Notes On Ship Construction 1z1t1c

LECTURE NOTES ON SHIP CONSTRUCTION Ship Construction – General General knowledge of the principal structural of a ship and the proper name for the various parts is essential for controlling the operation of the ship and care for persons on board at the operational level and to Maintain seaworthiness of the ship and actions to ensure and maintain the watertight integrity of the ship are in accordance with accepted practice. Stability conditions comply with the IMO intact stability criteria under all conditions of loading.

UNDERSTANDING THE VARIOUS STRUCTURAL : i.

Bracket – the girder, etc

ii.

Bulkheads – a vertical partition between compartments

iii.

Center girder – In lieu of longitudinal, provide longitudinal strength . Longitudinal framing in DB –

iv.

Floor – A vertical athwartships member in way of the doublebottom. It will run from the center girder out to the margin plate on either side of the vessel.

v.

Frame – Internal member for the shell plating. Vessel may be framed longitudinally or transversely.

vi.

Gusset – triangular plate for ing angle bar to a plate

vii.

Intercostals Side girder – A side girder in the fore and aft line sited either side of the keel. Integral connection with the tank top and the ship’s bottom plating and rigidly connected by floors

viii.

Keels – center line plate from stem to the stern frame.

. Flat plate keels – Generally used keel. Center girder is attached to the keel and inner bottom plating by continuous welding and no scallops permitted a. Duct keels – a form of flat plate keel with tow center girder. Often fitted between collision bulkhead and forward engine room bulkhead to provide tunnel for pipes and additional buoyancy. ix.

Lightening holes – holes cut into floors or intercostals to reduce weight and to provide access to tank areas

x.

Longitudinal – A fore and aft strength member connecting the athwartships floors. It must be continuous for ship > 215m. Additional longitudinal are to be found in pounding area

xi.

Margin plate – a fore and aft plate sited at the turn of the bilge. The upper edge is normally flanged to allow connection to the tank top plating, while the opposite end is secured to the inside of the shell plate by an angle-bar connection. It provides an end seal to the double bottom tanks, having all the floors ing at right angles, up to the collision bulkhead.

xii.

Panting beams – athwartships in the forepart introduced to reduce the in & out tendency of the shell plating, caused by varying water pressure on the bow.

xiii.

Panting stringers – internal horizontal plates secured to the shell plating and braced

xiv.

athwartships by the panting beams.

Scantlings – used to indicate the thickness of plates, angles and flanges.

xv.

Sheer strake – the continuous row of shell plates on a level with the uppermost continuous deck.

VARIOUS STRESSES ENCOUNTERED BY A SHIP Bending, Shear, Hogging, Sagging, Racking, Pounding, Panting. DEFINITIONS: HOGGING When the peak of a wave is amidships, causing the hull to bend so the ends of the keel are lower than the middle. The opposite of sagging. SAGGING: When the trough of a wave is amidships, causing the hull to deflect so the ends of the keel are higher than the middle. The opposite of hogging.

PANTING: The pulsation in and out of the bow and stern plating as the ship alternately rises and plunges deep into the water POUNDING: Pounding, as a boat will do in short seas when the bow lifts clear of the water then comes crashing down. The term is local to English ports on the North Sea. RACKING: The effect of distortion of a vessel into a parallelogram shape due to the low transverse strength of the vessel is called as racking. Racking stress occurs in a seaway when the vessel is sailing in heavy weather and encountered heavy accelerations due to roll and heave.

Racking stress and its causes In a seaway as a ship rolls from one side to the other the different areas of the ship have motion which are dependent on the nature of the subject area. The accelerations are thus not similar due to the various masses of the different sections (although ed together). These accelerations on the ships structure are liable to cause distortion in the transverse section. The greatest effect is under light ship conditions.

Local Stresses

Panting This is a stress, which occurs at the ends of a vessel due to variations in water pressure on the shell plating as the vessel pitches in a seaway. The effect is accentuated at the bow when making headway.

Pounding:

Heavy pitching assisted by heaving as the whole vessel is lifted in a seaway and again as the vessel slams down on the water is known as pounding or slamming. This may subject the forepart to severe blows from the sea. The greatest effect is experienced in the light ship condition.

Stresses caused by localized loading Localized heavy loads may give rise to localized distortion of the transverse section. Such local loads may be the machinery (Main engine) in the engine room or the loading of concentrated ore in the holds.

xvi.

