Offshore wind power has the characteristics of abundant resources, high power generation utilization hours, less impact on the surrounding environment, suitability for large-scale development, proximity to economically developed areas, proximity to power load, easy integration and consumption of wind power, etc. In recent years, many have turned the development of wind power to offshore. The foundation forms used by offshore wind power in China are divided into steel structure foundation and reinforced concrete foundation from the material point of view, among which steel structure foundation is widely used. Because the steel structure foundation is in harsh working conditions and difficult to maintain on site, it is generally designed according to the anti-corrosion life of no less than 15 years. The operation time of an offshore wind farm in China is far less than 15 years, but some steel structure foundation anticorrosive coatings have been damaged in large areas, which seriously affects the stable operation of wind turbines. This paper will analyze this situation in detail, find out the reasons and put forward corresponding solutions, so as to accumulate experience for the anticorrosion of steel structure foundation of subsequent offshore wind power projects.

Wind field situation

The wind farm with large-scale damage to the anticorrosion coating of steel structure foundation belongs to intertidal wind farm, which adopts paint anticorrosion. The anticorrosion scheme is designed by integrating ISO12944 Code for Anticorrosion Painting of Steel Structures, ISO20340:2009 Performance Requirements for Coastal and Related Structure Protective Coatings System and NB/T 31006: 2011 Technical Standard for Anticorrosion of Steel Structures in Offshore Wind Farms compiled by China Energy Administration. The anti-corrosion scheme is designed reasonably, the paint construction is carried out according to the construction technology provided by the paint supplier, the key working procedures are recorded correspondingly, and the dry film thickness of the paint meets the design requirements, thus eliminating the design and construction defects of anti-corrosion.

In order to know the specific situation of corrosion protection layer damage, we conducted a field investigation after the ebb tide, and found that the internal corrosion protection layer of steel structure foundation was intact, the external areas were damaged to varying degrees, some areas showed signs of impact, and a large number of marine organisms (mainly sea oysters) adhered to the external surface. In Figure 1, black objects on the outer surface of steel structure foundation are attached marine organisms, and Figure 2 shows the damage of marine organisms to the anticorrosion coating of steel structure foundation.

Cause analysis

Through the investigation of the field situation, we think that the impact of external forces and the attachment of marine organisms are the main reasons for the damage of the anticorrosive coating of offshore wind power steel structure foundation.

First, external force impact

We found that the ship berthing device designed for steel structure foundation with damaged anticorrosive coating is unreasonable. When the ship berths, it is affected by seawater flow, which makes the ship easily hit the steel structure foundation. In addition, there are many floating objects in the sea area in the wind place, which will also impact the steel structure foundation. However, in this project, the influence of external force impact on paint was not considered in the design of anti-corrosion scheme of steel structure foundation, so the paint layer was damaged and fell off when external force impact occurred in many areas, which affected the service life of anti-corrosion coating.

Second, the attachment of marine life

Because there are a large number of sea oysters (also called oysters) growing in the sea area where wind turbines are located, they have many unique living habits. The newly born oysters can swim freely in the water, but when they meet the right environment, they begin to parasitize on rocks or other hard objects in the sea, and live a fixed life all their lives, which is difficult to be removed, thus forming an external fouling layer on the steel structure foundation. After the oyster is attached to the steel structure, it can prevent the diffusion of oxygen molecules on the surface of carbon steel to the corroded surface and protect the corrosion of carbon steel to some extent. However, due to the impermeability of the attached layer and the respiration of aerobic bacteria in the outer fouling layer, the oxygen-deficient environment is formed on the surface of carbon steel, which is conducive to the growth of sulfate-reducing bacteria (SRB), thus accelerating the anaerobic corrosion of carbon steel.

According to the electrochemical corrosion principle and experimental facts, the corrosion mechanism of carbon steel induced by SRB is as follows: Fe-2e→Fe2+ (anodic reaction) 2H++2e→H2 (cathodic reaction) SO42-+8H→S2-+H2O(SRB cathodic depolarization) S2-+2H+→H2S (cathodic depolarization) Fe2++.

H+ required by the above reaction comes from the ionization of organic acid and water metabolized by SRB. From the above mechanism, it can be seen that sulfide (S2-), especially H2S, has both cathodic depolarization and anodic depolarization in the whole electrochemical corrosion process, thus accelerating the corrosion of carbon steel and seriously affecting the service life of steel structure foundation of offshore wind turbine.

Solution measure

In order to ensure the stable operation of wind turbines, we must take targeted measures to solve the problem after finding out the causes of damage to the anticorrosion coating of steel structure foundation.

First, measures to deal with external impact

(1) Optimize the design of the original ship docking device according to the water flow characteristics of the wind farm, and add corresponding protective fender at the ship docking place, which can absorb the impact energy when the ship docked, thus reducing the damage to the steel structure foundation;

(2) Considering the impact on the steel structure foundation when the ship is docked, it is necessary to select the paint with high impact resistance at the dock;

(3) In view of the fact that there are many floating objects in the sea area at the wind site, it is necessary to set isolation facilities around the steel structure foundation, so as to reduce the possibility of floating objects contacting the steel structure foundation.

