Current Status and Future Trends in Installation, Operation and Maintenance of Offshore Floating Wind Turbines

Abstract

The installation and operation phases are critical stages in the lifecycle of offshore wind turbines, with costs associated with the installation and maintenance of floating wind turbines accounting for approximately 50% of the total investment. This paper presents the latest advancements in the technologies for the installation and maintenance of floating wind turbines. First, it discusses the installation techniques and relevant research related to the foundations and components of floating wind turbines. Next, it explores various operational strategies for offshore wind turbines and studies on major component replacements. The interrelationship of research in the installation and maintenance fields for floating wind turbines is examined. Furthermore, this paper investigates various tools and equipment used for the installation and maintenance of offshore wind turbines. It also addresses the relevant regulations and standards governing offshore operations for floating wind turbines. Finally, this paper provides a forward-looking perspective on the installation and maintenance of floating wind turbines.

1. Introduction

In 2023, the global offshore wind power sector added 10.8 GW of installed capacity, bringing the cumulative global offshore wind capacity to 75.2 GW by the end of the year [1]. The next decade will be critical for harnessing the full potential of offshore wind technology and accelerating the energy transition [2]. To achieve the 1.5 °C climate target, approximately 380 GW of offshore wind capacity will need to be installed by 2030, which is five times the current level [1].

Floating wind power is now transitioning into commercial-scale development. In 2023, the remaining four spar-type floating wind turbines at the Hywind Tampen project in Norway were successfully installed. Outside of Norway, a total of 13 MW capacity of floating wind turbines was put into operation globally, including the 2 MW DemoSATH floating turbine in Spain, Mingyang’s 7.25 MW anti-typhoon turbine in China, and Shanghai Electric’s 4 MW turbine [3].

Compared with onshore wind turbines, offshore wind turbines are generally closer to the electricity load centers and have less impact on human and ecological environments [4,5]. However, offshore wind farm development in nearshore areas faces challenges such as resource scarcity, stringent ecological constraints, and dispersed site locations. When water depths exceed 60 m, the economics of fixed-bottom offshore wind farms decline significantly [6]. Currently, onshore, intertidal, and nearshore wind farm developments around the world are nearing saturation, and 70–80% of global wind resources are located in waters deeper than 40 m. Figure 1 shows the annual average energy production (AEP) potential of offshore wind farms in different categories of depth of several high-production regions and countries. Therefore, the development of wind farms in deepwater and far offshore areas has become an inevitable trend [7,8,9].

The lifecycle of offshore wind turbines consists of several stages, with installation and operations and maintenance (O&M) accounting for approximately 50% of the total investment cost. As offshore wind power development increasingly targets deeper and more distant waters, the prevalence of floating wind turbines is rising. While this shift offers numerous benefits, it also means that the installation and maintenance environments will be more challenging, increasing operational difficulties. For instance, the Hywind Scotland wind farm will employ the tow-to-shore (T2S) method to tow turbines back to Wergeland Port in Norway for major repairs over a four-month period in the summer of 2024 [12]. Currently, large-scale maintenance activities for floating wind turbines typically involve towing them back to port, a method that is time-consuming and costly. In contrast with conventional power generation approaches, the levelized cost of energy (LCoE) in offshore wind power remains considerably high. The initial capital expenditure (CapEx) has been on a steady downward trend; nevertheless, the operational expenditure (OpEx) remains high. At present, up to one-third of the energy cost can be ascribed to maintenance costs. Future offshore wind farms will also be located farther from shore, in more remote areas, with higher installed capacities [13]. Clearly, towing turbines back to port for maintenance is not a practical long-term solution.

There is considerable overlap in the technologies and operational procedures for floating wind turbine installations and O&M throughout their lifecycle. For example, both installation and maintenance processes require assessment and forecasting of marine conditions, involve the participation of lifting vessels or tugboats, and may utilize similar tools and equipment. Therefore, integrating research from these two domains for a comprehensive analysis is beneficial. Technical issues related to the on-site installation and maintenance of floating wind turbines remain a priority for current and future research. To date, there is a lack of a comprehensive overview that combines the studies of installation and O&M activities for floating wind turbines.

This paper systematically reviews all current topics related to the installation and O&M of floating wind turbines. By providing a broad overview of research on the installation and O&M of floating offshore wind turbines (FOWTs), our review aims to reveal the latest advancements, identify emerging trends, highlight key challenges and limitations, and propose noteworthy future research areas.

This study is organized as follows:

• Section 2: provides a detailed overview of the installation methods and relevant research for different types of floating wind turbine foundations and superstructures.
• Section 3: elaborates on various operations and maintenance (O&M) strategies and methodologies for floating wind turbines.
• Section 4: explores the interconnections between installation and O&M activities for floating wind turbines.
• Section 5: presents the types and characteristics of tools and equipment used for the installation and O&M of floating wind turbines.
• Section 6: discusses the regulations, industry standards, and studies related to the operability of offshore operations for floating wind turbines.
• Section 7: examines future development trends in the fields of installation and O&M of floating wind turbines.
• Section 8: concludes the paper.

2. Installation Operations of Floating Wind Turbines

The installation process for offshore wind turbines is critical to the success of offshore wind farm projects. The installation costs of China’s completed “Three Gorges Leading” and “HaiZhuang FuYao” prototype account for 21% and 26%, respectively, of the total development costs [14]. Efficient and effective installation not only minimizes costs and maximizes energy generation efficiency but also promotes the sustainable development of offshore wind power, significantly contributing to global clean energy goals.

Floating wind turbines consist of a floating foundation and a superstructure. Based on the type of foundation, the installation of floating wind turbines can be categorized into two classes: integrated transport installation and on-site installation. Integrated installation involves assembling the floating wind turbine at the port and then transporting it to the designated location using a tugboat, followed by anchoring at sea [15]. The on-site installation of floating wind turbines includes the installation of the floating foundation and the lifting of the superstructure. The specific operational methods depend on the type of floating wind turbine foundation being used.

2.1. Installation of Floating Wind Turbine Foundations

In recent years, with the increasing size of offshore wind turbines and the movement of wind farms into deeper and farther offshore areas, floating wind turbine technology has become a hot research topic [16,17,18,19]. Generally, based on the principles of static stability, floating wind turbines can be categorized into four types: spar, tension leg platform (TLP), semi-submersible, and barge, as illustrated in Figure 2.

In recent studies and projects, spar, semi-submersible, and tension leg platform (TLP) foundations are the most commonly considered types.

Table 1. Floating offshore wind turbine floater types and characteristics [20,21,22,23].

	Barge	Semi-Submersible	Spar	TLP
Stability	Hydrostatic	Hydrostatic	Ballast	Mooring tension
Towability	Great	Excellent	Good	Poor
Installation	Simple	Simple	Simple restricted	Complex
Water depth	>~30 m	>~30–60 m	>~50–100 m	>~40–60 m
Application	Some	Widely	Widely	A few

summarizes the characteristics of floating foundations in terms of stability, towability, installation, water depth, and application.

2.1.1. Spar

The spar is a long cylindrical floating structure anchored to the seabed by catenary mooring lines. The design of the platform features a center of gravity significantly lower than the center of buoyancy, allowing the rebalancing moment generated between these two points to counteract tilting. Additionally, the small waterline area reduces the platform’s heave motion [23]. However, the spar platform’s considerable draft imposes certain requirements on the operating waters, typically necessitating depths greater than 100 m.

In 2009, the world’s first 2.3 MW spar floating wind turbine, Hywind Demo, successfully underwent a trial operation off the west coast of Norway [24,25]. Its installation process involved several key steps: First, the s

Par foundation was towed from Finland to Norway by a tugboat, where it was then flipped and ballasted to achieve the design draft. Subsequently, the assembly of the tower and rotor took place. After assembly, the entire turbine was towed to the designated location to connect to the mooring system, with final adjustments made through ballasting [26]. Eight years later, building on the successful experience of Hywind Demo [27], the Hywind Scotland floating offshore wind farm was constructed near the coast of Scotland [28]. The specific steps are illustrated in Figure 3. In this project, although a semi-submersible crane vessel was used for lifting operations during the installation of the tower and rotor, the overall installation steps were largely consistent with those of Hywind Demo. During the ballasting phase, a rock installation vessel was used to load solid ballast into the foundation while simultaneously unloading to maintain a stable draft.