Panting beams – athwartships in the forepart introduced to reduce the in & out tendency of the shell plating, caused by varying water pressure on the bow.

xvii. Panting stringers – internal horizontal plates secured to the shell plating and braced athwartships by the panting beams. xviii. Sheer strake – the continuous row of shell plates on a level with the uppermost continuous deck. SOME IMPORTANT QUESTIONS & ANSWERS: Q.

describes racking stress and its causes

Q.

describes what is meant by 'panting' and states which parts of the ship is affected

Q.

describes what is meant by 'pounding' or 'slamming' and states which part of the ship is affected

Q.

describes what is meant by 'hogging' and by 'sagging' and distinguishes between them

Q.

describes the loading conditions which give rise to hogging and sagging stresses

1. Describe the circumstances which cause panting and pounding stresses. i.

Panting stresses is an in and out motion of the plating in the bows of a ship and is caused by unequal water pressure as the bow es through successive waves.

ii.

Pounding stresses is exist when ships is pitching. Ship’s bows lift clear of the water and come down heavily. It causes damage to the bottom and girder at the bow.

2. Sketch a transverse section through the forward part of a large cargo vessel, showing the structural arrangements which resist the stresses. i.

Panting stress ¾

Tiers of panting beams are fitted forward of the collision bulkhead below the lowest deck. These are similar to deck beam and are connected to frames by beam knees, but are only fitted at alternative frames. Tiers of beams are spaced 2 meters apart vertically and ed by wash plates or pillars.

¾

Panting stringers, similar to deck stringers, are laid on each tier of beams.

¾

To stiffen the t between each beam and the inner edge of the stringer, the plate edge may be shaped or gussets fitted.

¾

At intermediate frame without beams, the stringer is by a beam knee of half its depth.

¾

At fore ends, the stringers are ed by flat plate called “Breasthooks”.

ii.

Pounding stress is resisted by strong cellular double bottom. For a large cargo vessel, longitudinally framed bottom is used. ¾

The outer bottom plating covering the flat of the bottom must be thickened.

¾

The connections of the shell and inner bottom girder-work are made stronger

¾

Plate floors are fitted at alternate frames

¾

Longitudinal are stronger than normal

¾

Side girders are no more than 2.1 meters apart.

compensating stress

Racking Beam knee Beams Bulkheads Decks Floors Frames Long' girders Pillars Shell plating

Heavy Water weight pressure

Local Stress

Hoging & Sagging

SF

BM

Drydocking

Pounding

Panting

SOME MORE EXAMINATION QUESTIONS i. Describe the circumstances which cause panting and pounding stresses. a. Panting stresses is an in and out motion of the plating in the bows of a ship and is caused by unequal water pressure as the bow es through successive waves. b. Pounding stresses is exist when ships is pitching. Ship’s bows lift clear of the water and come down heavily. It causes damage to the bottom and girder at the bow. ii. Sketch a transverse section through the forward part of a large cargo vessel, showing the structural arrangements which resist the stresses in (1). a. Panting stress i. Tiers of panting beams are fitted forward of the collision bulkhead below the lowest deck. These are similar to deck beam and are connected to frames by beam knees, but are only fitted at alternative frames. Tiers of beams are spaced 2 meters apart vertically and ed by wash plates or pillars. ii. Panting stringers, similar to deck stringers, are laid on each tier of beams. iii. To stiffen the t between each beam and the inner edge of the stringer, the plate edge may be shaped or gussets fitted.

iv. At intermediate frame without beams, the stringer is by a beam knee of half its depth. v. At fore ends, the stringers are ed by flat plate called “Breasthooks”. b. Pounding stress is resisted by strong cellular double bottom. For a large cargo vessel, longitudinally framed bottom is used. i. The outer bottom plating covering the flat of the bottom must be thickened. ii. The connections of the shell and inner bottom girderwork are made stronger iii. Plate floors are fitted at alternate frames iv. Longitudinal are stronger than normal v. Side girders are no more than 2.1 meters apart. iii.

Sketch a longitudinal section of a bulk carrier, showing and naming all the main compartments. // Explain the reasons for this arrangement of compartments. a. Large, clear holds without tween deck – to load and discharge cargo quickly b. Large hatches with steel covers for safety c. Engine place aft d. Topside tank – enable water ballast to be carried high up to reduce GM e. Sloping side tanks at the bilge – assist in handling bulk cargo since it helps the self-trimming of cargo

iv.

List the structural of a ship which are designed to resist the main longitudinal stresses in a ships hull. // State

briefly how structural continuity is maintained in these to enable them to perform their designed function. a. Longitudinal stress: hogging, sagging b. Longitudinal work in the double bottom: 1. Deck stringer and sheer-strake thicken 2. Deck girder and longitudinal bulkhead 3. Special steel for sheer-strake and bilge strake 4. Longitudinal frames and beams in the bottom and under the strengthen deck 5. Stress is greatest amidships, so strengths of the parts is made greater amidships c. Hull is strengthen at about the half-depth of the ship to resist the shearing stress v.

