Global growth in offshore wind turbine technology

Due to the commissioning of floating wind units, the latest technological developments, significant growth, and improvements in turbines, developments in offshore wind power capacity are estimated to increase faster than in the last two decades. The total installed offshore wind power capacity, which is currently 35 GW, is predicted to be approximately 382 GW by 2030 and approximately 2002 GW by 2050. For this reason, attempts are proposed to lower levelized cost of electricity (LCOE) for offshore wind power generation more than for other energy sources. In this study firstly, the global growth in the nominal capacity and size of offshore wind turbines over the last twenty years is examined. Then, the effects of this increase in nominal capacity and size on the LCOE, total installation cost (TIC), and turbine capacity factor are investigated. In parallel with this development, the changes in distance to shore and water depth for installation offshore wind power plants are reviewed according to the years. In addition, the effects of this global growth on wind farm capacity, turbine-specific power capacity, number of turbines per GW, and area needed per GW are investigated and discussed in detail.


Introduction
One of the biggest threats of this century is climate change, and it is already affecting many parts of the world significantly. Increasing greenhouse gas emissions from the energy sector cover two-thirds of the global total and are among the important and influential sources on the effects of climate change. Therefore, it becomes imperative to make technological breakthroughs toward clean, 1 3 sustainable, and renewable energy sources by making radical changes for energy technologies worldwide, driven by the need to end climate change and provide affordable energy sources (IRENA 2021a).
In recent years, limiting global average temperature increment to no more than 1.5 °C above pre-industrial levels and ensuring that all sectors of the economy achieve zero CO 2 emissions by the second half of this century have been one of the most pressing challenges in the world (IRENA 2020a). According to the Baseline Energy Scenario (BES), energy-related emissions are predicted to increase from 34 Gt in 2019 to 43 Gt by 2050, at a compound annual rate of 0.7%. This expectation will likely result in a temperature increase of 3 °C or more in the second half of the century (IRENA 2020b). According to the Planned Energy Scenario (PES), energy-related emissions are expected to increase slightly by 2030 and drop to around 33 Gt, which is today's level, by 2050. This expectation will likely lead to a global temperature increase of 2.5 °C in the second half of this century (IRENA 2020b). On the other hand, according to IRENA's Transforming Energy Scenario, energy-related emissions will fall to about 10 Gt or 70% less than today's level by 2050 at a compound rate of 3.8% per year, and the predicted temperature increase will be well below 2 °C (IRENA 2020b). Mainly, policies in force during the Paris Agreement in 2015, developments in the energy system based on governments' current energy plans and other planned policies and targets, largely renewable energy sources and steadily improved energy efficiency, and countries' climate and energy targets or plans have played a crucial role in determining these three scenarios. In this respect, renewable energy sources, energy efficiency, and electrification in energy consumption are vital in achieving these emissions reductions (IRENA 2019a).
With batteries and other enabling technologies, renewable energy technologies have proved effective and costeffective in every country for a growing range of applications. Today, renewable energy sources-whether for direct energy use or feedstocks-show more potential in the world than ever before. This makes them very important to achieve zero emissions (IRENA 2020a). Currently, renewable technologies already dominate the global energy market for new sustainable energy production. Among renewable energy sources, wind energy is the fastest-growing electricity production technology (Singh and Parida 2013). Wind is an increasingly affordable and cost-effective source of electricity in many markets, and it will be completely costcompetitive in the coming years. Many innovative solutions are developed to make the power system and grids more resilient, allowing for higher and more cost-effective use and diffusion of onshore and offshore wind energy systems (IRENA 2020c).
Offshore wind energy technology is one of the rapidly developing renewable energy systems in the last decade. With remarkable technology cost reductions, deliberate breakthroughs and advancements, and increased supply chain efficiencies, it has seen significant acquisitions in different markets, which has paved the way for further investment. Thanks to these features, offshore wind has become an attractive technology worldwide. Because offshore wind often allows higher and more consistent wind sources to be used, countries have developed large-scale wind farm projects near to densely populated coastal locations common in many places around the world. This capability makes a critical contribution to the existing position of low-carbon technologies to decarbonize the power sectors of many countries. By the end of 2020, a total of 35.3 GW of offshore wind power capacity had been established in the world, of which 90% was operated in the North Atlantic Ocean and North Sea. The UK, China, Germany, the Netherlands, Belgium, and Denmark are leading this setup. Thus, significant developments have occurred in offshore wind in the last decade and this development is expected to continue to play an essential role in future power generation systems (IRENA 2021b).
