Fundamentally, the designer is left with the problem of finding an appropriate distribution, along the tower length, of mass and stiffness properties that ensure safe turbine operation under all prescribed external conditions, including actions from the environment and from the interaction with the grid and the control system. The turbine hub height dictates the necessary length of the tower, and ideally one would strike a balance between gains in energy capture at higher altitudes and the costs of a taller tower. Historically, tower lengths were set at approximately one rotor diameter. Installations at low wind sites and at sea no longer follow this rule of thumb. For wind development to be profitable in less windy, forested areas, for example, higher hub heights that take advantage of higher wind speeds and less turbulent atmospheric layers are required. At sea, on the other hand, lesser wind shear values and the SbS interface terminating at several meters above the still-water level (SWL) favor shorter towers than on land for a given hub height. Nonetheless, solving the design problem in either case is non-trivial. Offshore installations, given the inherent high balance of station (BOS) costs, promote the largest turbines and are generally characterized by more significant tower-head masses and ultimate thrust values than on land (eg, 350+ t and 1800 kN for a typical 6-MW offshore machine). These extreme loads are also to be combined with a particularly corrosive environment, the possible presence of other sources of loading (eg, floating ice), and extraordinary fatigue loads coming from some 10⁹ cycles due to rotor aerodynamics and some 10⁸ cycles due to wave loading alone. For these reasons, the designer must ensure that the overall system simultaneously meets several structural criteria: with regards to the response to external and internal excitation, the system must achieve the prescribed modal behavior, avoiding risk of instabilities or resonance; strength and deflection limit states must be verified while also ensuring an economically optimized load distribution and material utilization throughout the tower (and SbS below); manufacturability constraints, for instance on the weldable wall thickness of steel cans and the rollability of plates must be verified; finally, transportability and installation loads and processes must be examined and quantified. As far as detail design, important aspects must be covered such as: the interface with the RNA and the transition piece (TP), also flange and weld design, access door and manholes, the housing of power electronics, hoists and manlifts, and the needed coating protections. Loads, generally determined through aeroelastic simulations, are to be applied to finite element models to assess the three-dimensional (3D) stress state. The same models are used to confirm modal characteristics. In this case, an additional difficulty comes from the assessment of the effects associated with the soil–structure interaction (SSI) and of the overall stiffness offered by the SbS and pile subsystem.

It is clear that even confining one's view to the tower alone, multiple disciplines (eg, civil and mechanical engineering, structural dynamics, metallurgy) must be invoked to effectively and comprehensively tackle the design problem. This chapter offers an overview of the configurations and layouts currently available for wind towers, of their design process, and of the key engineering aspects that need to be addressed for a reliable and effective design. No individual reference or computer software can replace the experience and good judgment of a well-versed tower engineer, and this chapter wants to highlight the importance of accounting for the effects on system dynamics and installed costs of the SSt design choices.

The chapter is organized as follows. Section 10.2 presents a gallery of tower configurations and discusses the limitations for land-based systems that are pushing toward new materials and designs that can also be utilized offshore. Specific offshore requirements that make offshore wind turbine (OWT) SSts uniquely challenging, but also prone to a number of options and innovations, are also mentioned in this section. The main standards of reference for design and certification are presented in Section 10.3, which also discusses the importance of quantifying the reliability of offshore systems in tropical cyclone regions and to strike a balance between capital investment costs and those of repairs. In Section 10.4, the typical processes used to determine structural loads are presented, together with an overview of the sources of loading and the distinction between coupled and uncoupled load analyses and systems. The importance of controlling the system eigenfrequency is shown as being accompanied by the complex ramifications in terms of aerodynamic damping, fatigue loads from aerodynamics and hydrodynamics, and the effects on these aspects of turbine lifetime availability and site conditions. Section 10.5 discusses a possible approach to the preliminary sizing of the primary steel of a tubular tower. The major factors of the detailed design of flanges and welds are also presented. Secondary steel design, including damping devices and corrosion protection strategies, is discussed in Section 10.6. Finally, in Section 10.7, key facets of systems engineering and optimization are discussed, which emphasize the importance of a multidisciplinary optimization of the wind plant system over that of individual components, in order to achieve the ultimate goal of a minimum LCOE. Concluding remarks are presented in Section 10.8.