【作者】 黎超文；

【导师】 王勇；

【作者基本信息】 中国石油大学 ， 化工过程机械， 2011， 博士

【摘要】 在役焊接是一种安全、环保、经济、高效的管道维修技术,适用于原油、成品油、化工介质、天然气等多种介质管线的正常维修改造和突发事故的抢修（如带压抢修、更换腐蚀管段、加装装置、分输改造等作业）。在役焊接修复可以保持管线的连续运行,具有巨大的经济效益和广阔的应用前景。在役焊接过程中容易出现烧穿失稳和氢致裂纹,其中避免烧穿失稳是实施管线在役焊接安全修复首先需要解决的问题。本文根据管线在役焊接的实际工况,采用平板腔室的设计方法,搭建了压力管道在役焊接烧穿模拟试验装置,采用该装置对X70管线钢进行了在役焊接烧穿试验。对在役焊接温度场和烧穿失稳熔池尺寸进行测量,观察分析在役焊接接头失稳形貌和显微组织的精细结构,并对在役焊接烧穿失稳机制和影响因素进行深入的研究。深入分析了在役焊接接头与管道输送介质之间的换热过程和特点,在充分考虑管道材质和管内输送介质物性参数随温度变化的基础上,修正了现行采用的换热系数公式,采用有限元软件SYSWELD建立三维有限元模型,对X70长输管线在役焊接温度场进行模拟,将管道强度降低的高温区域等效为瞬态“体积型缺陷”,计算管线允许的最大安全操作压力从而对长输管线在役焊接烧穿失稳进行判定。研究结果表明:在役焊接由于内部介质带走焊接接头的热量,导致其熔池尺寸和高温区范围变小,其热循环参数t_H、t_8/5、和t₁₀₀均比常规焊接时小。在役焊接失稳模式主要有膨胀失稳和烧穿失稳,在介质压力作用下,焊接接头产生了“外凸”的膨胀变形,烧穿失稳的典型形貌为熔池前端高温区形成较小的烧穿孔洞。在役焊接膨胀变形与焊接温度场引起的热应力、内部介质的压力作用在管壁上产生的应力、管材自身的临界失稳应力有关,当前两者的综合作用应力超过材料临界失稳应力时,就会产生膨胀变形。烧穿失稳取决于电弧在管壁上形成的温度场和管内介质压力,当熔池区域的剩余强度不足以承载内部介质压力时,烧穿失稳就会发生。将材料在高温下的承载能力换算成在常温下的有效承载厚度,将局部高温引起的强度损失转换为常温下管壁的金属损失,这样将高温的在役焊接接头看成是含瞬态“体积型缺陷”的管道,该管道的最大安全操作压力可以通过DNV-RP-F101方法计算,从而判定烧穿发生的可能性。该方法在考虑管道内介质压力基础上,只需要计算温度场,从而能提高计算效率和预测精度。以VB6.0为平台设计了长输管线在役焊接烧穿失稳评定系统,该系统通过调用SYSWELD软件对在役焊接温度场进行求解,调用Matlab软件对管线最大安全操作压力进行分析计算,然后根据计算结果判定该工况下进行在役焊接的安全性。在0.1 MPa～5 MPa范围内,天然气管道在役焊接的最大安全操作压力随着内部压力的升高呈上升趋势;最大安全操作压力随着焊接热输入的增加而降低,在确保安全操作的条件下,应尽量采用较高的运行压力和较小的焊接热输入,减小在役焊接对管线正常运输的影响。通过正交设计试验可知,管道壁厚、焊接热输入和介质压力是影响在役焊接烧穿的主要因素,管道直径变化对在役焊接烧穿影响较小。更多还原

