In recent years, many countries have shifted the focus of wind power development to the sea. The basic form used in offshore wind power in China is divided into two forms from the perspective of materials: steel structure foundation and reinforced concrete foundation, among which steel structure foundation is widely used.

Because the working conditions of the steel structure foundation are relatively harsh and the field maintenance is relatively difficult, the design is generally based on the corrosion protection life of no less than 15 years. A domestic offshore wind farm run far from 15 years' time, but some based coating of steel structure has emerged the phenomenon of widespread damage, seriously affected the safe and stable operation of wind power unit, this paper will detailed analysis for this situation, find out reason and put forward the corresponding solution measures, for subsequent offshore wind project of steel structure anticorrosion accumulate experience.

Wind field situation

The wind field with large damage to the corrosion resistant layer of steel structure foundation belongs to the intertidal zone wind field, and the paint is used to prevent corrosion, the anti-corrosion plan is the comprehensive ISO12944 code for anti-corrosion coating of steel structure, ISO20340:2009And the design of NB/T31006:2011 technical standard for anti-corrosion of steel structure of offshore wind farm, prepared by China energy administration.

The design of anti-corrosion scheme is reasonable. Paint construction is carried out according to the construction process provided by the paint supplier. The key process is recorded accordingly, and the paint dry film thickness meets the design requirements, thus eliminating the design and construction defects of anti-corrosion.

To know details of anticorrosion layer damage, after the ebb tide situation has carried on the field survey on the spot, we found that the steel structure based on the internal coating in good condition, outer area appeared different degree of damage, some areas have suffered from the impact of signs, and the external surface of a large number of Marine organisms (mainly oysters) attached. In FIG. 1, the black objects on the outer surface of the steel structure foundation are the attached Marine organisms, while FIG. 2 shows the damage of Marine organisms to the anti-corrosion layer of the steel structure foundation.

Cause analysis

Based on the investigation on the site, we believe that the external force impact and the attachment of Marine life are the main reasons for the damage of the anti-corrosion layer of offshore wind power steel structure.

1. External force impact

We find that the ship docking device designed with the damaged steel structure foundation is not reasonable. When the ship is berthing, it is affected by the flow of sea water, and the ship is easy to hit the steel structure foundation. In this project, the impact of external force on the paint was not taken into account in the design of anti-corrosion scheme for steel structure foundation. Therefore, many areas suffered from the impact of external force, and the paint layer was broken and detached, thus affecting the service life of anti-corrosion coating.

2.Attachment of Marine life

Due to wind turbines in waters with a lot of oysters (also called the oyster), it has many unique life habits, just born baby oyster, can swim freely in the water, but when they meet the appropriate environment, began to parasitic on rocks or other hard objects in the sea, a lifetime living in fixation type, it is difficult to be cleared, which formed in the steel structure on the basis of the fouling layer.

Are attached to the steel structure on the basis of oysters, although can hinder oxygen molecules on the surface of the carbon steel corrosion surface diffusion, for carbon steel corrosion has certain protective effect, but with the attached the impermeability and the fouling layer of aerobic bacteria in respiration, made of carbon steel surface oxygen environment, conducive to the growth of the sulfate reducing bacteria (SRB), so as to accelerate the anaerobic corrosion of carbon steel.

According to the principle of electrochemical corrosion and the experimental fact, the mechanism of corrosion induced by sulfate reducing bacteria (SRB) is as follows: fe-2e to Fe2 (anode reaction) 2H 2e to H2 (cathode reaction) so42-8h to s2-h2o (SRB cathode depolarization) 2H to H2S (cathode depolarization) Fe2 S2- to FeS (anode depolarization) Fe2 H2S to FeS 2H

The required H from the SRB metabolism reactions of organic acid ionization and the ionization of water, can be seen from the above mechanism in the process of the electrochemical corrosion, sulfide (S2 -), especially the H2S both cathodic depolarization effect, but also has the anode depolarization effect, thus accelerated the corrosion of carbon steel, seriously affected the service life of the offshore wind turbines steel structure foundation.

The measures

In order to ensure the safe and stable operation of the wind turbine, we must take targeted measures to solve the problem after finding out the cause of the corrosion resistant layer damage on the steel structure foundation.

1. Measures for external force impact

(1) optimize the design of the original vessel docking device according to the flow characteristics of the wind field, and add the corresponding protective side at the ship docking place. The protective side can absorb the impact energy when the ship is docked, thus reducing the damage to the steel structure foundation;

(2) considering the impact on the steel structure foundation during ship docking, paint with high impact strength shall be selected at ship docking;

(3) in view of the large amount of floating objects in the sea area where the wind site is located, it is necessary to set up isolation facilities on the periphery of the steel structure foundation to reduce the possibility of contact between floating objects and the steel structure foundation.

