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The Beauty of Obsolete Oil Rigs



By Ashley Fowler, Peter Macreadie & David Booth

The ear bones of reef fish are telling marine ecologists which decommissioned oil rigs are creating a vibrant habitat and which need to be brought back to land for disposal.

The world’s offshore oil rigs are ageing. The reserves in the seabed beneath them are gradually drying up, and more than 6000 of these structures will become obsolete within the next two decades.

“Good riddance” is a common reaction to this news. Oil rigs are massive steel monstrosities, some as large as the Eiffel Tower with a footprint the size of a football field. They tower over our seabed as an ugly reminder of our reliance on fossil fuels – a reliance we can’t shake despite the enormous environmental cost. Removing old rigs would bring the marine environment a step closer to its once-pristine state and us a step closer to embracing sustainable energy.

At least that’s what marine scientists used to think.

Unlikely Habitats

It’s easy to look at the hard surfaces and unnatural shapes of oil rigs and dismiss them as foreign objects that have no place in the marine environment. Indeed, this assumption led to international policies that require oil companies to completely remove rigs from the ocean once they became unproductive.

But we now know that rigs are gradually colonised by a myriad of invertebrates, including corals and sponges, following their installation. In time, the smooth surfaces are almost completely encrusted with growth, attracting larger fauna like fish, turtles and even seals and whales.

This discovery led to a practice known as “rigs-to-reefs”. This involves the conversion of disused oil rigs into artificial reefs – leaving the rig in the marine environment to provide habitat for marine organisms rather than taking it to shore for scrap.

The practice is lauded as a win–win scenario. Rigs enhance the marine environment by providing much-needed habitat, and oil companies save billions of dollars in removal costs.

Since its commencement in 1979 in Florida, the practice has spread to other states surrounding the Gulf of Mexico and internationally to Brunei and Malaysia. More than 200 rigs have already been converted into artificial reefs.

Despite increasing popularity in some regions, rigs-to-reefs remains extremely controversial. Environmental conscience set the mood, but global opposition to rigs-to-reefs traces back to a single event that occurred in the North Sea in 1995.

Shell UK was required to dispose of an obsolete oil storage buoy called the Brent Spar. After commissioning nearly 30 studies exploring a wide range of considerations, the company decided that the best option was to dispose of the Brent Spar in the deep sea rather than bring it to shore. The decision was supported by the British government at the time because it appeared to be the “best practicable environmental option”. Not only would a deep sea disposal avoid contaminating more fragile coastal ecosystems, the operation was both safer for workers and cheaper for the company than shore disposal. Any effects on the deep sea and its inhabitants were expected to be minimal due to the small amount of residual oil left in the storage buoy.

Greenpeace then launched a worldwide campaign to prevent the deep sea disposal, which it considered “sea dumping”. Activists occupied the Brent Spar for more than 3 weeks to protest the decision, which coincided with a boycott of Shell’s products across northern Europe and even vandalism of Shell petrol stations. Finally, one day before the planned disposal, Shell UK bowed to international pressure and announced it would dispose of the Brent Spar on land.

But the damage was done. Shell’s environmental reputation was in tatters and international agreements were quickly established that prevent rigs-to-reefs in Europe to this day. The political and industrial impacts of this controversy were so strong that even nations not bound by North Sea agreements are hesitant to raise rigs-to-reefs as a decommissioning option.

Science Weighs In

Until recently, science has played a relatively minor role in the rigs-to-reefs debate. However, research was pivotal in a recent policy change in California where, after more than 10 years of strong opposition and emotive debate, the State finally passed what is known as its “rigs-to-reefs law” in 2010. This law now allows the State to consider leaving at least part of disused oil rigs in place, instead of completely removing them. Essentially, this law paves the way for a future rigs-to-reefs program in California, similar to the southern US states surrounding the Gulf of Mexico.

Two similar bills were previously defeated, so how did California’s rigs-to-reefs law finally succeed? Scientists discovered that California’s oil rigs are home to large numbers of rockfish, some species of which are in serious decline on natural reefs in the region. They estimated that the removal of just one rig would be the equivalent of removing nearly 30 hectares of natural habitat for a single rockfish species. They also found that the rigs provide nursery habitat for juvenile rockfishes, which might boost flagging populations in the region.

These discoveries and others indicated that leaving rigs in place would provide better environmental outcomes than removing them – a net benefit for the marine environment. In this instance, science tipped the balance in favour of rigs-to-reefs, but perhaps more importantly, California’s policy change indicated that historical controversy and entrenched ideas could be overcome by scientific evidence.