【Abstract】 In-service welding is a maintenance and emergency repairing technique for operating pipelines which features safe, environment protection, economic, high efficiency and suitable for pipeline proper maintenance, rehabilitation, rush to repair for sudden accident （such as emergency repair under pressure, replace the corroded pipe sections, fix additional devices and off-take pipeline etc）. The pipeline medium can be crude oil, product oil, chemicals, natural gas etc. This in-service welding repair can maintain continuous operation of pipelines and has good economic return and extensive prospect of application.There are two primary concerns with welding onto in-service pipelines. The first is for burn-through, where the welding arc causes the pipe wall to be penetrated, allowing the contents to escape. The second concern is for the integrity of the pipeline following repair. Preventing burn through is the first problems need to be solved during in-service welding of pipeline. In this dissertation, based on the actual operating condition of in-service welding onto pipelines, an in-service welding burn-through test device, which was designed and established by the design method of bead-on-plate welding on chamber, was used to conduct the in-service welding experimental study of burn through onto X70 pipelines. The temperature distributions and the size of failure’s molten pool were measured. The morphology and microstructure of the burn-through joints were observed by optical microscope and scanning electron microscopy. The mechanism of bulge and burn through failure during in-service welding and its affecting parameters were studied carefully. The heat transfer process and characteristic between in-service welding joint and pipeline contents were analyzed. Heat transfer formula adopted by former researchers was modified and new formula was deduced based on considering the thermophysical parameters of pipeline steel and pipeline contents varying with temperature. A three-dimension finite element （FE） model, which is integrated into commercial finite element analysis （FEA） software SYSWELD by means of user subroutines, was established to simulate the in-service welding of X70 pipeline steel and predict temperature distributions. The calculated transient‘volume-scale defect’in the pipe wall representing the loss of strength during in-service welding, which can be calculated from the results of the simulated temperature and the yield strength of the material. The maximum allowable operating pressure （MAOP） of a pipe with a volume-scale defect in its wall can be used to evaluate the safe of in-service welding of long-distance pipeline.The experimental results show that the pipeline contents create a large heat loss through the pipe wall, resulting in accelerated cooling of the weld. The size of heat affected-zone （HAZ） and high temperature region is relative small during in-service welding. The parameters of welding thermal cycle such as t_H, t_8/5 and t₁₀₀ for in-service welding are smaller than traditional welding. The main failure modes of in-service welding are bulge deformation and burn through. The welding deformation of in-service welding joint is external convex for the effect of pipeline content’s pressure. The typical appearance of burn through is only a small hole in the high temperature region of molten pool.The thermal stress causing by welding temperature, the load stress causing by the content’s pressure and the material critical failure stress have an effect on bulge deformation during in-service welding. If the former two types of stress are bigger than the material critical failure stress, the welding bulge deformations will occur. Burn through depends on the pipe’s internal pressure and the localized high temperature, which creates a local reduction of pipe-wall strength in the region of the welding pool. The pipe wall may burst if the effective strength of the wall in the region of the welding pool can’t carry the pipe’s internal pressure during the welding process. The reduction in strength in the weld can be represented by an effective reduction in thickness of the pipe wall, at its original strength. The reduction of strength was changed into a local thinning region in pipe-wall. The in-service welding pipe joint can be viewed as a pipe with local volume-scale defect. The MAOP of a pipe with a volume-scale defect in its wall can be calculated by using a procedure specified in the Standard DNV-RP-F101. Then the safety of in-service welding can be predicted. This approach only requires a thermal field calculation which can improve the efficiency. In addition, this technique accounts for internal pressure which can enhance the precision of prediction. The evaluation system of the safety of in-service welding onto long-distance pipeline was design on VB6.0. The system can call SYSWELD soft to calculate the temperature during in-service welding, and Matlab soft to calculate its MAOP. Then the safety of in-service welding under specific situation can be predicted.The MAOP increases with the increase of pipe internal pressure at 0.1MPa～5MPa. The MAOP decreases with the increase of heat input. While carrying a safety of in-service welding on pipeline, relative higher pipe’s internal pressure and smaller heat input should be taken for decreasing the effect on pipeline. Based on orthogonal experimental design, pipe wall thickness, heat input and internal pressure have much more great effect on the safety of in-service welding, while the influence of pipe diameter is relative small.更多还原