End of life or afterlife?

Susan Gourvenec offers a down under outlook for decommissioning offshore oil and gas facilities.

Operating pipelines provide habitat for marine life. Photos from Woodside.

Australia’s first offshore oil and gas facilities were constructed in the 1960s and the country is now facing the first wave of decommissioning projects.

A construction boom in the 1980s, and a more recent boom, which has seen the construction of multiple super projects, including Chevron’s Gorgon and Inpex’s Ichthys developments, means there is a sustained decommissioning challenge on the horizon.

As in other regions, Australia is questioning the rationale for complete removal of offshore oil and gas facilities at the end of field life and is looking to provide leadership across Australasia and Asia in decommissioning offshore oil and gas infrastructure.

Scale and cost

There are more than 100 offshore oil and gas platforms and subsea structures in Australia, including about 35 fixed platforms and 12 floating production facilities, many approaching the end of production life. Only a small number of facilities have been decommissioned to date, including the Jabiru and Challis floating production, storage and offloading (FPSO) developments.

Nearby, across Southeast Asia, there are more than 1700 offshore installations, nearly half of which are more than 20 years old and due to be retired. In Asia Pacific, more than 600 fields are expected to cease production in the next 10 years.

Australia has put a US$21 billion price tag on offshore decommissioning over the next 50 years, based on current policy and technology. Australia’s predicted liability accounts for about 10% of the estimated total global decommissioning spending in that period, and can be compared with a UK estimate of nearly $60 billion by 2050.

The scale of the decommissioning challenge is understood – less well understood are the best decommissioning options.

Platform structure decommissioned in situ augmented with artificial reef modules.  Image from Subcon.

Options

• Complete removal and disposal onshore The current base case for offshore oil and gas infrastructure at the end of field life in Australia is – as elsewhere – complete removal and disposal onshore.

While complete removal of offshore infrastructure poses many challenges, a solution can be found for most engineering challenges with sufficient investment. An example is Allseas’ Pioneering Spirit – the purpose built decommissioning vessel, built to remove (and install) topsides and jackets from the North Sea. It is 382m-long and 124m-wide and cost some $3 billion to build – although is forecast to save up to $12 billion in decommissioning costs in the North Sea.

But just because we can – should we?

There are challenges, risks and costs of removal and transport; challenges, risks and costs of disposal onshore, whether for landfill or recycling; and the destruction or disruption of ecosystems that have established around infrastructure over several decades of operation.

• In situ decommissioning There is much interest in Australia in the precedence of removal and relocation or disposal to another offshore location – most well-known is the US rigs-to-reefs program, through which more than 400 decommissioned structures have been converted to permanent reefs since 1986.

Removal, even for relocation, involves expense and risk and can damage the marine ecosystem that developed during the production life.

A version of the rigs-to-reefs approach is leaving part or all of the field architecture in situ – i.e. without relocation. This has the benefit of not needing to mobilize large vessels for removal, sea fastening and long-distance transport. Therefore, achieving a reduction in cost and risk, and leaving the established marine ecosystem intact.

If cost and risk of engineered removal are to be eliminated – the alternative must be demonstrated to be safe from an engineering and ecological perspective.

From an engineering perspective, the basis of design (BOD) for the afterlife of a structure, if left in situ, is quite different to the production life; tolerances on differential movements are less stringent due to reduced or absence of risk from loss of containment (once cleaned and flushed). The high-level BOD for the afterlife is perhaps limited to avoiding dispersal of the structure in large or small parts. Loading is less onerous in the afterlife due to the absence of operational loads and resistance can be increased relative to the design state due to marine growth, burial or embedment and increased seabed strength.

Viewed through the lens of removal – increased resistance adds to the challenge. Viewed through the lens of in situ decommissioning – increased resistance is beneficial.

Recent geotechnical research at the Centre for Offshore Foundation Systems at The University of Western Australia has shown the potential lifting force to remove a shallow foundation from a muddy seabed at the end of production life can be up to nine times the foundation weight, without even considering marine growth. Considering this in terms of removal presents a significant challenge in terms of crane and vessel requirements. On the other hand, in terms of in situ decommissioning, it would make the structure more stable – and safer for the afterlife.

Geotechnical research of pipelines on sandy seabeds has shown that pipes can self-bury – which can make them either harder, and more ecologically disruptive to retrieve – or more stable if left in situ.

Undoubtedly, the notion of returning the seabed to its initial state is borne out of the best intentions, but after infrastructure has been part of the marine ecosystem for several decades, removing it might not be the best option.

From an ecological perspective, can it do more harm than good removing infrastructure? And, equally important, what are the risks associated with leaving the infrastructure in situ?

Marine science research from the Oceans Institute at The University of Western Australia has shown that offshore infrastructure on the Northwest Shelf supports diverse invertebrate habitats and fish life and benefits commercial fisheries. The warm tropical waters are particularly conducive to marine growth and various studies have demonstrated a range of marine biota on structures in Australian waters.

Trawling in areas offshore Australia decimated seabed habitat in the 1960s and 1970s, which has started to renew due to the hard standing provided by oil and gas infrastructure. Returning the seabed to the condition it was in before the oil and gas infrastructure was installed may be an ecological step backwards.

Similar experiences have been reported in the North Sea. Initiatives there, such as the INSITE (Influence of man-made Structures In the Ecosystem) [OE: February 2017] and LiNSI (Living North Sea Initiative) projects provide invaluable data to the required evidence base to inform decommissioning options.

An ocean laboratory case study for in situ decommissioning, such as that proposed in the Dutch sector of the North Sea through Platforms Natuurlijk, (OE: December 2016) would be a welcome addition to the evidence base for the Australian marine environment.

• Augmentation There is also the option of augmenting oil and gas infrastructure left in situ after decommissioning with engineered artificial reef modules to optimize benefits to marine life. Artificial reefs are widespread in Australia and Asia, and as such this is an area where Australia and Asia may provide leadership.

Subsea structures decommissioned in situ augmented with artificial reef modules. Image from Subcon.

Challenges and opportunities

Australia faces some specific challenges, due to the geographical remoteness of the country and general sparsity of offshore infrastructure, which is spread around the vast coastline.

An emergent theme in Australia, as elsewhere, is the potential positive benefits of full or partial in situ decommissioning, and that cross sector transdisciplinary collaborative solutions may lead to win-win scenarios for all stakeholders and the marine environment.

With appropriate coordination and collaboration, it is possible that these efforts will transform the physical legacy of abandoned offshore oil and gas projects from a liability on the private and public purse, to an asset within the marine ecosystem creating a beneficial increase or concentration of the ocean ecosystem, benefiting a diverse range of stakeholders.

Susan Gourvenec is a Professor at the Centre for Offshore Foundation Systems at the University of Western Australia. Gourvenec has over 15 years’ of offshore engineering experience, with particular interest in offshore geotechnics. She is a consultant offshore geotechnical engineer to industry and member of the ISO and API Committees for Offshore Geotechnics.