2.Measures for the attachment of Marine organisms

Adhesion of Marine organisms, will not only damage based coating of steel structure and accelerate the corrosion of carbon steel, and difficult to solve, because the sea wind power in China started relatively late, the relevant specification and imperfect, considering the offshore oil drilling platform environment compared with offshore wind turbines are similar, in order to better solve the problem, we refer to a large amount of offshore oil drilling platform of Marine biological adhesion prevention measures. At present, the commonly used measures to prevent the attachment of Marine organisms in China's oil drilling platforms are as follows: using Marine dynamic Marine biological devices (MGP), using antifouling paint for Marine biological and electrolytic Marine biological.

(I) Marine power Marine biological device (MGP)

Marine dynamic Marine biological device (MGP) is made of engineering plastics with high elasticity, high durability, excellent uv resistance and fatigue resistance. The main body of the device is wrapped around the vessel framework, which is the place where Marine organisms are easy to attach and grow. By using the buoyancy force of sea water and natural forces such as ocean currents and waves, the device makes vertical, horizontal and rotational motion on the periphery of the pipe framework.

Through this compound movement, the impactor wheels on the device constantly collide and touch the surface of the catheter framework, thus making the sea creatures unable to adhere, and this method can also gradually remove the attached sea creatures. The biggest advantage of this device is that it is simple, effective, pollution-free and low cost, with the natural capacity of seawater as the power and no external force required. The disadvantage is that in the long-term use, such as in the case of strong wind and typhoon weather, under the combined action of waves and ocean currents, the device and the catheter structure parts will have a violent collision, if there is damage, maintenance and replacement will be very difficult.

(2) Antifouling paint antiseptic creatures

Antifouling paint is a kind of special coating applied on the vessel structure. Its main function is to release the poison gradually through the leakage, diffusion or hydrolysis of the paint membrane poisons, so as to prevent Marine organisms from attaching to the steel structure. Although anti-fouling paint has many advantages, such as meeting the environmental performance requirements; The paint film has a certain water permeability, which can guarantee the continuous leakage of toxicity. The paint film has a good impact resistance to sea water, in the long-term immersion in the water does not bubble, does not fall off. The downside of antifouling, however, is that it only prevents Marine life from attaching to the vessel's structural parts for a period of time, typically requiring repainting after three to five years.

(3) Electrolytic sea defence organisms

The electrolysis of Marine organisms is to produce toxic substances through electrolysis, thus forming an environment which is not suitable for Marine organisms to attach and grow. At present, there are two most commonly used methods: electrolyzing seawater to produce hypochlorous acid to kill Marine organisms;

Electrolytic copper and aluminum electrodes use seawater as a conductor to electrolyze metal ions, so that the concentration of metal ions in seawater reaches a certain level, thus killing Marine organisms. The technology has the advantages of safety, reliability, convenience and low operation cost, but it will kill Marine life in the surrounding sea.

Because this summer typhoon wind field is more, under the action of wind, existence of Marine power protection sea creatures based coating of steel structure may be damaged and the wind field surrounding a large number of other culture zones, is not suitable for using the method of electrolytic anti sea creatures, if directly based on besmear brushs antifouling paint on steel structure, due to the service life of antifouling is only 3 years to 5 years, at the scene of the antifouling paint to use fixed number of year when must besmear again, considering the working conditions on site is poor, it is difficult to guarantee the construction quality, thus directly based on the steel structure coating antifouling paint is not an ideal solution.

Against the wind field, our solution is used for making glass fiber reinforced plastic protection tube is used to package based outside the steel structure, and on the outer surface of the glass fiber reinforced plastic protection tube coating low surface energy antifouling paint to prevent adhesion of sea creatures, antifouling paint failure in the future, glass fiber reinforced plastic protection tube can be removed back to factory to besmear brushs antifouling paint, and once again used in steel structure foundation to prevent adhesion of Marine organisms. This scheme avoids the work of anti-fouling paint on site, which can effectively guarantee the construction quality, but the cost will be higher.

Conclusion

Offshore wind have broad prospects for development, but the current steel structure anticorrosion is still faced with many problems, especially the adhesion problem of Marine organisms have been unable to have a very good solution, although in industries such as ships, oil RIGS in China there are a large number of Marine biological adhesion prevention measures, but most of the measures do not meet the use requirements of the offshore wind.

As the offshore wind farms developed in China are close to the offshore wind farms, measures should be taken to avoid damage to the corrosion protection layer of Marine life while protecting the Marine life and the surrounding environment. In order to avoid the premature failure of anti-corrosion layer of offshore wind power steel structure foundation, we must take the influence of various factors into consideration when carrying out anti-corrosion design, and strictly follow the corresponding specifications in the construction process.