Floating wind turbine foundations allow the offshore wind sector to generate larger energy yields in locations where fixed foundations are unfeasible or uneconomical. Floating foundations present new risk management challenges but also significant opportunities for optimising risks which may have some benefits over fixed installations.
FLOATING FOUNDATION DESIGNS
Traditional offshore wind turbines rely on fixed-bottom foundations which are secured directly to the seabed. However, this method has its limitations, as it is only feasible in shallower waters.
By using buoyant platforms or semi-submersible structures, wind turbines can be located in deeper waters where stronger winds enable higher energy yields.
Three prominent floating foundation designs have emerged (see Figure 1):
- Spar: This is a large diameter, vertical buoyant cylinder. It is kept in position by catenary or taut spread mooring lines with drag or suction anchors. The spar foundation is very stable, enabling it to support large and heavy wind turbines in demanding conditions.
- Tension leg: This design is a buoyant platform tethered to the seabed by tensioned mooring lines or tendons, which maintain the platform’s stability and position while allowing it to move vertically with the water column.
- Semi-submersible: This consists of multiple columns to provide stability, and pontoons to provide buoyancy. The structure maintains its position through its mooring system, which typically comprises catenary or taut spread mooring lines and drag or suction anchors. These structures require optimisation to achieve stability through design and the use of ballasting systems. The overall stability of semi-submersibles is generally lower than spars and tension leg platforms, although the concept remains the most popular.
Figure 1: Floating Foundations Illustration
The unique aspects of the semi-submersibles and tension leg platforms are the wide footprint of the foundation above the water level, compared with the fixed foundation types which typically have a smaller diameter.
Floating offshore wind turbines arguably have a heightened risk profile over their static fixed foundation counterparts. Their mobility introduces unique risks such as drifting or capsizing due to mooring system failure. The high seas and harsh weather conditions in deeper waters, where floating turbines are typically located, increase the likelihood of structural, mechanical and electrical failures, with specific challenges around dynamic cable integrity.
These adverse environmental conditions increase the risks associated with safe emergency response, with the remote and inaccessible locations also complicating recovery. The installation, maintenance, and decommissioning of floating turbines can be more complex and hazardous due to their unique design, assembly methods and the challenging environment.
The acceleration of offshore wind and floating technologies presents other risk management challenges, including:
- Limited historical data and knowledge sharing: The floating sector lacks an established history of successful project delivery at scale. Offshore wind Original Equipment Manufactures (OEMs) and developers often work independently, which impacts the sharing of data to support risk assessment and decision making. For example, an industry wide approach to vessel transfer for deep water and floating assets.
- Evolving technologies: The drive to meet the needs of net zero includes ever larger turbines, which creates a constantly changing risk landscape. For instance, dynamic inter-array cables for floating wind applications, including the increased number of connectors and more connect-disconnect cycles than equivalent applications in the oil and gas sector.
- Workforce expertise: A skilled workforce is needed with expertise of the technical aspects of the technology and the management of its risks. The sector’s rapid expansion makes developing and retaining talent a challenge, with staff turnover impacting corporate knowledge.
- Regulatory uncertainty: Emerging industries and new technology are often subject to disparate regulatory frameworks and viewpoints, which can create uncertainties for developers and investors. Navigating this requires a deep understanding of the policy landscape and the ability to adapt quickly to new requirements.
The approach to managing these risk challenges must be proportional, fit for purpose and recognise the uncertainties.
Whilst there are inherent risks to floating foundations, the industry should identify the hazards and determine whether floating foundations can be designed to eliminate, reduce, or more effectively mitigate these hazards. This would benefit from closer collaboration between wind turbine OEMs and floating foundations designers to achieve success.
When considering the design, questions should be asked such as:
- Can the larger foundation structure accommodate safer personnel access methods?
- Can the transformer in the turbine tower be relocated externally on the foundation to isolate a major fire hazard from the primary escape route?
- Can the larger foundation structure be designed with a refuge, or other emergency equipment, enabling personnel to shelter safely for prolonged periods during an incident?
- Can the turbines be installed, maintained, and decommissioned nearshore with lower risks than performing the same work offshore?
It is important to question the design at the earliest phase of the project, where there is opportunity to modify the concept without significant impact on cost and programme.
LEARNING FROM EXPERIENCE
The offshore wind industry has an established history of low incident rates. But this success may introduce an element of complacency during the shift to larger scale floating wind farms and fail to adequately recognise and manage the new risks.
There are countless pan-industry incidents and well-developed good practice that can be learned from and applied to the floating offshore wind sector. This provides a good starting point for risk assessment and risk management. As the industry grows, offshore wind-specific good practice and standards for risk and safety management should be developed by learning from and sharing experience.
Failure to remain diligent when managing risk in the face of change will be an unforgivable act, especially given the maturity of risk management as a discipline and the major incidents in the energy sectors that have shaped modern risk management.
The industry has the opportunity to fundamentally rethink its approach to managing risk during the design and operation of floating wind technology. It must not be blinkered in its enthusiasm to deliver projects and pursue the simplest and quickest route of connecting established wind turbine models onto a floating foundation. It should not be complacent in recognising and seizing opportunities for fundamental or marginal gains in risk reduction. It should take a step back and positively embrace its differences with more traditional fixed bottom technologies to ensure projects are progressed in a way that is safe by design. This focus should be nurtured through strong collaboration between all stakeholders.