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Accidents during the offshore oil and gas development

by Stanislav Patin, translation by Elena Cascio
based on "Environmental Impact of the Offshore Oil and Gas Industry"

Below you will find a short overview of drilling, transportation and storage accidents during the offshore oil and gas activities. To find more information on Environmental Impact of the Offshore Oil and Gas Industry click on the links at the end of the page.

Oil and gas accidents

Accidents inevitably accompany offshore development. They are the sources of environmental pollution at all stages of oil and gas production. The causes, scale, and severity of the accidents' consequences are extremely variable. They depend on a concrete combination of many natural, technical, and technological factors. To a certain extent, each accidental situation develops in accordance with its unique scenario.

The most typical causes of accidents include equipment failure, personnel mistakes, and extreme natural impacts (seismic activity, ice fields, hurricanes, and so on). Their main hazard is connected with the spills and blowouts of oil, gas, and numerous other chemical substances and compounds. The environmental consequences of accidental episodes are especially severe, sometimes dramatic, when they happen near the shore, in shallow waters, or in areas with slow water circulation.

Drilling accidents

Drilling accidents are usually associated with unexpected blowouts of liquid and gaseous hydrocarbons from the well as a result of encountering zones with abnormally high pressure. No other situations but tanker oil spills can compete with drilling accidents in frequency and severity.

Broadly speaking, two major categories of drilling accidents should be distinguished. One of them covers catastrophic situations involving intense and prolonged hydrocarbon gushing. These occur when the pressure in the drilling zone is so high that usual technological methods of well muffling do not help. Lean holes have to be drilled to stop the blowout. The abnormally high pressure is most often encountered during exploratory drilling in new fields. The probability of such extreme situations is relatively low. Some oil experts estimate it at 1 incident for 10,000 wells [Sakhalin-1, 1994]. The need to drill lean holes emerges, on average, in 3% of accidental episodes.

The other group of accidental situations includes regular, routine episodes of hydrocarbon spills and blowouts during drilling operations. These accidents can be controlled rather effectively (in several hours or days) by shutting in the well with the help of the blowout preventers and by changing the density of the drilling fluid. Accidents of this kind are not so impressive as rare catastrophic blowouts. Usually, they do not attract any special attention. At the same time, their ecological hazard and associated environmental risk can be rather considerable, primarily due to their regularity leading, ultimately, to chronic impacts on the marine environment.

Transportation and storage accidents

Tanker transportation. Oil extracted on the continental shelf accounts for a considerable part (probably at least 50%) of annual volumes of oil transported by tankers (the latter constitute over 1 billion tons). On some fields, the shuttle tankers are the main way of delivering hydrocarbons to the onshore terminals.

The main causes of tanker accidents that lead to large oil spills include running aground and into shore reefs, collisions with other vessels, and fires and explosions of the cargo. According to official data [IMO, 1990], the amount of oil spilled during tanker accidents in 1989 and in 1990 were 114,000 and 45,000 tons, respectively. At the same time, the total volume of oil pollution caused by marine oil transportation was 500,000 tons a year.

Significantly, both large drilling accidents and large tanker catastrophes occur relatively rarely. The frequencies of such incidents as well as the oil volumes released in large spills differ from year to year.

The history of tanker accidents has been thoroughly described by both the scientific literature and the media. Analyzing the statistics and circumstances of such events indicates that they can hardly be avoided. Although the rate of tanker accidents has been declining over the past two decades, we should be prepared to deal with them in the future.

While speaking about the history of tanker transportation, we want to mention a sequence of large supertanker accidents starting with the catastrophic grounding of the tanker Torrey Canyon in the English Channel in 1967. The spill of 95,000 tons of oil caused heavy pollution of the French and British shores with serious ecological and fisheries consequences. This accident was followed by a number of other tanker accidents, including Amoco Cadiz (1978, 220,000 tons of oil spilled), Exxon Valdez (1989, 40,000 tons of oil spilled), and Braer (1993, 85,000 tons of oil spilled). Each of these episodes developed in accordance with its unique scenario. In all the situations, though, the levels of oil pollution reached lethal limits for marine fauna, mainly for birds and mammals. The consequences included much more damage than just ecological disturbances in the sea and on the shore. Chapter 7 will discuss this in more details.

In some cases, the tanker accidents occurred right in the zone of the oil field development. One of them happened in 1978 in the Shetland basin. The tanker Esso Bernica was holed during the mooring, and 1,100 tons of heavy oil fuel spilled into the coastal zone causing serious damage to nature and the local population.

One of the most dramatic situations developed in 1989 in the shallow waters of Prince William Sound near the Alaskan southern shore. The oil tanker Exxon Valdez ran aground and spilled over 40,000 tons of crude oil. As the oil spread along the coastline, it covered sea animals, birds, and plants. It turned hundreds of miles of this area (unique for its cleanness and biological resources) into an area of ecological disaster.

This relatively recent episode in the history of the offshore oil and gas industry causes an alarming association in the mind of a Russian reader. The Exxon Valdez catastrophe happened approximately at the same latitudes where the grand projects of the oil and gas developments on the Russian Arctic shelf have already been started (the shelves of the Barents and Kara Seas in vicinity of the White Sea). The association gets even stronger if we take into account that considerable amounts of hydrocarbons extracted here are going to be transported by the tanker fleet. This will include tanker shuttles (including the ice types), large tankers with dead weight up to 120,000 tons, and supertankers. Each of these vessels is going to make hundreds of trips a year. This regular transportation activity is going to take place with the rest of the traffic in the area of the oil field developments and in addition to the general intense shipping and fishing in this Arctic basin. All of these factors considerably increase the probability of accidental situations occurring in the region. We must remember the high productivity and high vulnerability of the Arctic marine ecosystems. This region contains unique natural resources that are comparable to the rich resources of the Alaskan shelf.