Much research has been done on offshore wind energy, and many publications have appeared on the subject (Argin et al. 2019;Emeksiz and Demirci 2019;Bosch et al. 2019;Tavares et al. 2020;Basset et al. 2021;Kılıç 2019;Tercan et al. 2020). However, limited studies have been conducted on the global growth in offshore wind turbine technologies. For example, Arshad and O'Kelly (2013) reviewed the offshore wind turbine structures in terms of geometric size and rated power generation capacity, cost, foundation options available, challenges, and attractions of wind power generation. Díaz and Soares (2020) aimed to determine the existing status of offshore wind farms worldwide and conducted an overview of critical types of equipment, including foundations, operators, turbines, etc. Cali et al. (2018) conducted the techno-economic feasibility analysis for offshore wind power plant projects in three of the Turkey's most promising wind locations. Soares-Ramos et al. (2020) analyzed the trends and status of existing offshore wind farms above 150 MW in Europe. In this work, the latest information and trends on wind turbine size and capacity, investment cost, turbine model, water depth, distance to shore, transmission technology, type of foundation, and voltage array systems were discussed. Lacal-Arántegui et al. (2018) analyzed cost reduction factors in installing foundations and turbines, which are shown as the most crucial cost elements of offshore wind farms. Igwemezie et al. (2019) reviewed the current offshore wind industry trend and material requirements for improving fatigue resistance in support structures of large wind turbines. Wu et al. (2019) reviewed the current state of knowledge regarding geotechnical and structural issues affecting the types of foundations considered for support structures of offshore wind turbines and made recommendations for future research and development. Li et al. (2020) reviewed the current status of offshore wind power technology, the development trend of offshore wind power, the complex marine environment, deep-sea power transmission, and expensive equipment installation costs faced by offshore wind conversion technology in the world. Jiang (2021) presented a review of the technical factors of the installation of offshore wind turbines with their state of the art. In this work, the type of offshore wind turbines, regulations and rules, structure vessels, and numerical modeling were firstly investigated. Then, various installation concepts and techniques and wind turbine foundations and components for floating and bottom-fixed wind turbines were critically examined.
Offshore wind energy technologies have developed tremendously globally in recent years, and their installation has increased. This development has resulted in a significant reduction in the levelized cost of electricity (LCOE) and the total installation cost (TIC) of turbines. Moreover, the evolution of large offshore wind farms has been leading to a revolution in the current scenario. This evolution also causes many research necessities for different participants of wind power projects such as turbine manufacturers, structures constructors, system planners and operators, and other participants. According to the literature study, socially and politically, offshore wind power plants have a negligible noise and visual impact with the distance to shore. However, the impact on marine ecosystems should be analyzed in more detail. In this study, for this reason, the development of offshore wind turbine technology was reviewed in detail. Detailed information on the global growth in offshore wind turbine technology and the key technological trends emerging in this energy system was provided. In offshore wind technology, deployed capacity, technological situation and outlook, anticipated future potential were reviewed. The novelty of this study is highlighted below: • To reveal the global growth in nominal capacity and size of offshore wind turbines over the two past decades, • To review the effects of this increase in nominal capacity and size on LCOE, TIC, and turbine capacity factors, • In parallel with this development, to determine the change of water depth and distance to the shore by years in the establishment of offshore wind power plants, • To examine and discuss the effects of global growth in offshore wind turbines' nominal capacity and size on wind farm capacity, turbine specific power capacity, the number of turbines per GW, and the area needed per GW.

Methodology
The primary purpose of this study is to examine and analyze the 162 offshore wind farms currently in operation worldwide, technically, technologically, and historically. For this, all offshore wind farms in the world, established from 1991, when the offshore wind farm was first established, to 2020, were identified. Then, the technological characteristics of these offshore wind farms and their turbines were determined as an annual weighted average. All necessary data for the characteristics of operational offshore wind farms and their turbines have been obtained in detail from the wind farm database 4C Offshore (2021), the Wind Power (2021), Ref (Díaz and Soares 2020;IRENA 2021c;Win-dEurope 2019, 2020GWEC 2021). Figure 1 shows some specifications of an offshore wind farm and turbine. Table 1 gives the offshore wind farms with a capacity of at least 500 MW. The following technical and technological data regarding offshore wind farms and turbines have been obtained and calculated over the years:

