Study On Safety Assessment Technology Of Submarine Pipeline With Internal Corrosion

With the occurrence of bad oil spill in domestic and international in recent years, the relevant state agencies and pipeline operators pay more attention to the security and reliability of the various aspects and a variety of facilities of offshore oil production process. Submarine pipeline is an important transportation and means of offshore oil production. Submarine pipeline safety is directly related to offshore oil production and the marine environment. However, according to the statistics, corrosion is one of the most important factors in causing submarine pipeline failure. There must be some means of detection to grasp the corroded defects state of the pipe and the appropriate safety assessment should be implemented. This paper mainly studies on the following aspects:First, based on extensive literature data collection and field research in domestic and international, the application status as well as research results of submarine pipeline corroded defects safety assessment technology at home and abroad were analyzed and compared; structural characteristics of submarine pipeline systems, transmission characteristics, corrosion characteristics and influencing factors and the type of corroded defects were summarized; the undersea pipeline corrosion failure mode features, submarine corrosion pipeline failure mechanism, the applicability of common standard to assess volumetric defects pipeline for submarine pipeline safety assessment were analyzed.Second, in view of DNV RP-F101Recommended Practice for Corroded Pipelines and combination of the actual detect situation of submarine pipeline, submarine pipeline corrosion defects was classified as single defect and interaction defects; calculation program for remaining strength of the corroded submarine pipeline was developed with Visual Basic6.0as a platform by subentry safety coefficient method and allowed stress method; the sensitivity of several major defects parameters (such as defect depth, defect length, adjacent defects spacing) were analyzed by above calculation program, it showed that the depth of corrosion defect of pipeline has the greatest impact on the allowable operating pressure of corroded submarine pipeline. Simultaneously, the applicability and difference of the two kinds of calculation methods of the residual strength are evaluated by calculation program, the subentry safety coefficient method is recommended to evaluate the interaction defects submarine pipeline, and in the evaluation of the single defect submarine pipeline, should choose the two method to calculate the value of smaller value as submarine pipeline corrosion of the allowable stress value.Then, bearing capacity of corroded submarine pipeline with single and double tube structure was studied by the finite element software ANSYS. To consider the actual shape of submarine pipeline corroded defects, shape of submarine pipeline corroded defects were simulated respectively by cuboid and hemisphere. Because of the special structure of the single-layer tube, special consideration was given to the influence of concrete with a heavy layer on the remaining strength of the defective pipeline in modeling, and simulation findings showed that heavy layer had a great impact on residual strength of single-layer tube. According to structural characteristics that inner tube of double submarine pipeline subjected to internal pressure, the outer tube withstand the external pressure, Finite element modeling was only carried on inner tube in the analysis of the residual strength of the double-layer pipe.Finally, in view of the stress concentration phenomenon generated by localized corrosion thinning, the probability of failure model of corrosion factors was corrected. Combining allowable operating pressure and maximum stress safety of submarine pipeline the conclusions was checked. Based on submarine pipeline safety standards in DNV OS F101Submarine Pipeline Systems, the safety process to assess the limit state of submarine corroded pipelines was proposed. The submarine corroded pipelines in Wei Zhou Operations District CNOOC Ltd Zhanjiang were assessed by this safety assessment process and probability of failure was calculated by DNV model. The evaluation showed that the security level of this submarine pipeline is low. Appropriate security maintenance measures needed to take.