The access hatches, manlifts, and interfaces to the SbS and to the nacelle must comply with workmanship safety regulations and must offer enough versatility to allow for maintenance tools to be moved safely and effectively.

Engineers should especially strive to find adequate solutions for guaranteeing safe and quick access to the nacelle. Although larger offshore wind turbines may make use of helipads on the nacelle, providing for a quick transportation to the top of the RNA is crucial to reduce off-line periods and revenue losses.

Solutions that allow reducing the welds inside the tower are often sought to thin the shell and save on material quantities. Some original equipment manufacturers (OEMs) have adopted the use of magnets to attach platforms and other internals to the walls of the tower. Others have successfully employed either guy-wires secured to flanges, or adhesives to the tower inner walls. Both strategies reduce welded fittings and achieve savings in labor and material costs.

TMDs (also known as vibration absorbers or dampers, see Fig. 10.30) can be used to reduce low- and high-frequency vibrations in towers and OWTs due to aeroelastic and inertial forcing. The function of a damper is based on a spring/mass system, which counteracts and reduces the structural response (see Fig. 10.31). Once the performance targets are determined (eg, deflections, loads and accelerations, resonance frequency), dampers can be designed based on first principles and vibrational engineering. In certain cases, more complex solutions with viscous dampers, force-amplifying truss systems, and actively controlled actuators are needed to achieve the identified targets. These solutions are more difficult to properly design for, because they require renewed attention to detail and FEAs analyses coupled to system aeroelastic simulations.

There are fundamentally two kinds of dampers: TMDs and active mass dampers (AMDs). Multiple tuned mass dampers (MTMDs) are of the first kind, but with multiple application points within the same structure.

The main parameters, ie, the mass of the damper and its displacement, are a function of the mass of the system being damped. Therefore, for large OWTs, TMDs may require sizable devices and strokes to be effective, which can be an impediment to the installation inside towers. Typical damper mass entities are 1–10% of the system modal mass. Realistically 2–3% is all that can be achieved inside the tower. The TMD geometry, however, can be devised to amplify the primary structural drift and to achieve higher damping coefficients, for example through the use of trusses, toggles, and scissor-jacks as shown in Fig. 10.32 [95–98].

Besides truss systems, other devices have recently received attention for wind tower configurations following their success in the civil and offshore engineering industries. Among those are the tuned liquid column damper (TLCD) [96,99–102] (see also Fig. 10.31(b)) and other kinds of semipassive devices, such as the ball vibration absorber tested in Refs [103,104]. These systems can overcome some of the logistical limitations of the simpler TMDs.

AMDs can be fairly effective [105,106] and bypass the same TMD limitations, but have an inherent power requirement and generally are more expensive. Partially active devices [96] retain advantages of the passive TMDs, but can also guarantee extra action on demand, as for example under sudden wind gusts.

The theoretical effect of TMDs (as shown in Fig. 10.31(c)) is the complete reduction of the first eigenfrequency resonance peak. The original undamped mode is split into two modes with equal damping ratios. While the TMD is normally tuned to the f₀ of the system, any misalignment between damper and SSt eigenfrequencies (for example due to soil conditions) can actually lead to increased loads. It might be more efficient to tune the TMD to the frequency spectrum of the exciting loads. Another advantage of the TMD over the other dynamic control strategies is the fact that it is effective during both operational and parked/idling cases.

For floating turbines, the role of the oscillation damper is primarily on the surge and pitch DOFs, but for fixed-bottom OWTs, the primary role could be on the sway or side–side (SS) DOF, which has very little aerodynamic damping. Increasing damping has the potential of reducing deflections and loads, and of increasing the lifetime of the entire OWT.

Direct consequences of increased damping on the tower translate to a reduced amount of required steel. Additionally, the reduction in tower deflection might have positive benefits on the blade-to-tower clearance, allowing for a lighter rotor. These effects may combine and allow for a lighter SbS as well.

The ramifications of these kinds of design choice can further expand in the realm of the BOS costs, as manufacturing and transportation costs may all be affected. It is then clear how attention to the design of certain components, even within the secondary steel group, can actually have an important repercussion on the entire system performance and LCOE.

