Contents 1The ship-its functions, features and types 2Ship stresses and shipbuilding materials 3Shipbuilding 4Welding and cutting processes 5Major structural items AKeel and bottom construction BShell plating, framing systems and decks CBulkheads and pillars DFore end construction EAft end construction FSuperstructures and accommodation 6Minor structural items 7Outfit 8Oil tankers, liquefied gas carriers and bulk carriers 9Ventilation 10Organisations and regulations 11Corrosion and its prevention 12Surveys and maintenance 13Principal ship dimensions and glossary of term 目录 1 船舶的功能、特点和类型 2 船舶应力和造船材料 3 造船 4 焊接和切割工艺 5 主要结构项目 A 龙骨和底部结构 B 船外板、骨架系统和甲板 C 舱壁和支柱+ D 艏端结构 E 尾端结构 F 上层建筑和住舱 6 次要结构项目 7 舾装 8 油轮、液化气运输船和散货船 9 通风 10 组织和法规 11 腐蚀及其预防 12 调查和维护 13 船舶主要尺寸和术语表
1 The Ship-its Functions, Features and Types Merchantships exist to carry cargoes across the waterways of the world safely, speedilyand economically. Since a large part of the world's surface, approximatelythree-fifths, is covered by water, it is reasonable to consider that themerchant ship will continue to perform its function for many centuries to come.The worldwide nature of this function involves the ship, its cargo and its crewin many aspects of international life. Some features of this internationaltransportation, such as weather and climatic changes, availability of cargo handlingfacilities and international regulations, will be considered in later chapters. The ship, in itsvarious forms, has evolved to accomplish its function depending upon three mainfactors - the type of cargo carried, the type of construction and materialsused, and the area of operation. Three principalcargo-carrying types of ship exist today: the general cargo vessel, the tankerand the passenger vessel. The general cargo ship functions today as a generalcarrier and also, in several particular forms, for unit-based or unitised cargocarrying. Examples include container ships, pallet ships and 'roll-on, roll-off’ships. The tanker has its specialised forms for the carriage of crude oil,refined oil products, liquefied gases, etc. The passenger ship includes, generallyspeaking, the cruise liner and some ferries. The type ofconstruction will affect the cargo carried and, in some generally internalaspects, the characteristics of the ship. The principal types of constructionrefer to the framing arrangement for stiffening the outer shell plating, thethree types being longitudinal, transverse and combined framing. The use ofmild steel, special steels, aluminium and other materials also influences thecharacteristics of a ship. General cargo ships are usually of transverse orcombined framing construction using mild steel sections and plating. Mosttankers employ longitudinal or combined framing systems and the larger vesselsutilise high tensile steels in their construction. Passenger ships, with theirlarge areas of superstructure, employ lighter metals and alloys such asaluminium to reduce the weight of the upper regions of the ship. The area of trade,the cruising range, the climatic extremes experienced, must all be borne in mindin the design of a particular ship. Ocean-going vessels require several tanksfor fresh water and oil fuel storage. Stability and trim arrangements must besatisfactory for the weather conditions prevailing in the area of operation.The strength of the structure, its ability to resist the effects of waves,heavy seas, etc., must be much greater for an ocean-going vessel than for aninland waterway vessel. Considerations ofsafety in all aspects of ship design and operation must be paramount, so theship must be seaworthy. This term relates to many aspects of the ship: it mustbe capable of remaining afloat in all conditions of weather: it must remain afloatfollowing all but the most serious damage; and it must remain stable and behavewell in the various sea states encountered. Some of the constructional andregulatory aspects of seaworthiness will be dealt with in later chapters.Stability and other design aspects are explained in detail in NavalArchitecture for Marine Engineers, by W. Muckle (Butterworths.1975). The development ofship types will continue as long as there is a sufficient demand to be met in aparticular area of trade. Recent years have seen such developments as verylarge crude carriers (VLCCs) for the transport of oil, and the liquefiednatural gas and liquefied petroleum gas tankers for the bulk carriage of liquidgases. Container ships and various barge carriers have developed for generalcargo transportation. Bulk carriers and combination bulk cargo carriers arealso relatively modern developments. Several basic shiptypes will now be considered in further detail. The particular features ofappearance, construction, layout, size, etc., will be examined for the followingship types: (1)General cargo ships. (2)Tankers. (3) Bulkcarriers. (4)Container ships. (5)Passenger ships. Many other typesand minor variations exist, but the above selection is considered to berepresentative of the major part of the world's merchant fleet.
General cargo ships Thegeneral cargo ship is the 'maid of all work', operating a worldwide 'goanywhere' service of cargo transportation. It consists of as large a clear opencargo-carrying space as possible, together with the facilities required forloading and unloading the cargo (Figure 1.1). Access to the cargo storage areasor holds is provided by openings in the deck called hatches. Hatches are madeas large as strength considerations will allow to reduce horizontal movement ofcargo within the ship. Hatch covers of wood or steel, as in most modern ships,are used to close the hatch openings when the ship is at sea. The hatch coversare made watertight and lie upon coamings around the hatch which are set somedistance from the upper or weather deck to reduce the risk of flooding in heavyseas. One or moreseparate decks are fitted in the cargo holds and are knows as tween decks.Greater flexibility in loading and unloading, together with cargo segregationand improved stability, are possible using the tween deck spaces. Variouscombinations of derricks, winches and deck cranes are used for the handling ofcargo. Many modern ships are fitted with deck cranes which reduce cargo-handlingtimes and manpower requirements. A special heavy-lift derrick may also befitted, covering one or two holds. Since full cargoescannot be guaranteed with this type of ship, ballast-carrying tanks must befitted. in this way the ship always has a sufficient draught for stability andtotal propeller immersion. Fore and aft peak tanks are fitted which also assistin trimming the ship. A double bottom is fitted which extends the length of theship and is divided into separate tanks, some of which carry fuel oil and freshwater. The remaining tanks are used for ballast when the ship is sailing emptyor partly loaded. Deep tanks may be fitted which can carry liquid cargoes orwater ballast. The accommodationand machinery spaces are usually located with one hold between them and the aftpeak bulkhead. This arrangement improves the vessel's trim when it is partiallyloaded and reduces the lost cargo space for shafting tunnels compared with thecentral machinery space arrangement. The current range of sizes for generalcargo ships is from 2000 to 15 000 displacement tonnes with speeds of 12-18knots. Figure1.1 General cargo ships Refrigerated general cargo ship Thefilling of refrigeration plants for the cooling of cargo holds enables thecarriage of perishable foodstuffs by sea. Refrigerated ships vary little fromgeneral cargo ships. They may have more than one tween deck, and all holdspaces will be insulated to reduce heal transfer. Cargo may be carried frozenor chilled depending upon its nature. Refrigerated ships are usually fasterthan general cargo ships, often having speeds up to 22 knots, and they may alsocater for up to 12 passengers. Tankers Thetanker is used to carry bulk liquid cargoes, the most common type being the oiltanker. Many other liquids are carried in tankers and specially constructedvessels are used for chemicals, liquefied petroleum gas, liquefied natural gas.etc. The oil tanker hasthe cargo-carrying section of the vessel split up into individual tanks bylongitudinal and transverse bulkheads (Figure 1.2). The cargo is discharged bycargo pumps fitted in one or more pumprooms either at the ends of the tanksection or sometimes in the middle. Each tank has its own suction arrangementwhich connects to the pumps, and a network of piping discharges the cargo to thedeck from where it is pumped ashore. No double bottom is fitted in thecargo-carrying section of an oil tanker. Fore and aft peak tanks are used forballast, with often a pair of wing tanks situated just forward of midships.These wing tanks are ballast-only tanks and are empty when the ship is fullyloaded. Small slop tanks are fitted at the after end of the cargo section andare used for the normal carriage of oil on loaded voyages. On ballast runs the sloptanks are used for storing the contaminated residue from tank-cleaningoperations. Large amounts ofpiping are to be seen on the deck running from the pump-rooms to the dischargemanifolds positioned at midships, port and starboard. Hose-handlingderricks are fitted port and starboard near the manifolds. The accommodationand machinery spaces are located aft in modern tankers. The range of sizes foroil tankers at present is enormous, from small to 700000 deadweight tonnes.Speeds range from 12 to 16 knots. Oil tankers are dealt with in more detail in Chapter8.
