Bridging decades – Anticipating decommissioning in offshore wind farm design
Designing offshore wind farms today for their safe and environmentally friendly decommissioning 40 years in the future presents a daunting challenge. UK regulations require this consideration before a new asset can be deemed safe by design. But how can designers account for unknown technological, regulatory, and environmental changes? They must strike a delicate balance between current practicalities and future uncertainties. A structured approach involving scenario development, risk assessment, and option analysis will fulfil regulatory expectations and enable the creation of adaptable designs.
Introduction
Over 40 years could pass between designing an offshore wind farm and its eventual decommissioning. Much can change over that period, including regulatory requirements, stakeholder expectations, technology advancements, the structural integrity of the asset, and even the environmental landscape. There are many known unknowns, and no doubt plenty of unknown unknowns too. But developers cannot simply shrug their shoulders and leave this problem for the next generation to solve.
Other energy sectors like oil and gas production and nuclear power generation are still coming to terms with the lack of decommissioning forethought from decades past. Offshore wind developers today must consider decommissioning during the design phase because regulations demand it (for example, see Ref. 1). Furthermore, it is reasonable to anticipate that offshore wind will be held to high stakeholder expectations given the significant role it is playing in the pursuit of carbon net zero. Developers are simply expected to look after people and the planet and to do the right thing.
What decommissioning entails
Decommissioning an offshore wind farm involves taking the wind farm out of service at the end of its operational life and removing its components from the land and marine environment. More specifically, it involves:
- Ceasing operations: Shutting down power generation and disconnecting the wind farm from the electrical grid.
- Dismantling: Removing the visible structures such as wind turbine generators (WTG), including blades, nacelles, and towers.
- Subsea removal: Extracting underwater components like foundations, subsea cables, and other infrastructure.
- Site clearance: Clearing the seabed of any remaining debris or materials associated with the wind farm.
- Environmental restoration: Taking steps to return the site to a state as close as possible to its original condition, or to a state agreed upon with the regulatory authority.
- Waste management: Properly disposing of, recycling, or repurposing the removed materials.
- Post-decommissioning monitoring: Assessing the site to ensure environmental recovery and compliance with regulations.
The goal of decommissioning is to minimise the long-term impact of the wind farm on the land and marine environment, as well as other sea users, after the project has reached the end of its useful life – and to do so safely and responsibly.
But much can change
There is a great deal to think about when designing for decommissioning that will not happen for decades. Many factors can change over such a long period – some will be beneficial, some not, and many others will have an unknown impact.
- Regulatory changes: It is reasonable to assume that safety and environmental regulations will be more stringent than today. There may even be new laws regarding offshore structures and marine conservation that impact the acceptable decommissioned state. As a hypothetical example, a new law may be introduced requiring the mandatory restoration of marine habitats where the removal of subsea components adversely impacts the habitat created during the asset’s life.
- Stakeholder expectations: Public perception and industry views on environmental responsibility will inevitably evolve, which will influence the extent of removal required. In 1995, the UK government approved Shell’s plan to sink the Brent Spar in deep Atlantic waters because it was considered the best option based on scientific and economic analyses at the time. However, Greenpeace launched a high-profile campaign against the deep-sea disposal plan which led to widespread public opposition and boycotts of Shell service stations, particularly in Germany. Shell eventually opted instead for onshore dismantling and recycling.
- Technological advancements: New decommissioning methods and equipment may emerge. Improved recycling technologies could change material handling. Advances in WTG technology might make repowering more attractive than decommissioning.
- Structural integrity: Corrosion and fatigue may be more severe than anticipated. In the case of a floating WTG foundation for example, fatigue could be compounded by the structure being significantly heavier at end-of-life due to build-up of marine growth. This could affect the difficulty and methods of removal. As another example, the potential degradation of Horizontal Directional Drilling (HDD) landfalls for offshore wind farm export cables, could create a flood path from the sea to inland areas if the installation is left in place after decommissioning. Conversely, corrosion and fatigue may be less severe than expected, encouraging the extension of the operational life of the wind farm.
- Environmental landscape: The site may become an artificial reef, complicating removal decisions. Climate change impacts could alter marine ecosystems and decommissioning approaches. Again, with HDD, it may not be appropriate to leave the HDD in-situ at some locations where coastal erosion may lead to the land simply no longer being there.
- Economic factors: Changes in material values could affect recycling economics, for better or worse. Energy market shifts might influence decisions on choosing to repower rather than decommission. The adequacy of decommissioning funds set aside initially may be affected by long-term economic changes.
- Marine spatial planning: Changes in shipping routes, fishing areas, or other marine activities may impact safe decommissioning logistics.
- Handling of hazardous materials: What is the asbestos of 2060? New information about long-term health or environmental effects of materials used could emerge which might necessitate special handling procedures to ensure risks are acceptable. Another consideration would be long-term trapped ballast seawater in floating foundations. Its toxicity will be unknown after multiple decades in-situ with rusting metal and degrading anodes.
- Vessel and equipment availability: The offshore industry’s capacity to handle decommissioning may change. Current types of specialised vessels might become obsolete whilst new types might be developed.
- Knowledge retention: The best time to produce detailed information about the wind farm’s design basis is during the design stage. This should include information that will be useful at end-of-life to support effective decision making. Ensuring that this detailed knowledge is preserved for decades can be challenging, both technologically and organisationally.
