OOW Ship Construction Notes Sept 2024 PDF

SHIP CONSTRUCTION Nautical Science Year 3 / Officer of the Watch Module Ship Construction Notes forming an element of the module Ship Stability & Construction...

SHIP CONSTRUCTION Nautical Science Year 3 / Officer of the Watch Module Ship Construction Notes forming an element of the module Ship Stability & Construction Kim Mulcahy September 2024 Contents Module introduction............................................................................................. 1 Exam question breakdown.................................................................................... 1 Ship construction assignment guidelines............................................................... 3 Ship design and building process........................................................................... 4 Ship Construction definitions & terminology.......................................................... 9 Ship stresses........................................................................................................13 Welding process...................................................................................................22 Corrosion.............................................................................................................29 Protective coatings..............................................................................................33 Classification, maintenance & survey...................................................................35 Design features....................................................................................................41 Ship framing systems...........................................................................................48 Double bottoms & duct keels...............................................................................50 Pipes & pumping arrangements...........................................................................55 Bilge systems.......................................................................................................56 Ship structures.....................................................................................................58 Side framing........................................................................................................63 Fore end structure................................................................................................77 Aft end construction.............................................................................................83 Steering gear.......................................................................................................87 Load line rules......................................................................................................88 Hull openings.......................................................................................................89 Ship construction sample questions......................................................................93 Module introduction Ship Stability & Construction is a shared module. The final exam is split between each section with a 60/40 split between stability and construction, respectively. For Ordinary Degree purposes a pass mark of 40% is required. However to be eligible to attend the oral examination with the Department of Transport in Dublin a pass mark of 50% is required. Only exams attempted with a valid Notice of Eligibility will be accepted by the Department of Transport. The learning outcomes for this module are: 1. Complete the calculations necessary to show compliance with IMO Intact Stability for typical loaded conditions on board cargo ships. 2. Outline the design and construction requirements for specialised ship types. 3. Illustrate structural arrangements for merchant vessels. 4. State the requirements for watertight integrity and subdivision. 5. Outline the design and operation of ancillary systems on merchant vessels. 6. Break down the processes of ship building and maintenance. The module will be assessed by two in class assessments and one end of term (final) examination. Stability: One assessment consisting of an open-book Examination using the Arklow Wave Stability Booklet. This is worth 10% of the overall module final grade. Construction: One assessment consisting of an assignment outlining the construction of specialised ship types. This is worth 10% of the overall module final grade. Final Exam: The final assessment method is a two-hour end of semester final exam worth 80% of the overall final grade. These three combine to give an overall final grade for this module. Reassessment (if required) is by means of a repeat examination. Exam question breakdown 1 Outline the design and construction requirements for specialised ship types. Draw transverse midship cross section of “X ship type”. List the design features of “X ship type”. 2 Illustrate structural arrangements for merchant vessels. You may be asked structural arrangements of a particular ship type. For example; Draw a duct keel. Draw a double bottom structure. Draw a bow section etc. High stress areas and methods of compensation The question might also involve a brief descriptive element. For example; briefly explain the main areas of structural concern on a bulk carrier / container ship / double hull tanker & how these are overcome. 1 3 State the requirements for watertight integrity and subdivision. Why do we have subdivision? How is it achieved? What makes a vessel watertight? Draw specific items that achieve water tightness, What would you look for on inspection of a hatch cover? What affects its integrity? 4 Outline the design and operation of ancillary systems on merchant vessels. Ballast systems Bilge systems Heeling systems Steering systems Rudders etc. 5 Break down the processes of ship building and maintenance. Modern Ship building process Onboard maintenance Corrosion control See the final section of the notes for sample examination questions. 2 Ship construction assignment guidelines Deadline Week 6, commencing Monday 14th October 2024. Length 1,500 words approx. with drawings / photos etc. The project should be based on specific items that are a design feature of a specialised vessel, for example Watertight integrity on oil tankers Stability for tankers and container vessels Structural strength for bulk carriers Safety (e.g. for cargo containment systems) Examples (Concentrate on construction aspects) o Cargo containment systems o Hatch covers / Shell openings etc. - specifically how water tightness is achieved o Bow construction - bulbous bows o Anti-heeling systems for container ships o Stabilisers on passenger ships o Specialised steering gear systems o Design features of Ro-Ro vessels Include why the specific item is required, what led to its current design, comparison with other similar designs, advantages / disadvantages, what it achieves and some latest thinking or possible future development. Photos and diagrams should be included but you should also include your own drawings. Remember to focus on the construction aspects. Try not to base it on a vessel you have just been on; it is supposed to be a learning process. (But if you do ensure to include your own experiences.) During the course you will be given drawings of various ship types, please do not return the class notes and diagrams in your assignment. The point of the assignment is to see what you understand about the areas that you choose to explore rather than copying various details in notes. Everyone has their own way of researching and explaining things, and that is what the assignment is examining. Lay out (include at least the following) Cover page Title Contents Introduction Main content Conclusion References (Book title, edition, author, chapter, page number, full web link etc.) Notes All work submitted is subject to viewing by an external examiner. Plagiarism will not be tolerated; software WILL be used to check ALL assignments. Ensure your name is on your assignment and do NOT leave work unattended on a lecture’s desk or elsewhere. A hard copy of the assignment is to be handed in on or before the deadline date AND an electronic word document submitted to Canvas. If you have issues with Canvas, it can also be e-mailed to [email protected] on/before the deadline. MTU Marking Late Penalty: up to 1 week -10%, 2 weeks -20%, more than 2 weeks 100% Name your assignment as follows: Surname, First Name, Project Title, Deadline date E.g. “Mulcahy Kim - Ice Class vessels, October 2024”. 3 Ship design and building process The design of a ship is a complex problem for which there is no ideal solution. Many conflicting requirements have to be met as fully as possible in order to produce a profitable ship which will also meet relevant safety standards. Given these requirements, however, it is possible to work towards the ideal design, and this becomes manifest in the ever-changing pattern of shipbuilding and design. This is a continuous process of change, caused by widely diverse factors, and has in the past been responsible for the change from wooden to steel ships and from sail to steam. The process still continues, and is currently showing a trend towards larger, faster, and more highly specialised ships. Factors Affecting Ship Design Economic The merchant ship is a commercial undertaking and must earn a profit in order to provide a continuous service. Consequently, before ordering a new vessel, the ship owner will carefully assess the economic prospects of the proposed vessel. This requires an estimate of the cargo to be carried in the future, and the probably freight rate and extent of competition. The most economic size of ship depends on the quantity of cargo carried and the length of voyages. For example, the ship owner may have to choose between smaller but faster vessels and fewer larger and slower vessels, both of which could carry the same amount of cargo in a year. In this case, the two alternatives must be compared for profitability, and the choice is inevitably made on economic grounds. Many factors influence this assessment: changing patterns in world trade, competition changes in operating costs etc. and this creates a very complex problem. Commercial The trade in which a ship will operate has a significant influence on the design, for the most efficient vessels in one trade may be entirely unsuitable for another trade. This decides the general arrangement and main dimensions of the vessel - these could be limited by port facilities. Once the main hull dimensions are fixed, they cannot readily be altered, for the proportions of the hull influence the resistance and required engine power. Certain combinations of dimensions are more efficient than others, for they give lower hull resistance, and it is necessary to balance between the most efficient proportions in term of resistance and those required by the trade. The type of cargo to be carried determines the general arrangement of cargo spaces and the cargo gear. Much emphasis is now placed a rapid port speed. Legal The design must meet the statutory requirements dealing with hull construction load line tonnage measurement, crew accommodation, safety equipment oil pollution etc., many of which form part of the International Convention in the Safety of Life at Sea, which aims at securing a uniform international standard. These regulations ensure uniform minimum standards of seaworthiness and safety. Ships are built in accordance with the rules of a classification society such as Lloyds Register of Shipping. This