The O&G industry has developed extensive experience with corrosion protection, and special coatings have been developed to protect offshore structures. In contrast to O&G platforms, however, turbine towers are unmanned structures without permanent inspection of corrosion protection systems (CPSs). Therefore, while O&G platforms are promptly repaired upon damage detection, towers cannot be repaired without incurring large costs [112]. Mühlberg [114] states that repair of CPSs at sea can cost up to 50 times more than the initial application during tower manufacturing. It is of note that repairs might be accompanied by expensive downtimes. Ref. [115] offers guidance on the inspection of offshore wind turbine structures, and particular attention is given to the CPS.

The physics and chemistry of corrosion are quite complicated; whereas many references are available on the subject, it is good practice to request the help of material and corrosion experts during the design phase. Corrosion may be described as a process system consisting of three subsystems: medium, material, and interphase. For offshore applications, the medium can be either air, water, a combination of the two as in sea spray, condensation water in internal spaces, and soil on foundations. In all cases, a high salt content should be expected. The materials include unalloyed, low-alloyed and stainless steels, cast iron, aluminum and copper alloys, and composites. For the scope of this chapter, tower sections are considered to be made from steel. The interphase consists of chemical compounds that form from the oxidation of the materials within the medium.

Depending on the environment and the loading level, various types of corrosion can be identified, for example, uniform corrosion, galvanic corrosion, pit corrosion, crevice corrosion, microbiologically influenced corrosion (MIC), stress (fatigue) corrosion, and erosion corrosion. All of these forms of corrosion can occur on offshore SSts.

In general, the presence of moisture in the medium (conductive environment) spurs an oxidation reaction. The metal loses electrons in the medium, but metal oxide is not formed, as opposed to dry oxidation, and the formation of the reaction product might not occur at the reaction site. This is the basic mechanism of uniform corrosion.

If another metal is in electrical contact with the primary one and located in the same environment, further (galvanic) corrosion will occur, as the metal at lower electrochemical potential will corrode (anode), protecting the other (cathode) as an electronic/ionic current is established. This explains why a zinc-coated (galvanized) steel tower performs efficient protection in the atmosphere where the moisture film is thin. Also, galvanic corrosion can occur at joints, where fasteners with different electrochemical potential are in contact with the tower main steel.

Because of the presence of the medium, cathodic and anodic reactions might happen on the surface of the same metal, as water and oxygen interact with metal ions to form more complex ions and change the thermodynamics of the reaction. This is what makes the splash zone so active from a corrosion point of view. The presence of salt accelerates the reaction and allows for the formation of metal chlorides (the interphase) that undergo hydrolysis and thus lower the pH of the medium solution. The galvanic corrosion rate is proportional to the current intensity, and this is inversely proportional to the area of the electrodes. Pits and crevices represent small anodic areas, where a high salt concentration and more extensive presence of moisture are to be expected. For these reasons, pit and crevice corrosion are examples of fast-rate corrosion processes.

MIC is a consequence of biological fouling, which can initiate and accelerate corrosion due to the interaction between microbial activity and steel. In general, underneath a biological layer that attaches to the SSt (eg, seaweeds and mussels), an anaerobic environment develops. Within the anaerobic environment, organisms like sulfate-reducing bacteria (SRB) enhance the development of MIC. Metal reacts with hydrogen sulfides forming metal sulfides and hydrogen as corrosion products. Hydrogen tends to permeate the metal matrix making it more brittle and prone to cracking. This type of corrosion can also occur near the seabed.

