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2017 | Buch

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Ship Construction and Welding

verfasst von: Nisith R. Mandal

Verlag: Springer Singapore

Buchreihe : Springer Series on Naval Architecture, Marine Engineering, Shipbuilding and Shipping

Enthalten in: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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Über dieses Buch

This book addresses various aspects of ship construction, from ship types and construction materials, to welding technologies and accuracy control. The contents of the book are logically organized and divided into twenty-one chapters.
The book covers structural arrangement with longitudinal and transverse framing systems based on the service load, and explains basic structural elements like hatch side girders, hatch end beams, stringers, etc. along with structural subassemblies like floors, bulkheads, inner bottom, decks and shells. It presents in detail double bottom construction, wing tanks & duct keels, fore & aft end structures, etc., together with necessary illustrations. The midship sections of various ship types are introduced, together with structural continuity and alignment in ship structures.
With regard to construction materials, the book discusses steel, aluminum alloys and fiber reinforced composites. Various methods of steel material preparation are discussed, and plate cutting and forming of plates and sections are explained. The concept of line heating for plate bending is introduced.Welding power source characteristics, metal transfer mechanisms, welding parameters and their effects on the fusion zone, weld deposit, and weld bead profile are discussed in detail. Various fusion welding methods, MMAW, GMAW, SAW, Electroslag welding and Electrogas welding and single side welding are explained in detail. Friction stir welding as one of the key methods of solid state welding as applied to aluminum alloys is also addressed.
The mechanisms of residual stress formation and distortion are explained in connection with stiffened panel fabrication, with an emphasis on weld induced buckling of thin panels. Further, the basic principles of distortion prevention, in-process distortion control and mitigation techniques like heat sinking, thermo-mechanical tensioning etc. are dealt with in detail.
In its final section, the book describes in detail various types of weld defects that are likely to occur, together with their causes and remedial measures. The nondestructive testing methods that are most relevant to ship construction are explained. Lastly, a chapter on accuracy control based on statistical principles is included, addressing the need for a suitable mechanism to gauge the ranges of variations so that one can quantitatively target the end product accuracy.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction to Ships

Abstract

Over the years as international trade increased and also bulk transportation of goods became more and more necessary, various types of ships came into being depending on the type of cargo that needs to be carried. This trade will naturally involve very high volume of transportation of all kinds of items starting from bulk grain, ore, coal to crude, automobiles and various other kinds of farm and engineering products. Passenger ships and inter island ferries also play a very important role in transportation as well as tourism. These vessels, their outward features, i.e. the hull form may not be very different, but the internal structural arrangement will be very much dependent on the type of cargo the vessel needs to carry. The internal structural arrangement should be such that it will facilitate loading, stowage and unloading of the cargo. And needless to say, should ensure safe transportation of the cargo. It naturally implies that the structure will be able to sustain all the service loads and also at the same time the hull form should be hydrodynamically efficient.

Nisith R. Mandal

Chapter 2. Characteristics of Shipbuilding Industry

Abstract

Shipbuilding industry has a very specific character as compared to that of any other manufacturing industry. Ship building involves usage of a wide range of equipment, materials and skills. The very size of ships makes it different from other industrial products. The huge size along with the required fittings and fixtures depending on the type of the vessel, it calls for a huge manhour requirement. It calls for a very wide variety of materials to be used in ship construction. As a very wide variety of materials are used it also calls for personnel with skills of various trades. It is not a mass production item, neither a show case item. It is a case of unit production. The ship builder gets only four inputs from the customer, i.e. the type of cargo, volume or weight of cargo to be carried, ship’s route of operation and ship’s cruising speed. Based on these, the builder needs to work out the entire design, build strategy, delivery schedule and cost of the ship. The ship builder also needs to guarantee that the vessel will deliver the required speed at the given loading condition.