During the installation process of floating wind turbines, traditional installation methods have become unsuitable due to the remote locations far from the coast. Consequently, the installation of the Hywind floating wind turbine referenced several innovative methods proposed in the past. For instance, it utilized barges with the ability to flip [31], employed reusable transport frames [32], incorporated vessels with loading and unloading systems [33], and opted for catamaran installation ships [34], along with floating docks or wind turbine shuttles [35]. Ultimately, a barge with flipping capabilities was selected [31,36]. This type of barge can load and transport pre-assembled turbine components horizontally, significantly enhancing the efficiency and reliability of offshore installations. Chen et al. [37] investigated the installation of spar platforms using the float-over method with twin-hull vessels, emphasizing the significant hydrodynamic interactions and gap resonance that occur during the operation. These complex hydrodynamic phenomena are critical to understanding the dynamic behavior of twin-hull vessels during installation, and similar studies on gap resonance between twin-hull vessels are further explored in Refs. [38,39,40,41,42].

In addition to the Hywind Demo and Hywind Scotland projects, several other initiatives have adopted spar platforms as the foundation for floating wind turbines, with specific project cases detailed in

Table 2. List of existing floating offshore wind projects with spar foundations.

Project Name	Country	Water Depth	Distance to Shore	Capacity	Commission	Decommission
Hywind Demo [27]	Norway	220 m	12 km	1 × 2.3 MW	2009	-
Hywind Scotland [28]	UK	95–120 m	25 km	5 × 6 MW	2017	-
Hywind Tampen [35]	Norway	260–300 m	140 km	11 × 8 MW	2022	-
Haenkaza/ Sakiyama [43]	Japan	100 m	5 km	1 × 2 MW	2013	-
Goto floating wind farm [44]	Japan	-	-	8 × 2.1 MW	2026	-
Fukushima Hamakaze [45]	Japan	48 m	23 km	1 × 5 MW	2016	2021
Kabashima/ Sakiyama [46]	Japan	76 m	1 km	1 × 2 MW	2013	-
TetraSpar demonstrator [47]	Norway	200 m	10 km	1 × 3.6 MW	2021	-

. These projects provide valuable experience for further validating the applicability and performance of spar platforms in different environments.

2.1.2. Semi-Submersible

Semi-submersible foundations typically consist of three or more buoyancy pontoons interconnected by structures above the waterline. When the turbine tilts, the distributed pontoon structure generates significant changes in the waterline area, creating a restoring moment that counteracts the platform’s tilting motion. During operation, the semi-submersible wind turbine relies on mooring chains for stabilization, with the geometric and material nonlinearities of the chains having a significant impact on both the mooring platform’s response and the overall behavior of the mooring system [48]. This type of platform is suitable for water depths greater than 40 m and offers a wide range of applicable depths. The platform exhibits moderate motion in all directions but is relatively sensitive to second-order forces from low-frequency waves. Semi-submersible platforms can be transported using the wet towing method, allowing for flexible deployment with relatively mature technology. Numerous concepts for semi-submersible platforms exist, such as the Ideol platform [49] and the V-shaped floating foundation [50].

The semi-submersible wind turbine foundation has a large waterline area, providing enhanced hydrodynamic stability. Liu et al. [51] reviewed popular semi-submersible floating offshore wind turbine (FOWT) designs, summarizing the advantages and disadvantages of various design concepts. A notable feature of semi-submersible offshore wind turbines is their superior towability, making installation, maintenance, and decommissioning simpler compared with other types of floating wind turbines. For instance, the WindFloat, which was commissioned in Portugal in 2011 [52], was assembled in a dry dock, and after that, it was towed to the site and connected to a pre-installed mooring system. Figure 4 illustrates the mooring installation process of the Haiyou Guanlan, where only three tugboats were utilized during transportation, with the connection to the mooring system assisted by three tugs. Chen et al. [53] analyzed the dynamic issues during the towing process of semi-submersible wind turbines, combining computational fluid dynamics (CFDs) with empirical formulas. Their study examined the dynamic response of the towing system under various operating conditions, providing valuable insights for practical engineering applications.

Currently, semi-submersible foundations are among the most widely used types of floating wind turbine foundations, having been extensively implemented in wind energy projects across countries such as China, Japan, the United Kingdom, France, and Norway. Specific project cases are detailed in

Table 3. List of existing floating offshore wind projects with semi-submersible foundations.

Project Name	Country	Water Depth	Distance to Shore	Capacity	Commission	Decommission
WindFloat 1 [54]	Portugal	45 m	5 km	1 × 2 MW	2011	2016
WindFloat Atlantic [54]	Portugal	85–100 m	20 km	3 × 8.4 MW	2019
Kincardine [54]	UK	60–80 m	15 km	5 × 9.5 MW	2021	-
Les Éoliennes Flottantes du Golfe du Lion [54]	France	70–100 m	16 km	3 × 10 MW	2024	-
Erebus [54]	UK	70 m	44 km	-	2027	-
Korea Floating Wind [54]	S Korea	250 m	80 km	-	2028	-
EOLINK demonstrator [55]	France	-	-	1 × 5 MW	2024	-
BLOW project [56]	Bulgaria	-	-	1 × 5 MW	2025	-
Fukushima Mirai [45]	Japan	120 m	23 km	1 × 2 MW	2013	2021
Fukushima Shimpuu [45]	Japan	32 m	23 km	1 × 7 MW	2015	2020
Yangxi Shapa III Demo [57]	China	30 m	28 km	1 × 5.5 MW	2021	-
FuYao prototype [58]	China	50–70 m	13 km	1 × 6.2 MW	2022	-
Haiyou Guanlan [59]	China	100+ m	136 km	1 × 7.25 MW	2023	-
New England Aqua Ventus [60]	US	60–110 m	3 km	2 × 6 MW	2024	-
Pentland FOWF [61]	UK	60–102 m	7.5 km	7 × 15 MW	2026	-
TwinHub demo [62]	UK	50–60 m	16 km	2 × 16 MW	2026	-

.

2.1.3. TLP

A tension leg platform (TLP) is a type of floating platform anchored to the seabed via mooring cables, relying on vertical tension in the mooring lines to balance the excess buoyancy of the structure, creating a “tightly stretched” configuration. TLP platforms exhibit excellent vertical motion performance and are suitable for environments with water depths greater than 40 m. Numerous design concepts for TLP platforms have been proposed in the literature [63,64,65,66]. Due to the complexity of TLP’s anchoring system and the cumbersome installation process, its commercial application is less common compared with spar and semi-submersible foundations. Nevertheless, research on TLP installation methods continues. Wybro et al. [67] suggested the use of a tensioning device with downpull lines to supplement the time-consuming depressurization process. The GICON TLP has also demonstrated a new installation procedure [68], as shown in Figure 5b. Although there have been some small-scale model tests, such as the Blue H [69], reports on the installation of commercial-scale TLP wind turbine platforms are scarce, and related experience remains limited [70,71].

Typically, the tension legs of a TLP platform are pre-deployed before the platform installation. The stability of the TLP platform during transportation and installation is a critical issue, depending on the design. Once the TLP platform is towed to the installation site, ballasting adjustments are performed to transition it from the initial free-floating state to the lock-off draft, after which the platform is secured to the pre-installed tendons. Finally, by removing ballast water from the platform, the tension legs are tightened. Figure 5a illustrates this installation method. Ongoing projects utilizing tension leg platforms (TLPs) are listed in

Table 4. List of existing floating offshore wind projects with TLP foundations.

Project Name	Country	Water Depth	Distance to Shore	Capacity	Commission
Provence Grand Large [72]	France	100 m	17 km	3 × 8.4 MW	2023
Bluewater TLP demonstrator [73]	Norway	200 m	-	1 × 6 MW	2024
PivotBuoy Project [74]	Spain	50 m	1 km	1 × 225 kW	2023
NextFloat Project [75]	France	-	-	1 × 6 MW	2025

.

2.1.4. Barge

Barge-type foundations are large flat structures that resemble barges floating on the water’s surface, utilizing buoyancy to counteract gravity, typically suitable for water depths greater than 30 m. This structural type is simple, easy to manufacture, and offers good stability. The most prominent feature of the barge-type platform is its central moonpool, which absorbs part of the wave loads, thereby reducing platform motion [76]. Similar to semi-submersible foundations, barge-type foundations are usually installed using the wet towing method. For instance, the Floatgen project [77] is France’s first floating wind energy project, aimed at demonstrating and validating the feasibility and commercial potential of floating wind power technology. This project is a collaboration between multiple companies and institutions, including Ideol, Bouygues Travaux Publics, and Centrale Nantes, and officially commenced in 2018. Located along the Atlantic coast near Le Croisic in France, the project involves pre-installing the Vestas V80-2.0 MW wind turbine tower, nacelle, and blades onto the barge-type foundation in port, before towing the floating foundation to the installation site for connection to the pre-arranged mooring system. Additionally, several other ongoing floating wind energy projects using barge-type foundations are detailed in

Table 5. List of existing floating offshore wind projects with TLP foundations.