So does this mean that the case is closed and rigs will now be left in oceans across the globe? Not by a long shot. Experts agree that each rig is unique, with some structures apparently supporting abundant and diverse reef communities while others appear to be quite poor habitats. Even rigs separated by less than 10 km can differ greatly in their associated flora and fauna.

Rigs-to-reefs decisions therefore need to made on a case-by-case basis, even if a regional rigs-to-reefs policy is adopted. But how do we work out which rigs are high-quality habitats worth keeping and which aren’t?

Residency on Rigs

Working out the habitat value of oil rigs seems simple enough: we just need to survey the organisms living on and around each rig. Shouldn’t rigs with abundant life be kept as reefs and those with minimal life be recycled on land? Unfortunately this task is complicated by the mobility of many marine organisms.

The observation of a mobile organism, like a fish, near a rig doesn’t necessarily mean that it lives there. Perhaps it is a temporary resident, or simply swam past at the time it was observed.

Without knowing the residency of organisms it is impossible to determine whether a rig represents a productive habitat capable of developing and sustaining marine life. Instead of providing quality habitat and boosting local fish populations, the rig may simply attract fish away from surrounding natural habitats. Attraction could alter food webs on natural habitats by removing top predators but, more worryingly, it may concentrate fish on rigs and make them more susceptible to fishing.

Whether rigs produce new biological resources like fish, or simply redistribute and endanger existing ones, is a key question for rigs-to-reefs that is difficult to answer.

We used tiny bones located in the inner ear of a fish to address this fundamental question of residency. Fish ear bones incorporate chemical elements from the surrounding seawater as they grow. Because the chemistry of seawater varies from place to place, the elements in a fish’s ear bone can be used to work out where it has been living. And because the ear bone lays down rings each year like a tree trunk, it acts like a time capsule that can be used to work out where the fish has been living in previous years. An ear bone’s shape is also useful for working out where a fish has been living because ear bones develop distinctive shapes that are specific to particular areas.

We wanted to test whether fish develop chemical “signatures” and ear bone shapes that can be used to identify the specific oil rig where they live. To do this, we collected specimens of a small reef fish, the red-belted anthias, from four oil rigs located in deep water off the coast of north-western Australia. This species was ideal for our test because it moves less than a few metres during its adult life. We could therefore be sure that individuals were long-term residents on the rigs where they were caught. Their ear bone characteristics should therefore reflect environmental conditions experienced on their “home” rig.

After carefully euthanising the specimens, we removed their ear bones and photographed them. The overall shape of each ear bone, as well as the bumps and notches surrounding their edge, was described using mathematical equations. We then cut the ear bones in half and used a powerful laser to remove small sections of bone from each growth ring, which were then fed through a mass spectrometer to determine their elemental composition.

Unique Signatures and Shapes

We found that the red-belted anthias possessed chemical signatures and ear bone shapes that were unique to their “home” rig. Differences in ear bone chemistry were even detected between rigs separated by as little as 10 km.

This finding brings us closer to making sound rigs-to-reefs decisions, because future environmental assessments could trace fish caught in oil fields back to specific rigs. This will allow researchers to estimate the biomass of fish supported by each rig, and therefore determine how valuable they are as habitat.

Previously, such information could only be obtained by physically tagging fish and tracking their movements over periods of months to years. Our approach is considerably less costly and time-consuming, which will be essential for assessing the thousands of rigs approaching obsolescence worldwide.

Analysis of previous growth rings in the ear bone indicated that each rig’s chemical signature was consistent through time, which suggests that a fish’s movements between rigs, and possibly between rigs and natural habitats, could be traced. This would allow residence histories to be recreated for fish species that move frequently, like jacks and snappers. Such species may call a group of rigs home, or only reside on rigs in particular seasons. Rigs may still form an important habitat component for these species, yet their behaviour usually makes it difficult to detect this type of habitat use.

Surprisingly, the chemical elements that helped distinguish home rigs were not the heavy metals you might expect from oil production, like nickel, copper or cadmium. They were elements that are naturally found in seawater, such as strontium and barium. The lack of “drilling” elements in the ear bones suggests that long-abandoned rigs with minimal contamination could still be assessed using our approach.

This also raises questions about the potential contamination of fish living near oil structures. Perhaps rigs are not a strong source of contamination for fish.

Our research provides a practical method for assessing the habitat value of oil rigs, but considerable research is needed to determine whether the approach translates to species of commercial and environmental significance. Our approach was also not completely accurate, and “home rigs” could not be determined for all individuals.

However, with further advancements in mass spectrometry and mathematical shape analysis, the humble fish ear bone may very well decide the fate of the world’s obsolete oil rigs.

Ashley Fowler is an Adjunct Associate from the Decommissioning Ecology Group at the University of Technology Sydney (