Trans RINA, Vol 152, Part A2, Intl J Maritime Eng, Apr-Jun 2010
steadily from the initial 15,000dwt to over 100,000dwt by 1968 although the 60-70,000dwt Panamax proved popular with a breadth of 104ft, later 106ft (32.2m), to get through the locks. The development of the combined carrier (ore/oil or ore/bulk/oil) in the late 1960s proved popular initially, offering the ability to switch cargoes between voyages, either to reduce ballast steaming or to take advantage of higher freight rates in the other market. But by the 1990s disadvantages outweighed advantages, with requirements for double hulls and high standards of hold cleaning between oil and dry bulk cargoes, so that few have been built since then. Although the smaller bulk carriers had their own cargo handling gear to enhance their ‘go anywhere’ capability, initially derricks then cranes, the bigger ships carried no gear, trading as they did between relatively few ports handling iron ore and coal, so relied on shore gear.
(parcels) carrier was Marine Dow-Chem built in 1954. Chemical resistant tank coatings such as epoxies and zinc silicate and some tanks built of stainless steel allowed a huge range of chemicals to be carried in bulk in a single ship. That also required the development of segregated pumping and piping systems, so that the submerged (deepwell) pump was born, hydraulically driven from the deck.
Liquefied Petroleum Gas (LPG) carriers were another 1950s development, where cargoes like propane could be liquefied either by cooling to about -48oC, or in smaller tanks by pressure alone. A more demanding technology was required for Liquefied Natural Gas (LNG) which liquefies at -163oC, so having heavily insulated tanks separate from the main hull structure. The first purpose built LNG ship was Methane Progress of 27,400m3 with aluminium alloy tanks in 1964. Spherical tanks were developed by Moss-Rosenberg, but
the most popular
design proved to be the membrane tank, a prismatic tank with a thin cryogenic lining supported all around by insulation and a secondary barrier
(in case of LNG
leakage causing brittle fracture). LNG carriers plateaued in size at around 130,000m3 for over two decades, but in recent years have broken the 200,000m3 barrier. Such vessels were for long the last outpost of steam turbine technology as the boil-off gas could be readily burned in the boilers. More recently diesel-electric machinery plus reliquefaction plant has been used.
Figure 11. The single deck bulk carrier developed
quickly in the 1960s as far more suited to cargoes like ore, coal and grain than tween-deckers. This view of 76000dwt Essi Kristine under construction at Harland & Wolff’s Belfast shipyard in 1967 shows the now classic hoppered hold shape with topside wing tanks. To the left is an early VLCC, Shell’s Myrina of 190000dwt. Such vessels were difficult to build on conventional slipways, so shortly afterwards H&W built their 556m x 93m building dock in the Musgrave Channel,
immediately
behind the tanker, with one (later two) 840ton goliath crane spanning the dock, permitting much larger prefabricated units than was possible with the berth cranes seen. Such building docks were widely constructed from the mid 1960s, usually in ‘greenfield’ shipyards – but often actually land reclaimed from the sea.
The late 1950s and 1960s saw a great increase in specialist ship types, as demand for large volumes of cargoes like export motor cars justified a tailor-made design rather than a general purpose ship. The growth in car exports initially from Europe, later from the Far East, brought about the pure car carrier, a floating multi-storey car park initially capable of carrying about 1000 cars, but around 8000 today. The first purpose-built chemical
The impetus for specialist ships comes when the volume of a particular trade expands to a level sufficient to support a custom-built fleet. Although specialist port facilities are also required, the increased efficiency and improved quality of cargo outturn outweigh the lack of flexibility for alternative cargoes. Flexibility was the key attribute of the general purpose (multi-deck general cargo) ship, which dominated such trades for over a century. It was the inefficiencies of the latter that spurred the development of the container ship to speed up general cargo handling, eliminate labour intensive methods and reduce port time from a week or so to a day or so. Although early vessels developed for Sea-Land’s US coastal trades were based on road trailer dimensions (but using the cellular stowage principle), standardisation of container dimensions by ISO in 1965 paved the way for its widespread adoption. The now ubiquitous 20ft and 40ft boxes (no-one talks of 6.1 or 12.2metre containers) allowed mechanised handling and stowage processes and equipment to be developed, ranging from gantry cranes to container spreaders to straddle carriers to lashing mechanisms, as well as intermodal transfers to road and rail vehicles. Steel hatch covers were designed to take containers stacked on them;
container load can be stowed on deck, the rest in cell- guides in the holds.
Such was the increase in productivity from larger faster container ships that one vessel replaced five or six break- bulk vessels, so all
the main trade routes had been
today up to half the
©2010: The Royal Institution of Naval Architects
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