Decommissioning of Offshore Renewables

All too often the challenge of decommissioning structures used in offshore renewables is underestimated, ignored or avoided, yet it is an integral part of a project. We completely understand, at the start of a project you are far more excited about how cutting edge, efficient and beneficial the project will be to focus too heavily on the end of life. However, it is a factor that needs to be considered in Levelised Cost of Energy calculations (this was ignored in a 2016 offshore wind study; is that fair on other renewables?). Now that some of the early offshore wind farms are getting towards the end of life, better and better levels of understanding of offshore decommissioning are being developed. Not to mention the Blackfish FoDTEC project that is currently ongoing (more info).

Using offshore wind as an example, we can look through some cost modelling that has been done for BEIS (who as a last resort may be required to decommission offshore wind farms) and understand the challenges and opportunities in decommissioning offshore structures (BEIS report).

As ever when working offshore the vessel costs and associated weather risks dominate the budget. For the purpose of this study the vessel cost was the entire cost, disregarding the cost of recycling/disposal of hardware and onshore transport. These costs will be significant especially in the case of the difficult to deal with composite blades. Considering the material make up of most offshore renewables is similar to that of an offshore wind turbine, we are looking at a cost around £250,000 per MW in vessel operations for decommissioning. This will be higher for smaller capacity devices and smaller for multi-MW turbines, but this provides some guidance.

The impact upon LCoE is minimal (~1%), mainly due to the costs being incurred so far in the future that they become heavily discounted. It should be noted that these predictions are highly uncertain, the costs of decommissioning vessels are still significant and onshore transport and doesn't factor in onshore transport, recycling and disposal costs.

Offshore Wind Decommissioning

Image Source: Wind Power Monthly

Tidal Turbine Decommissioning

Image from the FoDTEC Project

Having talked about the costs, let's not forget the risks and opportunities.

There are risks with any offshore operation, although over time installing and maintaining devices the offshore operational risks should be well understood and measures in place to reduce them.

In terms of opportunities decommissioning any offshore structure (or even a farm of structures) offers a great learning experience with regard to how these engineering solutions have performed during their designed lifetime in various marine zones (i.e. items on the seabed, submerged and in the splash zone). There is a vast amount to be learned about materials, coatings, connections (electrical and data) and structural geometry.

Making offshore renewables an affordable energy option is reliant on significant lifetimes - typically 20-25 years. We need to take the opportunity to learn from structures that have completed significant duration in marine environments to ensure future structures are designed for the best operation in these environments, either to reduce the capital and/or operational costs or to extend the lifetimes.

As with many other technologies that reach the end of their design life, it is worth considering how life can be extended. This is done for many different types of infrastructure (bridges, power stations, ships) and this will clearly form part of the decommissioning strategy when turbines begin to reach the end of their life. It might be possible to extend the lifetime of the tower by assessing the measured loads throughout the life and calculate that there is still fatigue margin left in the structure. It may also be possible to retrofit newer blades that impart lower loads on the structure in order to keep the turbines running.   Although it is impossible to know at the outset of a project what technologies will be available at the end of the design life that could be used to extend life, there will be solutions available.

Perhaps it's not the most exciting thing to think about at the start of a project, but it is important to consider in the lifetime costs and can certainly provide valuable information for future developments, helping to make offshore renewables even more competitive in the energy market.

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