gives the ship owner and insurance underwriter a form of guarantee that the ship is soundly designed and constructed. These guarantees are a commercial necessity. Technical Advances in technology give the designer more latitude in the design of the ship, and the ship owner more flexibility in operation. Perhaps the most significant developments 4 in recent years have been the application of computer aided design and computer aided manufacturing techniques to ship construction. This has led to new form of construction and to the reorganisation of shipyards on production line methods, using much more capital equipment than previously. Many other advances have also been incorporated in new vessels including welding and new materials. Summary A good ship design represents the best possible compromise between economic, commercial, legal and technical factors, in an attempt to provide the ship owner with a seaworthy ship which can operate at the highest possible level of economic efficiency. As these factors change the design of new ships will be constantly modified to reflect these changes and ensure a safe environment. Shipbuilding and processes Plan approval The fundamental design plans and basic construction details must receive classification society and the ship owner’s approval. Unusual aspects of design & construction methods and any departures from standard practice receive special attention. Progress is not hindered by the classification societies; whose main concern is the production of a safe ship. The ship owner will normally clearly indicate their requirements from the design stage and the approval of plans is usually straightforward. Plan issue With plan approval the ordering of equipment, machinery, steel section and plate etc. will begin. Plans will be issued to the various production departments in the shipyard. The classification society, owners and their representatives in the shipyard also receive copies of the plans. During the manufacturing processes, amendments may be made to the plans. A system of plan recall, replacement or modification must be available to ensure that any future sister ships do not have the same faults and corrective action has been taken. Steel ordering The ordering of steel to ensure availability in line with requirements is essential. It must begin at the earliest opportunity. (Possibly before plan approval, where delivery problems may occur.) The steel ordering requires involvement of the drawing office, planning and production departments and the steel supplier. The monitoring and control of stock is important as the steel for a ship is a substantial part of the ship’s final cost and stock held by a shipyard represents a considerable capital investment. Stockyard Usually about three months’ supply kept in stock. Steel plates and sections are usually stored in separate stockyards. Mangles Here plates from the stockyard are rolled as flat as possible. Loft work Would have taken place in a large, covered area with a wooden floor upon which the ship’s details are drawn to a smaller more convenient scale. Much of the traditional loft work is now done by computer. (Traditionally the lines plan & working drawing information was converted into full-scale lines drawn on the loft floor. A 1-50th scale half-block model was constructed, which had the exact lines of the ship and was used to mark out the actual plates on the shell. 5 Shot Blasting All materials plates and sections are in most cases shot blasted to remove rust and mill scale. Steel plates and sections are taken from the stockyards and fed into shot-blasting and priming machines. The plates are cleaned by abrasive shot/grit and coated with a suitable prefabrication priming paint to a limited thickness, for ease of welding. The material transfer before, during and after the various processes in shipbuilding utilises many handling appliances such as overhead travelling cranes, roller conveyors, forklift equipment. The various steel parts in plate and section form are now joined together by welding to produce subassemblies, assemblies and units. A subassembly is several pieces of steel making up a two-dimensional part which, together with other subassemblies, join to form a unit. Subassemblies may weigh up to 5 tonnes or more and examples would be transverses, minor bulkheads and web frames. An assembly consists of larger, usually three-dimensional, structures of plating and sections weighing up to 20 tonnes. E.g. Flat panels & bulkheads consisting of various pieces of shell plating with stiffeners and perhaps deep webs crossing the stiffeners. Units are complex built-up sections of a ship; perhaps the complete fore end forward of the collision bulkhead, and can weigh more than 100 tonnes, their size being limited by the transportation capacity of the yard’s equipment. Unit Fabrication The size of the units to be built is primarily dependent on the handling facilities available, both from a weight point of view, and the dimensions that can be handled. Generally unit sizes are in the 20 to 100 tonnes range. The various subassemblies, assemblies or units are moved on to the building berth or storage area until required for erection at the ship. At this stage items of pipe work and machinery may be fitted into the unit in what is known as pre-outfitting. Once erected at the berth the units are cut to size, where necessary, by the removal of excess material. The units are faired and tack welded one to another and finally welded into place to form the hull of the ship. 6 Shipbuilding processes Preliminary ship design  Drawing of detailed plans Steel ordered   Approval of plans and issue Steel delivered  Loft work and production table of offsets  Issue of steel and production begun  Material preparation - shot blasting and priming  Manufacture of plates and sections - marking, cutting, machining and shaping  Subassemblies and assemblies produced  Units fabricated and delivered to the berth  Units erected, faired and welded The flow chart shows progression through the various stages of production. Shipyard layout The shipyard layout is arranged to provide a logical, ordered flow of materials and equipment towards the final unit build-up, erection and outfitting of the ship. The various production stages are arranged in work areas or ‘shops’ and, as far as practicable in modern yards, take place under cover. The layout of the shipyard should aim to reduce materials handling to a minimum by suitable location of various workstations or areas. The building of large units and the capacity to transport them will reduce the number of items handled but will require greater care and sophisticated equipment. The building of a ship is as much governed by the shipyard layout as the materials handling equipment and its capacity. The shipyard layout is shown below. 7 Types and testing of steel Types of Steel - Because of the introduction of all-welded ships and the increase of their size, ordinary mild steel is no longer suitable for some purposes and special steels are often used for sheer strakes, bilge strakes and other highly- stressed parts of ships. Most of these steels are ‘notch-resistant’: that is, they are less likely to crack in the way of a notch when they are welded. Lloyds’ Rules now quote five general types: Grade A is ordinary mild steel. Grade B is generally similar to grade A but is notch resistant. Grades C, D and E are tougher steels and are also notch-resistant. Grade E is the strongest and grade C the least strong of the three. In addition, special high-tensile steels may be used for some purposes. These are generally similar to the ordinary grades but have greater tensile strength. They are denoted by the suffix 'H'; that is, AH, BH, EH, etc. All steel which is to be built into a ship must be tested at the place where it is manufactured and then stamped with a special mark, authorised by the Society. Test pieces of certain standard dimensions are to be used for this purpose. All steels, other than Grade A, must withstand an impact test, known as a ‘Charpy V Notch Test’, in order to determine their toughness. Short lengths of bar, of a length equal to twice their diameter, are to be compressed, cold, to half their length without fracturing. This is called a 'dump test'. Steel Castings-Steel castings must have a tensile strength of between 41 to 50 kg/mm", with an elongation of not less than 20 per cent. Test pieces of standard dimensions are to be taken and bent cold through an angle of 120°, without fracture, the internal diameter of the bend being not greater than 60 millimetres. All-important castings must undergo a magnetic test for flaws. 8 Ship Construction definitions & terminology Forward Perpendicular (FP) An imaginary line drawn perpendicular to the waterline at the point where the forward edge of the stem intersects the summer load line. After Perpendicular (AP) An imaginary line drawn perpendicular to the waterline at the point where the after edge of the rudder post meets the summer load line or if no rudder post is fitted, the centreline of the rudder stock is taken. Length Between Perpendiculars (LBP) The distance between the forward and after perpendiculars measured along the summer load line. Length Overall (LOA) The distance between the extreme points of the ship forward and aft. Amidships The point midway between the forward and after perpendiculars. A special symbol is used. Sheer The curvature of the deck in a fore and aft direction, rising from amidships to a maximum at the ends. The sheer forward is usually twice that aft. Sheer on exposed decks makes a ship more seaworthy by raising the deck at the fore and after ends further from the water and by reducing the volume of water coming on the deck. Rake The slope of the stem (bow), which is not usually vertical, but slopes aft from top to bottom. Draft Taken from the lowest point of the keel to the waterline. Freeboard The distance from the waterline to the top of the deck plating at the side of the deck amidships. Freeboard deck The freeboard deck is the uppermost continuous deck exposed to the weather and sea which has permanent means of closing all openings, and below which all openings in the ship’s side have watertight closings. Camber (or rounded beam) The transverse curvature of the deck from the centreline down to the sides. This camber is used to drive water to the sides of the ship. Most modern ships have decks which are flat transversely over the width of the hatch or centre tanks and slope down towards the side of the ship. Tumblehome An inward curvature of the midship side shell above the summer load line. (In the region of the upper deck) 9 Rise of the Floor The angle between the base line of the top edge of the keel and the bottom shell plating. The bottom shell of a ship is sometimes sloped up from the keel to the bilge to facilitate drainage. This rise of the floor is small 150mm being usual. Base line A horizontal line drawn at the top of the keel plate from midships. All vertical moulded dimensions are measured relative to this line. (Usually the lowest external edge of the keel) Breadth Extreme (B. Ext.) The greatest breadth of the ship, measured to the outside of the shell plating. Breadth Moulded (B. Mid.) The greatest breadth of the ship measured to the inside of the inside strakes of shell plating. Depth Extreme (Depth Ext.) The depth of the ship measured from the underside of the keel to the top of the deck beam at the side of the uppermost continuous deck amidships. Depth Moulded (Depth Mid.) The depth measured from the top of the keel. Flare This is the flaring out of the bows of a ship. It helps to prevent water from coming on board and gives more width to the foc’sle head. Super structure A structure on the freeboard deck extending from side to side, and enclosed by sides, end bulkheads and decks. Scantlings Term used to describe the measurements of steel sections used in ship construction. The dimensions and thicknesses of sections and plates which together compose the ship's main structure and provide the required strength. Scantlings are determined by the size of ship, type of cargo, service etc. and are laid down by classification societies. Intercostals Composed of separate parts, non-continuous. These are plates, angles etc. fitted down between the others or cut to allow other parts to pass through them. Side girders in the inner bottom are common examples. Other examples are stringers (horizontal plates) fitted between the ships side frames. Light Displacement Weight of a ship only with gear and machinery fitted (boilers filled to working level) but with no ballast. Load Displacement Weight of a ship and all on her when loaded to her marks. Deadweight The difference between load and light displacements. It is the carrying capacity of the ship. This is the number of tonnes of cargo, stores, bunkers, fresh water, crew, passengers, etc., on a ship when loaded to her marks. 