Control of factors affecting the construction quality of LNG storage tank pile foundation and application of regular inspection magnetic particle testing technology

Firstly, the factors affecting the construction quality of LNG storage tank pile foundation control. Low temperature storage tanks have high requirements for the bearing capacity of the foundation and foundation. Currently, many LNG storage tank foundations in various regions often use direct foundation or pile foundation. The Tianjin LNG storage tank is built in an underwater fan-shaped area formed mainly by sedimentation of silt in the Bohai Bay. This type of naturally growing foundation soil generally cannot meet the requirements for settlement and deformation caused by upper structural loads. A pile foundation is a deep foundation formed by connecting the top of several piles together with a bearing platform to jointly bear the load. Therefore, it has great integrity and stiffness, and can meet engineering requirements. The geology of Tianjin LNG station is different from other regions, with a pile foundation of 55m, making construction difficult. If the construction quality is not strictly controlled, it will leave hidden dangers to the engineering cost, construction period, and even the safety of LNG storage tanks in the later stage of use. Proper handling of quality issues and hidden dangers during pile foundation construction is crucial to prevent similar problems from occurring. Prepare technical documents, engineering materials (equipment), and construction machinery before construction. The control work of on-site construction personnel plays a very important role in smooth construction, and construction quality supervision is the key to protecting construction quality. A full process supervision team should be adopted. 1) Control of construction documents. Before construction, carefully carry out the drawing review work in accordance with the requirements of the program documents and relevant management regulations, carefully review and verify the corresponding construction drawings and construction plans, timely report design defects and quality problems in the plans, and avoid quality problems caused by technical defects. 2) Control of construction materials. The purchased materials meet the requirements specified in the design and contract, and timely witness sampling and re inspection of the incoming materials and equipment shall be carried out. Materials and equipment that do not meet the requirements shall be removed from the site; Materials that meet the requirements should be stored in a timely manner and recorded in a ledger. 3) Control of construction equipment. Choosing appropriate construction equipment plays a crucial role in protecting construction quality and improving construction efficiency. During the construction process, it is necessary to strictly strengthen the inspection of construction equipment, and construction equipment that does not meet the requirements for use shall not be used. 4) Control of construction personnel. Improve construction organization, clarify the responsibilities of construction personnel, and ensure that each person performs their duties. Operators should be familiar with the construction process, hold certificates, and pay special attention to the role of construction technicians and construction supervisors. 5) Control of construction environment. A good construction environment is an important condition for protecting the quality and performance of engineering projects. In actual engineering construction, attention should be paid to taking corresponding measures for abnormal environmental conditions, such as construction during winter and rainy seasons, hot seasons, and windy seasons. Appropriate control measures should be developed for different seasons to avoid hazards such as freezing damage, cracking, and erosion, thereby protecting the quality of the project. Secondly, the application of magnetic particle testing in tank inspection. Due to the fact that most of the materials used in liquefied natural growth gas storage tanks are carbon steel or low-alloy steel such as Q235 and 16MnR, which have the characteristics of low remanence and low coercivity, the magnetic particle testing of LNG storage tanks can only be carried out using a continuous method, that is, applying magnetic particles or magnetic suspensions to the workpiece for magnetic particle testing while magnetizing with an external magnetic field. There are various magnetization methods for magnetic particle inspection, which are generally selected based on the characteristics of the tested workpiece. Common methods for circumferential magnetization include conduction method, center conductor method, bias core rod method, contact method, induced current method, and cable winding method. Methods for longitudinal magnetization include coil method, magnetic choke method, and magnetic iron method. Methods for multi-directional magnetization include cross magnetic choke method and cross coil method. Different methods have different characteristics. For example, the contact method uses two rod contacts to contact the surface of the workpiece, magnetizes it, and generates a distorted circumferential magnetic field on a flat workpiece. It is used to discover discontinuities parallel to the line connecting the two contacts. With a smaller magnetizing current, the required magnetic field strength can be deeply applied locally to the workpiece. It has good sensitivity and is easy to use. Although there are various methods for magnetic particle inspection, the inspection of LNG storage tanks mainly focuses on welds, including butt welds, fillet welds, etc. Fixed equipment cannot be used, and only portable equipment can be used for segmented inspection, which limits the selection of magnetic particle inspection methods for LNG storage tank inspection. Currently, the commonly used methods are as follows. (1) Magnetic choke method: This method is widely used, with simple equipment and easy operation. The movable joint magnetic choke can detect corner welds. In order to detect defects in various directions during use, at least two perpendicular inspections should be performed on the same part. The weld should be divided into several inspection sections, and there should be a certain overlap during the inspection operation. This method is inefficient and improper operation may result in missed detections. (2) Cross magnetic choke method: Currently, it is a widely used method in container inspection, which can generate a rotating magnetic field, has good inspection efficiency, sensitivity, simple operation, and can detect defects in all directions with one magnetization. It is suitable for detecting long butt welds, but not for diagonal welds. However, due to the need for a 380V power supply, there are some restrictions on its use inside containers in the petrochemical industry. (3) Contact method: It belongs to the unidirectional magnetization method, and the electrode spacing can be adjusted. The electrode spacing and current size can be determined according to the inspection site and sensitivity requirements, and can be flexibly adjusted for fillet welds. This method, like the magnetic choke method, requires two intersecting and perpendicular inspections of the same area. (4) Coil method: For the circumferential and corner weld seams of pipelines, the cable winding method can be used for inspection, which belongs to longitudinal magnetization and can detect longitudinal cracks in the weld seam and heat affected zone. In the actual inspection of LNG storage tanks, the author's unit mainly uses two methods: magnetic choke method and cross magnetic method. For the longitudinal and circumferential welds of container docking, these two methods are easy to operate, have good sensitivity and efficiency, and occupy an irreplaceable position; For the fillet weld of the connecting pipe, the cross magnetic method cannot be used for inspection. The movable joint magnetic choke can better solve the problem of detecting the fillet weld of the connecting pipe perpendicular to the container cylinder, but there are difficulties in determining the fillet weld of the connecting pipe at a certain angle. The contact method and coil method can better solve this problem. In fact, due to the complex stress conditions at the junction, corner welds are more prone to problems than butt welds. Therefore, how to solve the problem of corner weld inspection and how to introduce and apply the contact method and coil method well are the future work and efforts.