nm209_marine design production_v01

NM 209

Principles of Marine Design

and Production

University of Strathclyde

2013

NM209 - Principles of Marine Design and Production __________________________________________________________________________

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Contents Contents ....................................................................................................................................... iList of Figures ........................................................................................................................... iiiList of Tables ............................................................................................................................. iv1. INTRODUCTION ........................................................................................................... 1

1.1 Goal of the Shipbuilder ................................................................................................. 11.2 Key Features of the Product .......................................................................................... 11.3 The Life Cycle of Ships ................................................................................................ 21.4 Characteristics of Shipbuilding ..................................................................................... 31.5 Key Activities in Shipbuilding ...................................................................................... 41.6 The Ship Design Process .............................................................................................. 61.7 Special Characteristics of an Offshore Construction Yard ........................................... 91.8 The Offshore Design and Production Process ............................................................ 10

2. FACILITIES .................................................................................................................. 122.1 Introduction ................................................................................................................. 122.2 Production Organisation ............................................................................................. 122.3 Shipyard Layout .......................................................................................................... 132.4 Development of Shipbuilding Yards .......................................................................... 152.5 Comments ................................................................................................................... 19

3. PRODUCTION TECHNIQUES ................................................................................... 213.1 Introduction ................................................................................................................. 213.2 Straightening Techniques ............................................................................................ 213.3 Methods of Cutting ..................................................................................................... 223.4 Forming Techniques ................................................................................................... 253.5 Welding Processes ...................................................................................................... 283.6 Minimising Distortion ................................................................................................. 363.7 Materials Handling ...................................................................................................... 393.8 Outfitting - Tasks involved ......................................................................................... 42

4. THE SHIPBUILDING PROCESS ................................................................................ 474.1 Introduction ................................................................................................................. 474.2 Traditional Processes and Modern Approaches .......................................................... 484.3 An Introduction to Group Technology ....................................................................... 494.4 Work Breakdown - A Group Technology Approach .................................................. 524.5 Build Strategy and Shipbuilding Policy ...................................................................... 544.6 Relationship between Shipbuilding Policy and Build Strategy .................................. 564.7 Integrated Hull Construction, Outfitting and Painting (IHOP) ................................... 564.8 Zone Outfitting Method (ZOFM) ............................................................................... 654.9 Zone Painting Method ................................................................................................. 694.10 Pipe Piece Family Manufacturing (PPFM) ............................................................. 70

5. PLANNING, SCHEDULING AND PRODUCTION CONTROL ............................... 745.1 Introduction ................................................................................................................. 745.2 Phases or Levels of Planning ...................................................................................... 755.3 Network Analysis ........................................................................................................ 76

5.3.1 Rules for Networks .......................................................................................................... 775.3.2 Populating the Network ................................................................................................... 785.3.3 Using the Network ........................................................................................................... 78

5.4 Progress Recording ..................................................................................................... 795.5 Monitoring .................................................................................................................. 795.6 Managing Production .................................................................................................. 80


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5.7 Tools ........................................................................................................................... 805.8 Planning Data .............................................................................................................. 80

6. SUPPORT FUNCTIONS .............................................................................................. 876.1 Quality ......................................................................................................................... 87

6.1.1 Introduction ..................................................................................................................... 876.1.2 Quality Control ................................................................................................................ 886.1.3 Quality Assurance ............................................................................................................ 896.1.4 Organisation for Quality .................................................................................................. 896.1.5 The Cost of Quality ......................................................................................................... 90

6.2 Commissioning and trials ........................................................................................... 956.2.1 Commissioning ................................................................................................................ 956.2.2 Trials ................................................................................................................................ 95

6.3 Material procurement (purchasing) and control ......................................................... 966.3.1 Material Control .............................................................................................................. 976.3.2 Material Definition .......................................................................................................... 996.3.3 Material Standardisation ................................................................................................ 100

6.4 Production engineering and design for production ................................................... 1066.4.1 The Production Engineer - Why a Modern Yard needs one. ......................................... 1066.4.2 Integration of Design and Planning ............................................................................... 1086.4.3 Production Engineering and Design for Production ...................................................... 1086.4.4 Practical Producibility ................................................................................................... 110

7. SHIPBUILDING COST .............................................................................................. 1127.1 Introduction ............................................................................................................... 1127.2 Cost and Sale Price ................................................................................................... 1127.3 Components of Cost .................................................................................................. 1127.4 Stages of Cost Estimation ......................................................................................... 1167.5 Cost Estimation Spreadsheet ..................................................................................... 1167.6 Some Factors Affecting Ship Cost ............................................................................ 1167.7 Special Features of the Shipbuilding Cost Model ..................................................... 117


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List of Figures Figure 1 Classification of different ship types ........................................................................... 2Figure 2 Classification of offshore structures ............................................................................ 2Figure 3 Different stages in the Life Cycle of a Ship ................................................................. 3Figure 4 Approximate work content for different ship types ..................................................... 4Figure 5 Ship as an overall product ............................................................................................ 5Figure 6 The ship design spiral .................................................................................................. 7Figure 7 Main shipbuilding tasks ............................................................................................... 8Figure 8 Main activities in ship construction (Functional) ........................................................ 9Figure 9 Fabrication of various typical parts of the ships hull structure ................................ 11Figure 10 number of items nvolved in a typical production process of a bulk carrier ship ..... 11Figure 11 Transition from craft to mass production ................................................................ 13Figure 12 Shipyard layout ........................................................................................................ 15Figure 13 First generation shipyard layout .............................................................................. 16Figure 14 Second generation shipyard layout .......................................................................... 17Figure 15 Third generation shipyard layout ............................................................................. 17Figure 16 Fourth generaion shipyard layout ............................................................................ 19Figure 17 Shipyard layout ........................................................................................................ 20Figure 18 Roller straightener (1: lower work rolls, 2: upper work rolls, 3: back-up rolls, 4: auxiliary rolls, 5: roller table) ................................................................................................... 21Figure 19 Straightening of thin plates (1: lower work rolls, 2: upper work rolls, 3: clamping rolls, 4: auxiliary roll) ............................................................................................................... 22Figure 20 Flame planner .......................................................................................................... 24Figure 21 Profile cutting machine ............................................................................................ 24Figure 22 Roll press operations (a: sheer strake rolling, b: half-round rolling, c: 90-degrees flanging, d: bhd flanging) ......................................................................................................... 25Figure 23 Frame bender (Inverse Curve (LHS); On Beds (Ctr); Hydraulic Bending (RHS)) Eyre .......................................................................................................................................... 26Figure 24 Frame bender operation (a: bow flare bend, b: initial position, c: bilge turn bend) 27Figure 25 Curvature from line heating ..................................................................................... 28Figure 26 Types of weld joints ................................................................................................. 29Figure 27 Metallurgical zones in welding ................................................................................ 29Figure 28 Different welding processes ..................................................................................... 30Figure 29 Submerged arc welding ........................................................................................... 33Figure 30 Gravity welding machine ......................................................................................... 34Figure 31 Basic types of distortion and distortion control strategy (Conrardy et al. 1997) ..... 37Figure 32 Steelwork material diagram ..................................................................................... 40Figure 33 Typical shipyard cranes ........................................................................................... 41Figure 34 Shipyard production process .................................................................................... 43Figure 35 Shipbuilding work stages ......................................................................................... 47Figure 36 Information flow in ship Design and Production ..................................................... 48Figure 37 Flow of material in a plate/section Preparation shop ............................................... 51Figure 38 Producton facilities based on a conventional functional layout .............................. 51Figure 39 Producton facilities based on a modern Group technology layout .......................... 52Figure 40 Typical Product structure for a ship ......................................................................... 55Figure 41 Components of the IHOP (Integrated Hull Outfitting Painting) approach .............. 57Figure 42 Hull block Construction Method (HBCM) manufacturing levels ........................... 58Figure 43 Combination of semi-blocks and blocks .................................................................. 62


