1. The document provides information on ship construction, including definitions of key ship design terminology and descriptions of basic ship types and designs. 2. It discusses the three main stages of initial ship design - concept, preliminary, and contract design. Key ship dimensions and specifications that are determined at each stage are outlined. 3. Ship types covered include liquid cargo ships, dry cargo ships, passenger ships, offshore vessels, fishing vessels, and naval vessels. The evolution of cargo ship designs over time is summarized.

1. 1
SHIP
CONSTRUCTION
By Jayan Pillai, MSc CEng FIMarEST MRINA MIBM
Ship Inspection & Technical Consultancy

2. 2
1.0 Ship
A ship is a vessel of considerable size for deep water navigation.
The Historic Ships Committee have designated a vessel below 40 tons and 40 feet in length as a boat.
Submarines and Fishing Vessels are always known as boats, whatever their size.
The Statutory Definition of a ship i the UK Me ha t “hippi g A t of 99 , is p o ided i s. “hip
i ludes e e des iptio of essel used i Na igatio .
2.0 Basic Design of a Ship:
The main requirement of a new ship is that it can trade profitably. The owner requires a ship that will
give the best possible returns for the initial investment and running costs. With LNG Ships, the vessel
could work the same route for its working life of 40 years. With the aid of computers it is possible to
make a study of a large number of design parameters, and hence design of a ship that is technically
feasible and economically efficient. The design should take into consideration first cost, operating cost
and future maintenance.
The initial design of a ship goes through 3 stages: concept; preliminary and contract design. A concept
design should, from the objectives, provide sufficient information for a basic techno-economic
assessment of the alternatives to be made. Preliminary design refines and analyses the agreed concept
design, fills out the arrangements and structure, and aims to optimize service performance. At this stage
the builder should have sufficient information to tender. Contract design details the final arrangements
and systems agreed with the owner and satisfies the building contract conditions.
Post-contract design requires confirmation that the ship will meet all operational requirements,
including safety requirements from regulators. It addresses design for production, where the structure,
outfit, and systems are planned in detail to achieve a cost and time effective building cycle. The post
contract design will also ideally consider the future maintainability of the ship in the arrangements of
equipment and services.
Information provided by design:
- Dimensions
- Displacement
- Stability
- Propulsive characteristics and hull form
- Preliminary General Arrangements
- Principal Structural details
Each item of information may be considered in more detail.
The dimensions of most ships are mainly influenced by the cargo carrying capacity of the ship and the
draft. Increase in length produces higher longitudinal bending stresses. Breadth may be such as to
provide adequate transverse stability. A minimum depth is controlled by the draft plus statutory
freeboard. Increase in depth is preferred to increase in length. Draft is limited by area of operation.
Displacement is made up of lightweight plus deadweight. Lightweight is the weight of the ship as built.
Deadweight is the difference between the lightweight and loaded displacement ( weight of cargo plus
weight of fuel, stores, water ballast, fresh water, crew and baggage).
In determining the dimensions, static stability is kept in mind, in order to ensure that this is sufficient in
all possible conditions of loading. Beam and depth are the main influences. Statutory freeboard and
shee a e i po ta t togethe ith the eight dist i utio i a a gi g the essel’s la out.

3. 3
Adequate propulsive performance will ensure that the vessel attains the required speeds. The hull
form is such that it offers a minimum resistance to motion.
Service Speed is the average speed at sea with normal service power and loading, under average
weather conditions. Trial Speed is the average speed obtained using the maximum power over a
measured course in calm weather with a clean hull and specified load condition. This speed may be a
knot or so more than the service speed. Unless a hull form similar to that of a known performance
vessel is used, a computer generated hull form and its predicted propulsive performance can be
determined. Propulsive performance can be confirmed by subsequent tank testing of a model hull. This
may lead to further beneficial modifications. The owner may specify their choice of propulsion plant.
The general arrangement is prepared in cooperation with the owner.
All LNG vessels are built to the requirements of a classification society. (Lloyds Register, American
Bureau of Shipping, Bureau Veritas, Det Norske Veritas, RINA). Class have rules on structural scantlings.
Computer programs can determine the minimum hull structural scantlings. Owners may specify
thicknesses and materials in excess of IMO and Class requirements.
2.1. Ship Contracts
Ship Owners may employ a firm of consultants to provide the preliminary design, prepare the tender
specifications, evaluate tenders and oversee the construction on their behalf.
The successful shipbuilder will prepare building specifications for approval by the owner. The technical
specification will normally include:-
1. Brief description and essential qualities and characteristics of the ship.
2. Principal dimensions
3. Deadweight, cargo and tank capacities, etc.
4. Speed and power requirements.
5. Stability requirements.
6. Quality and standard of workmanship.
7. Survey and certificates
8. Accommodation details
9. Trial conditions
10. Equipment and fittings
11. Machinery Details, including electrical installation.
Typical Payment Schedule
- 10% on signing contract
- 10% on arrival of materials on site
- 10% on keel laying
- 20% on launching
- 50% on delivery.

4. 4
2.2 Terminology
Aft Perpendicular (AP): A perpendicular drawn to the waterline at the point where the after side of
the rudder post meets the summer load line. Where no rudder post is fitted it is taken as the centre line
of the rudder stock.
Forward Perpendicular (FP): A perpendicular drawn to the waterline at the point where the fore-side
of the stem meets the summer load line.
Length Between Perpendiculars (LBP): The length between the forward and aft perpendiculars
measured along the summer load line.
Amidships: A point midway between the after and forward perpendiculars.
Length Overall (LOA): Length of vessel taken over all extremities.
Lloyd’s Length : Used for obtaining scantlings if the essel is lassed ith Llo d’s Registe . It is the sa e
as length between perpendiculars except that it must not be less than 96% and need not be more than
97% of the extreme length on the summer load line.
Register Length: Length of ship measured from the fore side of the head of the stem to the aft side of
the head of the stern post. In the case of a ship not having a stern post, to the fore side of the rudder
stock. If the ship does not have a stern post or a rudder stock, the aft terminal is taken to the aftermost
part of the transom or stern of the ship. This length is the official length in the register of ships
maintained by the flag state.
IMO Length: is defined as 96% of the total length on a waterline at 85% of the least molded depth
measured from the top of keel
Molded dimensions are taken to the inside of plating on a metal ship.
Base Line: A horizontal line drawn at the top of the keel plate. All vertical molded dimensions are
measured relative to this line.
Molded Beam: Measured at the midship section. This is the maximum molded breadth of the ship.
Moulded Draft: Measured from the base line to the summer load line at the midship –section.
Moulded Depth: Measured from the base line to the heel of the upper deck beam at the ship’s side
amidships.
Extreme Beam: The maximum beam taken over all extremities.
Extreme Draft: Taken from the lowest point of keel to the summer load line. Draft marks represent
extreme drafts.
Extreme Depth: Depth of essel at ship’s side f om upper deck to lowest point of keel.
Half Breadth: A ship’s hull is s et i al a out the lo gitudi al e t e li e , so half the ea o half
breadth in any section if given.
Freeboard: The e ti al dista e easu ed at the ship’s side et ee the su er load line and the
freeboard deck.
Freeboard Deck: Normally the uppermost complete deck exposed to weather and sea, and has a
pe a e t ea s of losi g all ope i gs, a d elo hi h all ope i gs i the ship’s side ha e
watertight closings.
Sheer: A rise in the height of the deck (curvature or in a straight line) in the longitudinal direction.
Measured as the height of deck at side at any point above the height of deck at side amidships.
Camber (Round of beam): Curvature of decks in the transverse direction. Measured as the height of
deck at centre above the height of deck at side. Straight line camber is often used on large ships to
simplify construction.

5. 5
Rise of Floor (Deadrise): The rise of the bottom shell plating line above the base line. This rise is
measured at the line of moulded beam. Large ships often have no rise of floor.
Half Siding of Keel: The horizontal flat portion of the bottom shell measured to port or starboard of the
ship’s lo gitudi al e t e li e. Useful to k o he d do king.
Tumblehome: The inward curvature of the side shell above the summer load line. Not common on large
modern ships.
Flare: the outward curvature of the side shell above the waterline. At the fore end of the ship.
Stem Rake: Inclination of the stem line from the vertical.
Keel Rake: Inclination of the keel line from the horizontal.
Parallel Middle Body: The length over which the midship section remains constant in area and shape.
Entrance: The immersed body of the vessel forward of the parallel middle body.
Run: The immersed body of the vessel aft of the parallel middle body.
Gross Tonnage: is a measure of the enclosed internal volume of the vessel (originally computed as 100
cubic feet per ton).
Suezmax: the largest tanker than can transit the current Suez Canal fully laden (150 000 dwt). Suez
maximum breadth & draft limits are 75 and 20 metres.
Aframax: (American Freight Rate Association) 80 000 to 115 000 dwt
Panamax: Vessel with beam & length restrictions of 32.2 and 275 metres. Upto 70 000 dwt
Handymax: Tankers of 35 000 to 45 000 dwt
Capesize: Ships too large to transit the Panama Canal, so have to voyage around Cape Horn.