Figure1.2 Oil tanker
Liquefied gas tankers
Liquefiedgas tankers are used to carry, usually at low temperature, liquefied petroleumgas (LPG) or liquefied natural gas (LNG). A separate inner tank is usually employedto contain the liquid and this lank is supported by the outer hull which has adouble bottom (Figure 1.3). LNG tankers carrymethane and other paraffin products obtained as a by-product of petroleumdrilling operations. The gas is carried at atmospheric pressure and temperaturesas low as -164°C in tanks of special materials (see Table 2.3), which canaccept the low temperature. The tanks used may be prismatic, cylindrical orspherical in shape and self-supporting or of membrane construction. Thecontaining tank is separated from the hull by insulation which also acts as asecondary barrier in the event of leakage. LPG tankers carrypropane, butane, propylene, etc., which are extracted from natural gas. Thegases are carried either fully pressurized, part pressurized-part refrigeratedor fully refrigerated. The fully pressurised tank operates at 18 bar andambient temperature, the fully refrigerated tank at 0.25 bar and -50°C.Separate containment tanks within the hull are used and are surrounded by insulationwhere low temperatures are employed. Tank shapes are either prismatic,spherical or cylindrical. Low temperature steels may be used on the hull whereit acts as a secondary barrier. Displacement sizesfor gas carriers range up to 60 000 tonnes, with speeds of 12-16 knots. Liquefiedgas carrier, are dealt with in more detail in Chapter 8.
Figure1.3 Liquefied petroleum gas (LPG) tanker (W.B. water ballast tanks)
Bulk Carriers Bulkcarriers are single-deck vessels which transport single-commodity cargoes suchas grain. sugar and ores in bulk. The cargo-carrying section of the ship is dividedinto holds or tanks which may have any number of arrangements, depending uponthe range of cargoes to be carried. Combination carriers are bulk carriersdesigned for flexibility of operation and able to transport anyone of severalbulk cargoes on anyone voyage, e.g. ore or crude oil or dry bulk cargo. The general-purposebulk carrier, in which usually the central hold section only is used for cargo,is shown in Figure 1.4 and 1.5(a). The partitioned tanks which surround it areused for ballast purposes either on ballast voyages or, in the case of thesaddle tanks, to raise the ship's centre of gravity when a low density cargo iscarried. Some of the double-bottom tanks may be used for fuel oil and freshwater. The saddle tanks also serve to shape the upper region of the cargo holdand trim the cargo. Large hatchways are a feature of bulk carriers, since theyreduce cargo-handling time during loading and unloading. An ore carrier hastwo longitudinal bulkheads which divide the cargo section into wing tanks portand starboard, and the centre hold which is used for ore. The high doublebottom is a feature of ore carriers. On ballast voyages the wing tanks anddouble bottoms provide ballast capacity. On loaded voyages the ore is carriedin the central hold, and the high double bottom serves to raise the centre of gravityof this very dense cargo. The vessel's behaviour at sea is thus much improved.The cross-section is similar to that of the ore/oil carrier shown in Figure 1.5(b).Two longitudinal bulkheads are employed to divide the ship into centre and wingtanks which are used for the carriage of oil cargoes. When ore is carried, onlythe centre tank section is used for cargo. A double bottom is fitted beneaththe centre tank but is used only for water ballast. The bulkheads and hatchesmust be oiltight. The ore/bulk/oilcarrier has a cross-section similar to the general bulk carrier shown in Figure1.4. The structure is, however, significantly stronger, since the bulkheadsmust be oiltight and the double bottom must withstand the high density ore load.Only the central tank or hold carries cargo, the other tank areas beingballast-only spaces, except the double bottom which may carry oil fuel or freshwater. Large hatches are afeature of all bulk carriers, to facilitate rapid simple cargo handling. Alarge proportion of bulk carriers do not carry cargo-handling equipment,because they trade between special terminals which have particular equipmentfor loading and unloading bulk commodities. The availability of cargo-handlinggear does increase the flexibility of a vessel and for this reason it is sometimesfitted. Combination carriers handling oil cargoes have their own cargo pumps, pipingsystems, etc., for discharging oil. Bulk carriers are dealt with in more detailin Chapter 8. Deadweight capacities range from small to 150000 tonnes dependingupon type of cargo, etc. Speeds are in the range of 12-16 knots.
Figure1.4 Bulk carrier Figure1.5 Tansverse sections: (a) bulk carrier; (b) ore /oil carrier
Container ships Thecontainer ship is, as its name implies, designed for the carriage of container.A container is a re-usable box of 2435 mm by 2435 mm section, with lengths of6055, 9125 and 12190 mm. Containers are in use for most general cargoes, andliquid-carrying versions also exist. in addition, refrigerated models are in use. The cargo-carryingsection of the ship is divided into several holds which have hatch openings thefull width and length of the hold (Figure 1.6). The containers are racked in specialframeworks and stacked one upon the other within the hold space. Cargo handlingtherefore consists only of vertical movement of the cargo in the hold.Containers can also be stacked on the hatch covers where a low density cargo iscarried. Special lashing arrangements exist for this purpose and this deckcargo to some extent compensates for the loss of underdeck capacity. The various cargoholds are separated by a deep web-framed structure to provide the ship withtransverse strength. The ship section outboard of the containers on each sideis a box-like arrangement of wing tanks which provides longitudinal strength tothe structure. These wing tanks may be utilised for water ballast and can bearranged to counter the heeling of the ship when discharging containers. Adouble bottom is also fitted which adds to the longitudinal strength andprovides additional ballast space. Accommodation andmachinery spaces are usually located aft to provide the maximum length of full-bodiedship for container stowage. Cargo-handling gear is rarely fitted, as theseships travel between specially equipped terminals for rapid loading and discharge.Container ship sizes vary considerably with container-carrying capacities from100 to 2000 or more. As specialist carriers they are designed for rapidtransits and are high powered, high speed vessels with speeds up to 30 knots.Some of the larger vessels have triple-screw propulsion arrangements. Figure1.5 Container ship Passenger ships Thepassenger liner, or its modern equivalent the cruise liner, exists to provide ameans of luxurious transport between interesting destinations, in pleasantclimates, for its human cargo. The passenger travelling in such a ship paysfor, and expects, a superior standard of accommodation and leisure facilities.Large amounts of superstructure are therefore an essential feature of passengerships. Several tiers of decks are fitted with large open lounges, ballrooms,swimming pools and promenade areas (Figure 1.7). Aesthetically pleasinglines are evident with usually well-raked clipper-type bows and unusual funnelshapes. Stabilisers are fitted to reduce rolling and bow thrust devices are employedfor improved maneuverability. Large passenger liners are rare, themoderate-sized cruise liner of 12000 tonnes displacement now being the moreprevalent. Passenger-carrying capacity is around 600, with speeds in the regionof 22 knots.
