HIGH PEAK STRAIN ON PIPELINE MATERIAL DURING REEL-LAY: ACCEPTABLE OR NOT?

摘要

The nominal strain occurring during installation of a pipeline by the reeling method is expected to be limited to 2-3% strain. This is only true if the pipeline has a perfect geometry (diameter, thickness) and homogeneous material properties along its length, resulting in a uniform bending stiffness. There will however always be a stiffness mismatch at the joints between pipes. Different scenarios can be considered as the cause of this stiffness mismatch: differences in average wall thickness and average yield stress of two pipe joints welded together, counterbored/machined pipe ends or field joint coating for pipes with thick coating. To some extent these scenarios can initiate high peak strains in the pipeline material far above the level of the expected nominal strain, exceeding in some cases 5% strain. Questions which might arise are: Could this high peak strain occurrence be ignored?, or: What is the impact of the high peak strain on the performance of the material after reeling ? This paper presents FEA results illustrating the concerns of the occurrence of large peak strains which can still be significant even after averaging these strains over the thickness as well as over a certain length of the pipe. The methodology of averaging strains, as proposed together with DNV GL, correlates the length of the averaging pipe section with the maximum length of the test specimen geometry as allowed when performing strain aging tests of specimens with high tension and compression strains. A series of cyclic plastic deformation tests with pre-strained specimens from 4% to 7% strain was performed with seamless pipe material, followed by the tensile, Charpy and hardness tests of strain aged samples. One of the challenges is the setup of the test machine to avoid buckling of specimens during high compression pre-straining. The results from material tests (tensile, hardness and Charpy) have been evaluated against the DNV-OS-F101 Supplementary requirement for plastic deformation (P). The consequences of material modification due to plastic strain is further discussed and evaluated referring to the DNV GL limit state design and criteria for pipeline installation after reeling as well as during the lifetime of the pipeline.