TANKER
AND LPG CARRIER
The absence of cargo hatches and external handling gear distinguish
ships that carry liquid cargo (most often petroleum and its
products) in bulk. They are also readily distinguishable by
the low freeboard, especially when fully loaded, the all-aft
construction with the funnel, or twin funnels, bridge and accommodation
all forming a single block on the poop and the catwalk. The
latter is a raised fore-and-aft gangway connecting the islands
to enable members of the crew to move from one island to another
with safety in rough weather. Additional characteristics are
the numerous small tank hatches, fore-and-aft pipes and, amidships,
the manifold with its complex arrangement of pipes and valves
connected to the ship's tank system. Each tank is fitted with
heating coils to facilitate pumping in cold weather. Within
the tanks are the main, or high-suction, pipes, running several
feet from the bottom to avoid sludge. Below them, low-suction
piping, or stripping lines, removes the lowest level of liquid
in the tank. The manifold is the main point of the loading operations
- by gravity or shore pumps - and discharging, by means of the
ship's own pumps. Close to it are the two light hose-handling
derricks. The tanker hull is divided by two longitudinal and
many transverse bulkheads forming up to thirty or so centre
and wing cargo tanks. The number of comparatively small compartments
minimizes the swishing effect of the liquid cargo and also means
that several grades of oil can be carried on one voyage. At
both ends of the cargo space there are double watertight bulkheads,
known as cofferdams, to reduce the risk of fire spreading to
the highly inflammable cargo. As the tanker transports her cargo
on only one way of a round voyage, her crew spends the "in
ballast" run cleaning the tanks and preparing for the next
cargo. The considerable use of automatic controls, however,
has reduced the size of the crew to a minimum.
The decline of crude oil prices after the petroleum crisis of
1979 led in turn to a decline in tanker size, but at that time
a few ships had reached 1,300 feet (400 metres) in length, 80
feet in loaded draft, and a deadweight of 500,000 tons.
Along with the great increase in numbers and size of tankers
have come specialized uses of tankers for products other than
oil. For shipment, gas is cooled and converted to liquid at
-260? F (-162? C) and is then pumped aboard a tanker for transit
in insulated tanks to prevent absorption of heat and to keep
the liquid from evaporating during the voyage.
The LPG carrier is distinguished from the normal oil tanker,
or bulk carrier, by the complex deck fittings and piping, and
the horizontal pressure cylinders. On some ships there is a
series of large conical or domed tank tops visible above deck
level. This type of vessel is specially designed for the carriage
of propane, butane and natural gas methane. Liquefied methane
is carried in insulated tanks at a very high pressure and is
cooled to its boiling point. The mast amidships contains the
vents from each tank. Like the oil tanker and bulk carrier,
the ship has a number of tanks for water ballast.