Second, measures to deal with the attachment of marine organisms

The adhesion of marine organisms will not only destroy the anticorrosive coating of steel structure foundation and accelerate the corrosion of carbon steel, but also be difficult to solve. Because the offshore wind power in China started late and the relevant specifications are not perfect, considering that the environment of offshore oil drilling platforms is similar to that of offshore wind turbines, in order to better solve this problem, we consulted a large number of measures for preventing marine organisms from adhering on offshore oil drilling platforms. At present, there are three commonly used measures to prevent marine organism adhesion on oil drilling platforms in China: using marine power marine organism prevention device (MGP), using antifouling paint to prevent marine organism and electrolyzing marine organism.

(1) marine dynamic marine biological protection device (MGP)

The marine power marine organism prevention device (MGP) is made of engineering plastics with high elasticity, high durability, excellent ultraviolet resistance and good fatigue resistance. The main body of the device surrounds the catheter frame, that is, the place where marine organisms are easy to attach and grow. The buoyancy of seawater, natural forces such as ocean currents and waves are used as power to make the device move vertically, transversely and rotationally around the jacket structure. Through this compound movement, the impact wheel arranged on the device constantly collides and contacts the surface of the catheter frame, so that the marine life can not be attached, and this method can gradually remove the attached marine life. The advantages of the device are that it takes the natural ability of seawater as power, does not need to apply external force artificially, and is simple, effective, with little pollution and low cost. The disadvantage is that in long-term use, in case of strong wind and typhoon, under the combined action of waves and currents, the device will collide violently with the jacket structure. If it is damaged, it will be very difficult to repair and replace.

(2) Anti-fouling paint against marine organisms

Anti-fouling paint is a special kind of paint coated on the pipe frame. Its main function is to gradually release toxic materials through the seepage, diffusion or hydrolysis of toxic materials in the paint film, so as to prevent marine organisms from adhering to the steel structure foundation. Although antifouling paint has many advantages, such as meeting the requirements of environmental protection performance; The paint film has certain water permeability, which can ensure the continuous leakage of toxicity; The paint film has good seawater impact resistance, and does not bubble or fall off when immersed in water for a long time. However, the disadvantage of antifouling paint is also very fatal. It can only prevent marine organisms from adhering to the pipe frame in a certain period of time, and it usually needs to be repainted after 3 -5 years.

(3) Electrolysis of marine organisms

Electrolysis of marine organisms produces toxic substances through electrolysis, thus forming an environment which is not suitable for the attachment and growth of marine organisms. At present, there are two commonly used methods, namely, electrolyzing seawater to produce hypochlorous acid to kill marine life; Electrolytic copper and aluminum electrodes electrolyze metal ions with seawater as conductor by means of applied current, so that the metal ions in seawater reach a certain concentration, thus killing marine life. This technology has the advantages of reliability, convenience and low operating cost, but it will kill the marine life in the surrounding sea area. Because there are many typhoons in summer in this wind farm, there is the possibility that marine dynamic marine biological devices will damage the anticorrosive coating of steel structure foundation under the action of strong winds. In addition, there are a large number of breeding areas around this wind farm, and it is not suitable to adopt electrolytic marine biological methods. If anti-fouling paint is directly applied on steel structure foundation, since its service life is only 3 -5 years, it must be re-applied on site when it reaches its service life. Considering the harsh working environment on site, it is difficult to guarantee the construction quality, so it is directly applied on steel structure foundation. In view of this wind farm, our solution is to make FRP protection pipes for wrapping the steel structure foundation, and apply anti-fouling paint with low surface energy on the outer surface of FRP protection pipes to prevent the adhesion of marine organisms. After the anti-fouling paint fails in the future, the FRP protection pipes can be removed and transported back to the factory for re-painting the anti-fouling paint, which can be used for the steel structure foundation again to prevent the adhesion of marine organisms. This scheme avoids painting antifouling paint on site, and can effectively guarantee the construction quality, but the cost will be higher. Offshore wind power has broad prospects for development, but there are still many problems in the anticorrosion of steel structure foundation, especially the adhesion of marine organisms. Although there are a lot of measures to prevent the adhesion of marine organisms in ships, oil drilling platforms and other industries in China, most of the measures can not meet the requirements of offshore wind power. At present, the offshore wind farms developed in China are mainly near sea breeze farms, so we should not only take measures to avoid the damage of marine organisms to the anticorrosive coating, but also give consideration to the protection of marine organisms and the surrounding environment when carrying out the anticorrosion design of offshore wind power steel structure foundations. In order to avoid premature failure of anti-corrosion coating of offshore wind power steel structure foundation, we comprehensively consider the influence of various factors during anti-corrosion design, and strictly follow the corresponding specifications during construction.