Offshore oil rigs have long been disliked by environmentalists because of their unsightliness and their demonstrated potential as sources of pollution. But over the years it has become obvious that many forms of sea life find the underwater portions of structures almost as habitable as nature's own reefs.
Now, with a generation of aging rigs ready to be phased out, oil companies, the US government, international regulators, and environmental groups must deal with the fact that many of the rigs have become home to entire reef communities, says Terry Henwood, a biologist with the National Maritime Fisheries Service.
The underwater portions of rigs, Mr. Henwood explains, are inhabited by starfish, algae, sea anemones, mollusks, worms, corals, sponges, reef fish such as snapper, grouper, and king mackerel - and even marine mammals.
A 1958 international treaty requires removal of the huge steel-and-concrete networks when they are no longer used for drilling operations. With 2,000 rigs on the US continental shelf due for retirement in the next 20 years - most of them in the Gulf of Mexico - oil companies, federal officials, and environmental groups are deliberating how to best dispose of the aging structures.
Wholesale demolition or disassembly continues to be the chief methods of rig ``abandonment,'' as the process is called in the industry. But in recent years, with the blessing of the government and many environmentalists, some oil operators have converted their rigs to artificial ocean reefs.
The conversions, so far fewer than a dozen, are very popular with Gulf Coast sport fishermen, who find that they can catch nearly twice as many fish near operating or decommissioned oil rigs as in open water.
Oil companies like the artificial reef plan because in many cases it is cheaper than rig removal. The cost of removing a rig from Gulf waters varies from just under $100,000 to several million dollars, depending on rig size, water depth, and other factors.
Environmental groups are generally supportive of artificial reefs. They say the rigs support both the crustaceans that otherwise would be discarded with the structures and the fish that congregate around them.
Some groups express concern that the use of explosives in dislodging rigs from the ocean floor has hurt larger marine animals - including the endangered loggerhead and Kemps-Ridley sea turtles. ``We were pretty appalled at this,'' says Lynn Davidson, marine habitat policy coordinator for the environmental watchdog group Greenpeace.
But government observers say that no turtles are known to have been harmed by rig removal since November 1986, when government regulations were implemented requiring an environmental impact report before removal can begin.
``I don't know of any injured turtles that have been sighted during removal operations - either by our observers or by [government] observers,'' says Rosemary Stein, a lawyer with Exxon.
For many rig operators, the environmental concerns have delayed rig-abandonment plans. The US Department of the Interior's Minerals Management Service estimates that more than 100 abandonment applications have been delayed while government regulators assess the environmental impact of rig removals on a case-by-case basis.
Although the vast majority of aging rigs are found in the Gulf of Mexico - 4,000 of the 6,000 rigs worldwide - and rig-to-reef conversions so far have occurred only in the gulf, some foreign countries are beginning to consider the merits of such conversions along with other abandonment alternatives.
In April, the International Maritime Organization (IMO), a London-based United Nations group, gave preliminary approval to a set of international guidelines for the removal of offshore installations, including oil rigs. The guidelines, if given final approval, would permit operators simply to abandon rigs, provided a ``new use'' could be found for them.
Not all rigs are capable of supporting reef communities. In the North Sea, for example, the water is too cold and deep for rigs there to become artificial reefs.
``There have been proposals,'' says Rear Adm. John Kime, the US Coast Guard's representative to the IMO talks, ``to convert some of the larger [rigs] to resort areas in some of the more temperate climates.''
Brian Hoyle, a State Department official who led the US delegation to the IMO talks, says there has been talk of converting oil rigs to ``offshore national security, military installations,'' as well suggestions that they could be used for casinos.
So far, no specific plans for these alternative uses have been initiated.