6. 6
2.3 Ship Types
Liquid Cargo Ships: Oil Tankers, Liquefied Gas Carriers , Chemical Tankers
Dry Cargo Ships: Tramps, Bulk Carriers, Cargo Liners, Container Vessels, Barge Carriers, Ro-Ro
Ships, Refrigerated Cargo Ships, Timber Carriers, Livestock Carriers, Car Carriers
Passenger Ships: Liners, Cruise Ships, Emigrant & Pilg i “hips “TP’s , C oss-channel Ferries,
Coastal Ferries, Harbour Ferries, Passenger Submarines
High Speed Craft: Multi-hulls including Wave Piercers, Small Waterplane Area Twin-hull (SWATH),
Surface Effect Ship (SES) & Hovercraft, Hydrofoil, Wing in ground effect craft (WIG)
Off Shore Vessels: Supply Ships, Pipe Layers, Crane Barges, Semi-Submersible Drill Rigs, Drill Ships,
Accommodation Barges, Production Platforms, Floating Storage Unit (FSU), Floating Production &
Storage unit (FPSO),
Fishing Vessels: Factory Ships, Trawlers, Purse Seiners
Harbour/ Ocean Work Craft: Tugs, Cable Layers, Dredgers, Salvage/buoy Vessels, Tenders, Pilot
Craft, Floating Dry Docks, Floating Cranes, Lightships, Wind Farm Support Vessels
Submersibles:
Warships: Air Craft Carriers, Helicopter Carriers, Destroyers, Frigates, Petrol Boats, Nuclear Submarines,
Submarines
Development of Cargo Ships:
1. Floating Logs
2. Canoes carved from Tree Trunks

7. 7
3. Wooden Sailing Ships
4. Metal Hulled Ships
5. Flush Deck Ship
6. Three Island Ship type
7. Combined Poop & Bridge
8. Raised Quarter Deck
9. Awning or Spar Deck
10. Open Shelter Deck
11. All Aft Cargo Ship
12. Oil Tankers
13. Chemical Tankers
14. Liquefied Gas Carriers
HULL FORM:
Between 1940 and 1970 there was a steady increase in the speed of the dry cargo ship. A much finer
hull is apparent in modern ships. Bulbous bow forms and open water sterns are used to advantage, and
considerable flare may be seen in the bows of container ships to reduce wetness on deck.
Oil Tankers
Until 1990, the form of vessels specifically designed for the carriage of oil cargoes has not undergone a
great deal of change since 1880. The growth in size from 1880 to 1945 was gradual, from 1 500 dwt to
about 12 000 dwt, reached 20 000 in 1953 and 30 000 in 1959. By 1979 the largest ULCCs reached 564
763 dwt, 458.45 metres in length, 70.06 metres beam and 28.50 draft. The Seawise Giant ( Jahre
Viking ) was the longest and had the greatest cargo carrying capacity, the Batilus had the deepest draft,
and the Sea World had the widest beam.
ULCC Jahre Viking (ex-Seawise Giant) DWT 564 763 Length 458.45 Beam 68.8 m Draft 22.60 m
Service speeds of tankers has increased since the 1940s from 12 knots to 17 knots and above. The
service speed is related to the optimum economic operation of the tanker. Optimum size of the tanker is
related to current market economics. The tanker fleet has grown enormously to meet expanding
demand for oil.

8. 8
Structurally, one of the greatest developments has been in the use of welding, and oil tankers were
amongst the first vessels to utilize the application of welding. Riveting is very labour intensive, and
maintaining oil tight joints more difficult. Welding allows cheaper fabrication methods in ship building.
The current trend is placing the machinery and propulsion plant aft. Most commercial ships have their
accommodation and bridge aft. After the tanker Exxon Valdez ran aground in 1989 in the Prince
William Sound in Alaska, double hull oil tankers were being built.
Oil tankers now generally have a single pump space aft, and just forward of the machinery space, and
specified slop tanks into which tank washings and oily residues are pumped. Tank cleaning may be
accomplished by water driven rotating machines on smaller tankers, but for crude oil tankers of 20 000
dwt and above, the tank cleaning system uses crude oil washing.
ULCC Al Rekkah
DWT 414 366 Length 366m Beam 70.06 m Draft 22.60 m
LNG Carriers
In 1959, the Methane Pioneer , a 5 034 dwt vessel was the first to carry a large cargo of LNG. Today the
largest LNG vessel can carry 266 000 cubic metres.

9. 9
Liquefied Natural Gas (LNG)
LNG is made from natural gas, which is clean burning source of heat energy with many applications,
including as fuel for power generation, industrial and home heating, and as a chemical feedstock.
Natural Gas is composed primarily of methane (typically 85 to 90%), but it also may contain ethane,
propane and heavier hydrocarbons (butane, pentane, hexane, etc). Small quantities of nitrogen,
oxygen, carbon dioxide and sulphur compounds are also found in most sources of natural gas.
Natural gas is transported by pipeline to its consumers, but when the distance between source and
consumption is great ( >1500 km by sea or >5000 km over land) then liquefaction of the gas to reduce its
volume by a factor of 600 becomes economic. The gas in liquid form can then be carried economically
by ships equipped with well-insulated tanks made from special steel.
Liquefied natural gas is formed when natural gas is cooled by the refrigeration process to temperatures
of - 1630
C, at atmospheric pressure, and the gas condenses to a liquid. Before natural gas can be
liquefied in this way, the impurities, including carbon dioxide, sulphur compounds, heavier
hydrocarbons and water must be removed by various processes. If nitrogen is present in the natural gas
at high levels, it may be removed at the end of the process as it condenses at an even lower
temperature (-196 0
C) than pure methane.
The liquefaction process can be designed to purify the LNG to almost 100% methane, or leave in more
ethane and some LPGs (propane and butane) to match the pipeline gas specifications in the receiving
gas system or country. Most gas distribution systems specify limits in terms of the heating (calorific)
value of the gas to maintain safe conditions in combustion equipment. The specifications are quite
different in Japan (the largest LNG importer) and the USA, the major emerging LNG import market.
LNG is about 47% as dense as water and is odourless, colourless, non-corrosive and non-toxic. When
vaporized it burns only in concentrations of 5% to 15% when mixed with air. Neither LNG nor its vapour
can explode in an unconfined environment.
The LNG chain from field production to pipeline gas consumers across oceans is as follows:-
Field Gas Production - Condensate & LPG Recovery – Liquefaction & Storage as LNG – Shipping -
Receive, Storage & Re-gasification - pipeline - Natural Gas distribution
In French, Spanish, Portuguese, or Italian-speaking countries, the abbreviation GNL is used in place of
LNG.
How is LNG made?
LNG involves the purification, chilling and liquefaction of natural gas by various processes including
refrigeration using hydrocarbon refrigerants. The first step is removal of carbon dioxide and other acid
gases such as hydrogen sulphide by a recirculating amine process. This is a very common process in
natural gas treatment plants producing pipeline gas and in petroleum refining and petrochemical plants.
The carbon dioxide is normally vented to the atmosphere. If carbon dioxide is not removed, it would
freeze solid in the heat exchangers involved in the liquefaction process and cause blockages. Water
would do the same, so it is removed using molecular sieve driers as commonly used in natural gas
treatment plants producing pipeline gas, Traces of mercury sometimes occur in natural gas and this is a
potential problem for LNG Plant, as mercury corrodes aluminium which is used in some of the
equipment. Mercury removal facilities are usually incorporated into LNG plants as a result.
The chilling of the gas to moderately low temperatures causes at first the condensation of heavy
hydrocarbons, which might also freeze and cause blockages of equipment at lower temperatures. These