Global growth in installed power capacity
The first offshore wind power plant was established in Vindeby in eastern Denmark in 1991. The Vindeby wind farm consisted of 11 onshore turbines producing 450 kW each and a cumulative capacity of 5 MW. Since then, the Vindeby wind farm has become an influential pioneer in the European offshore wind industry. With this breakthrough, offshore wind power plants in Europe showed significant development. In the 2000s, the installation of offshore wind farms in the Southern North Sea, Baltic Sea, and the Irish Sea progressed rapidly since these seas have strong winds on average of over 8 m/s and relatively shallow water depths of less than 50 m. The cumulative installed offshore wind power capacity in the world increased to 67 MW in 2000. Offshore wind power installation has grown excessively after 2007 as shown in Figs. 2 and 3. In 2013, 1,567 MW offshore wind installations contributed to 11,159 MW of installed wind energy capacity. As of 2015, 3,230 turbines were installed in 84 offshore wind farms with a total power capacity of 11,027 MW in 11 European countries.
By the end of 2017, the global installed offshore wind power capacity increased to 20 GW. A 4.3 GW offshore wind farm was installed in 2018. In 2020, 90% of the global offshore wind market was represented by European companies. Fifteen new offshore wind farms started generating electricity, and nearly 6.1 GW of offshore wind capacity was installed in the world in 2020. Currently, 162 offshore wind farms are installed worldwide, and 26 offshore wind power plants are under construction. The total installed offshore wind capacity was 35.3 GW at the end of 2020. As shown in Fig. 4, in 2020, cumulative capacity reached 10.2 GW in the UK, which has the world's largest offshore wind industry with the lowest costs. In addition, China, Germany, the Netherlands, Belgium, and Denmark were the countries that contributed to the increase in global installed capacity, enabling the offshore wind market to grow remarkably. Outside of Europe, China added approximately 3 GW in 2020, bringing its total installed capacity to approximately 10 GW. However, according to the International Renewable Energy Agency (IRENA), offshore wind will need to increase tenfold by 2030 to support the necessary energy sector transformation and meet the Paris Agreement goals. The total installed offshore wind power capacity is predicted to be approximately 382 GW by 2030 and approximately 2,002 GW by 2050 (Fig. 5) (IRENA 2021a).

Turbine capacity and size analysis
Due to its open and smooth sea location, ability to generate GWs quickly, and high energy output per m 2 , offshore wind energy is a highly viable alternative for cost-effectively powering densely populated coastal areas. Thanks to advances in installation, foundations, access, operation and system integration, and turbine technologies, it has made it possible to move into deeper waters and further offshore to achieve locations with more significant energy capacity. Fixed-base offshore wind turbines are routinely installed at water depths of up to 40 m and in some cases up to 60 m and up to 80 km from shore. Unlike fixed-base offshore wind turbines, which are limited to shallow water depths, floating foundations allow for installation in waters deeper than 60 m. In the last ten years, offshore wind energy technology has attained significant maturity and has become the most advanced technology among renewable energy sources. As shown in Fig. 6, wind turbines' nominal capacity and size have historically increased due to continuous and conscious research and development (R & D) processes (Quest Floating Wind Energy 2021). With this growth, foundation and cabling costs have been reduced, while at the same time, costs have been reduced by increasing the energy captured per MW of nominal capacity. The 15-MW turbine, which has the highest wind turbine rating globally, is scheduled to be first tested in 2022 and go into power generation in 2024.
The wind turbine capacity has shown a great improvement over time. Figure 7 shows the growth of offshore wind turbine capacity between 2000 and 2020. When the world's first offshore wind power plant was built in Denmark in 1991, the turbine power capacity was just 450 kW. Since this installation, the offshore wind turbine size has grown significantly. The global average offshore wind turbine size reached 1.5 MW in 2000. In Europe in 2009, the average turbine power capacity of an offshore wind turbine was about 3 MW. The average turbine size was higher than 7.2 MW for new installations in Europe in 2019 (Bosch et al. 2019). Global weighted average nameplate capacity increased 150% from 3 MW in 2010 to 7.5 MW in 2020. Wind farms installed in 2020 had a 10% higher wind turbine power capacity.
Growth in rotor diameter is of great importance as it causes greater power extraction from wind turbines and thus generates more energy throughout the year. Figure 8 presents the change in the global weighted-average turbine size for offshore wind over time (IRENA 2021b). The figure shows that rotor diameter and hub height increased in parallel with higher turbine capacity values. Rotor diameters expanded from 112 to 157 m in the decade from 2010 to 2019, an increase of 40%. On the other hand, hub height increased  Capacity factors of the wind turbines in offshore wind power plants vary due to the technology used, wind farm configuration, and differences in meteorology between wind farm sites. Between 2010 and 2020, the global weighted average capacity factor of yearly installed offshore wind turbines varied between 35 and 45%. The capacity factor for newly established projects in 2020 ranged from 33 to 47%. Wind turbines with larger swept areas and higher hub heights can have more electricity collection capability and increase their capacity.
Global R&D activities in recent years have focused on manufacturing larger wind turbines, as evidenced by the practices of turbine manufacturers. For instance, the Danish wind turbine market Vestas investigates a larger offshore wind turbine with a nameplate capacity of 17 MW for future release and installation. Currently, the largest wind turbine size of about 9.5 MW, the projects to be commissioned in 2025 are expected to include turbines with a rating of 12 MW and above. Recent developments under consideration have shown that turbine sizes can reach 15 to 20 MW within ten years (IRENA 2019b).

