The Invisible Enemy at Sea: How Modern Test Methods Are Revolutionizing the Service Life of Offshore Wind Farms

Hello and welcome to my blog! As a certified Frosio Level 3 inspector, it’s both my passion and my profession to safeguard the integrity of steel structures. Nowhere is this task more demanding—and more economically critical—than offshore. Offshore wind energy installations (OWEAs) are masterpieces of engineering and a crucial pillar of the energy transition. Yet they are under relentless attack—24/7, 365 days a year.

We’re talking about wind, waves, aggressive salty air, intense UV radiation, and even marine biology. This combination accelerates aging—so-called degradation. If a component fails, costs explode: a repair at sea can be 10 to 100 times more expensive than on land. Downtime also means massive revenue losses.

That’s why reliability and remaining service life are central questions for everyone involved—from planners and manufacturers to operators, banks, and insurers. To answer them credibly, we can’t rely on experience from onshore structures. We need precise data and a deep understanding of damage mechanisms at sea.

This is exactly where the groundbreaking research project “DegradO” (Degradation of Offshore Wind Energy Installations) from the renowned Fraunhofer Institute for Wind Energy Systems (IWES) comes in. In this article, I’ll give you, as a practitioner, a deep insight into this research and show what it means for our daily work in corrosion protection.

Why Offshore Reliability Is a C-Suite Issue

For the economics of offshore wind farms, every percentage point of availability counts. Repairs at sea quickly cost 10 to 100 times more than onshore interventions. At the same time, alternating mechanical loads (operation, wind, waves) and aggressive environmental conditions (salt, UV, humidity, fouling) act together—a turbocharger for degradation. If you don’t understand the long-term effects of these influences, you’re planning blind.

Key questions for all stakeholders:

Which components are critical weak points?
How fast does damage progress—and why?
How can remaining life be assessed credibly?
Which tests truly reflect offshore reality?

The Giants’ Achilles’ Heel: Why Welds Are in the Spotlight

The support structures of OWEAs are gigantic welded steel constructions. Welding is an efficient joining process, but it inevitably creates weak points. At a weld, different materials and zones meet: base metal, filler metal, and the heat-affected zone. This manufacturing-induced inhomogeneity changes material properties.

In addition, weld geometry, possible imperfections, and residual stresses from fabrication create stress concentrators (“notches”). These disturb load flow and cause local stress peaks. Under continuously varying loads from wind and waves, tiny fatigue cracks can initiate exactly at these spots.

Design rules account for this via fatigue verification. Corrosion, however, is often only indirectly considered by assuming perfect corrosion protection over at least 25 years. But what happens when that protection—perhaps due to a small mechanical defect—locally fails?

Mechanical cyclic loading then meets aggressive chemical attack. This phenomenon—known as corrosion–fatigue—dramatically accelerates crack growth and can drastically reduce a joint’s service life. The problem: until now, we’ve had very little reliable experimental data on this combined attack. The DegradO project set out to close exactly this knowledge gap.

Bringing Reality into the Lab: The “Maritime Reliability Laboratory” & DegradO at a Glance

To understand these complex processes, Fraunhofer IWES pursued a dual approach:

Field exposure: Deploy specimens under real conditions at the Bremerhaven, Helgoland, and Sylt sites.
Laboratory testing: Reproduce those real conditions in a controlled environment to accelerate aging and analyze it in a targeted way.

This iterative loop of field trials, lab analysis, and test-method refinement is the key to generating reliable data in a manageable timeframe.

The goal of DegradO is to develop new test methods that capture degradation processes of offshore OWEAs and to build a dedicated test & analysis infrastructure—the Maritime Reliability Laboratory.

Project building blocks:

Recreate real offshore loads (wind, water, salt, UV, biofouling) in the lab—also in accelerated form.
Test mechanical fatigue combined with corrosive/climatic conditions.
Field exposure (Bremerhaven, Sylt, Helgoland) ↔ lab: validate via load-spectrum comparison.
Build a characteristic data bank for damage images & material data → fast mapping, damage grade, maintenance/replacement recommendations.

