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.