Cost analysis
The changes in the global weighted average levelized cost of electricity (LCOE) and total installation cost (TIC) for offshore wind technology between 2010 and 2020 are shown in Fig. 9. As seen from the figure, the global weighted-average LCOE of newly installed offshore wind turbines decreased from USD 0.162/kWh in 2010 to USD 0.084/kWh in 2020, a decrease of 48% in 10 years. On the other hand, between 2007 and 2014, the LCOE increased as projects began to shift into deeper waters. For example, it peaked at 0.180 USD/kWh in 2007 and 0.179 USD/kWh in 2008, followed by a sharp decline after 2014. The LCOE for offshore wind in the world's leading countries has decreased sharply. For example, in 2020, the lowest weighted average LCOE value was obtained in China at USD 0.084/kWh.
The global weighted average TIC of offshore wind farms has increased from approximately USD 2592/kW in 2000 to over USD 5500/kW in 2008. Between 2008 and 2015, offshore wind farm installations were carried out in areas further from shore and into deeper waters, with a global weighted average TIC of around USD 5000/kW over that period. This cost started to decline after 2015 and fell relatively quickly to 3185 USD/kW in 2020. Installation costs of offshore wind technologies peaked in 2011-2012 due to projects being located in deeper waters, farther from shore, and using more advanced technology. In contrast with onshore wind installations, offshore wind farm installations became more costly and resulted in significantly longer lead times due to the difficulties of O&M installation in harsh marine environments. Also, the project development and planning for offshore wind power plants are more complex than for onshore projects. The fact that the construction is more and takes longer increases the TICs. Due to their offshore location, these projects also have higher construction costs and grid connectivity. However, effects such as standardization of turbine and foundation designs, increased industry maturity, industrialization of production for offshore wind farm components at local centers, and a decrease in the complexity of installation practices have resulted in cost savings. Installation times and costs per unit capacity have declined due to manufacturer experience, utilize of specialized vessels planned for offshore wind studies, and increases in turbine size for a turbine that amortizes installation attempts over larger capacities than ever before. In addition to the fact that offshore wind farm installations are in deeper waters and farther offshore, there has also been a tendency toward higher hub heights, more efficient and durable blades, and higher capacity turbines. Thanks to these technological features, these turbines, which are specially designed for the offshore sector, are in a position to capture more energy. This is very important for decreasing the LCOE of offshore farms.