The Test Material: A Mirror of Reality

Typical structural steel S355 was used. To obtain practice-relevant results, various market-established and offshore-certified coating systems were applied to the specimens:

Multi-layer epoxy (EP) system
Multi-layer epoxy duplex system
Multi-layer polyurethane (PU) system
Single-layer vinyl-ester resin system

Here’s where our expertise comes in: the entire process—from surface preparation (blasting) to final coating—was supervised and released by a FROSIO Level III inspector. This ensures top-tier industrial application quality—a decisive prerequisite for comparable results.

The researchers went one step further and deliberately created “imperfect” specimens we encounter in practice:

Over- and under-thickness in coating build.
Salt contamination of the substrate immediately before coating.
Artificial post-cure damage via a 50 mm wide scribe exposing the steel substrate.
Mechanical damage via impact tests (ball-drop).

These “imperfect” samples are gold—they show how a system behaves when ideal conditions no longer apply.

Front-Line Field Trials: What Really Happens in the North Sea?

Prepared specimens were deployed at Sylt, Helgoland, and Bremerhaven in different zones: from the spray zone through the tidal zone to the permanently submerged zone.

Over two years the samples were visually inspected and documented at regular intervals. After one and two years, specimens were taken and analyzed in detail using standard inspection methods:

Optical assessment for corrosion, blistering, etc.
Assessment of underfilm corrosion at the artificial scribe.
Holiday testing for coating defects.
Pull-off adhesion tests.

These field trials provide an invaluable baseline. They reveal how materials and coatings age under real, complex, and not always predictable conditions.

Fast-Forward: Accelerated Aging in a High-Tech Lab — The Maritime Reliability Laboratory

Waiting two years for results is too long for rapid development of materials and processes. Hence lab simulation is essential. The team used an impressive infrastructure:

Salt-spray and QUV chambers (UV & humidity) per standards such as ISO 20340.
An offshore climate chamber that can add surge/splash water with shock-like temperature changes.

The absolute highlight—and the project’s heart—is a globally novel test combo: a servo-hydraulic 1-meganewton test machine with an integrated climate chamber.

This setup is a true game-changer. For the first time, it allows a massive steel specimen (up to 3 m long) to be subjected simultaneously to mechanical cyclic loading (simulating wind & waves) while being exposed to typical offshore environmental conditions. The chamber can:

Generate temperatures from −30 °C to +100 °C.
Vary relative humidity.
Produce salt-spray.
Irradiate the specimen with UV.
Apply surge water or fully flood the specimen.

Mechanics: ±1 MN test force, ±50 mm stroke, specimen length up to 3 m, 4-column frame.
Standards: test programs are aligned with ISO 20340 and extended with offshore-typical loads (e.g., surge).

Important: In corrosive environments, time is the hidden parameter. Frequencies are therefore reduced (e.g., 0.3 Hz) so corrosion isn’t “sped away.”

With this setup, the team can target the feared corrosion–fatigue and understand how mechanical load, corrosion, and temperature interact.

The Results: What the Data Tell Us

The burning question: what did the tests show? The fatigue tests provide fascinating—and highly practical—insights.

Results are presented in an S–N diagram, showing how many load cycles a material can withstand at a given stress range before failure.

The key takeaways from the first two series under normal atmospheric conditions:

Surface preparation is everything: Coated specimens showed ~20% higher median fatigue strength than bare ones. Why? Pre-coat blasting not only cleans but also strengthens the surface and induces beneficial compressive stresses, delaying crack initiation. A clear scientific proof for high-quality blasting!
Even slight corrosion hurts badly: Bare specimens weathered for just six months at Sylt showed significantly lower fatigue strength than as-manufactured samples. The slightly corroded, roughened surface acts like countless small notches where cracks start earlier—underlining how critical even minor coating damage that exposes the substrate can be.
Is there “infinite” endurance? Even high-quality surfaces still failed at very high cycle counts (up to 17 million). Assuming a true endurance limit for OWEA structures must be treated very cautiously.

The truly groundbreaking results are now expected from the third series, where specimens are loaded mechanically and corrosively at the same time in the new test combo. These data will, for the first time, allow us to quantify corrosion–fatigue and integrate it into future life models.

