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STRENGTH OF SHIPS

KEY TO QUESTION PAPER

1(a) What are the functions of a ship structure? (2 Marks)

The main functions of the ship s

• To give longitudinal strength to the ship against the waves• To give transverse strength to the ship to resist • To withstand the Upward pressure,• To withstand the bending stresses• To withstand the bottom damage by grounding and underwater shock• To withstand the stresses & forces such as panting, pounding, raking,

slamming, whipping and springing.• To withstand the forces and stresses against the collisi

1(b) What are t he assu mptions of a longitudinal strength calculations.

(2

Marks)

The problem of calculating the ul basically, a kind of

usually with initial imperfection and residual stress.

The assumptions in the static

(i) Resulting stresses cannot be considered to be real but that they must be treated as comparative only

(ii) This comparison can be made between one ship to the other or between different loading conditions for the same ship.

1(c) What are the thermal effects on the hull girder? (2 Marks)

A thermal stress may be dened as a strenon-uniform temperature distribution within the body. Temperature grad ients

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in the ship’s hull will cause deections of the hull girder cau sed by thermalexpansion. Thereby th ermal stresses w ill be induced in the sh ip’s st eel.

As a result buckling of hull platwi

stresses can cause the body to have brittle fracture. In marine industry,heating due to the atmospheric temperatures is n ot an issue on ship’s hull. Whereas low temperature applications such as G Carr

are moving in cold weather con ditions can cause signicant failures to thestructure.

1(d) What are the forces acting on ship structure in head seas? (2 Marks)

During the head seas t he ship’s bow will be subjected to up and down motionsand thereby se vere p itching. This p itching will lift the fore b ody of the ves sel to

be completely come out of water and ther which is called as SLAMMING.

Due to slamming there will be n umber of stress t hat will be developed in theship’s st ructure a nd they a re a s follows:

(i) Panting(ii) Pounding

(iii) Whipping(iv) Springing

1(e) What are the factors affecting the superstructure effi ciency? (2

Marks)

The efficiency of superstructure is dened as

ή= σ 0 − σ

σ 0 − σ 1

Where, σ 0 is u pper deck st ress w ithout superstructure

σ is u pper d eck stress ca lculated

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σ 1 is t he u pper deck stress w ith fully effective su perstructure.

From the a bove formula it is cl ear t hat t he factors aff ecting the su perstructureefficiency are as follows:

(i) The Length of the Superstructure: When the superstructure is 15%-20% length of main hull, it can be regarded as a rel atively longsuperstructure

(ii) Fully effective superstructure may be obtained with 35% length ofmain hull.

1(g) Explain simple bending theory. (2 Marks)

Simple beam bending is often analyzed with the Euler-Bernoulli beamequation. The con ditions for u sing simple bending theory a re:

1.The beam is su bject to pure bending . This m eans that the shear force iszero, and that no torsional or a xial loads a re p resent.

2.The material is isotropic and homogeneous .

3.The material obeys Hooke's law (it is l inearly elastic a nd will not d eform

plastically).

4.The b eam is i nitially straight with a cross se ction that is co nstantthroughout the beam length.

5.The beam has an axis of symmetry in the plane of bending.

6.The proportions of the beam are such that it would fail by bending ratherthan by crushing, wrinkling or si deways buckling .

7.Cross-sections of the beam remain plane during bending.

http://en.wikipedia.org/wiki/Pure_bendinghttp://en.wikipedia.org/wiki/Shear_forcehttp://en.wikipedia.org/wiki/Isotropichttp://en.wikipedia.org/wiki/Homogeneous_spacehttp://en.wikipedia.org/wiki/Hooke's_lawhttp://en.wikipedia.org/wiki/Bucklinghttp://en.wikipedia.org/wiki/Shear_forcehttp://en.wikipedia.org/wiki/Isotropichttp://en.wikipedia.org/wiki/Homogeneous_spacehttp://en.wikipedia.org/wiki/Hooke's_lawhttp://en.wikipedia.org/wiki/Bucklinghttp://en.wikipedia.org/wiki/Pure_bending

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2(a) What are the forces acting on a ship at sea? Discuss with neat

diagrams. (7 Marks)

The hulls of ships are su bjected to a number of loads.

• Even when sitting at dockside or at anchor, the pressure of surrounding water displaced by the ship presses in on

• The weight of the hull, and of cargo and components within the bears down on the hull.

• Wind blows against the hull, and waves run into it.

• When a ship moves, there is additional hull drag, the water driven up against the bow.

• When a ship is loaded with cargo, it may have many times its wn empty weight of cargo pushing down on the structure.

If the ship's structure, equipment, and cargo are distributed unevenly theremay be large point loads into the structure, and if they are distributed

differently from the distribution of buoyancy from displaced water t hen thereare b ending forces on the h ull.

