Marine engineering technology, Exams of Engineering

Download Marine engineering technology and more Exams Engineering in PDF only on Docsity! 1 EMR 2304 MANUFACTURING TECHNOLOGY IN MARINE ENGINEERING Prerequisite: Introduction to Ship Technology Purpose The aim of this course is to enable the students to: 1. Understand applications of CAD/CAM in design and production processes 2. Understand basics of robotic and their application in manufacturing 3. Understand what a production facility and building cycle is in relation to work layout, material handling and storage 4. Understand design considerations for ship production, launching, float out and inspection Course objectives At the end of this course, the student should be able to; 1. Relate specific design and production tasks to appropriate CAD/CAM applications 2. Appreciate the application robotics in manufacture 3. Prepare a production plan applying building cycle in selecting work layout, material handling and storage. 4. Apply design considerations for ship building, launch, inspection and trials Course Description Application of CAD/CAM: architecture of mainframe systems. Applications to production planning and control, schedules, capacity estimation, process planning design, control of machine tools and manufacturing processes. Introduction to robotics. High power laser technology. Primary functions of a production facility and building cycle. Definition of piece parts, manufacturing of semi-fabricated materials. Steel works production machines, processes and technique. Assembly of hierarchy of components. Dimensional accuracy and quality control/assurance. Principles of work layout including work station philosophy and constraint on site. Materials handling and intermediate storage policies within plant. Inter-relation between construction technique and assembly plant. Design for production. Group technology: outfitting and machinery production activities. Launching and float out considerations. Ship and rig build strategy. Inspection and trials. Course text books 1. John W. and Gary W (2004), Design of Marine Facilities for the Berthing, Mooring and Repair of Vessels, American Society of Civil Engineers, 2nd Ed. 2. Gregory P. Tsinker (2004), Port Engineering Planning, Construction, Maintenance & Security, John Wiley & sons 3. Serope K. & Steven S. (2009), Manufacturing Engineering and Technology, Prentice Hall, 6th Ed. References 1. Ben C. Gerwick, Construction of Marine and Offshore Structures, 3rd Ed. 2. T. Graczyk, T. Jastrzebski and C.A. Brebbia, (1999) Marine Technology III (Marine and Maritime Vol 1). WIT Press. 3. T.J Wang and Alan Pillay, (2003) Technology and Safety of Marine Systems (Ocean Engineering Series), Elsevier Science. 2 Chapter 1 SHIP DESIGN 1.1 BASIC DESIGN OF THE SHIP The economic factor is of prime importance in designing a merchant ship. An owner requires a ship which will give him the best possible returns for his initial investment and running costs. This means that the final design should be arrived at taking into account not only present economic considerations, but also those likely to develop within the life of the ship. With the aid of computers it is possible to make a study of a large number of varying design parameters and to arrive at a ship design which is not only technically feasible but, more importantly, is the most economically efficient. 1.1.1 Preparation of the Design The initial design of a ship generally proceeds through three stages: 1) Concept 2) Preliminary 3) Contract design. Concept stage: A concept design should provide sufficient information for a basic techno- economic assessment of the alternatives to be made. Economic criteria is used to measure profitability, discounted cash flow or required freight rate. Preliminary stage: Preliminary design refines and analyses the agreed concept design, fills out the arrangements and structure and aims at optimizing service performance. At this stage the builder should have sufficient information to tender. Contract stage: Contract design details the final arrangements and systems agreed with the owner and satisfies the building contract conditions. Total design is not complete at this stage, it has only just started, post contract design entails in particular design for production where the structure, outfit and systems are planned in detail to achieve a cost and time effective building cycle. 1.1.2 Information Provided by Design When the preliminary design has been selected the following information is available: 1. Dimensions, 2. Displacement, 3. Stability 4. Propulsive characteristics and hull form, 5. Preliminary general arrangement 6. Principal structural details Dimension: The dimensions are primarily influenced by the cargo carrying capacity of the vessel. In the case of the passenger vessel, dimensions are influenced by the height and length of superstructure containing the accommodation. Breadth may be such as to provide adequate transverse stability. A minimum depth is controlled by the draft plus a statutory freeboard. Many vessels are required to make passages through various canals and this will place a limitation on the dimensions. The Suez Canal has a draft limit, locks in the Panama Canal and St. Lawrence Seaway limit length, beam and draft. 5Figure 1-2 Classification of Ship types After Perpendicular (AP): A perpendicular drawn to the waterline at the point where the aft side of the rudder post meets the summer load line. Where no rudder post is fitted it is taken as the center line of the rudder stock. Forward Perpendicular (FP): A perpendicular drawn to the waterline at the point where the foreside of the stem meets the summer load line. Length Between Perpendiculars (LBP): The length between the forward and aft perpendiculars measured along the summer load line. Amidships: A point midway between the after and forward perpendiculars. Length Overall (LOA): Length of vessel taken over all extremities. Base Line: A horizontal line drawn at the top of the keel plate. All vertical moulded dimensions are measured relative to this line. Moulded Beam: Measured at the midship section is the maximum moulded breadth of the ship. Moulded Draft: Measured from the base line to the summer load line at the midship section. Moulded Depth: Measured from the base line to the heel of the upper deck beam at the ship’s side amidships. Deadweight: Deadweight is the difference between the lightweight and loaded displacement, i.e. it is the weight of cargo plus weights of fuel, stores, water ballast, fresh water, crew and passengers, and baggage. Lightweight. The lightweight is the weight of vessel as built, including boiler water, lubricating oil, and cooling water system. Freeboard: The vertical distance measured at the ship’s side between the summer load line (or service draft) and the freeboard deck. Tonnage: Gross tonnage is a measure of the internal capacity of the ship and net tonnage is intended to give an idea of the earning or useful capacity of the ship. Various port dues and other charges may be assessed on the gross and net tonnages. 1.3 Development of Ship Types 6 1.3.1 Dry Cargo Ships Figure 1-3 Development of dry cargo ships The first steam ships followed in most respects the design of the sailing ship having a flush deck with the machinery openings protected only by low coamings and glass skylights. At quite an early stage it was decided to protect the machinery openings with an enclosed bridge structure. Erections forming a forecastle and poop were also introduced at the forward and after end respectively for protection. This resulted in what is popularly known as the ‘three island type’. A number of designs at that time also combined bridge and poop, and a few combined bridge and forecastle, so that a single well was formed. Another form of erection introduced was the raised quarter deck. Raised quarter decks were often associated with smaller deadweight carrying vessels, e.g. colliers. With the machinery space aft which is proportionately large in a small vessel 7 there is a tendency for the vessel to trim by the bow when fully loaded. By fitting a raised quarter deck in way of the after holds this tendency was eliminated. A raised quarter deck does not have the full height of a tween deck, above the upper deck. Further departures from the ‘three island type’ were brought about by the carriage of cargo and cattle on deck, and the designs included a light covering built over the wells for the protection of these cargoes. This resulted in the awning or spar deck type of ship. At a later date what are known as open/closed shelter deck ships were developed. Open shelter deck vessels were popular with ship-owners for a long period. However, during that time much consideration was given to their safety and the undesirable form of temporary openings in the main hull structure. Originally the machinery position was amidships with paddle wheel propulsion. Also with coal being burnt as the propulsive fuel, bunkers were then favorably placed amidships for trim purposes. Taking the machinery right aft can produce an excessive trim by the stern in the light condition and the vessel is then provided with deep tanks forward. This may lead to a large bending moment in the ballast condition, and a compromise is often reached by placing the machinery three-quarters aft. That is, there are say three or four holds forward and one aft of the machinery space. In either arrangement the amidships portion with its better stowage shape is reserved for cargo, and shaft spaces lost to cargo are reduced. 1.3.2 Bulk Carriers A series of bulk carriers were built for ore carrying purposes. The ore carrier were built with a double bottom and side ballast tanks in 1917, only at that time the side tanks did not extend to the full hold depth. Figure 1-4 Bulk Carriers To overcome the disadvantage that the ore carrier was only usefully employed on one leg of the voyage the oil/ore carrier also evolved at that time. The latter ship type carries oil in the wing tanks as shown in Figure 1-4(c) and has a passageway for crew protection in order to obtain the deeper draft permitted tankers. The common general bulk carrier takes the form shown in Figure 1-4(d) with double bottom, hopper sides and deck wing tanks. These latter tanks have been used for the carriage of light grain cargoes as well as water ballast. 10 up 11 to 50 knots many craft are of twin-hull form and include conventional catamarans, wave piercers with twin hulls and a faired buoyant bridging structure forward also small waterplane twin hulled (SWATH) ships. The latter have a high proportion of their twin-hull buoyancy below the waterline and very narrow twin-hull beam at the waterline Other high speed craft include hydrofoils, and various surface effect ships (SESs) including hovercraft which maintain a cushion of air, fully or partially, between the hull and the water to reduce (drag). 