Nisith R. Mandal

Chapter 3. Structural Requirement

Abstract

The basic challenge faced by the naval architect is to assess the uncertain loads that are coming on the structure. The solution to the structural requirement is not unique. It will have multiple solutions. One needs to figure out which one will be the most optimum one. A ship in a seaway experiences various kinds of motions causing dynamic loading on the ship structures. Along with this it is also subjected to static loading due to the self weight of the hull structure, cargo load, loading due to various machineries and equipment, buoyancy forces, etc. Thus it becomes an extremely challenging task to work out a structural solution satisfying this complex loading condition. All the loads coming on a ship structure on a seaway may be referred to as service loads. The response to these service loads is taken care of through four distinct strength considerations, longitudinal strength, transverse strength, torsional strength, local strength. Therefore the structural requirement calls for satisfying these four strength considerations and thereby takes care of the effect of service loads.

Nisith R. Mandal

Chapter 4. Basic Structural Components

Abstract

A ship is an assembly of various types of structural components. All these structural components provide the basic strength and support to the ship’s shell structure. These components broadly fall under either longitudinal or transverse components. These can be in the form of basic structural members, e.g. deck longitudinal, side shell frame, etc. or prefabricated components, e.g. plate floor, wing tank transverse, etc. In ship construction an orthogonal form of stiffening arrangement is followed. The stiffening members are never arranged in an arbitrary fashion nor in any oblique direction. The reason being, the various structural components are prefabricated and they are subsequently aligned and welded in position. Brackets play a major role in making the connections between various components and thus providing for the load path. The primary failure cause of these brackets is buckling. To prevent such failures, flanged brackets or using curling, providing higher corrosion margin or elimination of lighting holes and various cutouts in highly loaded areas are used.

Nisith R. Mandal

Chapter 5. Structural Subassemblies

Abstract

Shipbuilding involves assembly of various pre-fabricated structural components along with installation of various machineries and fitting out items. The products of first stage of assembly are referred to as subassemblies. These subassemblies put together yields assemblies and subsequently units, blocks and finally the complete ship. In the subassembly stage either flat or curved structures are fabricated. These are essentially two dimensional or simple three dimensional structures, comprised of flat or curved stiffened panels. These flat or curved stiffened panels would be either longitudinally framed or transversely framed.

Nisith R. Mandal

Chapter 6. Structural Assemblies

Abstract

Shipbuilding is essentially an assembly industry. The steel material flows through different workstations and gradually grows in size. The structural components are assembled together through these stages of assembly. The structural assemblies dealt here are double bottom structure, wing tank, duct keel, fore and aft end structure, bulbous bow and rudder. The double bottom provides for adequate longitudinal strength to the hull girder. For convenience of cargo stowage and its unloading, wing tanks are provided in cargo holds of bulk carrier. Duct keel is a structural arrangement within the double bottom forming a tunnel all along the length of a ship. The hull girder comprises of the middle body and the two end units, namely fore end and aft end. A bulbous bow is an extension of the hull just below the load waterline. The basic purpose is to create a low pressure zone to reduce or eliminate the bow wave and reduce the resulting drag. Rudder provides the ability to steer the ship to its destination. Rudders are designed as balanced, semibalanced or unbalanced types.

Nisith R. Mandal

Chapter 7. Midship Sections

Abstract

The longitudinal strength of hull girder depends on the section modulus of the midship section. This in turn depends on the scantlings and layout of the structural members in the midship region. The midship region extends one forth length of the ship forward and aft of midship. Over this midship region the scantlings of the structural members are kept the same. Maximum longitudinal bending moment is experienced by a hull girder within this midship zone. Therefore midship section plays an important role from longitudinal strength point of view, at the same time it depicts the structural layout depending on the type of cargo the ship is going to carry. Thus different types of ships have different midship sections. The structural arrangement and their scantlings are shown in these plans. These are statutory structural plans which are to be approved by the concerned classification society prior to actual construction of ship.

