海底双层混输管道内管外腐蚀原因与行为研究

发布时间：2018-09-14 12:42

【摘要】：海底管道损坏造成的油田停产、海域大面积污染事件的日渐增多。腐蚀是引起海底管道损坏的一个重要原因。M油田B平台至单点系泊系统(SPM)海底管道是双层管,但两次内检测中内管外壁腐蚀缺陷从732处增加至4618处。针对海管开展测试与实验研究,分析其外腐蚀的原因与行为,为海底管道的安全运行和预防发生腐蚀失效事故提供理论依据。本文通过内管内检测情况和现场样品分析确定了内管外腐蚀原因,通过焊接接头材料性能测试分析了外管焊接接头的力学性能和金相组织等,进行挂片腐蚀实验、电化学腐蚀实验和微区电化学腐蚀实验研究了内管外腐蚀影响因素的腐蚀规律和外管焊接接头的电偶腐蚀,通过管道剩余强度评价和剩余寿命预测判断了内管是否可以继续服役。本文的研究内容和结论主要包括以下5个方面:(1)通过对内管内检测情况、现场腐蚀产物成分、碳钢海水腐蚀研究、现场保温层样品外观和形状等方面的分析,确定了内管发生腐蚀的原因是外管焊缝开裂,海水与内管外壁发生电化学腐蚀。(2)外管焊接接头射线探伤结果表明焊接接头存在缺陷,不满足SY/T 4103标准;拉伸实验结果表明1-3焊接接头抗拉强度均值低于490MPa,不满足API Spec 5L标准;冲击实验结果表明平均冲击功大于50J,满足工程验收标准要求;硬度测试结果表明焊缝硬度高于母材和热影响区;慢拉伸实验结果表明海水增加了外管焊接接头的应力腐蚀敏感性。(3)外管焊接接头挂片实验结果表明外管焊缝的腐蚀速率高于母材和热影响区。极化曲线和交流阻抗谱测试和拟合结果表明焊接接头焊缝的腐蚀电流密度最大,容抗弧直径最小;电偶腐蚀实验结果表明焊缝-热影响区电偶电流最大。扫描开尔文探针(SKP)测试结果表明外管焊接接头焊缝和母材存在电偶腐蚀效应,焊缝作为阳极腐蚀加速。(4)内管挂片腐蚀实验结果表明随着浸泡时间的增加,试样腐蚀速率呈现降低的趋势;由扫描电镜(SEM)结果可知,试样表面腐蚀产物膜分为两层。电化学腐蚀实验表明不同因素对内管腐蚀规律的影响有所区别。通过电化学腐蚀实验,得到温度、Cl-和SO42-对于内管X65钢的腐蚀规律。扫描开尔文探针测试结果表明在24h内,X65电极试样局部腐蚀随浸泡时间的增加而加剧。(5)管道剩余寿命预测结果表明第一次内检测后管道剩余寿命为5.47年:第二次内检测后管道无剩余寿命,无法继续服役,提出对外管焊接接头断裂和内管腐蚀严重的管段进行更换的维修措施。
[Abstract]:Oil fields are shut down due to submarine pipeline damage, and a large number of pollution incidents in the sea area are increasing day by day. Corrosion is an important cause of submarine pipeline damage. The (SPM) pipeline from B platform to single point mooring system in M oilfield is a double layer pipe, but the corrosion defects of the inner and outer wall of the pipeline are increased from 732 to 4618 in two times of internal detection. In order to provide theoretical basis for the safe operation of submarine pipelines and the prevention of corrosion failure accidents, the causes and behaviors of external corrosion are analyzed. In this paper, the reasons for the corrosion of the inner tube and the outer tube are determined by the internal inspection and the field sample analysis. The mechanical properties and metallographic structure of the welded joint are analyzed by testing the material properties of the welded joint, and the corrosion test of the hanging sheet is carried out. The corrosion law of internal and external corrosion factors and the galvanic corrosion of welded joints were studied by electrochemical corrosion test and micro-area electrochemical corrosion experiment. The residual strength of pipeline and the prediction of residual life are used to judge whether the inner pipe can continue to be in service. The research contents and conclusions of this paper mainly include the following five aspects: (1) through the analysis of the condition of the inner tube, the composition of corrosion products in the field, the corrosion of carbon steel in seawater, the appearance and shape of the insulation layer sample in the field, etc. It is determined that the corrosion of inner tube is caused by weld cracking of outer tube and electrochemical corrosion between seawater and outer wall of inner tube. (2) Radiographic inspection results of welded joint of outer tube show that the welded joint is defective and does not meet SY/T 4103 standard; The tensile test results show that the mean tensile strength of 1-3 welded joints is lower than 490 MPA and does not meet the API Spec 5L standard, while the impact test results show that the average impact energy is more than 50 J, which meets the requirements of engineering acceptance standard. The hardness test results show that the hardness of weld is higher than that of base metal and heat affected zone. The results of slow tensile test show that seawater increases the stress corrosion sensitivity of welded joints of outer tubes. (3) the experimental results show that the corrosion rate of welded joints of outer tubes is higher than that of base metal and heat affected zone. The polarization curve and AC impedance spectrum test and fitting results show that the welding seam has the largest corrosion current density and the smallest capacitance arc diameter, and the galvanic corrosion test results show that the weld-heat-affected zone has the largest galvanic current. The results of scanning Kelvin probe (SKP) test show that there is a galvanic corrosion effect on weld and base metal of welded joint of outer tube, and the weld is accelerated as anode corrosion. (4) the corrosion test results of inner tube hanging sheet show that the corrosion rate increases with the increase of soaking time. The corrosion rate of the sample shows a decreasing trend and the corrosion product film on the surface of the sample is divided into two layers according to the (SEM) results of scanning electron microscope. Electrochemical corrosion experiments show that different factors have different effects on the corrosion law of inner tubes. The corrosion behavior of inner tube X65 steel at different temperatures was obtained by electrochemical corrosion experiments. The results of scanning Kelvin probe test showed that the local corrosion of X65 electrode samples increased with the increase of immersion time within 24 hours. (5) the predicted residual life of pipeline after the first detection was 5.47 years: within the second time. After testing, the pipeline has no residual life. This paper puts forward the maintenance measures to replace the welded joint of outer pipe and the seriously corroded inner pipe.
【学位授予单位】：西南石油大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TE988.2

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