Project Name	Country	Water Depth	Distance to Shore	Capacity	Commission
Floatgen [77]	France	33 m	22 km	1 × 2 MW	2018
Hibiki [78]	Japan	55 m	15 km	1 × 3 MW	2018
Eolmed [79]	France	55 m	16 km	3 × 10 MW	2024
DemoSATH [80]	Spain	85 m	3 km	1 × 2 MW	2023

.

2.2. On-Site Installation Operations of Floating Wind Turbines

In addition to integrated towing installation, floating wind turbines can also be installed using on-site methods at sea [81,82]. Wang et al. [83] categorized the on-site installation operations of floating wind turbines into integrated installation and component installation, analyzing the suitability of these methods for installing different numbers of offshore turbines. Zhu et al. [84] designed an invention patent for wind turbine installation, with Figure 6 illustrating the specific installation process. In Figure 6, number 60 refers to the wind turbine, number 43 refers to the inclined hydraulic installation system in the inclined push installation system, and number 51 refers to the midship pillar of the installation vessel. This method enhances the stability of the installation process, offering improvements in both efficiency and safety.

Offshore wind turbine installation methods are generally divided into integrated installation and split installation. Integrated installation involves assembling turbine components into a complete unit at the base and then transporting it using a specialized flat-top barge equipped with a transport vessel (capable of carrying 1–2 units at a time). Upon reaching the installation site, a crane vessel is used for the overall lifting operation. Split installation, on the other hand, may include using a jack-up crane vessel to insert the legs into the soil bearing layer, elevating the vessel above the water surface using a lifting system, or using a bottom-supported platform to submerge to the seabed through a ballasting system; alternatively, a floating crane vessel can be directly employed for split installation.

2.2.1. Integrated Lifting of Floating Wind Turbines

The integrated lifting operation of floating wind turbines involves three stages: dock assembly, commissioning and loading, and offshore transportation. At the port or dock, land-based lifting equipment is used for the assembly and testing of turbine components. Once assembled, the entire turbine is lifted onto a specialized transport vessel and secured using professional fastening equipment. Finally, a large crane vessel is used to lift the turbine onto its foundation.

The integrated installation method offers advantages such as shorter offshore operation times and reduced high-altitude work at sea, making it particularly suitable for turbine installation in deep sediment conditions. However, it has high demands for lifting equipment, and costs are constrained by vessel resources, typically making it slightly more expensive than split installation. Additionally, its feasibility in deep waters with poor sea conditions is lower. This method requires a well-equipped assembly base, transport vessels, and crane vessels, along with specialized installation techniques and operators. Although the application of integrated lifting in floating wind turbine projects is limited, it has been widely used in fixed wind turbine projects, such as the Beatrice Offshore Wind Farm in the UK, Donghai Bridge Wind Farm, and Xiangshui Wind Farm in China.

Several scholars have researched the integrated lifting method. For instance, Chen et al. [85] proposed a new integrated installation method based on a novel vessel design capable of simultaneously transporting four complete wind turbines in a single trip, aiming to study the motion control of offshore wind turbines during the installation process. Jiang et al. [34] investigated the motion responses during the integrated lifting process of floating wind turbines using a catamaran installation vessel, analyzing the relative motion between the vessel and the turbine, and the docking points between the turbine and the foundation. Ren et al. [86] proposed using a catamaran installation vessel for the integrated lifting of floating wind turbines, introducing a hydraulic active heave compensation control algorithm to mitigate relative motion at the docking points. Hong et al. [87,88] conducted a detailed study on the integrated lifting method, covering multi-turbine systems, mooring systems, and multi-body coupling with foundations. Liu et al. [89] compared the motion responses of two dynamically positioned installation vessels during the integrated lifting of floating wind turbines.

2.2.2. Split Installation of Floating Wind Turbines

The split installation technique involves transporting the various components of wind turbines (such as the tower, hub, main engine, and blades) to the installation site using transport vessels, followed by their assembly on-site. Compared with the conventional installation method, the split installation approach is more suitable for harsh marine conditions and imposes lower requirements on the vessel and equipment.

Currently, 15 MW wind turbine projects that have been implemented or are set to be implemented include the 1.1 GW project at Inch Cape in Scotland [90], the Empire Wind 1 and Empire Wind 2 projects in New York [91], the MunmuBaram offshore wind project in the Republic of Korea [92], the Atlantic Coast offshore wind project in New Jersey [93], and the He Dreiht offshore wind project in Germany [94].

Table 6. List of the specifications of offshore wind turbines.

Wind Turbine Model	Country	Year	Blade Length (m)	Hub Height (m)	Weight (Tons)
NREL 5 MW [95]	USA	2009	61.5	90	17.3
Sevion 5 MW [96]	Germany	2010	61.5	-	20.8
Siemens SWT 6 MW [97]	Germany	2013	75	-	-
FuYao prototype 6.2 MW [58]	China	2022	74	96	-
Haiyou Guanlan 7.25 MW [59]	China	2023	76.6	102	-
LEANWIND 8 MW [98]	Ireland	2016	80	110	35
DTU 10 MW [99]	Denmark	2013	86.4	119	41.7
MHI Vestas V164 10 MW [100]	France	2022	80	105	35
IEA 15 MW [101]	USA	2020	117	150	65.3
NREL 18 MW [102]	USA	2020	125	156	-
IEA 22 MW [103]	Denmark	2024	137.8	170	82.301

lists the key parameters of blades of representative OWTs. Figure 7 illustrates the current trends in blade length and hub height of wind turbines.

As the size and weight of the upper components of wind turbines continue to increase, the traditional installation operations will require larger vessels with greater lifting capacities, thereby highlighting the advantages of split installation for floating wind turbines.

Ahn et al. [104] proposed six different methods for offshore wind turbine split installation, as shown in Figure 8. Uraz et al. [105] differentiated these methods based on the characteristics of the installation site and the types of installation vessels. Considering the installation windows and associated costs, the most commonly used methods in Figure 8 are the third and fourth. The first three installation methods require single blade lifting, where the crane raises the blade to the installation height and maintains this position throughout the installation process [106]. In the single blade installation method, blades can be installed at horizontal, vertical, or inclined angles. Since blades are typically transported horizontally on the deck of the vessel, installing them vertically requires rotation, making this method relatively complex. Installing blades at an inclined angle necessitates lifting them above the hub height, which places higher demands on the installation equipment, especially in terms of lifting height. Overall, horizontal installation of blades is the optimal choice.

As the size of wind turbines continues to increase, the dimensions and installation heights of the blades also rise, resulting in significant movements at the installation height and imposing higher precision requirements for blade docking. To address these challenges, researchers have conducted extensive studies on blade motion control, docking precision, and aerodynamics.

Aerodynamic issues are critical in designing the loads that blades experience during lifting and docking. For actual single-blade installation operations, numerical simulation analysis of the lifting process is necessary. Gaunaa et al. [107] examined the accuracy of the cross-flow principle in calculating the aerodynamic loads on wind turbine blades and proposed a correction model that aligns the results for the DTU-10 MW blade under cross-flow conditions with CFD. Gaunaa et al. [108] also investigated the first-order aerodynamic and aeroelastic issues of blades in single-blade installation systems, presenting a simple engineering model and validating it against results from the aeroelastic code HAWC2 [109,110]. Yin et al. [111] established a numerical model that integrates mechanical, hydrodynamic, aerodynamic, and control analyses to examine the coupling effects during the lifting process of single blades on jack-up crane vessels.

The most hazardous phase in the single-blade lifting operation occurs during the docking of the blade with the hub, as shown in Figure 9. As the length and dimensions of the blades increase, their stiffness decreases. During high-altitude docking, if impacts on the docking bolt holes cannot be effectively mitigated, it may lead to damage at the blade root, resulting in substantial economic losses. This issue requires further investigation and resolution.

Zhao et al. [112] applied his self-developed wind turbine single-blade installation simulation tool, SIMO-Aero, to study the motion response of the DTU-10 MW blade during the single-blade installation process, as shown in Figure 10. Ren et al. [113] developed a framework for blade installation simulation, which models components such as the hook, blade, sling, lift wire, and tugger line, effectively simulating the installation process. Idres et al. [114] and colleagues created a nonlinear coupled dynamics model for cranes and vessels, incorporating six degrees of freedom for the vessel. Jiang et al. [115] simulated the relative planar motion between the blade root and the hub during the single-blade installation process. Verma et al. [116] and others established a collision model for the blade root and hub during installation, investigating the impact issues between the guide pin and the hub.

