Nature and Forms of Corrosion
There is a natural tendency for nearly all metals to react with their environment. The result of this reaction is the creation of a corrosion product which is generally a substance of very similar chemical composition to the original mineral from which the metal was produced. atmospheric corrosion Protection against atmospheric corrosion is important during the construction of a ship, both on the building berth and in the shops. Serious rusting may occur where the relative humidity is above...
Fabrication Of Large Aluminium Panels By Mechanised Mig Welding
Allday, 'Fabrication of Large Aluminium Panels by Mechanised MIG Welding', Lightweight Materials in Naval Architecture 1996, Royal Institution of Naval Architects Publications. 'Automatic Vertical Welding on the Berth', The Naval Architect, April, 1971. Chadburn and Salter, 'The Welding of Higher Tensile Shipbuilding Steels', The Naval Architect, January, 1974. Gilbert and Maughan, 'Economics of Welding from One Side', Shipping World and Shipbuilder, September, 1968. Gourd, 'Principles of...
Bulwarks
Bulwarks fitted on weather decks are provided as protection for personnel and are not intended as a major structural feature. They are therefore of light scantlings, and their connections to the adjacent structures are of some importance if high stresses in the bulkwarks are to be avoided. Freeing ports are cut in bulwarks forming wells on decks in order that water may Section through cross joint between panels Dumb panel raised by high-lift cylinders c WEATHER TIGHT GASKETS FOR Motorized panel...
Joining Ship Sections Afloat
Owing to the enormous increase in size of bulk carriers and tankers, shipyards with restricted facilities, berth size particularly, have examined various means of building these large ships in sections which are to be joined off the berth. In most cases the problem becomes one of joining the two hull sections afloat or in a dry-dock of sufficient size where available. Where the sections are to be joined afloat extremely accurate fit up of the sections is aided by the possibilities of ballasting...
Corrosion Control
The prevention of corrosion may be broadly considered in two forms, cathodic protection and the application of protective coatings. CATHODIC PROTECTION Only where metals are immersed in an electrolyte can the possible onset of corrosion be prevented by cathodic protection. The fundamental principle of cathodic protection is that the anodic corrosion reactions are suppressed by the application of an opposing current. This superimposed direct electric current enters the metal at every point...
Unit Erection
When any panel and the block assemblies are complete there will be some time buffer before their erection at the building berth or dock to allow for mishaps in the production schedule. Stowage is generally adjacent to the building hall and will vary in size according to the yard's practice, some yards storing a large number of units before transferring them to the berth or dock for erection in order to cut the berth dock time to a minimum. Sequences of erection for any particular ship vary from...
Further Reading Jib
Eidal, 'Recent Developments in Fire Safety for High Speed Craft,' Fire at Sea 1997, Royal Institution of Naval Architects Publications. Hoyning and Taby, 'Fire Protection of Composite Vessels Fire Protection and Structural Integrity an Integrated Approach', Fire at Sea 1997, Royal Institution of Naval Architects Publications. IMO 'International Code of Safety for High Speed Craft HSC Code ' 1994. Marchant, 'Meeting the Fire Requirements of the IMO High Speed Craft Code with Composite...
Stern and Bow Doors
Ro-ro vessels may be fitted with stern doors of the hinge down or hinge up type which if large are articulated. Bow doors are either of the visor type or of the side hinged type 'barn door' type . These are situated above the freeboard deck and where the bow doors lead to a complete or long forward enclosed superstructure Lloyd's require an inner door to be fitted which is part of the collision bulkhead. This would also be in keeping with the SOLAS requirements for passenger ships where the...
Stem
On many conventional ships a stem bar, which is a solid round bar, is fitted from the keel to the waterline region, and a radiused plate is fitted above the waterline to form the upper part of the stem. This forms what is referred to as a 'soft nose' stem, which in the event of a collision will buckle under load, keeping the impact damage to a minimum. Older ships had solid bar stems which were riveted and of square section, and as the stem had no rake it could cause considerable damage on...