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Figure 44 Combination of piece parts and sub-assemblies ...................................................... 63Figure 45 Examples of steel panel lines ................................................................................... 65Figure 46 Zone Outfitting Method (ZOM) manufacturing levels ............................................ 66Figure 47 Zone Painting Method (ZPM) manufacturing levels ............................................... 69Figure 48 PPFM method details ............................................................................................... 71Figure 49 Pipe Piece Family Method (PPFM) manufacturing levels ...................................... 72Figure 50 Pipe shop diagram .................................................................................................... 73Figure 51 Steel trades manhour comparison to basic SD14 ................................................... 109Figure 52 Outfit trades manhour comparison to basic SD14 ................................................. 109Figure 53 Shipbuilding cost ................................................................................................... 113

List of Tables Table 1 Typical Applications of Welding Processes in Shipbuilding ...................................... 32Table 2 Differences Between Design and Production Information ......................................... 48Table 3 Problem areas subdivisions ......................................................................................... 54Table 4 Parts Fabrication Characteristics ................................................................................. 59Table 5 Code areas/numbers for material procurement, production control and cost control . 99Table 6 Advantages and disadvantages of standardising plates and sections. ....................... 100


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1. INTRODUCTION 1.1 Goal of the Shipbuilder The main objective of a shipbuilder is to build ships at a profit which are of high quality and fulfil the needs of transporting commodities across the world sea routes, performing a specific work or task, and moving passengers not only between different destinations but also for leisure at a price that is acceptable to the market. In order to respond to the ever-changing challenges of the shipbuilding market, the shipbuilders specific goal is defined by Kuo (1997a):

to be competitive in meeting the clients specification with solutions that are cost effective at an acceptable level of safety.

This definition implies that the success in shipbuilding depends on simultaneously meeting the following four separate sets of criteria:

clients specification competitiveness cost-effectiveness safety

1.2 Key Features of the Product A ship is a floating structure that not only supports its own weight in water, but can safely carry a payload of cargo and passengers from one port to another. Ships and marine structures are also used for the exploitation of the ocean resources. Such marine structures or vehicles are either installed at a particular place or moored at an offshore site and moved occasionally from one position to another. Most marine vehicles will have some means of propulsion such as diesel engines, gas turbines, steam engines or even sails. The propulsion system will be driving screw propellers, water jets or paddles to generate the necessary thrust to propel the vehicle. The people who operate the vehicles, i.e. the crew have to live on board and accommodation and other support facilities must be provided. Ships have various roles to perform and can be:

a) Ships for transportation b) Ships for work c) Ships for other miscellaneous work

Ship types based and their intended service are shown in Figure 1.


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Figure 1 Classification of different ship types Offshore structures have played a key role in the exploitation of energy resources from the continental shelf and the deeper North Sea (Figure 2). These structures can be of the following types:

a) Fixed structures b) Compliant structure c) Mobile structures

Figure 2 Classification of offshore structures 1.3 The Life Cycle of Ships The product life cycle of a ship consists of the following stages:

Bid preparation Preliminary design


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Detailed design / Procurement Production Operational / Service (including maintenance) Scrapping / Disposal

The main stages of the product life cycle are shown in Figure 3.

Figure 3 Different stages in the Life Cycle of a Ship 1.4 Characteristics of Shipbuilding The shipbuilding process is more often a one of a kind production and therefore has significant differences from a mass production based industry. Some of the special characteristics of the shipbuilding industry are as follows:

The client or ship owner has a relatively large influence on the production The shipbuilding process has a number of intermediate production stages that are

dependent of each other Shipbuilding has varying manufacturing principles at different stages of

production and therefore is not suitable to a single flow line type of production Shipbuilding requires a high degree of craft skill A range of different types of equipment is needed for the fabrication process The design, planning and manufacturing processes have a high degree of overlap The working environment is harsh The shipbuilder gets to know the final definition of the ship only after the contract

is signed The shipbuilder has to make important decisions during the product definition

stage based on uncertain stochastic information It takes a long time to complete and deliver the product Ships have a high product value Ships are large in size in terms of both weight and volume Ships have a long product life of around 20 to 25 years

Ship types 10 20 30 40 50 60 70 80 90 100

Product tanker

Container ship

Cruise ship

Naval ship

SteelworkHull outfit & accommodationMachinery installation & pipeworkElectrical systems installationAuxilliary machinery & systems installation

Work % performed


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The above list, though not complete, gives some of the reasons why shipbuilding does not fully adopt the manufacturing principles of mass production. Ships could be as relatively simple as a bulk carrier or as complex as a drill ship, but all include a large number of systems and sub-systems performing interrelated functions which are. These functional systems in turn require different craft skills and manufacturing principles to produce them. However, in different ship types these systems require different workloads.

Figure 4 Approximate work content for different ship types Figure 4 shows how the workload will vary in four typical ships. The various systems in a ship can be as follows:

Steel hull structure Power generation system for main propulsion Power generation system for auxiliaries Power transmission system for main propulsion Power transmission for auxiliary requirements Manoeuvring and control system Cargo handling system Systems to distribute services: sea water and fresh water steam and condensate air fuel and diesel oil lubricating oil hydraulic oil

Systems for transmission of information communication monitoring and control

1.5 Key Activities in Shipbuilding

Ship types 10 20 30 40 50 60 70 80 90 100

Product tanker

Container ship

Cruise ship

Naval ship

SteelworkHull outfit & accommodationMachinery installation & pipeworkElectrical systems installationAuxilliary machinery & systems installation

Work % performed


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On obtaining the contract, the design department proceeds with the detailed design. At this stage the designer bases his calculation on the functional system of the ship. Design and drawings are produced to satisfy the owners requirements, classification society and other statutory bodies. There is a transition state where information generated for functional groups is mapped in terms of constructional groups based on the facilities, resources, production practices and constraints for the particular shipyard. Resource requirements in terms of manpower and facilities and due date for completion are calculated for each constructional group which can in turn be related to the total material requirement and the cost of the product. The interrelationship between functional, constructional and cost/quantity for the product ship is shown in Figure 5.

Figure 5 Ship as an overall product As ships are built by assembling blocks (constructional groups) which have a reasonable content of pre-outfitting, it is important that the functional groups must be further subdivided and distributed to the individual constructional blocks. In all these activities the design department has a key role to play. According to IHIs manual, the role of the design department can be defined as Designing is the beginning and the end of production engineering (Sasaki 1988). A design department has the following four tasks:

a) Determine the shape of the ship with defined functions and acceptable performance.

b) Examine with what materials, equipment, and methods a ship can be built at a

reasonable cost while at the same time satisfying the desired functions and performance specifications. The design department should express their results in terms of engineering documents and drawings.

c) Provide the Materials Procurement Department, within a defined time schedule,

information on specifications, quantities, and delivery dates for materials. They should also supply to the manufacturing Department drawings and work instructions for the different production processes within the defined time schedules.


