Advances in Offshore Wind and Wave Energies—2nd Edition

The urgent need to mitigate the severe environmental impacts of climate change urges for a global transition to low-carbon energy technologies, crucial for achieving global sustainability goals. Renewable energies arise as the most advantageous technologies to mitigate climate change, contributing to the necessary emission reductions [1]. Offshore renewables like offshore wind and wave energy are emerging as two of the most promising candidates for delivering clean, scalable, and secure energy. While offshore wind is already a mature industry, wave energy faces more challenges, progressing slowly toward commercial readiness. Despite the impressive advances of the past decade, there are technical, economic, and environmental issues that must be surpassed.

This Special Issue, Advances in Offshore Wind and Wave Energies—2nd Edition, provides an updated overview of progress in these areas and highlights promising directions for future research.

Over the past two decades, offshore wind has evolved from an experimental technology to the principal source of renewable energy generation. By 2023, global offshore wind capacity exceeded 64 GW, with projections exceeding 260 GW by 2030 and potentially 460 GW by 2050 if aligned with the Paris Agreement goals [2]. Floating offshore wind platforms allow access to high-quality wind resources far from shore, making them essential to offshore wind projects’ success [3]. Offshore wind is now among the most cost-effective large-scale renewable sources [4].

Presently, global energy demand increases dramatically. Therefore, turbine upscaling becomes a solution to optimize power generation and decreasing the Levelized Cost of Energy (LCoE), which represents the average cost of electricity over a project’s lifetime. Turbines started from 2 to 3 MW in the early 2000s, recently increasing above 15 MW prototypes, with rotor diameters exceeding 250 m [5]. Particularly in offshore wind systems, larger turbines create new engineering challenges in the design of supporting substructures (below seawater level) and structures (above seawater level). Regarding the substructures, monopiles dominate shallow waters (<40 m), jacket-frames are widely applied in intermediate depths, and floating platforms (spar, semi-submersible, tension-leg) extend feasibility to >60 m water depth [3]. Towers must support increased loads while maintaining structural integrity, cost-efficiency, and transportability [5].

Wave energy has an immense theoretical potential—estimated at more than 29,000 TWh/year globally [6]. Although due to well-known harsh challenges—severe environment conditions, survivability and reliability, huge economic costs, low energy conversion efficiency, and lack of standardization [7]—its development evolved slowly when compared with wind energy [6]. However, wave energy is gradually progressing to commercial demonstration projects. The availability of large-scale test sites promotes its development by providing real-sea validation of WEC prototypes [8].

Regarding the last drawback pointed above, the variety of wave energy converters (WECs) designs was gradually narrowed to promising categories such as oscillating water columns (OWCs), point absorbers, and overtopping devices [6,9]. OWCs are particularly attractive due to their simplicity and potential for hybridization with offshore wind systems. Advances include variable-geometry chambers, L-shaped ducts, and adaptive skirts that improve energy capture across wave conditions [9]. Improvements in OWC air turbines (especially self-rectifying turbines) and power take off (PTO) damping strategies contribute to an increase in reliability and energy conversion efficiency [6] which are also two of the disadvantages previously referred.

Currently, the combination of wind and wave renewable technologies into hybrid multipurpose offshore platforms offers several benefits, promising efficiency gains, cost reduction, complementarity and marine space optimization. Hybrid systems can achieve important efficiency gains when compared with independent WEC and wind turbine devices, especially when device geometry and PTO systems are optimized for varying sea states [10]. By sharing infrastructures, placing WECs on wind turbine substructures, foundations, electrical cabling, and maintenance logistics are shared, lowering costs [5]. Since wind and wave often reach its peak at different times, they create smoother combined outputs, thus reducing variability [11]. Hybrid wind–wave platforms decrease the seabed footprint, avoiding spatial conflicts with fisheries, shipping, or wildlife conservation [12].

Beyond wind–wave integration, the concept of multi-use ocean platforms is expanding to include aquaculture, hydrogen production, and maritime logistics. These synergies align with the European Union’s strategies for sustainable ocean space management [12].

However, this wind–wave synergy requires continuous investment in innovative R&D, policy support, and interdisciplinary collaboration between academia and industry. Important barriers still remain not only for standalone offshore wind (e.g., grid integration, environmental impacts on marine ecosystems) and wave energy (e.g., WEC survivability in extreme conditions, cost reduction, standardization), but also for hybrid wind–wave systems (e.g., structural design, dynamic load management).

Offshore wind has already reached a mature level, while wave energy demonstrates a sustainable progress. Consequently, the future of renewable energy may rely on synergies between offshore wind and wave systems. They can share infrastructures and complement each other to supply stable, reliable, cost-competitive, and large-scale renewable power, contributing to the reduction of fossil fuel dependency. Additionally, interdisciplinary collaboration between researchers, engineers, climate experts, and politics is essential to assure offshore renewable energy sustainability in the next future.

The manuscripts collected in this Special Issue reflect a broad spectrum of themes, ranging from fundamental modeling and control strategies to integrated energy system design and long-term resource assessment.

The first and last contributions of this Special Issue, from Liu et al. (Contribution 1) and Palma et al. (Contribution 8) address the control and operational resilience of floating offshore wind turbines, emphasizing the importance of developing fault-tolerant approaches and advanced forecasting methodologies. Those researchers are aware that the development of reliable and cost-effective floating technologies will be crucial for to take advantage of deep-water resources. Therefore, developed control strategies will contribute to improve efficiency and increase reliability in severe marine environments, where operational costs cannot be neglected.

Hybrid systems, combining wind and wave energy conversion, also receive attention in this Special Issue. Research from Pérez-Collazo et al. (Contribution 2) and Ahmad et al. (Contribution 4) demonstrate that integrated configurations can deliver smoother power outputs, improved capacity factors, and enhanced grid compatibility. Artificial intelligence and advanced control methods further reinforce the potential of these systems to become viable, scalable solutions in offshore renewable energy.

Another important area represented in the Special Issue is foundation stability and protection. Fixed-bottom offshore wind structures, which are the pillar of offshore deployment, suffer from scouring and seabed erosion. The review and design principles presented by Gao et al. (Contribution 3) provide valuable guidance for engineers and developers, supporting the safe and sustainable expansion of offshore wind systems.

Wave energy research is also represented through novel approaches to conversion and integration. WEC optimization and control methods investigated by Huang et al. (Contribution 5) and the development of energy storage and hydrogen production systems studied by Fayek et al. (Contribution 6), show that wave energy can contribute to future multipurpose energy systems, demonstrating the ability of offshore renewable energy to provide electricity combined with hydrogen supply solutions.

Tong et al. (Contribution 7) presents the long-term assessment of wave climate and resource variability for the South China Sea, supported by artificial intelligence and data-driven modeling, highlight the importance of understanding regional dynamics to the design of resilient systems. This research contributes to diminish the uncertainty in project planning and increase the predictability of offshore renewable production over extended periods.

Overall, contributions included in this Special Issue, ranging from the integration of novel control strategies, hybrid systems, to wave climate resource assessment, emphasize the multidimensional nature of offshore renewable energy research.