Discharges 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.

Ship-related 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.

Before 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.

  • Crude 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.
  • Segregated 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.
  • Clean ballast tanks. To have so-called dedicated clean ballast tanks (CBT) means that specific cargo tanks are dedicated to carry ballast water only.
  • Operational 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).

In 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.

Most 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 use CBT.

All 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.


Operational 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. Drilling muds are 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 and OEF).

Where one finds oil, one often also finds water. The 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. The 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.


There 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.