Farm capacity and installation analysis
Results show that the average wind farm capacity increased parallel with higher turbine capacity values and years. Figure 10 shows the change in the global weighted average wind farm capacity for offshore wind over time. It is evident from the figure that offshore wind farms established in Europe are slightly larger compared to the average offshore wind farm values in the world. The first large-scale offshore wind power plant started power generation in Denmark in 2002 with 160 MW. After that, globally installed offshore wind farm capacities have increased rapidly over the past two decades. In 2020, the average offshore wind farm size reached Europe currently has a total of 25 GW of offshore wind installed capacity, corresponding to 5,402 wind turbines in 12 countries. The total sea surface area covered by 116 offshore wind farms is about 3,820 km 2 . Figure 11 shows the evolution of the number of wind turbines and sea surface area needed to install offshore wind farms in Europe over time. As seen from the figure, in Europe, the number of offshore wind turbines and the sea surface area needed for offshore wind farm installation have increased over time. For example, while the number of turbines established in 2001 was 20 and the needed sea surface area was 0.3628 km 2 , 703 offshore wind turbines were installed on a sea surface area of 839.9 km 2 in 2018. In 2020, 358 offshore wind turbines were installed on an area of 303.4 km 2 . In addition, it is seen that the number of turbines and the needed sea surface area increase in parallel with the increase in wind turbine capacity over time.
Offshore wind projects worldwide were established in shallow nearshore waters in the early 2000s, while they were established later in deeper waters and further offshore. Figure 12 shows the evolutions of the global weighted average water depth and the distance from shore for offshore wind over time. It is seen that turbine capacities increase in parallel with this development. This growth has resulted in the increased foundation, grid connection, and installation costs. This increase accelerated the total installed cost of the offshore wind turbine as designs were developed. The offshore wind farms commissioned in 2000 were approximately 6.5 km from the shore, 8 m water depths, and an average turbine power capacity of 1.6 MW. These numbers have increased significantly over time. In 2020, the average offshore turbine power capacity reached 7.5 MW, while the weighted average distance to shore and water depth increased to 29 km and 33.6 m, respectively.

Technological growth analysis
Over three decades of research and development, offshore wind has established itself as a cost-competitive power generation option for mature industries and governments. This has resulted in a robust offshore wind supply chain in the countries bordering the North Sea and the Baltic Sea through collaboration between European markets and experienced stakeholders. Currently, a single offshore wind turbine now has more capacity than the combined output of the world's first two offshore wind farms. With new technological advances such as floating foundations, an alternative geographic range of opportunities is paving the way by enabling offshore wind turbines to be established in deeper waters. From a technological, location-specific, and technological connectivity perspective, it appears that the key emerging trends in offshore wind are primarily the fabrication of more extensive offshore wind turbine technologies, floating foundations, use of multifaceted foundations and structures, development of combined power generation technologies, creation of offshore energy centers for sustainable and renewable energy power generation, green hydrogen production with different offshore renewable energy technologies, and airborne wind power systems.  Fig. 11 Evolution of the number of wind turbines, and sea surface area needed to install offshore wind farms in Europe over time Fig. 12 Evolutions of the global weighted average water depth and distance from shore for offshore wind over time Offshore wind turbine technology achieves higher turbine capacity factors and more stable wind power output due to less wind shear and turbulence and higher average wind speeds. This effect causes offshore wind output to have a higher value for the electrical system than onshore wind. The most common trend in the design of wind turbine technologies today is to increase turbine capacity factor while reducing turbine costs. For this, the hub height and rotor sweep area of the turbines are increased. On average, if turbines' hub height and rotor swept area grow faster than their power capacity, the specific power capacity will decrease. This causes an increase in the capacity factor of the turbine. As in Europe, wind turbines in the latest offshore turbine technology models today have a larger rotor sweep area, using more wind and generating more electricity. This reduces the cost of renewable energy generation. Thus, offshore wind turbines are becoming more powerful all over the world.
Specific power is defined as a measure of the wind turbine nameplate capacity divided by the rotor swept area. Figure 13 shows the evolutions of the global weighted average capacity factor and specific power capacity for offshore wind over time. As seen from the figure, the global weighted average capacity factor and specific power capacity values for offshore wind turbines installed between 2010 and 2019 range between 35%-45% and 316 W/ m 2 -461 W/m 2 , respectively. The specific power capacity of the annual installed wind turbines in the world decreased from 461 W/m 2 in 2011 to 350 W/m 2 in 2019. On the other hand, the turbine capacity factor increased from 38 to 42% from 2011 to 2019. Since the wind turbine's power output is directly related to the rotor swept area, an increase in the rotor swept area results in more power output from the wind for a turbine of the same power capacity. This means that there is a decrease in the specific power capacity of the turbine. A decrease in the specific power capacity causes the turbine power output and power coefficient to increase significantly for the same wind speeds. This increase in the rotor swept area of turbines has also resulted in lower energy costs worldwide, as larger wind turbines can generally deliver electricity at a lower cost than smaller wind turbines.
As a result, the growth in turbine size results in increased wind turbine efficiency because larger turbines with larger swept areas provide higher capacity factors for the same resource quality. Technological developments that increase capacity factors and reductions in TICs, O&M costs, and capital costs have resulted in cost reductions for offshore wind farms. The larger rotor turbines have higher capacity factors because the spinning blades sweep a wider area and utilize more energy. By using taller towers to increase hub height in areas with positive wind shear, greater access to higher wind speeds was achieved, reducing wind energy costs. The growth in the rotor diameter and thus in the rotor swept area was remarkably rapid, as the wind turbines were mounted with longer blades. At the same time, there was a modest increase in the mean nameplate capacity. Results showed that the turbines with low specific power were initially designed for low wind speed sites and were increasingly installed throughout the country, even in areas with high wind speeds, because of their low cost in various wind speed regimes.
Results show that the number of turbines per GW has decreased since 2001 significantly. Figure 14 shows the decrease in the number of turbines per GW between 2001 and 2020 in Europe. While the number of turbines per GW of annual installed offshore turbines in 2001 was approximately 500, this value dropped to 122, decreasing by 76% in 2020. A decrease in this metric may suggest less visibility per power capacity. Figure 15 shows the change in the sea surface area needed per GW for yearly installed offshore wind farms between 2001 and 2020 in Europe. This figure shows that the area needed per GW increased from 129.19 km 2 in 2002 to 236.80 km 2 in 2019. However, this value has changed over the years since 2001, showing a fluctuating situation. For example, the area needed to establish a 1 GW offshore wind farm in 2020 is calculated as 103.56 km 2 . Fig. 13 Evolutions of the global weighted average capacity factor and specific power capacity for offshore wind over time