10 Student Drawing Ship Terminology longitudinal section Student Drawing Ship Terminology transverse section 11 Bale Capacity The cubic capacity of a cargo space in cubic metres using the breadth from inside the cargo batons, height from the top of the deck ceiling to the underside of the deck beams and the length from inside of the bulkhead stiffeners. There is a bale capacity for each cargo compartment which when added together with all the other compartments and spaces is the ship's bale capacity. Grain Capacity This is the cargo compartments cubic capacity, length, breadth and height measured from inside the shell plating or tank top less an allowance for frames etc. All the compartments, grin capacity added together give the ship's total grain capacity. Gross Tonnage This is a measurement of the space inside the ship. It is an indication of the size of the vessel. All ships Gross Tonnages are calculated with a special agreed formula. The tonnage is recognised worldwide. Although, it is called tonnage and the units are tons, it does not represent weight. The figure is used for many items including the application of regulations. Net Tonnage This is an indication of the cargo carrying capacity of the ship. It represents how much cargo can be carried in the cargo spaces. It does not represent any weight. It is used for calculating port dues. 12 Ship stresses The ship at sea or lying-in still water is being constantly subjected to a wide variety of stresses and strains, which result from the action of forces from outside and within the ship. The ship must always be able to resist and withstand these stresses and strains throughout its structure. It must therefore be constructed in a manner and of such materials that will provide the necessary strength. The ship must be able to function efficiently as a cargo-carrying vessel. The types of stress are compression, tensile, shear and torsion. Compression load Tensile load Shear load Point load Continuous load Static forces are due to the differences in weight and buoyancy which occur at various points along the ships structure causing shear forces within the ship. Dynamic forces result from the ship’s motion in a seaway and the action of the wind and waves. These will increase the effects of ship stresses. Stresses are divided into two broad areas. Structural Stress - those affecting the ship as a whole, longitudinally and transversely. Exterior forces include the hydrostatic pressure of the water on the hull and the action of wind and waves. Longitudinal forces are the greatest in magnitude and result in bending of the ship along its length. Local Stress - those affecting a particular part of a ship. Forces from within the ship result from structural weight, cargo, machinery, and the effects of operating machinery. 13 Structural Stress Hogging Hogging occurs when the bending moment is negative and tends to cause the ends to deflect downwards relative to the centre. This can be caused by excess weight in the ends of the vessel, and little weight amidships, where buoyancy is generally greatest. It must be remembered that ships in service will encounter waves, and this will alter the distribution of buoyancy, tending to accentuate hogging when a crest is situated amidships and a trough at each end. In estimating the required longitudinal strength of the hull, the most severe conditions must be provided for, and so it is standard practice to assume a wave with length equal to the length of the ship. Sagging Sagging occurs when the amidships part of the hull sags relative to the ends. This can be caused by excessive weight amidships and will be accentuated when the trough of the wave is amidships with a crest at each end. In the sagging condition, the deck will be in compression and the bottom in tension. Student Drawing Hogging & sagging In the case of both hogging and sagging, maximum stress will occur at the decks and bottom, passing from compression to tension at the passage of each wave. Maximum bending moment, and hence maximum stress, generally falls around amidships, and it is essential to ensure that there is sufficient continuous longitudinal material to prevent the stress becoming excessive. More material will be required at deck and bottom level because maximum stress occurs there, hence the need for sufficient stiffening by longitudinal material in the decks and bottom. In long vessels, the need for longitudinal strength becomes more acute, and these vessels will generally be framed longitudinally. Only continuous longitudinal material can be counted towards the longitudinal strength, hence the need for 14 continuity where these members are cut, for example, where longitudinal frames are cut at transverse bulkheads. Student Drawing Expansion & compression forces of hogging/sagging Shear Stress Ships are built with sufficient longitudinal strength to limit hogging and sagging stresses to reasonable levels which the structure can withstand, allowing a safety margin, and considering the additional stresses which may be induced by variation in the force of buoyancy as the ship encounters waves. It is assumed in all these design calculations that the ship will be properly loaded. A bad distribution of cargo can cause serious hogging or sagging. In order to assist the ship's officer in securing a good distribution of cargo, many ships are provided with loading calculators. The proposed loading is fed into the computer, which will then indicate the degree of hogging or sagging which will then take place, and if this is outside acceptable limits, suitable amendments in the distribution can then be made. Torsional Stress Torsional stresses are generated by wave actions on either ends of the ship in opposite directions causing a twisting motion along the length of the ship. Box girders are provided to accommodate the torsional stresses. (Reduce the twisting effect) Student Drawing Torsion Stress 15 Transverse Stresses Racking When a vessel is floating upright in still water, the forces of buoyancy are symmetrically distributed about each side of the ship’s centreline. Pressure on the port side is equal and opposite to pressure on the starboard side, and the resultant buoyancy force acts vertically upwards. When the ship is rolling in waves, however, the symmetrical distribution of buoyancy disappears, and the unsymmetrical distribution of buoyancy that is set up will cause transverse loading. The effect is at its greatest when the ship is in ballast. This transverse stress is termed racking and would result in transverse strain if the transverse structure were not stiffened. Transverse bulkheads are most effective in resisting racking, but these can only be fitted at intervals along the length of the hull. Intermediate spaces are strengthened against racking by transverse floors, beams and beam knees (joining transverse beams to side frames). Web beams, frames and transverse beams are fitted where additional transverse strength is required. Student Drawing Racking Water Pressure Fluid exerts a pressure on any submerged surface which increases with depth (pressure - depth x density) and always acts on the steel plating of the hull at right angles to the plating. Allowance must be made for stiffening the plating; otherwise the stress generated by the water pressure might become excessive. The bottom shell plating will therefore be subjected to water pressure stress in addition to the hogging and sagging stresses caused by longitudinal bending moments. The shell plating is stiffened against water pressure stresses by (a) Increasing the thickness of the plating, and (b) Fitting stiffeners at intervals. 16 Student Drawing Water Pressure Dry docking While the hull is afloat the water pressure which provides the upward force of buoyancy, is distributed over the entire underwater surface. When a ship is in dry dock, the weight of the ship is concentrated along the line of the keel which causes areas of the hull to bulge outwards. This would produce severe transverse strain if no support from shores or bilge blocks were provided. Internally, the ship is stiffened transversely by transverse floors and bulkheads web frames and tank side brackets. External support is provided for smaller vessels by erecting shores or blocks in the dry dock in the region of the bilge and shoring the sides of the ship from the side of the dock. For larger vessels, the keel blocks take the bulk of the ship’s weight, and additional support is provided in the region of the bilges of adjustable blocks which can be set by fluid pressure to provide a given amount of support at selected distances from the centreline. This provides a more effective form of support, as the distribution can be altered to suit different types of vessels. Student Drawing Dry Docking 1 of 2 Student Drawing Dry Docking 2 of 2 17 Local Stresses Panting Panting is the term used to describe the vibrations of the shell plating and, framing in the positions which are especially liable to fluctuating water pressures when the ship is at sea and encountering waves. Although all parts of the shell plating will be subject to fluctuating water pressures by the movement of the ship and waves, the effect will be most pronounced when the ship is pitching, and more severe forward than aft due to the ship’s forward motion. (It can be experienced at the stern when following seas) Student Drawing Local Stresses - Panting Water pressure on the forward shell plating will be increased when the bows drive into an oncoming wave, while seconds later, the crest of the wave will have passed aft, relieving the water pressure temporarily until the bows are driven into the next wave. This fluctuating pressure will cause a “panting” or “in and out” working of the side shell plating which will be increased by the forward motion of the ship. To resist this, the ship’s structure must be strengthened for 15 – 20% of the ship’s length with panting beams approx. 