Reading Passage 2.
ENVIRONMENTAL EFFECTS OF OFFSHORE DRILLING AND PRODUCTION

A main public concern about petroleum exploration and production seems to be that a blowout will cause a major oil spill.

Oil often exists in the subsurface at great pressure and, in the early days, when wells were drilled with only air or water in the hole, the oil could rush into and up the hole and 'blowout' at the surface.

For reasons of economy and safety, the early oil men soon put a. stop to that practice. Rotary drilling technology developed rapidly, including special drilling fluids with additives to control their density and consistency, and counterbalance the pressure of inflowing oil or gas. Modern drilling rigs are also fitted with blowout prevention controls: complex systems of metal clamps and shutters which can be used to seal the hole if unexpected high pressures are encountered.

There can be no denying that major blowouts still occur, and cause loss of life, as well as severe ecological trauma and economic loss.

AUSTRALIAN DRILLING RECORD

Fortunately, the available technology and proper precautions make them very rare events. Since offshore drilling commenced in Australia in the midtwentieth century, there has not been a single oil blowout.

On the other hand, six gas blowouts occurred during that time: five in Bass Strait and one in the Timor Sea. The Bass Strait blowouts were all controlled relatively quickly; the Petrel well in the Timor Sea flowed gas for 15 months.

It is a comment on improving technology and safety procedures that four of the incidents occurred in the initial decades of offshore drilling. The number of incidents, however, declined progressively over time.

The statistics on oil spills from offshore exploration and production in Australian Commonwealth waters are shown in the adjacent table. The total spill- age, over a 30-year period, is roughly equivalent in size to a large backyard swimming pool (70 kilolitres). The main spills have actually occurred in the loading of fuel onto production platforms; they had nothing to do with the oil well itself.

In addition to the oil spill issue, there are concerns about other discharges from the drilling and production facilities: sanitary and kitchen wastes, drilling fluid, cuttings and produced water.

Putrescible sanitary and kitchen wastes are discharged into the ocean but must be processed in accordance with regulations set by the Federal government. This material is diluted rapidly and contributes to the local food chain, without any risk of nutrient oversupply. All solid waste must be brought ashore.

The cuttings are sieved out of the drilling fluid and usually discharged into the ocean. In shallower waters they form a low mound near the rig; in deeper water a wider-spread layer forms, generally within one kilometre of the drill site, although this depends on a number of factors. Some benthic (bottomdwelling) organisms may be smothered, but this effect is local and variable, generally limited to within about 100 metres of the discharge point. Betteradapted organisms soon replace them and storm-driven wave activity frequently sweeps away the material.

Drilling fluid is also discharged directly into the ocean. Most of the common constituents of water-based fluids used in Australia have low-to-nil toxicity to marine organisms. Some additives are toxic but are used in small concentrations and infrequently. The small amounts of heavy metals present are not absorbed into the bodies of marine organisms, and therefore it is unlikely that they would pose a problem for animals higher up the food chain. Field studies have shown that dilution is normally very rapid, ranging to 1,000- fold within 3 metres of the discharge point. At Rivoli-1 well in Exmouth Gulf, the input was chemically undetectable 560 metres away.

Oil-based drilling fluids have a more toxic component, and discharge to the marine environment is more significant. However, they are used only rarely in Australia, and the impact remains relatively local.

At Woodside's North Rankin A Platform offshore Western Australia, the only facility currently using oil-based fluids, the discharge is diluted 2,000-fold within 1 kilometre downcurrent, and undetectable beyond 200 metres either side.

In the event of a discovery, the presence of a permanent production facility and the discharge of 'produced water are additional concerns. Produced water is the water associated with the oil or gas deposit, and typically contains some petroleum, dissolved organic matter and trace elements. Most produced water is effectively non-toxic but, even when relatively toxic, is quickly diluted to background levels.

The impact occurs mainly within about 20 metres of the discharge point, but is observable in some instances for about 1 kilometre downcurrent. Government regulations limit the oil content allowed to be discharged, and the produced water is treated on the platforms to meet those specifications, The discharge points are carefully selected to maximise dispersion and dilution, and avoid any particularly sensitive local environments.

Ultimately the best test of the real environmental effect of drilling and producing operations may be the response of the environment to the fixed production platforms. In many areas the platforms quickly become artificial reefs, with the underwater supports of the platforms providing a range of habitats, from sea-bottom to surface, and quickly colonised by a wide range of marine plants and animals.