This primary background information and general statistics about large tanker accidents (about 2% a year) allow us to conclude, without any calculations and modeling, that the risk of transportation accidents occurring on the Arctic shelves is going to be high. The consequences of these accidents can be catastrophic. Moreover, the environmental damage of possible accidents can exceed everything that has happened before in such cases, including the accidents on the Alaskan shelf.

Very dangerous situations can emerge in case of a gas tanker accident. Gas carriers are going to be used together with oil tankers in the Barents Sea as well as on the eastern shelf of Sakhalin to transport liquefied natural gas. Gas tanker accidents, although less probable than the accidents with oil tankers, can cause so-called flameless explosions. It happens due to the rapid evaporation of the liquefied gas on the sea surface and formation of pieces of ice and gas clouds followed by combustion and explosions. Such explosions can destroy everything alive in areas of up to 400 km2.

At last, the tragic apotheosis of possible outcomes is an accident involving a tanker that is transporting methanol - a rather toxic substance that is completely soluble in water. In case of an accident of such a vessel with a freight-carrying capacity of 35,000 tons, for example in the coastal zone of the Western Murman, the area of lethal impact to marine organisms will be from dozens and hundreds to thousands of square kilometers. In fact, it could cover the whole fisheries regions [Borisov et al., 1994].

Storage. Underwater reservoirs for storing liquid hydrocarbons (oil, oil-water mixtures, and gas condensate) are a necessary element of many oil and gas developments. They are often used when tankers instead of pipelines are the main means of hydrocarbon transportation. Underwater storage tanks with capacities of up to 50,000 m3 either are built near the platform foundations or are anchored in the semisubmerged position in the area of developments and near the onshore terminals. Sometimes, the anchored tankers are used for this purpose as well.

Of course, a risk exists of damaging the underwater storage tanks and releasing their content, especially during tanker loading operations and under severe weather conditions. However, no summarizing quantitative assessments and statistics of such events are available. After the spill of 1,200 tons of crude oil in 1988 from an underwater storage tank during a storm in the North Sea, some countries introduced restrictions on installing such structures near the shore [Cairns, 1992]. The most dangerous are the accidents involving underwater storage tanks that contain toxic agents, for example methanol. Such accidents are possible in the area of Shtokmanovskoe field developments in the Barents Sea where over 3,000 tons of methanol products are planned to be stored underwater.

Pipelines. Complex and extensive systems of underwater pipelines have a total length of thousands of kilometers. They carry oil, gas, condensate, and their mixtures. These pipelines are among the main factors of environmental risk during offshore oil developments, along with tanker transportation and drilling operations. The causes of pipeline damage can differ greatly. They range from material defects and pipe corrosion to ground erosion, tectonic movements on the bottom, and encountering ship anchors and bottom trawls. Statistical data show that the average probability of accidents occurring on the underwater main pipelines of North America and Western Europe are, respectively, 9.3x10-4 and 6.4x10-4. The main causes of these accidents are material and welding defects [Sakhalin-1, 1994].

Depending on the cause and nature of the damage (cracks, ruptures, and others), a pipeline can become either a source of small and long-term leakage or an abrupt (even explosive) blowout of hydrocarbons near the bottom. The dissolution, dilution, and transferring of the liquid and gaseous products in the marine environment can be accompanied in some cases by ice and gas hydrates formation. The intensity and scale of toxic impacts on the marine biota in the accident zone can be, of course, very different, depending on a combination of many factors.

Modern technology of pipeline construction and exploitation under different natural conditions, including the extreme ones, achieved indisputable successes. However, pipeline oil and gas transportation does not eliminate the possibility of serious accidents and consequences.

It is important to take into consideration that in a number of cases, the accidental oil and gas spills and blowouts on the onland main pipelines can pose danger to the coastal marine ecosystems. This can happen when onland pipeline accidents take place near big rivers or in locations of their crossing. Any pollution of river waters eventually affects the sea zone near the river mouth. Such a situation happened at the end of 1994 in the Usinsk area, Russia. An onland pipeline rupture led to the spill of more than 100,000 tons of oil with the danger of heavy pollution of the basin of Pechora River. The potential hazard of such situations can be even higher during oil and gas development on Sakhalin. The main pipelines are supposed to be laid along the entire eastern coast of the island, right across the main spawning rivers where reproduction of the unique populations of Sakhalin salmon takes place.

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Read other articles about Environmental Impact of the Offshore Oil and Gas Industry:

Oil and gas on the Russian shelf - click here if you want to learn about recent oil and gas developments on the Russian shelf.

Anthropogenic impact on the shelf and marine pollution - structure and scale of anthropogenic impact on the marine environment are considered. Marine pollution as the main, most wide spread and most dangerous factor of anthropogenic impact is discussed.

Oil pollution of the sea - oil pollution of the marine environment, including sources and volumes of oil input.

Decommissioning of offshore installations - click here if you want to learn about abandonment options and secondary use of offshore structures. Explosive activities to remove obsolete offshore installations and their impact on marine life are also discussed.

Gas impact on water organisms - gas impact on fish and other marine organisms is considered. Results of field and laboratory studies, including biological consequences of accidental gas blowouts are discussed.

Natural gas in the marine environment - chemical composition and biological impact of natural gas in the sea.

Spilled oil in the sea - fate, transformations and behavior of oil and oil hydrocarbons in the sea during an oil spill.

Waste discharges - sources, types and volumes of waste discharges during the offshore oil and gas activity are discussed. Chemical composition of discharged wastes (drilling muds, drilling cuttings and produced waters) is described. Atmospheric emissions and their impact on the marine environment are considered.

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