Challenges in offshore wind turbine technology and action
The primary elements of an offshore wind turbine are very similar to those of an onshore wind turbine. However, in addition, components of the offshore wind farm include the external cabling, the turbine foundation, and onshore and offshore substations. Generally, the critical challenges in offshore wind turbine technology include resource characterization, subsea cabling, turbine installation and foundation, transmission infrastructure development, operation, reliable integration into the national grid (Charles Rajesh Kumar et al. 2021). However, as shown in Fig. 16, based on the detailed analysis of the challenges in offshore wind turbine technology and action, the most substantial challenges have been assembled into five main aspects: i) External conditions, ii) Environmental, iii) Technological, iv) Operational and maintenance, v) Economical.

Challenges in external conditions
Environmental and external conditions play a very active role in the design of an offshore wind farm. Besides the design of the offshore wind farm, numerous external conditions must be considered for the connection to shore and the overall strategy of the plant's maintenance and operation. Soil conditions for the choice of foundation, turbulent wind, icing, wake turbulence, lightning, extreme waves, currents, tidal effects, earthquake, marine growth, tidal and storm surge, and water depth variation are examples of some of these external conditions. Very strong winds, extreme wave action, and even particularly hurricanes or heavy storms can significantly affect and harm offshore wind turbines.
Water depth is one of the most critical parameters in the design and installation of offshore wind turbines. Unfortunately, the water depths vary considerably in seas and oceans. Although offshore wind turbines are established in relatively shallow waters, the installation of these wind turbines in deep water has recently begun to increase significantly, because more than 80% of all offshore wind energy sources in the world are located in waters deeper than 60 m where bottom-fixed structures are not feasible. Unfortunately, it is very difficult to construct safe and robust wind farms in waters deeper than 60 m. Floating wind turbines are technologies that can overcome this challenge.
Offshore wind farms with fixed foundations are the most common type of installation. They are being routinely deployed in water depths of up to 40 m, and in some cases up to 60 m, and at up to 80 km distance from shore. A variety of fixed offshore wind turbines have been developed over time, with the most typical kinds being monopole foundations, gravity-based foundations, tripod foundations, and jacket foundations. Among these foundations, the monopile structure is the simplest and hence the Challenges in offshore wind turbine technology and action most common, but can only be utilized in shallow water up to 30 m in water depth. On the other hand, floating wind farms are one of the recent developments in offshore renewable energy technologies and offer several opportunities. In contrast with the fixed offshore wind farms that are limited to shallow water depths, floating foundations enable access to deeper waters. Floating foundationtype turbines are generally utilized at depths exceeding 50-60 m due to the high cost of fixed-base foundations in deeper waters.

Environmental challenges
One of the most common challenges of an offshore wind farm is the environmental and visual impacts. Noise from offshore wind turbines is generally a smaller issue compared with onshore since the wind turbines are installed further away from where people live. Furthermore, offshore wind farms create a barrier to natural water flow, and they can behave as obstacles for migrating birds. Offshore wind turbines constructed in a coastal view may not be popular with local residents and can visually influence tourism and property values. In this respect, opposition to offshore wind farms from residential areas can pose significant challenges that can slow growth.

