Bulk cargo carriersCargo which may be carried in bulk includes oil, ore, chemicals, vegetable oils,molasses, liquefied gas, coal, grain and forest products. Even cars and containers present a homogeneous cargo whose effects upon the ship are reasonablypredictable.Economic arguments indicate that most oil should be carried worldwide invery large crude carriers, VLCCs and some 500 such vessels ply the oceans.They may carry as much as 300,000 tonnes of crude oil in holds arrangedperhaps six longitudinally and three or four abreast. The block coefficient isoften around 0.8. Machinery aft is usually diesel driving a single shaft and aboiler produces steam for domestic use, for heating the bunker oil and forsteam cleaning cargo tanks. There is also an inert gas system for cargo tanks toprevent the build up of an explosive mixture above the cargo. There is asuperstructure aft with bridge and accommodation and a central walkwayalong the upper deck to protect personnel from the effects of a very smallfreeboard. Loss by structural failure is now fairly rare even though cracking is not and should be dealt with in good time. Most loss is attributable to collisionor grounding giving rise to severe environmental pollution. In 1990, the USAintroduced an Oil Pollution Act which required all oil tankers using their watersto have double skins and IMO followed in 1993. Unhappily, evidence suggeststhat this would not have helped to reduce pollution in many of the majorenvironmental disasters that the world has suffered and designers have soughtmore effective measures.

One way of containing at least some of the cargo after a grounding is tosubdivide the ship by additional oiltight decks. Spaces below would be maintained by regulated air vents at atmospheric pressure so that, should the bottombe pierced, there is a pressure differential forcing the cargo inwards. It is thenallowed to weir into empty tanks. There are several variations on this theme,which could work, at least partially until there is structural disintegration, eveif'in the dynamic fluid conditions that prevail. Of course, an increase in cost and areduction in payload is inevitable.Losses of ore carriers caused great anxieties during the 1980s. Many of themdisappeared without trace, presumably by structural failure. There is evidenceto suggest that the scarphing of the midships structure towards the ends into areduced section modulus was not, in the 1970 designs, carried out with sufficientcare and that local stress concentrations caused cracking which propagatedfast. Some 10 to 15 per cent of the length from the stern has been shown tosuffer high stresses in a seaway. Damage to structure by the huge grabs used tounload cargoes exacerbates the problem while some cargoes generate highlycorrosive fluids that further damage the structure with time. These matters haveled to strong pressures for more high quality steels in critical parts of the ship.More intensive survey is now adopted and cooperation within lACS hus led to

new regulations which provide good structural management, including hullcondition monitoring.Slamming can be troublesome in bulk carriers and masters need to be advisedon how to minimize these effects; it tends to be a highly tuned condition so thatsmall changes in course or speed are usually effective in reducing its incidence.Of particular importance to the bulk carrier is the sequence of loading andunloading. It is relatively easy to cause structural damage by large shear forcesbetween full and empty tanks as well as unacceptable still water hull flexure.Masters must be supplied with examples of these effects, often programmablereadily on a microcomputer, to guide decision making. Designers may also wishto present their classification societies with a direct structural analysis ratherthan simply following their rules .. This is entirely acceptable and can lead toimproved structure.Grain carriers, often affected by the need to sail the Great Lakes of NorthAmerica are common. Grain carriers introduce special problems of stabilitydue to the free surface effects of the grain. While shifting boards reduce the effects, large holds make these difficult to fit; covering the surface with baggedgrain damps the free surface but does not remove the problem. Masters arerequired to take the ullages (i.e. height above the free surface) of the grain andto apply these to diagrams supplied by the Authorities (see Chapter 14) fordetermining revised stability data. Expected angles of heel of the ship shouldnot exceed 12 degrees due to possible shifting of the grain (see also Chapter 5).A type of bulk carrier of increasing importance is the liquefied natural gascarrier. Natural gas, predominantly methane, is given off in vast quantities in afew areas of the world, particularly at oilfields. For use in other countries, itneeds to be transported economically. The gas is first liquefied by compressingit and cooling it to temperatures around minus 100C in which condition it ispumped into transit tanks. Such tanks are isolated from the ship's structure byvery thick insulation and the ship is fitted with double bottom and side protection. During its passage across the sea, certain of the gas boils off naturallyeither to waste or it may, economically, be used to help drive the ship, perhapsby gas turbine. To minimize this loss and avoid carrying refrigeration machinery, the ship needs to be fast and reliable and the characteristics of the ship are,again, determined by the economics of operation.Material for the liquefield gas tanks must not be brittle at these very lowtemperatures-which render even rubber brittle. Suitable materials are certainaluminium alloys and some nickel steel alloys. It is very important, of course, toprevent the mild steel of the ship from being reduced in temperature by leakageof the liquid, whereby it may become brittle (see Chapter 5 on brittle fracture).Every cargo brings its particular problems but none more than special liquidcargoes such as molasses, sulphuric acid and sulphur, including thermal stresses,explosive vapours and corrosive materials

Container shipsThere has been a revolution in the transportation of goods throughout theworld. Goods may now be collected at their point of origin by lorry or train andtaken to the port for sea transit. There they are deposited within a system ofgantries or cranes which take them to the ship or to temporary storage untilthey can be embarked. The containers are standard worldwide and are calledTEUs. Millions of the TEUs are available for hire in the knowledge that theywill fit into ships specially built to receive them and take them to their destinations, where the reverse process takes place The ships themselves adopt various systems to hold the containers. The shipsare like hollow shoeboxes, stowing containers below a single deck which hasvery large hatches. More of them may be carried above the deck in stacks.There has evolved also an open container ship constructed like a doubleskinned U without a deck. The containers may be locked together by fittings,such as twist locks and are lashed so that they are unable to move even in severeweather. Alternatively, ships may be fitted with long vertical stanchionsthroughout so that a container can be housed between four of them. Fig. 16.9shows a typical cross-section of an open container ship.A huge range of ships now exists. Small coasters may carry a few containerson deck while at the other end of the scale,' ships 350 metres long carrying10,000 containers are not uncommon.In large ships there may be 20 containers athwart ships in stacks 20 high, ofwhich a quarter at least may be above deck. Closed ships may also have stacksfive or six high above the hatches and lashed in various ways to avoid movement in bad weather.Regulations have, of course, evolved to make these ships relatively safe andlosses of complete ships are rare. Containers, however, are regularly lost overboard and present a significant hazard to small ships, especially yachts, and adrain on the insurance market. Forces on the lashings in rough weather arehigh and the efficiency of the devices depend much on the vigilance of the crew(twist locks alone are insufficient).Anxieties over safety have been expressed by Vossnack who points out thatthe statutory freeboard is extremely low, thereby keeping the underdeck tonnage (and harbour dues) low. As shown in Fig. 16.9 this gives an angle of deckedge immersion and uncontrollable flooding often around thirty degrees.Moreover, minimum crew numbers based on gross tonnage raise doubts abouttheir adequacy in foul weather.Further development of the carriage of standard packaged cargo such asmotor vehicles may be expected. There have been studies to provide a fasterservice across the oceans. Demands on the design for high speed ships could beconsiderable in terms of machinery and air intakes for gas turbines whichwould reduce the number of packages that could be carried. Structure to dealwith the enhanced impact loads at the bow and on the bottom would also haveto be more substantial. Of course, the final decision would rest uponthe predicted economy of trading off an amount of cargo against its earlierdelivery