Nisith R. Mandal

Chapter 8. Structural Alignment and Continuity

Abstract

A ship is assembled from various structural components. The question of structural alignment arises at the fit up stage of any production process. It can be alignment of two flat plates for butt welding, fitting of stiffeners of a stiffened panel, erection of the side shell panel above the double bottom unit or installation of the superstructure block above the main deck, etc. In all these cases, alignment of the mating edges and all the other associated stiffening members pose a serious challenge to the production team in the shop floor. This aspect of alignment is closely connected with structural continuity or discontinuity. Alignment is a production aspect whereas continuity needs to be implemented in the design stage. However they are interrelated, a structural misalignment during production will lead to a case of structural discontinuity. Thus to achieve structural continuity or to avoid serious discontinuities, precision structural alignment is a necessity.

Nisith R. Mandal

Chapter 9. Material of Construction

Abstract

Wide variety of materials is used in ship construction. For ship hull construction several materials are available, however one needs to select the one which suits the intending purpose the best. The objective is to have a product that will be economical to manufacture and maintain, should also be durable and reliable. Various characteristics of steels, marine grade aluminum alloys and glass fibre reinforced plastics as materials for ship construction have been dealt with in this chapter.

Nisith R. Mandal

Chapter 10. Steel Material Preparation

Abstract

Any fabrication activity is preceded by the steel material preparation activity. Steel plates and sections are received from the steel mills in as rolled condition. The very process of production in steel mills leads to formation of residual stress and a hard layer of oxides over the steel surface. Both this locked-in stress and the oxide layer, known as mill scale, needs to be removed before taking the plates and sections for further production operation. The handling of materials from steel mill to shipyard steel stockyard may also lead to surface deformation, hence straightening is required. Thus steel material preparation involves, straightening, stress relieving and mill scale removal. The plates are fed from the steel stockyard to the straightening machine and from there to the surface dressing station. Surface dressing is necessary to remove mill scale present on the plate surfaces. Mill scale is a layer of ferric and ferrous oxides formed on the plate surface during the hot rolling operation of the steel plates in steel rolling mills. There are various methods of surface dressing, however shot blasting and chemical pickling processes are the most efficient ones.

Nisith R. Mandal

Chapter 11. Plate Cutting

Abstract

In shipbuilding, cutting of various plate parts of varying thicknesses with accepted dimensional accuracy is required. Plate cutting is done either by mechanical means or by thermal processes. In mechanical process, cutting is done by applying a shearing force either using guillotine shear or high pressure water jet. Whereas in thermal processes, cutting is done either through oxidation, i.e. oxy-flame cutting or through fusion, i.e. plasma arc cutting or through sublimation i.e. laser cutting.

Nisith R. Mandal

Chapter 12. Plate and Section Forming

Abstract

In a ship structure the percentage of curved plates is generally not more than 15 % of the total plates used in the hull structure. Here each individual plate has a different curvature. The stiffeners are also required to be bent matching with the curvature of the plates at the respective sections. The plates in the forward and the aft end have compound curvature. Mathematically these curvatures are referred to as Non-Gaussian curvature. These are non-developable curvatures. Plate and section forming is carried out either by mechanical means or through a thermal process using line heating technique. The concept of matched die has been used in devising universal press. Here the die is made flexible, such that it can easily take the required curvature. Controlled heating is applied in line heating method along predetermined line segments. This causes differential shrinkage along the plate thickness, resulting in angular bending of the plate. By applying this bending at the required location, the target shape of the plate can be achieved. This method of line heating can be gainfully utilized using a numerically controlled heating torch to apply the required thermal load to achieve the desired target shape.

Nisith R. Mandal

Chapter 13. Fusion Welding Power Source

Abstract

In shipbuilding applications, electrical energy is most widely used in the form of arc for several welding processes and as resistance of molten slag in case of electro-slag welding. Arc welding requires a power source that can deliver electrical power at low voltage and high current such that it can establish and sustain an arc plasma column between the welding electrode and the work piece. The welding power sources can have output of alternating current or direct current. These power sources have volt-ampere characteristics typically of constant current or constant potential type. The output may also have a pulsing mode. The classification of the power sources are based on their static volt-ampere characteristics. Constant-potential power sources usually have near constant voltage output compared to constant-current sources having constant-current output. The fast response solid-state sources can provide power in pulses over a broad range of frequencies. These are known as pulsed power sources. The principal metal transfer modes in gas metal arc welding process are short circuiting transfer, globular transfer, spray transfer and Pulsed transfer.