Similarly, achieving stable alignment during the installation of large offshore wind turbine blades in high-altitude environments with significant wind loads is one of the pressing challenges that require further investigation. To address this issue, researchers both domestically and internationally have conducted extensive studies on blade motion control and the stability of the lifting boom during the installation of large wind turbine blades.

Fang et al. [117] proposed a nonlinear controller applied to crane lifting operations, demonstrating that this control method significantly improves lifting accuracy. Mansoud et al. [118] introduced a feedback control algorithm capable of precisely positioning the payload, achieving favorable control results in experiments using a 1/24 scale model of a lifting vessel. Ren et al. [119] utilized extended Kalman filtering to estimate blade motion and wind speed, proposing an active control scheme for the blade docking phase and validating its effectiveness. Sander et al. [120] studied the single-blade installation process in the North Sea offshore wind farm, confirming the effectiveness of tuned mass damping systems. Brodersen et al. [121] investigated an active tuned mass damper and implemented it in HAWC2 simulations, finding that the active tuned damper required less mass to achieve high damping compared with passive tuned dampers. Jiang [122] proposed the use of tuned mass dampers (TMDs) during the blade docking phase to reduce relative motion between the blade root and the hub, demonstrating its effectiveness through simulations. Similarly, Chen et al. [123] investigated the collision issues of installation vessels during the installation process based on the heave–roll–pitch coupling impact model derived from the Cummins equation.

2.3. Summary

The installation of floating wind turbines is a complex engineering task with high technical requirements, involving various advanced technologies. Compared with fixed turbines, floating wind turbines offer many advantages but also face numerous challenges. Currently, spar and semi-submersible foundations are the two most widely used and are objects of research types for floating wind turbines. The on-site installation methods for floating wind turbines are primarily categorized into integral and split installations. As the size of floating wind turbines continues to increase, the integral installation method demands higher lifting heights and capacities from vessels, limiting its applicability, while the split installation method is increasingly showing its advantages. However, maintaining blade stability and safety during lifting and docking in the split installation process remains a key area for further research. As floating wind turbines trend toward larger sizes and deeper offshore locations, the working environment for installation becomes more challenging. Integrated transport and installation methods are gradually losing their advantages, and on-site installation will become the mainstream approach. The further optimization of installation processes and the enhancement in efficiency and reliability will be critical factors driving the development of floating wind turbine technology.

3. Operations and Maintenance of Floating Wind Turbines

To reduce the levelized cost of energy (LCOE) for offshore wind turbines, floating wind turbines are expected to increase in both installed capacity and blade length, while their operational sites gradually move further offshore. Although this trend brings greater energy generation benefits, it is accompanied by higher failure rates and increased operation and maintenance (O&M) costs [124]. McMorland et al. [13] found that “assuming all variables are equal, operational expenditures (OpEx) typically halve when the offshore wind turbine installed capacity doubles”. However, this statement does not take into account practical limitations, such as increased supply chain competition and the rise in opportunity costs and downtime. Unlike fixed wind turbines, floating turbines face new challenges and constraints due to their buoyant nature. From an O&M perspective, the increasing distance from shore, harsher environmental conditions, and elevated blade lifting heights are critical considerations. Wang et al. [125], Henderson & Witcher [6], and Liu et al. [51] have all identified O&M as a key area for future research.

Due to prolonged exposure to harsh environments, the failure causes of floating wind turbines can generally be classified into two categories: one is the aging of electronic devices due to long-term operation [126], and the other is physical damage to turbine components. Continuous rotation of the rotor and drivetrain systems leads to wear and fatigue, which are common failure causes; some failures occur randomly and are difficult to predict. The most prone components are the turbine blades, with common issues including cracks, corrosion, and serious aeroelastic deflections [127,128,129]. Additionally, common gearbox and generator failures include wear, fatigue, overheating, and other thermal issues [130,131]. Various problems can also arise with the tower and nacelle [132,133,134,135]. O&M strategies vary depending on the type and severity of the failures. For issues such as wear or fatigue in the gearbox or generator, technicians are typically dispatched via service operation vessels to the turbine site for maintenance. In cases of significant damage requiring component replacement, large crane vessels may be sent to the site to install new turbine components, or towboats may be used to bring the floating turbine back to port or dry dock for repairs. For instance, the recent large-scale maintenance at the Hywind Scotland wind farm involved towing the floating turbines back to shore for inspection.

Different types of failures and damage levels necessitate tailored O&M strategies and methodologies. The subsequent sections will elaborate on various O&M strategies and the on-site replacement operations for major component failures.

3.1. Maintenance Strategies

In response to the various failure causes identified for floating wind turbines, multiple maintenance strategies have been proposed and implemented. These include corrective maintenance strategies (also known as reactive maintenance strategies) [136], preventive maintenance strategies [137], opportunistic maintenance strategies [138], and predictive maintenance strategies [139]. Each maintenance strategy directly impacts the overall operational efficiency, profitability, and safety of offshore wind farms. By selecting appropriate and effective maintenance strategies, significant reductions in downtime caused by equipment failures can be achieved [140,141].

3.1.1. Corrective Maintenance Strategy

Corrective maintenance strategy refers to the maintenance approach undertaken after a device has failed, allowing for targeted repairs. The advantage of this method lies in its ability to avoid unnecessary maintenance and inspections, thereby saving labor costs. However, the main drawback is the potential for extended downtime, which can adversely affect the normal operation of the wind farm. Consequently, corrective maintenance is suitable for small-scale offshore wind farms where the loss from turbine downtime is minimal, but this strategy is often inadvisable for large-scale offshore wind farms [136].

3.1.2. Preventive Maintenance Strategy

Preventive maintenance involves conducting regular inspections and replacing components before failures occur, thereby preventing minor issues from escalating into major failures and reducing downtime [142]. If a failure occurs between two maintenance intervals, the turbine will remain offline until the next scheduled maintenance. This strategy offers several advantages over corrective maintenance, e.g., (1) it provides sufficient weather windows for repairs; (2) it allows for better scheduling of maintenance vessels; and (3) it lowers maintenance costs [143].

Preventive maintenance strategies can be planned in advance based on the lifespan, reliability, and maintenance costs of turbine components to select the optimal maintenance approach. Santos et al. [144] compared preventive maintenance strategies based on component lifespan with corrective maintenance strategies, showing that preventive maintenance can reduce the usage and replacement costs of large vessels; despite increased personnel costs, the overall costs decreased by 24.2%. Dui et al. [145] and Nejad et al. [146] proposed a preventive maintenance strategy based on component reliability with cost as a control condition. Additionally, many researchers have further optimized preventive maintenance strategies using reliability or maintenance costs as decision-making criteria [147,148,149].

3.1.3. Opportunistic Maintenance Strategy

Opportunistic maintenance strategies were first introduced in the 1960s, yet a consensus on their definition has yet to be reached [150,151]. Typically, opportunistic maintenance combines preventive and corrective maintenance by conducting various maintenance activities on different turbine components within the same time frame [152]. This approach is also referred to as opportunistic preventive maintenance [138,153,154,155]. For instance, when a specific turbine component fails, corrective maintenance actions should be taken immediately; at the same time, opportunistic maintenance leverages this time window to perform preventive maintenance on other components. This strategy can significantly reduce preventive maintenance costs, potentially by as much as 43% [156].

Zhang et al. [138] proposed a method that combines opportunistic maintenance with reliability requirements, utilizing optimization techniques to determine the optimal maintenance schedule. Zhao et al. [157] introduced a state-based opportunistic maintenance strategy, where maintenance timing is decided by comparing component condition indices with maintenance thresholds. Li et al. [158] developed a novel opportunistic maintenance strategy by optimizing maintenance intervals for subsystems and demonstrated its effectiveness. Tao et al. [159] proposed a joint maintenance strategy that considers the concurrent opportunistic maintenance of two wind farms, showing through comparison with traditional strategies that this approach can lower costs. Su et al. [160] presented a framework for a joint opportunistic maintenance strategy that optimizes maintenance approaches based on the randomness of wind speed. Furthermore, Su et al. [161] optimized condition-based opportunistic maintenance strategies regarding maintenance intervals and resource allocation. Additionally, some researchers have also considered wind speed variability to propose innovative opportunistic maintenance strategies [162,163,164].

