Dear readers of the vgbe energy journal,

The expansion of offshore wind energy is progressing at a rapid pace – technologically, economically and in regulatory terms. However, as distances from shore increase, water depths grow and turbine dimensions continue to expand, the associated challenges are also shifting. One of these remains of central importance despite all technological advances: the corrosion protection of the predominantly steel-based support and foundation structures.

What was long regarded as a necessary protective measure is increasingly becoming a strategic factor for the availability, design life and economic viability of offshore wind turbines. Operating experience from recent years clearly shows that the protective effect is not determined solely by the basic protection concept, but above all by its detailed execution, the quality of application and the interaction of the individual protection systems under real offshore conditions.

Monopile and jacket-based foundation structures are typically protected in the atmospheric zone, the splash zone and the tidal zone by multi-layer, high-performance coating systems with total dry film thicknesses in the range of several hundred micrometres. In permanently submerged areas, supplementary cathodic protection systems are used, predominantly in the form of galvanic anodes, but increasingly also as impressed current systems or hybrid protection concepts in combination with organic coatings. Particular attention must be paid to the transition areas between coated and cathodically protected zones, where active electrochemical and passive barrier-based corrosion protection interact.

These protection concepts have generally proven effective in practice. At the same time, larger component dimensions, longer intended operating periods and stricter requirements regarding environmental compatibility and constituents mean that existing protection systems must be continuously developed further. Particularly for large-scale structures, the requirements for the design of cathodic protection systems are increasing in order to ensure the most uniform possible distribution of protection potentials. In addition, coating systems, especially in the splash zone, remain permanently exposed to high corrosive as well as mechanical and dynamic loads.

A look at operating practice shows, however, that typical damage patterns are rarely systemic in nature, but predominantly arise from local details. These include coating disbondment due to mechanical damage during installation or operation, underfilm corrosion at insufficiently prepared welds, and degradation phenomena in transition areas between coated and cathodically protected zones. Locally varying consumption rates of galvanic anodes, for example as a result of biofouling or flow-related effects, are also observed in long-term operation.

In addition, the lifetime extension of existing offshore wind farms is becoming increasingly important. This is driven in particular by economic requirements for longer operating periods and the aim of making long-term use of existing infrastructure. As a result, corrosion protection is increasingly becoming a key basis for decision-making in asset management. Whether recoating in the splash zone, the targeted addition of anodes or local reinforcement measures – many technical solutions are available, but they often involve considerable logistical effort. Restricted weather windows, limited options for surface preparation on existing structures and high mobilisation costs set tight economic constraints.

After operating periods of 10 to 20 years, ageing effects also become more apparent. Coating systems may show embrittlement and cracking in some areas, while adhesion strength may be reduced as a result of cyclic temperature and moisture exposure. At the same time, the boundary conditions for cathodic protection may also change, for example due to sediment relocation or oxygen gradients in greater water depths. Actual operating conditions therefore often prove to be significantly more heterogeneous than the original design assumptions.

This is precisely where modern monitoring approaches come in. In addition to conventional visual inspections, sensor-based methods are becoming increasingly established, for example for the continuous recording of protection potentials in cathodic protection systems or for assessing the degradation state of organic coatings. In future, electrochemical methods such as impedance spectroscopy, which is currently being tested, could also make it possible to qualitatively assess the barrier effect and permeability of coating systems. This would, for the first time, provide reliable information on the remaining protective performance.

In combination with digital evaluation models, this creates the prospect of a data-based understanding of degradation processes and condition developments. Corrosion protection is thus increasingly evolving from a purely pre-planned protection concept into an actively managed component of technical asset management.

At the same time, standardisation is becoming increasingly important. Guidelines such as VGBE-S-021 for the corrosion protection of offshore structures used for wind energy aim to bridge the gap between general corrosion protection standards, such as DIN EN ISO 12944, and the specific requirements of offshore applications. This not only provides technical clarity, but also forms an important basis for approval processes and for the assessment of protection systems during operation.

Experience from recent years has also shown that the actual operating costs (OPEX) of corrosion protection have often been underestimated. In particular, underwater inspections, logistically complex repairs and unplanned maintenance campaigns regularly lead to deviations from original planning assumptions. At the same time, it is becoming clear that early investment in quality assurance, monitoring and targeted maintenance measures can contribute significantly to stabilising operating costs in the long term.

The outlook is clear: extended operating phases, more sustainable coating systems and the increasing use of data-based methods will have a decisive influence on future corrosion protection concepts. The key will be to understand corrosion protection not as an isolated discipline, but as an integral part of holistic plant and asset management.

After all, the performance of the offshore wind sector depends not only on the turbines themselves, but also on the long-term durability of the supporting structures.

This article was prepared in collaboration with the vgbe Working Group on Corrosion Protection for Offshore Structures.