11 Chapter 2 COMPUTER BASED TOOLS The shipbuilding industry has used computer-based tools since the early 1950s, initially in accounting, and expanding during the early 1960s to certain design and fabrication activities, then by the early 1970s to the first CAD and CAM turnkey commercial systems. Computer based ship design and construction is an important aspect of making a shipyard more competitive in the world commercial market. Modern computer-aided systems can help address inefficiencies such as the following; 1. Multiple systems used within a single discipline, necessitating the storage of the same data in different places. Integration of work and ensuring consistency are difficult. 2. 2D drafting systems, causing difficulties when proceeding to the actual 3D ship design. 3. Separate hull and outfit designs, making integration of the final design and inclusion of future changes difficult and open to errors and no integrated planning during design. 4. Aging of the skilled workforce and difficulty in finding young workers willing to work in the traditional dirty, difficult and dangerous shipbuilding environment. 5. Inability to meet ever increasing demands by owners for ships of higher quality and shortened delivery times. Advantages to a shipyard using computers in ship design and construction include the following: 1. Quicker response to requests for quotes and shorter design and construction lead times, 2. Increased accuracy, 3. Availability of a reference database, 4. Availability of a product model to enhance concurrent engineering and production planning activities, 5. More flexibility in making design modifications, 6. A more controlled environment to help support standardization, 7. Improved cost control, 8. Elimination of many tedious manual and repetitive calculations, 9. Less rework in production, 10. Less skilled labor needs in production, 11. Storage of lifecycle data for the ship, and 12. Configuration arrangement of changes through design and life of the ship. 2.1 Computer Aided Design (CAD) Computer-Aided Design (CAD) is a direct outgrowth of the traditional drafting board approach to ship design. CAD depicts geometry and dimensions on a computer monitor and not directly on paper, though an important output of CAD is still paper drawings. Sophisticated CAD systems are much more powerful than computer versions of drafting boards. They may have extensive parts libraries, cut-and-paste capabilities, and efficient, menu-driven user interfaces. Stand-alone CAD is most appropriate for relatively simple designs such as those developed in the smaller shipyards. For more complex designs, CAE and product model programs are more appropriate. 14 10. Structure: Analysis of strength in smooth water and in waves and (for contained liquids) hydrostatic loading; optimization of weight, vertical center of gravity optimization; cost optimization; fatigue analysis; shock analysis; oil-canning calculations; and predictions of natural and forced vibration frequencies. Data may be presented in static and animated multicolor 3D models that show stresses, adequacy parameters and displacements of affected structure. Figure 2-2 shows such a structural model. 11. Maneuvering and control: Calculations are carried out of rudder geometry and ship maneuverability and control characteristics. Included is consideration of force, moment and motion in the horizontal plane for surface ships and the same considerations in three dimensions for submarines. 12. Propeller: Propeller selection, geometry and calculation of the propeller characteristics, such as thrust. 13. HVAC: Flow, heating and cooling calculations are carried out to help size fans, ducting and other HVAC components. 14. Launching: Calculations for launching over an inclined slipway may include (in a stepwise fashion) ship position, buoyancy, reaction of ground ways and rising and tipping moments. Static and dynamic stability and longitudinal strength may be calculated at the pivoting point and for the ship afloat. 15. Seakeeping: Ship motions in a seaway are predicted in six degrees of freedom moving forward at a fixed speed. Strip theory is typically used, with roll damping of appendages taken into account. Maneuvering with rudder is calculated. Figure 2-2 shows a visualization of a seakeeping analysis of a ship in oblique seas. 16. Noise: Airborne and waterborne noise levels are calculated for noise sources located in the ship. Effects of noise treatments such as isolation mounts and enclosures are calculated. In a class by itself are computer programs for the initial design and cost estimation. These programs are usually parametric, and produce their technical and cost estimates based on historical data. Some are quite sophisticated, with many input parameters. Their accuracy depends upon the validity of the parametric relationships, and they are useful only within their range of historical data. These programs are used to produce initial designs for trade-off analysis, and for quick initial response to ship owner inquiries. Figure 2-2 FEA Model, Seakeeping Visualisation and Ship Weight Distribution Visualisation 15 Examples of CAE programs that are in operation are; 1. BASCON - Korea Research Institute of Ships and Ocean Engineering, Korea. Integrated system to develop ship concept designs. 2. NavCad - Hydrocomp, Inc., United States. Resistance and power predictions and optimum propeller determinations. 3. GHS - Creative Systems, Inc., United States. Determination of ship hydrostatics, stability and longitudinal strength. 4. HICADEC-P - Hitachi Zosen, Japan, and Odense Steel Shipyard, Denmark. Pipe systems calculations, such as pressure drop. 5. HFDS (Hull Form Design System) - United States Navy. Develops predictions for powering, seakeeping, maneuvering and stack design through series data, parametrics and computational fluid dynamics. 6. MARINE (Mitsubishi Advanced Real-time Initial design and Engineering system) Mitsubishi Heavy Industries, Japan—Carries out initial design, naval architecture and ship performance calculations to support rapid response of marketing and proposal efforts. 7. MAESTRO—Optimal Structural Design, United States. Structural design, analysis and optimization program tailored to stiffened thin skin structures of ships. 8. NASTRAN—National Air and Space Administration (Original Version), United States. General purpose FEA program that may be used for ship structural analysis (12). 9. POSEIDON—Germanischer Lloyd, Germany. Software to develop structural design from a rules-based or rational (FEA) approach to aid in the classification process (27). 10. Safe Hull—American Bureau of Shipping, United States. Rationally-based FEA program to verify yielding, buckling and fatigue strength of ship structures (28, 29). 11. Ship Weight - BAS Engineering, Norway - estimates and follows up (during construction or design changes) weight and center of gravity of a vessel (30). A screen capture of this program is shown as Figure 13.6. 12. Weight prog - Germanischer Lloyd, Germany. Estimates steel and light-ship weights (31). 13. Vision (Virtual Integrated System for Shipbuilding Innovation), was developed by NAMURA Shipbuilding of Japan to respond quickly to inquiries from ship owners (32). 2.3 Computer Aided Manufacturing (CAM) Computer-Aided Manufacturing (CAM) programs help bridge the gap between ship design and construction. CAM programs develop data for use in areas such as welding, cutting, lifting, bending, forming, planning, and monitoring. Typical CAM systems have the following capabilities; 1. Accounting for weld shrinkage: Automatic calculations are made (and avoidance instructions may be developed) for angular distortion and buckling of plates (especially thin plates, 10 mm) caused by gas cutting and by welding stiffeners and other structure to a plate. 2. Dimensional control: Important dimensions for hull and outfit interfaces are monitored with technologies such as infrared and photogrammetry. 16 3. Interface between product model and robots: Data involving geometry, welding, cutting, assembly, testing and painting are transmitted from the product model to open 19 and production control, all using a common database. The most advanced shipyards today operate in an interfaced, but not totally integrated, CIM environment. A major objective of CIM is to minimize redundant operations within and between computer programs, particularly with regard to manual data input. CIM systems have some or all of the following capabilities; 1. Integration: The hallmark of CIM is a high level of communication and information management within and between technical and administrative programs and maintaining the information on a common database. 2. Management: Management is enhanced through increased capabilities in communication, tracking and reporting, within the shipyard and with customers, regulatory bodies and vendors. 3. Material Control: This applies to hull and outfit, at all stages of design and production, and may include procurement and inventory control and marking. 4. Scheduling: Schedules may be developed and modeled for overall ship construction purposes, management tracking and shop floor use. Graphical presentation is typical. By using the CIM context, scheduling may be made more efficient than when it is carried out as a separate function. 5. Production Planning: Included is consideration of time, resources, cost estimation, shop areas, and tracking by trade. Presentations may be graphical, especially for activity planning and detailed resource and workshop planning. 6. Production Automation: Automation through production- oriented data that is used in automated process equipment, including robots for processes such as cutting, welding and painting. 7. Purchasing: Regarding vendors, ship material and equipment specifications and purchase orders may be directly transmitted between yard and vendor. In addition, initiatives are being carried out with an aim to improve shipyard/ vendor communications and to establish strategic relationships. Such supply chain integration has been extremely successful in the automotive industry and steps are being taken in this direction by the United States aircraft industry. Potential payoffs include cost reduction and shorter cycle times. 