Figure1.6 Passenger ship
1 船舶功能、特点和类型
商船的存在是为了安全、快速和经济地将货物运送到世界各地的水道上。由于世界表面的很大一部分,大约五分之三,被水覆盖,因此可以合理地认为,商船将在未来许多世纪继续发挥其功能。这一职能的全球性质涉及船舶、货物和船员参与国际生活的许多方面。这种国际运输的一些特点,如天气和气候变化、货物装卸设施的可用性和国际条例,将在后面的章节中讨论。 船以各种形式发展以完成其功能,具体取决于三个主要因素 - 运载的货物类型,使用的结构和材料类型以及操作区域。 目前存在三种主要的载货船舶类型:杂货船,油轮和客船。如今,杂货船作为通用承运船,也以几种特定形式用于基于单元或单元的货物运输。示例包括集装箱船、托盘船和“滚装船”。油轮有其专门用于运输原油、成品油、液化气等的形式。一般来说,客船包括游轮和一些渡轮。 结构类型将影响所携带的货物,在某些一般内部方面,会影响船舶的特性。主要结构类型是指用于加强船体外板的骨架布置,三种类型是纵向、横向和组合骨架。低碳钢、特殊钢、铝和其他材料的使用也会影响船舶的特性。杂货船通常是使用低碳钢型材和板的横向或组合骨架结构。大多数油轮采用纵向或组合骨架系统,较大的船舶在其结构中使用高强度钢。上层建筑面积大的客船采用较轻的金属和合金(如铝)来减轻船舶上层建筑的重量。 贸易区域、续航里程、经历的极端气候,都必须在设计特定船舶时牢记在内。远洋船舶需要几个用于储存淡水和石油燃料的舱。稳性和纵倾布置必须符合运营区域普遍存在的天气条件。结构的强度,抵抗海浪,波涛汹涌等影响的能力,对于远洋船舶来说必须比内河船舶大得多。 船舶设计和操作各个方面的安全考虑必须是最重要的,因此船舶必须适航。该术语涉及船舶的许多方面:它必须能够在所有天气条件下保持漂浮:除了最严重的损坏外,它必须在所有损坏后保持漂浮;它必须保持稳定并在遇到的各种海况中表现良好。适航性的一些构造和监管方面将在后面的章节中讨论。稳性和其他设计方面在W. Muckle(Butterworths.1975)的Naval Architecturefor Marine Engineers中进行了详细解释。 只要在特定贸易领域有足够的需求需要满足,船型的发展就会继续下去。近年来,用于运输石油的超大型原油运输船(VLCC)以及用于散装液化气运输的液化天然气和液化石油气油轮等得到了发展。集装箱船和各种驳船已经发展用于普通货物运输。散货船和组合散货船也是相对现代化的发展。 现在将更详细地考虑几种基本船型。将检查以下船型的外观、构造、布局、尺寸等特殊特征: (1)杂货船。 (2)油轮。 (3)散货船。 (4)集装箱船。 (5)客船。 存在许多其他类型和微小变化,但上述选择被认为是世界商船队主要部分的代表。
杂货船
杂货船是“所有工作的女仆”,在全球范围内提供“随处可去”的货物运输服务。它由尽可能大的敞开载货空间以及装卸货物所需的设施组成(图1.1)。进入货物存储区或货舱的通道由甲板上的开口提供,称为舱口。舱口的尺寸要从强度考虑允许的情况下尽可能大,以减少货物在船内的水平移动。与大多数现代船舶一样,木头或钢制舱口盖用于在船舶出海时关闭舱口。舱口盖是水密的,位于舱口周围的围板上,这些罩子与上层或露天甲板有一定距离,以减少在汹涌的大海中发生进水的风险。 一个或多个单独的甲板安装在货舱中,称为中层甲板。使用中层甲板空间可以提高装卸灵活性,以及货物隔离和更高的稳性。井架、绞车和甲板起重机的各种组合用于货物的搬运。许多现代船舶都配备了甲板起重机,减少了货物装卸时间和人力需求。也可以安装一个特殊的重型起重井架,覆盖一个或两个货舱。 由于这种类型的船舶无法保证满载货物,因此必须安装压载舱。通过这种方式,船舶始终具有足够的吃水深度,以确保稳性和螺旋桨完全浸没。安装了首尾尖舱,这也有助于减小船舶纵倾。安装了一个双层底,延长了船的长度,并分为单独的舱,其中一些装有燃料油和淡水。剩余的舱用于船舶空载或部分装载时的压载物。可以安装深舱,可以装载液体货物或压载水。 住舱和机舱位于尾部,在它们和尾尖舱壁之间通常还有一个货舱。与中置机舱布置相比,这种布置改善了船舶在部分装载时的纵倾,并减少了由于轴隧产生货物空间损失。目前杂货船的尺寸范围为2000至15 000排水吨,速度为12-18节。
图1.1 杂货船
冷藏杂货船
用于冷却货舱的制冷设备使易腐食品的海上运输成为可能。冷藏船与杂货船差别不大。它们可能有多个中层甲板,并且所有保持空间都将绝缘以减少热量转移。货物可以根据其性质冷冻或冷藏运输。冷藏船通常比杂货船快,通常速度可达 22 节,它们还可以容纳多达 12 名乘客。
油轮
油轮用于运载散装液体货物,最常见的类型是装载石油的油轮。油轮也运输许多其他液体,专门建造的船只用于化学品,液化石油气,液化天然气等。 油轮的载货部分通过纵向和横向舱壁分成单独的舱(图1.2)。货物由安装在一个或多个泵房中的货泵排出,这些泵房位于舱段末端或有时位于中间。每个舱都有自己的吸入装置,连接到泵,管道网络将货物排放到甲板上,从那里被泵上岸。油轮的载货部分没有安装双层底。(此书成书于1970年代,现在MARPOL要求油舱有双壳保护。)首尾尖舱用于压载,通常有一对边舱位于船中部的前方。这些边舱是仅压载舱,当船满载时是空的。小型污油舱安装在货物部分的后端,用于装在载航程中正常运油。在压载航程中,污油舱用于储存舱清洁操作中受污染的残留物。 甲板上可以看到大量的管道,从泵房到位于船中部、左舷和右舷的排放总管。在总管附近的左舷和右舷安装有软管处理井架。住舱和机舱位于现代油轮的船尾。目前油轮的尺寸范围很大,从小吨到700000载重吨不等,速度范围从 12 到 16 节。第8章更详细地论述了油轮。
图1.2 油轮
液化气船
液化气船通常用于在低温下运载液化石油气(LPG)或液化天然气(LNG)。通常采用单独的内舱来容纳液体,该舱由具有双层底的外船体支撑(图1.3)。 液化天然气油轮运载甲烷,沼气。以及作为石油钻井作业副产品获得的其他石蜡链烷(属)烃产品。气体在大气压下输送,温度低至-164°C,采用特殊材料的舱体(见表2.3),可以接受低温。所使用的舱可以是棱柱形、圆柱形或球形,并且是自支撑的或膜结构的。容纳舱通过绝缘材料与船体隔开,绝缘材料在泄漏时也可用作辅助屏障。 液化石油气油轮运载丙烷、丁烷、丙烯等,这些是从天然气中提取的。气体要么完全加压,要么部分加压 -部分冷藏或完全冷藏。全加压舱在 18 bar 和环境温度下运行,全冷藏舱在 0.25 bar 和 -50°C 下运行。 使用船体内单独的容纳舱,并在使用低温的地方被绝缘材料包围。舱形状为棱柱形、球形或圆柱形。船体用低温钢,船体也作为辅助屏障。 气体运输船的排水量可达60000吨,速度为12-16节。液化气体运输船,在第8章中有更详细的论述。
图1.3 液化气船(W.B. 压载水舱)
散货船
散货船是运输谷物、散装糖和矿石等单一商品货物的单甲板船舶。船舶的载货部分分为货舱或,根据要运载的货物范围,可以有任意数量的液舱布置。组合运输船是为操作灵活性而设计的散货船,能够在任何航行中运输几批散装货物中的任何一种,例如矿石或原油或干散货。 通用散货船的中央货舱部分通常只用于转载货物,如图1.4和1.5(a)所示。围绕它的分隔舱用于压载目的,要么是在压载航行中,要么对于鞍形舱,在运输低密度货物时提高船舶的重心。一些双层底舱可用于燃料油和淡水。鞍形舱还用于塑造货舱的上层区域并修剪货物。大型舱口是散货船的一个特点,因为它们减少了装卸过程中的货物装卸时间。
图1.4 散货船 矿石运输船有两个纵向舱壁,将货物部分分为左舷和右舷边舱和用于装载矿石的中央货舱。高双底是矿石运输船的一个特点。在压载航行中,边舱和双底提供压载能力。在装载的航行中,矿石在中央货舱中运输,高双底用于提高这种非常密集的货物的重心。因此,船舶在海上的表现得到了极大的改善。横截面与图1.5(b)所示的矿石/石油载体的截面相似。两个纵向舱壁用于将船舶分为中舱和用于运输石油货物的边舱。运输矿石时,只有中舱部分用于货物。中舱下方装有双层底,但仅用于压载水。舱壁和舱口必须油密。
图1.5 横剖面 (a)散货船; (b)矿石/石油运输船 矿石/散装/石油运输船的横截面类似于图1.4所示的一般散货船。然而,该结构明显更坚固,因为舱壁必须油密,双层底必须承受高密度矿石负载。只有中舱或货舱运载货物,其他舱区域是压载空间,除了双层底舱可能携带油燃料或淡水。 大型舱口是所有散货船的特点,便于快速简单的货物处理。很大一部分散装货船不携带货物装卸设备,因为它们在拥有装卸散装商品的特殊设备的特殊码头之间进行贸易。提供货物装卸设备确实增加了船舶的灵活性,因此有时会安装它。装卸油品的组合运输船有自己的货泵、管道系统等,用于卸油。散装货船将在第8章中更详细地讨论。载重量从小到150000吨不等,具体取决于货物类型等,速度在 12-16 节的范围内。
集装箱船
顾名思义,集装箱船是为集装箱运输而设计的。集装箱是截面为2435 毫米 x 2435 毫米的可重复使用的箱子,长度为 6055、9125 和 12190 毫米。大多数普通货物都使用集装箱,也存在液体运输版本。另外,冷藏型号也正在使用中。 船舶的载货部分分为几个货舱,货舱的整个宽度和长度都有舱口(图1.6)。集装箱被架在特殊的框架中,并在货舱空间内一个接一个地堆叠。因此,货物装卸仅包括货舱中货物的垂直移动。集装箱也可以堆放在运输低密度货物的舱口盖上。为此目的存在特殊的绑扎设施,这种甲板货物在一定程度上弥补了甲板下容量的损失。 各个货舱由深强肋骨结构隔开,为船舶提供横向强度。集装箱的船体剖面每侧外侧都有箱状的边舱布置,为结构提供纵向强度。这些边舱可用于压载水,并且可以布置成在卸货集装箱时对抗船舶的倾斜。还安装了双层底,增加了纵向强度并提供额外的压载空间。 住舱和机舱通常位于船尾,以提供集装箱装载的全体船舶的最大长度。货物装卸设备很少安装,因为这些船舶在专门配备的码头之间快速装卸。集装箱船的尺寸差异很大,集装箱运载能力从100到2000或更多。作为专业承运船,它们专为快速运输而设计,是速度高达 30 节的高功率高速船舶。一些较大的船只具有三螺旋推进装置。
图1.6 集装箱船
客船
客轮,或其现代等价物邮轮,的存在是为了在宜人的气候下,在有趣的目的地之间为其人类货物提供一种豪华的运输方式。乘坐此类船舶的乘客付费并期望获得更高标准的住舱和休闲设施。因此,大量的上层建筑是客船的基本特征。多层甲板配有大型开放式休息室、宴会厅、游泳池和海滨长廊区(图 1.7)。 通常倾斜良好的剪刀式船首和不寻常的烟囱形状,线条美观。安装稳定器以减少横摇,并采用艏侧推以提高机动性。大型客轮很少见,排水量为12000吨的中型邮轮现在更为普遍。载客能力约为600,速度在22节左右。
图1.7 客船 2 Ship Stresses and Shipbuilding Materials Theship at sea or lying in still water is being constantly subjected to a widevariety of stresses and strains, which result from the action of forces fromoutside and within the ship. Forces within the ship result from structuralweight, cargo, machinery weight and the effects of operating machinery.Exterior forces include the hydrostatic pressure of the water on the hull andthe action of the wind and waves. The ship must at all times be able to resistand withstand these messes and strains throughout its structure. It musttherefore be constructed in a manner, and of such materials, that will providethe necessary strength. The ship must also be able to function efficiently as acargo-carrying vessel. The various forcesacting on a ship are constantly varying as to their degree and frequency. Forsimplicity, however, they will be considered individually and the particularmeasures adopted to counter each type of force will be outlined. The forces mayinitially be classified as static and dynamic. Static forces are due to thedifferences in weight and buoyancy which occur at various points along thelength of the ship. Dynamic forces result from the ship's motion in the sea andthe action of the wind and waves. A ship is free to move with six degrees offreedom - three linear and three rotational. These motions are described by theterms shown in Figure 2.1. These static anddynamic forces create longitudinal, transverse and local stresses in the ship'sstructure. Longitudinal stresses are greatest in magnitude and result in bendingof the ship along its length.
Figure 2.1 Shipmovement – the six degrees of freedom
Longitudinal stresses
Static loading
