The extremely high cost of decommissioning and removal off offshore installations led to the need to revise some of the national and international regulations adopted about 40 years ago. Such a revision covered, in particular, the requirement set by the Convention on the Continental Shelf (Geneva, 1958) and the United Nations Convention on the Law of the Sea (Montego Bay, 1982) to remove abandoned offshore installations totally. At present, a more flexible and phased approach is used. It suggests immediate and total removal of offshore structures (mainly platforms) weighing up to 4,000 tons in the areas with depths less than 75 m and after 1998 - at depths less than 100 m. In deeper waters, removing only the upper parts from above the sea surface to 55 m deep and leaving the remaining structure in place is allowed. The removed fragments can be either transported to the shore or buried in the sea. This approach considers the possibility of secondary use of abandoned offshore platforms for other purposes.
From the technical-economic perspective, the larger the structures are and the deeper they are located, the more appropriate it is to leave them totally or partially intact. In shallow waters, in contrast, total or partial structure removal makes more sense. The fragments can be taken to the shore, buried, or reused for some other purposes.
From the fisheries perspective, any options when the structures or their fragments are left on the bottom may cause physical interference with fishing activities. In these cases, the possibility of vessel and gear damages and corresponding losses does not disappear with termination of production activities in the area. Instead, abandoned structures pose the threat to fishing for many decades after the oil and gas operators leave the site. The obsolete pipelines left on the bottom are especially dangerous in this respect. Their degradation and uncontrolled dissipation over wide areas may lead to the most unexpected situations occurring during bottom trawling in the most unexpected places. At the same time, national and international agreements about the decommissioning and abandonment of offshore installations refer mostly to large, fixed structures like drilling platforms. The fate of underwater pipelines is still not affected by clear regulations.
Secondary use of offshore fixed platforms
The options of reusing abandoned platforms, their foundations, and other structures that are out of service have been actively discussed for the last 10 years.
An analysis of scientific potential of research stations permanently based on abandoned oil platforms in the Gulf of Mexico revealed several promising directions of marine research at such stations [Dokken, 1993; Gardner, Wiebe, 1993]. These include studying regulation of the marine populations and coral reproduction, making underwater observations, monitoring the sea level, and collecting oceanographic and meteorological information within the framework of international projects. Some other suggestions consider transformation of abandoned platforms into places for power generation using wind/wave and thermal energy [Rowe, 1993]. These platforms also could be used as bases for search and rescue operations or centers for waste processing and disposal [Side, 1992].
From the fisheries perspective, the most interesting projects are the ones aimed at converting the fixed marine structures into artificial reefs. Artificial reefs are known to be one of the most effective means of increasing the bioproductivity of coastal waters by providing additional habitats for marine life. They are widely and effectively used on the shelves of many countries.
The offshore structures can undoubtedly attract many species of migrating invertebrates and fish searching for food, shelter, and places to reproduce. In particular, observations in the Gulf of Mexico revealed a strong positive correlation between the amount of oil platforms, growing since the 1950s, and commercial fish catches in the region. It became one of the reasons to suggest the positive impact of offshore oil and gas developments on the fish populations and stock. Wide popularization of this fact led to the mass movement using the slogan "From rigs - to reefs" in the USA in the mid-1980s.
However, further analyses of the fishing situation in the Gulf of Mexico showed that the growth of the fish catch in this case was connected not with increasing the total stock and abundance of commercial species but with their redistribution due to the reef effect of the platforms. A critical point here was the use of static gear methods of fishing (e.g., lines and hooks) instead of trawl gears. Besides, the areas around the platforms became very popular places of recreational and sport fishing. This also made a significant contribution to the total catch volumes. Nothing similar was noted in the North Sea, where the number of oil platforms has also been growing since the 1960s. However, the total catch did not correlate with this growth at all and even decreased. This fact indicates the absence of any positive impact of the reef effect of oil platforms on the commercial fish catches in areas where the main way to fish is trawling.
At the same time, we should not forget about the danger that abandoned offshore oil platforms and their fragments pose to navigation and trawling fishing. With an abundance of such artificial reefs, this problem requires special regulations for negotiating the inevitable conflict of interests. One such regulatory program has been developed and applied in the USA in the Gulf of Mexico on the shelf of Louisiana [Pope et al., 1993]. It requires mapping the area to indicate the locations of platforms, underwater pipelines, and other structures left on the bottom. The program also includes monitoring, collecting data, developing a warning system, and other activities necessary to control the situation and ensure safety in the region.
Complete or partial removal of steel or concrete fixed platforms that weigh thousands of tons is practically impossible without using explosive materials. Bulk explosive charges have been used in 90% of cases. This is very powerful, although short-term, impact on the marine environment and biota, which should not be neglected.
It is extremely difficult to get any reliable estimates of possible mortality of marine organisms, especially fish, during an explosive activity even if the initial data, such as the type of explosive, depth of the water, bottom relief, and others, are known. This large uncertainty is connected, in particular, with the high heterogeneity of fish distribution that strongly depends on specific features of fish schooling behavior. Calculations show that with a 2.5-ton (TNT equivalent) charge, the mass of killed fish will be about 20 tons during each explosion. At the same time, if, for example, a school of herring happens to get into that zone, the fish kill figure may be much higher [Side, Davies, 1989].
One of the few known observations of fish damage in zones of explosive activity was done in 1992 in the Gulf of Mexico near the shore of Louisiana and Texas [Gitschlag, Herczeg, 1994]. In order to remove over 100 fixed platforms and other structures, more than 12,000 kg of plastic charges were exploded. The amount of dead fish floating on the surface was visually recorded after the explosions. It totaled to about 51,000 specimens. The actual number of killed fish was undoubtedly higher because many specimens could not float to the surface or did not get in the zone of visual observation.
Whatever number of adult fish actually died during the explosions, it will hardly influence the total abundance of commercial species. Much more hazardous for the fish stock are explosive impacts on fish larvae and juveniles. The threshold of lethal impacts for the younger organisms weighing up to several grams is tens of times lower than that for adult specimens [Yelverton et al., 1975; Side, 1992]. Thus, the zone of mortality of fish at the early stages of development is respectively wider. The quantitative estimates of possible effects at the populational level are even more complicated because of the absence of corresponding data and methods. Nevertheless, enough evidence exists to enforce strict regulations of explosive activities and to forbid them in areas and in seasons of spawning and fry development of commercial fish.
Removal of the offshore structures also decreases the number of habitats for structure-related fish. For example, in the mostly soft-bottom environment of the Gulf of Mexico, these structures provide hard substrates for marine organisms. The decline of stocks of reef fish observed in this region within the past decade can be connected, in particular, with elimination of over 400 oil-related structures that had served as an artificial habitat for marine life [MMS, 1995].