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Side girder Lightening longitudinals hole ALTERNATE FRAME SPACE Figure 16.6 Bulk carrier double bottom construction
Propellers
Ship propellers may have from three to six similar blades, the number being consistent with the design requirements. It is important that the propeller is adequately immersed at the service drafts and that there are good clearances between its working diameter and the surrounding hull structure. The bore of the propeller boss is tapered to fit the tail shaft and the propeller may be keyed onto this shaft a large locking nut is then fitted to secure the propeller on the shaft. For securing the...
Welding Practice
In making a butt weld with manual arc welding, where the plate thickness exceeds say 5 to 6 mm it will become necessary to make more than one welding pass to deposit sufficient weld metal to close the joint. With the higher current automatic welding processes thicker plates may be welded with a single pass, but at greater thicknesses multi-pass welds become necessary. In ship work unless a permanent backing bar is used, or the 'one sided' welding technique Chapter 9, Submerged Arc Welding is...
Frame Bending
The traditional system of bending side frames may still be in use for repair work, and is described as follows. A 'set-bar' which is a flat bar of soft iron is bent to the scrieve line of the frame on the scrieve board and then taken to the frame bending slabs. On these solid cast-iron slabs pierced with holes the line of the frame is marked, and modified to agree with the line of the toe of the frame. As the heated frame on cooling will tend to bend, the set-bar is sprung to allow for this...
Bulbous Bows
Vessels operating at higher speeds, and those with high block coefficients, are often found to have a bulbous or protruding bow below the waterline. The arguments for and against fitting some form of bulbous bow are the province of text-books on naval architecture, but it may be indicated that like most peculiarities of the immersed hull form this feature is usually intended to reduce the vessel's resistance to motion under certain conditions. From the construction point of view the bulbous bow...
Shaft Bossing and A Brackets
Twin-screw or multi-screw vessels have propeller shafts which leave the line of shell at some distance forward of the stern. To support the shaft overhang, bossings or 'A' brackets may be fitted. Bossings are a common feature on the larger multiple-screw passenger ships and are in effect a moulding of the shell which takes in the line of shaft for some distance. Access from inside the hull is thus provided to the shaft over a great proportion of its length, and it is afforded greater...
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Heavy plate introduced into deck girder flange in way of pillar intersection in the double bottom below. Where this is not possible partial floors and short intercostal side girders may be fitted to distribute the load. Machinery space pillars are fabricated from angles, channels, or rolled steel joists, and are heavily bracketed to suitably stiffened members Figure 18.6 . SMALL PILLARS Within the accommodation and in relatively small vessels solid round steel pillars having diameters seldom...
Oil Tankers 1
Small tankers, not exceeding 75 metres in length, involved principally in the coastal trade have a single longitudinal bulkhead on the centre line providing two athwartship tanks. The machinery is aft, and an expansion trunk, if fitted, is on the centre line in the way of the tank spaces see Figure 22.1 . Larger ocean-going tankers have at least two longitudinal bulkheads providing three athwartship tanks, and the machinery is again arranged aft see Figures 22.2 and 22.3 . This chapter is...
Refrigerated Container Ships
Many of the container ships operating on trade routes where refrigerated cargoes were carried in conventional refrigerated cargo liners 'reefer ships' have provision for carrying refrigerated containers and have in many cases replaced the latter. The ISO containers usually 20 foot size since with most refrigerated cargoes 40 foot size would be too heavy are insulated, and below decks the end of each hold may be fitted with brine coolers which serve each stack of containers. Air from the brine...
Decks
The weather decks of ships are cambered, the camber being parabolic or straight. There may be advantages in fitting horizontal decks in some ships, particularly if containers are carried and regular cross-sections are desired. Short lengths of internal deck or flats are as a rule horizontal. Decks are arranged in plate panels with transverse or longitudinal stiffening, and local stiffening in way of any openings. Longitudinal deck girders may support the transverse framing, and deep transverses...
Pillars
The prime function of the pillars is to carry the load of the decks and weights upon the decks vertically down to the ship's bottom structure where these loads are supported by the upward buoyant forces. A secondary function of pillars is to tie together the structure in a vertical direction. Within the main hull of a cargo ship two different forms of pillar may be found, those in the holds invariably fulfilling the first function, and those in the machinery spaces fulfilling the latter...