If the ship is consideredfloating in still water, two different forces will be acting upon it along itslength. The weight of the ship and its contents will be acting vertically downwards.The buoyancy or vertical component of hydrostatic pressure will be actingupwards. in total, the two forces exactly equal and balance one another suchthat the ship floats at some particular draught. The centre of the buoyancyforce and the centre of the weight will be vertically in line. However, atparticular points along the ship's length the net effect may be an excess ofbuoyancy or an excess of weight. This net effect produces a loading of thestructure, as with a beam. This loading results in shearing forces and bending momentsbeing set up in the ship's structure which tend to bend it. The static forcesacting on a ship's structure are shown in Figure 2.2(a). This distribution ofweight and buoyancy will also result in a variation of load, shear forces andbending moments along the length of the ship, as shown in Figures 2.2(b)-(d).Depending upon the direction in which the bending moment acts, the ship willbend in a longitudinal vertical plane. This bending moment is known as thestill water bending moment (SWBM). Special terms are used to describe the twoextreme cases: where the buoyancy amidships exceeds the weight, the ship is saidto 'hog', and this condition is shown in Figure 2.3; where the weight amidshipsexceeds the buoyancy, the ship is said to 'sag', and this condition is shown inFigure 2.4.
Figure 2.2 Static loadingof a ship’s structure
Figure 2.3 Hoggingcondition
Figure 2.4 Saggingcondition
Dynamic loading
If the ship is nowconsidered to be moving among waves, the distribution of weight will still bethe same. The distribution of buoyancy, however, will vary as a result of thewaves. the movement of the ship will also introduce dynamic forces. The traditionalapproach to solving this problem is to convert this dynamic situation into anequivalent static one. To do this, the ship is assumed to be balanced on astatic wave of trochoidal form and length equalto the ship. The profile of a wave at sea is considered to be a trochoid. Thisgives waves where the crests are sharper than the troughs. The wave crest is consideredinitially at midships and then at the ends of the ship. The maximum hogging andsagging moments will thus occur in the structure far the particular loadedcondition considered, as shown in Figure 2.5. The total shearforce and bending moment are thus obtained and these will include the stillwater bending moment considered previously. If actual loading conditions forthe ship are considered which will make the above conditions worse, e.g. heavyloads amidships when the wave trough is amidships, then the maximum bending momentsin normal operating service can be found. The ship'sstructure will thus be subjected to constantly fluctuatingstresses resulting from these shear forces and bending moments as the wavesmove along the ship's length.
Figure 2.5 Dynamicloading of a ship’s structure: (a) still water condition; (b) saggingcondition; (c) hogging condition
Stressing of the structure
The bending of aship causes stresses to be set up within its structure. When a ship sags,tensile stresses are set up in the bottom shell plating and compressive tressesare set up in the deck. When the ship hogs, tensile stresses occur in the decksand compressive stresses in the bottom shell. This stressing, whethercompressive or tensile, reduces in magnitude towards a position known as theneutral axis. The neutral axis in a ship is somewhere below half the depth andis, in effect, a horizontal line drawn through the centre of gravity of the ship'ssection. The fundamentalbending equation for a beam is M / I = σ / y whereM is The bending moment, I is the second moment of area of the section aboutits neutral axis, σ is thestress at the outer fibres, and y is the distance from the neutral axis to theouter fibres. This equation hasbeen proved in full-scale tests to be applicable to the longitudinal bending ofa ship. From the equation the expression σ = M / I/y isobtained for the stress in the material at some distance y from the neutralaxis. The values M, I and y can be determined for the ship, and the resultingstresses in the deck and bottom shell can be found. The ratio I/y is known asthe section modulus, Z, when y is measured to the extreme edge of the section.The values are determined for the midship section, since the greatest momentwill occur at or near midships (see Figure 2.2). A more detailed explanation ofthis process is given in Muckle's work, Naval Architecture for MarineEngineers, previously cited. The structuralmaterial included in the calculation for the second moment I will be all thelongitudinal material which extends for a considerable proportion of the ship'slength. This material will include side and bottom shell plating, inner bottomplating (where fitted), centre girders and decks. The material forms what is knownas the hull girder, whose dimensions are very large compared to its thickness.
Transverse stresses
Staticloading
A transversesection of a ship is subjected to static pressure from the surrounding water inaddition to the loading resulting from the weight of the structure, cargo, etc.Although transverse stresses are of lesser magnitude than longitudinalstresses, considerable distortion of the structure could occur, in the absenceof adequate stiffening (Figure 2.6). The parts of thestructure which resist transverse stresses are transverse bulkheads, floors in thedouble bottom (where fitted), deck beams, side frames and the brackets betweenthem and adjacent structure such as tank top flooring or margin plates
Figure 2.6 Staticwater pressure loading of a ship’s structure
Dynamicstresses
When a ship is rollingit is accelerated and decelerated, resulting in forces in the structure tendingto distort it. This condition is known as racking and its greatest effect is feltwhen the ship is in the light or ballast condition (Figure 2.7). The bracketsand beam knees joining horizontal and vertical items of structure are used toresist this distortion.
Figure 2.7 Racking
Localised stresses
The movement of aship in a seaway results in forces being generated which are largely of a localnature. These forces are, however, liable to cause the structure to vibrate andthus transmit stresses to other pans of the structure.
Slamming or pounding
In heavy weather,when the ship is heaving and pitching, the forward end leaves and re-enters thewater with a slamming effect (Figure 2.8). This slamming down of the forwardregion on to the water is known as pounding. Additional stiffening must befitted in the pounding region to reduce the possibility of damage to thestructure. This is discussed further in Section A of Chapter 5.