2 – 2.5m apart and stringers secured to the shell fitted forward of the collision bulkhead. The thickness of the side plating in the panting region may also be increased. Student Drawing - Panting Arrangements 18 Pounding When a ship is lightly loaded and forced at speed through waves, the fore end tends to rise out of the water on the wave, and then beat down heavily into the crest of the following wave. This is known as pounding, and subjects the bottom of the hull, for about 1/5th of the ships length, to a series of impulsive forces which would result in buckling of the plating if stiffening was inadequate or pounding excessive. The forward part of the bottom structure is stiffened to resist pounding by increasing the thickness of shell plating and stiffening it with additional force and aft and transverse plate floors. Lightly loaded ships are particularly susceptible to pounding in head seas, and this can always be remedied by a timely reduction of speed, which can avert damage. Student Drawing Local Stresses - Pounding Local Loading Ship structural members are often subjected to high stresses in localized areas, and great care is required to ensure that these areas are correctly designed. This is particularly the case where various load carrying members of the ship intersect e.g. longitudinals meeting transverse bulkheads, intersections of longitudinal and transverse bulkheads. Other highly stressed areas occur where there is a discontinuity of the hull girder at ends of deck house structures which can cause cracking. Sharp discontinuities are avoided by using gradual tapers and thicker plating. High local stresses are minimised at openings cut in decks by using rounded corners and sometimes doubling plates. Concentration of weights of machinery, derrick posts and masts, deck cargoes etc. will produce a concentration of stress which requires local stiffening e.g. the engine room must be stiffened to support the concentrated weight of machinery over comparatively small areas and be able to withstand the vibration produced by the machinery. Vibration is also present in the after part of the hull in the region of the propeller, where additional strengthening is provided by increased thickness of shell plating and a system of deep floors. 19 Student Drawing Local Loading Members compensating for stress Heavy Water Local Hog & Dry- Racking SF BM Pounding Panting weight pressure Stress Sag docking Beam knee    Beams     Bulkheads       Decks      Floors       Frames     Long'       girders Pillars     Shell           plating Ships structures to withstand stresses Shell Plating Compensates for all ships' stresses. Plating is thickened about the keel area in the garboard strake and in the sheer strake. It also is given increased thickness to combat localized stresses as would be expected in the region of shell door openings. Frames Compensate for water pressure, panting effects and dry-docking stresses. They maybe compared to the ribs of the human body which stiffen the body overall. Ships may be constructed with longitudinal or transverse framing reinforcing the outer hull of the vessel. Longitudinal Girders Compensate for "Hogging and Sagging" influences. Dry docking and pounding stresses together with localized shearing influences. Beams Compensate for racking stresses and influences from water pressure. Also local stresses incurred from heavy weights. 20 Pillars Extensively found in general cargo vessels in lower hold structures compensate for stresses caused by heavy weights, racking, dry docking and water pressure influences. Bulkheads Tend to compensate for most stresses, including racking, hogging and sagging, shearing dry docking and from heavy concentrated weights. Beam Knees Compensate for heavy weight and localized stresses as well as racking. Floors Compensate for pounding and vibration, dry dock stresses and localized stresses from water pressure, heavy weights and racking. Decks Compensate for hogging, sagging, shearing, bending and water pressure stresses. Decks and deck stringers provide compensation to stresses incurred from heavy loading. Monitoring Ship Stresses at Sea To improve safety during shipboard operations real time motion and stress monitoring information equipment can be fitted to a ship. This requires the fitting of strain gauges to the deck structure, an accelerometer and a personal computer with software that displays ship stress and motion readings on the bridge. An alarm is activated if the safety limits are exceeded, enabling remedial action to be taken. 21 Welding process Oxy-acetylene welding A gas flame produced by the burning of oxygen and acetylene is used in this process. A hand-held torch is used to direct the flame around the parent metal and filler rods provide the metal for the joint. Gas welding is little used, having been superseded by the faster process of electric arc welding. Outfit trades, such as plumbers, may employ gas welding or use the gas flame for brazing or silver soldering. Electric Arc Welding An electric arc is formed when an electric current passes between two electrodes which are separated by a short distance from each other in air. One electrode is the welding rod and the other is the metal to be welded. The arc is started by momentarily touching the metal with the rod and then withdrawing it to about 3 or 4mm from the plate. When the electrode touches the metal a current flows; when it is withdrawn from the plate a current continues to flow in the form of a spark. The electrical energy is converted into heat and light (temp. 3500 - 4000 C). This heat melts the parent metal and the rod, to form the weld. Metal is transferred from the electrode to the plate in the form of drops or globules which vary in size according to the current and type of rod used. When a bare wire electrode is used, it is found that they are difficult to control. The globules are exposed to atmosphere, oxygen and nitrogen are absorbed and this tends to make the weld porous and brittle. Electric Arc welding Shielding the arc can reduce these faults, and this is done by coating the rod with a flux. This-flux releases inert gas which does not combine with the atmosphere. The shielded arc coating also melts at a higher temperature than the metal core and the coating extends a little beyond the core, concentrating and directing the arc stream. The slag covering also protects the metal from the atmosphere while cooling and is easily removed, by chipping, after the weld has cooled. 22 Processes using gas These are welding processes employing a bare electrode or welding wire with a gas shield. Automatic or semi-automatic operation is usual. With automatic operation, once set the process is controlled by the machine. In semi-automatic operation certain machine settings are made but the torch is handheld and the process is to some extent controlled by the operator. Tungsten inert gas (TIG) This is a process for thin sheet metal such as steel or aluminium. A water-cooled non- consumable tungsten electrode and the plate material have an arc created between them by a high frequency discharge across the gap. The inert gas shield is usually argon gas. Metal Inert Gas (MIG) Process The MIG process makes use of an inert gas to shield the arc and the weld pool while filler material is provided by a bare wire. The process also differs from manual metal arc processes by using a D.C. power source. MIG was first introduced for welding aluminium and is suitable for magnesium alloys, alloy steels, stainless and heat resistant steels, copper and bronze. Different types of filler wires and shielding gas are used. In the MIG process a wire feed motor supplies 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 contact tube and work piece. Thermit Welding Basically a fusion process that takes place as a result of the heat released by a chemical reaction between iron oxide and powdered aluminium. Thermit steel is pure and contains few inclusions. Extremely useful for welding large sections such as stern frames and rudders. The molten steel and slag from the chemical reaction is formed in a crucible and then poured into the mould. The ends of the parts to be welded are built into a sand or graphite mould and the superheated metal is run into the mould. The molten metal fuses the parts to be welded. Types of Welds Butt Weld A butt weld is used for joining two plates or angles edge to edge. For thin plates, the plates are placed in contact and welded with a single run, but for thicker plates, it is necessary to leave a gap between the plates in order that the penetration should extend throughout the thickness of the plates, otherwise the joint will be weakened by lack of penetration. In many cases, where several runs of welding are required to complete the butt joint, the edges are formed to give a V or U shaped gap in order to ensure thorough penetration and reduce distortion. For V welds, it is usual, after completion of welding from one side, to cut out the root of the weld and apply a back sealing run from the other side. Fillet Weld A fillet weld is A fillet weld is used to join two surfaces at right angles to each other. For thin sections, the fillet weld can be completed at a single run, giving ample penetration. For heavier sections however, it is usual to V out the vertical member on one or both 23 sides in order to secure adequate penetration and a sound weld. These welds generally require several runs of welding before they can be completed. Single Fillet up to 6mm no edge preparation Single "Vee" Fillet - 6mm and over - vertical leg of the joint is used so that the weld metal can penetrate right through. Double Fillet - for stronger connections and water tightness Lap Fillet Weld Used for overlapping plates, e.g. onto beams and frames etc. Fillet welds may be continuous or intermittent depending on the structural effectiveness of the member to be welded. Where fillets are intermittent, they may be staggered or chain welded. When welding greater thickness than about 6mm manually it becomes necessary to make more than one welding pass to deposit enough weld metal to close the joint. An automatic welding machine can weld thicker plates with one pass but a stage of thickness will be reached when more than one pass is required. In order to reduce the amount of edge preparation required, deep penetration electrodes have been developed suitable for welding reasonably thick plates as a single run. Student Drawing Types of Welds 24 Edge Preparation Square Butt up to 6mm no edge preparation required. Single Vee 6/18mm these plates are prepared by "veeing" the plate edges so that the angle between them is 60°. Double Vee 18mm + used for manually welding thicker plates. Student Drawing Edge Preparation Typical Weld Faults Lack of Fusion The weld metal does not fuse with the parent metal throughout the depth of the weld, leading to a weakness in the joint. Lack of Penetration Due to failure of the weld to reach through the joint. Undercut The plate is melted, reducing it below its original form, leaving a possible trap for slag inclusion. Slag Inclusions Where slag has been trapped beneath a later run of welding, causing a weak spot. Porosity Small spherical or tubular spaces within the weld caused by entrapment of gas in the weld. Cracks Either surface or internal, can arise where the joint is restrained and cannot contract on cooling. 