222
A. NELSON-SMITH
hull. Burning is considered to be a last resort, since it renders the ship
worthless ; experience with “ Torrey Canyon ” shows that it is possible,
but requires the addition of much additional inflammable material and
frequent reignition.
The compartmental construction of tankers may permit salvage of
an undamaged portion by “ explosive surgery ”, using shaped plastic
charges, if the fire risk can be minimized. The after half of the “Anne
Mildred Brovig ” was saved in this way and, in general, it is more useful
after a collision than a stranding. Once a tanker has sunk in fairly deep
water, it appears that oil remaining in her is released slowly but steadily
through deck vents. The U S . Coast Guard (1959) estimated that, at
the end of the Second World War, at least 61 tankers still containing
210 million gal of oil lay along the Atlantic and Pacific coasts of the
United States; but recent dives on carefully selected wrecks off the
north-east Atlantic coast failed to produce even a small sample for
analysis (U.S. Coast Guard, 1968). The fuel-tanks of other ships may
retain their contents for longer ; the German cruiser “ Blucher ”, sunk
in Oslo Fjord during April 1940, began to leak oil in June 1969.
A source of pollution from tank-shipping which is less obvious but,
until recently, was much more important than accidental damage, is the
need for a tanker to carry sea-water ballast on her return journey. This
is usually accommodated in the oil tanks and occupies up to one-third
of the cargo space. The ballast has to be discharged at the loading port
as oil is taken on. Each tank contains the residues of its previous cargo ;
in all 0.3-0*5% of the cargo is left behind-about 200 tons in a 50 000ton tanker. The discharge of even a third of this amount of oil in the
ballast water would not be permitted at the loading port, nor would it
be practical to build shore separating facilities capable of dealing with
such large volumes ; so it was normal practice to wash the tanks at sea,
pumping the washings overboard and then taking ballast into the clean
tanks. From recent shipping figures, this would result today in the
discharge into the sea of some 6 000 tons of crude oil per day-nearly
2+ million tons per year. Fortunately, the major tanker operators
introduced the “ load-on-top ” system in 1964. This is described in
detail by Kluss (1968a) ; in brief, the washings are pumped to one tank
(acting as a slop-tank) where the oil is separated, often by the use of
demulsifying chemicals or heating coils. When separation is as nearly
complete as possible, the water is pumped from below the oil and discharged into the ship’s wake. At the destination port, a fresh cargo
is loaded on top of the recovered oil. This procedure requires care, since
only a little excess salt water can be dangerous in some refining processes; it also takes time and t,rouble, although this is balanced on
A. NELSON-SMITH
hull. Burning is considered to be a last resort, since it renders the ship
worthless ; experience with “ Torrey Canyon ” shows that it is possible,
but requires the addition of much additional inflammable material and
frequent reignition.
The compartmental construction of tankers may permit salvage of
an undamaged portion by “ explosive surgery ”, using shaped plastic
charges, if the fire risk can be minimized. The after half of the “Anne
Mildred Brovig ” was saved in this way and, in general, it is more useful
after a collision than a stranding. Once a tanker has sunk in fairly deep
water, it appears that oil remaining in her is released slowly but steadily
through deck vents. The U S . Coast Guard (1959) estimated that, at
the end of the Second World War, at least 61 tankers still containing
210 million gal of oil lay along the Atlantic and Pacific coasts of the
United States; but recent dives on carefully selected wrecks off the
north-east Atlantic coast failed to produce even a small sample for
analysis (U.S. Coast Guard, 1968). The fuel-tanks of other ships may
retain their contents for longer ; the German cruiser “ Blucher ”, sunk
in Oslo Fjord during April 1940, began to leak oil in June 1969.
A source of pollution from tank-shipping which is less obvious but,
until recently, was much more important than accidental damage, is the
need for a tanker to carry sea-water ballast on her return journey. This
is usually accommodated in the oil tanks and occupies up to one-third
of the cargo space. The ballast has to be discharged at the loading port
as oil is taken on. Each tank contains the residues of its previous cargo ;
in all 0.3-0*5% of the cargo is left behind-about 200 tons in a 50 000ton tanker. The discharge of even a third of this amount of oil in the
ballast water would not be permitted at the loading port, nor would it
be practical to build shore separating facilities capable of dealing with
such large volumes ; so it was normal practice to wash the tanks at sea,
pumping the washings overboard and then taking ballast into the clean
tanks. From recent shipping figures, this would result today in the
discharge into the sea of some 6 000 tons of crude oil per day-nearly
2+ million tons per year. Fortunately, the major tanker operators
introduced the “ load-on-top ” system in 1964. This is described in
detail by Kluss (1968a) ; in brief, the washings are pumped to one tank
(acting as a slop-tank) where the oil is separated, often by the use of
demulsifying chemicals or heating coils. When separation is as nearly
complete as possible, the water is pumped from below the oil and discharged into the ship’s wake. At the destination port, a fresh cargo
is loaded on top of the recovered oil. This procedure requires care, since
only a little excess salt water can be dangerous in some refining processes; it also takes time and t,rouble, although this is balanced on