Recovery Study Of Smooth Dent Defects On Long-distance Pipelines

Elastoplastic deformation of the pipe wall will be caused because of the occurrence of dent defects.When the external load is removed or internal pressure is changed,a certain degree of re-rounding will occur in the dented area,leading to variations in dent depths and dent lengths and affecting the evaluation and management of dents.The investigation of dent re-rounding is a necessary and basic work rather than a direct method of dent evaluation.As a result,it is essential to introduce the dent re-rounding into the evaluation of dented pipelines.This paper initially investigated a large number of domestic and aboard literatures,followed by introducing the definition and main types of dents.Dent re-rounding procedures were described and in-line inspection methods often used in dented pipes were also concisely overviewed.A three-dimensional finite element model with an indenter and a pipe was established by ABAQUS that is a well-known finite element software.After validation of the FE model,the effects of wall thickness,internal pressure,indenter size,dent location and dent depth on the dent re-rounding behavior were analyzed.The significance of each factor was analyzed by combining an orthogonal experimental design with the Grey correlation degree.Nonlinear regression analysis was used to obtain formulas for the re-rounding coefficient as a function of the influential factors.In addition,the value range of dent length was determined associated with the in-line inspection data of Guangyuan-Jiangyou section of LanChengYu product oil pipeline.The effects of the ratio of diameter to wall thickness and the internal pressure on the re-rounding amount of dent depth and dent length were investigated and the formulas for the re-rounding amounts of dent length as a function of the influential factors.Finally,the effect of dent re-rounding on the ductile damage of the dented pipe was analyzed in terms of the ductile damage model and the ductile damage level of the dented pipeline under different influential factors was calculated.The main conclusions of the above research are as follows:(1)The dented area rebounded elastically after removing the indenter from the pipe.With the respective growth of loading depth,wall thickness and indenter size,re-rounding coefficients(the ratio of the dent depth after re-rounding to the dent depth before re-rounding)increased continuously,which indicated that the dented area rerounded difficultly during this process.Besides,the location of indentation formed on the pipe was closer to the bottom of the pipe,the amount of dent rerounding also decreased.(2)After pressurization,difficulty arose for the deep dent re-rounding in comparison to the shallow dent.A thicker wall thickness led to less dent re-rounding.With the growth of internal pressure,the dent depth was greatly reduced,which correspondingly decreased the re-rounding coefficient.In addition,a larger indenter size led to less dent re-rounding.Furthermore,the closer the indenter was to the bottom of the pipe,the dented area was difficult to reround plastically.Consequently,attention should be paid to the dents at nought o'clock in the cross section of pipes because the dent defects here almost rerounded freely.The order of importance of the influential factors from largest to smallest was dent depth,internal pressure,dent location,wall thickness and indenter size.(3)After de-pressurization,re-rounding occurred in the dented area.The re-rounding coefficient decreased with increasing of dent depth,wall thickness,indenter size and dent location.However,the re-rounding coefficient increased with increasing internal pressure.The re-rounding coefficients were concentrated between 1.05 and 1.35.Accordingly,the literature values of the re-rounding coefficient without internal pressure of 1.43 are not very accurate and not applicable to all cases.The order of importance of the influential factors from largest to smallest was the internal pressure,dent depth,wall thickness,indenter size and dent location.(4)According to sensitivity of geometric inspection tool(20#CalScan)to the detection of dent defects,the range of a dent length was determined to be the distance between two points both with a depth of 4.064mm on the outer wall at both ends of a dent.The amounts of the dent depth re-rounding and the dent length re-rounding increased with increasing deformation degree of a dent while decreased with increasing wall thickness and internal pressure.However,the re-rounding of dent length was more affected by internal pressure.(5)The ductile damage degree of dented pipeline would reduce during the first and third dent re-rounding while the second dent re-rounding would increase the ductile damage of dented pipeline due to pressurization.The ductile damage raised with increasing dent depth in regards to running dented pipes.If there were dent defects with same depth on the pipes,a thicker wall thickness led to larger ductile damage.Besides,a smaller indenter size led to more serious ductile damage of the pipe.Therefore,dent magnitude should be considered in the evaluation of a dented pipe.The closer the dent was to the bottom of the pipe,the greater the ductile damage caused by the indenter,that was to say,the easier the pipe to crack.Consequently,the effect of the dent location should be taken into account in the safety assessment of dented pipes.It was noteworthy that a certain internal pressure could restrain the deformation of dent and protect the pipes.On the contrary,the excessive internal pressure led to increase the ductile damage of the dented pipes due to the dent re-rounding.