84
Lesson N°14
Oil pollution
It is difficult to calculate the total quantity of petroleum hydrocarbons
entering the sea . Estimates have varied widely over the years.
Tanker operations :
After a tanker has unloaded its cargo of oil , it has to take on
seawater as ballast for the return journey to the oilfields. The ballast
is usually
stored
in the cargo compartments , which previously
contained the oil.
During unloading , a certain amount of oil remains clinging to the walls
of the compartment ; the ballast water inevitably becomes
contaminated with this clingage , which , in any case , has to be
removed from the compartments before they can be filled with a fresh
cargo of oil.
Formerly , this dirty ballast and clingage were discharged to sea during
the return voyage , and were responsible for much of the casual oil
pollution of the world ’s oceans .
Measures have been introduced to reduce this input.
A further development has been the introduction
of
segregated
ballast , in which ballast water is carried in separate compartments
and does not come in contact with the oil , so that it can be discharged
to sea without causing pollution.
Dry docking :
All ships , including oil tankers , require
periodic dry docking for
servicing , repairs , cleaning the hull , and so on .
Ship scrapping :
Ships are brought ashore and broken up to salvage metals . Oil and
toxic materials remaining in the ships are generally lost to sea.