3.1.4. Predictive Maintenance Strategy

Due to the harsh environments in which floating wind turbines operate, equipment failures are frequent, with common causes including fatigue, corrosion, erosion, and wear. Among the three previously mentioned maintenance strategies, only corrective maintenance allows for accurate fault assessment prior to implementation; both preventive and opportunistic maintenance lack this capability. As a result, predictive maintenance strategies have emerged. This approach minimizes turbine downtime by integrating real-time data from condition monitoring systems (CMSs) with fault analysis results, thereby reducing maintenance frequency and significantly enhancing equipment reliability, ensuring that maintenance is conducted only when necessary.

In a CMS, sensors play a critical role, enabling technicians to monitor the operational status of offshore turbines in real time, particularly focusing on the health of key components such as bearings [165], gearboxes [166], generators [167], energy conversion systems [168], and drive systems [169]. These sensors facilitate feature extraction and the timely detection of potential faults [170,171,172].

Asensio et al. [173] validated the effectiveness of predictive maintenance strategies from the perspective of the total lifecycle cost of the monitoring system. Walgern et al. [174] found that a combined strategy integrating predictive maintenance with weekly scheduled maintenance was the most cost-effective option when compared with corrective and preventive strategies. Kang et al. [175] proposed a novel algorithm to evaluate the failure rates of turbine components and optimized the condition-driven maintenance strategy based on this failure rate.

3.1.5. Summary

A summary of different maintenance strategies is tabulated in

Table 7. Comparison among different maintenance strategies.

	Brief Introduction	Advantages	Disadvantages
Corrective maintenance	Repairs after a failure occurs	Targeted repairs, avoiding unnecessary maintenance	Can lead to longer downtime
Preventive maintenance	Regular inspections and part replacements before failure	Provides sufficient maintenance windows, reduces costs	May result in longer downtime
Opportunistic maintenance	Conducts preventive maintenance during corrective actions	Efficient scheduling, combining benefits of both types	Unable to predict specific turbine faults in advance
Predictive maintenance	Real-time monitoring of turbine status to detect potential faults	Minimizes downtime and reduces maintenance frequency	High technical difficulty

.

3.2. On-Site Operation and Maintenance of Floating Wind Turbines

As water depths and offshore distances increase, the operation and maintenance (O&M) of floating wind turbines have become more complex. Traditional major component replacement vessels [104] or the tow-to-shore (T2S) [176] technique are becoming less applicable. The T2S method is typically suited only for shallow-draft structures and requires the floating platform to remain stable without mooring, such as semi-submersible platforms. Furthermore, the feasibility of this method is constrained by port facilities, which must have adequate water depth and related equipment to support T2S operations. Consequently, as the O&M locations for floating wind turbines gradually extend into deeper waters, efficiently conducting offshore maintenance and repairs has emerged as a new challenge.

In response to this challenge, various potential solutions have been proposed within the industry, such as utilizing large floating crane vessels for on-site operations or towing floating wind turbines to shallow waters suitable for heavy lift vessels (HLVs) and jack-up vessels (JUVs). Figure 11 illustrates the on-site maintenance approach implemented at the Kincardine offshore wind farm, marking the world’s first instance of major component replacement in a floating offshore wind farm. This operation was completed by an offshore support vessel, which eliminated the need to tow the turbine back to port. Instead, a temporary tower crane was installed on top of the turbine to remove the old generator and install a new one. This method effectively replaces traditional tow-to-shore (T2S) operations and provides a significant reference for O&M activities in floating wind turbines, promising to drive further innovation in the industry [177]. However, most European ports currently lack the facilities needed to meet the installation and maintenance demands of large floating wind turbines. Brons-Illing [178] compared the costs of on-site and onshore O&M methods for wind farms at different offshore distances, revealing that as the distance increases, the economic advantages of on-site O&M become more pronounced. Thus, effectively conducting on-site maintenance operations in harsh marine environments has become a critical issue requiring urgent research in the floating wind turbine maintenance sector.

On-site maintenance of floating wind turbines includes routine maintenance and major component replacement. Routine maintenance involves specialized personnel traveling to offshore wind farms aboard dedicated service operation vessels, using wave compensation gangways and other docking devices to access the turbine internals for repairs. Li et al. [179] studied the operability of walk-to-work operations for floating wind turbines and service operation vessels through numerical simulations, focusing on the limiting wave height. Special attention was given to the impact of second-order drift motion, RAO fluctuations, shielding effects, and the effect of drift motion on the failure mode of operations. Li et al. [180] studied the hydrodynamic interaction between a semi-submersible floating wind turbine and an offshore support vessel during walk-to-work operations. Similarly, Li et al. [181] investigated the impact of motion response from offshore wind farm service vessels with DP dynamic positioning systems on the active compensation of gangways. Major component replacements necessitate the involvement of large crane vessels due to the lifting operations required.

Such research not only encompasses technical innovations and optimizations but also considers the synergistic development of O&M costs, equipment resources, and infrastructure. Particularly in Europe, enhancing port capacity and related infrastructure will directly impact the future efficiency and economic viability of floating wind turbine O&M.

For routine maintenance, selecting the appropriate transportation method is crucial, as it can enhance accessibility and minimize additional downtime costs. Depending on the nature of the maintenance work, various transportation modes are typically employed to transport personnel and components, including helicopters, supply vessels, and multipurpose boats. However, environmental factors such as wind speed, wave height, and visibility significantly influence the safety and efficiency of helicopter or vessel access to the wind farm [150,182]. If maintenance activities are delayed, turbine downtime will extend, leading to increased costs and substantial economic losses. Therefore, accurate sea condition measurements and forecasts are essential. Many researchers [183,184,185,186] have developed effective models for monitoring and predicting environmental factors such as wind speed and wave height to assist maintenance decision making. Shi et al. [187] proposed a method of mooring floating wind turbines to floating crane vessels to enhance operational stability. Numerical simulations confirmed that this approach significantly improves stability, providing a practical solution for stabilizing O&M activities in floating wind farms.

In cases involving major component replacement, large crane vessels are used to remove damaged turbine components and install new ones. Compared with onshore lifting operations, offshore lifting is significantly affected by wave and wind conditions, increasing operational difficulty. To address this challenge, some scholars have developed new processes and devices. For instance, certain studies have proposed built-in lifting devices specifically designed for gearbox replacements, which can meet replacement requirements while reducing maintenance costs [188]. Additionally, other researchers [116,189,190] have studied the weather windows and operational limits for major component replacements, further optimizing the feasibility and safety of offshore maintenance operations. Through appropriate transportation choices and major component replacement strategies, the maintenance efficiency of offshore wind farms can be significantly improved, reducing downtime caused by environmental factors and lowering maintenance costs. These studies and innovations are critical in addressing the challenges of turbine maintenance in adverse sea conditions.

3.3. Intelligent Operation and Maintenance of Floating Wind Turbines

Floating wind farms in deep offshore environments face harsher conditions than onshore and nearshore installations, resulting in significantly higher failure rates and more complex failure causes, along with longer accident response times [191,192]. To address these challenges, a maintenance strategy focused on condition monitoring and fault diagnosis, supplemented by periodic inspections, is essential. By analyzing online monitoring data and historical records of potential faults, more intelligent O&M solutions can be developed.

Intelligent early warning systems can alert operators to potential failures in critical turbine components, allowing for better scheduling of major maintenance and replacement operations, thereby reducing offshore construction time and O&M costs. For instance, Anaya-Lara et al. [193] utilized SCADA systems for the remote monitoring of wind farms, enabling the real-time tracking of equipment status. Helsen et al. [194] employed big data techniques to analyze sensor data from turbines to optimize O&M strategies. Wu et al. [195] proposed an artificial intelligence-based method for optimizing wind turbine layouts to enhance O&M efficiency. Lin et al. [196] developed a deep learning neural network based on SCADA systems to efficiently predict the state of wind farms, reducing computational time costs. Yin et al. [197] combined convolutional neural networks (CNNs) with long short-term memory (LSTM) networks to create a novel deep neural learning (DNL) architecture, enhancing fault prediction accuracy in offshore wind farms.

