非常规油气压裂泵头体裂纹扩展规律研究

The exploitation of unconventional oil and gas as the great strategic task of our country is highly depend on the fracturing technology, and the key equipment of fracturing is fracturing pump. Furthermore, the working life of fracturing pump is depend on that of pump head, and cracking failure has become the main factor to restrict the working life of pump head. However, at present the research on crack propagation of pump head is so few that the influencing factors and the change rules are still uncertain. By taking some typical features of the fracturing pump head, such as the complex inner structure, severe working conditions, complicate fracture modes and various influence factors into account, the research in the program is developed from three parts: theoretical description of plastic zone of crack front, crack initiation condition and direction, crack growth rate and direction. By combining the theoretical analysis, numerical simulation and experimental verification, a comprehensive insight into the change laws of the crack propagation is achieved. Through this study, the basic scientific theoretical system of crack propagation of pump head is formed, which provides the guidance for designing structure and working parameters as well as improves the accuracy of fatigue life prediction. Although the high pressure fracturing pump head is selected as the research object in this program, the research results can also have significant theoretical meaning and application value to the research on crack propagation and life prediction of high pressure vessel used in other fields.

非常规油气开采是我国的重大战略，其关键技术在于压裂，压裂的核心设备是压裂泵，压裂泵的使用寿命取决于泵头体，泵头体开裂失效已成为制约其工作寿命的主要因素，而目前对于泵头体裂纹扩展的研究甚少，其影响因素和变化规律未知。本项目针对压裂泵泵头体内腔结构复杂，承受高压脉动循环载荷，断裂模式复杂，断裂影响因素多等特点，从裂尖塑性区形态，复合裂纹起裂条件和方向，疲劳裂纹扩展速率和扩展方向等三个方面，采用理论分析，数值模拟并结合实验验证对泵头体裂纹扩展规律进行全面深入的研究。通过本研究将形成泵头体裂纹扩展的基础科学理论体系，为泵头体的结构设计和工作参数提供依据，能更准确预测泵头体的疲劳寿命。本研究以超高压泵头体为研究对象，但其科学问题的研究成果对其它高压容器的裂纹扩展和寿命预测也具有重要的理论意义和应用价值。

非常规油气开采是我国的重大战略，其关键技术在于压裂，压裂泵作为压裂技术的核心设备，其使用寿命取决于泵头体，泵头体开裂失效已成为制约其工作寿命的主要因素。而唯有准确掌握疲劳裂纹的扩展规律，才能分析疲劳开裂原因，提高泵头体的工作寿命。基于此，本项目研究压裂泵泵头体的疲劳裂纹扩展规律。根据泵头体相贯内腔结构的特征，采用理论分析和数值计算，研究了泵头体相贯内腔裂尖应力场和应力强度因子的变化规律，揭示了影响泵头体开裂失效的主要因素；针对泵头体的主要失效形式，基于弹塑性断裂力学理论及有限元法分析了不同裂纹长度及工作压力下裂纹前缘塑性区长度，根据最大张开位移建立了裂尖塑性区长度多项式公式；基于泵头体材料的疲劳断裂参数，根据裂纹扩展过程中试样柔度与裂纹扩展量之间的关系标定了泵头体材料的内聚力参数；基于双线性内聚力牵引分离法则及Roe-Siegmund疲劳损伤演化方程，开发了适用于2D/3D问题的循环内聚力UMAT子程序，并利用CT试样的疲劳裂纹扩展试验验证了程序的有效性；基于此程序，采用位移差值法建立了泵头体的有限元局部模型，对泵头体微小裂纹的疲劳裂纹扩展进行了分析；最后建立了泵头体的疲劳裂纹扩展寿命预测公式。相关研究结果表明：对于泵头体这类相贯内腔结构，采用K-t11-t33三参数模型能更准确描述裂尖附近区域的应力场；将无缺陷相贯内腔的周向应力作为名义应力，能更好的描述相贯内腔结构裂尖应力强度因子的变化规律；在泵头体裂纹扩展过程中位于排出腔的裂纹最先发生扩展，其次位于堵头腔的裂纹发生扩展，位于初始裂纹前缘中部靠近堵头腔的裂纹扩展最慢；随着裂纹的不断演化，泵头体的疲劳裂纹扩展速率随之增大；当泵头体相贯线处微裂纹尺寸小于临界尺寸时，泵头体周期性的工作压力并不是驱动疲劳裂纹扩展的主要因素；疲劳裂纹扩展寿命预测公式在裂纹长度小于有效厚度一半时比较适用。研究成果为压裂装备中的泵头体的结构设计，工作参数，疲劳寿命预测提供了科学依据和技术指导。