It is useless to make one part very strong if an adjacent part which has to resist the same stress is weak. Hence, it is important to maintain structural continuity

vi.

When material has to be cut away, compensations must be made to preserve continuity of strength. Square corners should be avoided as far as possible since it has been found that these are always a source of weakness.

vii.

Parts which are very strong compared to the neighboring parts should not be ended suddenly, as there would be a tendency for them to tear away where they end. They should be gradually tapered off

viii.

and merge into the weaker parts.

List the functions of : bilge wells, stern tubes. a. Stern tube: to the shaft and to make a watertight t where the shaft enters the hull.

i. Steel tube. The fore end with flange bolted to after peak bulkhead and a large nut in aft end ii. Inside tube, a brass bush which has grooves in it iii. Strips of lignum vitae in grooves act as bearing for shaft iv. Studding box to prevent water getting into hull b. Bilge well: when the cellular double bottom extends out to the ship’s side there are no proper bilges. In this case the holds drain into bilge well, which are sunken compartments in the double bottom. It is to collect water from rain, cargo sweat, ship sweat, refrigerated container water, etc

ix.

Sketch a cross section of a rudder carrier.

x. Sketch a transverse midship section of a general cargo ship constructed on a combined framing system. List the functions of longitudinal framing. xi. Functions of longitudinal framing: Resist hogging and sagging, water pressure, pounding, dry-docking and shear stresses. xii. Draw a sketch showing the between a steel hatch cover and the hatch coaming indicating how watertightness is achieved. a. The lower rollers are mounted on an eccentric bush which enables them to be raised or lowered. This enables the hatch covers to be raised for rolling and stowage, or lowered so that they can be secured and made watertight.

b. The hatches are made watertight by rubber ting, being pull down by cleats and cross-t wedges.

xiii. Sketch a longitudinal section of a VLCC showing and naming all the

main

compartments.

Explain

the

reasons

for

this

arrangement of compartments. xiv. List the functions of floors and double bottoms in a general cargo ship. Floor: resist water pressure, dry-docking stresses, heavy weights, local stresses, racking, vibration and pounding. Double bottom: resist pounding, etc

xv. Draw

a

sketch

showing

the

layout

of

the

bilge

piping

arrangement in a general cargo ship with the engine room amidships. // Explain the reason for separate pipelines to each bilge. a. Not through deep tank, double bottom tanks and oil fuel bunkers b. Bilge pipes are fitted with non-return valve to avoid hold flooding c. Suction are usually placed at the aft end of each hold since ship normally trim by the stern and water can be collected at the aft end of the bilges d. Separate pipelines to each bilge since:

xvi. Sketch a transverse section through a cargo vessel showing and naming the structural which resist: Racking stresses and Water pressure. a. Racking – Resisted by tank side brackets and beam knees. Also, transverse bulkhead, web frames or cantilever frames, floor, shell plating and pillar. (Ship racked by wave action or rolling. Stresses come on the corners.) b. Water pressure – Resisted by bulkheads and by frames and floors, also beam, deck, longitudinal girder, pillar and shell plating (water pressure push-in the side and bottom of ship) xvii.

List and briefly describe the main drawings and plans available on board ship.

xviii.

Draw sketches showing the transverse stresses which can be exerted on a ship’s hull and name the structural which resist these stresses.

xix.

Sketch and list out the various part of a general dry container. Describe the various types of container, their sizes, and their usage. State the precautions that should be observed on Container Stowage before and after operations in a cellular container ship.

xx.

List the function of : longitudinal bulkheads and longitudinal framing

xxi.

Sketch a transverse section of a double bottom tank in way of a bracket floor, naming the main structural .

xxii.

Sketch a transverse midship section of a general cargo ship constructed on a combined framing system. (OR) List the three basic types of ship construction. a. Transverse system – closely spaced transverse frames to hold the planks together so that the seams could be caulked. It provides considerable transverse strength to resist the racking stress. Mostly for small ship and sailing ships b. Longitudinal system – has longitudinal frame at the bottom, sides and decks, ed by widely spaced transverse web. Strong longitudinal strength resists hogging and sagging stresses for long ships. c. Combination system – longitudinal frames in the bottom and strength deck, transverse frames on the ship side where longitudinal stresses are smaller. Plate floor and transverse beams are fitted at intervals to give transverse strength.

Q.

Sketch a transverse section of a double bottom tank in way of

a plate floor. Q.