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d) The design department should make a systematic analysis of the differences between estimates and actual figures in terms of costs, quality and performance both during the building process and at the completion of the ship. The Department should endeavour to incorporate improvements on the basis of this experience in future building strategy.

The design engineers cannot contribute to cost reduction as long as they consider their job as simply producing drawings. The design department must take the responsibility towards cost reductions by aiming at minimising production man-hour requirements. The cost of materials can be as much as 60% of a ships cost. It is therefore vital that the design department plays a key role in the reduction of material costs. The design department should not only try to reduce the quantity of materials but also make an effort towards selecting quality materials that are affordable and easily available. The design department has an important responsibility towards providing timely and appropriate information to the Material Procurement and the Manufacturing Departments. 1.6 The Ship Design Process The design of a ship is an iterative process carried out in different stages, which may be identified as

a) Concept Design, which translates the mission requirements of the ship into its design characteristics

b) Preliminary Design / Contract Design, which provides a precise definition of ship

leading to a set of drawings and specifications forming an integral part of the shipbuilding contract, and

d) Detail Design, which delineates design details of the ship, and then develops

working drawings and work instructions for ship production, and is sometimes not regarded as a part of the basic design process (Taggart 1980).

This ship design process can be represented by the design spiral as shown in Figure 6.


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Figure 6 The ship design spiral There are several features of the ship design process and of the design information it generates which are of importance in the context of ship production:

The information produced by the design process defines the finished product: the ship.

Ship design information is arranged in terms of the functional systems and subsystems of the ship (Figure 7, 8).


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Figure 7 Main shipbuilding tasks


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Figure 8 Main activities in ship construction (Functional)

Design information is not always absolutely complete or explicit. Design information is often developed independent of the production processes

and the facilities of the shipyard building the ship, although some design for production considerations are usually involved at the design stage.

Design information does not normally contain production-related parameters such as work content which are necessary for planning, scheduling and production control.

Among the different stages of the ship design process, the detail design stage concerned largely with transforming design information into working drawings and instructions involves an overwhelming proportion of the design effort

1.7 Special Characteristics of an Offshore Construction Yard The objective of an offshore construction yard is the same as described in Section 1.1, but with certain special features. The business of offshore construction has certain special features and these are:

the structure is usually stationed at a particular offshore site for a considerable portion of its working life and therefore cannot be dry docked for periodic maintenance and repair

the steel plates used in offshore construction are of relatively higher thickness


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the top sides of offshore structures are considerably heavy than the superstructures of ships and are often manufactured and installed separately from the main structure

the structure is designed and constructed to withstand long term environmental loads

considerations for decommissioning the structure at the end of its productive life 1.8 The Offshore Design and Production Process The offshore design process is similar to that of ships. It is initiated on enquiry from governmental or industrial/commercial organisation. As an example, an offshore oil company wanting to install and commission an offshore structure for exploiting oil and gas resources from the sea initiates the process with an offshore fabricator. The next step is to analyse the problem in sufficient details so as to arrive at a clear statement of the problem. The design criteria that must be satisfied and possible materials to be used need to be identified. For an offshore structure design criteria include environmental loads (wave, wind and current), the statutory and regulatory codes of practices of the various classification societies (e.g. Lloyds Register, Det Norske Veritas, and American Bureau of Shipping) and governmental bodies. The design should also satisfy the cost constraints both for the initial fabrication and installation cost and for subsequent maintenance, inspection and repair cost. Offshore structures should be able to undergo a successful decommissioning process that satisfies the regulations of the country and the international regulatory bodies. Once the design problem has been clearly stated, several conceptual designs are identified through brain storming, literature searches and reviewing past designs. A number of such conceptual designs are analysed using engineering techniques and experience to determine their feasibility. This is an iterative process and during the course of concept formulation, the designer may eliminate one or more concepts or even add a new one that need to be analysed. This iterative process is illustrated with the help of a design spiral in Figure 6 showing the various stages in the design process of a submersible. A rational method for evaluating the different concepts should be used. The criteria for evaluating the different designs must be outlined and the performance of each design should be assessed by apportioning weightings to each criterion. The concept design thus selected can now be further developed in details with a final design description and detailed drawings required for fabrication (Figures 9, 10).


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Figure 9 Fabrication of various typical parts of the ships hull structure

Figure 10 number of items nvolved in a typical production process of a bulk carrier ship


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2. FACILITIES 2.1 Introduction There are two basic activities involved in shipbuilding; namely hull construction and outfitting. Dramatic changes have taken place over the years in the methods employed to carry out these two major shipbuilding activities. In the recent past, new and reconstructed yards have attempted to achieve an assembly line type of material flow to, and within, large assembly areas and to maintain, in turn, an unhindered and regular flow of major assemblies to the building berth. In a highly competitive shipbuilding market, it is difficult for a shipyard to specialise in a particular size and category of ship. Any yard must be able to manufacture a variety of ship sizes and types. Thus it is difficult to justify the introduction of special and expensive assembly line equipment to suit the production of a particular ship type. When looking at the layouts of different shipyards and their facilities, one finds many of them have some characteristic similarities, e.g.

Small and medium sized shipyards that are located in restricted space and have scattered building and launching facilities.

Bigger shipyards that have one or more building berths/docks and have more space available and their layout is designed for a smooth flow of materials.

2.2 Production Organisation Any production organisation can be divided into the following five categories:

a) Craft based (or job shop) organisation: Craft based production organisation uses well-trained and skilled workers to perform a wide variety of tasks in one or several locations. The work object is fixed at least for a certain time, whereas equipment and workers have to be moved to it. In such an organisation production decisions are taken by the craftsman, who may approach each job in the way he feels is best suited from his experience. Planning and control is difficult in such a production structure (Figure 11).

b) Semi-process Organisation: This form of production organisation, like the craft

based organisation, also utilises well-trained and skilled workers to carry out similar processes at specific areas. Semi-process production organisations require more planning effort for scheduling and control of the processes. Engineering information has to be more detailed to enable the planners to break up the job into suitable work packages.

c) Process (or Batch) Organisation: In process (or batch) organisation, specific areas

are used for specialised activities. Here, the equipment is fixed whereas the work object and the worker who is trained for a specific task are moved to the production equipment. Planning and control is more difficult in than craft or semi-process production organisation. The engineering information required is for the specialised task being performed on a particular piece of equipment rather than for the total product.


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Figure 11 Transition from craft to mass production

d) Product (or Group) Organisation: Product (or group) production organisation

focuses on the intermediate product and all the processes required completing it. In such an organisation the worker and his equipment is fixed and the working object moves from one workstation to another. The intermediate products produced in this manner are combined to make the final product. For such an organisation planning is simpler as the work processes are arranged logically. The engineering information needed by the worker is for his particular workstation alone.

e) Mass Production Organisation: Mass production organisation maximises the use

of mechanisation, continuous flow lines, and specialisation of activities at sequential workstations. Material handling is decided at the time of facility design. Engineering information is required for machine operation, jigs and tooling and tolerances for quality control.