Rudders
Many of the rudders which are found on present-day ships are semibalanced, i.e. they have a small proportion of their lateral area forward of the turning axis less than 20 per cent . Balanced rudders with a larger area forward of the axis 25 to 30 per cent , and un-balanced rudders with the full area aft of the axis are also fitted. The object of balance is to achieve a reduction in torque since the centre of lateral pressure is brought nearer the turning axis. However the fully balanced rudder...
Outfit Modules
Units of machinery, pipework and other outfit systems required for a specific zone can be planned and built up into modules and installed as such into a block fabrication. Pipework in particular lends itself to this form of assembly and can, with careful planning from the CAD stage, be arranged in groupings so that pipe bank modules can be arranged for a particular zone. Modules can range from a small pipe bank supported by light framing of pipe hangers, or a complete auxiliary machinery unit...
Classification Society Tests for Hull Materials
Both mild steel and higher tensile steel plates and sections built into a ship are to be produced at works approved by the appropriate classification society. During production an analysis of the material is required and so are prescribed tests of the rolled metal. Similar analyses and tests are required by the classification societies for steel forgings and steel castings, in order to maintain an approved quality. Destructive tests are made on specimens obtained from the same product as the...
Shell Plating
The bottom and side shell plating consists of a series of flat and curved steel plates generally of greater length than breadth butt welded together. The vertical welded joints are referred to as 'butts' and the horizontal welded joints as 'seams' see Figure 17.1 . Stiffening members both longitudinal and transverse are generally welded to the shell by intermittent fillet welds with a length of continuous weld at the ends of the stiffening member. Continuous welding of stiffening members to the...
Cargo Restraint
In ro-ro and container ships the lashing of cargo is an important safety consideration and usually calls for fittings which will permit rapid and easy but effective securing of the cargo because of short ship turn around times. The shipbuilder is responsible for the deck and perhaps hatch fittings for the securing devices and will look to the ship operator for guidance on their type and positions. On the decks of ro-ro ships where the direction of lashing is unpredictable and vehicles must...
Steering Gear
Unless the main steering gear comprises two or more identical power units, every ship is to be provided with a main steering gear and an auxiliary steering gear. The main steering gear is to be capable of putting the rudder over from 35 on one side to 35 on the other side with the ship at its deepest draft and running ahead at maximum service speed, and under the same conditions from 35 on either side to 30 on the other side in not more than 28 seconds. It is to be power operated where...
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 Figure 8.1 Vertical shear and longitudinal bending in still water bending moment, vertical shear force, and stresses. Classically the extreme effects can be illustrated with the vessel balanced on a wave of length equal to that of the ship. If the crest of the wave is amidships the buoyancy forces will tend to 'hog' the vessel if the trough is...
Single Bottom Structure
In smaller ships having single bottoms the vertical plate open floors are fitted at every frame space and are stiffened at their upper edge. A centre line girder is fitted and one side girder is fitted each side of the centre line where the beam is less than 10 m. Where the beam is between 10 and 17 m two side girders are fitted and if any bottom shell panel has a width to length ratio greater than four additional continuous or intercostal stiffeners are fitted. The continuous centre and...
Testing of Materials
Metals are tested to ensure that their strength, ductility, and toughness are suitable for the function they are required to perform. In comparing the strengths of various metals stresses and strains are often referred to and require to be defined. 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, is often stated. The load per unit area is simply obtained by dividing the applied load by the...
Bilge Keel
Most ships are fitted with some form of bilge keel the prime function of which is to help damp the rolling motion of the vessel. Other relatively Rounded sheerstrake . m grade 'E' Deck plating Rounded sheerstrake . m grade 'E' Deck plating Bottom plating 14mm Bottom longitudinals 225mm x 11mm O.B.P Figure 17.7 Bulk carrier midship section Bottom plating 14mm Bottom longitudinals 225mm x 11mm O.B.P Figure 17.7 Bulk carrier midship section minor advantages of the bilge keel are protection for the...