The various forces, moments and stresses moving in a sea-way a re as f ollows:

(i) Panting(ii) Pounding(iii) Racking(iv) Slamming(v) Whipping(vi) Springing(vii) Sagging(viii) Hogging

http://en.wikipedia.org/wiki/Hull_(ship)http://en.wikipedia.org/wiki/Hull_(ship)

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2(b) Discuss t he distortion of a sh ip structure with application of theory

and experience. (7 Marks)

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3(a) How do you carry out the 3-dimensional analysis of a ship structure?

Explain. (7 Marks)

The three-dimensional hydro-elasticity include the effect of uid compressibility. This enables the a coustic resp onsesof a ship structure induced by the machinery or wave excitations beingpredicted with the inclusion of the free su rface a nd the forward speed effect. Amethod for el iminating the irregular frequencies in the numerical analysis isproposed.

To develop a 3-D model, there is a need of an integrated software tools for n aval architects and ship design engineers. It is used for t hepreliminary design of a ship's hull structure and supports key decisionsregarding naval arch itectural characteristics. C lassication drawings, st eelquantity est imates, weld lengths, and Weights an d Centre of Gravity reports a reproduced within the m odel. In today’s h ighly concurrent shipbuilding projects,structural engineers m ust work in parallel with the d etailed designers.

Once the 3D model is generated the global and local strength analysis can beeasily performed. In conclusion, by using the 3D model and FEM software, thenumerical FEM analysis provides reliable data for the ship strength

assessment (under equivalent quasi-static head waves), having a goodconcordance between the structural models developed. Same model can also beused for fatigue a nalysis, risk assessm ents a nd sensitivity analysis a lso can beperformed.

3(b) Explain the use of probability theory in the assessment of

longitudinal strength of ship. (7 Marks)

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4) PROBLEM :

Given Data is: Let upper deck height = x mSuper-structure d eck height = 2.6mDeck beam = 13mDeck Thickness = 12 m mMaterial of the deck = Aluminium Alloy ( Density = 1/3 of steel)Ship’s Sagging Moment = 450 MN-mFrame Spacing = 730 m mStress in the upper deck = 55 MN/m2 because of added superstructure.

FORMULA:

The efficiency of superstructure is dened as:

ή= σ 0 − σ

σ 0 − σ 1

Where,σ 0 is upper deck stress without super structure = 5 5 MN/m2

σ is u pper deck stress calculated = 44 MN/m2σ 1 is t he u pper de ck stress w ith fully effective su perstructure

= 40 MN/m2

Theefore,

ή=55 − 44

55 − 40 = 11/ 15 = 73%

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5(A) Briey explain the modeling of ship hull girder as a be am with neat

diagram. (7 Marks)

When the ship’s strength is assessed, strengths, which are longitudinal strength, transverse strength and localstrength. Among these, longitudinal strength, that is h ull girder st rength, is t hemost fundamental and important strength to ensure the safety of a shipstructure

Hull girders are ver y important components in ship structures, and thereforethey should be designed for a set of failure modes that gover n their st rength.

The modes of failures can be classied tserviceability limit states. Strength limit states are based on safetyconsideration or u ltimate load-carrying capacity of the hull girders and theyinclude plastic st rengths, buckling, and permanent deformation.

After modeling a simple Euler-Beam theory can be

5(b) Differentiate between the buoyancy curve and weight curve with

diagrams. (7 Marks)

Buoyancy Curve :The up-thrust at any one metre length of the sh ip dependsupon the immersed cross-sectional area of the sh ip at t hat po int. If the va luesof up-thrust at different positions a long the length of the sh ip are p lotted on a

base representing the ship’s NCY CURVE is formed.

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This curve increases from zero aparallel mid-ship portion. The a rea of this cu rve r epresents t he t otal up-thrustexerted by the water on the sh ip.

Weight Curve : The weight curve starts and nishes at the extremes of theship’s st ructure

The buoyancy or the weight along the l At various points along the

an excess of weight.

Hence: Difference (weight-buoyancy) = resultant load

Below gure indicates both Buoyancy Curve an d Weight curves on a single

base.

6(a) Explain the application of plastic theory to ship structures. (7

Marks)

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6(b) Explain the procedure for calculation of hull girder section modulus

and null deection. (7 Marks)

7 (a) Briey discuss t ransverse l oads on the ship’s hull with neat

diagrams. (7 Marks)

7(b) Briey explain the inuence of different bracketed connections in

ship structures with diagrams. (7 Marks)

8 (a) Explain the thin plate theory (3 ½ Marks each question)

In continuum mechanics , plate theories ar e mathematical d escriptions of t hemechanics of at plates that draws on the theory of beams . Plates a re denedas plane structural elements with a small thickness compared to the planardimensions. The typical thickness to width ratio of a plate structure is lessthan 0.1.

A plate theory takes advantage of tfull three-dimensional solid mechanics problem to a two-dimensional problem.

The aim of plate theory is to deformation and stresses in a platesubjected to loads.