Figure 2.8 Pounding
Panting
The movement ofwaves along a ship causes fluctuations in waterpressure on the plating. The tends to create an in-and-out movement of theshell plating, known as panting. The effect is particularly evident at the bowsas the ship pushes its way through the water. The pitching motionof the ship produces additional variations in water pressure, particularly atthe bow and stem, which also cause panting of the plating. Additionalstiffening is provided in the form of panting beams and stringers. This is discussedfurther in Section D of Chapter 5.
Localisedloading
Heavy weights, suchas equipment in the machinery spaces or particular items of general cargo, cangive rise to localised distortion of the transverse section (Figure 2.9).Arrangements for spreading the load, additional stiffening and thicker platingare methods used in dealing with this problem.
Figure 2.9 Localisedloads tending to distort the ship’s structure
Superstructuresand discontinuities
The ends of superstructuresrepresent major discontinuities in the ship's structure where a considerable changein section modulus occurs. Localised stresses will occur which may result in crackingof adjacent structure. Sharp discontinuities, are therefore to be avoided bygradual tapers being introduced. Thicker strakes of deck and shell plating may also be fittedat these points. Any holes oropenings cut in decks create similar areas of high local stress. well-roundedcomers must be used where openings are necessary, and doubling plates may alsobe filled. in the case of hatchways the bulk of the longitudinal strength materialis concentrated outboard of the hatch openings on either side to reduce thechange in section modulus at the openings. This is discussed further in SectionsB and F of Chapter 5.
Vibrations
Vibrations set upin a ship due to reciprocating machinery, propellers, etc., can result in thesetting up of stresses in the structure. These are cyclic stresses which couldresult in fatigue failure of local items of structure leading to more generalcollapse. Balancing of machinery and adequate propeller tip clearances can reducethe effects of vibration to acceptable proportions. Apart from possible damageto equipment and structure, the presence of vibration can be most uncomfortableto any passengers and the crew. The design of the structureis outside the scope of this book. The various shipbuilding materials used to providethe structure will now be considered.
Steel
Steel is the basicshipbuilding material in use today. Steel may be regarded as an iron-carbonalloy, usually containing other elements, the carbon content not usually exceedingabout 2%. Special steels of high tensile strength are used on certain highlystressed parts of the ship's structure. Aluminium alloys have particularapplications in the construction of superstructures, especially on passengerships.
Production
'Acid' or 'basic'are terms often used when referring to steels. The reference is to theproduction process and the type of furnace lining, e.g. an alkaline or basiclining is used to produce basic steel. The choice of furnace lining is dictatedby the raw materials used in the manufacture of the steel. There are three particularprocesses currently used for the manufacture of carbon steel namely the open hearthprocess, the oxygen or basic oxygen steel process and the electric furnace process.in all these processes the hot molten metal is exposed to air or oxygen whichoxidises the impurities to refine the pig iron into high quality steel. In the open hearthprocess a long shallow furnace is used which is fired from both ends. A highproportion of steel scrap may be used in this process. High quality steel is producedwhose properties can be controlled by the addition of suitable alloyingelements. In the oxygen orbasic oxygen steel process the molten metal is contained in a basic linedfurnace. A jet of oxygen is injected into the molten metal by an overheadlance. Alloying elements can be introduced into the molten metal and a highquality steel is produced. In the electricfurnace process, an electric arc is struck between carbon electrodes and thesteel charge in the furnace. Accurate control of the final composition of thesteel and a high standard of purity are possible with this process.
Finishing treatment
Steels from theabove-mentioned processes will all contain an excess of oxygen, usually in theform of iron oxide. Several finishing treatments are possible in the finalcasting of the steel. Rimmed steel is producedas result of little or no treatment to remove Oxygen. in the molten state theoxygen combines with the carbon in the steel, releasing carbon monoxide gas. Onsolidifying, an almost pure iron outer surface is formed. The central core ofthe ingot is, however, a mass of blow holes. Hot rolling of the ingot usually 'weldsup' these holes but thick plates of this material are prone to laminations. Killed steel is producedby fixing the Oxygen by the addition of aluminium or silicon before pouring thesteel into the mould. The aluminium or silicon produces oxides reducing theiron oxides to iron. A homogeneous material of superior quality to rimmed steelis thus produced. Balanced or semi-killedsteels are an intermediate form of steel. This results from the beginning ofthe rimming process in the mould and its termination by the use of deoxidisers. Vacuum degassedsteels are produced by reducing the atmospheric pressure when the steel is illthe molten state. The equilibrium between carbon and oxygen is thus obtained ata much lower level and the oxygen content becomes very small. Final residualdeoxidation can be achieved with the minimum additions of aluminium or silicon.A very 'clean' steel is produced with good notch toughness properties andfreedom from lamellar tearing problems (lamellartearing is explained in Chapter 4). The composition ofsteel has a major influence on its properties and this will be discussed in thenext subsection. The properties of steel are further improved by various formsof heat treatment which will now be outlined. in simplified terms the heattreatment of steels results in a change in the grain structure which alters themechanical properties of the material.
Normalising.The steel is heated to a temperature of 850-950°C depending upon its carboncontent and then allowed to cool in air. A hard strong steel with a refinedgrain structure is produced.
Annealing.Again the steel is heated to around 850-950°C, but is cooled slowly either in thefurnace or in an insulated space. A softer more ductilesteel than that in the normalised condition is produced.
Hardening.The steel is heated to 850-950°C and then rapidly cooled by quenching in oil orwater. The hardest possible condition for the particular steel is thus producedand the tensile strength is increased.
Tempering.This process follows the quenching of steel andinvolves reheating to some temperature up to about 680°C. The higher thetempering temperature the lower the tensile properties of the steel. Oncetempered, the metal is rapidly cooled by quenching.
Composition and properties
Various terms areused with reference to steel and other materials to describe their properties.These terms will now be explained in more detail
Tensilestrength. This is the main single criterion with reference to metals. It is ameasure of the material's ability to withstand the loads upon it in service.Terms such as stress, strain, ultimate tensile strength, yield stress and proofstress are all different methods of quantifying the tensile strength of thematerial. The two main factors affecting tensile strength are the carboncontent of the steel and its heat treatment following manufacture.
Ductility.This is the ability of a material to undergo permanent changes in shape withoutrupture or loss of strength. It is particularly important where metals undergoforming processes during manufacture.
Hardness. This is a measure of the workability of thematerial. It is used as an assessment of the machinability of the material andits resistance to abrasion.
Toughness.This is a condition midway between brittlenessand softness. It is often quantified by the value obtained in a notched bartest.
Standard steel sections
A variety ofstandard sections are produced with varying scantlings to suit theirapplication. The stiffening of plates and sections utilises one or more ofthese sections. which are shown in Figure 2.10.
Figure 2.10 Standardsteel sections: (a) flat plate; (b) offset bulb plate; (c) equal angle; (d)unequal angle; (e) channel; (f) tee
Shipbuilding steels
The steel used in shipconstruction is mild steel with a 0.15-0.23% carbon content. The propertiesrequired of a good shipbuilding steel are:
(1) Reasonablecost. (2) Easily weldedwith simple techniques and equipment. (3) Ductility andhomogeneity. (4) Yield point tobe a high proportion of ultimate tensile strength. (5) Chemicalcomposition suitable for flame cutting without hardening. (6) Resistance to corrosion.
These features areprovided by the five grades of mild steel (A-E) designated by the classificationsocieties (see Chapter 10). To be classed, the steel for ship construction mustbe manufactured under approved conditions, and inspected, and prescribed testsmust be carried out on selected specimens. Finished material is stamped withthe society's brand, a symbol with L superimposed on R being of Lloyd’s Register.The chemical composition and mechanical properties of a selection of mild steelgrades are given in Table 2.1.
Table 2.1PROPERTIES AND COMPOSITION OF SOME MILD STEELS
Table 2.2PROPERTIES AND COMPOSITION OF SOME HIGH TENSILE STEELS
Table 2.3PROPERTIES AND COMPOSITION OF LOW TEMPERATURE CONSTRUCTIONAL MATERIALS
Developments in steelproduction and alloying techniques have resulted in the availability of higherstrength steels for ship construction. These higher tensile strength (HTS)steels, as they are called, have adequate notch toughness, ductility andweldability, in addition to their increased strength. The increased strengthresults from the addition of alloying elements such as vanadium, chromium,nickel and niobium. Niobium in particular improves the mechanical properties oftensile strength and notch ductility. Particular care must be taken in thechoice of electrodes and welding processes for these steels. Low hydrogenelectrodes and welding processes must be used. Table 2.2 indicates the chemicalcomposition and mechanical properties of several high tensile steel grades. Aspecial grade mark, H, is used by the classification societies to denote highertensile steel. Benefits arisingfrom the use of these steels in ship construction include reduced structuralweight, since smaller sections may be used; larger unit fabrications arepossible for the same weight and less welding time, although a more specialisedprocess, is needed for the reduced material scantlings. Cryogenic or lowtemperature materials are being increasingly used as a consequence of thecarriage of liquefied gases in bulk tankers. Table 2.3 details the propertiesand composition of several of these cryogenic materials. The main criterion ofselection is an adequate amount of notch toughness at the operating temperatureto be encountered. Various alloys are principally used for the very lowtemperature situations, although special quality carbon/manganese steels havebeen used satisfactorily down to -50°C.