2.3 Shipyard Layout In the traditional shipbuilding countries of Europe and North America, most shipyards were established over 100 years ago in locations which were suitable for building small ships and using production methods which have now been outdated. With the rapid growth in the size of ships since World War II and the changes in production technology most of these shipyards to survive had to be rebuilt, renewed or extended to allow for larger size of ships and modern production methods. The layout of the modified shipyard should be such that it allows for an easy flow of materials from one production area to another with the elimination of possible bottlenecks. Several modern shipyards have been redesigned or are in the process of redesigning their layout and the associated production facilities on group technology concepts.


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A careful examination of the potential shipbuilding market and the market share of the likely product range of ships that the shipyard intends to build is needed when considering whether to build a new shipyard or modify an existing one. Other considerations that need be examined are:

availability of vendors and subcontractors environmental considerations energy costs availability of land and its cost

Geographical and urban factors that affect the siting of a new shipyard include:

proximity of the site to a river having sufficient draught or sea with protected water front

the shipyard should be located in close proximity to highway, rail, water transport and airport

proximity to technical schools and university The following factors have to be examined when planning a new shipyard or modifying an existing one:

Size and type of ship being built Material throughput per year Material handling methods Fabrication and assembly process Size of unit in terms of weight and volume to fabricated and welded Extent of outfitting work up to the pre-launch stage Information requirements for modern production methods Administration support and necessary including planning and control

A medium size shipyard is likely to specialise in ship types over a narrow product mix and will have a fairly high throughput so that one covered building dock or partially covered building berth is sufficient. As mentioned earlier, since the capacity and annual throughput of the shipyard is based on the total market analysis, the layout of the shipyard should be examined as a total system. The individual facilities in the yard should be such that a balanced flow of materials is achieved between interim products. The following should be examined when considering the shipyards facility layout (Figure 12):

Optimising material and work-in-progress inventory by adopting just-in-time concepts. This may lead to a situation when there could be a risk that a part or component is not there when needed.

Minimise the buffer storage and marshalling areas so that the interim product from one production stage is absorbed without delay by the subsequent stage.

Materials handling both in terms of number of lift and the distance moved for all the intermediate products must be kept to a minimum.


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Figure 12 Shipyard layout 2.4 Development of Shipbuilding Yards The historical development of shipbuilding yards in Europe can be divided into four stages:

1. First Generation Shipyards - prior to World War II The first generation yards that were in existence prior to World War II had the following characteristics (Figure 13):

Component production at the steel shop was the dominant production facility with the components being directly assembled on the open building berth or next to it.

Limited facilities for storage and shops Lifting capacity limited to 5 tonnes to 10 tonnes cranes A large number of building berths Space for outfitting a number of ships in the outfitting basin Outfitting shops located near the outfitting basin All engineering work and outfitting work was performed post launch A good balance between steel (hull) work and outfitting work Wide ranging craft skills required at all stages of production Environmental protection was minimal as work was done predominantly in the open

under harsh environment


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Organisation of work was simple as little planning effort was needed Most of the work was concentrated around the building berth and the outfitting basin Ships were predominantly of riveted construction.

Figure 13 First generation shipyard layout

2. Second Generation Shipyard (post World War II till about 1960) The adoption of welding in shipbuilding and better production technology for cutting and forming led to the concept of prefabrication whereby work was shifted from the building berth to the protected environment of the steel shop and fabrication shop for the second generation shipyards (Figure 14). The other characteristics of these shipyards were:

Work organised on the basis of processes in the steel shop Sections and plates were marked, cut and formed in the shops and transported to the

slipways where they were erected directly or assembled into smaller units before erection on to the berth

Mechanisation, materials handling and environmental protection were mainly adopted at the component production stage

Work areas were defined with some flexibility Efforts for scheduling and control were applied at the component production stage

where the craft skills were less when compared with the first generation shipyards that had a wholly a craft based production organisation

An imbalance of facilities between component production and other stages such as erection and outfitting

Provision for storage and marshalling areas Better materials handling as a result of improved crane capacity Improved production methods for cutting, e.g. optically controlled oxygen-gas

cutting in conjunction with 1/10th scale lofting and better welding technology Smaller number of slip ways Little or no change in the nature of outfitting work.


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Figure 14 Second generation shipyard layout

3. Third Generation Shipyards (from 1960s to 1980s) The growth in world trade in the 1950s and 60s provided the impetus for building ships that were not only larger in size but were designed to carry specialised cargo. Shipyards tended to specialise in building one or more of the emerging ship types and planned the layout and the facilities of the shipyard so as to gain competitive edge in their chosen market segment. There was a trend towards having the organisation suited for a flow line production. Figure 15 shows the layout of a third generation shipyard.

Figure 15 Third generation shipyard layout


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The main characteristics of these shipyards were:

Work stations clearly defined and fixed Advanced technology introduced in hull production The production of intermediate modules arranged as an assembly line Scheduling and control became more important as there was greater interdependency

between the different stages of production Concepts of standardisation in were introduced There was a quantum jump in the materials handling capability, especially at the

erection stage. Cranes of 200 tonnes lifting capacity became quite common. Some shipyards were equipped with cranes capable of lifting blocks up to 1600 tonnes. Transporters capable of moving 600 tonnes block from the fabrication areas to the erection berth (or building dock) were employed.

Prefabrication shops having areas for sub-assembly, assembly and large block assembly became the focal point of shipbuilding activity in the shipyard

A large number of shipyards moved away from slipways to building docks. The panel line fabrication method was introduced for flat stiffened panels The lofting method was altered from the 1/10th scale optically controlled to

numerical controlled lofting The productivity of burning machines increased with the use of plasma-arc burners Modern semi-automatic and automatic welding methods produced better quality

welds faster. Pre-outfitting and advanced outfitting concepts introduced to correct the imbalance

between the highly mechanised hull construction and the craft based outfitting Outfitting work was planned on a zone basis so that outfit workers specialising in a

particular trade could accomplish their work in less time Covered building docks or partially covered inclined berths were built to provide

protection from the environment. The closing of the Suez Canal in 1967 resulted in crude oil carriers having to sail round the Cape of Good Hope in order to deliver crude from oil fields in West Asia to European ports. The economics of the longer route resulted in large size tankers, which came to be known as Very Large Crude Carriers (VLCCs) or Ultra Large Crude Carriers (ULCCs) with a carrying capacity of 250,000 to 450,000 tonnes of crude oil. This required a very large jump in the building capacity for the existing shipyards. Some shipyards in Europe invested huge sums of money to upgrade their facilities to accommodate these huge ships. A few overcame the problem by launching the ship in two parts and subsequently welding them together in the water. The third generation shipyards that successfully produced supertankers and large bulk carriers had a very high degree of mechanisation, materials handling and work protection resulting in extremely high throughput and productivity. These yards were very rigid and inflexible as they could only produce ships of a particular shape and type. The steep increase in the price of crude oil in the mid-1970s led to a slump in the demand for supertankers. The shipyards that had invested heavily in infrastructure and facilities could not build ships of a relatively smaller size and type and therefore had to close down or be nationalised.


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4. Fourth Generation shipyards (1980s till date) The fourth generation shipyards were the ones that avoided the pitfalls of the third generation yards building super tankers and large bulk carriers (Figure 16).