10. 10
o po e ts of the gas a e e o ed i a s u olu alo g ith so e of the LPG p opane and
butane) as the gas is cooled to about minus 35 degrees C.
Chilling and then liquefaction is accomplished by a refrigeration process powered by a large compressor
typically driven directly by a gas turbine or steam turbine. In principle, the refrigeration process is no
different from that in a domestic refrigerator or air conditioner, but the difference in scale is immense.
Cooling and condensation of the high pressure refrigerant gas is accomplished in air coolers or water
cooled heat exchangers. When the high pressure liquid refrigerant is depressured through an expansion
valve, the drop in temperature is used to extract heat energy from the process gas through a heat
exchanger. Usually there are at least two main compressors. Usually there are at least two main
compressors with multiple stage of heat exchangers for maximum efficiency in liquefying the methane
and ethane in the purified natural gas to make LNG.
How is it stored?
LNG is stored in shore tanks at both ends of its sea voyage to accumulate sufficient volumes for
economic shipping. The tanks are of double walled construction with extremely efficient insulation
between the walls. Large tanks tend to have a low aspect ratio (height to width) and are cylindrical in
design with a domed roof. Storage pressures in these tanks are very low, less than 5 psig. The outer
walls are made of reinforced concrete and are designed to safely contain the contents of the inner tank
in the extremely unlikely event of it developing a leak.
How is it kept cold?
The sho e ta k o ship ta k’s i sulatio , as effi ie t as it a e, ill ot keep the LNG old e ough to
e ai as a li uid itself. LNG is sto ed as a oili g oge , that is, it is a e old li uid at its
boiling point for the pressure at which it is being stored. Storage of LNG utilizes a phenomenon called
auto- ef ige atio , he e the LNG sta s at ea o sta t te pe atu e if kept at o sta t p essu e.
This constant temperature occurs as long as the LNG vapour boil off is allowed to leave the storage tank.
The vapour is either removed and used as fuel or re-liquefied and returned to the tank.
Is LNG safe?
It is important to remember that LNG is a form of energy and must be respected as such. Today LNG is
transported and stored as safely as any other liquid fuel. Before the storage of cryogenic liquids was
fully understood, there was a serious incident involving a LNG storage tank failure killing 128 people in
Cleveland, Ohio in 1944. This incident virtually stopped all development of the LNG industry for 20
years.
The race to the Moon led to a much better understanding of cryogenics and cryogenic storage with the
expanded use of liquid hydrogen (- 2520
C) and liquid Oxygen (- 1820
C). LNG technology grew from the
advancements developed by NASA for the space programme.
Today the LNG industry maintains an excellent safety record by incorporating many years of experience
and engineering solutions and safety codes into the design and operation of LNG liquefaction, storage
and re-gasification plants around the world.
Where does LNG come from?
LNG primarily comes from areas where large gas discoveries have been made, such as Algeria, Australia,
Brunei, Egypt, Equatorial Guinea, Indonesia, Libya, Malaysia, Nigeria, Norway, Oman, Qatar, Trinidad
and the United Arab Emirates. Some LNG is produced in the US (Alaska) and Europe. LNG can be
p odu ed he e e atu al gas is a aila le. H d auli f a tu i g o Fra king” is also used to produce
natural gas. Fracking is the process of propagation of fractures in a rock layer by a pressurized fluid.

11. 11
Materials and Strength of Ships
Basically a ship can be built of any material provided it can float on water and navigate safely from one
port to another. However the most common materials used are steel, aluminium, wood, GRP and FRP.
Steel is used for the hulls of all large ships, because it is strong, cheap and easily available. Steel alloys
are more expensive but have some advantages over mild steel. Mild steel is not suitable for LNG tanks.
At low temperatures mild steel becomes brittle and cracks.
Aluminium and its alloys are 10 times more expensive than mild steel, lighter and not as strong. It is
used for smaller craft and for the accommodation blocks of some passenger ships. Aluminium has a
lower melting point than steel. Because of their cryogenic properties, some LNG tanks are made of
aluminium.
Glass Reinforced Plastic and Fibre Reinforced Plastic is used for smaller hulls. They are flammable and
lose their strength properties above ambient temperature.
Wood was the traditional material for building ships until the 20th
century. Some small boats and Fishing
Vessels are still made of Wood. Wood boat building is an ancient art and does require a lot of training
and skill.
Steel
The production of all steels used for shipbuilding purposes starts with the smelting of iron ore and the
making of pig-iron. The iron ore is smelted in a blast furnace, which is a large, slightly conical structure
lines with a refractory material. Coke is used to provide the heat for smelting, and limestone is also
added. This makes the slag formed by the incombustible impurities in the iron ore fluid, so that it can be
drawn off. Air for combustion is blown in through a ring of holes near the bottom., and coke, ore and
limestone are charged into the top of the furnace in rotation. Molten metal is drawn off at intervals
from a hole or spout at the bottom of the furnace, and run into molds formed in a bed of sand or into
metal molds. The resultant pig-iron contains 92-97% iron, the remainder being carbon, silicon,
manganese, sulphur and phosphorus. In the subsequent manufacture of steels the pig-iron is refined
and the impurities are reduced.

12. 12
Steels are alloys of iron and carbon. The carbon percentage varying from 0.1% (mild steel) to about 1.8%
in some hardened steels. Iron has a melting point of 15100
C.
The four processes for producing steel are:
1. Open Hearth Process
2. Electric Furnaces
3. Oxygen Process
4. Bessemer Converter Process
The Bessemer Converter Process is not used for shipbuilding steels.
Chemical Additions to Steels: Chemical elements are added to steels during the above processes. They
may be used to deoxidise the metal, remove impurities and bring them out into the slag, and to bring
about the desired composition. The amount of deoxidizing elements added determines whether the
steels a e i ed steels o killed steels .
Rimmed steel is unsuitable for thicker plate, as it contains gas blow holes near the centre of the ingot.
The te killed steel i di ates that the etal has solidified i the i got old ith little o o e olutio
of gas. Addition of sufficient quantities of deoxidising material (silicon or aluminium) has reduced the
gas content. Steel of this type has a high degree of chemical homogeneity, and killed steels are superior
to rimmed steels. Semi-killed steel has more gas content in the ingot.
Heat Treatment of Steels: the properties of steels may be altered greatly by the heat treatment to
which the steel is subjected. Heat treatment brings about a change in the mechanical properties, by
odif i g the steel’s st u tu e.
Annealing: This consists of heating the steel at a slow rate to a temperature of 850-950 0
C, and then
cooling it in the furnace at a very slow rate. Annealing relieves any internal stresses and softens the
steel.
Normalizing: This is carried out by heating the steel slowly, similar to annealing, and allowing it to cool
in air. The faster cooling rate produces a harder, stronger steel, and also refines the grain size.
Quenching: Steel is heated similar to annealing and normalizing, and then quenched in water or oil. The
fast cooling rate produces a very hard structure with a higher tensile strength.
Tempering: Quenched steels may be further heated to 6800
C, and some alloy steels are then cooled
fairly rapidly by quenching in oil or water. This treatment is to relieve the severe internal stresses
produced by the original hardening process and to make the material less brittle but retain the higher
tensile stress.
Stress Relieving: To relieve internal stresses the temperature of the steel is raised so that no structural
change of the material occurs and then it may be slowly cooled.
Steel Sections
A range of steel sections are rolled from ingots. Flat Bars, Offset Bulb Plate, Angle Bar, Tee Bulb, Channel
Bar, Tee Bar are some of the main sections used in shipbuilding.
(drawings of steel sections) ***
Shipbuilding Steels
Mild Steel containing 0.15 to 0.23% carbon (and a reasonably high manganese content) is generally
used for hull construction. Both sulphur and phosphorus are kept to a minimum (less than 0.05%).

13. 13
Higher concentrations of both are detrimental to the welding properties of the steel. Cracks can occur
during the rolling process if the sulphur content is high.
“teel fo a ship lassed ith Llo d’s Registe is p odu ed a app o ed a ufa tu e . I spe tio a d
prescribed tests are carried out at the steel mill before dispatch. All certified materials are marked with
the so iet ’s a d, a d othe pa ti ula s as e ui ed the ules. F o 9 9 ajo so ieties ag eed to
standardise their requirements. IACS graded steels are A, B, C, D & E. Grade A is ordinary mild steel to LR
requirements and generally used in shipbuilding. Grade B is a better quality than Grade A, and specified
where thicker plates are required in the more critical regions. Grade C, D and E possess increasing
notch-tough characteristics. Grade C being to ABS requirements. Normal mild steel are not used for LNG
tanks or barriers, as they turn brittle at low temperatures.
High Tensile Steels have a higher strength than mild steel and could be used in the more highly stressed
regions of large ships. It also allows reduction in thickness of deck, bottom shell and framing. They do
cause larger deflections, rusting of thinner plate is an issue and have reduced fatigue life compared to
mild steel. They are also susceptible to stress corrosion cracking. LNG ships with membrane tanks
generally do not have these steels, as the flexing can damage the tanks.
Corrosion Resistant Steels: (stainless steels) steels with alloying elements are too costly for normal ship
building. They are used for the tanks carrying corrosive chemicals. They could be used for Liquefied Gas
tank barriers due to their cryogenic properties.
Steel Sandwich Panels: Proprietary manufactured steel sandwich panels are available for deck repairs
and naval ship construction, where lighter weight is a bonus. They are not used on LNG ships.
Steel Castings
Stern Frames, Rudder Frames, Spectacle Frames for bossings, and other structural components may be
produced as castings. Molten steel produced by the Open Hearth, Electric furnace or Oxygen Process, is
poured into a carefully constructed mold and allowed to solidify to the shape required. After removal
from the mold, heat treatment is required – annealing or normalising and tempering to reduce
brittleness.
Steel Forgings
Forging is a method of shaping metal by heating it to a temperature where it becomes more or less
plastic and then hammering or pressing it to the required form. Forgings are manufactured from killed
steel made from the Open Hearth, Electric Furnace or Oxygen Process. Where possible the working of
the metal is such that metal flow is in the most favourable direction with regard to the mode of
stressing in service. Large engine crankshafts are forged.
Aluminium
Aluminium is a relatively new discovery. Mainly because it was difficult without electricity to achieve the
high temperatures, 2000 0
C, required to melt Bauxite. It was first used for small craft in 1891 and for
experimental naval vessels in 1894. It has not been a significant material for ships until comparatively
recently. Aluminium (2723 kg/m3) is lighter than steel(7840 kg/m3). With an aluminium structure 60%
of the weight of a steel structure may be saved. Aluminium has a high resistance to corrosion and is
non-magnetic.
A major disadvantage of aluminium alloys is the higher initial and fabrication costs. Aluminium ( 660.30
C) also has a lower melting point than steel (about 1370 0
C). Aluminium is used for the construction of