25 Student Drawing Typical Weld Faults These various faults may be due to several factors, such as bad design, use of wrong materials, incorrect welding procedure or bad workmanship. Since defective welds are potentially dangerous, a thorough inspection of welded work is carried out by competent persons to detect any flaws. Non-Destructive Testing 1. Visual examinations 2. Dye penetrant 3. Magnetic particle 4. Radiographic 5. Ultrasonic Visual Examination The trained inspector and surveyor will notice surface defects in a weld such as undercutting, incorrect bead shape, bad stop and start points, incorrect alignment and surface cracks. This is a cheap method of inspection and will be quite effective if such inspections are regularly carried out before, during and after welding. Radiographic Inspection Sub-surface and internal defects cannot be observed visually, but such defects' can be detected by use of X-rays. A surveyor may request such examination especially in known regions of high stress to ensure a high standard of workmanship. Stresses and Distortion in Welding Good design should ensure that welded joints should be accessible, preferably for down hand welding. As welding proceeds, heat flows out from the pool of molten metal, resulting in cooling of the joint at a rate depending on the size of the piece, type of electrode, and current used. Varying conditions of expansion and contraction surround the weld, setting up forces in the weld and parent metal. The forces which remain will set up stresses usually referred to as residual stresses. It is generally known that a weld on cooling shrinks and tends to pull the plates with it. This may result in a structural deflection depending on the amount of restraint. 26 Shrinkage occurs in a butt weld principally along the length of the weld, and to a lesser extent across it. If high restraint is relied on to control distortion, then the structure will contain high residual stresses, which should be avoided if possible. Student Drawing Stress and distortion in welds The correct procedure in welding can do much to reduce distortion. The fewer runs involved in a welded joint the less will be the distortion. Symmetrical welding either side of a joint with a double-vee will also produce a weld with very little distortion hence this technique is sometimes employed. Backstep method to reduce distortion Welding aluminium A number of processes for welding steel can also be used to weld aluminium, e.g. tungsten inert gas. Stir friction welding is a relative new technique utilising friction to weld aluminium. The material is clamped in a jig or fixture whereby a rotating probe is forced into the aluminium under high pressure. The metal is heated by friction until it becomes soft and unites behind the tool as it moves along the joint. The metal temperature remains below melting point throughout the process. When metal plates are greater than 20 mm thick, the process is undertaken from both sides. Metals up to 30 mm thick can be joined with a full penetration joint. 27 Student Drawing Welding Aluminium to Steel Sealing compound Welding aluminium to steel The bimetallic joint where, for example, an aluminium superstructure joins a steel hull, was previously bolted with appropriate insulation to prevent galvanic corrosion. Now, a welded joint is generally used with a transition bar fitted between the two materials. The transition bar is an explosively bonded laminate of steel and aluminium alloy with pure aluminium used at the interface. One example is ‘Tri-Clad’, this is initially a 'sandwich' of aluminium alloy, pure aluminium and steel, with polystyrene spacers between and a layer of dynamite on top. When the dynamite is detonated the plates are forced together and, in effect, welded into a single transition plate. After welding, or cladding as this process is often called, the plates are flattened and 100 per cent ultrasonically tested. This material has been accepted for marine use by most classification societies. Corrosion is not a problem with transition plates as they form an extremely hard and inert corrosion product; aluminium oxide hydrate. This acts as a seal at any unpainted or exposed part of the plate and renders the system passive. Differential expansion is also not a problem, as only a relatively small force will build up, even with a significant change in temperature. 28 Corrosion Definitions Corrosion The destruction of a metal by chemical or electro-chemical reaction with its environment. Erosion The destruction of a material by abrasive action of a liquid or gas, often accelerated by the presence of solid particles of matter in suspension. The wastage or wearing away is purely mechanical with no electro-chemical action whatever. Corrosion/Erosion The combined effects of corrosion and erosion in which the surface film at the metal/environment interface is continually removed by mechanical means making fresh unprotected metal available for further attach by corrosion. Nature of Corrosion The basic cause of the corrosion of metals is their spontaneous tendency to return to their stable state, i.e. the natural ore. Where this reaction takes place in the atmosphere it is thought of as ‘rusting’ and as a chemical reaction explained by the simple formula [ Fe + 2 H 2 O = F2 O2 + 2 H 2 ] This means that rust will not form unless both oxygen and water are present. The rate of rusting will vary with humidity, where it is less than 65%, rusting will hardly occur at all. Where liquid water is present, the attach will be comparatively rapid – water pockets and absorbent filling materials should be avoided. The amount of oxygen present will also be a factor. Rusting will occur faster near the surface and hardly at all at great depths. An increasing temperature of the seawater will also increase the rate of rusting, however, the solubility of oxygen in seawater decreases as the temperature rises counteracting the original effect. The velocity of the water passing along the steel surface is another factor, the faster the more plentiful the oxygen supply. Corrosion due to rusting is of a general nature and is not the major cause for concern because the rate of uniform corrosion is relatively small. Local corrosion occurs at a faster rate and is the result of an electro-chemical process. In this process we have two reactions, the anodic reaction is the dissolution of the iron; the cathodic involves the consumption of oxygen. The reaction may occur on different parts of the surface and can result in the local corrosion or ‘pitting’. This electro-chemical reaction is increased where dissimilar metals are placed in an electrolyte. The corrosion rates of metals and alloys have been extensively investigated and a table drawn up known as the galvanic series. 29 Corrosion – Galvanic Cell Student Drawing Corrosion – Corrosion Cell Parts of ship most liable to corrosion (a) Areas where dissimilar metals are present: (i) Stern region due to bronze propeller (ii) Vicinity of E/R inlets and discharges (iii) Valve fittings in tanks (iv) Welded or riveted seams (v) Aluminium superstructure (vi) Steel lifting hooks in aluminium lifeboats (vii) Openings in superstructures, portholes and windows (b) Areas in which a metal has a different electrical potential. (c) Areas where water would accumulate i.e. deck stringer etc. 30 Mill-scale This is the bluish-black scale of iron oxide that forms on steel plates during hot rolling at the steel mill. Because of the fact that the scale is very corrosion resistant, the anodic process of corrosion cannot take place where the mill-scale is pore-free and adherent. It is unlikely that mill-scale will be continuous over a surface; pores and fissures will exist particularly at welded seams, burned edges and areas where vibration takes place. This could result in severe corrosion occurring at these places and the shipyard practice of using ‘weathered’ plates with the mill-scale still present has been replaced by a system whereby the plates are shot-blasted and then dipped in a shop primer paint. Corrosion Control The prevention of corrosion may be broadly considered in two forms, cathodic protection and the application of protective coatings. Modern design has also played a part in reducing parts likely to corrode. Cathodic protection This can only be used where metals are immersed in an electrolyte. The fundamental principle is that the anodic corrosion reactions are suppressed by the application of an opposing current. This superimposed direct electric current enters the metal and lowers the potential of the anode metal so that they become cathodic. The two main types of cathodic protection used are sacrificial anode systems and impressed current systems. (i) Sacrificial anodes These are metals of alloys, usually zinc or magnesium ignoble metals that are attached to the hull, i.e., less noble than steel when immersed in salt water. These anodes will supply the cathodic protection current, but will be consumed in doing so, and will require replacement usually at each dry-docking. This system is used in the vicinity of the bronze propeller and other immersed fittings. It is also used in ballast tanks and in cargo tanks of tankers. Student Drawing Corrosion Control – Sacrificial Anodes 31 (ii) Impressed current systems These systems can only be used in the protection of the immersed external hull only. The principle is that a voltage difference is maintained between the hull and fitted anodes, which will protect the hull against corrosion. Student Drawing Corrosion Control – Impressed Current System For normal operating conditions, the potential difference is maintained by means of an externally mounted silver/silver chloride reference cell detecting the voltage difference between itself and the hull. An amplifier controller is used to amplify the micro-range reference cell current, and it compares this with the pre-set protective potential value, which is to be maintained. Using the amplified dc signal from the controller, a suitable reactor controls a larger current from the ship’s electrical system, which is supplied to the hull anodes. Design Factors to Minimise Corrosion 1) Reduction in the use of dissimilar metals or different grades of steelwork, except where it is fundamentally required, will reduce corrosion sources. 2) Use of insulating materials between dissimilar metals e.g., aluminium/steel. 3) Careful use of sacrificial anodes, in ballast tanks directly attached to the hull can retain steelwork quality. 4) Suitable paint coatings applied to exposed hull areas or in cargo tanks (e.g., deep or chemical tanks) will provide additional protection against corrosive effects. 5) Adequate drainage features incorporated at the design stage, from decks, tanks, wells etc., will reduce the possibility of an accumulation of slack water and associated corrosion effects. 6) Use of corrosion resistant alloy steels, like with stainless steel bearings or valve seats, etc. 7) Fitting adequate insulation where extreme temperature ranges are expected to be experienced can expect to reduce brittle fractures. 8) The fitting of rubbing strakes or doubling plates to absorb wear and tear can provide some structural protection. 9) Removing mill scale from new steel work prior to fitting. 10) Inclusion of measures to minimise the likelihood of fatigue caused by welding techniques, vibration and employing stress relieving methods. 