Offshore wind energy plants work under harsh weather conditions. Wind, waves and UV radiation create punishing loads, and corrosion induced by salt water attacks structural components such as tubes and nodes. The anti-corrosion coatings used until now on offshore foundation structures were not designed for a service life of 20 to 25 years and hence needed considerable maintenance under the described conditions.
"What‘s more, conventional coatings based on epoxy resins are quite time-consuming, as they have to be applied in several layers, each of which must set and harden," pointed out Hanno Schnars, at the start of the project responsible for the KOWIND project at the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES). KOWIND stands for Korrosionsschutz für Offshore-Windenergieanlagen, which is German for Corrosion Protection for Offshore Wind Energy Plants. Besides the IWES, partners in the project sponsored by the German Federal Ministry of Education and Research include the chemical companies Evonik and TIB Chemicals, Mannesmann Line Pipe, the engineering firm Prof. Bellmer, the research institute IFINKOR, and the University of Duisburg-Essen.
Maintenance-free protection for 25 years
The objective of the project is to coat the tubes and components of offshore foundations with a thermoplastic layer of polyamide 12, in order to ensure maintenance-free corrosion protection for 25 years. Significantly longer intervals between inspections, more economical production of the foundation structures, and substantially lower operating costs due to reduced maintenance requirements also played an important role in the research project.
Corrosion protection based on polyamide and PUR coatings
Within four years, the project partners developed a corrosion protection strategy for welds and node structures, which – unlike tubes - cannot be coated using the extrusion technique. Dr. Hans-Jürgen Kocks, who was involved in the project on the side of Mannesmann Line Pipe, explains: "To start with, the component to be coated is shot-blasted to the applicable specification and provided with a primer that matches the PA12 coating. Subsequently, a cold-ground UV-stable PA12 powder is flame-sprayed onto the primer, of course also to tested specifications."
Tests on the demonstrator
For verification of the resistance under dynamic loads, especially in the weld areas, a hollow section node in a double-K configuration was chosen as a test specimen. Scaled approximately 1 : 2.5, it was subjected to robustness and fatigue tests. A tangential brace was subjected to sinusoidal oscillations, which created a real mechanical load that acted not only on the steel structure, but also on the coating. No peeling was observed during the fatigue test. In order to document potential progressive damage to the steel structure, more than 80 strain gauges were applied in the weld area, which is particularly susceptible to fatigue.
Positive upshot of the project
"A registered increase – or drop - in strain could have indicated signs of cracking and thus initial damage," is how Mareike Collmann from IWES explained the testing technique. Under the load level defined in the project, there was, however, no abnormality observed in the steel node strructure nor in the coating throughout the duration of the test. "So the efficiency of the new corrosion protection strategy has been successfully proved for large components as well," is Mareike Collmann‘s positive conclusion.
With the new corrosion protection strategy, we are putting a tailored, sustainable innovation at our customers‘ disposal. Within the framework of KOWIND, it has been possible to define the requirements on an extruded PA12 coating in a detailed mill standard, which is now available to our customers as a technical delivery condition, and that constitutes a contractual basis. At the same time, this standard prepares the ground for the certification of this maintenance-free corrosion protection for offshore jacket structures.