Castings and forgings
The larger castingsused in ship construction are usually manufactured from carbon or carbonmanganese steels. Table 2.4 details the composition and properties of thesematerials. Examples of large castings are the stern frame, bossings, A-bracketsand parts of the rudder. The examples mentioned may also be manufactured as forgings.Table 2.4 also details the composition and properties of materials used forforgings.
Table 2.4PROPERTIES AND COMPOSITION OF CASTING AND FORGING MATERIALS
Table 2.5PROPERTIES AND COMPOSITION OF ALUMINIUM ALLOY CONSTRUCTIONAL MATERIALS
Aluminium alloys
The increasing useof aluminium alloy has resulted from its several advantages over steel.Aluminium is about one-third the weight of steel for equivalent volume ofmaterial. The use of aluminium alloys in a structure can result in reductionsof 60% of the weight of an equivalent steel structure. This reduction in weight,particularly in the upper regions of the structure, can improve the stability ofthe vessel. This follows from the lowering of the vessel's centre of gravity,resulting in an increased metacentric height. Stability is discussed in detailin Muckle's Naval Architecture for Marine Engineering. The corrosion resistanceof aluminium is very good but careful maintenance and insulation from theadjoining steel structure are necessary. The properties required of analuminium alloy to be used in ship construction are much the same as for steel,namely strength, resistance to corrosion, workability and weldabilily. Theserequirements are adequately met, the main disadvantage being the high cost ofaluminium. The chemicalcomposition and mechanical properties of the common shipbuilding alloys areshown in Table 2.5. Again these are classification society gradings where thematerial must be manufactured and tested to the satisfaction of the society. Aluminium alloysare available as plate and section, and a selection of aluminium alloy sectionsis shown in Figure 2.11. These sections are formed by extrusion, which is theforcing of a billet of the hot material through a suitably shaped die.Intricate or unusual shapes to suit particular applications are thereforepossible. Where aluminiumalloys join the steel structure, special insulating arrangements must be employedto avoid galvanic corrosion where the metals meet (see Chapter II). Whererivets are used, they should be manufactured from a corrosion-resistant alloy(see Table 2.5).
Figure 2.11 Aluminiumalloy sections
Materials testing
Various qualitiesof the materials discussed so far have been mentioned. These qualities aredetermined by a variety of tests which are carried out on samples of the metal. The terms 'stress'and 'strain' are used most frequently. Stress or intensity of stress, itscorrect name, is the force acting on a unit area of the material. Strain is thedeforming of a material due to stress. When the force applied to a materialtends to shorten or compress the material, the stress is termed 'compressivestresses’. When the force applied to a material tends to lengthen the material,the stress is termed 'tensile stresses’. When the force tends to cause thevarious parts of the material to slide over one another the stress is termed'shear stress'. The tensile test isused to determine the behaviour of a material up to its breaking point. A speciallyshaped specimen piece (Figure 2.12) of standard size is gripped in the jaws ofa testing machine. A load is gradually applied to draw the ends of the barapart such that it is subject to a tensile stress. The original test length L1of the specimen is known and for each applied load the new length L2can be measured. The specimen will be found to have extended by some smallamount L2 -L1. This deformation, expressed as Extension /Original length isknown as the linear strain. Figure 2.12 Tensiletest specimens: (a) for plates, strips and sections (a = thickness ofmaterial); (b) for hot-rolled bar Additional loadingof the specimen will produce results which show a uniform increase of extensionuntil the yield point is reached. Up to the yield point the removal of load wouldhave resulted in the specimen returning to its original size. Stress and strainare therefore proportional up to the yield point, or elastic limit as it is alsoknown. The stress and strain values for various loads can be shown on a graphsuch as Figure 2.13. Figure 2.13 Stress/ Strain graph for mild steel Figure 2.14 Stress/ Strain graph for higher tensile steel
If the testing werecontinued beyond the yield point the specimen would 'neck' or reduce in cross-section.The load values divided by the original specimen cross-sectional area wouldgive the sharp shown in Figure 2.14. The highest value of stress is known asthe ultimate tensile stress (UTS) of the material. Within the elasticlimit, stress is proportional to strain, and therefore Stress / Strain =constant Thisconstant is known as the 'modulus of elasticity' (E) of the material and hasthe same units as stress. The yield stress is the value of stress at the yieldpoint. Where a clearly defined yield point is not obtained a proof stress valueis given. This is obtained by a line parallel to the stress-strain line beingdrawn at some percentage of the strain, such as 0.1%, The intersection of thisline with the stress-strain line is considered the proof stress (see Figure2.14). The bend test is usedto determine the ductility of a material. A piece of material is bent over aradiused former, sometimes through 180 degrees. No cracks or surface laminations should appear in the material. Impact tests canhave a number of forms but the Charpy vee-notch test is usually specified. Thetest specimen is a 10 mm square cross-section, 55 mm in Length. A vee-notch is cutin the centre of one face, as shown in Figure 2.15. The specimen is mountedhorizontally with the notch axis vertical. The test involves the specimen beingstruck opposite the notch and fractured. A striker or hammer on the end of aswinging pendulum provides the blow which breaks the specimen. The energyabsorbed by the material in fracturing is measured by the machine. A particularvalue of average impact energy must be obtained for the material at the testtemperature. This test is particularly important for materials to be used in lowtemperature regions. For low temperature testing the specimen is cooled byimmersion in a bath of liquid nitrogen or dry ice and acetone for about 15minutes. The specimen is then handled and tested rapidly to minimise anytemperature changes. The impact test, in effect, measures a material'sresistance to fracture when shock loaded. Figure 2.15 Charpyimpact test
A dump test is usedon a specimen length of bar from which rivets are to be made. The bar is compressedto half its original length and no surface cracks must appear. Other rivetmaterial tests include bending the shank until the two ends touch without anycracks or fractures appearing. The head must also accept flattening until itreaches two and a half times the shank diameter.
