I. Introduction of Offshore Wind Power System and Existing Corrosion Problems
Wind power generation is the fastest growing green energy technology in the world. Because of the abundant wind energy resources at sea and the feasibility of today’s technology, the ocean will become a rapidly developing wind power market. European and American offshore wind farms have been on the eve of large-scale development. China’s eastern coastal water depth within 50 m of the vast sea area, and close to the power load center (coastal economic developed power shortage area), with the development of offshore wind farm technology mature, wind power will become an important source of sustainable development of the eastern coastal areas of China. Offshore wind power has the characteristics of abundant resources, high generation and utilization hours, no land occupation and suitable for large-scale development, and is close to the power load center, which is the latest frontier of wind power development in the world.
Because offshore wind power pile foundation is directly exposed to seawater environment, steel pile foundation is seriously threatened by corrosion, because of the special geographical environment and technical requirements of offshore wind power, the maintenance cost of its pile foundation is extremely high. Therefore, corrosion prevention of steel pile foundation is a prominent problem that must be considered in the design, production, operation and maintenance of offshore wind power, and corrosion prevention design has become one of the important links in offshore wind farm design.
II. Corrosion and cathodic protection
For offshore wind power pile foundation, the main corrosion is the electrochemical corrosion of the underwater part of the pile foundation:
Hull corrosion of metals in seawater is an electrochemical phenomenon caused by many primary batteries formed on the surface of hull metal. As happens in batteries, the corrosion current flows from the more active part of the metal (or anode) to the less active part of the metal (or cathode) through seawater. The primary cell can be formed not only between different metals, but also between different regions of the same metal. In this case, the less protected area formed by spraying becomes an anode relative to the better protected area. it turns out that the amount of corrosion of the metal is proportional to the magnitude of the corrosion current.
The protective potential of steel in seawater can only be measured by test. the experiments show that the natural corrosion potential of A3 steel in seawater is about -0.6 v relative to the silver/silver chloride reference electrode and 0.45 V relative to the zinc reference electrode. when the cathodic potential of the protected object is polarized to -0.80~-1.0 v (relative to the silver/silver chloride reference electrode), or 0.25~0.15 v (relative to the zinc reference electrode), the protection efficiency can reach 94%~99%.
The cathodic protection of wind power pile foundation is based on electrochemical principle, by applying external current, the electrode potential of the protected object moves in the negative direction, thus reducing the corrosion rate. According to the source of protection current, there are two kinds of cathodic protection: external (forced) current and sacrificial anode :
1. Applied current cathodic protection is through the applied dc power supply and auxiliary anode, which is to supplement the metal with a large amount of electrons, so that the whole protected metal is in the state of electron excess, so that the points on the metal surface reach the same negative potential, and the protected metal structure potential is lower than the surrounding environment. the applied (forced) current cathodic protection is to set an external power supply and auxiliary anode. the external power supply, the protection object (wind power pile foundation) and the auxiliary anode are formed through the seawater circuit to provide the protection current by the external power supply, so as to achieve the anti-corrosion effect. in the system, another reference electrode is set to detect the potential of the protected object relative to the seawater. the external power supply compares the detected reference potential through the set protection potential, thus adjusting the output protection current to make the system achieve the best protection effect.
2. Sacrificial anode cathodic protection is to connect a metal with a more negative potential (more active metals such as zinc alloy, aluminum magnesium alloy, etc.) to the protected metal and be in the same electrolyte, so that the electrons on the metal are transferred to the protected metal, so that the whole protected metal is at a more negative same potential, thus inhibiting the corrosion of the latter. This method is simple and easy to operate, without the need of external power supply, and rarely produces corrosion interference. According to the domestic reports, there are many failure lessons for the use of sacrificial anode, it is considered that the service life of sacrificial anode is generally not more than 3 years, up to 5 years.
III. Choice of cathodic protection
For the cathodic protection of offshore wind power piles, the difference between applied current cathodic protection and sacrificial anode cathodic protection is as follows :
According to the above comparison, from the cost, function, convenience of use, service life of equipment, equipment construction, maintenance and maintenance, environmental protection and other aspects, the use of applied current cathodic protection mode has obvious advantages over sacrificial anode protection mode. Based on this, Shanghai Rongde Electromechanical Engineering Equipment Co., Ltd. chose the applied current cathodic protection as the solution to the corrosion protection of offshore wind power pile foundation.
The applied current cathodic protection system is one of the important products of Shanghai Rongde Electromechanical Engineering Equipment Co., Ltd., which has been widely used in the corrosion protection of ships, offshore platforms and other underwater steel structures. As an applied current cathodic protection system for offshore wind power piles, it mainly includes the following functions:
1. protection potential setting;
2. total output current display;
3. output voltage display;
4. reference electrode potential indication;
5. protection potential overshoot alarm;
High temperature alarm 6. potentiostatic control box;
7. constant potential control box temperature high stop output;
8. potentiostatic control box power failure alarm;
9. realize remote setting of protection potential of multiple wind power piles and remote centralized monitoring of protection state in shore management system, including output voltage, protection current, reference potential, fault state, etc., and can realize the export function of monitoring data.
IV. System composition
The offshore wind power pile applied current cathodic protection system developed by Shanghai Rongde Electromechanical Engineering Equipment Co., Ltd. usually consists of a potentiostatic control box, auxiliary anode and reference electrode.
the potentiostatic control box applies a protective current to the underwater metal structure to be protected by an auxiliary anode, and can monitor the potential difference of the protected steel structure relative to seawater over the reference electrode, so that the potential difference between the protected structure and seawater remains at the set optimal protection potential. shore management system is composed of computer and signal receiving device. it can communicate with the constant potential control box installed on each wind power pile through internet or wireless network. it can set the protection potential of each wind power pile, display the output voltage, output current, reference potential and each alarm signal of each constant potential control box in real time, and realize the download of all data.
The shape (or structure) of the typical pile foundation structure and the main equipment of the system are as follows :
The typical system structure of offshore wind power pile applied current cathodic protection system is shown in the following figure :