32 11) Installation of an impressed current system for monitoring of corrosion cell activity. (Include internal piping protection) 12) Taking advantage of an oxygen deficient atmosphere to reduce corrosive effects. (Installation of efficient scrubber, in an inert gas system.) Protective coatings Steel Mild steel for its strength to weight ratio and cheapness is one of the most widely used construction materials. However, it readily rusts and must be painted to prevent this corrosion and to provide to it a decorative appearance. Mill scale found on new steel is a hard, brittle coating of several distinct layers of iron oxides formed during processing of steel such as hot rolling girders, tank plates and other structural shapes. Usually bluish black in colour, mill scale cracks and fissures readily, and is permeable to both air and moisture. Mill scale is cathodic to the steel substrate and if left in place, corrosion will occur as a result of the electrical potential difference between them. Rust is an oxide of iron formed by the action of air and water. It is voluminous and occupies one and three-quarter times the volume of the steel from which it originated. Rust forming under a paint coating or through breaks in the coating can burst through and may creep under the coating resulting in flaking so that repair is both difficult and costly. It may cost a little more for a better surface preparation, but as the paint coating will last many times longer, the overall cost saving in maintenance will justify the initial expense. Paint Paint is the primary method of providing a protective coating for metal. It has two ways of approaching the problem of corrosion. By providing a barrier to prevent the electrolyte (sea water) from coming in contact with the metal, thus preventing cell formations. Normal paints are not sufficiently impervious to water to do this, but bituminous materials applied sufficiently thickly (about 2.5 mm=at least five coats) are reasonably water resistant. However, if the coating is damaged, exposing the metal, attack is highly concentrated; further, rust will tend to form behind the coating. By including in the paint, materials which stifle the electrochemical action. For this purpose, rust-inhibitive pigments, e.g. red lead, zinc chromate are used. Paints of this type depend for their effectiveness on the passage of water through the film in small amounts. They will not prevent corrosion completely, but they have the advantage over bituminous coatings of helping to stifle corrosion should the film be damaged. Traditional paint is a composition of the following ingredients: Pigment The pigment gives the paint its colour and covering capacity, in the case of primer, it is largely contributory to the active corrosion-preventing properties of these paints. Pigments for primers mainly extracted from lead, calcium, zinc and aluminium. For other paints, the pigments are obtained from many different materials depending upon the properties desired. Binding Agent 33 The binding agent or vehicle depending upon the proportion of it used in comparison with the amount of pigment will determine the consistency and application of the paint. The most important groups are those based on oils or alkyd resins. Paints based on linseed oil are dying out because of the comparatively long drying time (24 hours). Alkyds, a modern synthetic resin combined with vegetable oils and can be made to have faster drying, better durability, better gloss retention, greater waterproofing qualities, and better resistance to oil and chemicals. Adhesion to the surface is rather good but lacks the penetration quality of linseed oil. Solvent The solvent is added under certain conditions to make the paint easy flowing, care must be taken not to use excessive amounts of thinner. Nowadays each paint usually has its own special thinner. Drying Agent The drying agent is added in certain cases to accelerate drying of the paint and is included only in oil and alkyd paints. Surface preparation The shell plating should be examined for areas which need to be repaired. The whole surface of the shell is then cleaned and prepared for recoating. In some cases the hull may be cleaned down to the bare metal and completely recoated. Most situations will only require preparation of the surface for recoating. Proper surface preparation is essential for the success of any protective coating scheme. The importance of removing oil, grease, old coatings and surface contaminants (such as mill scale and rust on steel and zinc salts on galvanised surfaces) cannot be over emphasised. The performance of any paint coating is directly dependent upon the correct and thorough preparation of the surface prior to coating. The most expensive and technologically advanced coating system will fail if the surface preparation is incorrect or incomplete. Several methods are used for cleaning the ship's hull prior to recoating: Manual wire brushing and scraping with steel scrapers usually takes place on the wet surface as the water level drops in the dock. The finish is poor, operation is slow and the effectiveness varies according to the skill / effort of the operatives involved. Power discing or wire brushing uses an electric or pneumatic handheld machine. The method is slow but provides a relatively good finish. High pressure water jetting is used for hull cleaning. Water at pressures of 150-500 bar is directed on to the hull by a steel lance. The lower pressure is sufficient to remove weak fouling growths, while the higher pressure will clean the hull down to the bare metal. The results from this method are excellent and very fast (although time is lost while waiting for the hull to dry) It is a skilled operation requiring trained personnel for efficient safe performance. Shot-blasting or abrasive-only cleaning utilises a jet of abrasive at 5-7 bar pressure. This rapidly produces a clean dry surface ready for painting. The dusty, dirty nature of the work stops any other activities in the area. Abrasive and water-blasting combines in effect the foregoing two methods and claims the advantages of each. The method is fast, clean and effective, the abrasive speeding the cleaning and the water suppressing the dust. With this method and water jetting, 34 corrosion inhibitors are added to the water to allow time between cleaning, drying and painting. Paintwork Successful application of paint requires the correct technique during painting and suitable conditions during application. Painting should take place in warm dry weather. The presence of moisture in the air or on the metal surface may damage the paintwork or slow down its curing process. Where poor conditions are unavoidable, especially formulated paints for curing under these conditions should be used. Using shelters or awnings and a supply of warm air will improve curing & adhesion of the paint. Scuppers, discharges or overflows which direct water on to the surface being painted should be blocked or diverted before work begins. The main methods of paint application are the spray, roller and brush. Brush and roller application are used where rough surfaces exist and small often inaccessible areas are to be covered. This method is slow, labour intensive and difficult with certain types of paints. Airless spray is the fastest and cleanest application method. Throughout the preparation and painting of a ship the need for good safe, suitable means of access is paramount. Freedom of movement to maintain the correct distances for water jetting and paint spraying is essential. Free-standing scaffolding and hydraulically operated mobile platforms may be used. Paints in their various forms can be poisonous, skin irritants and of a highly inflammable nature. Adequate protection and ventilation are necessary. Care is required in the location and operation of equipment to avoid the possibility of fires and explosions. Most manufacturers apply their own symbols to paint containers to indicate the various hazards, in addition to any mandatory requirements on labelling. Classification, maintenance & survey The International Convention for the Safety of Life at Sea (SOLAS) is an international maritime treaty which sets minimum safety standards in the construction, equipment and operation of merchant ships. The SOLAS convention (Chapter 2 - Construction - Subdivision and stability, machinery and electrical installations) requires signatory flag states to ensure that ships flagged by them comply with at least these standards. All of the various types of ships must be surveyed and certified that they meet the requirements of the convention. The vast majority of vessels are registered with one or other of the Societies. Registration brings with it many advantages for the ship owners. A Classification Society can be defined as an independent organization which develops and updates published rules, regulations and standards for the safe design, construction and maintenance of ships capable of international trade. Classification societies certify that the construction of a vessel comply with relevant standards and carry out regular surveys in service to ensure continuing compliance with the standards. They are world- wide organizations which employ their own exclusive staff to inspect and assist the shipping industry to meet required standards. Classification surveyors inspect ships to make sure that the ship, its components and machinery are built and maintained according to the standards required for their class. They set technical standards for the building of ships and provide design advice, oil analysis and quality accreditation National administrations recognize various Classification Societies as official certification bodies. They delegate part of their authority to them to verify implementation of national and international regulations for marine safety and environmental protection. 35 These include such international conventions as Safety of Life at Sea (SOLAS), Load Lines (LL), Prevention of Pollution from Ships (MARPOL) and Tonnage Measurements (TONNAGE). Well known examples include American Bureau of Shipping (ABS) Bureau Veritas. (BV) BV is the largest organisation for classification. The International Association of Classification Societies (IACS-) is a non-governmental organization that plays a role within the International Maritime Organization (IMO). It provides technical support and guidance and develops unified interpretations of the international statutory regulations. Work of Classification Societies The Registers provides a reliable reference which supports the construction and repair to recognised standards, for their ships. Without the organisation of Classification Societies, many organisations such as underwriters, bankers, shippers and charter brokers would have great difficulty in conducting their businesses. The work of the Classification Societies includes (but is not limited to) the following: 1) Classification and survey of ships and floating structures. 2) Classification of nuclear ships. 3) The Classification of "Ice Breakers" and of ice strengthened ships. 4) ISM and ISPS certification. 5) The survey of container units. 6) Certification of Training Centres. 7) Carrying out Risk Assessments. 8) Consultation services. 9) Conducting international seminars on maritime affairs. 10) ISO certification. 11) Human element studies. 12) Provision of expertise for Mobile Offshore Drilling Units (MODU). 