埋地金属管道腐蚀的浅部瞬变电磁响应数值模拟及检测研究

The nondestructive testing of buried metal pipeline corrosion in service can be achieved by the transient electromagnetic method in trenchless. However, its theory research is insufficient. The signal amplitude is small，and the errors of test results are greatly affected by noise. This project is mainly to study the numerical simulation of shallow transient electromagnetic response and high precision detection technique of the corrosion model with special cavity structure of buried metal pipeline. Firstly, the 3d finite difference time domain algorithm of transient electromagnetic fields will be derived. The gradient non-uniform grid subdivision technology will be used. The fine grid subdivision will be carried on at the position of excitation source and the pipe wall. Conformal grid recursive algorithm will be achieved basing on the equivalent medium parameter, with the purpose of good transition of medium in the pipe wall. Secondly, study the truncation boundaries conditions on absorbing boundary. According to the established principle of one-way wave absorbing boundary conditions, absorbing boundary of low frequency electromagnetic diffusion equation will be established. Three types of boundary should be considered in the model. Thirdly, study the processing method of multi-turn small loop source affected by turn-off time to simulate the effects of primary field truly. At last, the receiving coil will be improved using a magnetic core. The interference of noise will be restrained effectively by digital signal processing. The shallow transient electromagnetic response characteristics of buried metal pipeline corrosion will be revealed accurately for numerical results and physical model test and verify each other. The study results will evaluate that the metal pipeline corrosion detecting by transient electromagnetic method is credible, practical and scientific.

瞬变电磁法用于埋地金属管道腐蚀的非开挖在役无损检测，其理论研究不足，且信号幅值较小，受噪声影响检测结果误差大。本课题研究埋地金属管道特殊腔体结构腐蚀模型的浅部瞬变电磁响应数值模拟和高精度管体腐蚀在役检测技术。首先，理论推导3维地电模型的瞬变电磁场时域有限差分算法，建立渐变非均匀网格剖分技术，在激励源和管壁处进行精细网格剖分，利用等效介质参数递推得到共形网格算法，实现管壁内外交界面处介质的良好过渡。其次，研究三类边界条件的设置，重点是根据单向波吸收边界条件建立的原理，得到低频电磁场扩散方程的吸收边界。再次，研究受关断时间影响的小回线源的处理方法，真实模拟一次场的影响。最后，研究改进接收线圈，加入聚磁磁芯，利用数字信号处理的方法有效抑制噪声的干扰，建立高精度的检测方法。数值结果与模型试验互相验证，揭示埋地金属管道腐蚀的浅部瞬变电磁响应特征，论证瞬变电磁法检测埋地金属管道腐蚀的可行性和可靠性。

检测埋地金属管道的腐蚀状况是管道安全性评价的关键依据。瞬变电磁法用于埋地金属管道腐蚀的非开挖在役无损检测，其理论研究不足，且信号幅值较小，受噪声影响检测结果误差大。基于此，本课题主要研究埋地金属管道特殊腔体结构腐蚀模型的浅部瞬变电磁响应数值模拟和高精度管体腐蚀在役检测技术。研究内容及重要结果如下：1、基于广义麦克斯韦方程组推导了适用于三维截断空间的非均匀网格算法及基于非均匀网格技术的时域有限差分方程，实现了三维截断空间模型的建立、电磁场信号的提取及不同类型腐蚀模型的数值模拟，获得了时域有限差分算法建模仿真的模型结果。设置不同腐蚀长度和腐蚀程度的埋地管道模型、不同传输介质的埋地管道模型、不同管径与埋深的埋地管道模型和不同属性腐蚀物质的埋地管道模型进行仿真计算，得到各类模型的瞬变电磁响应特征，总结相关参数变化引起的感应电动势变化规律。2、进行埋地管道模型的瞬变电磁检测试验研究，验证数值模拟结果的准确性，模拟实际工况下的检测试验，获得合适的检测方式、检测参数和数据处理方法。发射线圈的参数直接影响观测结果，线径越大、匝数越多、激励电流越大，关断时间越长，观测到的响应信号中有用成分减少，会降低检测结果的可靠性。对于不同参数的接收线圈，需要调节阻尼电阻使其工作于临界阻尼状态，此时接收端可获得最佳观测信号。当接收线圈边长为0.5m、匝数为60，且发射线圈边长为1.0m、匝数为30时，实际检测的效果较好。在此基础上，研制了新型的加磁芯线圈装置，利用铁氧体磁芯的“聚磁”效果加大接收信号的幅值，提高信噪比，改善检测精度，但铁氧体和线圈需有合适的匹配，不然会适得其反，对检测产生极大干扰。3、将瞬变电磁法应用到石油管道、天然气管道和城市热力管道的检测中，部分进行了开挖验证，取得了较好的工程应用效果。数值模拟和模型试验的结果和结论论证了瞬变电磁法检测埋地金属管道腐蚀的可行性和可靠性，为实践应用提供了科学的理论支撑。