Of the numerous plate theories that ha ve been developed since the late 19thcentury, two a re widely accepted and used in engineering. These are

http://en.wikipedia.org/wiki/Continuum_mechanicshttp://en.wikipedia.org/wiki/Bendinghttp://en.wikipedia.org/wiki/List_of_structural_elementshttp://en.wikipedia.org/wiki/Continuum_mechanicshttp://en.wikipedia.org/wiki/Deformation_(mechanics)http://en.wikipedia.org/wiki/Continuum_mechanicshttp://en.wikipedia.org/wiki/Bendinghttp://en.wikipedia.org/wiki/List_of_structural_elementshttp://en.wikipedia.org/wiki/Continuum_mechanicshttp://en.wikipedia.org/wiki/Deformation_(mechanics)http://en.wikipedia.org/wiki/Stress_(mechanics)

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• the Kirchhoff theory of plates (classical plate th eory)

• The Mindlin – Reissner theory o f plates ( rst-order sh ear p late t heory)

The Kirchhoff – Love theory is an extension of Euler–Bernoulli beam theory tothin plates. It is a ssumed that a mid-surface plane can be u sed to representthe three-dimensional plate in two-dimensional form.The following kinematicassumptions that are m ade in this theory.

• straight lines normal to the mid-surface remain straight afterdeformation

• straight lines normal to the mid-surface remain normal to the mid-surface a fter deformation

• the thickness of the plate does n ot change during a deformation.

8(b) Discuss the bending and membrane stresses in ship plates

A "plate" is a solid body bounded by two urtwo surfaces d enes the thickness of the plate, which is assu med to be smallcompared t o the lateral dimensions.

Membrane and Plates have basic differences in structural behaviour.Membrane sustains transverse loading using in-plane stresses. While platessustain loading using bending stresses. 1-D analogous exam ples ar e difference

between a thread/cable and beam. If the bending stiffness of te reduced to zero, it will act like a membrane with due con sideration of boundaryconditions.

8(c) Write about t he structural discontinuities in a ship’ s h ull.

Due to poor structural desi gn, there may be several types of discontinuitiesthat will be formed in ship’s st ructure. The gen eral discontinuities that aremostly observed are as f ollows:

http://en.wikipedia.org/wiki/Gustav_Kirchhoffhttp://en.wikipedia.org/wiki/Raymond_Mindlinhttp://en.wikipedia.org/w/index.php?title=Eric_Reissner&action=edit&redlink=1http://en.wikipedia.org/wiki/Gustav_Kirchhoffhttp://en.wikipedia.org/wiki/Augustus_Edward_Hough_Lovehttp://en.wikipedia.org/wiki/Beam_theoryhttp://en.wikipedia.org/wiki/Gustav_Kirchhoffhttp://en.wikipedia.org/wiki/Raymond_Mindlinhttp://en.wikipedia.org/w/index.php?title=Eric_Reissner&action=edit&redlink=1http://en.wikipedia.org/wiki/Gustav_Kirchhoffhttp://en.wikipedia.org/wiki/Augustus_Edward_Hough_Lovehttp://en.wikipedia.org/wiki/Beam_theory

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Ends of superstructure : When the poop deck, and forecastle and theaccommodation superstructure. When seen from the prole view, thesestructures are su dden elevations in the prole of a ship. The effect of stressconcentration here, is in addition to the maximum bending moment at the

midship region, therefore m aking it highly p rone t o failure.

g2

Ends o f Longitudinal Girders: The girders that run longitudinally along the length of

the ship, for example the centre girder, and the side girders, are to be terminated at thefore and aft perpendiculars.

Deck Openings an d other openings: A ship, in general, is a lways characterised by a

deck opening. Every d eck opening is b asically a hole cut within a steel plate, be it in the

form of a manhole, or a hatch opening. discontinuities ca n appear in a ship structure.

Special Case- Container S hips: A special case arises i n case of container ships,

which are equipped with torsion boxes. Due to a ne hull form, it gets d ifficult to taper

the cross sectional areas of the torsion boxes in the aft and for’d regions, thus giving

rise to severe structural discontinuity.

Design methodologies to reduce structural discontinuities

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It is by now evident, that discontinuities in ship structures are unavoidable, but they can

certainly b e reduced to considerable limits. So it’s t ime we discuss a few interesting

ways t he industry h as a dopted to reduce the effect of discontinuities:

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8 (d) Distinguish between the st rength and stiffness of a hull girder.

Stiffness and strength are two of the most commonly confused terms in theship structural , and many people u se t hem interchangeably. But these ar e twodifferent things, and building the best-performing ship requires knowing thedifference.

Strength: A measure of the maximum load that can be placed on a material before it permanently deforms or breaks. Engineers of

stress, σy, as a measure of a material's st rength.

Stiffness: A measure of the amount of deection that a load causes in amaterial. Engineers u se a value ca lled Young's m odulus, E, for s tiffness.

For m ore clarity in understanding, consider a p iece of rubber su rgical tubinghas very low stiffness because it d eects a lot u nder l oad, but it is relativelystrong. A piece of glass lament is the opposite — it deects very little underload but might not carry a huge load before it breaks.