Offshore wind farms offer a promising avenue for generating clean and low-carbon energy rapidly. Several countries, such as the European Union and the United Kingdom, have adopted ambitious net-zero plans to decarbonize their economies by 2050 ^[1]. Ref. ^[2] examined the main features of offshore wind projects in Europe that are operational or under development (e.g., countries, installed capacity, number of turbines, water depth, project area, distance offshore, transmission technology, and investment costs). They reported that offshore wind in Europe has expanded at an average annual rate of 36.1% since 2001. There are currently 76 offshore wind projects in European waters with a cumulative installed capacity of 7748 MW and an additional 3198 MW under construction.

Despite having a larger carbon footprint than onshore wind projects, offshore wind projects have almost achieved similar Energy Payback Periods (EPP) due to increased renewable electricity production resulting from better wind resources ^[1]. The offshore wind power industry has to overcome various obstacles and seize potentials, such as marine geological survey, floating foundation design, flexible DC transmission technology, intelligent O&M, standardization, parity, and research and development ^[3]. First, marine geological surveys are conducted to better determine the site selection, design, and construction of wind farms. The second is the floating foundation design, a structure that can float on the surface of the water in the deep water area and is connected to the wind turbine, called the floating foundation. The third is the flexible DC transmission technology, which is an efficient, low-loss, and easy-to-control transmission technology. Intelligent operation and maintenance refer to the use of digital, intelligent, automation, and other technologies to achieve real-time monitoring, fault diagnosis, predictive maintenance, and other functions of wind turbines and other equipment.

Before implementing an offshore wind power project, it is necessary to evaluate its technical and economic feasibility, whether the project can operate stably technically and is economically cost-effective ^[4]. Ref. ^[5] proposed a design model for offshore wind power plants that can be derived from either a systematic theoretical analysis based on a comprehensive understanding of the system dynamics or a systematic experimental analysis using identification methods. The factors affecting the cost of offshore wind power were studied, scenarios with different wind turbine capacities and wind farm sizes were set up, and then the average electricity price under each scenario was calculated using a technical economic model ^[6].

Offshore Wind Turbines (OWT) are broadly divided into stationary and floating types, and Floating Wind Turbines (FWT) can overcome the environmental impact and cost constraints of Conventional Stationary Wind Turbines (CSWT) ^[7]. Strong winds and wave effects combine to shock OWT, creating vibration, fatigue, and heavy loads on the structure and other components of the wind turbine. From a control point of view, cost reduction can be achieved by operating the turbine close to its optimal operating point in the partial load, ensuring reliability by reducing the structural load, and regulating the power generated under strong wind conditions ^[8][9]. Economically, the support structure affects the cost of system balances and O&M. The purpose of cost reduction can be realized by operating the turbine close to its optimal operating point in partial load. The cost of the support structure and environmental factors significantly impact the energy parity level of offshore wind power ^[10]. Life-Cycle Engineering Services (LCES) is a method of evaluating and optimizing the O&M of wind turbines. A generic LCES method has been proposed, and a case study of an offshore wind farm gearbox has been presented ^[11]. A life-cycle cost analysis framework of offshore wind farms has been developed to help wind farm developers reduce costs of the medium to long term ^[12].

Historically, the approach to maintenance has been purely passive, and there is a shift towards a more active, condition-based approach to maintenance ^[13]. Offshore wind farms need better methods of O&M to improve economics and sustainability ^[14]. Some researchers have proposed a method of Condition-Based Maintenance (CBM) that can predict and prevent failures based on operational data ^[15]. Two different maintenance strategies have also been introduced, one predictive and the other prescriptive. They also explain how to optimize the maintenance measures to make them more suitable for the actual situation.

2. Optimization of Offshore Wind and Wave Energy Utilization

The ocean is the largest reservoir of renewable energy resources on the earth, which contains huge wind, wave, tidal and current energy, and other forms of energy. Offshore wind and wave energy utilization refers to the use of OWT and wave energy converters and other devices to convert wind and wave energy in the ocean into electricity, which has the advantages of being clean, efficient, and sustainable, and is one of the important directions of future energy transformation. However, offshore wind and wave energy utilization also face many challenges, such as the complexity and badness of the marine environment, the inefficiency and instability of power conversion and transmission, and the high cost and high risk of system construction and O&M. Wind energy resources and their utilization were evaluated by analyzing wind speed probability distribution, average wind speed, average wind energy density, effective utilization time of wind energy, and wind power output ratio ^[16]. Therefore, how to optimize the technology and strategy of marine wind and wave energy utilization and improve its economy and reliability is the hot and difficult point of current research. This section aims to comprehensively analyze and summarize their main contents and contributions to the optimization of marine wind and wave energy utilization, analyze the correlation and differences between them, point out the current research gaps and deficiencies, and propose some feasible methods and strategies to optimize the utilization of marine wind and wave energy, including power converter, ocean-atmospheric boundary layer, Offshore Pumped Storage (OPS), rotary energy harvesting, fault-tolerant control, energy storage concept, wave energy converter, etc.

