Anchor Bolt Foundation Project Of The Wind Turbine
The quality of the construction of the prestressed anchor bolt foundation of the wind turbine directly affects the construction progress of the entire wind farm. In this type of onshore wind turbine foundation, the anchor bolt is a key component connecting the foundation and the tower, and it is of great significance to ensure that it meets the construction quality requirements. Suppose the positioning is inaccurate or the overall positioning is affected during the foundation construction process.
In that case, the anchor bolts at the top of the turbine base foundation will have position deviations (this phenomenon has been demonstrated in the actual project construction), making it difficult to install the tower. For anchor bolts with position deviations, if the deviation size is too large, the foundation cannot continue to be used; if the deviation range is small, the anchor bolts can be adjusted by taking appropriate correction schemes so that the wind tower can be installed smoothly.
Solution For Anchor Bolts With Position Deviation
The commonly used solution is to apply external force directly to guide the anchor bolt, so that the anchor bolt will bend to a certain extent, but this will generate additional bending moment in the anchor bolt. After the bending, the upper and lower sections of the anchor bolt are not subjected to forces in a straight line. The tension generated by the preload and the external load will generate a couple in the anchor bolt, and the additional bending moment generated by the couple will increase the stress of the anchor bolt.
After the anchor bolt with a large degree of eccentricity is bent, it will come into contact with the anchor bolt hole of the bottom flange, and local stress concentration will occur at the contact position. At the same time, the extrusion and friction between the anchor bolt and the bottom flange will cause wear on the structure and reduce the service life of the anchor bolt and the bottom flange. Therefore, it is necessary to ensure that the ultimate strength and fatigue life of the anchor bolt after correction can still meet the standard requirements through detailed calculation. The engineer calculates the ultimate strength and fatigue strength of the anchor bolt after correction through theoretical analysis, determines the applicability of the correction scheme, and proposes to formulate a real-time monitoring scheme to ensure the safe operation of the wind turbine to a greater extent and eliminate the owner’s safety concerns after the anchor bolt is corrected.
How To Properly Install Anchor Bolts In Anchor Cage Foundation
The prestressed anchor cage foundation is a structure that fixes concrete and anchors together, and is composed of a lower anchor plate, an upper anchor plate, a PVC protective pipe, and anchors. As shown in Figure 1, a PVC protective pipe is used between the lower anchor plate and the upper anchor plate to isolate the concrete from the anchors to prevent the concrete from entering the protective pipe during the pouring process and corroding the anchors. When the anchor is subjected to tension, the entire anchor becomes an elastic body above the lower anchor plate, and there is no interface between the elastic part and the rigid part, thereby avoiding stress concentration. Since the concrete foundation is always in a compressed state after the prestressing force is applied to the anchor, the wind turbine foundation using prestressed anchors will not be damaged due to the stress concentration of the concrete on both sides of the foundation ring. Therefore, the application of prestressed anchor foundations is becoming more and more widespread.
Construction Process Of Wind Turbine Foundation
The construction process of prestressed anchor wind turbine foundation is relatively complicated, and the construction process is as follows: Construction preparation → Retention of embedded parts → Installation and level control of lower anchor plate → Positioning anchor and adjustment of upper anchor plate → Alignment and assembly of upper and lower anchor plates → Adjustment of anchor assembly and process acceptance → Rebar construction → Formwork construction → Concrete construction.
Ensure the feasibility of Anchor Bolt Quality
If the anchor bolts fail to meet the inspection standards during the construction process, the anchor bolt components are touched during the steel bar binding process, or the steel bars are directly placed on the anchor bolt components, the positioning of the anchor bolt components will be affected, causing the top of the anchor bolt to deviate from the design expectations, resulting in difficulties in the bottom tower hoisting.
If no solution is adopted, the foundation will lose its use value, causing serious economic losses to the company; if the anchor bolts are corrected by adopting a customized solution, their safety (ultimate strength and fatigue strength) must be calculated in the early stage to ensure the feasibility of the correction solution.
Through the analysis of the ultimate strength of the pilot anchor bolt, it is known that the pilot bolt will increase the average stress of the anchor bolt, and the average stress will always exist during the entire life cycle. Since the increase in average stress will lead to a decrease in the fatigue performance of the anchor bolt, the impact cannot be ignored. Therefore, the DC value of the pilot anchor bolt is corrected by referring to the Goodman average stress correction of the casting.
Case Study of M36 Anchor Bolt
Taking the M36 anchor bolt of a wind farm as an example, the anchor bolt hole diameter is 42 mm, the anchor bolt center deviation is 7mm, the forced displacement caused by the guide is 4 mm, and the free deflection height is 800 mm. The above method is used to calculate the ultimate strength and fatigue strength of the anchor bolt.
- Ultimate strength According to the above calculation method, the additional stress generated during the correction process is 69.5MPa. The anchor preload is 360kN. According to the stiffness distribution, the maximum anchor tension generated by the external load is 386.1kN, and the stress under the bending moment generated by the maximum tension is 337.2MPa. The maximum anchor tension generated by the external load is 386.1kN, and the corresponding anchor stress is 472.6MPa. Therefore, the total stress under the limit working condition is 879.3MPa, which is less than the design allowable stress of the anchor of 940MPa (because the tension method is used when the anchor is preloaded, the material safety factor of 1.1 can be ignored), and the ultimate strength meets the design requirements.
- Fatigue strength The fatigue calculation of the anchor bolts after alignment using the Markov matrix is carried out by the above method, and the maximum fatigue damage is 0.78, which meets the design requirements. Since the extrusion between the anchor bolts and the flange is not considered in the engineering algorithm, it is recommended to take measures to ensure that there is no contact between the anchor bolts and the bottom flange during the operation of the unit, to avoid stress concentration.
Stress Monitoring Of Anchor Bolts During Operation
Due to the increase in average stress caused by the correction process and the possible stress concentration and initial defects, the fatigue damage results calculated using the engineering algorithm will have certain deviations. To ensure the safe operation of the unit, it is recommended to strengthen the stress monitoring of the anchor bolts, timely understand the status of the anchor bolts, and make emergency plans for anchor bolt failure.
The anchor bolt monitoring system consists of ultrasonic sensors, media, data cables, temperature compensators, data acquisition software and computers. The ultrasonic sensor is fixed on the object to be measured to capture the time for the sound wave to propagate and reflect in the anchor bolt; the data acquisition instrument collects the data transmitted by the ultrasonic sensor and converts the data into the computer to obtain intuitive data. The operation process of ultrasonic monitoring is shown in Figure 5.
As an important component connecting the wind turbine to the foundation, the safety of the anchor bolt is related to the normal operation of the entire wind turbine. Therefore, the anchor cage foundation construction should be carried out in strict accordance with the operating procedures to avoid anchor bolt deviation.