14. 14
MOSS Spherical Tanks for the carriage of LNG. These tanks can be as large as 800 tonnes, 45 metre
diameter, 32mm wall thickness and 160mm thickness at the equatorial ring.
Because of the low melting point of aluminium, fire protection is more critical on ships built of
aluminium.
Fibre - Reinforced Composites (FRCs)
Composite materials combine two or more elements with very different characteristics to provide a
material with good structural capability. The fibre provides the strength and the matrix in which it is
contained, usually a plastic, holds the fibre in place. The fibre can be arranged to provide directional
strength so the composite can be tailored to very specific structural requirements.
Composite technology is a very ancient art. Bricks and concrete are composites. For marine applications
Glass fibre reinforced plastic (GRP) was first introduced in the 1950s. It is now the main material for
small boats. Some boats are still operational after 50 years. GRP is generally light and durable. The major
advantages of GRP/ FRP for small vessels include low weight, combined with high strength and stiffness.
The disadvantages are: It is labour intensive, dependent on the skill level of workmanship, is flammable,
loses its mechanical properties above ambient temperatures.
Gas Carrier Rules & Regulations
Regulations & Codes of Practice governing Gas Carriers
It is IMO’s (International Maritime Organization) global responsibility to develop international
standards and adopt conventions on many aspects of marine operations, including the carriage of
liquefied gas. This responsibility is discharged through a number of Codes, Recommendations, Treaties
and Guidelines.
The main conventions that apply to all vessels, including Gas Carriers are:
1. International Conference on Load Lines, 1966
2. International Convention on Safety of Life at Sea (SOLAS), 1974 as amended.
3. International Convention on Marine Pollution Pollution 1973/78 (MARPOL)
4. International Convention on Training & Certification of Seafarers, 1978 (STCW), as amended in
1995 STCW’95
Ships built after 1986, are required to comply with the International Gas Carrier Code (IGC), and its
International Certificate of Fitness. This compliance is necessary to gain entry to a foreign port.
Ships built before 1986 were recommended to comply with the IMO Gas Carrier Code (GC), and before
1976 with the Existing Gas Carrier Code (EGC), in addition to a Certificate of Fitness.
Packaged gases carried on other ship types (Ro-RO’s o Co tai e ships a e o e ed i the IMDG Code
Class 2.
The underpinning philosophy of the IGC Code is to relate the ship type to the hazards of each of the
products covered by the Code, including cargoes transported under cryogenic or pressurised conditions.
The code is based on established naval architectural and engineering principles, together with the best
understanding available of product hazards. The code is a live document, as it reflects the continuing
development as it reflects the continuing development of gas carrier technology.

15. 15
The Classification Societies
Classification Societies are organisations that establish and apply technical standards in relation to the
design, construction and survey of marine related facilities, including ships and offshore structures. The
standards are issued as published rules. A Certificate of Class is issued when a vessel is designed and
built to the appropriate rules of the society.
Such a certificate does not imply, and should not be construed as, an express warranty of safety, fitness
for purpose or seaworthiness of the ship. It only attests that the vessel is in compliance with the
standards developed and published by the society issuing the classification certificate.
Classification Societies are independent, self regulating and no commercial interests relating to ship
design, ship building, ship ownership, ship operation, ship management, ship maintenance or repairs,
insurance or chartering. In drawing their rules they consult with members of the industry who are
considered expert in their field. Classification rules are developed to contribute to the structural
st e gth a d i teg it of esse tial pa ts of the ship’s hull, appe dages a d the elia ilit a d fu tio of
the propulsion and steering systems, power systems, power generation and auxiliary systems that
maintain essential services on board.
A Ship is maintained in class, provided that relevant rules have been complied with, and approved
surveys carried out in accordance with its rules. Classification societies also maintain significant research
departments that encourage innovative developments in the maritime industry. All classification
societies act on behalf of Flag Administrations in carrying out statutory surveys and inspections.
Inter- Industry Organisations
All existing International Conventions, Codes and Regulations deal with the legislation of the ships. The
right equipment and qualifications are recommended to achieve safe operations, but no operational
guidance or recommendation is given.
A number of Inter-Industry Organisations are involved i p odu i g Codes of P a ti e that suppo t the
operator in complying with legislation.
1. ICS - International Chamber of Shipping.
2. OCIMF – Oil Companies International Marine Forum
3. SIGTTO – Society of International Gas Tanker and Terminal Operators
International Chamber of Shipping (ICS)
The IC“ is ade up a d suppo ted y the “hipow e ’s Asso iatio s of the e e ou t ies. Thei Head
Quarters are based in London, England. The structure of the ICS is based on committees that deal with
the various branches of maritime activity (Nautical, Construction, Tanker Safety, Operational Pollution,
Liquefied Gas and Chemical Panels, etc.)
Some of the codes of practice produced by the ICS for Gas Carriers are:
- Tanker Safety Guide (Liquefied Gases)
- International Safety Guide for Oil Tankers & Terminals (ISGOTT)
- Ship to Ship Transfer Guide (Liquefied Gasses).

16. 16
Oil Companies International Marine Forum (OCIMF)
Only Oil Companies can be members. Their primary aim is to foster safe and pollution free operations for
all types of tankers at sea and in port.
Some of their codes of practice specifically for Liquefied Gas Carriers are:
- Safety Inspection Guidelines and Terminal Safety Check-list for Gas Carriers.
- Standardisation of Manifolds for Refrigerated Liquefied Gas Carriers (LNG).
- Standardisation of Manifolds for Refrigerated Liquefied Gas Carriers for Cargoes from 0 degrees
to -104 degrees C.
- Design and Construction Specifications for Marine Loading Arms.
- Safety Guide for Terminals Handling Ships Carrying Liquefied Gasses in Bulk.
Society of International Gas Tankers & Terminal Operators (SIGTTO)
The majority of Gas Companies are members of SIGTTO, covering gas carrier owners as well as liquefied
gas producers and users.
The ai “IGTTO pu li atio is Li uefied Gas Ha dli g P i iples o “hips & Te i als . I 2005
“IGTTO pu lished LNG “hippi g “uggested Co pete y “ta da ds . The sta da ds we e w itte using
the “TCW o pete e ased ethodology a d a e p ese ted y “IGTTO as guida e to the i dust y.
Ship Inspectorate Groups : SIRE and CDI
Commercial inspectorate groups act on behalf of charterers. Ship Inspecting and Reporting (SIRE) are
mainly involved with oil and Liquefied Gas Carriage. The Chemical Distribution Institute (CDI) deal with
chemical tankers and those gas carriers carrying butadiene vinyl chloride and other chemicals. Their
aim is to improve the standard of ships.
SOLAS International Convention on Safety of Life at Sea 1974
SOLAS and its amendments, main objective is to lay down the basic design and
equipment requirements to ensure that all ships comply with basic safety standards.
Chapter II-1 : Construction – Subdivision and Stability, Machinery and Electrical
Installations.
Chapter II-2 : Construction, Fire Protection, Fire Detection and Fire Extinction.
The fire main on gas carriers should be capable of at least 5 bar to adequately supply the
water spray system.
Chapter III : Life Saving Appliances
Chapter IV : Radio communications
Chapter V: Safety of Navigation
Chapter VII : Carriage of Dangerous Goods
Chapter IX : Management for the Safe Operation of Ships (ISM Code)
Chapter XI-1 and XI-2 : Special Measures to Enhance Maritime Safety.