Technological challenges
The design of offshore foundations creates a more complex situation than onshore. The loads are a combination of normal load and extreme operating loads such as wind, waves, currents, ice. In addition, detailed calculations should be made for different water depths and ground conditions. The installation and fabrication of electrical cables under the seafloor used to transmit electricity back to land can be quite expensive. Installation of blades and nacelles requires equipment such as cranes and hoists, and a helicopter is needed when accessing the turbine from ships. During the process, there is a possibility of occupational accidents such as collisions, occupational accidents, structural failures, blade failures, and ice throwing.
Offshore wind turbine manufacturers or suppliers have focused on building the larger and taller wind turbine model in recent years. However, offshore wind turbine models with longer blades and taller towers have presented challenges in everything from design to production, transportation, and installation. To overcome the challenges associated with transporting taller towers, a mobile concept-based plant setup was considered, which enables concrete towers to be produced and installed locally, reducing logistics costs as well as transport distances (REN21 2021).

Operational and maintenance challenges
The operation of wind turbines in the harsh marine environment poses great challenges in their maintenance and repair. Offshore wind turbines can be difficult and expensive to install and maintain. As offshore wind farms are encountered in remote areas and operate under extreme atmospherically and environmental conditions, the need for high reliability and low operational costs becomes higher compared with onshore wind farms. Lack of trained personnel with basic technological capabilities, training, and expertise can be seen for turbine design, installation, maintenance of submarine electrical and communication cables, turbine foundation construction, defects, and repair.

Economic challenges
Offshore wind energy economics is a versatile and important field. Evaluating the economy involves considering a range of issues, including the cost of capital of offshore wind power projects, power generation, the market value of the energy produced, operation and maintenance, and the monetization of environmental benefits. Offshore wind also has some inherent higher costs compared to onshore wind. Higher project costs result from service vessels and equipment, specialized installation, and more expensive support structures.

Conclusion
Wind power has now become one of the most increasing renewable energy sources in the world, and it is expected to transform the global electricity industry in the coming years. In addition, offshore and onshore wind will produce more than one-third (35%) of total electricity worldwide by 2050, thus becoming a significant generation source. The global cumulative installed offshore wind capacity is estimated to reach 382 GW by 2030 with an increase of more than tenfold and to reach 2002 GW by 2050 (almost 60-fold) compared to installed capacity in 2020 (35 GW).
In recent years, one of the general trends in wind turbine technology is to produce larger machines. Parallel to this development, the size of the turbines has been increasing quickly to lower costs by scale and standardization. This trend is expected to continue toward longer blades, larger rotor sizes, extended machines, and higher hub heights; especially, offshore developers will continue to benefit from larger turbines as far as possible. It has also been observed that offshore wind farms are moved to deeper waters and farther seas to obtain more power output from the wind.
The global cumulative installed offshore wind power capacity has increased more than 11-fold from 3.1 GW in 2010 to 35 GW in 2020. The global weighted-average LCOE of offshore wind declined by 48% between 2010 and 2020, from USD 0.162 to USD 0.084/kWh, with a 9% reduction year on year in 2020. Between 2010 and 2020, global weighted-average TICs declined 32% from USD 4706/kW to USD 3185/kW. The global weighted-average TIC peaked at USD 5513/kW in 2008 and then decreased by 42% to USD 3185/kW in 2020. Advances in technology, including longer blades with higher hub heights, larger turbines, and reach to better wind speeds, have increased offshore wind farms' global weighted average capacity factor. For example, the global weighted average capacity factor increased from 29% in 2001 to 45% in 2017, and then, this factor dropped to 40% in 2020 as China increased its share in global distribution. TIC and LCOE reductions were driven by technological advances and the industry's growing maturity. Many factors have contributed to cost reductions, including greater product standardization, developer experience, manufacturing industrialization, economies of scale, and regional production and service centers. In addition, this cost reduction has been facilitated by the benefits of generation policies that support the growth of renewable energy, precision distribution, and the scale seen in the industry today.
As result, the trend of the rated capacity of installed wind turbines increased linearly relative to the hub height and rotor diameter. In addition, a linearly increasing trend between the rotor diameter and hub height was observed. Together with the tendency toward larger-sized turbines, there has also been a continuing increase in turbine capacity.