8. Data States: Data states may be associated with each part or component in a ship during the course of a project. During design, the data state may move from conceived, to decided (by designer) to broadcast (for review), to approved (by project management). Once approved, the data state may be on hold, or it may progress to planned (purchase and installation), to implemented (installed), to tested, and finally, to as-built. Typically, CIM programs comprise interfaced combinations of stand-alone programs. Examples of such programs include interfaced combinations of programs described in preceding sections as well as the following: 1. MHI’s CIM - Mitsubishi Heavy Industries, Ltd., Japan - an interfaced combination of MARINE, selected CAE systems, MATES, Factory Automation/Robotics systems, DAVID, and Production Management System. 2. SUMIRE—Sumitomo Heavy Industries, Ltd., Japan—an interfaced combination of conventional CAE systems, SUMIRE-VPS, Basic Design System, Steel Material Procurement System, SUMIRE-H, CAM systems, SUMIREF, Production Planning System, and fitting and equipment procurement systems. 20 3. MACISS (Mitsui Advanced Computer Integrated Shipbuilding System)—Mitsui Engineering and Shipbuilding Co., Ltd., Japan—addresses hull design, outfitting design, assembly procedures, scheduling of jobs, process control and distribution control of parts and components. 4. IHI’s CIM—Ishikawajima Harima Heavy Industries Co., Ltd., Japan—composed of four major subsystems: AJISAI (Advanced Joint less Information System by Assimilation and Inheritance), PE (Production Engineering), KLEAN (Kure LEAN production scheduling) and the FA (production data information system for Factory Automation). Chapter 3 HIGH POWER LASER TECHNOLOGY LASER is an acronym for Light Amplification by Stimulated Emission of Radiation which describes the theory of laser operation. Albert Einstein published the theoretical basis for the laser in 1917, but it was only in 1960 that the first functioning laser was constructed by Theodore Maiman in California, using a ruby crystal to produce laser light. 21 3.1 Components of a Laser 1. A lasing medium or “gain medium”: May be a solid (crystals, glasses), liquid (dyes or organic solvents), gas (helium, CO2 ) or semiconductors (GaAs- Gallium Arsenide, AlGaAs- Aluminium Gallium Arsenide, GaP-Gallium Phosphate, InGaP-Indium Gallium Phosphate, GaN-Gallium nitride, InGaS, GaInNaS, InP, GaInP). 2. An energy source or “pump”: May be a high voltage discharge, a chemical reaction, diode, flash lamp or another laser. 3. An optical resonator or “optical cavity”: Consists of a cavity containing the lasing medium, with 2 parallel mirrors on either side. One mirror is highly reflective and the other mirror is partially reflective, allowing some of the light to leave the cavity to produce the laser’s output beam – this is called the output coupler. Figure 3-1 Componentsof a Laser 3.2 Laser Operation The laser active medium (e.g. a crystal) is excited with a flash lamp (optical pumping), its atoms absorb the optical energy and the electrons move from a so-called “ground state” energy level to a high energy “excited state”. This electron can randomly return to its low energy “ground” state level by releasing a photon of light. This process is called spontaneous emission. When an electron is still in the excited high energy state, and another photon passes nearby, it can cause the excited atom to emit a photon of the same frequency, in the same direction and in phase with the incoming photon. This process is called “stimulated emission”. When the external excitation is sufficient so that the majority of atoms are in the excited state, any photon emitted within the active medium will stimulate other excited atoms to emit. The laser cavity serves the purpose of bouncing back and forth only those photons that propagate along the cavity axis. This creates a sustained avalanche-like process, which allows a laser beam to be generated through the partially reflecting mirror. 3.3 Characteristics of laser light Unlike other forms of light, laser light has special properties which make it significantly more effective and dangerous than conventional light of the same power. The laser light particles (photons) are usually: 24 power air-cooled systems are used in a wide variety of OEM applications. Unlike the HeNe laser, ion lasers operate with a high-intensity low-pressure arc discharge (low voltage, high current). 3.4.1.4 Carbon Dioxide Lasers Because of their ability to produce very high power with relative efficiency, carbon dioxide (CO2) lasers are used primarily for materials-processing applications. The standard output of these lasers is at 10.6 mm, and output power can range from less than 1W to more than 10 kW. Unlike atomic lasers, CO2 lasers work with molecular transitions (vibrational and rotational states) which lie at low enough energy levels that they can be populated thermally, and an increase in the gas temperature, caused by the discharge, will cause a decrease in the inversion level, reducing output power. To counter this effect, high-power CO2 lasers use flowing gas technology to remove hot gas from the discharge region and replace it with cooled (or cooler) gas. With pulsed CO2 lasers that use transverse excitation, the problem is even more severe, because, until the heated gas between the electrodes is cooled, a new discharge pulse cannot form properly. A variety of types of CO2 lasers are available. High-power pulsed and cw (continuous wave) lasers typically use a transverse gas flow with fans which move the gas through a laminar- flow discharge region, into a cooling region, and back again. Low-power lasers most often use waveguide structures, coupled with radio-frequency excitation, to produce small, compact systems. 3.4.2 Semiconductor Diode Lasers The means of generating optical gain in a diode laser, the recombination of injected holes and electrons (and consequent emission of photons) in a forward-biased semiconductor p-n junction, represents the direct conversion of electricity to light. This is a very efficient process, and practical diode laser devices reach a 50-percent electrical-to-optical power conversion rate, at least an order of magnitude larger than most other lasers. Over the past 20years, the trend has been one of a gradual replacement of other laser types by diode laser based–solutions, as the considerable challenges to engineering with diode lasers have been met. At the same time the compactness and the low power consumption of diode lasers have enabled important new applications such as storing information in compact discs and DVDs, and the practical high- speed, broadband transmission of information over optical fibers, a central component of the Internet. 3.5 Industrial Applications Lasers have become so much a part of daily life that many people may not realize how ubiquitous they are. Every home with a CD player has a laser; hardware stores are now selling a wide variety of laser levels; many, if not most, computers, printers, and copiers are using laser technology. Laser applications are so numerous that it would be fruitless to try to list them all; however; one can give some illustrative examples of how lasers are used today. High-power lasers have long been used for cutting and welding materials. Today the frames of automobiles are assembled using laser welding robots, complex cardboard boxes are made with laser-cut dies, and lasers are routinely used to engrave numbers and codes on a wide variety of products. Some less well-known applications include three-dimensional stereo lithography and photolithography. 25 3.5.1 Three-Dimensional Stereo lithography Often a designer, having created a complex part on a CAD machine, needs to make a prototype component to check out the dimensions and fit. In many cases, it is not necessary for the prototype to be made of the specified (final) material for this checking step but having a part to check quickly is important. This is where rapid prototyping, i.e., three-dimensional stereo lithography, comes in. The stereo lithography machine consists of a bath of liquid photopolymer, an ultraviolet laser, beam-handling optics, and computer control. When the laser beam is absorbed in the photopolymer, the polymer solidifies at the focal point of the beam. The component design is fed directly from the CAD program to the stereo lithography computer. The laser is scanned through the polymer, creating, layer by layer, a solid, three-dimensional model of the part. Figure 3-4 A laser stereo lithography system for rapid prototyping of three-dimensional parts 3.5.2 Photolithography Lasers are used throughout the manufacture of semiconductor devices, but nowhere are they more important than in exposing photo-resist through the masks used for creating the circuits themselves. Originally, ultraviolet mercury lamps were used as the light sources to expose the photo-resist, but as features became smaller and more complex devices were put on a single wafer, the mercury lamp’s wavelengths were too long to create the features. Approximately ten years ago, manufactures started to switch to ultraviolet lasers operating at approximately 300 nm to expose the photoresist. Manufacturers are now using wavelengths as short as 193 nm to get the resolution needed for today’s semiconductor integrated circuit applications. 3.5.3 Marking and Scribing Lasers are used extensively in production to apply indelible, human and machine-readable marks and codes to a wide variety of products and packaging. Typical applications include: 1. marking semiconductor wafers for identification and lot control 2. removing the black overlay on numeric display pads 3. engraving gift items 4. Scribing solar cells and semiconductor wafers. 26 The basic marking system consists of a laser, a scanning head, a flat-field focusing lens, and computer control. The computer turns the laser beam on and off (either directly or through a modulator) as it is scanned over the surface to make the mark. Depending upon the application, scanning may occur in a raster pattern (typical for making dot-matrix marks) or in a cursive pattern, with the beam creating letters one at a time. The mark itself results either from ablation of the surface of the material, or by a photochemically induced change in the color of the material. Another marking technique, used with high-energy pulsed CO2 and excimer lasers, is to shine the light through a mask containing the marking pattern and focusing the resulting image onto the marking surface. Laser scribing is similar to laser marking, except that the scan pattern is typically rectilinear, and the goal is to create micro-scoring along the scan lines so that the substrate can be easily broken apart. A wide variety of materials, including metal, wood, glass, silicon, and rubber, are amenable to laser marking and scribing. Each material has different absorption and thermal characteristic, and some even have directional preferences due to crystalline structure. Consequently, the type of laser used depends, to some extent, on the material to be marked (e.g., glass transmits the 1.06 mm output from a yttrium–aluminum–garnet (YAG) laser but absorbs the 10.6 mm output from a CO2 laser). Other considerations are: 1. The size of the pattern 2. The speed of the scan 3. Cosmetic quality 4. Cost 3.5.4 Noncontact measurement There are many types of laser-based noncontact measurement techniques in use today including scatter measurement, polarimetry and ellipsometry, and interferometric measurement. 