The ultimate strength of metallic pipelines will be inevitably affected when they have suffered from structural damage after mechanical interference. The present experiments aim to investigate the residual ultimate bending strength of metallic pipes with structural damage based on large-scale pipe tests. Artificial damage, such as a dent, metal loss, a crack, and combinations thereof, is introduced to the pipe surface in advance. Four-point bending tests are performed to investigate the structural behavior of metallic pipes in terms of bending moment-curvature diagrams, failure modes, bending capacity, and critical bending curvatures. Test results show that the occurrence of structural damage on the pipe compression side reduces the bending capacity significantly. Only a slight effect has been observed for pipes with damage on the tensile side as long as no fracture failure appears. The possible causes that have introduced experimental errors are presented and discussed. The test data obtained in this paper can be used to further quantify damage effects on bending capacity of seamless pipes with similar D/t ratios. The comparison results in this paper can facilitate the structural integrity design as well as the maintenance of damaged pipes when mechanical interference happens during the service life of pipelines.

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On the basis of an experimental investigation [1], numerical investigation is conducted in this paper on damaged seamless metallic pipelines with metal loss (diameter-to-thickness ratio D/t around 21) through nonlinear finite element method (FEM). Numerical models are developed and validated through test results by using the measured material properties and specimen geometry, capable of predicting the residual ultimate strength of pipes in terms of bending capacity (M_cr) and critical curvature (κ_cr). By changing the metal loss parameters, i.e. length (l_m), width (w_m) and depth (d_m), a series of numerical simulations are carried out. Results show that the larger the d_m or l_m is, the less the bending capacity will be. The increase of notch width slightly reduces the pipe strength, presenting a linear tendency. Based on the FEM results, empirical formulas are proposed to predict the residual ultimate strength of metallic pipes with metal loss under pure bending moment. The prediction results match well with the results from the tests, the numerical simulations as well as the theoretical derivation. Such formulas can be therefore used for practice purposes and facilitate the decision-making of pipe maintenance after mechanical interference.

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The combination damage induced by mechanical interference, in reality, is more likely to happen. In this paper, numerical models on pipes with combined dent and metal loss in terms of a notch are developed and validated through tests (diameter-to-thickness ratio D/t of test pipes around 21), capable of predicting the residual ultimate strength of pipes in terms of bending moment (M_cr) and critical curvature (κ_cr). The effect of residual stress is explored, assuming a linear distribution in the pipe hoop direction. Investigations of damaged pipes with different D/t (15–50) are carried out. Through changing damage parameters in the combinations, i.e. dent depth (d_d) or metal loss depth (d_m), the corresponding effects of damage are clarified. Results show that the combined dent and notch damage is a more severe type of damage on pipe strength compared with other damage types (excluding fracture). The dent in combined damage plays a more dominant role on the pipe residual strength. Empirical formulas are proposed to predict residual ultimate strength of damaged metallic pipes (D/t around 21) with combined dent and metal loss under bending moment, which can be used for practical purposes. The application domain can be expanded to pipes with D/t up to 30 based on simulations.

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Numerical investigation is conducted in this paper on both intact and dented seamless metallic pipelines (diameter-to-thickness ratio D/t around 21), deploying nonlinear finite element method (FEM). A full numerical model is developed, capable of predicting the residual ultimate strength of pipes in terms of bending capacity (M_cr) and critical curvature (κ_cr). The simulation results are validated through test results by using the measured material properties and specimen geometry. An extensive parametric investigation is conducted on the influences of material anisotropy, initial imperfection, friction of the test set-up and dent parameters. It is found that the structural response is quite sensitive to the frictions that have been introduced by the test configuration. For a pipe with a considerable dent size, the effect of manufacturing induced initial imperfection is insignificant and can be neglected in the FEM simulation. The material yield stress in the pipe longitudinal direction dominates the bending capacity of structures. In the end, formulas are proposed to predict the residual ultimate strength of dented metallic pipes under pure bending moment, which can be used for practical purposes. A satisfying fit is obtained through the comparison between the formulas and FEM methods.

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A dent is one of the main structural damages that may affect ultimate strength. In this paper, the residual ultimate strength of dented metallic pipes subjected to a bending moment is quantitatively investigated. The numerical model is developed accounting for the variation of the dent length (l_d), dent depth (d_d), dent width (w_d), dent rotation angle (θ_d) and dent location based on ABAQUS Python. The numerical model is validated by test results from a four-point bending test. Subsequently, a parametric investigation is performed on the effects of wave-type initial imperfection, D/t and dent geometrical parameters. It is found that both l_d and d_d have a significant effect on the residual ultimate strength of dented metallic pipes, while the effect of w_d is slight. Finally, an empirical formula with respect to l_d and d_d has been proposed for the prediction of bending moment, which can be deployed for practical purposes.

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This thesis deals with the structural damage and its corresponding effect on the residual ultimate strength of seamless metallic pipes. The structural damage is here characterized as the type of damage produced by external mechanical interference, considerably undermining the strength and/or stability of the pipes. Not only the aesthetic appearances of the pipes are affected by the occurrence of damage, but most importantly the pipes’ functional demands are jeopardized. This research project aims to develop an effective prediction model to predict the residual ultimate strength of damaged metallic pipes subjected to a bending moment. To be able to effectively predict the residual strength, the types of structural damage need to be first categorized and simplified. Three typical types of damage are deployed for the further quantification of the effects, including a dent, metal loss in terms of a notch, and a combined dent and notch on the compression side of pipes. Meanwhile, the damage in terms of a crack, a notch and the combined notch and crack on the tensile side of pipes is qualitatively investigated. Once it occurs, the fracture failure of pipes is likely to happen, which may introduce a detrimental consequence. @en

The ultimate strength of metallic pipelines will be inevitably affected when they have suffered from structural damage. The present experiments aim to investigate the residual ultimate bending strength of metallic pipes with structural damage based on large-scale pipe specimens. Artificial damage such as dent, metal loss, crack and combinations thereof is introduced on the pipe surface in advance. The entire test project consists of 34 seamless pipes with a relative low Diameter-to-thickness (D=t) ratio around 21.3, among which four intact specimens and thirty damaged specimens have been carried out for mutual comparison. Extensive measurements on structural damage and pipe geometries including wall thickness and outer diameter are performed. The material properties are measured by tensile tests with specimens from both pipe longitudinal and hoop direction. The four-point bending tests are performed to investigate the structural behaviors of metallic pipes. The bending strength associating with failure mode of each specimen is documented extensively, and the bending moment-curvature curves are presented and discussed. The fundamental research of experiments on damaged pipes in the present paper will be deployed for the following numerical and analytical research in the near future.

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While the offshore pipelines are designed to withstand an extensive range of combined loads such as the internal/external pressure, axial force and bending moment, the residual strength of these structures accounting for impact induced damage is one of the most detrimental situations in limit state design. The impact might be caused by machinery such as trawl gears or anchors, which could introduce mechanical damage. One specific case is the interaction between the pipelines and fishing trawl boards when they are towed underwater. Depending on the pipe material properties and the impact forms, mechanical damage could be a dent, metal loss or even a crack. Therefore, the aim of this paper is to investigate the residual strength of metallic pipelines with impact induced dent or combined dent & crack subject to dominated bending moment with a combination of axial force and internal pressure under a certain level of absorbed energy. The nonlinear finite element method is deployed and the numerical models accounting for damage are developed. Simulation results are compared with each other and the most unfavorable damage pattern of the pipelines is determined. Research found that the combined dent & crack damage significantly decreased the residual strength of pipe structure under combined loads, and single dent damage had slight effect on pipe residual strength.@en