The document discusses ship construction and design. It describes the process of designing a ship including determining dimensions and purposes. It then explains how a ship is constructed through building units that are welded together and outfitted. The document also covers principles of ship strength, loads on the hull, and primary, secondary and tertiary structural analysis of bending in the hull.
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Nmlc ef4 module1 day01 ships stresses
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Function 4:
Controlling the operation of
the ship and care for
persons onboard at the
management level.
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PRINCIPLES OF SHIP CONSTRUCTION
INCLUDING THEORIES AND FACTORS
AFFECTING TRIM AND STABILITY
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In order to begin the design of a ship a naval architect
must meet with the owners of the planned vessel and
establish exactly what it is the owner wants his vessels
to do. The naval architect is responsible for determining
the size (length, breadth, and depth), shape (hull form) ,
power requirements, and general arrangement of decks
and compartments, and general arrangement of decks
and compartments. To do this he must have a very clear
idea of what its is the owner wishes to with his vessel. He
will then produce concept designs based on the owner’s
needs, ideas from similar ship types that have already
been built and the incorporation of new technology which
might make for a better ship.
Ship Construction
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Once the owner has selected a basic design he thinks
best suits his needs, work starts on refining the basic
design, estimating vessel costs and planning for the
production of the ship. Naval architects refine the hull
design and general arrangement while marine engineers,
marine systems designer and production engineers work
to design the systems which will turn the naval architects’
hull into an operating ship. Engines are selected,
propulsion systems designed, fuel, oil, water, electric
power production systems, heating, ventilation, air
conditioning, cargo handling, anchoring and mooring
systems all must be designed or purchased to suit the
vessel and its purpose.
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Once a contract has been awarded to a shipyard to build
the new ship, people in the design office (the naval
architects and marine system designers) work to prepare
more detailed production drawings. These drawings are
used by the production department to plan how they will
employ the hundreds of people working to turn those
drawings into reality. The purchasing department begins
"sourcing" equipment required for the ship, purchasing all
the materials and equipment needed to fulfill the design
requirements. Others work on financing arrangements to
pay for the construction of the ship, and all the usual
activities required to operate any organization the size of
a shipyard.
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The actual construction process consists of steel
fabrication and outfitting. Steel must be cut in various
shapes which will be welded together to form the hull,
bulkheads, and decks of the ship. This is done by
computer controlled cutting machines working from data
produced at the naval architect's computer. Outfitting
means the installation of all the pumps, piping, heat
exchangers, motors, engines, generators, cabling,
machinery and bridge control equipment, insulation, and
everything else that goes into a ship.
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The ship will be built in "units,” or blocks of the ship that will be built
independently and then welded together to form the final product. By
building the ship this way workers will have easier access to the
interior of the ship, welding can be more easily carried out, the
installation of equipment is simpler, in other words each unit can be
"pre-outfitted" more quickly and with less effort than if the hull was
completed first. This makes the ship quicker and less costly to produce,
a very important consideration in the competitive shipbuilding industry.
The units are welded together to form sub-assemblies which are then
lowered into a dry dock and welded to other subassemblies until the
ship is complete. The size of the sub-assemblies is usually only limited
by the capacity of the equipment used to transport them to the dock
and lower them into place. Parts of the ship may even be built at other
shipyards and floated on a barge to the lead shipyard for assembly.
Once all the units are together the ship is "floated up" in the dock and
tugs will move it to an outfitting pier where the all the remaining work
is finished. Hundreds of people are involved in the building of a ship;
pipe fitters, machinists, electricians, welders, joiners, draughts men,
sheet metal workers, riggers, painters and others virtually swarm the
dockyard when construction is underway. Companies overseas may
send people to install and "set-to-work" equipment the shipyard has
purchased for the ship. Supervisors and quality assurance technicians
are kept busy ensuring work proceeds according to the designs
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The main dimensions decide many of the ship's
characteristics, like stability, power requirements, hold
capacity, sea keeping, and even economic efficiency. The
length (L), breadth (B), depth (D), draught (T), freeboard (F), and
block coefficient (CB) are the main dimensions, and are
decided first during ship design. Optimization is used to
achieve the best possible combination.