Figure 16 Fourth generaion shipyard layout The fourth generation shipyards did not go for a large materials-handling capacity and large size module fabrication, but concentrated their effort on improving the integration and management of the current production technology. These shipyards incorporated the concept of group technology at the different production stages. Block size was optimised to accommodate zone outfitting and to facilitate the erection at building berth or the building dock. The materials handling facilities and marshalling areas were designed so as to integrate with the production facilities. Such shipyards had the flexibility to build ships of different types and sizes. 2.5 Comments As the types and sizes of ships and the equipment and methods for building them have changed, the nature of the optimum shipyard layout has also changed. As a result, older shipyards have tended to become inefficient and non-competitive. Attempts to remedy this by modernising the shipyard have not always been successful because the limitations imposed by the existing site make all the necessary changes in the layout very difficult, and the mere use of modern equipment ineffective. In Japan, the usual solution has been to construct new


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shipyards for building the larger ships. To further avoid restrictions imposed by the topography of a site, most of these new shipyards have been constructed on reclaimed land. The main developments in shipyard layout may be summarised as follows:

The concentration of production into a few costly and productive work areas instead of the several less costly and less productive areas, e.g. one or two building docks instead of a large number of building berths.

Separate, specific and specialised locations for different production activities, e.g. in steel work, separate areas for plate treatment, profile treatment, each of the different production activities, flat panel assembly, web and transverse assembly, panel build up, curved panel assembly and block assembly.

Careful attention to the flow of material between work stages to simplify material handling and make it more efficient.

Integration of outfitting and engineering shops with steel shops to facilitate advance outfitting, e.g. pipe shop adjacent to the steelwork areas.

Large block storage areas alongside and at the head of a building dock or berth to facilitate advance outfitting.

Areas in between major manufacturing facilities for future expansion

Figure 17 Shipyard layout


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3. PRODUCTION TECHNIQUES 3.1 Introduction The nature of the production organisation and development of facilities and layout for a modern shipbuilding yard were discussed in the previous chapter. A number of basic facilities are needed to build ships or offshore structures, although certain specific facilities may depend on the type of ship or offshore structure being built. This chapter proceeds to examine the production methods and their associated facilities that are necessary for fabrication of steel and outfit in a shipyard under the following heads:

Straightening - Plates and Sections Cutting, Forming and Welding - Plates and Sections Handling and transportation of materials and units Outfitting - Tasks involved

3.2 Straightening Techniques The plates and sections supplied by the steel mill arrive at the shipyard usually in a deformed condition due to handling and transportation. Accurate marking and cutting is difficult on a deformed plate or stiffener. The distortion can be removed by cold or hot straightening processes. In the cold process the straightening of plates is carried out using a plate-straightening machine (Figure 18). The vertical gap between the top and bottom rollers can be adjusted by moving the top rollers up and down. The centres of the bottom rollers can be moved horizontally. A number of smaller supporting rollers are positioned around the five main rollers. The deformed plate is passed between the upper and lower rollers that are spaced according to the thickness of the plate. The centre rolls produce several bends whereas the other rolls straighten the plate. A number of passes between the rollers may be required to straighten a plate.

Figure 18 Roller straightener (1: lower work rolls, 2: upper work rolls, 3: back-up rolls, 4: auxiliary rolls, 5: roller table) The larger the deformation and the thinner the plate, the greater is the number of passes required to straighten the plate. For thin deformed sheet metal plates, straightening is done in machines having a large number of rollers with one pair acting as clamps. The cross-sectional


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view of such a roller configuration is shown in Figure 19. The cold straightening of sections can be done by one of the following processes:

Thin sections are straightened on the anvil by hammering Bending press for straightening heavier sections Straightening machines fitted with rollers that are specially configured to the size

of the section being straightened.

Figure 19 Straightening of thin plates (1: lower work rolls, 2: upper work rolls, 3: clamping rolls, 4: auxiliary roll) Deformed plates and sections can also be straightened by selective heating. The process is similar to the one used for hot forming of plates and sections and is discussed in Section 3.4(b). 3.3 Methods of Cutting Steel and aluminium plates and sections arrive at the shipyard from the factories in standard sizes and scantlings. On straightening these plates and sections have to be cut into different shapes. The cutting of plates and sections in a shipyard is performed by one of two processes: the Mechanical Cutting (cold process) and the Thermal Cutting (hot process).

1. Mechanical Cutting Mechanical cutting is employed for cutting structural plates and sections of lower thickness and for thin sheet metal. The resulting cut edges are smoother and the plates and sections have lesser distortion. Two types of machines are used for mechanical cutting in shipyards and these are:

Shearing Machines - for heavier structural members Band Saw - for lighter members and sheet metals

Heavy duty shears (also called guillotine shears) are used for cutting plates with straight edges. Press shears are used for cutting plates with curved edges of larger radius of curvature whereas rotary disk shears are used for cutting curved edges having a small radius of curvature or if the edges are of irregular shape.

2. Thermal Cutting All the hot thermal cutting processes use a heat source as a means for melting the metal and then separating them by means of kinetic energy. The source of heat could be one of the following:

oxygen - gas ; (the gas used is either acetylene or propane) plasma arc air carbon arc


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laser beam

i) Oxy-acetylene / Oxy-propane Gas Cutting: In this cutting process a torch burning acetylene or propane gas in oxygen is used to melt a narrow strip of the metal, which is then blown away by the kinetic energy of the gas stream. Mild steel plates up to 300mm thick can be cut by this process. This process is not suitable for cutting Aluminium alloy, alloy steel, stainless steel or titanium. The commonly used equipment is the hand held and manually operated oxygen gas burning torch. The quality of cut may be poor and the distortion in the plate is generally on the higher side.

ii) Plasma-arc Cutting: In the plasma arc cutting process, a high velocity jet of

ionised gas first melts the metal with a constricted arc which is then blown away by the kinetic energy of the ionised gas. This process provides a smoother cut to that obtained by the oxy-gas process. To reduce the effect of distortion, the work piece is submerged in a water bath during the cutting process. All metals can be cut with the plasma-arc cutting process. Mild steel up to 20mm can be cut faster by this process than by an oxy-gas cutting process. The Plasma-arc cutting process is however noisier and consumes more electricity resulting in a higher operating cost.

iii) Carbon-arc Cutting Process: The carbon-arc cutting process uses an arc to melt

the metal, which is subsequently removed from the kerf by a high velocity jet of compressed air. This process can cut grooves in metal of up to 16mm deep in a single pass. However, the process is noisy and expensive and therefore used mainly to rectify weld defects or to remove excess metal from a casting.

iv) Laser-beam Cutting Process: This process uses a laser light beam as a source of

heat for melting and evaporating material from the region that is being cut. Laser beam cutting process has the following advantages over other thermal cutting process:

ability to cut all metals and certain non-metals like carbon and ceramics produces a narrower kerf and heat affected zone compared with other thermal

cutting processes has relatively higher cutting speeds for plates up to 13mm thick than other

processes

The major disadvantage of the laser beam cutting process is the relatively high capital cost but also it is not effective for cutting plates of greater thickness.

Thermal Cutting Machines

Besides the hand held burning torch two thermal cutting machines are popularly used in shipyards for cutting flat plates. These are

i) Flame Planer ii) Flame Profiler


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i) Flame Planer: The flame planer is used to cut rectangular flat plates and is also

used for edge preparation for welding. The burning torch uses either oxygen-gas or plasma-arc for cutting plates (Figure 20).