Nisith R. Mandal

Chapter 14. Welding Parameters

Abstract

Weld quality and weld metal deposition are both influenced by the various welding parameters. These parameters are, welding current, arc voltage, welding speed, electrode feed speed, electrode extension, electrode diameter, electrode orientation, electrode polarity, shielding gas composition. Each of these parameters has influence to a varying degree on the deposition rate, weld bead shape, depth of penetration, cooling rate and weld induced distortion. To achieve a sound welded joint proper selection of welding parameters is needed. Hence a proper understanding of the effects of these parameters or process variables is required. Weld penetration in a welding process is driven by electromagnetic force, surface tension gradient, buoyancy force and impinging force of arc plasma. High temperature gradients within the molten weld pool results in variation of the surface tension over the weld pool surface. This surface tension gradient changes the convection mode of the molten metal in the weld pool. This is known as Marangoni convection. The cooling rate of the weld zone determines the metallurgical structure of the weld metal and the HAZ. As adverse microstructural transformations may take place in the HAZ, it is preferable to choose such welding parameters which also keep the HAZ to a minimum.

Nisith R. Mandal

Chapter 15. Fusion Welding Methods

Abstract

Welding is a joining process by which two separate parts can be joined to make one integral part. Ideally there should be complete continuity between the parts and the joint area should be indistinguishable from the parent metal of the individual parts. The joining of two parts can be achieved if the electrons can be shared by the atoms across the interface. This is achieved by applying external heat leading to fusion of the parts to be joined. All fusion welding processes must satisfy four basic requirements, supply of energy to achieve union by fusion, mechanism for removing superficial contamination from the joint faces, avoidance of atmospheric contamination, control of weld metallurgy to achieve desirable microstructure. This can be achieved in many ways, which give satisfactory service. Not every welding process is equally suitable for all types of metals or all types of joints. It is the knowledge base of a welding engineer which helps him to decide the appropriate welding process which will satisfy all the essential and necessary fabrication and operation requirements.

Nisith R. Mandal

Chapter 16. Solid State Welding

Abstract

The welding processes that produce sound joints at temperatures much below the parent metal melting temperature are referred to as solid state welding. Since there is no melting taking place, solid-state welded joints are usually free from all the defects that may occur in case of fusion welding, like, porosity, slag inclusion, hot cracking, undercut, etc. At the same time it does not require any shielding medium as well as any kind of filler metal. Using solid state welding, dissimilar materials can also be effectively joined which may not be possible through fusion welding. Friction stir welding is one such method of solid state welding. In the heart of this method lies a cylindrical shouldered tool with a profiled pin. The pin while rotating is inserted into the joint line between the two pieces, clamped rigidly to a base plate. The welds are created by the combined action of frictional heating and mechanical deformation due to the rotating tool. The majority of the heat is generated from the friction. It softens the material but does not attain the melting temperature. The pin traverses the entire joint resulting in the required bonding.

Nisith R. Mandal

Chapter 17. Welding Residual Stress and Distortion

Abstract

The panels, subassemblies and assemblies being welded are subjected to thermal cycles of heating followed by cooling. It causes shrinkage forces to develop in the welded panels. The shrinkage forces tend to cause different degrees of distortion. Non uniform shrinkage forces across thickness may lead to angular deformation, whereas the inplane compressive forces in plate panels made of thinner plates tend to buckle. In a welded joint, the base metal away from the weld zone remains at room temperature throughout the welding operation and is not subjected to any expansion or contraction. This ‘cold’ part of the base metal restrains the welded zone and the adjacent heated base metal from free expansion and contraction. This leads to stresses of near yield point magnitude in the weld. Under this stress the weld deposit and the adjacent heated base metal yields resulting in plastic strains. As the weld metal and the base metal cool down to the room temperature residual stresses are formed. If some of the external restraints such as clamps or welded lugs are removed, the residual stresses may find partial relief by causing the base metal to further deform.

