Study On Failure Mechanism For Submarine Pipeline With Corrosion Defects

Submarine pipelines have been widely used as one of the most economical ways of transmitting oil and gas from the offshore oil fields to the terminals. The damage of submarine pipelines could cause not only economical loss but also severely environmental pollution especially in the harsh marine environment. Corrosion is a kind of major degradation form for submarine pipelines, which accounts 24 per cent of all offshore pipeline accidents. This project is funded by National Natural Science Foundation of China (Grant No. 50439010), and Ministry of Education of China (Grant No. 305003). The objectives are to study the failure mechanism for submarine pipelines with corrosion defects.Based on the practical submarine pipeline installed in ShengLi offshore oil fields, ANSYS software is used to study the free spanning submarine pipeline with corrosion defects, which is the critical case. The results from FE analysis are compared with B31G code and DNV-RP-F101 criteria. The research is composed of the following works.1. A review of the study on the damage mechanism of corroded submarine pipeline under complex loadings are introduced, and the aims of the thesis are presented firstly.2. Based on the theories of geometric non-linearity and the material non-linearity, the finite element analysis is performed to study failure mechanism and residual strength of submarine pipeline with infinite-long longitudinal corrosion defects under internal pressure. The effects of corrosion sizes such as corrosion depth and width on residual strength are discussed. The results from FE analysis are compared with B31G code, DNV-RP-F101 criteria and the analytical formula deduced from assumption of plane strain. The FE analysis method is feasible to analyze the residual strength of corroded pipelines.3. The ultimate load and failure behavior of submarine pipeline with finite-long corrosion defects subject to internal pressure are studied. The effect of corroded length, width, depth and the ratio of out-diameter to thickness on failure pressure are discussed. Predicted fitting formula for failure pressure of corroded submarine pipeline was proposed, which is validated by comparing with the burst test results, codes and methods proposed by the other scholars.4. The FE analysis and damage mechanism of submarine pipeline with corrosion defects exposed to complex loadings such as combined bending moment and internal pressure or combined axial force and internal pressure, are studied. The results from FE analysis are compared with DNV-RP-F101 code.5. The ultimate capacity and failure behavior of submarine pipeline with double corrosion defects along axial and circumferential direction are analyzed due to internal pressure respectively. The interaction relationships of submarine pipeline with double corrosion defects along axial and circumferential directions are proposed respectively. The interaction relationship proposed in the thesis may give more accurate results than the methods used in the existing codes.