End Launches
On release of a holding mechanism the launching cradle with the ship slides down the ground ways under the action of gravity. When the stern has entered the water the vessel is partly supported by buoyancy and partly by the ground ways. If this buoyancy is inadequate after the centre of gravity of the ship has passed the way ends, the ship may tip about the way ends causing large pressures on the bottom shell and on the ends of the ground ways. To avoid this the greatest depth of water over the...
Deep Tanks
Deep tanks were often fitted adjacent to the machinery spaces amidships to provide ballast capacity, improving the draft with little trim, when the ship was light. These tanks were frequently used for carrying general cargoes, and also utilized to carry specialist liquid cargoes. In cargo liners where the carriage of certain liquid cargoes is common practice it was often an advantage to have the deep tanks adjacent to the machinery space for cargo heating purposes. However in modern cargo...
Ramps
Ro-ro ships fitted with ramps usually have a stern ramp, but some vessels fitted with bow doors may also have a bow ramp which doubles as the inner weathertight door see above and is lowered onto a linkspan when the bow visor or side hinged doors have been opened. Ramps may also be fitted internally to give access from deck to deck. These can be hydraulically or mechanically tilted to serve more than one deck and can be fixed in the horizontal position to serve as decks themselves see Figure...
Gas Welding
A gas flame was probably the first form of heat source to be used for fusion welding, and a variety of fuel gases with oxygen have been used to produce a high temperature flame. The most commonly used gas in use is acetylene which gives an intense concentrated flame average temperature 3000 C when burnt in oxygen. An oxy-acetylene flame has two distinct regions, an inner cone, in which the oxygen for combustion is supplied via the torch, and a surrounding envelope in which some or all the...
Cutting Processes
Steel plates and sections were mostly cut to shape in shipyards using a gas cutting technique, but the introduction of competitive plasma-arc cutting machines has led to their widespread use in shipyards today. GAS CUTTING Gas cutting is achieved by what is basically a chemical thermal reaction occurring with iron and iron alloys only. Iron or its alloys may be heated to a temperature at which the iron will rapidly oxidize in an atmosphere of high purity oxygen. The principle of the process as...
Flat Plate Keel
At the centre line of the bottom structure is located the keel, which is often said to form the backbone of the ship. This contributes substantially to the longitudinal strength and effectively distributes local loading caused when docking the ship. The commonest form of keel is that known as the 'flat plate' keel, and this is fitted in the majority of ocean-going and other vessels see Figure 16.1 a . A form of keel found on smaller vessels is the bar keel Figure 16.1 b . The bar keel may be...
Framing
The bottom shell may be transversely or longitudinally framed, longitudinal framing being preferred particularly for vessels exceeding 120 m in length. The side shell framing may also be transversely or longitudinally framed, transverse framing being adopted in many conventional cargo ships, particularly where the maximum bale capacity is required. Bale capacities are often considerably reduced where deep transverses are fitted to support longitudinal framing. Longitudinal framing may be...
Bulkheads 1
Bulkhead spacing throughout the cargo tank space is determined by the permissible length of cargo tanks. MARPOL requires that the length of each cargo tank shall not exceed the greater of 10 metres or a length expressed as a percentage of the ship's length which is dependent on the number of longitudinal bulkheads fitted and the minimum distance from the ship's side of the outer longitudinal bulkhead. Tankers with two or more longitudinal bulkheads may have wing and centre tank lengths up to 20...
Double Bottom Structure
An inner bottom or tank top may be provided at a minimum height above the bottom shell, and maintained watertight to the bilges. This provides a considerable margin of safety, since in the event of bottom shell damage only the double bottom space may be flooded. The space is not wasted but utilized to carry oil fuel and fresh water required for the ship, as well as providing ballast capacity. The minimum depth of the double bottom in a ship will depend on the classification society's...
Construction in Tank Spaces
Ocean-going tankers have a longitudinally framed bottom shell and deck through the tank spaces. The side shell may however be either longitudinally framed or transversely framed, and the longitudinal bulkheads longitudinally or vertically stiffened in other than the larger tankers. Lloyd's Register generally requires full longitudinal framing once the vessel's length exceeds 150 m. transverse side framing Where transverse framing is adopted in smaller and medium size tankers the frames are...