13) Provision of expertise for Floating Production, Storage and Offloading Systems (FPSO's). Conditions for Classification Ships which are constructed in accordance with the Rules and Regulations of the Classification Society will be assigned a class in the Register Book. They will continue to retain this class provided they are found by examination and survey, to be maintained in accordance with the rules. Compliance with conditions for the ship's hull and machinery is a condition for classification. Vessels are said to be in class when their hull, structures, machinery, and equipment conform to International Maritime Organization and MARPOL standards. Any damage or defect occurring to the vessel which could invalidate the conditions of the issue of class, must be reported to the Society. Suspension or Withdrawal of Class There are several reasons why the classification of a ship could be withdrawn or suspended. Where an infringement of the Rules and Regulations is incurred, class may be withdrawn. Where a vessel sails overloaded or with incorrectly marked load lines, class may be suspended. If a ship operates in waters for which she was not classed, then automatic suspension of class could be the result. 36 Types of Ship Survey Initial Survey: A complete Inspection of all items relating to the particular certificate before the ship enters service to ensure that it is in a satisfactory condition for the intended service. Periodical Survey: An inspection of items which relate to a particular certificate to ensure that the items are in a satisfactory condition and fit for the intended service. Renewal Survey: As for the periodical survey but leads to the renewal of the certificate. (5 years) Annual Survey: (+/- 3 months) A general inspection of the items which relate to a particular certificate to ensure that those items have been maintained and remain III a satisfactory condition for the service the ship is intended for. Additional Survey: An inspection either general or partial according to the circumstances, to be made after a repair resulting from a casualty, or whenever any important repairs or renewals are made. A ship requires regular overhaul and maintenance due to the severe operating conditions. Berthing, cargo loading & discharge, constant immersion in sea water and the variety of climate extremes take their toll on the structure and its protective coatings. Classification societies have requirements for examination or survey of the ship at set periods throughout its life. The nature and extent of the survey increases as the ship becomes older. Classification Societies introduced an Enhanced Survey Programme which expands and emphasises the existing survey requirements for annual, intermediate and special surveys, with particular attention being paid to the hull structure in the cargo hold region. One specific requirement is that ships must apply a protective coating to the structure in water ballast tanks, where they form part of the hull boundary, side shell & transverse watertight bulkheads in holds. The condition of these coatings is graded and recorded at the Special Survey and the nature and extent of future annual and intermediate surveys depends on the protection provided by these coatings. Additional checks are made for oil tankers, including ore/oil and ore/bulk/oil ships, dry bulk cargo ships and chemical tankers which include Close-up Surveys and thickness measurements. The numbers and extent of structural members examined increases as the vessel ages. Where coatings are found to be in GOOD condition the extent of Close- up Surveys may be specially considered. Continuous Surveys may be permitted at the request of an Owner during which all compartments of the hull are opened for survey and examination in turn. An interval of five years is permitted between the examinations of each part. All ships must be surveyed annually to ensure that they comply with the conditions of assignment as stated in the Merchant Shipping (Load Line) Rules of 1968. Protective coatings Protective coatings of sea water ballast tanks have come in for special examination by Classification Societies and are generally considered to be hard paint coatings. When surveyed, their condition is assessed as GOOD, FAIR or POOR. 37 Lloyd's definitions of these terms: GOOD: Condition with only minor spot rusting affecting not more than 20% of areas under consideration, e.g. deck transverse, side transverse, total area of plating and stiffeners on the longitudinal structure between these components, etc. FAIR: Condition with local breakdown at edges of stiffeners and weld connections and/or light rusting affecting 20% or more of areas under consideration. POOR: Condition with general breakdown of coating affecting 20% or more of areas under consideration or hard scale affecting 10% or more of area under consideration. Hull surveys of very large crude carriers The very size of these ships requires planning and preparation prior to any survey. Large amounts of staging are necessary to provide access to the structure. Good lighting, safe access and some means of communication are also required. Surveys are often undertaken at sea, with the gas freeing of the tanks being one of the main problems. In- water surveys of the outer hull are also done, but the hull plating surface must be clean prior to survey. Bulk carrier surveys Mechanical damage during cargo handling can lead to side shell failure. Cargoes such as iron ore and coal can bring about corrosion of the structure which can cause failure. High losses of this type of vessel in recent years have resulted in classification societies paying particular attention to problem areas during surveys. Main side frames with end connections are prone to cracks beginning at the toe or root of the lower bracket connection to the hopper tank. These cracks may propagate during heavy weather movements of the ship and bring about separation and then similar action at the upper bracket connections. The unsupported shell plating then begins to crack and a major failure may follow. The cross-deck strips between hatches provide the upper support to vertically corrugated bulkheads. If this welded joint cracks the bulkhead may buckle, possibly upwards, causing the hatch covers to become detached. Corrosion may also occur where the bulkhead joins the deck or its stool or the stool joins the tank top. The bulkhead may then fail in shear due to excessive loading on one side. Transition zones are particularly prone to cracking. The change in cross-section forward and aft of the cargo hold areas may be significant. These regions may have been hand welded during the ship's construction making them further suspect. The ends of the upper and lower hopper tanks are also problem areas. Cracks may begin at the termination points against the transverse bulkheads. Water leaks may then occur causing corrosion which will hasten the failure. Ballast tanks may corrode if the protective coatings fail or are not maintained. Where these ballast tanks act as support for other structural elements, they must be inspected very carefully. The various areas which should be examined are shown below: 38 Bulk carrier surveys cont’d 1) Side shell plating. 2) Connection of bulkhead plating to side shell. 3) Connection of side shell frames and end brackets to shell plating and hoppers side tank plating by close-up inspection. 4) Connection of side shell frames and end brackets to the shell plating and topside tank plating. Examination in dry dock The dry docking of a ship provides a rare opportunity for examination of the underwater areas of a ship. Every opportunity should be taken by the ship's staff, the shipowners and the classification society to examine the ship thoroughly. Some of the more important areas are listed: Shell plating The shell plating must be thoroughly examined for any corrosion of welds, damage, distortion and cracks at openings or discontinuities. Any hull attachments such as lugs, bilge keels, etc., must be checked for corrosion, security of attachment and any damage. All openings for grids and sea boxes must also be examined. Cathodic protection equipment Sacrificial anodes should be checked for security of attachment to the hull and the degree of wastage that has taken place. With impressed current systems the anodes and reference anodes must be checked, again for security of attachment. The inert shields and paintwork near the anodes should be examined for any damage or deterioration. Rudder The plating and visible structure of the rudder should be examined for cracks and any distortion. The drain plugs should be removed to check for the entry of any water. Pintle or bearing wear down and clearances should be measured and the security of the rudder stock coupling bolts and any pintle nuts should be ensured. 39 Stern frame The surface should be carefully checked for cracks, particularly in the areas where a change of section occurs or large bending moments are experienced. Propeller The cone should be checked for security of attachment and the rope guard. Blades should be examined for corrosion and cavitation damage, and any cracks or damage to the blade tips. It is normal to examine any tail shaft seals and measure the tail shaft wear down. Anchors and cables Cable should be laid out or 'ranged' in a dry dock and the various lengths (shackles) transposed. The individual links should then be examined for wear and the joining shackles should be opened up and examined. Every link should be hammer tested to ensure it is sound. The chain locker should meanwhile be thoroughly cleaned out and the cable securing arrangement overhauled. The anchor should be cleaned and examined to ensure free movement of the head pivoting mechanism. The mechanism should be greased after examination. Computer-aided Survey and Maintenance Several computer-aided hull maintenance programmes have been developed to enable owners, classification societies and statutory authorities to monitor and record hull details enabling maintenance to be reliably programmed. The International Association of Classification Societies (IACS) and the IMO have various requirements for enhanced detailed annual inspections of suspect areas on particular types of ships and require maintenance planning procedures to be completed. Most leading classification societies have developed computer-based systems to assist owners in the design, construction, safe operation and maintenance of their ships. For example Lloyd's “ShipRight” system offers Construction and Hull Monitoring procedures, where Structural Design and Fatigue Design Assessment procedures have been followed during the ship design stage. The computer software allows planning, inspection, maintenance and monitoring of structural condition and for repairs to be scheduled and incorporated with the planned maintenance for the ship's structure. The monitoring system enables planning of various inspections, documentation of thickness measurements, coating inspections and recording of fractures. A 'survey status' option provides details of all previous, and the next due, survey, together with the class requirements and critical area details. An 'executive hull summary' gives details of previous special surveys as required by IACS. Maintenance planning requires the appropriate sequencing of events: to plan the survey, enter survey data and then display and record all survey results and allow a summary of hull condition obtained and viewed. These hull condition monitoring systems are a powerful tool for planning and documenting surveys in accordance with new procedures for tankers and bulk carriers. They provide continuity in the surveying process and maintain accurate records in what is a very flexible system. 