船舶应力和造船材料
在海上或静水中的船舶不断受到各种应力和应变的作用,这些应力和应变来自船舶内外的力的作用。船内的力来自结构重量、货物、机器重量和操作机器的影响。外力包括水对船体的静水压力以及风和波浪的作用。船舶必须始终能够抵抗和承受整个结构中的这些应力和应变。因此,它必须以提供必要强度的方式和材料来构造。船还必须能够有效地作为载货船只。 作用在船上的各种力在程度和频率上不断变化。然而,为了简单起见,它们将被单独考虑,并且将概述为对抗每种类型的力而采取的特定措施。 力初步可以分为静态力和动态力。静力是由于重量和浮力的差异而产生的,这种差异出现在沿船舶长度方向的不同点上。动力来自船在海里的运动以及风和波浪的作用。一艘船可以自由移动,有六个自由度——三个线性自由度和三个旋转自由度。这些运动用图2.1所示的术语来描述。
图2.1船的运动-6个自由度
这些静态和动态力在船舶结构中产生纵向、横向和局部应力。纵向应力最大,导致船舶沿其长度弯曲。
图2.2船体结构的静态载荷
纵向应力
静态载荷
如果船被认为是漂浮在静止的水中,两种不同的力将沿着它的长度方向作用于它。船的重量及其内容物将垂直向下作用。静水压力的浮力或垂直分量将向上作用。总的来说,这两个力正好相等并相互平衡,这样船就能在特定的吃水深度下漂浮。浮力的中心和重量的中心将垂直地在一条线上。然而,在沿船舶长度的特定点上,净效应可能是浮力过剩或重量过剩。这种净效应产生了结构的载荷,就像梁一样。这种负载导致船舶结构中产生剪切力和弯矩,从而使其弯曲。作用在船舶结构上的静力如图2.2(a)所示。如图2.2(b)-(d)所示,重量和浮力的这种分布也会导致载荷、剪切力和弯矩沿船舶长度的变化。根据弯矩作用的方向,船将在纵向垂直平面内弯曲。这个弯矩被称为静水弯矩(SWBM)。特殊术语用于描述两种极端情况:当船中部的浮力超过重量时,船舶被称为“中拱”,这种情况如图2.3所示;当船中部的重量超过浮力时,船舶被称为“中垂”,这种情况如图2.4所示。
图2.3中拱状态 图2.4中垂状态
动态载荷
如果船现在被认为是在波浪中运动,重量的分布仍然是一样的。然而,浮力的分布会因波浪而变化。船的运动也将引入动态力。 解决这个问题的传统方法是将这种动态情况转换成等效的静态情况。为了做到这一点,假设船舶在与船舶长度相等的余摆线形式的静态波浪上保持平衡。海上波浪的轮廓被认为是次摆线。这就产生了波峰比波谷更尖锐的波浪。波峰最初被认为是在船的中部,然后是在船的两端。如图2.5所示,在所考虑的特定载荷条件下,最大中拱和中垂力矩将出现在结构中。
图2.5船体结构的动态负载:(a)静水状态;(b)中垂状态;(c)中拱状态
这样就得到总剪力和弯矩,其中包括前面考虑的静水弯矩。如果考虑到船舶的实际装载工况,这将使上述状态变得更糟,例如,当波谷位于船中部时,船中部的重负载,则可以找到正常运行服务中的最大弯矩。 因此,当波浪沿着船的长度移动时,船的结构将受到由这些剪切力和弯矩产生的不断波动的应力。
结构的应力
船的弯曲使其结构内部产生应力。当船中垂时,船底外板会产生拉应力,甲板会产生压应力。当船中拱时,甲板上会产生拉应力,船底会产生压应力。这种应力,无论是压缩应力还是拉伸应力,都朝着称为中性轴的位置减小。船的中性轴在一半深度以下的某个地方,实际上是一条穿过船身重心的水平线。 梁的基本弯曲方程为 M / I = σ / y 其中M是弯矩,I是截面绕中性轴的二阶面积矩,σ是外层纤维的应力,y是中性轴到外层纤维的距离。 该方程在实船试验中已被证明适用于船舶的纵向弯曲。从方程中,得到表达式 σ = M / (I/y) 是在离中性轴一定距离y处的材料中的应力。可以确定船舶的M、I和y值,并且可以找到甲板和船底外板中的合成应力。当y测量到截面的最边缘时,比值I/y称为截面模量Z。这些值是对于船中截面确定的,因为最大力矩将出现在船中或靠近船中 (见图2.2)。这个过程的更详细的解释在前面引用的Muckle的著作《海洋工程师的船舶建筑》中给出。 在计算二阶面积矩I时,包括的结构材料将是延伸相当一部分船舶长度的所有纵向材料。这种材料将包括船舷和船底外板、内底板(如有安装)、中纵桁和甲板。这种材料形成了所谓的船体梁,与厚度相比,它的尺寸非常大。
横向应力
静态载荷
除了由结构、货物等的重量产生的载荷之外,船的横截面还受到来自周围水的静压力。尽管横向应力比纵向应力小,但如果没有足够的加强,结构可能会发生相当大的变形(图2.6)。 抵抗横向应力的结构部分是横向舱壁、双层底的肋板(如安装)、甲板横梁、舷侧肋骨和它们之间的肘板以及相邻结构,如舱顶肋板或边缘板。
图2.6横向船体结构的静水压力载荷
动态应力
当船横摇时,它被加速和减速,导致结构中的力倾向于扭曲它。这种情况被称为倾侧,当船舶处于空载或压载状态时,其影响最大(图2.7)。连接水平和垂直结构的肘板和梁弯头用来抵抗这种变形。
图2.7倾侧
局部应力
船舶在海上的运动导致产生的力主要是局部性质的。然而,这些力容易导致结构振动,从而将应力传递到结构的其他部分。
砰击或冲击
在恶劣天气下,当船舶发生垂荡和纵摇时,艏端离开并重新进入水中,产生砰击效应(图2.8)。这种艏部区域向水中的猛烈撞击被称为砰击(冲击)。必须在砰击区域安装额外的加强件,以降低损坏结构的可能性。这将在第5章A节中进一步讨论。
图2.8砰击
拍击
波浪沿着船的运动导致了板上水压的波动。倾向于产生外板的进出运动,称为拍击(振颤)。当船在水中前进时,这种效应在船头尤其明显。 船舶的纵摇运动会产生额外的水压变化,尤其是在船头和船尾,这也会导致船板拍击。额外的加强以抗震横梁和纵材的形式提供。这将在第5章D节中进一步讨论。
局部负载
较重的重量,如机舱中的设备或某些杂货,会引起横截面的局部变形(图2.9)。处理这一问题的方法是分散载荷、额外的加强和加厚板。
上层建筑和不连续性
上层建筑的末端代表了船舶结构中的主要不连续性,在此处,截面模量发生了相当大的变化。将出现局部应力,这可能导致相邻结构开裂。因此,通过引入渐变来避免尖锐的不连续性。在这些点上也可以安装较厚的甲板列板和船外板。 甲板上的任何孔洞或开口都会产生类似的高局部应力区域。在需要开口的地方必须使用圆角,也可以安装复板。对于舱口的情况,大部分纵向强度材料集中在舱口开口两侧的舷外侧,以减少开口处截面模量的变化。这将在第5章B和F节中进一步讨论。
图2.9趋使船体结构扭转变形的局部载荷
振动
由于往复机器、螺旋桨等在船上产生的振动。会导致结构中产生应力。这些是循环应力,可能导致结构局部的疲劳破坏,进而导致更普遍的倒塌。机器平衡和适当的螺旋桨桨尖间隙可以将振动的影响降低到可接受的程度。除了可能损坏设备和结构之外,振动的存在可能会使乘客和船员感到非常不舒服。 结构的设计超出了本书的范围。现在将考虑用于提供该结构的各种造船材料。
钢铁
钢是今天使用的基本造船材料。钢可以被认为是一种铁碳合金,通常含有其他元素,碳含量通常不超过2%。高抗拉强度的特种钢被用在船体结构的某些高应力部位。铝合金在建造上层建筑,尤其是客轮上有特殊的用途。
生产
“酸性”或“碱性”是指钢时经常使用的术语。参考生产工艺和炉衬类型,例如碱性炉衬用于生产碱性钢。炉衬的选择取决于炼钢所用的原材料。目前有三种用于制造碳钢的特殊方法,即平炉法、氧气或碱性氧气炼钢法和电炉法。在所有这些过程中,熔融金属暴露在空气或氧气中,氧化杂质,将生铁炼成高质量的钢。 在平炉工艺中,使用一个从两端燃烧的长而浅的炉子。在这个过程中可能会用到大量的废钢。生产高质量的钢,其性能可以通过添加合适的合金元素来控制。 在氧气或碱性氧气炼钢法中,熔融金属装在碱性炉衬炉中。用头顶的喷枪将氧气喷入熔融金属中。合金元素可以被引入到熔融金属中,从而生产出高质量的钢。 在电炉法中,电弧在碳电极和炉中的钢料之间产生。用这种方法可以精确控制钢的最终成分和高标准的纯度。
整理处理
上述过程产生的钢都含有过量的氧,通常以氧化铁的形式存在。在钢的最终铸造中,可以进行几种精整处理。 沸腾钢是由于很少或根本没有脱氧处理而产生的。在熔融状态下,氧与钢中的碳结合,释放出一氧化碳气体。凝固时,形成几乎纯铁的外表面。然而,钢锭的中心是一大堆气孔。钢锭的热轧通常会“焊住”这些孔,但这种材料的厚板容易分层。 镇静钢是通过在将钢倒入模具之前添加铝或硅来固定氧而生产的。铝或硅产生氧化物,将氧化铁还原成铁。这样就生产出了质量优于沸腾钢的同质材料。 平衡钢或半镇静钢是钢的一种中间形态。这是由于模具中沸腾过程的开始和终止阶段使用脱氧剂。 当钢处于熔融状态时,通过降低大气压力来生产真空脱气钢。碳和氧之间的平衡因此在低得多的水平上获得,并且氧含量变得非常小。添加最少量的铝或硅就能达到最终的残余脱氧。一种非常“干净”的钢被生产出来,具有良好的缺口韧性,并且没有层状撕裂问题(层状撕裂在第4章中解释)。 钢的成分对其性能有重大影响,这将在下一小节中讨论。钢的性能通过各种形式的热处理得到进一步改善,下面将对此进行概述。简而言之,钢的热处理会导致晶粒结构的变化,从而改变材料的机械性能。