3.5.4.1 Scatter Measurement In the semiconductor industry, patterns of material are deposited on a wafer substrate using photolithographic processes. This involves scanning the wafer with a laser and measuring backscatter with a very sensitive photo-detector array. Lasers used in this application have to have excellent pointing stability, constant wavelength and power stability to calculate the correct size of the defects through complex algorithms, and low noise so the little scatter the defect makes can be distinguished from the background laser light. 29 Chapter 4 INDUSTRIAL ROBOTS 4.1 Introduction An industrial robot is a general purpose, programmable machine possessing certain anthropomorphic characteristics. The most typical anthropomorphic or human like, characteristics of a robot is its mechanical arm. Such anthropomorphic characteristics include the mechanical arms or manipulator, used for various industry tasks, or sensory perceptive devices, such as sensors, which allow robots to communicate and interact with other machines and make simple decisions. The technology is quite similar to numerical control, as it has followed the same developmental path, and its history is related. Both robots and Numerical Control are similar in that they seek to have coordinated control of multiple moving axes (called joints in robotics). Both use dedicated digital computers as controllers. Robots, however, are designed for a wider variety of tasks than numerical control. This arm, together with the robots capacity to be programmed, make it ideally suited to a variety of production tasks, including machine loading, spot welding, spray painting and assembly. Robots are typically used as substitutes for human workers in these tasks. The robot can be programmed to perform sequence of mechanical motions, and it can repeat that motion sequence over and over until programmed to perform some other job. Definition: An industrial robot is a reprogrammable, multifunctional manipulator designed to move material, parts, tools or specialized devices through variable programmed motions for the performance of a variety of tasks: Robot Institute of America, 1979. Figure 4-1 Examples of Industrial Robots How are robots used? 1. Industrial robots do tasks that are hazardous or menial. 2. Exploratory robots explore environments that are inhospitable to humans such as space, military targets or areas of search and rescue operations. 3. Assistive robots help handicapped individuals by assisting with daily tasks including wheelchair navigation and feeding. 30 4.2 Robot Anatomy An industrial robot consists of a mechanical manipulator and a controller to move it and perform other related functions. 1. The mechanical manipulator consists of joints and links to position and orient the end of the manipulator relative to its base. 2. The controller operates the joints in a coordinated fashion to execute a programmed work cycle. 3. A robot joint is similar to a human body joint as it provides relative movement between two parts of the body. 4. Typical industrial robots have five or six joints. 4.2.1 The Mechanical Manipulator The mechanical manipulator consists of joints and links; Figure 4-2 Parts of an industrial Robot The mechanical Manipulator consists of joints and links  Joints provide relative motion  Links are rigid members between joints  Each joint provides a “degree-of-freedom”  Most robots possess five or six degrees-of-freedom Robot manipulator consists of two sections:  Body-and-arm – for positioning of objects in the robot's work volume  Wrist assembly – for orientation of objects 4.2.1.1 Manipulator Joints A robot joint is a mechanism that permits relative movement between parts of a robot arm. There are six basic motions or degrees of freedom, which provide the robot with the capability to move the end effectors through the required sequences of motions. These six degree of freedom 31 are intended to emulate the versatility of movement possessed by the human arm. Not all robots are equipped with the ability to move in all six degrees. The six basic motions consist of three arm and body motions and three wrist motions. The basic movements required for the desired motion of most industrial robots are: 1. Rotational movement: this enables the robot to place its arm in any direction on a horizontal plane; rotation about the vertical axis (right or left swivel of the robot arm) 2. Radial movement: this enables the robot to move its end-effectors radically to reach distant points i.e. extension and retraction of the arm (in and out movement). 3. Vertical movement: this enables the robot to take its end-effectors to different heights. Up and down motion of the arm is caused by pivoting the entire arm about a horizontal axis or moving the arm along a vertical slide. 4.2.2 Types of Manipulator Joints The manipulator joints can be of the following types depending on the type of motion; 4.2.2.1 Translational motion 1. Linear joint (type L) The relative motion between the input link and the output link is a translational sliding motion with the axes of the two links being parallel. 2. Orthogonal joint (type O) The relative movement between the input link and the output link is a translational sliding but the output link is perpendicular to the input link. 4.2.2.2 Rotary motion 1. Rotational joint (type R) This provides rotational relative motion with the axis of rotation perpendicular to the input link. 34 4. Jointed-Arm Robot Jointed-arm configuration: is combination of cylindrical and articulated configurations. The arm of robot is connected to the base with a twisting joint. Rotate joints connect the links in the arm. The most popular robot, which is very close to this configuration, is SCARA (selective compliance assembly robot arm).Notation TRR: Figure 4-6 Jointed Arm Robot 5. SCARA Robot SCARA stands for Selectively Compliant Assembly Robot Arm. Similar to jointed-arm robot except that vertical axes are used for shoulder and elbow joints to be compliant in horizontal direction for vertical insertion tasks. In the SCARA robot rotations take place in horizontal planes, thus reducing the possibility of large deformations in the links. Notation VRO Figure 4-7 SCARA Robot T R R T 35 R T Example  Sketch following manipulator configurations  (a) TRT:R, (b) TVR:TR, (c) RR:T. R T R R V (a) TRT: R T (b) TVR:TR (c) RR:T Example 2 Designate the robot configuration shown below using the robot notation scheme. a). OO b). RRR c)TO 4.3 Motion system Robots can also be classified depending on the method used to control the motions. They include; 1. Point-to-point (PTP) control robot: is capable of moving from one point to another point. The locations are recorded in the control memory. PTP robots do not control the path to get from one point to the next point. Common applications include component insertion, spot welding, hole drilling, machine loading and unloading, and crude assembly operations. 2. Continuous-path (CP) control robot: with CP control, the robot can stop at any specified point along the controlled path. All the points along the path must be stored explicitly in the robot’s control memory. Typical applications include spray painting, finishing, gluing, and arc welding operations. 3. Controlled-path robot: the control equipment can generate paths of different geometry such as straight lines, circles, and interpolated curves with a high degree of accuracy. All controlled-path robots have a servo capability to correct their path. 36 4.4 Fundamentals of Robotics and Robotics Technology The key components include; Figure 4-8 Components of a Robot 4.4.1 Power sources for robots There are three basic drive system used in commercially available robots: 1. Hydraulic drive: gives a robot great speed and strength. These systems can be designed to actuate linear or rotational joints. The main disadvantage of a hydraulic system is that it occupies floor space in addition to that required by the robot. 2. Electric drive: compared with a hydraulic system, an electric system provides a robot with less speed and strength. Accordingly, electric drive systems are adopted for smaller robots. However, robots supported by electric drive systems are more accurate, exhibit better repeatability, and are cleaner to use. 3. Pneumatic drive: are generally used for smaller robots. These robots, with fewer degrees of freedom, carry out simple pick-and-place material handling operations. 4.4.2 End Effectors End effectors can be defined as a device which is attached to the robots wrist to perform a specific task. The task might be work part handling, spot welding, spray painting, or any of a great variety of other functions. The possibilities are limited only by the imagination and ingenuity of the application engineers who design robot systems. The end effectors are the special purpose tooling which enables the robot to perform a particular job. It is usually custom engineered for that job, either by the company that owns the robot or company that sold the robots. Most robot manufacturer has engineered groups which design and fabricate end effectors or provide advice to their customers on end effectors design. For purpose organization, we will divide the various types of end effectors into two categories: grippers and tools. 1. Grippers: are generally used to grasp and hold an object and place it at a desired location. Grippers can be classified as; Mechanical grippers, Vacuum or suction cups, Magnetic grippers, Adhesive grippers, Hooks, Scoops, etc 2. Tools: A robot is required to manipulate a tool to perform an operation on a work part. Here the tool acts as end-effectors. Spot-welding tools, arc-welding tools, spray painting nozzles, and rotating spindles for drilling and grinding are typical examples of tools used as end-effectors. 39 through. The main concern is getting the position sequence correct. The walk through method would be appropriate for spray painting and arc welding. 