Floating Offshore Wind: Technology Innovation and System Design Challenges

Global offshore wind installed capacity has grown rapidly in recent years, increasing from 14 GW in 2016 to 67 GW in 2023. This figure is projected to exceed 400 GW by 2032 (WFO, 2024). Currently, most offshore wind farms are bottom-fixed, with floating offshore wind (FOW) accounting for less than 0.3 GW. However, FOW is anticipated to be a key technology for harnessing offshore wind power, as 80% of the world's offshore wind resources are found in waters deeper than 60 meters (GWEC, 2022).

The key difference between FOW and bottom-fixed offshore wind is that FOW turbines are mounted on floating platforms anchored to the seabed with mooring lines. These dynamic characteristics affect both the design of the foundation and other subsystems. This article explores the design challenges associated with FOW components.

System Design

Floating offshore platforms could be new to the offshore wind industry but have been used in the oil and gas industry since the 1950s. It has already been installed at a water depth of over 2,000 meters. Leveraging experience from the oil & gas industry can inform the design of mooring systems for FOW. However, the design philosophy of the floating platform and mooring system in these two industries are quite different. The oil and gas industry has high safety standards due to the hazards from the processing materials, and a redundant mooring design could be affordable due to the high economic value of the industry. On the other hand, cost-effectiveness is essential for the offshore wind industry. Innovation is one of the keys to reducing costs without sacrificing safety and system performance. This provides options to the wind farm developers but also creates a problem - which platform design should be taken.

Floating Foundation

The design of floating platforms can be grouped into 4 major categories (i.e. SPAR, Semi-submersible, Barge, and Tension-Leg Platform), but there are over 200 different floating platform concepts on the market[1]. Each of them has specific features to bring some advantages but also limitations. For example, semi-submersible and barge can be manufactured and installed at shallower water (e.g. <100m), but the former is more complicated to manufacture, and the latter would need a complex mooring system due to its sensitivity to wave-induced motions.

Like many other merging technologies, only a few designs will stay in the market once the technology is mature. With more floating platforms demonstrated in the field, the key design consideration is moving from the stability of the system to the manufacture and installation of the system. For example, the early design of semi-submersible platforms had the wind turbine generators mounted in the middle of the platform due to the consideration of platform stability. Today, most designs have WTGs at the side or corner because they are easy to assemble for large-scale turbines that easily over 200 meters heights. After all, today's cranes can not lift thousands of tons of components over a hundred meters heights and extend far from the quayside.

Mooring System

Various crucial factors must be considered when designing mooring systems for floating offshore wind[2]. The choice of a floating foundation significantly influences the mooring system design. Tension-leg platforms (TLP) rely on taut mooring lines to maintain their stability, and the rest typically employ catenary, semi-taut, or taut mooring lines. Water depth and environmental conditions determine how robust the mooring systems must be to withstand extreme loads and fatigue. Seabed conditions such as soil type and potential for liquefaction during seismic events are crucial in selecting appropriate anchors.

Innovative solutions for mooring systems have been explored recently[3]. New materials (e.g. synthetic ropes) and load-reduction devices can significantly reduce the cost and mass of the mooring lines. Shared anchors or shared moorings configuration (e.g. connecting multiple FOWTs to a single anchor point or sharing mooring lines between turbines using mid-water buoys) show great potential for cost reduction but still need further research.

Similar Components as Bottom-fixed but With Dynamic Characteristics

Some components of the FOW system, such as wind turbine blades, nacelle, and towers, are fundamentally the same as bottom-fixed offshore wind. However, some components have similar functions, but the design must be changed to address the dynamic characteristics of the floating structure.

Dynamic Cable

Dynamic cables are critical components for transmitting the generated electricity to the onshore grid. Unlike bottom-fixed offshore wind farms, dynamic cables must withstand the continuous bending, twisting, and tension cycles associated with floating structures[4]. Increasing voltage from 33 kV to 132 kV makes the cable design more challenging.