To ensure smooth O&M operations for floating wind turbines, it is crucial not only to monitor turbine performance but also to track and predict local weather conditions, wind speeds, and wave heights to determine optimal operation windows. Trombe et al. [198] and Nicolaos et al. [199] monitored weather conditions at the Horns Rev and Horns Rev2 offshore wind farms to optimize management strategies. Japar et al. [200] employed five different machine learning methods to estimate power losses due to wake effects, enhancing short-term forecasting accuracy for wind energy. Chao et al. [201] introduced a sequential Markov chain Monte Carlo (MCMC) model that accounts for the time correlation between weather conditions and component repair processes in reliability assessments. Brusch et al. [202] analyzed satellite imagery to predict severe weather, providing reliable support for offshore wind farm operations.

Moreover, the safety and efficiency of O&M operations largely depend on the selection of vessel types [203,204]. Choosing the appropriate vessel types and fleet combinations for various O&M tasks can enhance safety while effectively reducing downtime and costs. Gundegjerde et al. [205] proposed a three-phase stochastic programming model to determine the optimal fleet size and vessel type combinations for improved O&M efficiency. Stamatlhane et al. [206] designed a two-stage stochastic programming model to optimize vessel selection based on weather conditions and failure occurrence times. Li et al. [207] introduced a discrete event simulation-based model to study the optimal fleet size and composition under opportunistic maintenance strategies, aiming to minimize vessel costs. Gutierrez-Alcoba et al. [208] developed a heuristic method for simulating fleet optimization during actual scheduling processes, aiding in more accurate cost estimates for O&M. By thoroughly investigating weather conditions, vessel types, and fleet combinations, the O&M efficiency of deep offshore floating wind farms can be significantly enhanced, resulting in lower operational costs and improved overall reliability and economic viability.

3.4. Summary

The operation and maintenance of offshore wind turbines are crucial for the development of offshore wind farms. As floating wind farms gradually expand into deeper waters, they face more complex environmental conditions, presenting significant challenges for O&M strategies. While the traditional tow-to-shore (T2S) method has some applicability in nearshore settings, its use for deep offshore floating wind farms will likely lead to substantial increases in vessel and downtime costs due to greater distances. Consequently, a gradual shift toward on-site maintenance strategies is anticipated, which can reduce costs and shorten repair response times. In particular, traditional O&M strategies may prove inadequate in addressing the diverse failures faced by floating wind turbines under complex conditions. Future O&M strategies will increasingly focus on condition monitoring and fault diagnosis, supplemented by periodic inspections. By employing advanced monitoring technologies to track equipment health in real time and integrating historical data for fault prediction and condition assessment, more intelligent and effective O&M solutions can be developed that are especially suited for deep offshore floating wind farms.

4. The Relationship Between Installation and Maintenance of Floating Wind Turbines

The installation and maintenance of floating wind turbines share several similarities, allowing insights from installation research to inform maintenance practices. One installation method involves assembling turbine components onshore and then transporting the entire assembly to the installation site via towing. Similarly, during maintenance, turbines can be towed back to shore for repairs. Another installation approach entails transporting individual components to the site for assembly; similarly, maintenance can involve on-site replacement of major components by disassembling damaged parts and installing new ones. Although the processes of assembly and disassembly can be viewed as reverse operations, they are not entirely equivalent.

As floating wind turbines increasingly develop into larger models and extend into deeper waters, future installation methods will likely focus on on-site modular lifting, particularly single-blade lifting techniques. Concurrently, maintenance will also depend more on on-site operations. Both installation and maintenance require monitoring and forecasting of weather conditions, such as wind speed and wave height, to determine operational weather windows, ensuring the safety and effectiveness of the tasks.

However, significant differences exist between the installation and maintenance of floating wind turbines. Maintenance requires not only monitoring weather and sea conditions but also real-time tracking of the turbine’s overall performance and the condition of critical components to predict potential failures. This proactive approach enables the formulation of maintenance strategies before issues arise. Maintenance is a long-term complex task that spans the entire lifecycle of the turbine. Various types of failures may occur during operation that do not manifest during installation.

Therefore, while similarities exist in the replacement of key components, directly applying installation research findings to maintenance practices has limitations and in-depth research into maintenance technologies will be an essential trend for the future.

5. Equipment for Installation and Maintenance of Floating Wind Turbines

Currently, the global floating wind power industry aims to reach a capacity of 270 GW by 2025, which translates to the installation of 20,000 large floating wind turbines, each with a capacity of 10 MW or more, in the coming years. Alongside this ambitious development goal, the industry faces significant operational and maintenance (O&M) demands. However, the rapid growth of the floating wind power sector starkly contrasts with the shortage of specialized installation vessels. The installation and maintenance of floating wind turbines require not only large crane vessels and specialized operational equipment but also auxiliary tools and intelligent maintenance equipment, such as O&M robots, to facilitate the automation and smart management of the entire maintenance process. In this section, we will introduce various commonly used tools and equipment for installation and maintenance operations. These include the following:

5.1. Installation and Maintenance Vessels

The involvement of large vessels is crucial in the installation and operation maintenance (O&M) processes of floating wind turbines. Compared with traditional fixed wind turbines, the installation and O&M of floating wind turbines present unique challenges, particularly in offshore environments. The deployment of floating wind turbines necessitates vessels with robust lifting capabilities, capable of operating in various water depths to transport and install large floating turbines and their components. Common vessels used for offshore installation and maintenance operations are shown in Figure 12, where (a) is the service operation vessels, (b) is the tugboat, (c) is the heavy lift vessel, (d) is the jack-up crane vessel, and (e) is the semi-submersible crane vessel.

Vessel selection should be based on factors such as the type of operational task, sea conditions at the work site, market availability, and budget constraints. Tugboats, lacking lifting capabilities, are primarily used for towing floating turbine foundations or for overall tow-in installation. Jack-up crane vessels can elevate their hulls above the water using adjustable legs, creating a stable working platform suitable for nearshore projects. However, their operational depth is limited to a maximum of 70 m due to leg length, and their slow movement results in longer repositioning times, making them unsuitable for deep-sea areas [213,214]. In contrast, floating crane vessels offer high mobility, allowing flexible operations across varying water depths; however, their stability is inferior, and safety and precision diminish significantly in adverse sea conditions [215]. Semi-submersible crane vessels combine the advantages of jack-up and floating crane vessels, featuring high stability, substantial lifting capacity, and operational flexibility, although they are complex to operate and come with high rental costs [216]. The trend in daily charter rates for crane vessels shows a sudden spike around a lifting height of 85 m [217]. These studies [178,218,219,220,221] provide detailed information on vessel rental costs. The daily rate for tugboats ranges from USD 1000 to USD 5000 [104]. Crane barges have daily rates between USD 80,000 and USD 100,000 [222,223,224], while jack-up crane vessels range from USD 100,000 to USD 250,000, and semi-submersible crane vessels from USD 280,000 to USD 500,000 [104,225]. A brief overview of the advantages and disadvantages of representative vessels, along with their daily charter rates is summarized in

Table 8. Comparison between different vessels.

Type	Advantages	Disadvantages	Day Rate
Tugboat	High maneuverability, versatile	Slow speed, no lifting capability	USD 1000–5000
Jack-up crane	Stability, reduced environment impact, self-elevating	Limited mobility, shallow operational depth, slow transfer speed	USD 100,000–250,000
Heavy lift vessel	High mobility, flexibility, effective in various water depths	Weather sensitivity, limited stability	USD 80,000–100,000
Semi-submersible crane vessel	Stability, heavy lifting capacity, versatility, dynamic positioning capability	High operational cost, limited mobility	USD 280,000–500,000

.

According to the Global Wind Energy Council (GWEC) update of September 2023 for the Global Offshore Wind Turbine Installation Vessel (WTIV) database, the majority of jack-up and heavy-lift vessels are operated by China and Europe [3]. As shown in Figure 13, there are currently 194 specialized installation vessels globally, with an additional 54 under construction or in planning. As the installed capacity of offshore wind turbines continues to grow, the weight of various turbine components is also increasing. The nacelle of a 10–15 MW offshore turbine weighs between 500 and 800 tons, while the tower for a 14 MW turbine weighs approximately 2000 tons, with nacelle heights for 8–14 MW turbines ranging from 109 to 150 m. Consequently, the lifting capacities and height requirements for installation vessels have also escalated. Current lifting capacities of installation vessels are illustrated in Figure 14. In Figure 13 and Figure 14, “Asia ex China” refers to all countries in Asia except China. Among existing jack-up vessels, only 4 meet the 3000-ton lifting requirement, and 23 can lift over 2000 tons, with only three vessels capable of installing 14 MW turbines [226]. As the operational water depths for floating wind turbines increase, the limitations of jack-up vessels become increasingly apparent, rendering them inadequate for deep-sea operations. Therefore, there is an urgent need to develop specialized installation and maintenance vessels with greater lifting capacities and adaptability to deeper waters to meet the rapidly evolving demands of the offshore wind industry.