4.5.3 Lead through method: The lead through method makes use of a teach pendant to power drive the robot through its motion sequence. The teach pendant is usually a small hand held device with switches and dials to control the robots physical movements. Each motion is recorded into memory for future playback during work cycle. The lead through method is very popular among robot programming methods because of its ease and convenience. 4.5.4 Off- line programming: This method involves the preparation of the robot program off-line, in a manner similar to NC part programming. Off-line robot programming is typically accomplished on a computer terminal. After the program has been prepared, it is entered in to the robot memory for use during the work cycle. The advantaged of off-line robot programming is that the production time of the robot is not lost to delay in teaching the robot a new task. Programming off-line can be done while the robot is still in production on the preceding job. This means higher utilization of the robot and the equipment with which it operates. Another benefit associated with off-line programming is the prospect of integrating the robot into the factory CAD/CAM data base and information system. 40 Chapter 5 WELDING Initially welding was used in ships as a means of repairing various metal parts. During the First World War various authorities connected with shipbuilding, including Lloyd’s Register, undertook research into welding and in some cases prototype welded structures were built. However, riveting remained the predominant method employed for joining ship plates and sections until the time of the Second World War. During and after this war the use and development of welding for shipbuilding purposes was widespread, and welding totally replaced riveting in the latter part of the twentieth century. There are many advantages to be gained from employing welding in ships as opposed to having a riveted construction. These may be considered as advantages in both building and in operating the ship. For the ship builder the advantages are: 1. Welding lends itself to the adoption of prefabrication techniques. 2. It is easier to obtain water tightness and oil tightness with welded joints. 3. Joints are produced more quickly. 4. Less skilled labor is required. For the ship-owner the advantages are: 1. Reduced hull steel weight, therefore more deadweight. 2. Less maintenance from slack rivets, etc. 3. The smoother hull with the elimination of overlapping plate joints leads to reduced skin friction resistance, which can reduce fuel costs. Other than some blacksmith work involving solid-phase welding, the welding processes employed in shipbuilding are of the fusion welding type. Fusion welding is achieved by means of a heat source that is intense enough to melt the edges of the material to be joined as it is traversed along the joint. Gas welding, arc welding, laser welding, and resistance welding all provide heat sources of sufficient intensity to achieve fusion welds. 5.1 Gas welding A gas flame was probably the first form of heat source to be used for fusion welding, and a variety of fuel gases with oxygen have been used to produce a high-temperature flame. The most commonly used gas in use is acetylene, which gives an intense concentrated flame (average temperature 3000 C) when burnt in oxygen. An oxyacetylene flame has two distinct regions: an inner cone, in which the oxygen for combustion is supplied via the torch; and a surrounding envelope, in which some or all the oxygen for combustion is drawn from the surrounding air. By varying the ratio of oxygen to acetylene in the gas mixture supplied by the torch, it is possible to vary the efficiency of the combustion and alter the nature of the flame (Figure 5-1). If the oxygen supply is slightly greater than the supply of acetylene by volume, what is known as an ‘oxidizing’ flame is obtained. This type of flame may be used for welding materials of high thermal conductivity, e.g. copper, but not steels, as the steel may be decarburized and the weld pool depleted of silicon. With equal amounts of acetylene and oxygen a ‘neutral’ flame is obtained, and this would normally be used for welding steels and most other metals. Where the acetylene supply exceeds the oxygen by volume a ‘carburizing’ flame is obtained, the excess acetylene decomposing and producing submicroscopic particles of carbon. These readily go into solution in the molten steel and can produce metallurgical problems in service. The outer envelope of the oxyacetylene flame by consuming the surrounding oxygen to some extent protects the molten weld metal pool 41 from the surrounding air. If unprotected the oxygen may diffuse into the molten metal and produce porosity when the weld metal cool. Figure 5-1 Gas Welding With metals containing refractory oxides, such as stainless steels and aluminum, it is necessary to use an active flux to remove the oxides during the welding process. Both oxygen and acetylene are supplied in cylinders, the oxygen under pressure and the acetylene dissolved in acetone since it cannot be compressed. Each cylinder, which is distinctly colored (red— acetylene, black— oxygen), has a regulator for controlling the working gas pressures. The welding torch consists of a long thick copper nozzle, a gas mixer body, and valves for adjusting the oxygen and acetylene flow rates. Usually a welding rod is used to provide filler metal for the joint, but in some cases the parts to be joined may be fused together without any filler metal. Gas welding techniques are shown in Figure 5-1. Oxyacetylene welding tends to be slower than other fusion welding processes because the process temperature is low in comparison with the melting temperature of the metal, and because the heat must be transferred from the flame to the plate. The process is therefore only really applicable to thinner mild steel plate, thicknesses up to 7 mm being welded using this process with a speed of 3–4 meters per hour. In shipbuilding oxyacetylene welding has almost disappeared but can be employed in the fabrication of ventilation and air-conditioning trucking, cable trays, and light steel furniture; some plumbing and similar work may also make use of gas welding. These trades may also employ the gas flame for brazing purposes, where joints are obtained without reaching the fusion temperature of the material being joined. 5.2 Electric arc welding The basic principle of electric arc welding is that a wire or electrode is connected to a source of electrical supply with a return lead to the plates to be welded. If the electrode is brought into contact with the plates an electric current flows in the circuit. By removing the electrode a short distance from the plate, so that the electric current is able to jump the gap, a high-temperature electrical arc is created. This will melt the plate edges and the end of the electrode if this is of the consumable type. Electrical power sources vary, DC generators or rectifiers with variable or 44 the unfused flux may be recovered before cooling. This is the most commonly used process for downhand mechanical welding in the shipbuilding industry, in particular for joining plates for ship shell, decks, and bulkheads. Figure 5-3 Manual arc welding Figure 5-4 Automatic arc welding. 45 5.1.2 Gas-shielded arc welding processes The application of bare wire welding with gas shielding was developed in the 1960s, and was quickly adopted for the welding of lighter steel structures in shipyards, as well as for welding aluminum alloys. Gas-shielded processes are principally of an automatic or semi-automatic nature. 5.1.2.1 Tungsten inert gas (TIG) welding In the TIG welding process the arc is drawn between a water-cooled non-consumable tungsten electrode and the plate. An inert gas shield is provided to protect the weld metal from the atmosphere, and filler metal may be added to the weld pool as required. Ignition of the arc is obtained by means of a high-frequency discharge across the gap, since it is not advisable to strike an arc on the plate with the tungsten electrode. Normally the inert gas shield used for welding aluminum and steel is argon. Only plate thicknesses of less than 6 mm would normally be welded by this process, and in particular aluminum sheet, a skilled operator being required for manual work. This may also be referred to as TAGS welding, i.e. tungsten arc gas shielded welding. 5.1.2.2 Metal Inert Gas (MIG) Welding This is in effect an extension of TIG welding, the electrode in this process becoming a consumable metal wire. Basically, the process is as illustrated, a wire feed motor supplying wire via guide rollers through a contact tube in the torch to the arc. An inert gas is supplied to the torch to shield the arc, and electrical connections are made to the contact tube and workpiece. Welding is almost always done with a DC source and electrode positive for regular metal transfer, and when welding aluminum to remove the oxide film by the action of the arc cathode. Although the process may be fully automatic, semi-automatic processes as illustrated with hand gun are now in greater use, and are particularly suitable in many cases for application to shipyard work. Figure 5-5 Metal inert gas welding. 46 Initially aluminum accounted for most of the MIG welding, with argon being used as the inert shielding gas. Much of the welding undertaken on aluminum deckhouses, and liquid methane gas tanks of specialized carriers, has made use of this process. Early work on the welding of mild steel with the metal inert gas process made use of argon as a shielding gas, but as this gas is rather expensive. Research in this direction was concentrated on the use of CO2 as the shielding gas, and the MIG/CO2 process is now widely used for welding mild steel. Using higher current values with thicker steel plate a fine spray transfer of the metal from the electrode across the arc is achieved, with a deep penetration. Wire diameters in excess of 1.6 mm are used, and currents above about 350 amps are required to obtain this form of transfer. Much of the higher current work is undertaken with automatic machines, but some semi-automatic torches are available to operate in this range in the hands of skilled welders. Welding is downhand only. On thinner plating where lower currents would be employed, a different mode of transfer of metal in the arc is achieved with the MIG/CO2 process. This form of welding is referred to as the dip transfer (or short-circuiting) process. The sequence of metal transfer is: 1. Establish the arc. 2. Wire fed into arc until it makes contact with plate. 