17. 17
IGC The International Code for the Construction & Equipment of Ships
Carrying Liquefied Gases in Bulk
Chapter 1 - General
Deals with:-
- Requirements for the specification of the Code
- Date of entry 1st
July 1986
- Definition of Hazards
- Survey and Certification
- Maintenance after Survey, so that the ship is maintained to conform with the provisions of the
Code
Chapter 2 - Ship Survival Capability & Location of Cargo Tanks
For each cargo, the Code aims to achieve a high level of safety by specifying the criteria for ship
survivability after damage and the tank location and cargo containment standards.
Ships subject to the Code are classed as either Type 1G, 2G or 3G.
The intact stability of gas carriers in all conditions, including damage stability, should be positive (GM
0.15m). The bottom and side damage that a gas carrier should be able to survive are as shown in the
Figures below:

18. 18
Chapter 3 - Ship Arrangements
Outlines the segregation standards for the ship, covering:
- Cargo Tanks
- Hold Spaces
- Cargo Piping
- Accommodation
- Compressor Rooms
- Motor Rooms
Chapter 4 - Cargo Containment
Details the requirements for the cargo containment systems and types of tank:
- Integral
- Membrane
- The independent tanks of Types A, B & C
It includes specific details for design loads, structural analysis, allowable design stress, secondary
barrier, thermal calculations, materials, construction and testing.
All categories of tanks, except for independent type C, are generally restricted to a maximum vapour
pressure of 0.25 bar, although type B may be allowed to hold slightly more pressure.
For temperatures between -10 and -55 0
C - the hull may act as a secondary barrier
For temperatures below -55 0
C - a secondary barrier is required.
For temperatures below -100
C - suitable insulation around the tank is required.
Integral tanks are usually limited to the carriage of cargoes that have a temperature of -100
C or higher at
atmospheric pressure.
Chapter 5 - Process Pressure Vessels and Liquid and Pressure Piping System
This chapter details the requirements for the design, construction and testing of cargo piping systems.

19. 19
Chapter 6 - Materials
Gives the requirements for plates, sections, pipes, forgings, castings and elements used in the
construction of cargo tanks, cargo and process piping, secondary barriers and the hull structure.
Chapter 7 - Cargo Pressure / Temperature Control
This chapter covers the equipment and arrangements necessary for containment when the cargo is
carried at a pressure below that corresponding to the ambient temperature. It covers boil-off gas by
either reliquefaction or by combustion in the ships boilers, engines and similar equipment.
Only LNG can be used in the ship’s oilers or engines, as the vapour is lighter than air, and any leaks
would rise out of the Engine Room.
Chapter 8 - Cargo Tank Vent System
The requirements are given for pressure relief systems serving cargo tanks, spaces surrounding cargo
tanks and cargo piping. This includes the capacity and arrangements of relief valves. Unless the cargo
tank is smaller than 20 m3
, each cargo tank must be fitted with two pressure relief valves.
Chapter 9 - Environmental Control
Provides standards for the environmental control of :
- Cargo tanks and piping systems
- Interbarrier spaces and void spaces
- Vapour spaces of loaded cargo tanks
- Holds, when inerting is not required
The quality and control of the inert gas produced onboard is also covered in this chapter.
Chapter 10 - Electrical Arrangements
This chapter defines the hazardous zones created by the products and the class of electrical equipment,
including instrumentation that is required in them. Requirements for cargo pump room and electric
motor rooms are specifically addressed.
Chapter 11 - Fire Protection and Fire Extinction
This chapter details standards for structural fire protection, fire protection arrangements in cargo
handling spaces, sea water extinguishing systems and water spray systems in the cargo tank area. A dry
po de e ti guishi g s ste is spe ified fo s all fla a le gas fi es. Re ui e e ts fo fi e e ’s
protective clothing and outfits is included.
Chapter 12 - Mechanical Ventilation in the Cargo Area
Covers the spaces normally entered during cargo handling operations and the spaces that are not
normally entered. Location of intakes and exhaust, number of air changes and materials of construction
for fans are also covered.
Chapter 13 - Instrumentation
Requirements are given for cargo tank level indicators and alarms, pressure gauges and temperature
indicators in cargo systems and vapour detection equipment.

20. 20
Chapter 14 - Personnel Protection
This chapter discusses requirements for protective clothing, gas masks, breathing apparatus, first aid,
resuscitation equipment, decontamination showers, eye wash facilities and personnel rescue
equipment.
Chapter 15 - Filling Limits for Cargo Tanks
Limits are specified to prevent a tank becoming liquid full by thermal expansion after loading. The
maximum limit is considered to be 98% (of the volume of the tank) liquid full at the reference
temperature. Some administrations allow for greater tank volume when carrying LNG, with some ships
permitted to fill to 99% volume. This limit takes account of the expected boil-off of vapours from the
cargo tanks during the loaded voyage.
Chapter 16 - Use of Cargo as fuel
Describes the conditions under which LNG boil-off gas may be used as fuel. The necessary arrangements
of piping, valves and gas detecting equipment for safe delivery of the gas to the machinery are specified,
as are the ventilation systems to ensure that there is no accumulation of hazardous vapours.
Chapter 17 - Special Requirements
The code incorporates special requirements for cargoes that have unusual characteristics.
Chapter 18 - Operating Requirements
Deals with the operation of a gas carrier. It highlights the regulations in other chapters that are
operational in nature and includes other requirements unique to gas ship operation.
Chapter 19 - Summary of Minimum Requirements
Links the product hazards to the ship design. In determining the applicable ship type requirements,
products are categorised as high hazard, medium hazards or low hazard.
MARPOL 73 - International Convention for the Prevention of Pollution from Ships
Annex I Regulations for the Prevention of Pollution by Oil (2-10-1983)
Annex II Regulations for the Control of Pollution by Noxious Liquid Substances in Bulk (6-4-1987).
Annex III Regulations for the Prevention of Pollution by Harmful Substances Carried by Sea in
Packaged Form (1-1-1992)
Annex IV Regulations for the Prevention of Pollution by Sewage from ships.
Annex V: Regulations for the Prevention of Pollution by Garbage from Ships (31-12-1988)
Annex VI Controlling atmospheric pollution by the prevention of release of volatile organic
compounds and NOX or SOX engine exhausts. (19-5-2005)
STCW International convention on Standards of Training, Certification &
Watchkeeping for Seafarers, 1978 as amended 1995.
The convention is a framework of agreed international standards of training, certification &
watchkeeping for seafarers. It covers all of the qualifications required for Masters, Chief Engineers, Deck
and Engine Officers, Radio Operators and Ratings.

21. 21
They contain special requirements for Masters, Officers and ratings on Oil Tankers (Regulation V/1), on
Chemical Tankers (Regulation V/2) and Liquefied Gas Tankers (Regulation V/3)
Regulation V/3 specifies:
Officers and ratings who are to have specific duties and responsibilities in connection with cargo and
cargo equipment on liquefied gas tankers, and who have not served on liquefied gas tankers as part of
the regular complement, shall have completed an appropriate shore-based fire-fighting course. They will
also have:
- An appropriate period of supervised shipboard service in order to acquire adequate knowledge
of safe operational practices.
- An appropriate liquefied gas tanker familiarisation course which includes basic safety and
pollution prevention precautions and procedures, the layout of different types of liquefied gas carriers,
types of cargo, their hazards and their handling equipment, general operational sequence and liquefied
gas tanker terminology.
Masters, Chief Engineers, Chief Officers, Second Engineers and any person with the immediate
responsibility for loading, discharging and care in the transit or handling of cargo, in addition to the
provisions stated above shall have:
- Relevant experience appropriate to their duties on liquefied gas tankers.
- Completed a specialised training programme appropriate to their duties including liquefied gas
tanker safety, fire safety measures and systems, pollution prevention and control, operational practice
and obligations under applicable laws and regulations.
According to the Convention requirements, every officer who has completed a specialised level training
ou se, ust ha e his li e e e do sed to se e eithe o a oil, he i al o li uefied gas a ie ,
depending on the nature of the cargo being carried.
STCW 1995 was a revision to the existing Annex, no change was made to the 1978 convention.
STCW 95 provided a complete, consolidated text of the STCW Convention, including its original articles,
a revised Annex and the supporting STCW Code. Mandatory technical standards are contained in Part A
of the STCW Code. Part B of the Code provides guidance to assist those involved in educating, training or
assessing the competence of seafarers or those who are otherwise involved in applying STCW
Convention provisions.
Testing of Materials
Metals are tested to ensure that their strength, ductility and toughness are suitable for the function
they are required to perform.