Local Strengthening of Shell Plating
The major region in which the shell plating is subjected to local forces at sea is at the forward end. Strengthening of the forward bottom shell for Figure 17.3 Web frame and tank side bracket Figure 17.3 Web frame and tank side bracket pounding forces is dealt with in Chapter 16. Panting which is discussed in Chapter 8 will also influence the requirements for the scantlings and strengthening of the shell forward and to a lesser extent at the aft end. Where a ship is to navigate in ice a...
The IMO International Gas Carrier Code
In 1975 the 9th Assembly of 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 which 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 Lloyd's and other classification societies. The code covers...
Brittle Fracture
With the large-scale introduction of welding in ship construction much consideration has been given to the correct selection of materials and structural design to prevent the possibility of brittle fracture occurring. During the Second World War the incidence of this phenomenon was high amongst tonnage hastily constructed, whilst little was known about the mechanics of brittle fracture. Although instances of brittle fracture were recorded in riveted ships the consequences were more disastrous...
Hatches
The basic regulations covering the construction and means of closing hatches in weathertight decks are contained within the Conditions of Assignment of Freeboard of the Load Line Rules 1968 see Chapter 31 . Lloyd's Register provides formulae for determining the minimum scantlings of steel covers, which will be within the requirements of the Load Line Rules. Only the maximum permitted stresses and deflections of covers under specified loadings are given by the Load Line Rules. Under these...
Stern Construction
As the cruiser stern overhang may be subjected to large slamming forces a substantial construction with adequate stiffening is required. Solid floors are fitted at every frame space, and a heavy centre line girder is fitted right aft at the shell and decks. The stern plating is stiffened by cant frames or webs with short cant beams supporting the decks and led to the adjacent heavy transverse deck beam. Further stiffening of the plating is provided, or adopted in lieu of cant frames, by...
Insulation
As the steel hull structure is an excellent conductor of heat, some form of insulation must be provided at the boundaries of the refrigerated compartments if the desired temperatures are to be maintained economically. Cork, glass fibre, and various foam plastics in sheet or granulated form may be used for insulating purposes, also air spaces which are less efficient. Figure 28.2 Midship section of refrigerated cargo ship Figure 28.2 Midship section of refrigerated cargo ship Glass fibre is...
Aft End Structure
Considerable attention is paid to the overall design of the stern in order to improve flow into and away from the propeller. The cruiser stern see Figure 21.1 was for many years the favoured stern type for ocean going ships, but today most of these vessels have a transom stern see Figure 21.2 . A cruiser stern presents a more pleasant profile and is hydrodynamically efficient, but the transom stern offers a greater deck area aft, is a simpler construction, and can also provide improved flow...
Shipbuilding Steels
Steel for hull construction purposes is usually mild steel containing 0.15 per cent to 0.23 per cent carbon, and a reasonably high manganese content. Both sulphur and phosphorus in the mild steel are kept to a minimum less than 0.05 per cent . Higher contents of both are detrimental to the welding properties of the steel, and cracks can develop during the rolling process if the sulphur content is high. Steel for a ship classed with Lloyd's Register is produced by an approved manufacturer, and...
Side Doors and Loaders
Side door ramps are available for ro-ro operations and are similar to stern door ramp installations. Most side door installations, however, are intended for quayside fork lift operations with palletized cargo being loaded onto a platform at the door by the quayside forklift and stowed in the ship by another forklift truck. Instead of a loading platform on ships trading to ports with high tidal ranges a ramp onto which the quayside forklift truck drives may be fitted. Elevator platforms may be...
Watertight Doors
in order to maintain the efficiency of a watertight bulkhead it is desirable that it remains intact. However in some instances it becomes necessary to provide access between compartments on either side of a watertight bulkhead and watertight doors are fitted for this purpose. A particular example of this in cargo ships is the direct means of access required between the engine room and the shaft tunnel. in passenger ships watertight doors are more frequently found where they allow passengers to...