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Oil Spills and the Marine Environment
with it, making a line of upwelling water. When the water
reaches the surface, it spreads horizontally in the two directions
perpendicular to the submerged pipe. Experiments indicate that it
is possible to stop oil from crossing this two-way current, which
has been done for some time in European harbors built especially
for the unloading of oil tankers. There, the places where pollution
must be stopped are quite narrow and the water especially calm. The
air-curtain concept is particularly useful since it can be
installed across the mouth of a harbor and still allow ships to
pass it. Considerable laboratory research has been done to
determine requirements for air barriers (). One conclusion from
this work is that fairly large amounts of compressed air are
needed, typically one horsepower per foot of barrier. An
air-curtain barrier was used at the Santa Barbara oil spill (). It
was not successful owing to operational problems, and thus the
effectiveness of using an air barrier at sea could not be
determined conclusively.
OIL-SKIMMING DEVICES
In addition
to oil pollution booms, numerous companies have designed and built
devices for skimming oil from the surface of the sea without first
doing the necessary background work. Although some of them are
effective for very small harbor spills, their use at sea was a
failure. One important thing that did not receive sufficient
consideration for years is that, for effective oil pickup, a large
amount of sea surface must be swept; however, the sweeping speed
must be kept moderately slow to keep large amounts of oil from
going past the skimming device. It has been conclusively shown that
the fluid velocity into any type of oil-pickup device is limited by
the depth of the oil (): the deeper the pool of oil at the pickup
device, the higher the allowable velocity. Once this limiting
velocity is reached, a greater velocity will not increase oil
pickup, but merely pick up more air and water. Therefore, in order
to be able to pick up a substantial amount of oil, a fairly deep
pool at the pickup device is needed. These facts indicate that the
most effective type of pickup device is a boom having a collector
inside it, with the oil driven into the boom either by current or
by towing the boom. The boom provides a large surface area from
which to collect the oil into a relatively deep pool at a skimmer,
which actually carries out the collection. The collection of 18,000
gallons of soybean oil () was done like this, as was most of the
collection carried out during the Chevron oil spill ().
Although an
oil collection device covering a large area is required for large
spills at sea, there are some instances where the skimming of
smaller slicks would be facilitated by a collection device that
could be effective without a boom.
Suction-Type Skimmers
A
suction-type skimmer is a device that floats on the sea and
has