese and nickel, can stabilize the austenitic structure, facilitating heat-treatment of low-alloy steels. In the case of austenitic stainless steel, much higher alloy content makes this structure stable even at room temperature. On the other hand, such elements as silicon, molybdenum, and chromium tend to de-stabilize austenite, raising the eutectoid temperature (the temperature where two phases, ferrite and cementite, become a single phase, austenite).

Austenite can contain far more carbon than ferrite, between 0.8% at723°C and 2.08% at (1148°C). Thus, above the critical temparture, all of the carbon contained in ferrite and cementite (for a steel of 0.8% C) is dissolved in the austenite.

      II.            Cementite          Cementite is iron carbide with the formula Fe₃C. It is a hard, brittle material, essentially a ceramic in its pure form. It forms directly from the melt in the case of white cast iron. In carbon steel, it either forms from austenite during cooling or from martensite during tempering. Cementite contains 6.67% Carbon by weight; thus above that carbon content in the Fe-C phase system, the alloy is no longer steel or cast iron, as all of the available iron is contained in cementite. Cementite mixes with ferrite, the other product of austenite, to form lamellar structures called pearlite and bainite. Much larger lamellae, visible to the naked eye, make up the structure of Damascus steel. Fe₃C is also known as cohenite, particularly when found mixed with nickel and cobalt carbides in meteorites

  [ 2 Marks]

2Ekg 20175

 12

Ships hull is designed to withstand  stresses caused due to external forces such as weather therefore normally  a ship structure can remain intact provided that the load distribution requirements are met.

However in nomal operation it is quite difficult to maintain uniform load distribution  due to the nature of the loading unloading programmes, type of cargo or pre stowage of cargo as in the case of containers

In bulk cargo vessel Pouring the cargo through a shooter or via a conveyor belt does the loading. while doing so it is difficult to achieve even load distribution.In loading high density cargo such as steel makes it even more difficult. Above work is made worse as the owners are always trying to load the maximum cargo capacity.

In container cargo vessels load distribution makes more difficult due poor load ditrbution of cargo inside the container. Also due Wrong weight declaration. in general cargo ship different typs of cargo loaded in the same hold make it difficult to obtain the required load distribution.

In order to satisfy the stability requirements when water ballast is taken the always ditrbute on large tank areas making it difficult to reach the right balance. Some additional stress are induced to the hull when the cargo is loading and discharging as port operation demands fast turnaround of ships

As per above given conditions in operation of a ship it is difficult to meet the load distribution intended by the designer. the gap between designer anticipated load distribution and actual load distribution  is widened.. More than the external factors these factors contribute to structural failures of the hull

Large bulk carries transporting either liquid or dry cargos bear ample evidence of the irralavnce of  sheer

26.7.6 Correction for sheer profile (Regulation 38) Sheer is defined as being the curvature of the freeboard deck in a fore and aft direction. Benefits of sheer include: * Greater reserve buoyancy at the ends of the ship, particularly forward, ensuring good lift in a head/following sea; * Reduces water shipped on deck; * Reduces risk of foredeck being submerged after collision thus improving survivability in the damaged condition and helps to maintain an acceptable angle of heel at which progressive downflooding takes place. The tabular freeboards are based upon a standard sheer profile (standard ship), measured at seven equally spaced stations along the hull. A process based on Simpson’s 1331 Rule of area estimation is applied separately to the sheer measurements from the aft Fig. 26.17 perpendicular to amidships and the forward perpendicular to amidships to produce measures of effective sheer aft and forward respectively. Any deficiency in sheer will result in an increase in freeboard. Excess sheer will result in a deduction in freeboard. The amount of the deduction or increase in freeboard is determined by formulae in regulation 38.

indentations,sea chest anchor and chain etc

1             a)   Describe possible faults which may be found during a dry dock inspection of the following and the remedial action taken.

1. Hull indentations                   
2. Anchor and chain                   
3. sea chest                              

hull indentations-

the primary factors to be considered in assessing the effect of indents are as follows

• measure the area and depth od the indentation and record in shell expansion drawing check Whether internal structural members in way.

• Whether internals in way are significantly affected i.e. tripped or adrift from bottom plating. Smooth sccttered indent with small depth with the internals remain intact can be left after the recording. If the depath is large renewal of the section with the similar plate having minimum 300x300mm  should be done any damaged or detached internals must be renewed.

 Anchor and chain

Identification of repairs are done after laying in dock bottom. Washing ,removal of any rust makes it easier for the inspection and caliberation

If the wear down is exceeded the recommended value those chain length are to be renewed with similar chain lengths with classification certificates .

Any missing lugs are to be replaced and loose lugs are to be re welded  joining shakeles securing arrangement and the pin and their end sealing arrangement with lead is checked

Anchor is checked for worn out fluke pin worn out shank pin and hole weight measurement is requirement in some classification societies

A           Steam outlet pipe section inside the sea chest corroded and wasted

B.             Pipe connecting the sea chests are thinned down or erroded (stub pieces are thinned down )

C.               Usually vent pipe is find chocked due to the small pipe diameter  the pipe get chocked

D.             In high sea sucion  upper area corrosion due to the accumulated air in the sea chest may find if the vent               holes are not provided on the top

E.           Sea chest top plates from the engine room side may get thin down due to the water leakages of glands and        accumulated water.

F.            Fouling

G.            Mgps sytem not funtioning

lll          locking arrangements of the gratings   wasted anodes   

 thhic  thickness measurement of sea chest plats and stub pieces should be done overhauling all opening mounted on the sea chest is done steam discharge line should be check anodes fitted are renewed. securing arrangements of the gratings are checked and ensure anodes are uncovered before fitting back the gratings.

s

a

hull plate renewal

1     As a chief officer write a dry dock specification for the repair for the repair of following damage that has occurred, stating what factors must be considered when costing the repairs.” approx. 2 meters of damage to Forepeak tank shell plating must be removed and replaced along with relevant damaged stiffeners. In your specification, include the services required, accessory work required and final tests required to ensure satisfactory completion of the repair. 

Steel renewal in no fore peak  ballast tank

Please quote for following scope of work to be carried out during the period of dry docking 

tank capacity 5000 m³(sea water)

open manhole covers 02 nos on deck size 680mmX480mm for the inspection and to be boxed back after the repair with new jointings indicate repair cost for replacement of studs.

provide ventilation blowers for the tank during repair indicate cost per day

provide lighting to the tank

provide man entry certificate and hot work permit during the period of repair

clean the ballast tank and hose down prior to commencement of the repair

erect staging from frame no 46 to 51 at second stringer level to required height and to remove on completion

erect staging from frame no 46 to 51 for the outside shell on port side to 8meter height from ships baseline and to remove on completion

crop and renew the damage section of approx.; 02 square meters side shell plating with damaged frames in way of the plating area with classification approved materials. the material specification andthickness is clealy marked on the shell expansion drawing provided and details of the frames are included in the drawing for preparation.

Marerials certificates welding procedure and qualified welders are to be provided during the repair

work to be completed to the satisfaction of the classification society surveyor

on completion of the plate renewl vaccum test or hose test to be done on the plates.

Apply paint scheme   with inside the tank as per the attached specification

Paint supplied by ship

Outside the hull apply the yard supplied paint two coats of anticorrosive and anti-fouling paint coating thickness 300 microns are to applied paint supplied should have anti fouling approval certificate

Clean all debris generated during repair from the tank 

Tank will be pressure tested by ship staff on completion of the repairs by filling with sea water

Al above wok to be carried out to the satisfaction of the classification society surveyor