5.2 Corrosion Aspects and Biofouling
5.2.1 Corrosion Mechanisms and Corrosivity Zones
for Offshore Structures
In general, the same mechanisms that can damage offshore structures like wind
turbines and platforms can also damage aquaculture structures that are made of the
same or similar material. Offshore structures are exposed to different and varying
corrosive environmental conditions.
Based on theory and practical experience with offshore structures, in total eleven
different corrosion zones of offshore wind structures can be identified. The most
critical zones are the splash/tidal zone and closed compartments filled with seawater
(e.g. the internal of a monopile or jacket foundation structure). Design specifications for steel structures define a corrosion allowance. In case of uniform corrosion
this is an applicable design tool. However, when local corrosion mechanisms like
microbial corrosion (MIC), galvanic corrosion or corrosion fatigue occur, the
structural integrity of the steel structure must be evaluated. The offshore wind
structure design is determined by fatigue load. Local defects like pitting attack may
act as initiation sites for fatigue cracking. For this reason special attention should be
given to local defects in the foundation and the tower structure.
5.2.2 Corrosion Risks in Currently Used Offshore Wind
Turbines
The offshore wind energy market is young, compared to the offshore oil and gas and
shipment markets; the first offshore wind farm was installed in 1991. The most
important lesson learned from the first generation offshore wind turbines is: wind
turbines based on onshore technology are not suitable for offshore application. The
first offshore wind farm, Horns Rev (D), suffered from a major coating failure of
eighty wind turbine foundations. The coating on the transition pieces broke down
and resulted in unexpected repair and maintenance costs. The reason was a combination of wrong coating selection and improper application of the coating. This
points out to the key issue: a lack of conformity between the manufacturer, coating
applicator and coating supplier.
Other corrosion related problems reported are failing cathodic protection systems, corroding boat landings by combination of wear, impact and seawater and
corroding secondary structure components like ladders and railings. The impact of
corrosion damage varied from increased safety risks for maintenance personnel to
re-evaluating the structural integrity of the foundation structure because of local
pitting attack.
Local corrosion attack by MIC has been noticed on the internal surface of
different monopile foundations on different locations in the North Sea. With
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117
5.2.1 Corrosion Mechanisms and Corrosivity Zones
for Offshore Structures
In general, the same mechanisms that can damage offshore structures like wind
turbines and platforms can also damage aquaculture structures that are made of the
same or similar material. Offshore structures are exposed to different and varying
corrosive environmental conditions.
Based on theory and practical experience with offshore structures, in total eleven
different corrosion zones of offshore wind structures can be identified. The most
critical zones are the splash/tidal zone and closed compartments filled with seawater
(e.g. the internal of a monopile or jacket foundation structure). Design specifications for steel structures define a corrosion allowance. In case of uniform corrosion
this is an applicable design tool. However, when local corrosion mechanisms like
microbial corrosion (MIC), galvanic corrosion or corrosion fatigue occur, the
structural integrity of the steel structure must be evaluated. The offshore wind
structure design is determined by fatigue load. Local defects like pitting attack may
act as initiation sites for fatigue cracking. For this reason special attention should be
given to local defects in the foundation and the tower structure.
5.2.2 Corrosion Risks in Currently Used Offshore Wind
Turbines
The offshore wind energy market is young, compared to the offshore oil and gas and
shipment markets; the first offshore wind farm was installed in 1991. The most
important lesson learned from the first generation offshore wind turbines is: wind
turbines based on onshore technology are not suitable for offshore application. The
first offshore wind farm, Horns Rev (D), suffered from a major coating failure of
eighty wind turbine foundations. The coating on the transition pieces broke down
and resulted in unexpected repair and maintenance costs. The reason was a combination of wrong coating selection and improper application of the coating. This
points out to the key issue: a lack of conformity between the manufacturer, coating
applicator and coating supplier.
Other corrosion related problems reported are failing cathodic protection systems, corroding boat landings by combination of wear, impact and seawater and
corroding secondary structure components like ladders and railings. The impact of
corrosion damage varied from increased safety risks for maintenance personnel to
re-evaluating the structural integrity of the foundation structure because of local
pitting attack.
Local corrosion attack by MIC has been noticed on the internal surface of
different monopile foundations on different locations in the North Sea. With
5 Technical Risks of Offshore Structures
117