Since the first offshore wind farm was deployed in Denmark in the early 1990s, utilising stronger and steadier wind energy offshore to generate electricity has always been part of the wind industry development agenda.

With the confidence and technical competency that has been accumulated through the experience of onshore wind development, it can be seen that the offshore wind industry started to grow significantly in the middle of the 2000s, doubling the total capacity every 2–4 years. Fig. 1.1 shows an analysis presented by the European Wind Energy Association (EWEA) on offshore wind installations in Europe since 1993 [1]. Based on the Global Wind 2014 statistics [2], more than 90% of all offshore wind installations were in European waters, spread across the North Sea (63.3%), the Atlantic Ocean (22.5%) and the Baltic Sea (14.2%). The United Kingdom accounts for over half of the total European offshore wind capacity installed to date, with an accumulative capacity of 4494 MW. Outside Europe, countries have set aggressive plans to promote their wind industry and offshore wind has become a new focus. China in particular installed close to 230 MW of offshore wind in 2014 alone, which makes it the third largest annual market globally after the United Kingdom and Germany.

The earlier offshore wind farms were typically within 10 km of the shore, with a water depth of less than 20 m. However, as the availability of such sites has been exhausted, new offshore wind farms have moved to further and deeper locations. For example, the ‘Dogger Bank’ wind farm, one of the world's largest wind farms under development in the United Kingdom, is located more than 100 km from the shore with what is currently the longest edge-to-edge distance of 260 km. Generally, developing larger wind farms further offshore could allow a higher rate of energy harvesting and hence better financial returns.

Increases in farm size and distance to shore are both inevitable in future offshore wind farm developments. To reduce the dependency on shallow water sites and to explore high wind resources in further offshore and deeper water regions, floating wind turbines have been proposed and have achieved good development breakthrough in the past few years. As described in detail in Chapter 11 ‘Design of floating offshore wind turbines’, despite the leaps in recent development, there are challenges on issues such as load stress reductions, design margin calculation, operational stability and so on, and these are yet to be addressed to realise the practicality of floating wind turbines.

Efficiencies, including design efficiency, system efficiency and operational efficiency, together with system availability that is dependent on the subsystem reliability, are the key elements in the offshore wind energy cost battle and are the main focus of this book. Amongst other parameters, the annual availability and OPEX (operation and maintenance expenditure) of the power plants would be one of the easiest measurable performance indicators that will influence energy cost. The offshore wind industry, in addition to its high capital expenditure (CAPAX), also suffers from higher OPEX as compared to its onshore counterpart. Lifetime OPEX of offshore wind is close to 90% of its CAPAX. Unlike the large mechanical drive-train components, such as gearboxes and bearings, the design philosophy adopted by the power electronics system industry, that is, power converters, power conditioners, etc., to overcome the fragile nature and maintain high system availability, is usually to modularise the system with easily swappable subsystems. This concept has proven to be effective in many of the onshore projects as on-site repairs can be performed without the need for major system replacement. However, when it comes to offshore, due to the extremely costly offshore logistics and highly weather-dependent vessel scheduling, any intermittent failures of power electronic systems that require manual reset or component replacement would have a significant impact, possibly as great an impact as failures of mechanical components, to the plant O&M in terms of OPEX.

In recent years, there have been a number of UK- and European-funded research projects looking into system robustness improvement, health condition monitoring and lifetime prognosis methodologies, to improve overall wind turbine availability. Deployment further offshore in deeper waters would potentially further increase the OPEX. How condition monitoring techniques can help on these issues is addressed in Chapter 18 ‘Condition monitoring of offshore wind turbines’, in Part IV of the book, which includes three other chapters to discuss other aspects about the installation and operation of offshore wind farms:

These are complemented by three chapters in Part I of the book which addresses the resource and siting criteria for offshore wind:

In the wind industry, as analysed in detail in Chapter 2, scale is important to achieve the economic benefits. There is a constant increase in wind farm size and, at the same time, larger wind turbines are being promoted and deployed offshore to improve the return of investments. Larger wind turbines, however, have introduced technical challenges in the substructures or subcomponents, such as rotor blades, towers and potentially the foundation designs. The rotor blade, and the building materials, as some of the heaviest components in the existing wind turbines are discussed in Chapter 5 ‘Developments in materials for offshore wind turbine blades’ and Chapter 6 ‘Design of offshore wind turbine blades’, respectively. These chapters in Part II of the book also address the issues of how to improve the mass density and reliability, and discuss designs to accommodate larger wind turbines for the future offshore wind industry.

Increasing turbine size and head mass will have a direct impact on the tower as well as the foundation. This, together with the harsh offshore environment, wind and wave loadings, as discussed in Chapter 10 ‘Design of offshore wind turbine towers’ and Chapter 19 ‘Health and Safety of Offshore Wind Farms’, would create new design challenges to the offshore wind turbine developers.

Turbine technologies are mostly covered in Part II of this book. In addition to Chapters 5, 6, 10 and 11, Part II also includes the following three chapters on other key components:

Given the uncertainties involved in the future development of the offshore wind industry, many aspects of current practice in terms of planning, design, deployment and operation could be open for argument. Most of the chapters in this book include discussions on the factors that affect the current practice and that may vary in the future.