5.2. Blade Lifting Equipment

The trend towards larger individual offshore wind turbines has become an irreversible aspect of offshore wind power development. The increase in turbine capacity corresponds to larger and heavier blades, making the development of blade lifting equipment compatible with high-capacity turbines an essential task.

Currently, single-blade lifting devices are among the most critical technical equipment in the blade installation process. Depending on the turbine drive type and lifting conditions, blade lifting methods can be categorized into horizontal, diagonal, and vertical orientations. Accordingly, lifting device types are primarily classified as rotating and horizontal [116,227], as illustrated in Figure 15.

Horizontal lifting devices typically consist of a lifting beam, slings, and structures or straps used to support the blade, featuring a relatively simple design. Since horizontal lifting devices generally lack rotational adjustment capabilities, the hub angle must be adjusted during lifting operations. Notable examples of horizontal lifting devices include the C-yoke-Basic developed by Eltronic [230] and Siemens’ Janett lifting yoke [231].

In contrast, rotating lifting devices are more complex, typically integrating clamping mechanisms, hydraulic rotation systems, and center of gravity adjustment systems. These devices allow for the adjustment of blade installation angles, significantly enhancing installation efficiency, though they generally have a higher cost and greater weight. Statistics indicate that using rotating lifting devices can save approximately 10 h of installation time per turbine [232]. Additionally, most current lifting devices have a maximum average operating wind speed of 12–15 m/s, with extreme wind speeds reaching up to 18 m/s. To ensure successful blade lifting and docking under higher wind conditions, a wind rope control system is often required. For example, the Blade Eagle lifting device is typically used in conjunction with the Tagline System developed by Liftra to ensure lifting stability.

Table 9. Comparison of key parameters for typical single-blade lifting devices.

Name	Manufacturer	Type	Rotation Range (°)	Max Wind Speed (m/s)	Features
LT1600 Blade Hawk [233]	Liftra	Horizontal	±5	12	Clamping structure; fine adjustment; weight 15 t, max load 30 t
LT975 Blade Dragon [229]	Liftra	Rotating	−215~35	12	Clamping structure; single-point suspension; max load 65 t
LT5061 Blade Eagle II [234]	Liftra	Rotating	−60~30	12	C-shaped; weight 160 t, max load 60 t
Simple C-yoke-Basic [230]	Eltronic	Horizontal	0	15	C-shaped; basic Eltronic C-series
SC-yoke [235]	Eltronic	Horizontal	±6	15	C-shaped; clamping; vertical lift with auxiliary crane
Janett lifting yoke [231]	Siemens	Horizontal	0	14	Clamping and sling; specific blades
Rotor Blade Clamp-D [236]	ema Tech	Rotating	±35	-	Hydraulic clamping pad; max load 50 t

presents a comparison of key parameters for typical single-blade lifting devices available in the market, highlighting differences in performance, wind speed adaptability, and structural complexity across various models.

In the installation process of wind turbine blades, both horizontal and rotating lifting devices have their advantages and play critical roles in different operational scenarios. Horizontal lifting devices are favored for their simplicity and relatively low cost, making them suitable for environments with limited budgets and favorable working conditions. However, their stability is compromised under poor sea conditions or high wind speeds, restricting their application to relatively calm offshore environments. While horizontal devices can successfully lift blades in these situations, their lack of rotational adjustment capabilities can lead to significant impacts from wind speed, complicating the docking process. In contrast, rotating blade lifting devices offer enhanced wind resistance. By adjusting the blade to an optimal wind-facing angle, these devices can effectively mitigate the effects of wind speed on the lifting process, thereby improving safety and stability. Additionally, rotating lifting devices can reduce adjustment times during blade docking, significantly boosting overall operational efficiency. Thus, they are particularly well-suited for high-precision large-scale projects, especially in complex sea conditions and high wind speed environments found in deep offshore wind farms. However, the complex structure of rotating lifting devices results in increased operational difficulty and higher costs, limiting their widespread use primarily to large wind power projects.

5.3. Maintenance Robots

Despite the numerous challenges posed by harsh marine environments for the operations and maintenance (O&M) of offshore wind farms, the use of repair robots and drones is gradually gaining traction. These technologies not only significantly reduce safety risks for personnel but also greatly enhance operational precision. Consequently, robotic systems have become a key component of O&M activities in offshore wind farms.

The underwater environment is complex and hazardous, and robots can operate in these conditions for more extended periods of time and at greater depths than humans. For example, BladeBUG [237] is a tracked robot designed to climb on turbine blades, scanning for surface defects and performing repair tasks. Meanwhile, the ROV II underwater vehicle is capable of underwater environmental exploration and executing other complex underwater operations. Figure 16 illustrates the BladeBUG and MIMRee projects.

The use of drones and autonomous vessels [239] is also steadily increasing, significantly enhancing O&M efficiency. Drones can monitor the status of turbine towers, nacelles, and blades during operation without requiring shutdowns, thereby further reducing downtime. Autonomous vessels are specifically designed to inspect the underwater portions of turbine foundations or equipment, such as the systems in the Watereye project [240], which can efficiently assess infrastructure. In harsh sea conditions where vessels cannot reach the work site, drones can be deployed for inspection or repair. For instance, the ongoing MIMRee project [241] is validating the feasibility of drones and autonomous vehicles in such scenarios. These technological advancements not only accelerate O&M activities and reduce costs but also allow access to areas that are difficult for humans to reach, such as high-risk environments like offshore wind turbine blades at great heights or submerged offshore wind turbine foundations. Moreover, the integration of advanced sensors and real-time data analysis allows for precise monitoring, quick identification of faults, and enhanced decision making. Furthermore, autonomous systems, through their increased collaboration, form an integrated O&M solution where drones can provide data and visual inspections, while robots, such as BladeBUG, can carry out physical repairs or maintenance tasks. This collaboration not only optimizes the use of technology but also significantly improves operational efficiency. As such, these systems lay the groundwork for effective, safer, and cost-efficient future operations in offshore wind farms.

6. Standards and Guidelines

The installation and maintenance operations of offshore wind turbines involve lifting components weighing from hundreds to thousands of tons, with lifting heights typically exceeding 100 m. Environmental conditions, such as ocean waves, significantly impact the operations of lifting vessels, increasing the risks associated with installation and maintenance. Therefore, key factors such as the operational window and the relative motion between lifted objects and docking points must be carefully considered [242]. Additionally, strict adherence to regulatory guidelines and industry standards is a core requirement for ensuring the safety of offshore wind turbine operations.

Table 10. Standards and guidelines for offshore wind turbine installation.

Content	Standards	Title
Environmental conditions	DNVGL-RP-C205	Environmental conditions and environmental loads
	DNVGL-RP-H103	Modeling and analysis of marine operations
	DNVGL-ST-0437	Loads and site conditions for wind turbines
Operation process	DNVGL-ST-N001	Planning and execution of marine operations
	DNV-OS-H204	Offshore installation operations
	ISO 29400:2015	Ships and marine technology, offshore wind energy, port and marine operations
	DNVGL-ST-0054	Transportation and installation of wind power plants
	DNVGL-OS-H205	Lifting operations
Crane	DNVGL-ST-001	Marine operations and marine warranty
	Noble Denton-0027	Guidelines for lifting operations by floating crane vessels
	DNVGL-RP-H201	Lifting appliances used in subsea operations

lists the standards related to offshore operations, categorized by their guidance content. For instance, DNVGL-RP-C205 [243] provides essential background on environmental conditions applicable in DNV offshore regulations, supplementing relevant national and international regulations. DNVGL-RP-H103 [244] simplifies formulas used to calculate loads during lifting and towing operations, although it is not specifically focused on offshore wind turbines. DNVGL-ST-0437 [245] offers guidance on loads and operational conditions for wind turbines and is often used in conjunction with offshore wind turbine design standards [246,247]. During the installation of complete units, it is crucial to strictly follow offshore operation standards to ensure that lifting operations are conducted within safe limits [248,249]. The updated DNVGL-ST-N001 [250] replaces the earlier DNV-OS-H series [251,252], establishing a unified standard for offshore lifting operations that stipulates safe distances between objects during lifting and provides detailed guidelines for offshore structures.