Sensing & Other Observations

The research also yielded valuable insights into monitoring systems:

Electrical strain gauges: failures in some cases after 1–2 weeks.
Humidity sensors: failed very early; temperature sensors survived.
Fiber-optic sensors: functional at test end despite intensity losses/spectral deformation.
Biofouling (barnacles): massively damaging in reality (undermining/prying off protection layers), only partly reproducible in the lab.
Seawater ingress along cables → connector-end corrosion/salt crystallization: address in protection design!

From Research to Practice: The Damage Database

What good are the best data if they gather dust? A central goal of DegradO is to make the knowledge directly usable via a digital database.

The database follows a fault-tree logic. An inspector on site can enter a damage image (e.g., a specific crack, blistering, delamination) with its features. The software matches this input to the many analyzed field and lab cases characterized via high-resolution microscopy (SEM/FIB) and chemical analysis (EDX).

The system can then:

Map damage images quickly and reliably,
Identify likely causes,
Assess damage severity,
Forecast remaining life,
Recommend maintenance or replacement.

Purpose: fast, software-assisted mapping of images/measurements to damage grade, cause, and actions.

Contents & features:

Images, data series, descriptions (type/cause/appearance/size/count)
Interactive fault-tree prompts → refined diagnosis
Applications: electrical & fiber-optic sensors, covers/coatings/sealants, components, materials
Output: remaining-life indication, maintenance/replacement suggestion, field ↔ lab linkage

This tool has the potential to shift offshore maintenance from reactive repairs to predictive, condition-based strategies.

Practical Value — What Operators, Manufacturers, and Certifiers Can Do Now

For operators & O&M:

Align condition monitoring toward fiber-optic sensing; treat connector corrosion and protection layers as design points.
Prioritize the tidal/splash zone (inspection, protection, repair concepts).

For manufacturers & coaters:

Treat surface prep (blasting) & system selection as a fatigue lever.
Embed quality assurance (FROSIO) consistently; address salt pre-loading & local defects (scribes/impacts) realistically.

For planning & certification:

Anchor combined mechanical–climatic tests in specs (beyond ISO 20340).
Build S–N data under corrosive conditions systematically and feed them into verifications.

For insurers & financiers:

Use database-supported risk and reliability analyses to set premiums/coverage based on engineering reality.

My Conclusion as a Frosio Inspector & Outlook

DegradO is more than academic research. It’s a decisive step toward greater safety, reliability, and thus the economics of offshore wind. For me as a corrosion-protection specialist, the results so far clearly confirm the principles we stand for every day:

Quality matters: Excellent surface preparation and flawless coating application are not cost items but the best investment in long life. The tests show a demonstrable fatigue benefit.
Vigilance is mandatory: Even tiny defects can become fatal when combined with corrosion. Regular, detailed inspections are essential to catch issues early—before critical failure.
Knowledge is power: Projects like DegradO deepen our understanding of the mechanics–corrosion interplay. Bundled in tools like the new damage database, this enables better decisions to run assets safely across their planned life—and possibly beyond.

Next up are extensive corrosion-condition fatigue tests (low frequency, realistic cycles). This will expand the data basis on corrosion–fatigue—today’s major gap in design. The goal is a continuous chain from testing → mechanism model → remaining life → action, making offshore maintenance more plannable and wind power cheaper.

Fraunhofer IWES’s work provides the scientific foundation that underpins our practical work—and helps place the energy transition on a safe, long-lived footing.

Acknowledgments & Source

The DegradO project was funded by Germany’s Federal Ministry for Economic Affairs and Climate Action (BMWK) under the 6th Energy Research Program “Research for an environmentally friendly, reliable and affordable energy supply.”
This post is based on work by C. Kupferschmidt, M. Collmann, O. Kranz, Fraunhofer IWES, Bremerhaven.

What are your thoughts on this topic? Have you faced similar challenges with corrosion–fatigue or coating system evaluation in your practice? I look forward to your comments and an active exchange!

Need a professional inspection or consultancy for your project? Feel free to contact me for a non-binding inquiry.

(For further information, I recommend visiting Fraunhofer IWES’s website or looking up ISO 20340, which covers performance testing of corrosion-protection coating systems for offshore installations.)

Outsmarting the Sea: Longer-Life Offshore Wind