2.1. Sea Wind and Wave Utilization Optimization

The various aspects of optimizing the utilization of wind and wave energy at sea can be divided into the following categories:

• Power converters: ^[17] provided a comprehensive overview of power converters used in high-power wind turbines, analyzing key challenges and potential solutions for improving system efficiency, reducing costs, and enhancing flexibility. A fault-tolerant control strategy based on reconfiguration control was proposed to improve the reliability of parallel converters in permanent magnet synchronous generator wind power generation systems

• Power converter: The power converter is the core component of the offshore wind and wave energy utilization system, and its performance directly affects the efficiency, cost, and flexibility of the system. To improve the performance of the power converter, the following methods can be used:

• ^[18].

• (a)

  Selection of appropriate topologies and control strategies to accommodate different types of generators and loads and to improve the power density, efficiency, and reliability of the converter ^[17];

  (b)

  The use of modular, integrated, and intelligent technologies to reduce the volume, weight, and heat dissipation requirements of the converter, and improve the maintainability and fault tolerance of the converter ^[17];

  (c)

  Multi-stage, multi-port, and multi-function technologies are utilized to achieve collaborative control between converters and to improve the flexibility and compatibility of converters ^[17].

• Marine Atmospheric Boundary Layer (MABL): An experimental platform for characterizing the structure and dynamics of the MABL was presented to support offshore wind energy research
• ^[19]. The platform includes an unmanned aerial system, a weather tower system, and a remote sensing system that can provide MABL data with high spatiotemporal resolution.

• MABL: The MABL is the operating environment of the offshore wind and wave energy utilization system, and its structure and dynamics have an important impact on the output power, stability, and lifetime of the system. To improve the characterization of MABL, the following methods can be used:

  (a)

  A variety of platforms and means such as unmanned aerial vehicles, meteorological towers, and remote sensing are used to obtain MABL data with high spatiotemporal resolution, and perform data fusion and analysis ^[19];

  (b)

  Use physical models, numerical simulation, machine learning, and other methods to establish accurate and real-time MABL prediction models, and conduct model validation and optimization ^[19];

  (c)

  Use MABL data and models to guide the siting, design, control, and operation of offshore wind and wave energy utilization systems and to evaluate their performance under different MABL conditions ^[19].

• OPS: A predictive operation strategy based on an event-triggered Model Predictive Control (MPC) approach was proposed to achieve the complementary power of OPS and real-time offshore waves
• ^[
• ^20]. This strategy can effectively smooth output power fluctuation and improve the operation efficiency of the OPS system. Similar to the concept of energy storage, Ref. ^[21] discussed the use of wind energy in low wind speed areas to provide microgrid solutions for offshore oil and gas platforms to improve the timeliness of wind energy utilization.

• OPS: OPS is a technology that uses water pressure differences for energy storage and release, which can effectively smooth the output power fluctuations of offshore wind and wave energy utilization systems and improve the operational efficiency of the system. To improve the performance of OPS, the following methods can be used:

• (a)

  Select suitable energy storage media (such as air, water, or liquid metal) to improve energy storage density, efficiency, and safety ^[20];

  (b)

  Select suitable energy storage structures (such as spherical tanks, cylindrical tanks, or underwater caves) to reduce energy storage costs, risks, and environmental impacts ^[20][23][28].

  (c)

• The application of rotary energy harvesting technology in the field of self-powered sensing was reviewed in detail in
• ^[
• ²²
• ^]. Its performance characteristics at different scales, frequency ranges, and operating modes were analyzed, and its application in rotary machines and renewable energy systems was discussed.
• Energy storage concept: A novel offshore wind energy storage concept was proposed, whereby excess wind power is stored in underwater spherical tanks through compressed air and released through turbines to meet demand ^[23]. This concept can significantly reduce rated power costs and improve system stability. The studied Reversible Solid Oxide Cell (rSOC) system is compatible with the auxiliary system requirements of 2.3 MW wind turbines and can cover the auxiliary needs during wind speed shortages or maintenance ^[24].

• Advanced control methods such as predictive control and event-triggered control are utilized to realize the power complementary between OPS and real-time offshore waves, and to optimize the operation strategy of the OPS system

• ^[

• ²⁰

  ^]

  .

