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Offshore Wind Journal

Offshore Wind Journal

Designing for decommissioning can reduce costs later

Tue 11 Jul 2017 by David Foxwell

Designing for decommissioning can reduce costs later

In the offshore oil and gas industry decommissioning is in ever-sharper focus, particularly in the North Sea, where massive structures such as Shell’s Brent topsides have begun to be brought ashore to be disposed of.

As I noted in our sister journal, Offshore Support Journal, the cost of decommissioning is a major issue. In the offshore oil and gas industry, greater co-operation to share good practice, lessons learned and decommissioning campaign management is needed to bring costs down. Cost reductions of around 35 per cent are being targeted. Operators need to look for synergies with other energy industries, it is said, and new technology and different working practices will be required to make the most expensive parts of decommissioning less expensive.

The offshore wind industry won’t have to deal with some of the high-cost aspects of decommissioning an oil field – such as plugging and abandoning infrastructure in which hydrocarbons are involved – but as a number of papers at Offshore Wind Energy 2017 highlighted, the industry needs to begin to focus on decommissioning now if costs aren’t to escalate later.

As experts from DNV GL told the conference, from the early stages of designing marine structures, decommissioning should be considered to minimise the impact on lifetime costs. Prudent consideration of end of life costs during the design and operation phases and the efficient execution of the decommissioning phase when it comes can reduce the cost of energy and liability. Similarly, owners of existing offshore windfarms need to make cost-efficient decisions regarding life extension, late-life operation and ultimately decommissioning and removal.

If you think that decommissioning isn’t an issue for the offshore wind industry yet, think again. A handful of projects have been dismantled in the last two years (such as Yttre Stengrund, Lely and elements of others) and others are approaching the end of their design lives. 

As DNV GL pointed out, decommissioning in the offshore oil and gas sector began more than 30 years ago and a large body of experience and specialised techniques have been developed and continue to evolve. With care, the offshore wind sector can benefit greatly from this extensive experience, while keeping aware of distinct differences between the two sectors.

At a high level, the main similarities with offshore oil and gas structures are the range of foundation types (jackets, piles, gravity base, suction buckets) and the site conditions. Much knowledge is transferable, says DNV GL, but there are significant differences between the two sectors, primarily from the much greater risks of pollution and environmental impact with oil and gas installations, requiring much greater consideration during dismantling and removal; and from the scale and numbers whereby windfarms comprise multiple installations that are essentially identical compared with the single complex entity at an oil installation. 

The oil and gas industry has also made advances in subsea cutting technologies; in remotely operated subsea vehicles; and in heavy lift vessels to serve the needs of decommissioning. Developments needed for offshore wind include cutting techniques for the largest monopiles, and alternative techniques such as vibro-removal may be embraced if complete foundation removal is desired.

“In terms of planning ahead, ease of decommissioning should be built into the design, such as breaking down large structures into modules, or releasable joints,” says DNV GL. “Throughout the lifetime, structural changes need to be recorded in detail. The marine logistics during decommissioning need careful optimisation as costs are particularly sensitive to the use of expensive specialised vessels. 

“Without considering decommissioning at all stages in the life of an offshore wind project, the overall cost of energy may be higher than necessary. The offshore oil and gas industry has learned pertinent lessons in the need to plan years ahead before cessation of production and preferably at the design stage. It has developed techniques and methodologies in certain key areas. For offshore wind, these methods need to be supplemented by enhanced and new techniques. Optimisation of marine operations is essential, for example taking advantage of the programme nature and scale, working with multiple vessels, and recognising their major contribution to the overall costs.”

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