22. 22
The Strength of the material is its ability to resist deformation. Yield Stress and Ultimate Tensile
Strength measure the ability to resist forces on the structure.
Hardness is the ability to resist abrasion. Hardness is usually measured on Rockwell or Brinell scale,
based on test results.
Ductility is the ability of a material to be deformed before it fails.
Brittleness is the opposite of ductility and describes a material that fails under stress because it cannot
deform. Softer metals, like aluminium are ductile. Hard materials like cast iron are strong but brittle.
Toughness is the ability of a material to absorb energy.
Stresses and strains are often referred to when comparing the strengths of various metals. Stress is a
measure of the ability of a material to transmit a load, and the intensity of stress in the material, which
is the load per unit area.
Total strain is defined as the total deformation that a body undergoes when subjected to an applied
load. Strain is the deformation per unit length or unit volume.
Stress is directly proportional to strain. Stress is equal to a constant, which is the slope of the straight
line part of the graph.
This constant is referred to as the Modulus of Elasticity (E). (for mild steel it is about 21 100 kg/mm2
).
The Yield Stress for a metal corresponds to the stress at the yield point, that is the point at which the
metal no longer behaves elastically. Ultimate Tensile Stress is the maximum load to which the metal is

23. 23
subjected, divided by the original cross-sectional area. Beyond the yield point the metal behaves
plastically.
Proof Stress is obtained by setting off on the base some percentage of the strain, say 0.2%, and drawing
a line parallel to the straight portion of the curve. The intersection of this line with the actual stress-
strain curve marks the proof stress.
“hip’s st u tu e is desig ed fo o ki g st esses that a e ithi the elasti a ge and much lower than
the ultimate tensile strength of the material to allow a reasonable factor of safety.
Classification Society Tests for Hull Materials
Mild Steel and Higher Tensile Steel plates and sections built into a ship are produced at works approved
by a classification society. During production an analysis of the material is required, and so are
prescribed tests of the metal. Similar tests are also required for steel forgings and steel castings, in order
to maintain an approved quality. Destructive Tensile and impact tests are also carried out.
Tensile Test: A specimen of given dimensions are subject to an axial pull and a minimum specified
yield stress, ultimate tensile stress, and elongation must be obtained. The specimen has a gauge length
5.65 times the square root of the cross sectional area, which is equivalent to a gauge length 5 times the
diameter
Impact Test: The Charpy V-notch test or Charpy U-notch test is commonly specified . The impact test is
to determine the toughness of the material, that is, its ability to withstand fracture under shock loading.
The specimen is placed on an anvil and the pendulum is allowed to swing so that the striker hits the
specimen opposite the notch and fractures. Energy absorbed in fracturing the specimen is automatically
recorded by the machine. Making allowance for friction, the energy absorbed in fracturing the specimen
is the difference between the potential energy the pendulum possesses before being released, and that
which it attains in swinging past the vertical after fracturing the specimen.

24. 24
Stresses on Ships
Ships experience stresses when floating in still water and when underway at sea.
- Vertical Shear & Longitudinal Bending in Still Water
- Bending Moments in a seaway
- Longitudinal Shear Forces
- Bending Stresses
- Transverse Stresses (Racking & Torsion)
- Local Stresses (Panting, Pounding & other local stresses)
- Brittle Fracture
- Fatigue Failures
- Buckling
Vertical Shear & Longitudinal Bending in Still Water
For a homogeneous body of uniform cross section and weight that is floating in still water, at any
section the weight and buoyancy forces are equal and opposite. There is no resultant force at a section,
and the body will not be stressed or deformed. A ship floating in still water has an unevenly distributed
weight owning to both cargo distribution and structural distribution. The buoyancy distribution is also
non-uniform since the underwater sectional area is not constant along the length. Total weight and total
buoyancy are balanced, but at each section there will be a resultant force or load, either in excess of
buoyancy or excess of load. Since the vessel remains intact, there are vertical upward and downward
forces tending to distort the vessel, which are referred to as vertical shearing forces, since they tend to
shear the vertical material in the hull.

25. 25
The ship shown will be loaded in a similar manner to the beam shown, and will tend to bend in a similar
manner owing to the variation in vertical loading. A vessel bending in this manner is said to be hogging.
If there is excess weight amidships the vessel is sagging. When sagging the deck will be in compression
and the bottom shell in tension.
Bending Moments in a seaway
When a ship is in a seaway, the waves with their troughs and crests produce a greater variation in the
buoyant forces, and therefore can increase the bending moment, vertical shear force, and stresses. In a
seaway the overall effect is an increase of bending moment from that in still water when the greater
buoyancy variation is taken into account.
Longitudinal Shear Forces
When the vessel hogs and sags in still water and at sea, shear forces similar to the vertical shear forces
will be present in the longitudinal plane. Vertical and longitudinal shear stresses are complementary and
exist in conjunction with a change of bending moment between adjacent sections of the hull girder. The
magnitude of the longitudinal shear force is greatest at the neutral axis and decreases towards the top
and bottom of the girder.
Bending Stresses
From bending theory, the bending stress ( a ) at any point in a beam is given by:
a = M/I x y
M = applied bending moment
I = second moment of area of cross-section of beam about the neutral axis
y = distance of point considered from neutral axis
When the beam bends, say in the case of hogging, it is in tension at the top and in compression at the
bottom. Somewhere between the top and bottom is a line that is neither in tension or compression. This
position is called the neutral axis. The neutral axis also contains the centre of gravity of the cross-
section of the beam. The greater the value of the second moment of area, the less the bending stress
will be. The second moment of area of the section varies as the depth squared and therefore a small
increase in depth of section can be very beneficial in reducing the bending stress.
The Ship as a Beam
Ships bend like a beam does. The hull can be considered as a box-shaped girder for which the position of
the neutral axis and second moment of area can be calculated. The deck and bottom shell form the
flanges of the hull girder, and are far more important to longitudinal strength than the sides that form
the e of the gi de a d a the shea fo es. The o shaped hull gi de a d a o e tio al I
gi de a e o pa ed elo .

26. 26
In a ship the neutral axis is generally nearer the bottom, since the bottom shell will be heavier than the
deck, having to resist water pressure as well as bending stresses. In calculating the second moment of
an area of the cross-section, all longitudinal material is of greatest importance and the further the
material is from the neutral axis, the greater will be its second moment of area about the neutral axis.
However, at greater distances from the neutral axis, the sectional modulus will be reduced and
correspondingly the higher stress may occur in extreme hull girder plates such as the deck stringer,
sheer strake, and bilge. These strakes of plating are generally heavier than other plating.
Bending stresses are greater over the middle portion of the length and it is owing to this variation that
Llo d’s Registe gi e a i u s a tli gs o e % of the le gth a idships. Othe s a tli gs a tape

27. 27
towards the ends of the ship, apart from locally highly stressed regions where other forms of loading are
encountered.
Strength Deck
The deck forming the uppermost flange of the main hull girder is often referred to as the strength deck.
All continuous decks should be strength decks. Along the length of the ship (the top flange of the hull
girder, ie the strength deck), may step from deck to deck where large superstructures are fitted or there
is a natural break, for instance in way of a raised quarter deck. Larger superstructures tend to deform
with the main hull and stresses of appreciable magnitude will occur in the structure. Early vessels fitted
with large superstructure of light construction demonstrated this to their cost. Attempts to avoid
fracture have been made by fitting expansion joints, which made the light structure discontinuous.
These were not entirely successful and the expansion joint may itself form a stress concentration at the
strength deck., which one would want to avoid. In modern construction the superstructure is usually
made continuous and of such strength that its sectional modulus is equivalent to that which the
strength deck would have if no superstructure were fitted.
Transverse Stresses
Whe a ship e pe ie es t a s e se fo es, these te d to ha ge the shape of the essel’s oss-
sections and thereby introduce transverse stresses. These forces may be produced by hydrostatic loads
and impact of seas or cargo and structural weights, both directly and as the result of reactions due to
change of ship motion.
Racking
When a ship is rolling, 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 deformation is known as racking. Transverse
bulkheads primarily resist such transverse deformation. When transverse bulkheads are widely spaced,
deep web frames and beams may be introduced to compensate.
Torsion
When a body is subject to a twisting moment (torque) , the body is said to be in torsion. Beam seas
cause torsion to happen to ships. In most ships these torsional moments and stresses are negligible, but
ships with extremely wide and long deck openings they are significant.
Panting
Panting refers to a tendency for the shell plating to work in and out in a bellows like fashion, and is
caused by the fluctuating pressures on the hull at the ends when the ship is in rough seas. These forces
are most severe when the ship is running into waves and is pitching heavily, the large pressures
occurring over a short time cycle.
Pounding
When a ship is driven into head seas, severe local stresses occur in way of the forward bottom shell and
framing. These pounding stresses are likely to be more severe in a lightly ballasted condition, and occur
over an area of the bottom shell aft of the collision bulkhead. Additional stiffening is required in this
area.