Figure 20 Flame planner

ii) Flame Profiler: The flame profiler is used for cutting complicated non-rectangular shapes on a flat plate (Figure 21). Such shapes occur in floor plates, deep web frames, etc. The burning torch of the flame profiler can move in any direction on the two-dimensional plane of the plate. This machine consists of a robust portal frame that can move on rails in the longitudinal direction. One or more sets of motorised burners mounted on the portal frame can move transversely.

The movement of the burner in the two-dimensional plane of the plate can be controlled by an optical eye, which traces a 1:1, 1:5 or 1:10 drawing but is more likely to be numerically controlled. Edge preparation can also be performed while cutting by having more than one burning nozzle attached to the burner. Two sets of motorised burners are usually mounted on the frame so that two plates are cut simultaneously (one being for the port side and other, a mirror image, for the starboard side).

Figure 21 Profile cutting machine


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3.4 Forming Techniques In shipbuilding, a considerable number of plates and sections have to be formed/bent to a particular shape. As in the cutting process, the forming process used in a shipyard could either be a cold mechanical forming or the hot thermal forming (more popularly referred as the line heating process).

a) Mechanical Forming The machines used in a shipyard under this category are all hydraulically operated and are Bending Rolls, Brake Press or Gap Press and Frame Bender

1. Bending Rolls Bending rolls, similar to those described earlier for the plate straightening process, are primarily used to provide steel plates with a cylindrical or conical shape. The desired curvature on the plates can be obtained by adjusting the horizontal distance between the bottom rollers. The rolls can perform the flanging of steel plates, required for corrugated bulkheads, by fitting a flanging bar and bottom block to the top and bottom rollers respectively. Figure 22 shows the diagram of a bending roll and the cross-sectional view of the rolls fitted with flanging bar and bottom blocks for flanging of steel plates. The operation of a gap press is similar to that of a brake press except that the former is used for shaping plates of a smaller size and thickness.

Figure 22 Roll press operations (a: sheer strake rolling, b: half-round rolling, c: 90-degrees flanging, d: bhd flanging)


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2. Press: Many of the steel plates used in the forward and aft regions of the ships hull have compound curvature. Such plates can be shaped by using a brake press. Special die sets fitted on the brake press are used by the operator to get the desired shape of the plate. Knuckling is a process by which a plate is bent slightly then repositioned and then bent again till the desired angle is achieved. Brake presses also use the knuckling process to get the desired compound curvature on the plate

3. Frame Bender: Ships frames are usually fabricated from rolled sections (e.g. bulb sections, ordinary angle or T-sections). These rolled sections obtained from steel mills have to be bent to the correct shape. In shipbuilding, a hydraulically operated frame bender (for heavier sections) or a beam bender (for lighter sections) is used for this purpose. Three in-line clamps hold the initial straight frame in position. The main ram then moves the outer two clamps forwards or backwards to bend the frame to the desired shape.

Figure 23 Frame bender (Inverse Curve (LHS); On Beds (Ctr); Hydraulic Bending (RHS)) Eyre


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The section can be shifted to allow the ram force to be applied at a different position along the length of the frame. The frame bender can bend pairs of unsymmetrical sections (bulb or ordinary angle), placed back to back, so that port side and starboard side frames can be produced simultaneously. Traditionally wooden templates were used to check whether the frame had been bent to the correct shape. In modern machines, the correct curvature of the frame is checked by the inverse curve method or by numerical control (Figures 23, 24).

Figure 24 Frame bender operation (a: bow flare bend, b: initial position, c: bilge turn bend)

b) Thermal Forming (Line heating technique)

A plate will have induced stress when subjected to non-uniform heating. This in turn will produce strain, i.e. permanent deformation similar to the deformation of a plate subjected to mechanical forces. The Line Heating technique produces permanent deformation in the plate based on this principle. Heat is applied along a relatively narrow region of the plate, which is immediately cooled by water or air. The local heat thus applied causes thermal stress in a small region of the plate. The Youngs modulus and the elastic limit of the material both decrease with the increase in temperature. The narrow heated line of material is prevented from expanding by the adjacent cold material (the adjacent region can be cooled by air or water). The constrained heated region swells beyond the elastic limit perpendicular to the plane of the plate. On cooling, contraction will occur with the bulged region contracting more than the other side. This results in the plate bending and becoming concave on the side to which the heat was applied. The plate also undergoes some overall shrinkage. By making a pattern of such heat lines on the plate an experienced operator can produce controlled distortion of the plate to obtain the desired shape (Figure 25). Line heating is dependent on the following factors:

material type and thickness shape of the deformed plate heat input amount and rate cooling process employed.

The rate and amount of heat input to which the plate is subjected to depend on:

torch tip and size


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distance between torch tip and plate torch travel speed.

The extent of thermal deformation also depends on the cooling process and can be controlled by:

use of water or air rate at which the coolant is applied distance between the torch heating region and the cooling region.

Line heating is also used for fairing a plate or stiffener to the correct shape after the plate or stiffener has been given a rough shape by a mechanical forming process.

Figure 25 Curvature from line heating 3.5 Welding Processes Welding is the accepted means of joining metals in the shipbuilding industry. It can be defined as a process for joining two metal pieces wherein a heat source is used to melt the edges of the joint thereby permitting them to fuse with the molten weld metal so as to produce a joint which is as strong as, or stronger than, the parent metal.


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a) Types of Joint

The common types of welded joints in shipbuilding are (Figure 26):

i) Butt Welds: The two pieces of metal that are to be joined together are lying approximately in the same plane.

ii) Fillet Welds: The two metal pieces that are to be joined together are approximately at right angles to each other.

iii) Lap Welds: The edges of the two metal pieces to be joined together are lying one on top of the other.

Figure 26 Types of weld joints Welding by fusion creates three different metallurgical zones (Figure 27):

the fusion zone - the zone that was melted on application of heat and contains the weld metal and the parent metal

the heat affected zone (HAZ) - the region just adjacent to the fusion zone which has not melted on application of heat by the welding process

the unaffected parent metal.

Figure 27 Metallurgical zones in welding

b) Welding Processes

The welding processes can be broadly divided into the following three categories (Figure 28): i) Gas Welding ii) Electric Resistance Welding iii) Electric Arc Welding i) Gas Welding: In gas welding heat is generated by combustion of oxygen-

acetylene fuel that melts the work edges to be joined to form a molten puddle along with the filler material to fill the gaps or grooves. This process is very slow and is rarely used in shipbuilding for fabrication purposes.


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Figure 28 Different welding processes


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ii) Electric Resistance Welding: In electric resistance welding two metal pieces are fused together by the heat generated due to the resistance of the work piece to an electric current. Mechanical or hydraulic pressure must be applied to the two pieces being welded. Shipyards rarely employ electric resistance welding except for the electroslag process, described later, which can be used for joining two blocks on the erection berth by a vertical butt weld.

iii) Electric Arc Welding: In electric arc welding a high current is passed between the

electrode and the work piece being welded with a small distance separating the two. On completion of the electric circuit a high current arc is generated that produces sufficient heat to melt the edges of the work piece and the tip of the electrode so that fusion occurs between the edges of the work piece. During the welding process, the molten metal in the weld pool will have a tendency to react with the oxygen and nitrogen in the surrounding atmosphere to form oxides and nitrates. This in turn will cause a degradation of the weld quality. The electric arc welding process prevents the degradation of the weld by providing a shield of flux or gas or both around the molten weld pool during the welding process.