• Rotational energy harvesting: Rotational energy harvesting is a technology that uses rotational motion to generate electrical energy, which provides a continuous and reliable power source for self-powered sensors in offshore wind and wave energy utilization systems. To improve the performance of rotational energy harvesting, the following methods can be used:

  (a)

  Select an appropriate energy harvesting mechanism (such as electromagnetic induction, piezoelectric effect, electrostatic induction, etc.) to adapt to rotational motion at different scales, frequency ranges, and operating modes ^[22];

  (b)

  Techniques such as multi-physical field coupling, non-linear vibration, and bi-stable state are used to improve the output power and frequency bandwidth of the rotating energy collector ^[22];

  (c)

  Technologies such as energy management, power matching, and load regulation are utilized to improve the electrical matching and synergy between the rotating energy collector and the self-powered sensor ^[22].

• Energy storage concept: The concept of energy storage refers to the use of different physical or chemical principles for energy storage and release technology, which can effectively improve the economy and reliability of offshore wind and wave energy utilization systems. To improve the performance of the energy storage concept, the following approaches can be adopted:

  (a)

  Select suitable energy storage media (such as compressed air, underwater vehicles, or liquid metals) to improve energy storage density, efficiency, and safety ^[23][28].

  (b)

  Select suitable energy storage structures (such as underwater spherical tanks, underwater caves, or underwater reservoirs) to reduce energy storage costs, risks, and environmental impacts ^[23][28];

  (c)

  Optimization algorithms, multi-objective planning, and other technologies are used to achieve optimal matching and coordinated control between the energy storage concept and the offshore wind and wave energy utilization system ^[23][28].

• Wave energy converter: A wave energy converter is a device that uses wave motion to generate electricity, which can effectively use the abundant wave resources in the ocean and complement OWT. To improve the performance of wave energy converters, the following methods can be used:

  (a)

  Select suitable wave energy conversion mechanisms (such as oscillating water columns, point absorbers, or underwater vehicles, etc.) to adapt to different types and strengths of waves ^[25][26][27];

  (b)

  The use of non-linear vibration, bi-stable, chaos, and other technologies to improve the output power and frequency bandwidth of the wave energy converter ^[25][26][27];

  (c)

  The use of array layout, phase control, power regulation, and other technologies to improve the synergy between wave energy converters and the overall efficiency ^[25][26

• Wave energy converter: ^[25] evaluated the potential for offshore wind and wave energy utilization on a global scale and compared differences across regions and seasons. The Life Cycle Assessment (LCA) of a tidal stream power generation array composed of multiple underwater vehicles was carried out to analyze its performance in terms of environmental impact, resource consumption, and economic benefits ^[26]. Strategies to improve the sustainability of Wave Energy Converters (WEC) and offset their high initial capital expenditures are explored, including technological innovation, strategy support, and social engagement ^[27].

2.2. The Offshore Wind and Wave Energy Utilizes Optimized Methods and Strategies

Combined with the previous analysis on the optimization of wind and wave energy utilization at sea, this paper summarizes some corresponding feasible methods and strategies to optimize the utilization of wind and wave energy at sea through the investigation of the literature:

• ^]

• ^[

• ²⁷

• ^]

• .

These methods and strategies design, analyze, evaluate, and optimize the marine wind and wave energy utilization system from different angles and levels, involving many factors such as system components, operating environment, storage mode, and conversion efficiency. They not only show the progress and achievements of offshore wind and wave energy utilization technology in recent years, but also reveal the problems and challenges in theoretical models, experimental verification, engineering implementation, and other aspects. In the future, there are still many directions and challenges worth further research in the optimization of marine wind and wave energy utilization, such as:

• How to comprehensively design, model, control, and evaluate multiple types of marine renewable energy collaborative utilization systems, such as mixed wind–wave–tidal current systems;
• How to systematically compare and analyze the optimization performance of offshore wind and wave energy utilization systems at different scales (such as individual devices, arrays, or regions), different scenarios (such as normal operation or fault conditions), and different objectives (such as maximum power or minimum cost);
• How to comprehensively assess and optimize the sustainability of offshore wind and wave energy utilization projects taking into account social and economic factors (such as job creation, community participation, etc.);
• How to improve the intelligence level of marine wind and wave energy utilization systems based on big data analysis and artificial intelligence technology.

The explicit methodologies employed in the process of the optimization of sea wind and wave energy utilization are selection of appropriate topology and control strategy, energy storage medium, and energy storage structure; and the use of various platforms and means such as drones, meteorological towers, and remote sensing, non-linear vibration, bi-stable state, chaos, and other technologies, and advanced control methods such as predictive control and event-triggered control. These key technologies can guide the siting, design, control, and operation of offshore wind and wave energy utilization systems and evaluate their performance under different ocean-atmosphere boundary layer conditions. They can also help offshore wind energy and wave energy utilization systems improve their economy and reliability, reduce their costs and risks, and enhance their flexibility and compatibility. They can promote the innovation and development of offshore wind energy and wave energy utilization technology and enhance its important role and value in future energy transformation and low-carbon development.