The DNVGL.ST-001 [253] guidelines require that the amplification factor (DAF) for lifting vessel loads be considered during offshore lifting analyses [254]. Furthermore, the Noble Denton guidelines [255] cover considerations for lifting slings and the lifting process in offshore operations. Other standards, such as DNVGL-RP-H201 [256], apply to underwater installation operations, while DNVGL-OS-H205 [254] provides detailed guidance for land, nearshore, and offshore lifting operations, both aerial and underwater. For jacket foundation installations, DNV-OS-H204 [257] specifies particular requirements and recommendations. ISO 29400:2015 [258] pertains to various offshore structures for port and offshore operations. DNVGL-ST-0054 [259], published in 2017, offers safety guidelines specifically for the transport and installation of onshore and offshore structures, covering the safe transport and installation of turbine components, foundations, and substations.

The classification services of the American Bureau of Shipping (ABS) have been expanded to include various vessels supporting offshore wind farms, such as wind turbine installation vessels (WTIVs), service operation vessels (SOVs), cable laying vessels (CLVs), and crew transfer vessels (CTVs) [260].

While existing standards and guidelines provide clear directions for certain engineering phases and scenarios, there remains room for further supplementation and improvement. In addition to these regulations and guidelines, many scholars, drawing on industry experience, safety protocols, and domain knowledge, have proposed research findings for assessing the operability and operational limits of offshore activities.

Collu et al. [261] combined the maritime commercial vessel safety regulations framework with advanced design standards from the oil and gas industry, along with stability research methodologies, to propose a new standard for stability safety assessment applicable to the towing of floating wind turbines. Wang et al. [262] analyzed the lifting of objects by crane vessels, identifying safe sea conditions and using the movements of vessels and lifted objects as evaluation criteria, while incorporating real wave data to assess the operability of offshore operations. Although various methods exist for evaluating allowable sea conditions for offshore operations, the core requirement is to ensure that the maximum response values meet safety standards. Additionally, EI Mouhandiz and Bokhorst [263] used numerical simulations and field monitoring to determine allowable sea conditions for float-to-install operations. Chen et al. [264] conducted an in-depth analysis of the complex dynamic responses between jacket structures and floating crane vessels during the installation process. Through extensive steady-state time-domain simulations, they employed extreme value distribution models to assess the peak responses at target probabilities. This approach provided critical insights into how varying wave parameters influence the dynamic behavior of the jacket structure, offering valuable guidance for offshore engineering applications. Guachamin-Acero et al. [265] proposed a general method that utilizes extreme sea conditions, such as significant height (Hs) and peak period (Tp) to evaluate structural responses and movements, which has been applied in various offshore operations [266,267,268,269]. Li et al. [270] conducted spectral analysis of response extremes based on the Rayleigh distribution for offshore maintenance operations, assessing operability in conjunction with real sea conditions. These studies enrich existing standards while providing a scientific basis for complex offshore wind operations, thereby promoting the ongoing development of the offshore wind industry. Chen et al. [215] investigated the lifting operations of jacket foundations on barges, conducting extreme value analysis based on the Gumbel distribution and performing extreme value forecasting analysis using real sea condition data.

7. Future Outlook

The trend of offshore wind power moving into deep waters is unstoppable, presenting various technical challenges for the installation and operation of floating wind turbines. Effectively reducing the installation and operational costs of deep-water wind power projects to ensure their economic viability remains a critical area for research. Based on the preceding discussions, several technology areas closely related to the installation and operation of floating wind turbines are expected to rapidly develop.

7.1. On-Site Installation and Operation Methods

Currently, the installation and operation of semi-submersible and spar-type floating wind turbines typically employ tow-to-shore (T2S) methods. After all turbine components are assembled in port, tugboats tow them to the operational site. If a floating turbine suffers damage or requires extensive repairs, it must also be towed back to port for maintenance. Considering factors such as towing costs, downtime losses, operational window constraints, and customs procedures, these methods are not optimal for actual operations. Future research should focus on on-site installation and operation methods for floating wind turbines, particularly the stability issues of floating crane vessels during on-site installation and maintenance operations.

7.2. Specialized Installation and Maintenance Vessels

As the size and capacity of floating wind turbines continue to grow, with future rotor diameters expected to exceed 280 m and hub heights surpassing 170 m, existing installation vessels cannot meet the requirements for operational water depths and lifting heights. Therefore, there is a need to build vessels with greater lifting capacities, higher lifting heights, wider deck spaces, and enhanced operational stability. Additionally, various new maintenance equipment suitable for multiple operational modes must be developed to adapt to the ongoing growth of the offshore wind power industry.

7.3. Intelligent Maintenance

As floating wind power expands into deeper waters, the failure rates of turbines are expected to rise significantly, with more complex failure causes leading to longer accident response times. To address these challenges, robots and drones will play a key role in driving the offshore wind industry towards greater intelligence and automation in future O&M operations. Intelligent maintenance solutions tailored to the unique operating environments and common failure causes of floating wind turbines need to be developed. Establishing intelligent warning systems for real-time monitoring and alerting of critical turbine components is essential. Efficient scheduling of maintenance and major component replacement plans, along with coordination among various maintenance phases, is also necessary. In situations where severe sea conditions prevent maintenance vessels and personnel from accessing the operational area, the future development of more sophisticated maintenance robots will be crucial for achieving remote intelligent turbine maintenance. These technologies will not only improve operational efficiency but also significantly reduce costs, driving the development of the offshore wind sector and supporting its long-term sustainability.

7.4. Floating Wind Turbine Operational Standards

The installation and operation of floating wind turbines, like other offshore operations, must adhere to industry norms and standards. However, comprehensive regulations covering the entire process of floating wind turbine installation and maintenance are not yet fully developed. Establishing detailed industry standards will help standardize operational processes and minimize safety risks. As floating wind turbine technology becomes a focal point in the industry, there is an urgent need to create dedicated standards for the installation and maintenance of floating wind turbines, unifying regulations for offshore wind farm construction and turbine maintenance processes. This will promote the standardization and regulation of operational procedures, thereby enhancing operational efficiency and safety while reducing overall costs.

8. Concluding Remarks

This paper provides a comprehensive review of the installation and operation technologies for floating wind turbines. It begins by outlining the development background of floating wind turbines, including installation vessels, maintenance equipment, installation methods, relevant international standards and guidelines, and current operational strategies. Subsequently, it reviews and summarizes existing technologies and academic research, exploring future development trends in the installation and operation of floating wind turbines. Finally, this paper looks forward to the field of floating wind turbine installation and maintenance. The main conclusions are as follows:

• Floating wind power breaks the limitations of traditional fixed wind turbines concerning water depth and offshore distance. With continuous technological advancement, it is regarded as the primary means for future deep-water offshore wind development. Although some projects have demonstrated significant potential, widespread application still faces numerous challenges. Traditional towing installation and T2S operation methods are susceptible to adverse weather and narrow operational windows, especially against the backdrop of increasingly larger floating wind turbines. The installation and operation of floating offshore wind farms are rapidly developing. Despite notable progress, achieving sustainable development remains a substantial research gap. Future studies should focus on enabling safer and more reliable on-site operational methods to reduce costs and enhance operational efficiency.
• Single-blade lifting technology has shown significant advantages in turbine installation and major component replacement, applicable to both fixed and floating wind turbines. Currently, installation primarily relies on jack-up vessels for static operations; however, the future focus will shift to dynamic operations involving floating crane vessels—specifically the floating-to-floating operational mode. To address higher wind speeds and complex sea conditions while minimizing manual intervention, various innovative processes and tools have been developed. Nevertheless, most technologies are still at a low maturity stage and require further development for practical application.
• As the operational areas of floating wind turbines gradually move into deeper waters, the harshness of the working environment and water depth demands higher safety and feasibility standards for installation and maintenance. There is an urgent need to develop new installation and maintenance vessels capable of maintaining operational stability under adverse conditions, adapting to deep-sea environments, and meeting the demands of larger turbines to ensure safe and efficient operational processes.
• Intelligent condition monitoring and digital operations can effectively enhance fault diagnosis and prevention capabilities of wind power equipment, thereby reducing maintenance costs. By employing digital monitoring and control systems, real-time monitoring and analysis of turbine conditions can promptly identify potential issues, preventing further damage or downtime incidents. Integrating intelligent maintenance strategies can significantly improve operational efficiency, avoid unnecessary blind repairs, and reduce long-term maintenance costs.

In the long term, floating wind power holds vast development potential and will undoubtedly become a crucial support for the energy transition in various countries. However, current cost issues remain prominent, and the technological maturity is still developing. Future research needs to concentrate on innovating installation and maintenance methods for floating wind turbines, addressing key technological challenges for on-site maintenance and installation to promote ongoing progress in this field.