The common electric arc welding processes used in shipbuilding are:

Shielded Metal Arc Welding (SMAW) which could be one of the following: Manual Shielded Metal Arc Welding Semi-automatic/Automatic Submerged Arc Welding Semi-automatic Gravity Welding

One Sided Welding Flux Cored Arc Welding (FCAW) / (FUSARC) Gas Metal Arc (GMA) Welding / Metal Inert Gas (MIG) Welding Gas Tungsten Arc (GTA) Welding / Tungsten Inert Gas (TIG) Welding Electrogas Welding Electroslag Welding

In the above welding processes, shielded metal arc welding is a flux shielded process, while one-sided welding and flux-cored arc welding could be either flux shielded or gas shielded. The last four are gas-shielded processes with the electroslag welding process using electrical resistance to generate heat. In the gas shielded welding processes, the arc is shielded from the atmosphere by an inert gas such Argon, Helium or Carbon Dioxide, which is supplied externally. A summary of these welding processes and their application in shipbuilding is given in Table 1.


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Table 1 Typical Applications of Welding Processes in Shipbuilding Sl. No.

PROCESS APPLICATIONS

1 Shielded Metal Arc - Manual All position welding 2 Gravity Welding Joining pre-assembled stiffener frameworks to plating and

other horizontal and vertical fillet joints 3 Submerged Arc Welding Butt weld of flat plate panels with two side welding. 4 Flux-cored Arc Welding Outdoor work instead of submerged arc process, such as

joints of deck plating using portable equipment. 5 Gas Shielded Welding

(MIG / TIG) Short welds, joints in curved panels and similar work inside the hull, protected from wind

6 One Sided Welding Automatic or semi-automatic one-sided welding can be used for butt joints of flat panels using FCB or FAB backing. Portable equipment may be used for deck plating joints with FAB for seams and Kataflux for butts, and also for long continuous bottom shell joints. Manual one sided using special brick-backing may be used for making joints in bottom plating where these are interrupted by longitudinals and other structures, and for side shell seams and joints in vertical plating where the fit up is not very good.

7 Electrogas Welding Long vertical butt joints of the side shell plating or longitudinal bulkhead, and vertical fillet welds joining transverse bulkheads to longitudinal bulkheads or side shell plating.

8 Electroslag Welding Long vertical butt joints of side shell plating. Consumable nozzle electroslag process can be used for short vertical welds, such as joints of deck or bottom longitudinals and their junctions with transverses or bulkheads. The process is also used for heavy work such as rudders and stern frames.

c) Shielded Metal Arc Welding

The shielded metal arc welding process could be one of the following:

i) Manual Shielded Metal Arc Welding Process: This method uses an electrode consisting of a core wire of rimming steel with a flux coating around it. The flux coating is generally a mixture of mineral silicates, fluorides, carbonates, hydrocarbons, and powdered metal alloys plus a liquid binder. During the welding process, the high temperature of the generated by the electric arc causes the flux to melt and form a layer of slag over the molten pool and at the same time providing an envelope of gas around the arc. The manual shielded welding process can be used in any one of the following weld positions:

downhand vertical horizontal overhead

ii) Submerged Arc Welding: This is a semi-automatic/automatic flux-shielded electric

arc welding process used to weld two plates together by means of a butt weld. This process is always carried out in the downhand position. It uses a bare wire electrode automatically fed along with a separately fed granulated flux. During the welding process, the flux melts to provide a gas shield for the arc along with a covering of slag on the weld pool (Figure 29).


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The high heat concentration permits heavy weld deposition at relatively high speeds without air entering the weld pool. The submerged arc welding process is very efficient for straight downhand welding and can be used for single pass or multiple pass welding. The unused flux can be recovered for reuse. The main advantage of the submerged arc welding process is its speed. It can be used, for example, to butt weld 25 mm plates in one pass on either side at speeds of 1.3 - 1.6 metres per minute. The cost of consumables is lower than in the manual metal arc welding process, but because the equipment is relatively costly, the process is economical only with high equipment utilisation. The portable submerged arc-welding machine takes some time to set up and hence its use is restricted to long butt joints or long continuous fillet welds. The submerged arc welding process also requires an accurate edge preparation and good fit up. In joints where the root gap is not uniform, an initial run or two may be done by a manual welding process before the submerged arc welding process is used to complete the joint.

Figure 29 Submerged arc welding

iii) Gravity Welding: The gravity welding method is a semi-automatic flux-shielded electric arc welding process typically used in shipyards for welding stiffeners to plates by means of fillet welds (Figure 30). The machine consists of a tripod, one leg of which acts as a rail for the sliding holder of the flux coated electrode. The electrode is positioned and the electric arc is struck. The weight of the electrode and the holder causes it to slide down the rail and deposit the weld along a straight line. The angle of sliding will determine the amount of weld metal deposited. The machines are provided with a mechanism to break the arc after a


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particular length of welding is performed. The machines can be moved from one position to another and several such machines can be operated by a single operator, who need not be a skilled welder. The consumption of electrodes is about 10% less than in the (manual) shielded metal arc welding process. One advantage of gravity welding is that welding over paint primer does not cause porosity, a defect that occurs if submerged arc fillet welding is used.

Figure 30 Gravity welding machine

d) Flux Cored Arc Welding (FCAW)

The flux-cored arc welding process uses a continuous flux coated electrode. It is a mechanised process wherein the electrode wire fed from a reel is coated with a suitable flux to produce a slag covering the weld pool. The process, which is suitable for outdoor welding, is faster than the manual submerged arc welding process but is slower than the submerged arc welding process.

e) Gas Metal Arc (GMA) Welding / Metal Inert Gas (MIG) Welding The metal inert gas welding process is used mainly with semi-automatic welding techniques for producing butt and fillet joints involving longitudinals, floors, girders and brackets. The electrode is continuous to allow for a longer duty cycle. The weld pool is shielded by CO2 gas when using the spray transfer and the dip transfer welding process to deposit the weld metal. A gas shield of Argon or Helium gas is preferred when using the pulsed arc transfer process. When welding mild steel, a shielding gas of CO2 is more economical than using Argon or Helium as gas shields. The disadvantage of this process is that it requires trained welders to perform the task. Also, when welding outdoors, wind and draught may cause porosity in the weld. To avoid porosity in the weld, the semi-automatic no gas process is used. In this process a self-shielded flux


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cored electrode is obtained from a wire fed gun. The flux decomposes in the arc to provide a gas shield. Accurate joint preparation is required to use this process.

f) Gas Tungsten Arc (GTA) Welding / Tungsten Inert Gas (TIG) Welding

In this process an arc is generated between the work piece and a tungsten electrode which is not consumed during the welding process. The arc and the molten weld pool are protected from the atmosphere by an inert gas shield of either Argon or Helium. An externally fed filler rod may supply the filler metal to the molten weld pool. The non-consumable tungsten electrode is provided with a water cooling system. The arc from the tungsten electrode melts the work pieces to fuse together. A cleaner weld is produced as filler metal is normally not used. The tungsten inert gas welding process is used in shipyards and offshore yards for welding sheet metal, smaller diameter pipes and to provide the root pass on a multiple pass weld for larger diameter pipes. It is also a process suitable for welding aluminium.