28. 28
Other Local Stresses
Ship structural members are often subjected to high stresses in localized areas, and these have to be
correctly designed. Examples are where various load-carrying members of the ship intersect, where
longitudinal meet at transverse bulkheads, and at intersections of longitudinal and transverse
bulkheads. Another highly stressed area occurs where there is a discontinuity of the hull girder at ends
of deck house structures, also at hatch and other opening corners, and where there are sudden breaks
in the bulwarks.
Brittle Fracture
When welding of ships began to overtake riveted ships in the 1940s, brittle fracture occurred more
frequently. Welded joints provided metal continuity, and cracks propagated more rapidly that on riveted
ships. Brittle fracture occurs when an otherwise elastic material fractures without any apparent sign or
little evidence or material deformation prior to failure. Fracture occurs instantaneously with little
a i g a d the essel’s o e all st u tu e eed ot e su je t to a high st ess at the ti e. Mild steel is
particularly prone to brittle fracture. The following factors influence the possibility of brittle fracture:
1. A sharp notch is present in the structure from which the fracture initiates.
2. Tensile stress is present.
3. There is a temperature above which brittle fracture will not occur.
4. The metallurgical properties of the steel plate.
5. Thick plate is more prone.
Brittle fracture is distinguishable from ductile failure by the lack of deformation at the edge of the tear,
and is bright granular in appearance. Brittle fracture is also distinguished by the apparent chevron
markings, which aids location of the fracture initiation point since these tend to point in that direction. A
ductile failure has a dull grey appearance. Sharp corners and notches where cracks may be initiated
should be avoided in ship construction. Because ships are very large structures, faults in welds will
occur, and a complete weld examination is time consuming and costly. Steel specified for hull
o st u tio should ha e goof ot h du tilit at the se i e te pe atu es, pa ticularly where thick
plate is used.
Crack arrester was the practice of introducing a riveted seam in cargo ships to subdivide the vessel into
welded substructures, so that any possible crack propagation was limited to the substructure. Such a
crack arrester was usually specified in the sheerstrake/stringer plate area of large ships. With modern
ships, st akes of highe ot h tough ess steel a e e ui ed to e fitted i su h a eas. Llo d’s Registe
require the mild steel sheerstrake and stringer plate at the strength deck, over the midship portion of
ships over 250 metres to be Grade D if less than 15mm thick, and Grade E if of greater thickness.
Fatigue Failures
Fatigue failure occurs very slowly and can take years to propagate. Fatigue failure occurs at low stresses
that are applied to a structure repeatedly over a period of time. A fatigue crack, once initiated, may

29. 29
grow unnoticed until the load bearing member is reduced to a cross-sectional area that is insufficient to
carry the applied load. Fatigue failures are associated with sharp notches or discontinuities in structures,
and are especially prevalent at hard spots (regions of high rigidity in ship structures. With the growth in
size of oil tankers, bulk carriers, and container ships, there has been increasing use of higher yield
strength steels in their hull structures. The classification societies have subsequently places special
emphasis on analysis of the fatigue performance of these larger structures, usually over a 25 year life
cycle, as part of their approval process.
Buckling
With the substantial increase in size of ships, greater attention has had to be given to the buckling
strength of the stiffened plate panels constituting the shell. Buckling of a structural member loaded in
o p essio a o u at a st ess le el that is su sta tiall lo e tha the ate ial’s ield st ess. The
load at hi h u kli g o u s is a fu tio of the st u tu al e e ’s geo et a d the ate ial’s

30. 30
modulus of elasti it athe tha the ate ial’s st e gth. The ost o o e a ple of u kli g failu e
is the collapse of a pillar under a compressive load. A stiffened plate panel in compression will also have
a critical buckling load, whose value depends on the plate thickness, unsupported dimensions, edge
suppo t o ditio s, a d the ate ial’s odulus of elasti it .
Monitoring Ship Stresses at Sea
To enhance safety during shipboard operations, real-time motion and stress, monitoring information
e uip e t a e supplied lassifi atio so ieties su h as Llo d’s Registe to a ship. This ea s
fitting of strain gauges to the deck structure, an accelerometer, and a computer with software that
displays ship stress and motion readings on the bridge. Where this equipment is fitted, the notation
SEA(R) is assigned.
Welding and Cutting

31. 31
Ship Yard Layout
Design Information for Production
In order to build a ship, it is necessary to develop the initial structural and arrangement
design into information usable for production. Initial design is focused on the operation
of the ship once completed. Production requires information in a form that relates to the
manufacturing, assembly and construction processes. Traditionally this was done by
creating a full size model of the structure, in the mold loft, from which dimensions could
be lifted, and templates could be made. The relatively simple outfitting of early ships was
carried out on the completed ship structure, with pipes and other connectors run where
space was available. Computer modelling has replaced some of these traditional
methods.

32. 32
Plate & Section Preparation
(info to type in)

33. 33
Assembly of Ship Structure
Historically, ships were built on a slipway piece by piece. The transition from wood to iron and then to
steel was somewhat similar in the construction process. During the 1940s a large number of merchant
and war ships were built in a short period of time. Welding speeded the construction process. And mass
production of sections was possible, prior to erection. Sections could be built away from the slipway and
transported to be assembled and welded together. Welding and construction faults resulted because of
the rapid building of ships that were needed during the Second World War. Since then, quality has
improved in leaps and bounds, and larger and larger prefabricated sections were being built. By 1970 a
200 000 dwt tanker could be built and launched in 2 to 3 months. Outfitting then took place. On modern
ships, considerable amount of the outfitting and pipework was incorporated in the sections and blocks,

34. 34
and married up when the blocks were welded together to form the ship. Computer Aided Design and
Computer Aided Manufacturing techniques smartened up the whole process of designing and building
the ship.
A typical bulk carrier of 250 metres and 32 metres beam would require about 10 000 tons of steel.
Assembly would take the following pattern:-
Minor Assembly: Brackets, intercostal floors or girders, bulwarks, two dimensional assemblies and a
maximum size of 2 by 5 metres and < 2 tons
Subassembly: A flat or curved panel, box structure 12 by 12 metres and < 20 tons.
Unit Assembly: A structure that can be erected in the building dock < 60 tons.
Block: An assembly of two or more units into a bottom or side block, weighing up to 200 tons.
Modern Ship Yards may employ automatic welding processes where feasible. Units of machinery,
pipework, and other outfit systems required for a specific zone can be planned, built into modules and
installed as such into a block fabrication.

35. 35
Ship Structure
Ships are the largest, mobile man-made structures in the world. The largest ships are
comparable with major buildings, but have additionally to withstand enormous forces
from wind and waves. Their structural arrangements are of great importance. As
commercial ships have to earn by carrying cargo, their structure has to be both strong
and light as safely possible.
The structure of a ship is based on flat and curved steel plate panels, with primary and
secondary stiffening. The complete hull structure has to fulfil several important
requirements.
- Maintaining watertight integrity
- Maintain fire integrity
- Separating internal spaces, for example cargo, fuel oil, ballast water.
- Resisting forces on the ship from rough seas.
The structure also has to conform to the hull shape dictated by hydrodynamics – the
performance in terms of speed and seakeeping specified by the hull designer.
The forces on the ship and the capability of the structure have some uncertainty, which
must be allowed for in the design. The structure may suffer from changes in material
properties over the life of the ship. Some causes of structural failure are difficult to
predict (fatigue and corrosion). The quality of workmanship can also vary considerably.
Bottom Structure

36. 36

37. 37

38. 38
Shell Plating & Framing

39. 39

40. 40
Bulkhead & Pillars

41. 41

42. 42
Decks, Hatches & Superstructures

43. 43

44. 44
Fore End Structure

45. 45
Aft End Structure

46. 46
Tanker Construction

47. 47
Launching
Launching involves the transference of the weight of the ship from the keel blocks on which it is
supported during construction, to a cradle on which it is allowed to slide into the water. Normally the
vessel is launched end on, stern first. Shipyards located on rivers or narrow channels are obliged to
launch sideways.

48. 48
Liquefied Gas Carriers
A large number of ships are in service for carrying gases in liquid form in bulk. Many of the smaller ships
are designed to carry Liquefied Petroleum Gas (LPG). There are close to 400 ships that carry Liquefied
Natural Gas (LNG).
Liquefied Petroleum Gas (LPG)
LPG is the name given by the oil industry to a mixture of petroleum hydrocarbons (Propane and
Butane, and a mixture of the two. LPG is used as a clean fuel for domestic and industrial purposes.
These gases may be converted to the liquid form and transported in one of three conditions:
1. Solely under pressure at ambient temperature.
2. Fully refrigerated at their boiling point ( -30 to -480
C )
3. Semi-refrigerated at reduced temperature and elevated pressure.
Other gases with similar physical properties (ammonia, propylene and ethylene) are also shipped on LPG
carriers. These gases are liquefied and transported in the same conditions as LPG, except ethylene,
which boils at a much lower temperature ( -1040
C), and is carried in the fully refrigerated or semi-
refrigerated condition.
Liquefied Natural Gas
LNG is natural gas from which most of the impurities (sulphur and carbon dioxide) have been removed.
It is cooled to or near its boiling point of -1650
C, or at near atmospheric pressure and is transported in
this form as liquid methane. Methane has a critical pressure of 45.6 kg/cm2
, and a critical temperature
of -82.50
C ( the pressure and temperature above which liquefaction cannot occur). So the methane can
only be liquefied by pressure at very low temperatures.
The IMO International Gas Carrier Code
In 1975 the 9th
Assembly of the IMO adopted the Code for the Construction and Equipment of Ships
Carrying Liquefied Gases in Bulk, A.328(IX), which provides international standards for ships that
transport liquefied gases in bulk. It became mandatory in 1986, and is generally referred to as the IMO
International Gas Carrier Code. The requirements of this code are incorporated in the rules for ships
carrying liquefied gases published by Classification Societies.
The code covers damage limitations to cargo tanks and ship survival in the event of collision or
grounding, ship arrangements for safety, cargo containment and handling materials of construction,
environmental controls, fire protection, use of cargo as fuel and more. The code defines the basic cargo
containment types a d i di ates a se o da a ie is e ui ed, to p ote t the ship’s hull st u tu e f o

49. 49
the embrittling effect of the low temperature should cargo leak from the primary tank structure. The
cargo containment types are described below:
Integral Tanks
These ta ks fo a st u tu al pa t of the ship’s hull a d a e i flue ed i the sa e a e a d the
same loads that stress the hull structure. These are used for the carriage of LPG at or near atmospheric
conditions (Butane for example), where no provision for thermal expansion and contraction of the tank
is necessary.
Membrane Tanks
These are non-self-supporting tanks consisting of a thin layer (membrane) supported through insulation
by the adjacent hull structure. The membrane is designed in such a way that thermal and other
expansion or contraction is compensated for without undue stressing of the membrane. Membrane
tanks are primarily used for LNG cargoes.
Semi-Membrane Tanks
These are non-self-supporting tanks in the load condition. The flat portions of the tank are supported,
transferring the weight and dynamic forces through the hull, but the rounded corners and edges are not
supported so that tank expansion and contraction is accommodated. Such tanks were developed for the
carriage of LNG, but have been used for a few LPG ships.