Dynamic cable connectors are essential components that connect and disconnect the cables between turbines and substations, and they can be classified into 2 types: dry-mat and wet-mate. Dry-mate connectors are mature but require lifting to the surface for disconnection. Alternatively, wet-mate connectors can be connected or disconnected underwater and therefore facilitate the installation and maintenance operations, but still in the early stages of development. Regarding the inter-array cable layouts, non-traditional designs (e.g. fishbone, star) can enhance redundancy and flexibility but require further technological development in subsea hubs and wet-mate connectors.

Offshore Substation

Different substation designs have been proposed to collect and transmit the electricity from the FOW. Bottom-fixed substations can still be installed if the water is not too deep - there are offshore oil and gas platforms at water depths up to 400 metres. As floating wind farms move into deeper waters, developing cost-effective and reliable floating substations is important but still has many challenges such as the feasibility of dynamic export cables. Instead, subsea substations are being explored as a potential alternative and a pilot project in Norway is testing this concept.

Accelerate the FOW Industry Development

The Importance of the Systems Approach

Applying a systems approach that considers the entire system instead of individual components in isolation can reduce the time and costs of FOW development. Floating offshore wind involves the development of different subsystems and components, and their design could affect each other. For example, the design of a floating structure (dynamic motion) could affect and design of mooring systems and dynamic cables. Typically, each component is designed by separate technology providers, and it is fine-tuned after being integrated into the whole system. This design cycle could take years and be costly. Encouraging systems thinking and industry collaboration at the earlier stage can enhance the design change later.

Moreover, to optimise the design of the whole FOW system, NREL developed a holistic simulation framework, Wind Energy with Integrated Servo-control (WEIS), which integrates models at different development stages of FOW design and enables the co-design of multiple components in the FOW system[5]. The tool allows engineers to understand and optimise the complex interactions between various components.

Support Needed for Innovation and Technology Development

Today, numerous challenges must be addressed to make FOW a cost-effective energy source. However, this early phase also offers significant opportunities for innovation and leadership in both market and technology advancements. Collaborative research and development between academia and industry are important as many challenges remain unaddressed. Scientific research can provide the foundation for future technology development and improvement.

Pilot and demonstration projects are essential for testing and validating the performance, reliability, and safety of new FOW technology in real-world environments. They offer valuable data and insights for refining designs and addressing technical challenges, which is vital for acquiring the knowledge and experience needed for scaling up to commercial deployments.

Governments play a pivotal role in fostering FOW innovation by providing long-term policy certainty, creating enabling regulatory frameworks, and backing research and development[6]. Providing robust financial support mechanisms such as contracts for difference and feed-in tariffs is particularly important for attracting developers to participate in early-stage pilot projects.

Summary

Floating offshore wind system design is a complex and evolving field that requires a holistic approach, considering the interactions between various components and optimizing for performance, stability, and cost-effectiveness. We need collaborative research to tackle the remaining challenges across different domains and pilot and demonstration projects to drive innovation, build confidence, and reduce risk and uncertainty. Together, these actions will drive us closer to a future where floating offshore wind is at the forefront of our energy solutions.

Reference:
[1] RWE, Shaping the future of floating wind
[2] WFO, Mooring systems for floating offshore wind
[3] DNV, Mooring systems
[4] WFO, Floating offshore wind dynamic cables
[5] [Barter et. al., 2020
[6] [DNV, 2022b

GWEC (2022). New GWEC report identifies floating offshore wind's critical role in long-term global decarbonisation efforts.
https://gwec.net/report-outlines-enormous-potential-for-floating-offshore-wind-in-energy-transition/

WFO (2024). Global Offshore Wind Report 2023.
https://wfo-global.org/wfo-global-offshore-wind-report-2023-published/

DNV (2022). Mooring systems: Floating wind and solar research needs.

DNV (2022b). Floating offshore wind: the next five years

Barter et. al. (2020). A systems engineering vision for floating offshore wind cost optimization.
https://www.sciencedirect.com/science/article/pii/S1755008420300132

WFO (2022). Mooring systems for floating offshore wind: integrity management concepts, ricks and mitigation