So, what realistically can be done?
These factors underscore the importance of flexible decommissioning plans that can adapt to changing circumstances over the decades-long lifespan of an offshore wind farm. Balancing current design decisions with future uncertainties in decommissioning is clearly a complex challenge. So how does the developer go about considering these known unknowns, let alone the unknown unknowns?
It is not reasonable for a designer to be expected to develop a detailed plan for end of life and then designing the offshore wind farm around this plan – there are far too many unknowns. A reasonable expectation, however, is for a designer to apply a structured process that creates a wind farm that is well-prepared for various decommissioning scenarios, without unnecessarily constraining future options or introducing unacceptable risks. This process should involve scenario development, risk assessment and option analysis.
- Scenario development: Create multiple potential end-of-life scenarios based on current knowledge and trends, and consider various technological, regulatory, and environmental possibilities.
- Risk assessment: Identify potential hazards associated with each scenario, evaluate the likelihood and potential impact of these hazards, and focus on risks that could arise from design choices.
- Option analysis: Develop a range of design options that address identified risks and assess how each option might affect future decommissioning flexibility.
Some key factors to consider during this process include:
- Best practice integration: Incorporate current industry best practices for decommissioning-friendly design and stay informed about emerging trends in sustainable design and circular economy principles. For example, prioritising modular designs that allow for easier component replacement or removal and building in adaptability to accommodate technological advancements, as well as choosing materials with known long-term behaviour and minimal environmental impact.
- Stakeholder consultation: Engage with regulators, safety and environmental experts, and industry peers, to incorporate diverse perspectives to broaden the consideration of potential risks.
- Technology neutrality: Where possible, make design choices that do not lock into specific decommissioning technologies. This allows for the adoption of improved methods in the future.
- Regulatory compliance and future proofing: Meet current regulations while building in compliance margins to accommodate potentially stricter future requirements.
- Performance monitoring: Integrate monitoring technologies and data management systems into the design to collect and analyse data throughout the wind farm’s operational life, for example structural health, environmental impact, and WTG performance. This long-term performance data will inform decisions about life extension, repowering, or decommissioning.
- Trade-off analysis: Weigh the costs of over-engineering against the benefits of future flexibility, aiming for a design that is robust enough to handle various scenarios without being excessively complex or expensive.
- Document and knowledge management: Maintain comprehensive records of design decisions and rationales and ensure this information will be accessible for future decommissioning activities. The UK requires a Health and Safety File to be maintained for an asset. Collating the decommissioning information in a retrievable form, for example a pull-out annex rather than scattered across multiple documents, will greatly benefit future plans.
This approach balances current knowledge with future uncertainties, allowing for responsible design decisions in the face of long-term unknowns.
The offshore wind sector cannot wait for a fully developed, effective and efficient circular economy before starting new build projects. Clean energy is needed now to tackle the climate crisis. Decommissioning should be approached with the confidence that the sector is experienced at finding solutions to the challenges it faces.
Phased transition
Whilst decommissioning and disposal will be the fate of all offshore wind farms, life extension or repowering could be viable intermediate stages to maximise the economics of the asset.
Repowering potential is an important consideration in offshore wind farm design, as it can significantly extend the life of an asset and delay the need for full decommissioning. Repowering involves upgrading or replacing existing WTGs with newer, more efficient models without completely dismantling the entire wind farm. However, designing for repowering also presents challenges, such as higher initial costs and the need to predict future technological advancements. Designers must balance these factors against the potential benefits when considering repowering in their initial plans.
Consideration should also be given to the decommissioning sequence. There will not be a sudden shutdown of the offshore wind farm. The transition from the operations and maintenance phase into end-of-life will involve some WTGs remaining operational while others are shutdown ready for decommissioning, life extension or repowering. It is important that any hazards associated with this transitional phase are risk assessed as part of the wider offshore wind farm “simultaneous operations” study, allowing measures to be introduced into the design to reduce the risks to acceptable levels.
Operations and maintenance
Whilst the primary focus during the operational life is on supporting safe and reliable operations, owner-operators should keep one eye on the end game. Monitoring and analysing operational performance of, for instance, structural integrity, equipment reliability, environmental impacts, and energy generation, will provide invaluable insights for end-of-life decision-making, whether that involves decommissioning, life extension, or repowering.
As assets approach end-of-life, there can be a temptation to back-off on routine maintenance to reduce cost and maximise generating output. Whilst reaping short-term benefits, this approach can be counter-productive in supporting effective end-of-life decision making. Due and appropriate consideration should always be given to the end game throughout the operational life of the assets.
Conclusion
The designer of an offshore wind farm will have a significant bearing on the risk posed to people and the environment during decommissioning. It has a responsibility to ensure these risks are reduced to acceptable levels.
But the designers cannot be expected to predict the 2060s – the technologies that will be available to conduct decommissioning, the regulations in place, public perceptions, the materials that can be recycled, and the appearance of the physical landscape.
Nevertheless, a reasonable expectation is that, during the design phase, adequate consideration is given to decommissioning, as well as any life extension or repowering, based on the knowledge and experience available today. A process of scenario development, risk assessment and option analysis will enable this in a structured way. The asset can then be justified as safe by design regarding decommissioning.
