Small amounts of oil and natural gas seep through and are found near the Earth's surface, but almost all oil and gas is found in deep underground reservoirs on land and in the seabed (on the continental shelf). Offshore oil production accounts for about 30 per cent of the total world oil production, and offshore gas production for about half of the world production of natural gas. According to a recent report by the U.S. National Research Council (NRC), there were about 8,300 fixed or floating offshore platforms worldwide in 1999. The UNEP Offshore Oil and Gas Environment Forum (OEF) gives these figures: more than 6,500 offshore oil and gas installations worldwide, about 4,000 of which in the U.S. Gulf of Mexico, 950 in Asia, 700 in the Middle East and 400 in Europe.

OFFSHORE DRILLING: Offshore drilling for oil and natural gas on the continental shelf is carried out in many waters and at large depths. As described by the U.S. National Ocean Industries Association (NOIA): "Today, offshore activities take place in waters of more than half the nations on earth. And no longer do primitive, shore-bound wooden wharves confine offshore operators. Instead, they drill wells from modern steel or concrete structures. These structures are, in many cases, movable. What is more, they can float while being moved, and often while drilling. Further, offshore rigs have drilled in waters over 7,500 feet (over 2,200 meters) deep and as far as 200 miles (over 300 kilometers) from shore". It should also be noted that the lifetime of a rig is generally about 20 years. At the end of that time, unless it is re-used or redeveloped, it must be decommissioned.

The major potential environmental effects from offshore drilling occur from the discharge of wastes, including drilling fluids (also referred to as drilling muds), drill cuttings and produced formation water. The decommissioning of platforms/rigs is also a potential environmental problem.

The full text of the following description of offshore drilling in Australian waters can be on the education site of the Australian Petroleum Production and Exploration Association (APPEA). See also information on exploration and drilling, including technical description of platforms and production methods, on the site of the United Kingdom Offshore Operators Association (OKOOA), as well as the report "Environmental management in oil and gas exploration and production", by UNEP IE and the E&P Forum.

"Drilling or digging for oil has occurred in one way or another for hundreds of years. The Chinese, for instance, invented a bamboo rig to obtain oil and gas for lighting and cooking. But only in the last 40 years has humankind been able to efficiently extract petroleum from beneath the seas — an achievement to rank with this century's mightiest technological triumphs.

The search is difficult, extremely expensive, and often fruitless. Locating an oil and gas "trap" (as it is known) and extracting the oil and gas is difficult enough on land. But offshore, in deep and often stormy waters, it becomes an awesome undertaking. Potential traps are identified by analysing seismic survey data but whether they contain oil or gas won't be known until a drill bit penetrates the structure. Directing the drill bit to a precise location, perhaps several kilometres away, requires sophisticated computer technology. A navigation device installed above the drill bit feeds back information which enables the exact position of the well to be measured and monitored. A steerable motor within the drillpipe can be remotely controlled to adjust the direction of the drill.

FOUR DRILLING TYPES: In the early days of offshore drilling, explorers simply fitted a derrick to a barge and towed it to their site. Today, four types of offshore rigs are used to drill wildcat or exploration wells:

Submersibles: These are rarely used. They can be floated to shallow water locations then ballasted to sit on the seabed. Jackups:Usually towed to a location. Their legs are then lowered to the seabed and the hull is jacked-up clear of the sea surface. Used in waters to about 160 metres deep. Drill ship: These look like ordinary ships but have a derrick on top which drills through a hole in the hull. Drill ships are either anchored or positioned with computer-controlled propellers along the hull which continually correct the ships drift. Often used to drill "wildcat" wells in deep waters. Semi submersible:. Mobile structures, some with their own locomotion. Their superstructures are supported by columns sitting on hulls or pontoons which are ballasted below the water surface. They provide excellent stability in rough, deep seas.

PRODUCTION PLATFORMS : Once oil or gas is discovered, the drilling rig is generally replaced by a production platform, assembled at the site using a barge equipped with heavy lift cranes. Platforms vary in size, shape and type depending on the size of the field, the water depth and the distance from shore. In Australia's medium to large fields, fixed production platforms are commonly used. These are made of steel and fixed to the seabed with steel piles. These platforms house all the processing equipment and accommodate up to 80 workers who typically work a 12 hour day, one week on and one week off. There are also concrete structures which are big enough to store oil. Gravity holds them on the seabed. The world's biggest platforms are bigger than a football field and rise above the water as high as a 25 storey office tower. They are home to 500 workers. If the field is in shallow water and near land or another platform, small remotely controlled monopod platforms may be used. Another system is a floating structure, either anchored or tethered, called a Floating Production Storage Offloading (FPSO) vessel. Another platform type, suitable for deep water production, is the Tension Leg platform, built of steel or concrete and anchored to the sea floor with vertical "tendons".

DRILLING FOR OIL: The first stage of drilling is called "spudding" and drilling starts when the drill bit is lowered into the seabed. The bit can be of two types: a roller cone or rock bit which usually has three cones armed with steel or tungsten carbide teeth or buttons; or a diamond bit, imbedded with small industrial diamonds.

The drill bit is attached to drill pipe (or a drill string) and rotated by a turntable on the platform floor. As the hole deepens, extra lengths of drill pipe are attached. A length of drill pipe is 30 feet long, or 9.1 metres (oil workers use the old imperial measurement system). The drill bit ranges in diameter from 36 inches or approximately 91.4 centimetres (used at the start of the hole) to eight and a half inches (approximately 21.5 centimetres). Drilling may take weeks or months before the targeted location is reached.

PREVENTING BLOW-OUTS: The weight of the drilling fluid acts as the first line of well control by keeping underground pressures in check. If an influx of pressurised oil or gas does occur during drilling, well control is maintained through the rig's blowout prevention system (BOP). This is a set of hydraulically operated valves and other closure devices (rams) which seal off the well, and route the wellbore fluids to specialised pressure controlling equipment. Trained personnel operating this highly reliable equipment minimise the possibility of a blowout, or an uncontrolled flow of fluids from a well.

DIRECTIONAL DRILLING: Drilling an oil well may not be a case of going straight down. Directional drilling has been developed where drill bits are steered laterally over a distance of up to several kilometres towards the petroleum reservoir. One production platform is often used to drill a number of wells, in a variety of directions and inclinations. To steer the drill bit, a downhole motor may be used. It is turned by pumping the drilling mud through it.

COMPLETING THE WELL: When the well has been drilled to its target depth, production casing is set and cemented. Tubing is lowered into the hole together with "packers" which seal the space between the tubing and the casing. Finally, at the end of the well, the casing is perforated at predetermined depths by small explosive charges detonated remotely. The small holes in the casing allow the oil or gas under its natural pressure to flow to the surface.

A DRY WELL: If the drilled hole is considered to be "dry" or not worth developing, the decision is made to "plug and abandon" it. This involves setting several cement plugs in the well."