Oil and gas has been a major industry in the North Sea for over 40 years.

The extensive and complex infrastructure associated with the industry, including oil rig structures and a vast network of pipelines, is now coming to the end of its productive life and the process of decommissioning these huge structures – recycling, reusing or disposing on land – is now in discussion.

Weighing up the options

At present, operators are under a legal obligation to remove all platforms and associated structures from the seabed, eventually leading to approximately 470 installations, 10,000 km of pipeline and 5,000 wells having to be decommissioned. Additionally, all drill cuttings piles and accumulated debris will have to be cleared. It is a massive undertaking, both technically and financially, that will take decades to complete.

Conservative estimates put the cost of decommissioning at £40 billion, of which the UK and Scottish Governments have agreed to provide tax reliefs, meaning the taxpayer could end up paying half of this cost.

From an environmental perspective, complete removal of all artificial structures would seem on the surface to be the most appropriate approach, but it’s not quite that straightforward. Finding a balance between the environmental and socio-economic costs and benefits of decommissioning is challenging and highly complex. The financial benefits are clear – leaving a structure in place eliminates removal costs – but the environmental costs and benefits are more complicated.

Decommissioning of oil rigs, even if carried out sensitively, will create a range of ecological disturbances – release of trapped chemicals, seafloor disturbance, increased sedimentation, and removal of an established marine community growing on the structure. In addition to the physical removal of the structure, all materials brought to shore (the physical structure, chemical waste, and biological growth on the structure) need to be recycled, re-used or disposed of in landfill, which can have environmental impacts of their own.

The potential for artificial reefs

The alternative to removal is leaving the structure as it is. Although this may seem like abandonment and dumping at sea, there can be ecological benefits to leaving oil rigs in place. For example, fish and other marine organisms are often attracted to large structures such as the ‘legs’ of oil rigs, using them as artificial reefs, and species such as the rare cold-water coral Lophelia pertusa have been found growing on oil platforms in the North Sea.

There can be ecological benefits to leaving oil rigs in place.

Additionally, fishing activity is restricted up to 500m from oil platforms. These restricted zones make up approximately 1% of the North Sea area and could provide important refuge areas for fish. Once the oil rig is removed, these marine communities disappear and fishing returns to the area.

Identifying the most environmentally positive approach for decommissioning in the North Sea is a controversial issue and there are many differing views. The principal interest for the Scottish Wildlife Trust is identifying the option that provides the most benefits (or least damage) for the environment (direct and indirect). The best option must be identified by a detailed environmental assessment of the whole decommissioning process, on a case-by-case basis. When the best option identified is leaving a structure in place, the Trust proposes that any financial savings go into a ‘marine stewardship fund’ that supports marine research and conservation projects that aim to enhance the health of Scotland’s seas.

The Scottish Wildlife Trust is currently engaging closely with the oil industry, Government and academic institutes on how best to assess the different options for decommissioning and consulting on decommissioning plan proposals.

North Sea Oil Platform
A North Sea oil platform. © Andy Muir, CC BY 2.0

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