3. Resistance heating of wire in contact with plate. 4. Pinch effect, detaching heated portion of wire as droplet of molten metal. 5. Re-establish the arc. To prevent a rapid rise of current and ‘blast off’ of the end of the wire when it short-circuits on the plate, variable inductance is introduced in the electrical circuit. Smaller wire diameters, 0.8 and 1.2 mm, are used where the dip transfer method is employed on lighter plate at low currents. The process is suitable for welding light mild steel plate in all positions. It may be used in shipbuilding as a semi-automatic process, particularly for welding deckhouses and other light steel assemblies. The pulsed MIG/argon process, developed for positional welding of light aluminum plate, may be used for positional welding of light steel plate but is likely to prove more expensive. Use of the MIG semi-automatic processes can considerably increase weld output and lower costs. This form of welding may also be collectively referred to as MAGS welding, i.e. metal arc gas- shielded welding. 5.1.2.3 Plasma Welding This is very similar to TIG welding as the arc is formed between a pointed tungsten electrode and the plate. But, with the tungsten electrode positioned within the body of the torch, the plasma arc is separated from the shielding gas envelope. Plasma is forced through a fine-bore copper nozzle that constricts the arc. By varying the bore diameter and plasma gas flow rate, three different operating modes can be achieved: 1. Microplasma—the arc is operated at very low welding currents (0.1–15 amps) and used for welding thin sheets (down to 0.1 mm thickness). 2. Medium current—the arc is operated at currents from 15 to 200 amps. Plasma welding is an alternative to conventional TIG welding, but with the advantage of achieving deeper penetration and having greater tolerance to surface contamination. Because of the bulkiness of the torch, it is more suited to mechanize welding than hand welding. 49 Figure 5-7 Metal cutting processes. This gas is ionized in the first place by a subsidiary electrical discharge between the electrode and the nozzle. Plates are cut by the high-temperature concentrated arc melting the material locally (Figure 5-7). The plasma-arc process may be used for cutting all electrically conductive materials. Cutting units are available with cutting currents of 20-1000 amps to cut plates with thicknesses of 0.6–150 mm. The plasma carrier gas may be compressed air, nitrogen, oxygen, or argon/hydrogen to cut mild or high alloy steels, and aluminum alloys, the more expensive argon/hydrogen mixture being required to cut the greater thickness sections. A water-injection plasma-arc cutting system is available for cutting materials up to 75 mm thick using nitrogen as the carrier gas. A higher cutting speed is possible and pollution minimized with the use of water and an exhaust system around the torch. Water cutting tables were often used with plasma-arc cutting, but more recent systems have dispensed with underwater cutting. Cutting in water absorbed the dust and particulate matter and reduced the plasma noise and ultraviolet radiation of earlier plasma cutters. 5.3.3 Gouging Both gas and arc welding processes may be modified to produce means of gouging out shallow depressions in plates to form edge preparations for welding purposes where precision is not important. Gouging is particularly useful in shipbuilding for cleaning out the backs of welds to expose clean metal prior to depositing a weld back run. The alternative to gouging for this task is mechanical chipping, which is slow and arduous. Usually, where gouging is applied for this purpose, what is known as ‘arc-air’ gouging is used. A tubular electrode is employed, the electrode metal conducting the current and maintaining an arc of sufficient intensity to heat the workpiece to incandescence. Whilst the arc is maintained, a stream of oxygen is discharged from the bore of the electrode that ignites the incandescent electrode metal and the combustible 50 elements of the 51 workpiece. At the same time the kinetic energy of the excess oxygen removes the products of combustion, and produces a cut. Held at an angle to the plate, the electrode will gouge out the unwanted material. A gas cutting torch may be provided with special nozzles that allow gouging to be accomplished when the torch is held at an acute angle to the plate. 5.3.4 Laser cutting Profile cutting and planning at high speeds can be achieved with a concentrated laser beam and has increasingly been employed in a mechanized robotic form in the shipbuilding industry in recent years. In a laser beam the light is of one wavelength, travels in the same direction, and is coherent, i.e. all the waves are in phase. Such a beam can be focused to give high energy densities. For welding and cutting the beam is generated in a CO2 laser. This consists of a tube filled with a mixture of CO2, nitrogen, and helium that is made to fluoresce by a high-voltage discharge. The tube emits infrared radiation with a wavelength of about 1.6 mm and is capable of delivering outputs up to 20 kW. Laser cutting relies on key holing to penetrate the thickness, and the molten metal is blown out of the hole by a gas jet. A nozzle is fitted concentric with the output from a CO2 laser so that a gas jet can be directed at the work coaxial with the laser beam. The jet can be an inert gas, nitrogen, or in the case of steel, oxygen. With oxygen there is an exothermic reaction with the steel, giving additional heat as in oxy-fuel cutting. The thermal key holing gives a narrow straight-sided cut compared with the normal cut obtained by other processes relying on a chemical reaction. 5.3.5 Water jet cutting The cutting tool employed in this process is a concentrated water jet, with or without abrasive, which is released from a nozzle at 2½ times the speed of sound and at a pressure level of several thousand bar. Water jet cutting can be used on a range of materials such as timber, plastics, rubber, etc., as well as steels and aluminum alloys. Mild steel from 0.25 to 150 mm in thickness and aluminum alloys from 0.5 to 250 mm in thickness can be cut. Being a cold cutting process, the heat-affected zone, mechanical stresses, and distortion are left at the cut surface. Water jet cutting is slower than most thermal cutting processes and is not a portable machine tool. 5.4 Welding practice and testing welds The strongest welded joint that may be produced in two plates subsequently subjected to a tensile pull is the butt joint. A butt joint is one where the two joined plates are in the same plane, and in any welded structure it is desirable that butt joints should be used wherever possible. In mild steel the weld metal tends to have a higher yield strength than the plate material. Under tension it is found that initial yielding usually occurs adjacent to a butt weld in the plate when the yield strength of the plate material is reached locally. Since a good butt weld in tension has a strength equivalent to that of the mild steel plate, it is not considered as a line of structural weakness. Lapped joints, where fillet welds are used to connect the plates, should be avoided in strength members of a welded structure. As the fillet welds are in shear when the plates are intension, the strength of the joint is very much less than that of the plate material or butt joint. Fillet welds are unavoidable where sections or plates are connected at an angle to an adjacent plate, but often there is not the same problem as the loading is different. The fatigue strength of 54 finish. Plates of varying thickness may be butt welded together at different locations, a good example being where heavy insert plates are fitted. Insert plates are preferred to doubling plates in welded construction, and the heavy plate is chamfered to the thickness of the adjacent thinner plate before the butt edge preparation is made. 5.4.2 Welding automation Larger shipyards with a large production line throughput of welded panels use automated welding systems to produce the stiffened panels. To join the plates, highspeed one-sided submerged arc welding is used. The required welding parameters are set in advance in the operation box and linked to a computer. The operator selects the plate thickness and starts the machine. The machine automatically controls the welding parameters for the weld crater and stops when the run-off tab is reached at the end of the plates. Fig 5.4.2 Plate edge preparation. 5.4.3 Welding distortion During the welding process the metal is heated, which causes expansion and the metal then contracts on cooling. The initial rapid heating causes the welded area to expand locally. The slower cooling of the weld causes the plate to move as the weld contracts. The result is a distortion of the part, and this is a major cause of extra work during assembly of units and construction of the ship. The need to adjust distorted parts so that they fit correctly can take considerable time and effort. In-plane distortion is basically shrinkage of the plate. For repeatable processes, which are usual in shipbuilding, the shrinkage can be measured and sufficient data built up to allow the shrinkage to be predicted. Computer-aided design systems can now include an allowance for shrinkage, so that the part as modeled in the system can then be adjusted during the generation of cutting information. The plates are cut oversized and the shrinkage after welding brings it to the correct size. More recently, work has been carried out to model shrinkage of more complex parts, for example 55 structural webs with face flats. Again, these parts can then have their dimensions adjusted prior to cutting so that the effect of shrinkage is to bring them to the correct shape. Out-of plane distortion is much more difficult to predict and manage. The cause is the same as in-plane shrinkage, but the distortion is often associated with fillet welds used to attach stiffeners to plates. The fillet welds, as they shrink, pull the plate out of plane, resulting in the typical appearance of a welded hull with indentations between the frames. The effect is much more noticeable for thin plate structures, for smaller ships, and for superstructures. Restraining the plate during assembly and welding is one commonly used solution. The causes of distortion are complex and also include any residual stresses in the steel plate as a result of the steel mill rolling and cooling. Some of the stress may be relieved by rolling the plate prior to production, but distortion remains a significant problem for many shipbuilders. 