50. 50
Independent Tanks
These are self-supporting and independent of the hull. They are completely self supporting and do not
fo pa t of the ship’s hull. There are three types, depending upon the design pressure.
Type A : designed using standard traditional methods of ship structural analysis. LPG at or near
atmospheric pressure or LNG may be carried in such tanks

51. 51
Type B: are designed using more sophisticated analytical tools and methods to determine stress levels,
fatigue life, and crack propagation characteristics. The overall design concept of these tanks is based on
the a k dete tio efo e failu e p i iple , hi h pe its thei use ith a edu ed se o da a ie .
LNG is normally carried in these tanks

52. 52
Type C: are designed as pressure vessels, the dominant design criteria being the vapour pressure.
Normally used for LPG and Ethylene.

53. 53
Internal Insulation Tanks
These are non-self-supporting and consist of thermal insulation materials, the inner surface of which is
exposed to the cargo supported by the adjacent inner hull or an independent tank. There are two types:
Type 1: the insulation or combination of insulation and one or more liners act only as the primary
barrier. The inner hull or independent tank forms the secondary barrier.
Type 2: the insulation or combination of insulation and one or more liners act as both the primary and
secondary barriers and are clearly distinguishable as such.
Liners on their own do not act as liquid barriers and therefore differ from membranes. These tanks are a
later addition to the Code, and Type 1 has been used for the carriage of LPG.
Secondary Barrier Protection
The requirements for secondary barrier protection are given in the table below:

54. 54
Liquefied Natural Gas Ships
There are over 20 approved patent designs of containment vessel for LNG ships. The majority fall into
the membrane or independent tank categories. LNG ships are of the double-hull construction, within
which are fitted the cargo tanks and the secondary barrier system.
At the beginning of 2011, some 350 large LNG ships were trading. Older ships have independent Type B
tanks of the Kvaerner-Moss design, with most being of the membrane type. The GAZ Transport
membrane system is twice as prevalent as other membrane systems.
Sea Trials
“ea t ials a e o du ted to easu e a essel’s pe fo a e a d ge e al sea o thi ess. Testi g of a
essel’s speed, a oeuverability, equipment and safety features are usually conducted. In attendance
are Technical representatives from the builder, flag and certification officials, and representatives of the
owners. Successful sea trials lead to a vessel’s e tifi atio fo o issio i g a d a epta e its
owner.
Speed Trial: The vessel is ballasted or loaded to a predetermined draft and the propulsion machinery is
set to the contracted maximum service setting, usually a percentage of the a hi e ’s a i u
o ti uous ati g 9 % MCR . The ship’s headi g is adjusted to ha e the i d a d tide as lose to o -
on as possible. The vessel is allowed to come to speed, and the speed is continuously recorded using
differential GPS. The ship is then turned through 1800
and the procedure is followed again. This reduces
the i pa t of i d a d tide. The fi al T ials “peed is dete i ed a e agi g all of the easu ed
speeds during each of the runs. This process may be repeated in various sea states.

55. 55
Ship Inspections
Surveys fall into two types: those required by Class or for compliance with other
regulatory bodies, and those requested by Owners for structural assessment purposes.
Class and statutory surveys include annual and intermediate surveys., bottom/ docking
surveys, special or periodical surveys and occasional surveys. Owners surveys are based
on a requirement to assess general condition, corrosion rate, detailed condition or for
repair assessment.
Statutory surveys for fulfilling Convention requirements of Load Line and SOLAS, are very
similar to Class requirements. So they are generally done at the same time, to minimise
duplication. Flag Administrations may delegate Class to carry our statutory surveys.
Special (Periodical) surveys are generally required at four year intervals, but may be
postponed for up to one year if a General Examination is satisfactorily completed. The
scope of the General Examination varies according to the age of the ship, and would
include an overall survey of ballast tanks and other tanks if deemed necessary., together
with thickness measurements and tank testing required for the forthcoming Special
Survey as far as the tanks are accessible.
Instead of carrying out the survey at one time, it is possible to do it on a continuous basis
over a five year cycle. Societies are also prepared to do surveys at sea.

56. 56

57. 57
Dry Docking & Ship Repairs
Ships are to be examined in Dry Dock at intervals not exceeding two and a half years. At
the dry docking survey, particular attention is paid to the shell plating, stern frame and
rudder, external and through hull fittings, and all parts of the hull particularly liable to
corrosion and chafing, and any unfairness of the bottom.
Class may accept In-water Surveys in lieu of any one of the two dockings required in a
five year period. The in-water survey is to provide the information normally obtained for
the docking survey. Generally, consideration is only given to an in-water survey where a
suitable high-resistance paint has been applied to the underwater hull.
When a vessel requires repairs to damaged equipment or to the hull, the work has to be
carried out to the satisfaction of the Class. In order that the ship maintains its class,
approval of the repairs undertaken must be obtained from the surveyors, either at the
time of the repair or at the earliest opportunity.

58. 58
References
1. IGC Code International Code for the Construction & Equipment of Ships carrying
Liquefied Gases in Bulk. (1993 Edition) 166 pages plus
ISBN 978 92 801 1277 1 (IMO 1104E) £ 14.00
2. Liquefied Gas Carriers Your Personal Safety Guide (2nd
Edition) (2013)
SIGTTO ISBN 978 1 85609 5723 50 pages £ 15.00

59. 59
3. Guidance Manual for the Inspection & Condition Assessment of Tanker Structures
Tanker Structure Co-operation Forum (Witherby ) 2009 ISNN
13: 978 0 948691 119 A4 200 pages £ 50.00
4. Guidelines on the Enhanced Programme of Inspections During Surveys of Bulk Carriers & Oil
Tankers. 2008 Edition (IMO IA265E) ISBN 978 92
801 14966 357 pages £ 14.00
5. Ship Construction DJ Eyres & GJ Bruce (7th
Edition) 2012
ISBN 978 008 0972398 388 pages £ 49.99
6. Ballast Water Management Convention (2009 Edition) (IMO I621E)
ISBN 978 92 801 15031 234 pages £ 10.00
7. Tanker Safety Training (Liquefied Gas) : Specialised Level
Witherby Publishers 312 pages £ 142.80 ISBN
13:978 1856 093 415 (2007) A4 312 pages
8. Specialized Training for Liquefied Gas Tankers, 1999 (IMO:TA106E)
ISBN 978 92 801 61090 220 pages £ 33.68
9. Design & Operation of Gas Carriers. 2004 Conference Proceedings RINA
Publications £ 95.00
Web Sites
1. SIGTTO http://www.sigtto.org/
2. Merchant Navy http://www.merchantnavy.com
3. Warsash Nautical Bookshop http://www.nauticalbooks.co.uk
4. IMO http://www.imo.org
5. Marine Equipment Directive http://www.mared.org
6. Lloyds List http://www.lloydslist.com
7. Marine Accident Investigation Branch http://www.maib.dft.gov.uk
8. Maritime & Coastguard Agency http://www.mcga.gov.uk
9. Chemical Data Sheets http://hazard.com/msds
10. Data Sheets (Oxford University) http://physchem.ox.ac.uk/msds/#msds
11. Dept. of Environment (DTi) http://www.shipping.dft.gov.uk/
12. Health & Safety Executive http://www.hse.gov.uk
13. US Coast Guard http://www.uscg.mil/
14. US Code of Federal Regulations http://www.access.gpo.gov/nara/cfr/cfr-table-
search.html
15. International Association of
16. Classification Societies www.iacs.org.uk
17. Lloyds Register www.lr.org
18. American Bureau of Shipping www.eagle.org
19. Bureau Veritas www.bureauveritas.co.uk
20. Det Norske Veritas www.dnv.co.uk

60. 60