40 Design features Design features of General Cargo vessels Designed to carry maximum diversity of cargo types Speed 12 – 18 knots Capacity varies between 2000 – 15000 dwt Medium block coefficient One or more tween deck offers flexibility (flush hatches) Large single or twin hatches Hatches served by cargo derricks or cranes (electric) Transversely framed with corrugated bulkheads Modern vessels have accommodation and engine room aft Access to cargo storage areas Can carry deck cargo High hatch coamings Double bottoms for fuel oil or ballast Heavy lift derricks may be fitted Deep tanks may be fitted to carry liquid cargoes Variation includes refrigeration equipped vessels for carriage of perishable foodstuffs Student Drawing General Cargo Vessel 41 Design features of Ro-Ro Carriers Designed to carry wheeled cargo that can be loaded horizontally into vessel Speeds 18 – 22 knots Sizes vary, commonly 20,000 dwt Medium hull coefficient Cargo carried above waterline but below upper deck Deck heights sufficient to accommodate various types of vehicles Up to 10 vehicle decks on designated car carriers Transverse strength maintained by deep close spaced web frames in conjunction with deck beams Lower decks subdivided by watertight bulkheads with watertight hydraulic operated doors Moveable ramps fitted to allow access to different levels and create a greater loading area Bow and/or stern doors may be fitted. Stern door may be set at an angle to ships centre line Fitted with low height medium speed diesel engines On Ro-Ro ferries passenger accommodation extends along the vessel’s length above vehicle decks Some Ro-Ro vessels may be designed to carry containers on deck Student Drawing Ro-Ro Vessel 42 Design features of Oil Tankers Designed to carry petroleum liquids in bulk Double hull design required for new buildings Speed 14-15 knots High block coefficient Machinery and accommodation aft, with cofferdam or pump room separation. Twin longitudinal bulkheads creating centre and wing tanks Longitudinal strength main deck and outer bottom Side shell stiffened by longitudinals or transverse frames High powered cargo pumps in pump room Crude oil tankers up to 400,000 dwt (ULCC) Oil tight bulkheads usually corrugated Cofferdams fitted for’d and aft of cargo space Small circular oil tight hatch covers Open rails fitted at least half of length of weather deck Strong permanent fore and aft gangway fitted at level of superstructure deck Fitted with numerous outfit items i.e. inert gas plant, gauges, tank washing machines etc. Student Drawing Double Hull Oil Tanker 43 Design features of Liquefied gas Carriers Specialist carriers designed to liquid gas in bulk Separate inner tank Secondary barrier Double hull structure with water ballast space Up to 60,000 dwt / 140,000m3 Speeds of 12-16 knots Accommodation and engine room aft Expensive insulation in non-pressurised vessels LNG carried at - 164°C May be Self-supporting/membrane or spherical type LPG carried to -45°C Can be carried in fully pressurised or semi pressurized/semi refrigerated ships. If fully pressurised up to 18 bar Prismatic, spherical or cylindrical Bulkheads and cofferdams between cargo tanks Cargo pumping pipe work no interconnection with other systems Reliquifaction plants may be fitted (LPG) Student Drawing Liquefied Gas Carrier 44 Design features of Container ships Specialist carriers designed to carry containers High powered with speeds up to 30 knots on larger vessels Either single hull with heavy web frames or double hull Hatchway sometimes divided into sections using long hatch girders Underdeck passageways Wings utilised for water ballast and/or heeling tanks Hatchways as large as possible (up to 80% breath of main deck) High tensile steel used in upper deck and sheer strake Hatch covers have provisions for suitable container securing arrangements Hatch covers strengthen to take high deck loading due to containers on deck Container spaces suited either for 20 ft (6. 10m) or 40 ft (12.20m) units Watertight bulkheads fitted as required by rules Robust container guides consist of heavy-duty angle bars Reinforced stools at base of cell guides Open or plate floors may be used Generally do not have cargo-handling equipment Hatch less holds used on some Student Drawing Container Vessel 45 Design features of Bulk Carriers Designed to carry maximum deadweight of any type of bulk cargo (grain, coal etc.) Construction of bulk carriers varies considerably High block coefficient Speed 15-17 knots Clear deck with accommodation and engine room aft Hopper / Saddle tanks Deep transverses in wing tanks Bulkheads generally corrugated Large high hatchways Double bottoms for fuel oil or ballast Few vessels have cargo handling equipment Double hull on larger vessels Up to 200,000 dwt Ore carriers are significantly stronger Ore/oil carriers’ longitudinal subdivision similar to tankers Some smaller vessels have movable bulkheads Student Drawing Bulk Carriers 46 Design features of Supply Vessels Accommodation and superstructure on raised bows Long clear afterdeck sheathed in timber Crash barriers for crew protection and securing points Bulwarks with large freeing ports Low freeboard, large roller full width of transom Twin screw CPP, twin rudders, bow/stern thrusters Dual wheelhouse controls Extra-long anchor cables Large cement/bulk chemical facility Flume tanks Student Drawing Platform Supply Vessel 47 Ship framing systems The bottom shell and side plating are framed, i.e. stiffened along their length, against the compressing forces of the sea. Two different types of framing, or a combination of the two, are employed. These are known, respectively, as transverse, longitudinal and combined framing. The side shell framing may also be transversely or longitudinally framed. Transverse framing is generally adopted in many conventional cargo ships, particularly where the maximum bale capacity is required. Bale capacities are often considerably reduced where deep transverses are fitted to support longitudinal framing. Longitudinal framing may be adopted in larger container ships and larger bulk carriers. It is common within the hopper and topside wing tanks of bulk carriers. Transverse frames are then fitted at the side shell between the hopper and topside tanks. But generally, considerations of longitudinal strength are the deciding factor. Transverse framing Transverse framing of the shell plating consists of vertical stiffeners, either of bulb plate or deep-flanged web frames, which are attached by brackets to the deck beams and the flooring structure. The scantlings of the frames depend on their position, spacing and depth. Particular locations, such as at the ends of hatches, require frames of increased scantlings. Very deep web frames are often fitted in the machinery space. Frame spacing is generally not more than 1000mm but is always reduced in the pounding region and at the fore and aft ends in the peak tank regions. Transverse framing system 48 Longitudinal framing Longitudinal framing of the side shell uses horizontal offset bulb plates with increased scantlings towards the lower side shell. Continuity of strength must be maintained. Transverse webs are used to support the longitudinal frames, their spacing being dependent upon the type of ship and the section modulus of the longitudinals. Student Drawing Longitudinal framing system Combined framing Student Drawing Combined framing system 49 Double bottoms & duct keels Double Bottoms Advantages Disadvantages Greater safety - ship can float on inner Reduces volume of cargo space bottom if outer bottom is damaged. Contributes to longitudinal strength. Adds weight to structure. Provides convenient tanks for carriage of Adds cost to initial build oil, water, and ballast. An inner bottom (or tank top) may be provided at a minimum height above the bottom shell and maintained watertight to the bilges. This provides a considerable margin of safety, since in the event of bottom shell damage only the double bottom space may be flooded. The space is not wasted but utilized to carry oil fuel and fresh water required for the ship, as well as providing ballast capacity. The minimum depth of the double bottom in a ship will depend on the classification society’s requirement for the depth of centre girder. It may be deeper to give the required capacities of oil fuel, fresh water, and water ballast to be carried in the bottom. Water ballast bottom tanks are commonly provided right forward and aft for trimming purposes and if necessary, the depth of the double bottom may be increased in these regions. In way of the machinery spaces the double bottom depth is also increased to provide appreciable capacities of lubricating oil and fuel oil. The increase in height of the inner bottom is always by a gradual taper in the longitudinal direction to avoid sudden discontinuities in the structure. The double bottom is divided by the vertical centre girder (/s) and extends out to the ship's side, where it meets the margin plate. The margin plate is generally at right angles to the bilge strake of platin g, to which it is welded. Vertical transverse plate floors are provided both where the bottom is transversely and longitudinally framed. At the ends of bottom tank spaces and under the main bulkheads, watertight or oil tight plate floors are provided. Within the double bottom, longitudinal frames are fitted on the outer and inner bottom, while additional longitudinal strength is provided by a fore and aft girder, known as the intercostal side girder, which extends from transverse solid floor to solid floor. These transverse solid floors are fitted at intervals to provide sufficient transverse strength; the spacing varies according to the loads supported and local stresses experienced. At intermediate frame spaces between the solid plate floors, ‘bracket floors’ extending from margin plate and centre girder to the nearest longitudinal frame are fitted. The bracket floor consists simply of short transverse plate brackets fitted in way of the centre girder and tank sides. At the margin plate, the side frame is joined to the tank side bracket, which is in turn welded to the margin plate in line with the bracket inside the double bottom. Transversely framed double bottom When transversely framed, the double-bottom structure consists of solid plate floors and bracket floors with transverse frames. The bracket floor is fitted between the widely spaced solid floors. It consists of transverse bulb angle sections stiffening the shell and inner bottom plating. Vertical support is provided by brackets at the side shell and centre 50 girder, any side girders and intermediate struts. The number of intercostal side girders fitted is determined by classification society rules. Solid and bracket floors for a transversely framed vessel are shown in the diagram. Student Drawing Transversely framed double bottom Longitudinally framed double bottom This is the system favoured as a result of tests and it provides adequate resistance to distortion on ships of 120m in length or greater. Offset bulb plates are used as longitudinal stiffeners on the shell and inner bottom plating, at intervals of about 1 m. Solid floors provide support at transverse bulkheads and at intervals not exceeding 3.8 m along the length of the ship. The unsupported span of the bottom longitudinals should not exceed 2.5m, vertical angle or channel bar struts may be provided to support the longitudinals between widely spaced solid floors. Intercostal side girders are again fitted, their number depending upon classification society rules. Student Drawing Longitudinally framed double bottom 51 Manholes provide access into the tanks and lightening holes are cut in the solid floors to provide access within and reduce steel weight. Small air and drain holes may also be drilled at the top and bottom respectively of the solid plate floors in the tank spa