正常化。根据钢的含碳量,将钢加热到850-950 ℃,然后在空气中冷却。生产出具有精细晶粒结构的高强度钢。
退火。钢再一次被加热到850-950℃左右,但是在炉子里或者在一个隔热的空间里慢慢冷却。生产出比正常状态下更软、更有韧性的钢。
硬化。钢被加热到850-950 ℃,然后通过在油或水中淬火来快速冷却。因此产生了特定钢的最坚硬的可能条件,并增加了抗拉强度。
回火。这一过程在钢淬火后进行,包括加热到680℃左右。回火温度越高,钢的拉伸性能越低。一旦回火,金属通过淬火迅速冷却。
成分和性质
在描述钢和其他材料的特性时,使用了不同的术语。现在将更详细地解释这些术语。
拉伸强度。这是关于金属的主要单一标准。这是衡量材料在使用中承受负荷的能力。诸如应力、应变、极限拉伸强度、屈服应力和验证应力等术语都是量化材料拉伸强度的不同方法。影响抗拉强度的两个主要因素是钢的碳含量及其制造后的热处理。
延展性。这是一种材料在不破裂或不损失强度的情况下永久改变形状的能力。在制造过程中金属经受成形过程的地方,这一点尤其重要。
硬度。这是对材料可加工性的一种度量。它用于评估材料的可加工性及其耐磨性。
韧性。这是介于脆性和柔软性之间的一种状态。它通常由缺口棒试验中获得的值来量化。
标准型钢
各种各样的标准节生产与各种构件尺寸,以适应他们的应用。板和型材的加强利用了这些型材中的一个或多个。如图2.10所示。
图2.10标准型钢:(a)扁钢;(b)球扁钢;(c)等边角钢;(d)不等边角钢;(e)槽钢;(f)T型材
造船用钢
用于造船的钢是含碳量为0.15-0.23%的低碳钢。一种好的造船用钢所需的性能是:
(1)成本合理。 (2)易于焊接,工艺和设备简单。 (3)延展性和均匀性。 (4)屈服点是极限抗拉强度的一个高比例。 (5)化学成分适合火焰切割,无需硬化。 (6)抗腐蚀性。
这些特性由船级社指定的五个等级的低碳钢(A-E)提供(见第10章)。要进行入级,造船用钢必须在批准的条件下生产,并进行检验,而且必须对选定的样品进行规定的试验。成品材料上印有该协会的品牌,这是一个L叠加在R上的劳氏船级社标志。表2.1给出了一系列低碳钢的化学成分和机械性能。
钢铁生产和合金化技术的发展使得更高强度的钢可用于造船。这些所谓的高抗拉强度(HTS)钢,除了增加强度之外,还具有足够的缺口韧性、延展性和可焊性。强度的增加是由于添加了合金元素,如钒、铬、镍和铌。铌尤其提高了拉伸强度和缺口延展性的机械性能。对于这些钢,在选择电极和焊接工艺时必须特别小心。必须使用低氢焊条和焊接工艺。表2.2显示了几种高强度钢的化学成分和机械性能。船级社使用特殊的等级标记H来表示高强度钢。 在船舶建造中使用这些钢的好处包括减少结构重量,因为可以使用更小的截面;对于相同的重量和更少的焊接时间,更大的单元制造是可能的,尽管减少材料尺寸需要更专业的工艺。 由于用散装油轮运输液化气体,低温材料正越来越多地被使用。表2.3详细列出了几种低温材料的特性和成分。选择的主要标准是在工作温度下要有足够的缺口韧性。各种合金主要用于非常低的温度环境,尽管特殊质量的碳/锰钢已经在低至-50℃的情况下得到令人满意的应用。 表2.1某些低碳钢的化学成分和机械性能 表2.3某些高碳钢的化学成分和机械性能 表2.3某些低温材料的化学成分和机械性能
铸件和锻件
用于造船的大型铸件通常由碳钢或碳锰钢制成。表2.4详细说明了这些材料的成分和特性。大型铸件的例子有船尾肋骨、桨毂、A字支架和舵的零件。提到的例子也可以制造成锻件。表2.4还详细说明了锻件所用材料的成分和性能。 表2.4铸件和锻件的化学成分和机械性能 表2.5铝合金材料的化学成分和机械性能
铝合金
铝合金的使用越来越广泛,这是因为它比钢有几个优点。对于同等体积的材料,铝的重量大约是钢的三分之一。在结构中使用铝合金可以使同等钢结构的重量减少60%。这种重量的减少,特别是在结构的上部区域,可以提高船的稳性。这是由于船只重心降低,导致稳心高度增加。稳性在Muckle的《海洋工程船舶建筑》中有详细讨论。铝的耐腐蚀性非常好,但小心维护和与相邻钢结构的绝缘是必要的。用于造船的铝合金所要求的性能与钢的性能大致相同,即强度、耐腐蚀性、可加工性和可焊性。这些要求已充分满足,主要缺点是铝的成本高。 常用造船合金的化学成分和机械性能见表2.5。同样,这些是船级社分级,材料必须按照船级社的要求进行制造和测试。
图2.11铝合金型材
铝合金可用于板材和型材,图2.11显示了铝合金型材的选择。这些部分通过挤压形成,挤压是将热材料的坯料挤压通过合适形状的模具。因此,适合特定应用的复杂或不寻常的形状是可能的。 在铝合金连接钢结构的地方,必须采用特殊的绝缘装置,以避免金属接触处的电化腐蚀(见第二章)。如果使用铆钉,它们应该由耐腐蚀合金制成(见表2.5)。
材料试验
到目前为止所讨论的各种质量的材料已经提到。这些质量是由对金属样品进行的各种试验来确定的。 “应力”和“应变”这两个词用得最多。应力或应力强度,它的正确名称是作用在材料单位面积上的力。应变是材料因应力而产生的变形。当施加在材料上的力趋向于缩短或压缩材料时,这种应力称为“压应力”。当施加在材料上的力趋向于拉长材料时,这种应力称为“拉应力”。当力趋向于使材料的不同部分相互滑动时,这种应力称为“剪切应力”。
图2.12拉伸试验试样:(a)板、条、型材(a = 材料厚度);(b)热轧棒材 拉伸试验用于确定材料在断裂点之前的行为。标准尺寸的特殊形状试样(图2.12)被夹在试验机的钳口中。逐渐施加载荷以拉开杆的端部,使其承受拉伸应力。本的原始测试长度L1是已知的,并且对于每个施加的载荷,新的长度L2可以被测量。通过将L2 - L1人们将会发现,这个样本已经延伸了某个很小的量。这种变形表现为 延伸量/原始长度 被称为线性应变。 对试样加额外的载荷将产生结果,该结果显示延伸均匀增加,直到达到屈服点。在屈服点之前,移除载荷会导致试样恢复到其原始尺寸。因此,直到屈服点或弹性极限,应力与应变成正比。各种载荷下的应力和应变值可以显示在图2.13中。
图2.13低碳钢的应力-应变图 图2.13高拉力钢的应力-应变图
如果试验持续到超过屈服点,试样的横截面就会“收缩”或缩小。载荷值除以原始试样的横截面积,就得到图2.14所示的曲线。应力的最高值称为材料的极限拉伸应力(UTS)。 在弹性极限内,应力与应变成正比,因此 应力/应变=常数 该常数被称为材料的“弹性模量”(E),其单位与应力相同。屈服应力是屈服点处的应力值。如果没有获得明确定义的屈服点,则给出一个验证应力值。这是通过在应变的某个百分比(如0.1%)画一条平行于应力-应变线的线获得的,这条线与应力-应变线的交点被认为是验证应力(见图2.14)。 弯曲试验用于确定材料的延展性。一块材料在一个弧形样板上弯曲,有时弯曲180度。材料中不应出现裂纹或表面分层。
图2.15夏比冲击试验 冲击试验可以有多种形式,但夏比v形缺口试验通常是指定的。试样是一个10毫米见方的横截面,长55毫米。在一个面的中心切出一个v形切口,如图2.15所示。试样水平安装,缺口轴垂直。该试验包括在缺口对面撞击样品并使其断裂。在摆动的钟摆末端有一个撞针或锤子,它提供了击碎样品的力量。破裂时材料吸收的能量由机器测量。必须获得材料在试验温度下的特定平均冲击能量值。该测试对于在低温区域使用的材料尤为重要。对于低温试验,将样品浸入液氮或干冰和丙酮浴中冷却约15分钟。然后快速处理和测试样本,以最大限度地减少任何温度变化。冲击试验实际上是测量材料在冲击载荷下的抗断裂能力。 倾倒试验用于制作铆钉的一段棒材样品。棒材被压缩至其原始长度的一半,并且不得出现表面裂纹。其他铆钉材料测试包括弯曲铆钉杆,直到两端接触而不出现任何裂纹或断裂。头部也必须接受展平,直到它达到柄直径的两倍半。
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