Australasian Science Magazine

Two tugboats pull the Perdido spar from Texas shore to Alaminos canyon, where it was secured to the seafloor in ~2450 metres of water. Photo: Shell

By Ashley Fowler, Peter Macreadie & David Booth

Some 6500 oil rigs are due for decommissioning by 2025 at a cost of $100 billion. Would they be more useful as artificial reefs?

Ask anyone whether obsolete oil industry structures should be left in the ocean, and the answer would likely be a resounding “Absolutely not!”. These large mechanical structures, including platforms, rigs and pipelines, are an unnatural intrusion into a pristine environment. Add to this the potential for active rigs to cause environmental catastrophes such as the Deep Horizon spill in the Gulf of Mexico last year and it’s no wonder they are considered necessary evils.

So, when rigs reach the end of their production lives, an alternative to complete removal is rarely considered. However, one alternative could see rigs playing an unexpected role in the conservation of vulnerable deep sea communities.

In a recent review article published in the journal Frontiers in Ecology and the Environment, we identified the global issues surrounding the use of rigs as artificial reefs in the deep sea, specifically. So, what are the ecological risks and benefits of this option?

An Overwhelming Issue
The ever-increasing need for fossil fuels has resulted in a dramatic increase in the number of rigs deployed throughout the world’s oceans over the past few decades. There are currently more than 7500 rigs in operation, and all these rigs will eventually become obsolete because oil and gas reserves are finite.

In keeping with the general philosophy of “leave it as you found it”, international treaty provisions stipulate that decommissioning of obsolete rigs should usually involve complete removal of the structure and disassembling it on land. This process involves severing the rig from the sea floor, raising it onto a large vessel and transporting it back to shore for disassembly and scrapping.

This is incredibly labour-intensive and, with the number of rigs due for decommissioning expected to reach 6500 by the year 2025, is likely to cost the oil industry in excess of US$100 billion. Concern is growing over whether such a momentous task can be achieved in a timely manner.

Australia has more than 60 production wells operating in offshore waters, and many of these are approaching decommissioning in the next decade. Australian government policy currently follows international treaty provisions, requiring that any damage to the seabed be rectified (essentially “leave it as you found it”) and prohibiting the dumping of structures at sea when it is not necessary.

However, the policy does not provide specific guidelines on possible alternatives to complete removal. For this reason, the Department of Resources, Energy and Tourism has drafted a public discussion paper on rig decommissioning as a first step in developing a final policy.

The “Rigs-to-Reefs” Alternative
Instead of expending considerable money and resources on complete removal, rigs can be converted into artificial reefs. The superstructures of rigs, including offices and living quarters, are removed and the lower sections are either toppled in place or towed to other locations, where they are left indefinitely. This alternative to removal reduces decommissioning costs to industry, because disassembly and scrapping on land are not required. It also provides critical reef habitat for communities of marine organisms.

The first rig-to-reef conversion was undertaken off the coast of Florida in 1979. Since then, hundreds of obsolete rigs have been turned into artificial reefs in the USA, Mexico and South-East Asia. Other nations, including Australia and nations bordering the North Sea, are currently debating the implementation of such programs.

However, substantial controversy surrounds the issue, with conservationists and scientists describing rigs-to-reefs programs as a simple excuse to dump large amounts of industrial waste. The controversy is largely due to the lack of evidence that artificial reefs actually benefit marine communities. There are even claims that artificial reefs may actually hinder marine organisms rather than help them.

As oil and gas reserves in shallow water are gradually consumed, and the number of rigs operating in the deep sea (>500 metres depth) increases, there is mounting pressure for rigs-to-reefs programs in the deep sea. Due to the heightened vulnerability of deep sea communities to human disturbance, there is an urgent need for consideration of the performance of rigs as artificial reefs, and the potential issues surrounding their use as reefs in the deep sea.

Do Artificial Reefs Actually Work?
The answer is a decided “maybe”. Success depends heavily on why the reef was installed in the first place.

Artificial reefs are usually installed to increase fish numbers. Although there is little direct evidence that this actually works, research on rigs in the USA indicates that the practice has potential.

Dr Stephen Szedlmayer and colleagues found that the addition of large amounts of artificial habitat off the coast of Alabama, including hundreds of rigs, coincided with the establishment of a substantial red snapper fishery. Catches of snapper in the area prior to this were relatively minor.

In southern California, Dr Milton Love and colleagues found that eight rigs supported 20% of the juvenile bocaccio rockfish surviving each year over the entire geographic range of this species. They estimated that the removal of one rig from this area would be the equivalent of removing 29 ha of natural rockfish habitat.

Artificial reefs may also increase invertebrate numbers, because many invertebrates are heavily reliant on reef habitat for survival. Fishermen in Hokkaido, Japan, reported increased catches of Pacific giant octopus following the addition of a large number of artificial reefs. This increase was attributed to the octopus’ need for shelter space, which was provided by the numerous holes in the artificial reef habitats.

In addition to octopus, species from nearly all invertebrate groups have been observed colonising and growing on artificial reefs, including corals, anemones, sponges, crabs, and mussels.

