of oil from shipping, offshore extraction of oil, and transport
of oil in pipelines is the result of either accidents or "normal",
deliberate operational discharges. Accidental
discharges (oil spills) occur when vessels collide
or come in distress at sea (engine breakdown, fire, explosion)
and break open, or run aground close to the shore, or when there
is a blowout of an offshore oil well, or when a pipeline breaks.
Much can be done to avoid accidents, but there will always be
unfortunate circumstances and situations that cause accidents
to happen. Operational discharges, on the other hand,
are mostly deliberate and "routine", and can to a
very large extent be effectively controlled and avoided. It
is much a question combining available technical solutions with
information, education and a change of attitude among ship-owners,
mariners, offshore platform and pipeline operators.
operational discharges of oil include the discharge of bilge
water from machinery spaces, fuel oil sludge, and oily ballast
water from fuel tanks. Also other commercial vessels than
tankers contribute operational discharges of oil from machinery
spaces to the sea. Cargo-related operational discharges from
tankers include the discharge of tank-washing residues and
oily ballast water.
international regulations were introduced to prevent oil pollution
from ships, the normal practice for oil tankers was to wash
out the cargo tanks with water and then pump the resulting
mixture of oil and water into the sea. Also, oil cargo or
fuel tanks were used for ballast water and, consequently,
oil was discharged into the sea when tankers flushed out the
oil-contaminated ballast water to replace it with new oil.
oil washing systems (COW) means that
the cargo tanks, where tankers carry the oil they transport,
are cleaned by means of high-pressure flushing with crude
oil ("oil to remove oil") or crude oil plus water.
This reduces the quantity of oil remaining on board after
discharge. The residues from such tank washing are pumped
into slop tanks and left in a reception facility in port.
ballast tanks (SBT). Ballast water is taken on board
to maintain stability, such as when a vessel is sailing
empty to pick up cargo or after having unloaded cargo. Ballast
water contained in segregated ballast tanks never come into
contact with either cargo oil or fuel oil.
ballast tanks. To have so-called dedicated clean ballast
tanks (CBT) means that specific cargo tanks are dedicated
to carry ballast water only.
oil separation and filtering equipment with an automatic
stopping device. Bilge water is produced when the machinery
spaces of a vessel are cleaned. Leaking cooling water often
becomes contaminated with fuel oils and lubricant oils.
Vessels in operation produce oil-contaminated bilge water
to a variable extent. With the right equipment on board,
dirty bilge water can be processed in a way that separates
most of the oil from the water before it is discharged into
the sea. If the oil content exceeds the limit, the discharge
is automatically stopped (bilge alarm).
a sea area with Special Area status
under the international MARPOL Convention Annex I (so far,
only the Mediterranean Sea area, the Baltic Sea area, the
Red Sea area, the Gulf of Aden area, the Antarctic area, and
the North West European waters) , it is altogether forbidden
for oil tankers to discharge oil, oily sludge and oil-contaminated
residues from tank washing, or heavily oil-contaminated ballast
water. All oily wastes (mixtures) must be kept on board and
stored in so-called slop tanks until the vessel reaches a
reception facility in port. Furthermore, it is not allowed
to discharge bilge water unless it has been properly cleaned
and contains no more than 15 mg of oil per litre.
sea areas are not Special Areas, but in accordance with international
regulations under MARPOL, attempts are nevertheless made to
make large oil tankers and product carriers have equipment
for crude oil washing and segregated ballast tanks. According
to MARPOL Annex I, adopted in 1978, all new crude oil tankers
of 20,000 dwt and above, and all new product carriers (30,000
dwt and above), must have SBT. Existing tankers over 40,000
dwt must be fitted either with SBT or with COW systems. For
an interim period it was also allowed for some tankers to
oil tankers and other large vessels must be fitted with the
equipment described above for bilge water cleaning. However,
it has been emphasized that bilge water also contains traces
of detergents used in the cleaning process. When mixed, the
residues of oil and detergents form a stable emulsion with
another density than oil. This sometimes milk-like but highly
oil-contaminated mixture is not always "recognized"
by the separation and filtering equipment, and thus discharged
into the sea.
discharges in the offshore exploration for and extraction
of oil and natural gas include operational wastes, such
as drilling fluids/drilling muds, produced formation waters
and formation cuttings, and machinery space discharges.
During drilling, specially formulated drilling fluids/drilling
muds are used to cool and lubricate the drill bit, control
pressure and bring the cuttings (rock or sand from a borehole)
back to the surface. The mud is pumped down the drill pipe
and into the hole at high velocity through nozzles in the
drill bit. Drilling muds are usually a mixture of water, clay,
a weighting material (usually barite), and various chemicals.
most commonly based on water (WBM), but in some cases mineral
oil (OBM) or synthetics (SBM) is believed to be more biodegradable.
Although the use of WBM is preferred, sometimes OBM or SBM
has to be used when drilling conditions are more difficult.
Inevitably, when drilling with OBM, rock cuttings are contaminated
with oil from the muds. In the past when working offshore,
these cuttings were often discharged into sea, in accordance
with local regulation. Where OBM is used offshore, the spent
mud and cuttings are now re-injected or transported to shore
for treatment and disposal, or recycling. (The fluid is recycled
through a circulation system where equipment mounted on the
drilling rig separates out the drill cuttings and allows the
clean fluid to be pumped back down into the hole.) The objective
of environmental management of drilling operations is to attempt
to minimise the potential environmental impacts (see APPEA
Where one finds oil, one often also finds water.
water is either naturally present or has been injected into
the reservoir to maintain pressure for production. The proportion
of water produced increases as the oil field matures. As oil
is drawn from a reservoir, it is therefore necessary to separate
the water and return it to the ocean. This is what is known
as produced formation water (PFW). Great emphasis is
placed on ensuring that the water returned to the ocean is
as free as possible from oil and chemicals. Strict regulations
apply on how much petroleum hydrocarbon is contained in PFW.
objective of environmental management of produced water is
to reduce the quantity and to improve the quality of discharged
produced water (see APPEA and OEF).
As an oil well is drilled, the drill cuttings, consisted
of crushed rock and clay, are brought to the surface by the
drilling fluid/drilling mud and discharged overboard. New
measures to reduce discharges, such as re-injecting the cuttings
into the well, and slim hole drilling, are being examined
and tested by the industry (see APPEA).
According to Statoil, it ranks today
as a tried and tested technology.
Similar to the bilge water from machinery spaces in ships,
oil platform machinery space drainage is an oil-containing
mixture. Operational oil separation and filtering equipment
is thus needed to clean the drainage before it is discharged
into the sea. If such equipment is not installed, the oily
drainage should be kept on board until it can be transported
to a reception facility.
is no certain figure of how many miles of offshore pipelines
there is in the world today. One estimation, published in
the 2002 U.S. National Research Council (NRC)
report, is 82,748 miles (about 52,000 kilometres) of pipelines.
Operational discharges from offshore oil pipelines usually
consist of chemical discharges during construction, hydrostatic
testing, commissioning, pigging, and maintenance of the pipeline
systems. Pipeline discharges usually contain corrosion and
scale inhibitors, biocides, oxygen scavengers, and other agents.
However, pipelines can also continuously leak oil in small
quantities, although the line is intact. (When a pipeline
breaks, however, the spill will be an acute one, like any
other accidental oil spill.) Technical systems to detect and
locate such smaller leaks are required as a means to avoid
discharge of oil into the marine environment as well
as to save money for the pipeline operator.