storage tanks, shipbuilders minimise
the effects of sloshing (see ‘Slosh
dy namics’ box). While a smaller tank
filled to capacit y won’t slosh and shift its
weight on the high seas, a large,
half-empt y tank could slosh w ith
enough force to capsize even a
supertanker. Once the ship reaches its
destination, a powerful on-board pump
sucks the oil from the tanks and
transports it to an on-shore pipeline,
storage facilit y or to a smaller tanker.
Safet y is a major consideration on a
supertanker. First and foremost, you are
transporting massive quantities of a
highly flammable liquid. (Ever y oil
tanker features a large stencilled ‘No
smoking’ sign over the crew quarters!) It
turns out that the greatest danger is not
the oil itself, but the vapours that can
become trapped in the partially filled
tanks. That’s why modern oil tankers
employ an automated inert gas system
that fills unused portions of a storage
tank with a cocktail of gases that render
the vapour inflammable.
Oil leaks and spills are another big
concern, both for economic and
environmental reasons. In the wake of
the infamous Ex xon Valdez oil spill in
1989, all modern oil tankers are required
to have double-hull construction. The
inner hull containing the storage tanks
is protected by an outer hull; these are
div ided by a three-metre (ten-foot) gap.
When the tanker is f ull, the space
bet ween the hulls is left empt y, forming
an effective crumple zone. When the
tanker unburdens its load of oil, the
space is filled w ith water to act as ballast.
Temperature is another serious
concern for supertankers. Crude oil and
other fuel products can get thick and
sticky if they are allowed to become too
cold, making them nearly impossible to
unload. When supertankers cross
through near-frozen arctic waters, they
maintain the desired oil temperature by
pumping hot steam through coils
underneath each storage tank.Slosh dynamics
What is
crude oil?
Despite their incredible size and weight, supertankers are surprisingly
vulnerable to capsizing. That’s because they are filled with liquid cargo, which
sloshes about w ith great force, dangerously altering the ship’s centre of grav it y.
The worst scenario is a large storage tank only partially filled. The liquid in this
‘slack tank’ will slosh and shift with sudden manoeuvres of the ship or outside
forces like strong waves or w ind gusts. Since the liquid sloshes in the same
direction as the roll, it exaggerates the pitch of the vessel, creating something
called the free surface effect. As the vessel tries to right itself to centre, the liquid
sloshes even more violently in the opposite direction, initiating a positive
feedback loop that can eventually lead to disaster. To mitigate the dangers of the
free surface effect, supertankers use several smaller storage tanks and either fill
them to the top (a ‘pressed up’ tank) or leave them empt y.Crude oil is the raw, unprocessed
petroleum that is pumped out of
the ground through oil drilling.
The composition of crude oil
varies greatly with the location of
the underground oil deposit. The
main ingredient of crude oil is
carbon, which makes up 83-87
per cent of the mi x. There are
also natural gases bubbling
through the thick liquid such as
methane, butane, ethane and
propane, composed of hydrogen,
nitrogen, ox ygen and sulphur in
var y ing quantities. The black/
brow n crude is shipped to oil
refineries, where it is purified
and separated into commodities
like gasoline, diesel fuel,
kerosene and liquid natural gas.Slack tank
The free surface effect
is exaggerated in a
partially filled tank,
where liquid moves
freely over a large area.Slosh
If the ship’s manoeuv ring
or an outside force tips it
starboard, the liquid will
slosh in the same direction,
deepening the roll.Displacement
Normally, a slight roll is
counteracted by the upward
pressure of the water
displaced. Sloshing liquid acts
against that correcting force.Centre of gravity
If enough liquid sloshes
with enough force, it can
alter the vessel’s centre of
grav it y and leave the ship
unable to right itself.Rocking the boat
The free surface effect
can be mitigated by
using smaller,
off-centre tanks and
filling them to capacity.Deadweight
tonnage
Following the principle of
Archimedes’ “Eureka!”
moment, if you lower a floating
vessel into water, a force called
buoyancy pushes upwards on
the hull with a force equal to
the weight of the water it
displaces. Buoyancy only
works on objects that are less
dense than water. It is the huge
volume of air in the hull that
allows supertankers to float.
Because displacement equals
weight, we can figure out the
total weight of a ship – known
as deadweight tonnage – by
measuring the height of the
waterline against markers
painted on the ship’s hull.A bird’s eye v iew of the
prow of an oil tankerCr ude oil is a mi xture
of compounds known
as hydrocarbonsDID YOU KNOW? A supertanker transporting liquid natural gas has more energy potential than six Hiroshima-scale bombs