Artificial reefs have also been used to protect marine communities from trawl fishing, and this application has proven quite successful. Trawlers cannot fish where artificial reefs have been deployed because their nets become entangled in the structures. For this reason, artificial reefs have been successfully used to prevent illegal trawling in seagrass beds in western Europe, and to passively enforce fishery-protected areas in Hong Kong.

What’s Different about the Deep Sea?
Almost all research on the performance of rigs as reefs has been done in relatively shallow (<200 metres) coastal waters, and this makes it difficult to predict how rigs will affect deep sea organisms. Compared with coastal environments, the deep sea is vast, and this has prompted suggestions that the addition of rigs to the deep sea would only be a “drop in the ocean”: even a 100 metre long rig would represent such a minute increase in habitat that any beneficial effect would be immeasurably small.

However, because reef habitat in the deep sea is rare compared with soft bottom habitat (<4%), rigs may still boost the amount of reef habitat within particular regions as long as rigs-to-reefs programs are large enough.

Reefs in the deep sea can be extremely isolated from each other due to the wide expanses of soft sediment between them, and this renders inhabiting organisms particularly vulnerable to environmental disturbance. Lost individuals on a reef cannot easily be replaced by a supply of planktonic larvae or the immigration of adults. If a disturbance like prey shortage continues, the population will eventually decline to an irreversibly low level.

But the addition of rigs in the deep sea may allow organisms to move more easily between reefs by acting as “stepping stones” in an inhospitable landscape of soft sediment. In this way, the strategic placement of rigs in areas between known reefs may increase the resilience of deep sea organisms and assist in the conservation of deep sea communities. However, the flip side of this is it may also assist the spread of invasive species.

In addition to their isolation, the biology of many deep sea organisms also renders them vulnerable to over-exploitation. They are often slow-growing, produce fewer offspring and reproduce later in life than their shallow-water counterparts. These characteristics mean that populations take a long time to replenish themselves following a reduction in numbers.

One example is the orange roughy, a fish that takes 30 years to reach maturity and is easily targeted by fisherman due to the formation of large spawning aggregations around seamounts. Stocks of this species in Australian and New Zealand waters have declined substantially in the past few decades and, despite reductions in allowable catches, they have not yet recovered.

Rigs may afford such species considerable protection from fishing because their three-dimensional shape and large number of internal spaces would make fish inaccessible to trawl nets. Similarly, rigs could also be used to exclude trawling from deep-water coral reefs, which support a great diversity of deep sea species. These reefs can take thousands of years to develop, and are unlikely to recover within a sufficient time following degradation from fishing pressure.

The Environmental Balance
Like all man-made intrusions into nature, the benefits of rigs as reefs in the deep sea must be weighed against potential environmental costs. Most of the foreseeable costs involve the relocation of obsolete rigs to new locations, and include:

• smothering of soft sediment organisms within the deployment zone;

• release of contaminants during initial rig removal;

• facilitating the spread of invasive species; and

• adverse changes to the natural community structure.

Although the effects of smothering and contaminant release are likely to be localised around the sites of disturbance, little research has been done on any of these potential threats. Invasive species have been observed on rigs but it is unknown whether rigs assist their invasion of surrounding natural habitat.

Perhaps one of the most insidious risks is the possibility that the addition of a large amount of reef habitat to areas previously dominated by soft sediment may alter the natural community structure and bio­diversity. Since indirect effects – such as reductions in soft sediment invertebrate numbers due to increased predation – could go unnoticed, any addition of rigs to the deep sea must first be done in small steps and monitored closely.

The gradual deterioration of rigs over hundreds of years must also be considered, and would likely mirror the gradual breakup of a steel shipwreck. Over time, rusting of rigs may affect the encrusting communities that inhabit them. If this is the case they may not be the most suitable long-term material for deep sea artificial reefs. Conversely, if rigs end up supporting thriving communities of deep sea organisms, some thought needs to given to their eventual replacement.

The Verdict
Rigs-to-reefs programs could be useful for conserving deep sea reef communities throughout the world’s oceans, but considerable scientific research is required before rigs should be deployed on a large scale. Will they even do what we want them to? If so, does this benefit outweigh the potential environmental costs involved?

At the very least it seems rigs could be useful in excluding destructive fishing activity from fragile deep sea coral reefs. However, will their presence translate to wider-reaching benefits for deep sea communities?

In Australia, the current policy of “leave it as you found it” was established more than two decades ago, and has been little revised over that time. Although our offshore waters hold far fewer rigs than places like the Gulf of Mexico, certain areas have large numbers of oil structures that will all become obsolete one day. Some obsolete rigs on the north-western continental shelf are already due for decommissioning, and research done by ourselves and colleagues at the University of Sydney has shown that the removal of these structures can result in the entire destruction of thriving reef communities. This suggests that leaving the environment in a pristine state may not be straightforward, and removing rigs without regard for the communities they now support could be detrimental.

As more and more obsolete rigs in Australia reach the end of their lives, it is increasingly important that we choose the best solution for both us and the environment.