Main Dimensions and their Effects in Ship Design
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Increasing length of a conventional ship (while retaining
volume and fullness) increases the hull steel weight and
decreases the required power.
Length is often restricted by the slipway, building docks,
locks or harbours.
Length on the Waterline (LWL) is the length from the
forward most point of the waterline measured in profile to
the stern-most point of the waterline.
Ship's length (L)
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When varying B at the design stage, T and D are generally
varied in inverse ration to B. Increasing B in a proposed design,
while keeping the midship section area constant have the
following effects (ex: BWL is the maximum beam at the
waterline)
(a) Increased resistance and higher power requirements.
(b) Small draught restricts the maximum propeller dimensions.
This usually means lower propulsive efficiency (except in case
where a smaller propeller diameter is chose for other reasons).
(c) Increased scantlings in the bottom and deck result in
greater steel weight (function of L/D).
(d) Greater initial stability: KM becomes greater.
(e) The righting arm curve has steeper slope (greater GM), but
may have decreased range.
(f) Smaller draught-convenient when draught restrictions exist.
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The depth D is used to determine the ship's volume and
the freeboard and is generally closely related to the
draught. The depth should be considered in the context of
longitudinal strength. If the depth is decreased, the upper
deck and bottom must be strengthened to maintain the
section modulus. In addition, the side-wall usually has to
be strengthened to enable proper transmission of the
shear forces.
Ship's depth (D)
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The draught is often restricted by insufficient water
depths, particularly for large ships like ULCCs.
The advantages of large draughts are (i) low resistance
and (ii) the possibility of installing a large propeller with
good clearances.
Ship's draught (T)
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The International Load Line Convention 1966 stipulates
the minimum freeboard requirements for ships. On most
occasions the freeboard is more than the minimum
requirement as in many cases the ship is cheaper to built
with more freeboard than the minimum required by the
rules.
Ship's freeboard (F)
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The block coefficient considerably affects resistance. For a
ship with constant displacement and speed the resistance
increases with CB.
If CB is decreased, B must be increased to maintain
stability. These changes have opposite effects on
resistance in waves, with that of CB dominating.
Ship's block coefficient (CB)
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The Strength of Ships is a topic of key interest to Naval
Architects and shipbuilders. Ships which are built too
strong are heavy, slow, and cost extra money to build
and operate since they weigh more. Ships which are built
too weakly suffer from minor hull damage and in some
extreme cases catastrophic failure and sinking.
Strength of Ships
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The hulls of ships are subjected 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
its hull.
The weight of the hull and of cargo and components
within the ship 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
force of propellers, water driven up against the bow.
When a ship is loaded with cargo, it may have many
times its own empty weight of cargo pushing down on the
structure.
Loads on Ship Hulls
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If the ships structure, equipment, and cargo are
distributed unevenly there may be large point loads into
the structure, and if they are distributed differently than
the distribution of buoyancy from displaced water then
there are bending forces on the hull. When ships are dry-
docked, and when they are being built, they are
supported on regularly spaced posts on their bottoms.
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Diagram of ship hull (1) Sagging and (2) Hogging under loads.
Bending is exaggerated for demonstration purposes.
The primary strength, loads, and bending of a ship's hull are
the loads that affect the whole hull, viewed from front to
back and top to bottom. Though this could be considered to
include overall transverse loads (from side to side within the
ship), generally it is applied to Longitudinal Loads (from
end to end) only. The hull, viewed as a single beam, can
bend
Primary Hull Loads, Strength, and Bending
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1. down in the center, known as sagging
2. up in the center, known as hogging.
This can be due to:
hull, machinery, and cargo loads
wave loads, with the worst cases of:
• sagging, due to a wave with length equal to the ship's
length, and peaks at the bow and stern and a trough
amidships
• hogging, due to a wave with length equal to the ship's
length, and a peak amidships (right at the middle of the
length)
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Primary hull bending loads are generally highest near the
middle of the ship, and usually very minor past halfway
to the bow or stern.