Sketch a transverse section of a double bottom tank in way of

a bracket floor. Q.

Sketch a transverse section of a double bottom tank in way of

a solid water tight floor. Q.

Sketch a transverse section of a double bottom tank in way of

a solid not-tight floor.

SHEAR FORCE AND BENDING MOMENTS:

When a section such as a beam is carrying a load there is a tendency for some parts to be pushed upwards and for other parts to move downwards, this tendency is termed Shearing. The Shear force at a point or station is the vertical force at that point. The shear force at a station may also defined as being the total load on either the left hand side or the right hand side of the station; load being defined as the difference between the down and the upward forces, or for a ship the weight would be the downward force and the buoyancy would be the upward thrust or force. The longitudinal stresses imposed by the weight and buoyancy distribution may give rise to longitudinal shearing stresses. The maximum shearing stress occurs at the neutral axis and a minimum at the deck and keel. Vertical shearing stresses may also occur.

Bending Moment The beam, which we have been considering, would also have a tendency to bend and the bending moment measures this tendency. Its size depends upon the amount of the load as well as how the load is placed together with the method of . Bending moments are calculated in the same way as ordinary moments that is multiplying force by distance, and so they are expressed in weight – length units. As with the calculation of shear force the bending moment at a station is obtained by considering moments either to the left or to the right of the station. Hogging and sagging Hogging – When a beam is loaded or other wise is subjected to external forces such that the beam bends with the ends curving downwards it is termed as hogging stress. For a ship improper loading as well as in a seaway when riding the crest of a wave the uned ends of the ship would have a tendency similar to the beam above.

Sagging – In this case the beam is loaded or other wise subjected to external forces making the beam bend in such a way that the ends curve upwards, this is termed as sagging. Similar with a ship if improper loaded or when riding the trough of a wave – with crests at both ends then the ship is termed to be sagging.

For Hogging the ship ends to curve downwards would mean that the weight/ load amidships is much less than at the end holds/ tanks. For Sagging the ship would have been loaded in such a manner that a greater percentage of the load is around the midship area. In a seaway the hogging and the sagging stresses are amplified when riding the crests and falling into the troughs. Thus especially for large ships there are two conditions in the stability software – Sea Condition and Harbour condition.

A ship loaded while set in the harbour condition may allow loading with hogging/ sagging stresses reaching a high level, when this state of loading is transferred to a Sea condition in the software the results would be catastrophic since now the wave motions have also been incorporated. Thus planning a loading should always be in the Sea Condition.

Discharging in port may be planned in the Harbour Condition. Hogging and sagging cause compressive and tensile stresses on the ship beam – notably on the deck and the keel structure. Water pressure and Thrust

Pressure is force per unit area and water pressure is dependent on the head of the water column affecting the point of the measurement of the pressure. Let us assume an area of 1sq.m. then this area of water up to a depth of 1 m below the surface would have a volume of 1sq.m. x 1m = 1cbm and the weight of this volume would be 1cbm x density of the water = 1MT (assuming that it is FW) or 1000kgf, therefore the pressure exerted by this mass would be 1000kgf/sq.m. Similarly if now the depth of measurement is increased to 3m then the volume of this area subtending up to the 3m mark would be 1sq.m x 3 = 3cbm and the weight of the water would be 3MT or 3000kgf and the pressure exerted would be 3000kgf/sq.m. If now the liquid had not been FW but any other then the weight would be found by multiplying the volume by the density of the liquid. And thus the pressure exerted would be found. If we now increase the area of the square of water plane would it make a difference in the pressure? Let us consider a area of 2000sq.m then the volume of this water at a depth of 1 m would be 2000cbm and the weight would be 2000MT (consider FW) and the pressure exerted would be 2000,000kgf/ 2000sq.m which would give us again 1000kgf/sqm, thus the pressure is independent of the area of the water plane. Thrust however is different, thrust is taken to be the total weight of the liquid over an area. Thus for the previous example the thrust would be 2000 tonnes.

Thus the thrust is given by: the area of the water plane x pressure head x density of the liquid. Thrust always acts at right angles to the immersed surface and for any depth the thrust in any of the directions is the same. The pressure head which is used in the above calculation of thrust is the depth of the geometrical centre of the area below the surface of the liquid. For a ship the thrust on the ship side changes as the depth increases, however the bottom is affected uniformly for a set depth. Centre of pressure of an area is the point on the area where the thrust could be considered to act. It is taken that the centre of pressure is at 2/3rds the depth below the surface for ordinary vertical bulkheads and at half the depth in the case of collision bulkheads.