Stress, or fatigue corrosion, occurs when any of the other types of corrosion is accompanied by cyclic loading. Corrosion-fatigue fracture surfaces may or may not be coated with corrosion product depending on the relative effects of corrosion and stress (see Fig. 10.36). More evidence of corrosion can be expected at lower stress levels or lower frequencies of stress cycling, because of the increased time of exposure to an aggressive environment. Finally, erosion corrosion is associated with the abrasive action due to boat impacts, ice floe impact, surface icing, and biological fouling. Standards for corrosion protection (eg, Ref. [25]) subdivide the media and the environments in terms of corrosivity categories and exposure classes, as seen in Table 10.7. For offshore installations, category C5-M applies, and corrosion rates can be determined based on recommendations in Refs [1,3,11,25]. Further attention must be paid to the climatic region of the installation, as warmer climates, as for example in tropical and subtropical regions, give rise to higher corrosion rates, as do environments with higher salinity. In Arctic conditions, ice scoring can also increase corrosion rates through the removal of coatings and oxidation layers, but boat impacts can be even more damaging (see Fig. 10.37).

Conventionally, five corrosion zones are identified for wind turbines (see also Fig. 10.38): underwater corrosion zone (UZ); intermediate corrosion zone (IZ); splash corrosion zone (SZ); atmospheric corrosion zone (AZ) (C5-M); and internal or nacelle corrosion zone (NZ) (see also Refs [1,11]). For each of the zones, an appropriate design corrosion rate can be assigned, and an adequate CPS could be devised. CPSs include: design choices (eg, through the choice of structural materials and approaches to drainage), coating application, electrochemical protection, and monitoring and regular inspection [115,118,119].

The AZ, above the SZ, is exposed to uniform corrosion and must be protected by coatings as specified by standards such as Ref. [25]. For external surfaces, such as the tower outer surface, a zinc-based primer is normally covered by additional epoxy and polyurethane coatings. Coatings need to be inspected and repaired at prescribed intervals. For internal surfaces that can be exposed to external air, such as nacelle and tower inner surfaces, corrosion protection through coatings is commonly employed. Corrosion allowance can be granted in place of CPSs for components of minor significance in the AZ [3] and where inspections and repairs are possible. Corrosion-resistant materials for fastening devices (eg, stainless steel) and grating (GFRP) are acceptable as well.

Corrosion rates are highest in the SZ, where wave loading is also maximum, thus particular attention to the CPS must be paid in this zone. The SZ is bound by the 1-year crest height at highest still-water level (HSWL) and 1-year trough at lowest still-water level (HSWL). Here, maintenance of the CPS is not very effective, and neither is cathodic protection (CP). Coatings are mandatory and should be made of materials of proven reliability and following the usual standards [24–26], but they cannot be the only defense against corrosion. Therefore, corrosion allowance (CA) should be used for components such as towers, SbSs, and transition pieces if in the SZ. CA can be calculated as [11]:

In the IZ and UZ, CP is necessary, and coatings are optional and used to reduce the required CP capacity. Internal surfaces should be protected by either CP or CA. Ref. [11] recommends using a corrosion rate of Ċ ≥ 0.10 mm/year.

CP makes use of aluminum- or zinc-based, sacrificial anodes fastened to the main steel between the seabed and the MSL. The CP system must be designed for a minimum lifetime equal to the OWT's, and guidance is offered by various standards (eg, Ref. [3]) on how to size anodes based on I, a protective current density (A/m²), and Q, a practical current capacity (Ah/kg) based on anode manufacturer specifications:

Impressed current cathodic protection (ICCP), where the current is produced by a dedicated power source, can be used in lieu of the galvanic CP, and in principle it could be provided by tapping the grid connection. Dedicated cables, rectifiers, control, and monitoring devices are also needed. The extra costs associated with these devices should be weighed against the advantages of a more controlled cathodic polarization of the structure through ICCP, and the generally fewer anode elements that can also be located off the main structure.

One must recognize that because of the presence in the structure of hatches and openings, perfect water- or air-tightness is almost impossible to achieve, even in zones that are initially thought of as such, as for instance the foundation cavity near the mudline. Moreover, air-sealed zones in towers and SbSs can still give rise to corrosion due to infiltration of seawater and the presence of SRB, and due to contamination of external air any time maintenance access needs to be granted. If a seal fails, tidal variations can occur within the structure cavities. This situation then resembles conditions near ports and harbors where accelerated corrosion occurs due to alternating wet and dry cycles with semistagnant conditions.