Primary strength calculations generally consider the
midships cross section of the ship. These calculations
treat the whole ships structure as a single beam, using
the simplified Euler-Bernoulli beam equation to
calculate the strength of the beam in longitudinal
bending. The moment of inertia (technically, Second
moment of area) of
the hull section is calculated, by finding the neutral or
central axis of the beam and then totaling up the quantity
for each section of plate or girder making up the hull, with
Iy being the moment of inertia of that section of material, b being the
area of the section, h being the height or distance of the center of that
section from the neutral axis.
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Primary (1), Secondary (2), and Tertiary (3) structural
analysis of a ship hull. Depicted internal components
include a watertight bulkhead (4) at the primary and
secondary level, the ship's hull bottom structure including
keel, keelsons, and transverse frames between two
bulkheads (5) at the secondary level, and transverse
frames (6), longitudinal stiffeners (7), and the hull plating
(8) at the tertiary level.
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Primary strength loads calculations usually total up the
ships weight and buoyancy along the hull, dividing the
hull into manageable lengthwise sections such as one
compartment, arbitrary ten foot segments, or some such
manageable subdivision. For each loading condition, the
displaced water weight or buoyancy is calculated for that
hull section based on the displaced volume of water
within that hull section. The weight of the hull is similarly
calculated for that length, and the weight of equipment
and systems. Cargo weight is then added in to that
section depending on the loading conditions being
checked.
The total still water bending moment is then calculated by
integrating the difference between buoyancy and total
weight along the length of the ship.
For a ship in motion, additional bending moment is added
to that value to account for waves it may encounter.
Standard formulas for wave height and length are used,
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For a ship in motion, additional bending moment is added
to that value to account for waves it may encounter.
Standard formulas for wave height and length are used,
which take ship size into account. The worst possible
waves are, as noted above, where either a wave crest or
trough is located exactly amidships.
Those total bending loads, including still water bending
moment and wave loads, are the forces that the overall
hull primary beam has to be capable of withstanding.
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The secondary hull loads, bending, and strength are those
loads that happen to the skin structure of the ship (sides,
bottom, and deck) between major lengthwise subdivisions or
bulkheads. For these loads, we are interested in how this
shorter section behaves as an integrated beam, under the
local forces of displaced water pushing back on the hull,
cargo and hull and machinery weights, etc. Unlike primary
loads, secondary loads are treated as applying to a complex
composite panel, supported at the sides, rather than as a
simple beam.
Secondary loads, strength, and bending are calculated
similarly to primary loads: you determine the point and
distributed loads due to displacement and weight, and
determine local total forces on each unit area of the panel.
Those loads then cause the composite panel to deform,
usually bending inwards between bulkheads as most loads
are compressive and directed inwards. Stress in the
structure is calculated from the loads and bending.
Secondary Hull Loads, Strength, and
Bending
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Tertiary strength and loads are the forces, strength, and
bending response of individual sections of hull plate
between stiffeners, and the behaviour of individual
stiffener sections. Usually the tertiary loading is simpler
to calculate: for most sections, there is a simple,
maximum hydrostatic load or hydrostatic plus slamming
load to calculate. The plate is supported against those
loads at its edges by stiffeners and beams. The deflection
of the plate (or stiffener), and additional stresses, are
simply calculated from those loads and the theory of
plates and shells.
Tertiary Hull Loads, Strength, and Bending
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Ship hull structure elements
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This diagram shows the key structural elements of a
ship's main hull (excluding the bow, stern, and
deckhouse).
Deck structure
Transverse bulkhead
Inner bottom shell plating
Hull bottom shell plating
Transverse frame (1 of 2)
Keel frame
Keelson (1 of 4)
Longitudinal stiffener (1 of 18)
Hull side beam
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The depicted hull is a sample small double bottom (but
not double hull) oil tanker.