5.4.4 Testing welds For economic reasons much of the weld testing carried out in shipbuilding is done visually by trained inspectors. Spot checks at convenient intervals are made on the more important welds in merchant ship construction, generally using radiographic or ultrasonic equipment. Welding materials are subjected to comprehensive tests before they are approved by Lloyd’s Register or the other classification societies for use in ship work. Operatives are required to undergo periodical welder approval tests to ascertain their standard of workmanship. 5.4.5 Weld faults Various faults may be observed in butt and fillet welds. These may be due to a number of factors: bad design, incorrect welding procedure, use of wrong materials, and bad workmanship. Different faults are illustrated in Figure 5-9. The judgment of the seriousness of the fault rests with the weld inspector and surveyor, and where the weld is considered to be unacceptable it will be cut out and rewelded. 5.5 Nondestructive testing For obvious reasons some form of nondestructive test is required to enable the soundness of ship welds to be assessed. The various available nondestructive testing methods may be summarized as follows: 1. Visual examination 2. Dye penetrant 3. Magnetic particle 4. Radiographic 5. Ultrasonic. Of these five methods, the dye penetrant and magnetic particle tests have few applications in ship hull construction, being used for examining surface cracks in stern frames and other castings. Visual, radiographic, and ultrasonic examinations are considered in more detail, as they are in common use. 56 Figure 5-9 Weld faults Magnetic particle testing is carried out by magnetizing the casting and spreading a fluid of magnetic particles (e.g. iron fillings suspended in paraffin) on the surface. Any discontinuity such as a surface crack will show up as the particles will concentrate at this point where there is an alteration in the magnetic field. A dye penetrant will also show up a surface flaw if it remains after the casting has been washed following the application of the dye. To aid the detection of a surface crack the dye penetrant used is often luminous and is revealed under an ultraviolet light. Visual inspection of welds is routine procedure, and surface defects are soon noticed by the experienced inspector and surveyor. Incorrect bead shape, high spatter, undercutting, bad stop and start points, incorrect alignment, and surface cracks are all faults that may be observed at the surface. Subsurface and internal defects are not observed, but the cost of visual inspection is low, and it can be very effective where examination is made before, during, and after welding. The principle of radiographic inspection is simply to subject a material to radiation from one side, and record the radiation emitted from the opposite side. Any obstacle in the path of the radiation will affect the radiation density emitted and may be recorded. As radiation will expose photographic plate, for all practical weld test purposes this is used to record the consistency of the weld metal. The photographic plate records changes in radiation density emitted; for example, a void will show up as a darker shadow on the radiograph. Either X-ray or gamma-ray devices may be used to provide the source of radiation. X-ray equipment consists of a high- voltage power source (50–400 kV), which is used to provide potential between a cathode and target anode in a 59 Chapter 6 SHIP PRODUCTION FACILITIES AND SHIPBUILDING CYCLE. 6.1 Shipyard Layout Until the advent of steel ships, a shipbuilding operation could be set up almost anywhere close to the sea or a river and to trees for the main construction materials. Iron, rapidly followed by steel, construction resulted in shipbuilding moving to areas where raw materials, primarily coal and iron ore, were available. These were also often areas where the basic metalworking skills were to be found as the basis for the labour force, or in some cases a labour force was moved into the area. Shipyards were usually found along river banks, or in protected harbors, giving sheltered water and their basic arrangement did not vary. Figure 6-1 shows the typical arrangement of a shipyard up to around 1960. The slipways on which the ships are constructed piece by piece are supported by small and simple workshops. There was a relatively small initial investment and the output could be varied by opening or closing building slipways and taking on or laying off labour. Figure 6-1 Traditional Riverside Layout The layout of a shipyard did not vary significantly until the mid-1950s. A relatively small number of shipyards engaged in capital warship construction or passenger ships, where the product is significantly larger and more complex than average commercial ships. These had extensive outfitting workshops and quays, as well as larger slipways. Large cranes, almost always fixed in position, were available to lift heavy items, perhaps 200 tonnes for large, complex ships. However, for the lifting of hull parts and most outfitting the available lifting capacity was usually below 10 tonnes. The major change in the shipyards came about initially because of rapid increases in commercial ship size after 1950. At that time a typical cargo ship was of below 10,000 deadweight tonnes, and a tanker of 20,000 deadweight tonnes was considered large. By 1958, the first tanker over 100,000 deadweight tonnes was in operation and the first over 200,000 tonnes deadweight by 1966. By 1970, 250,000-tonne and larger ships were being built. An important aspect of these newer ships was a tenfold increase in steel weight, from a typical 3000 tonnes to 30,000 tonnes. Also, the largest ships were in excess of 300 meters in length. As such, they were too large for most existing shipyards’ slipways and the lifting capacities of the small cranes usually available would have meant an excessive construction time. 60 The result was that existing shipyards reduced the number of slipways and increased the size of their cranes as the ship size increased. This allowed them to construct the ships in a shorter time, so keeping the construction time acceptable. The contemporary shipbuilding practices and production methods have improved but the basic technology and main equipment has been consistent for the last half century. A number of traditional shipbuilders, which were often based on river banks, also established new yards where they could build larger ships and/or exploit the new technology and production methods. In general, the smaller shipbuilders have been able to re-configure their site in order to utilize new technology and improve production, whilst continuing to build smaller and medium-sized ships. An ideal layout for a modern shipyard is based on a production flow basis, as in Figure 6-3, with the shipyard built on a greenfield site and no longer, as with existing shipyards, having to follow the river bank. This removes typical restrictions on old sites, restricted by their location in a built-up area or the physical river bank slope from extending back from the river, so that modified production flow lines are required. The sequence of layout development is outlined below. It should be noted that particular locations and circumstances can dictate significantly different arrangements that may not be ideal but that do work. Planning a new shipyard, or re-planning an existing one, will involve decisions to be made on the following: 1. Size and type of ship to be built 2. Number of ships per year to be achieved 3. Breakdown of the ships into structural blocks and outfit modules (interim products) 4. Material handling equipment required for the interim products 5. Part production and assembly processes to be installed 6. Amount of outfit and engine installation to be undertaken 7. Control services to be supplied 8. Administration facilities required. Shipyards usually have a fitting out basin or berth where the virtually completed ship is tied up after launching and the finishing off work and static trails may be carried out. Some of the facilities identified may be omitted from the shipyard and a sub-contractor used instead. This will depend on the location, availability of subcontractors, and the economics of the alternatives. 6.2 Ship Building Process Before considering the actual layout of the shipyard, it is essential to consider the relationship of the work processes involved in building a ship. At this point it may be convenient to mention the advantages and disadvantages of building docks as opposed to building berths. Building docks can be of advantage in the building of large vessels where launching costs are high, and there is a possibility of structural damage owing to the large stresses imposed by a conventional launch. They also give good crane clearance for positioning units. The greatest disadvantage of the building dock is its high initial cost. However, the dock is the usual choice for new shipyards, especially for larger ships. The level base simplifies the construction process, for example alignment of structural blocks. Also, the dock is more flexible in operation, for example in some cases ships are built in two stages with the outfit and labor-intensive stern, containing machinery and accommodation, built as a single structure, then moved along the dock for the cargo-carrying part to be constructed. This doubles the available time for outfitting the ship. Plate Section Outfit materials stowage stowage and bought in ftoms Shotblasuprime ——_ Shet-blast/prime Cais cies Pipe bending Marking cutting: Shaping Ournt fabrication and assembly sub- Panel Matri< Curved assemblies assemblies assemblies unit as | assemblies Out palletization ‘and modules Block assembly — Block and unit ‘erection Final outrit Figure 6-2 Shipyard layout Pee and section stockyard Marshaling and preparation Adri ottices, Pane Jone section | machining IModure, assembly unit assombiy| areas ed pale Block fabrication Pipe and engine shop] Coworod ‘Outtit building shops dock Fitting out basin Figure 6-3 Shipbuilding process 61 64 Figure 6-4 Plate profiling and Cutting layout 6.4.2 Planning Machines Profiling machines are essentially for use where a plate requires extensive shaping with intricate cuts being made. Many of the plates in the ship’s hull, particularly those in straightforward plate panels, decks, tank tops, bulkheads, and side shell, will only require trimming and edge preparation, and perhaps some shallow curves may need to be cut in shell plates. This work may be carried out on a planning machine, usually a flame or plasma-arc planer. A flame planer consists basically of three beams carrying burning heads and running on two tracks, one either side of the plate-working area. One beam carries two burning heads for trimming the plate sides during travel, whilst the other two beams have a single burning head traversing the beam in order to trim the plate ends. 6.4.3 Drilling machines Some plates or sections may need to have holes drilled in them, for example bolted covers and portable plates. Drilling machines generally consist of a single drilling head mounted on a radial arm that traverses the drill bed. 6.4.4 Guillotines Smaller ‘one-off’ plate shapes such as beam knees, various brackets, and flat bar lengths may be cut in a hydraulically operated guillotine. Plate feed to the guillotine is usually assisted by the provision of plate-supporting roller castors, and positioning of the cut edge is by hand. 6.4.5 Presses Hydraulic presses may be extensively used in the shipyard for a variety of purposes. They are capable of bending, straightening, flanging, dishing, and swaging plates (see Figure 6-5). All of the work is done with the plate cold, and it is possible to carry out most of the work undertaken by a set of rolls. This is done at less capital cost, but the press is slower when used for bending and requires greater skill. 