g) One Sided Welding This electric arc welding process can be either flux or gas shielded. Here a back or sealing run is not required. The plates to be welded are brought together and the weld made from one side with flux or refractory backing material on the other side. There are several types of backing: water-cooled copper plate, flux copper backing (FCB), flux asbestos backing (FAB), and resin-bound sand (Kataflux). The welding can be done manually or automatically using the same equipment as in conventional two sided welding. Accurate edge preparation is desirable to avoid over-penetration, which causes erosion of the backing material. The advantages of one side welding are that it reduces handling - it avoids the need to turn the plate in a panel line - less time is required to make a joint since the back of the weld does not require dressing, accurate joint preparation is not absolutely necessary and the fatigue strength of the joint is better than with two sided welding. Among the disadvantages are the cost of flux backing materials, the necessity to start and finish the weld on a run-off plate and a greater risk of distortion.

h) Electrogas Welding

Electrogas welding is an electric arc process for the automatic vertical butt welding of blocks on the erection berth. In this process, the weld pool is held in place by copper shoes and shielded by carbon dioxide fed through holes in the shoes. The welding equipment consisting of the filler wire on a reel, the feed motor, copper shoes, shield gas supplying device, and control equipment, is carried in a cage which is hoisted up automatically as the welding progresses. The process can cope with small amounts of curvature in the plating, and with small inclinations from the vertical due to declivity. It can also tolerate small variations in the edge gap. Electrogas welding machines have been developed for use on vertical welds inside the hull, as well as outside. The equipment can be set up quickly, is compact and easy to transport and store, and can be operated by one man. The advantages of the electrogas welding process are that it is five or six times faster than the manual metal arc-welding process, and is economical in the use of consumables. Compared to the electroslag welding process (another automatic vertical butt welding process and described subsequently), electrogas gives better


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metallurgical properties and is easier to start, and to restart in the middle of a weld. The disadvantages of the electrogas welding process are that it is adversely affected by wind and draughts and can only be used economically for long welds in thick plates.

i) Electroslag Welding

Another automatic process for the vertical welding of butt joints is the electroslag process which uses the principle of electric resistance heating. Heat is generated by resistance heating within a bath of molten slag on the top of the weld pool held in place by copper shoes. Normally, a square edge preparation is used, and the weld metal is provided by feeding a solid wire into the slag bath. The welding head moves up along the weld automatically on a carriage moving in a tower. The electroslag welding equipment takes a long time to set up, so that its use is only economical for plate thicknesses over 15 mm and weld lengths over 3.5 metres. For shorter joint lengths of thick plates, which would require a large number of runs by a manual process, the consumable nozzle electroslag welding process is used. Electroslag welding is much faster than manual welding, gives welds of better appearance, and does not require accurate joint preparation. Its main disadvantages are poor notch toughness in the weld, and restarts are difficult if the process accidentally stops in the middle of a weld.

3.6 Minimising Distortion

In the modern yard, ships are built by assembling large blocks at the building dock or berth. The blocks or units in turn are assembled from smaller assemblies and sub-assemblies. Such production methods involve:

sub-assembly of plates and stiffeners assembling the piece parts and the sub-assembled parts into assemblies and into larger

block assemblies. erection of assemblies and blocks

It is obvious that each stage of the assembly process introduces welding stresses and strains which result in shape distortions. The cumulative effect of all these distortions can result in a significant amount of re-work. Any readjustment and rework at the later stages costs the shipyard both in terms of time and money. Over the years, shipbuilders have come to expect some degree of distortion and have learned to live with the consequences. At the panel fabrication and block erection stage, shipyards employ the following practices to correct inaccuracies in the shape of the panel or blocks:

Flame straightening to remove distortion. Stiffener ends are left unwelded to aid in fitting. Panels are made slightly over-size and are subsequently trimmed to shape.

In recent years, ship panel distortion has become a major problem with the increasing use of thin plates (plate thickness of less than 10 mm) in panel fabrication. The significantly increased distortion has resulted in a large increase in man-hours for fitting, flame straightening, and rework following flame straightening. Shipbuilders are generally concerned with two forms of thin panel welding distortion (Conrardy et al. 1997):


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* In-plane distortion

In-plane distortion is the shrinkage in the overall panel dimensions. This type of distortion is not helpful for producing neat cuts at the fabrication stage.

* Out-of-plane distortion Out-of-plane distortion is the edge waviness of the fabricated panels. This type of distortion requires flame straightening and needs extra man-hours for fitting.

The in-plane and out-of-plane distortions are not mutually separate. Some of the in-plane shrinkage observed in ship panels is a due to the out-of-plane distortion. This is because a deformed plate is shorter than a flat plate. Also, flame straightening to correct out-of-plane distortion, if done prior to panel/block erection will impart additional in-plane shrinkage for thin panels.

Figure 31 Basic types of distortion and distortion control strategy (Conrardy et al. 1997)

a) Types of Welding Distortion The basic types of welding distortion occurring in ship panels are (Figure 31):

Transverse shrinkage Longitudinal shrinkage Rotational distortion Angular distortion Longitudinal bending distortion Buckling distortion

In thin panels, all of the above forms of distortion are likely to occur. However, studies


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carried out by Conrardy et al. (1997) suggest that buckling is the primary contributor to thin panel distortion. The buckling is due to the longitudinal shrinkage forces that are generated as a result of reaction to the residual tensile stress during the welding of longitudinal stiffeners. Buckling occurs when this compressive load exceeds the critical buckling load for the panel and the system becomes unstable. The critical buckling load for a panel depends on the stiffness of the panel which in turn is a function of the panel plate thickness, the panel size and the spacing of the stiffeners. A recommended distortion control strategy for thin panels proposes is to first eliminate the buckling distortion arising from welding longitudinal stiffeners and subsequently control the angular distortion resulting from the welding of both longitudinal and transverse stiffeners. In this respect, the techniques available for reducing buckling distortion include the following:

Modifying panel design - This can be best achieved by increasing the thickness of the plate and reducing the spacing of the longitudinal stiffeners.

Reducing welding heat input - Implementation of low heat input welding process

will not only reduce buckling distortion but also angular distortion.

Using intermittent welding - Significant reductions in panel distortion can be achieved by replacing continuous welds with intermittent welds having the same total strength. However, concern regarding fatigue and corrosion problems and the fact that intermittent welding is not conducive to welding automation has been a major hindrance to this technique of distortion control.

Using an egg-box method of construction - In the egg-box method of construction,

first the longitudinal stiffeners are welded to the transverse stiffeners. Then, the stiffener assembly is welded to the assembled plate. This technique is useful as a distortion control technique because the stiffener assembly is rigid and partitions the plate into small areas which have a higher resistance to buckling.

Weld quenching - This technique is debatable and shows more promise for

distortion control of aluminium alloys than for steels. Though there is a significant reduction in buckling distortion as a result of quenching (application of a water spray on the back side of the plate at the stiffener location), under certain conditions cracking can occur if the weld experiences excessive cooling rates.

Thermal tensioning - In thermal tension

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