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The total load on a particular section of a ship's hull is the
sum total of all primary, secondary, and tertiary loads
imposed on it from all factors. The typical test case for
quick calculations is the middle of a hull bottom plate
section between stiffeners, close to or at the midsection
of the ship, somewhere midways between the keel and
the side of the ship.
Total Loads, Bending, and Strength
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Explain with the aid of neat labeled sketches
how hogging,sagging,racking and torsion
affect the ship’s structure.
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1. Hogging due to waves
If the wave crest is considered at mid-ships then the
buoyancy in this region will be increased. With the
wave trough positioned at the ends of the ship, the
buoyancy here will be reduced. This loading condition
will result in a significantly increased bending moment,
which will cause the ship to hog. This will be an
extreme condition giving the maximum bending
moment that can occur in the ship’s structure.
HOGGING
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2. HOGGING DUE TO DISCONTINUITY IN LOADING
Consider a ship loaded with the weights concentrated at
the bow and the stern, which tends to droop. This leads
to hogging of the ship hull.
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1. Sagging due to waves
In a heavy seaway, a ship may be supported at the ends by the
crests of waves while the middle remains unsupported. If the wave
trough is now considered at midships then the buoyancy in this
region will be reduced. With the wave crest positioned at the ends
of the ship, the buoyancy here will be increased. This loading
condition will result in a bending moment which will cause the ship
to sag.
SAGGING
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2. Sagging due to discontinuity in loading
Consider heavy weights concentrated at the midships of
a ship. The middle hull part tends to droop more than the
ends. This causes sagging of ship hull.
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When a ship rolls in a seaway, it results in forces in the structure
tending to distort it transversely and may cause deformation at the
corners. The deck tends to move laterally relative to the bottom
structure, and the shell on one side to move vertically relative to
the other side. This type of deformation is referred to as “racking”.
RACKING
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When any body is subjected to a twisting moment,
which is commonly referred to as torque, that body is
said to be in ‘torsion’. A ship heading obliquely to a
wave will be subjected to righting moments of opposite
direction at its ends twisting the hull and putting it in
‘torsion’. In most ships, torsional moments and stresses
are negligible but in ships with extremely wide and long
deck openings they are significant.
TORSION
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Explain with aid of neat labeled
sketches stresses due to water
pressure, dry-docking, localized
loading, discontinuity in loading.
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Stresses due to water pressure
Water pressure increases with depth and tends to set in
the ship’s plating below the water line. A transverse
section of a ship is subjected to a static pressure from
the surrounding water in addition to the loading resulting
from the weight of the structure, cargo, etc. Although
transverse stresses are of lesser magnitude than
longitudinal stresses, considerable distortion of the
structure could occur, in absence of adequate stiffening.
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Stresses due to water pressure
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Stresses due to dry-docking
Dry- docking tends to set the keel upwards because of
the up-thrust of the keel blocks. There is a tendency for
the ship’s sides to bulge outwards and for the bilges to
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Stresses due to localized loading
Heavy weights, such as equipment in the machinery spaces are
particular items of general cargo, can give rise to stresses due to
localized distortion of the transverse section. The fitting of
transverse bulkheads, deep plate floors and web frames reduce
such stresses.
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Stresses due to discontinuity
A stress concentration is a localized area in a structure at which
the stress is significantly higher than in the surrounding
material.
■ Two types of discontinuity in ships are
■ Built into ship unintentionally by the methods of construction
e.g. rolling, welding, casting.
■ Introduced into structural design deliberately for reasons of
architecture, use, access, e.g. superstructures, deckhouses,
hatch openings, door openings.
Examples
1.If the ends of the superstructures are ended abruptly, there is
a major discontinuity of the ships structure, which may give rise
to localized stresses resulting in cracking of the plating.
2.Holes cut in the deck plating create areas of high local stress.
due to discontinuity created by the opening.
3.The high stresses at the corner of the hatch may result in
cracking.