65 Figure 6-5 Gap Presses 6.4.6 Plate rolls Heavy-duty bending rolls used for rolling shell plates etc. to the correct curvature are hydraulically operated. Two lower rolls are provided and are made to revolve in the same direction so that the plate is fed between them and a slightly larger diameter top roll that runs idly (see Figure 6-6). Either or both ends of the top roll may be adjusted for height, and the two lower rolls have adjustable centers. With modern bending rolls, plates up to 45 mm thick may be handled and it is possible to roll plates into a half circle. These large rolls are also supplied with accessories to allow them to undertake heavy flanging work with the pressure exerted by the upper beam, for example ‘troughing’ corrugated bulkhead sections. Shorter pyramid full-circle rolls are also used in shipyards, these being very useful for rolling plates to a full circle. This may be done to obtain large mast and derrick post sections for example or bow thruster tunnel. Arrangements are made for removing the rolled full-circle plate by releasing the top roller end bearing. Vertical rolls are also available and may be used to roll plates full circle, but can be much more useful for rolling heavy flats used as facing bars on transverses of large tankers, etc. 66 Figure 6-6 Plate Rolls 6.4.7 Heat-line bending The ‘heat-line’ bending procedure is a widely used technique to obtain curvature in steel plates for shipbuilding purposes. It is, however, a process that until recently relied on highly skilled personnel and did not guarantee constant accuracy of shapes formed. Heat is applied in a line to the surface of a plate by a flame torch, with immediate cooling using air or water. The narrow-heated line of material is prevented from expanding in the direction of the plate surface by the large mass of cold plate, and therefore expands outwards perpendicular to the plate surface. On cooling, contraction will take place in the direction of the plate surface, causing the plate to become concave on the side to which heat was applied (see Figure 6-6). Heat-line bending may be used after cold forming to obtain improved accuracy. It is also used where a double curvature is required. In recent years fully automated heat-line bending systems have been developed and installed in shipyards. These numerically controlled heat-line bending machines permit highly accurate, reproducible thermal forming of any steel plate using the data originating from the shipyard’s CAD system. 6.4.8 Cold frame bending It is now almost universal practice to cold-bend ship frames using commercially available machines for this purpose. The frames are progressively bent by application of a horizontal ram whilst the frame is held by gripping levers (Figure 6-7). Any type of rolled section can be bent in 69 ground is 1.25– 1.5 m, giving reasonable access, but not too high so that a large amount of packing is required. At the bow the height of the keel must be sufficient to allow the ship’s forefoot to dip the required amount without striking the ground during pivoting when the stern lifts at launch. To suit the declivity of the launching ways determined beforehand, the keel is also inclined to the horizontal at about 1 in 20, or more where the shipyard berths have a larger slope. To transfer the ship from the building blocks to the launching cradle, the commonest practice is to drive wedges into the launching cradle. This lifts the ship and permits the removal of the keel and bilge blocks together with the shores. In large ships it becomes necessary to split the blocks to remove them, but several types of collapsible blocks have been used to overcome this difficulty. One type is the sand box, which contains sand to a depth of 80–100 mm held in a steel frame located between two of the wooden blocks. This steel frame may be removed and the sand allowed to run out. Another type is a wooden block sawn diagonally, the two halves being bolted so that they collapse on removal of the bolts. Figure 7-1 Building Blocks 7.3 Launching ways and cradle The fixed ground ways or standing ways on which, the cradle and ship slide may be straight or have a fore and aft camber. Transversely the ground ways are normally laid straight but can be canted inwards to suit the ship’s rise of floor. Usually, the ground ways have a small uniform fore and aft camber, say 1 in 400, the ways being the arc of a circle of large radius. This means that the lower part of the ways has a greater declivity (say1in16) than the upper part of the ways (say1in25). As a result a greater buoyancy for the same travel of the ship beyond the way ends is obtained, which will reduce the way end pressures. Additional advantages are increased water resistance slowing the vessel and a bow height that is not excessive. The slope of ground ways must be adequate to allow the vessel to start sliding and, if too steep, a large amount of shoring will be required to support the bow; also, the loads on the releasing arrangements will be high. Straight sliding ways have declivities of the order of 1 in 25 to 1 in 16. To guide the sliding ways as they move over the ground ways, a rib band may be fitted to the outer edge of the ground ways. This could be fitted to the inner edge of the sliding ways, but when fitted to the ground ways has the advantage that it aids retention of the lubricating grease. Finally, the ground ways are shored transversely to prevent sideways movement and longitudinally to 70 prevent them from moving down with the ship. The sliding ways, covering about 80% of the length of the vessel, form the lower part of the cradle, the upper part consisting of packing, wedges, and baulks of timber with some packing fitted neatly to the line of the hull in way of the framing. In very fine-lined vessels the forward end of the cradle, referred to as the forward poppet, will need to be relatively high, and may be built up of vertical timber props tied together by stringers or rib bands. This forward poppet will experience a maximum load that may be as much as 20– 25% of the ship’s weight when the stern lifts. It is therefore designed to carry a load of this magnitude, but there is a danger in the fine-lined vessel of the forward poppets being forced outwards by the downward force, i.e. the bow might break through the poppets. To prevent this, cross ties or spreaders may be passed below the forefoot of the vessel and brackets may be temporarily fastened to the shell plating at the heads of the poppets 7.4 Lubricant For the ship to start sliding on release of the holding arrangements, it is necessary for the ship to overcome the coefficient of friction of the launching lubricant. To do this the slope of the ways under the vessel’s center of gravity must exceed the lubricant’s coefficient of friction. An estimate of the frictional resistance of the grease must be made before building the ship, since the declivity of the keel is dependent to a large extent on the slope of the ways. Formerly, melted tallow was applied to the ways, allowed to harden, and then covered with a coat of soft soap. Since the mid 1900s patent mineral-based greases have been applied to the ground ways, these greases being virtually unaffected by temperature changes and insoluble in water whilst adhering firmly to the ways. A commercially marketed petroleum-based launching grease is applied over the mineral grease base coat. This has a coefficient of friction that is low enough to allow initial starting and the maintenance of sliding until the initial resistance of the base coat is overcome by frictional heat. To prevent the petroleum-based grease from soaking into the sliding ways, a base coat may be applied to them. Standing ways that extend into the water may be dried out at low tide prior to the launch and the base coat and grease applied. 7.5 Releasing arrangements Small ships may be released by knocking away a diagonal dog-shore fitted between the sliding and standing ways. In most cases, however, triggers are used to release the ship. There are several types available, hydraulic, mechanical, and electrical-mechanical triggers having been used. Electrical-mechanical triggers are commonly used for rapid simultaneous release in modern practice. The hydraulic trigger is less easily installed and less safe. The electrical- mechanical trigger illustrated in the Figure 7-3 below is generally located near amidships and a small pit is provided in the berth to accommodate the falling levers. A number of triggers will be fitted depending on the size of the vessel to be launched; in the case of the 75,000-tonne bulk carrier for which a launching sequence is given below, six triggers were fitted for the launch. These triggers are in effect a simple system of levers that allow the large loads acting down the ways to be balanced by a small load on the releasing gear. The principle is often compared with that of a simple mechanical reduction gearing. 71 Figure 7-2 Launching ways-fore poppet Figure 7-3 Release arrangement for ship launch Simultaneous release of the triggers is achieved by means of catches held by solenoids wired in a common circuit. These are released immediately the circuit current is reversed. 7.6 Launching sequence As a guide to the procedure leading up to the launch, the following example is given for the launch of a 75,000-tonne bulk carrier. The launch ways have been built up as the ship is erected from aft; the ways have been greased and the cradle erected.