Nisith R. Mandal

Chapter 18. Distortion Control and Mitigation

Abstract

Out-of-plane distortion results from buckling and angular deformations of panel edges as well as in the panel in between the stiffeners. The most effective way of reducing distortion is to control the formation of plastic stains produced in regions near the weld. If the appropriate distortion control is applied, the final distortion will be reduced. On the other hand improper control may lead to higher distortion. Control of distortion needs to start from implementation of appropriate design and fabrication techniques. Reducing welding heat input reduces all types of weld-induced distortions. Distortion mitigation through heat sinking involves removal of heat from the welded region such that it does not get spread in the plates away from the weld zone thus reducing residual stresses and distortion. Mitigation of buckling distortion of stiffened panels can be done by applying an active distortion mitigation technique known as Thermo-Mechanical Tensioning (TMT). In this method through the use of restraining and tensioning lugs, the tensile residual stress field generated by welding of stiffeners is reduced. This leads to reduction of the balancing compressive stress field.

Nisith R. Mandal

Chapter 19. Welding Defects

Abstract

In a welding process, there can be flaws in the welded joint. The flaws depending on their, size, location and type may be considered as defects. Once these defects are detected, remedial measures are to be implemented to remove them, as because a structure cannot be put to service with defects. Flaws are nothing but imperfections in a welded joint. Welding procedure, joint features, access and welding technique will have direct effect on fabrication imperfections. Incorrect procedure or poor technique may produce imperfections leading to premature failure in service. The majority of the defects encountered in welded structures are primarily due to improper welding procedure. Once the causes are established, the operator can easily correct the problem. Most encountered welding imperfection/defects are lack of penetration (lack of root fusion), lack of fusion, slag inclusion, undercutting, porosity, and weld cracks.

Nisith R. Mandal

Chapter 20. Nondestructive Testing

Abstract

Welding being a process of joining, the end user would like to assure himself of the quality of the joint that has been achieved though welding. There should be some mechanism which will give assurance to this aspect of no-defect in the welded joint. Hence the necessity of non-destructive testing (NDT). Through this the welded joints are inspected to determine the presence of any flaw, its type, extent and its location, such that, if it qualifies to be an unacceptable flaw, i.e. a defect, then necessary remedial measures could be undertaken to eliminate the defect. Thus it will ensure that the structural component is free from any welding defect. Various standards have been worked out by various regulatory authorities. The non-destructive testing (NDT) is carried out to verify compliance to the standards by suitable methods of examination of the surface and subsurface of the welded joint. The NDT methods that are commonly used to examine finished welds in shipbuilding are: visual, dye penetrant, magnetic particle, radiographic and ultra-sonic testing.

Nisith R. Mandal

Chapter 21. Accuracy Control

Abstract

Accuracy Control is based on the basic fact that there is no such thing as absolute accuracy. Whatever be the method of production, variations from design dimensions are unavoidable. However these variations are measurable and at the same time they are anticipated. Hence it is necessary to know the ranges of variations so that one can quantitatively target the end product accuracy. A system is needed to monitor and control the accuracy of interim products. Otherwise, work in succeeding stages of production will be adversely affected by inaccurate interim products. The reasons for dimensional variations in any work process can be attributed to either some Common Cause or some Special Causes. Variations caused by the so-called Common Causes reflect the status of accuracy level of the existing manufacturing process, which includes production process, machineries used etc. Whereas the so-called Special Cause variation points directly to some fault in the production line. For implementation of accuracy control system, it is necessary to have an accuracy control data base. This gives the statistical history of the accuracy level of the work processes employed in the shipyard.

Nisith R. Mandal

Titel: Ship Construction and Welding
verfasst von: Nisith R. Mandal
Verlag: Springer Singapore
Electronic ISBN: 978-981-10-2955-4
Print ISBN: 978-981-10-2953-0
DOI: https://doi.org/10.1007/978-981-10-2955-4

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