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Residual ultimate strength of seamless metallic pipelines with structural damage
... Residual stresses can be defined as auto-balancing stresses which are locked into a material when it is free from external forces (Cai, 2018). In submarine pipelines, extensive residual stresses may exist due to many factors such as fabrication, pipe joining processes, welding, heat treatment, mechanical interferences, forming processes and long term service conditions (Jr et al., 1990;Amirat et al., 2004;Pirling et al., 2011). ...
... Meanwhile, it is accepted that the influence of lateral geometric imperfections dominates the influence of residual stresses when it comes to the axial buckling capacity of conventionally fabricated tubular columns (Bjoerhovde, 1973;Toma and Chen, 1979). This phenomenon has been also observed throughout the analysis of a damaged pipe under pure bending, accounting for both residual stresses and initial imperfections (Cai, 2018). ...
This paper consists of a literature review on the latest research progress about the lateral buckling of submarine pipelines under high temperature (HT) and high pressure (HP). First, the main general assumptions and simplifications made in the context of pipe lateral buckling are summarized in order to better understand the practical behavior of submarine pipelines. The governing equations of pipelines under uniaxial compression are then derived. Next, the controversial but widely deployed concept of effective axial force under complex sea environment is introduced. Influential parameters including initial imperfections, pipe-seabed interactions and residual stresses are elaborated and discussed. Furthermore, numerical simulation methods and experimental
tests dealing with the lateral buckling of pipelines are presented as well. Controlled methods which are practically used for buckle initiation are described. This paper also reveals the remaining challenges and new tendencies, such as the use of data-driven methods for the smart prediction of pipe buckling. Finally, a specific case study is numerically conducted. It is found that the effect of axial friction variation can be
generally ignored in practical calculations. This paper may provide a guidance for the design and research on pipelines in the future.
Due to increasing urbanisation rates worldwide combined with growing transportation demand, liveability of the urban environment is under pressure (UN, 2018). In response, many governments worldwide have set goals for increasing the share of trips made using sustainable modes of transport, such as walking and cycling. The use of active modes (i.e. walking and cycling) provides health benefits for individuals due to increased activity levels, and on a network level these modes (standalone or in combination with public transport) can potentially reduce traffic jams and the associated externalities (including air and noise pollution) when substituting the car. To achieve the desired increase in active mode shares, targeted policies need to be implemented. This requires a better understanding of who currently uses these modes, who could be persuaded to switch to active modes, and which determinants are driving active mode choice. This intended change towards active modes requires an adequate representation of walking and cycling in the transportation planning models in order to assess the effect of active mode policies on modal shares and distribution over the network. However, this is often not the case. Moreover, integration of active modes in these models occurs very slowly. Walking and cycling are often missing in transportation planning models, treated as a ‘rest’ category, or combined into slow/active modes, all of which result in incorrect estimates of the active mode shares, making it impossible to correctly identify the impact of potential policy measures on active mode shares. Examples of these policy measures are introduction of new infrastructure or changes to existing infrastructure, which impact route choice and distribution over the network, and reimbursement of using the bicycle to go to work, which impacts the mode choice of individuals.Investigating mode and route choice of active mode users increases the knowledge on active mode choice behaviour. By bridging this gap, the transportation planning models can potentially be improved. The objective of this thesis is ‘to understand and model mode and route choice behaviour of active mode users’. We identify six topics that are imperative to travel choices. First, we investigate the daily mobility patterns of individuals in relation to attitudes towards modes, because attitudes are considered to influence travel behaviour (Chapter 2). Afterwards, we zoom in on individual trips. We aim to understand which determinants drive the choice to walk or cycle (Chapter 3). In this topic we define the mode choice set as all feasible modes per individual and trip. However, not all feasible modes are used by individuals. Therefore, the third topic focuses on modes used over a long period of time, which we coin the experienced choice set. We investigate which determinants are relevant for including or excluding modes in this choice set (Chapter 4). Regarding cyclists’ route choice, we investigate the determinants influencing this choice (Chapter 5). This research is based on the experienced choice set. Accordingly, we compare this method to frequently used choice set generation methods to identify the added value of the experienced choice set (Chapter 6). Finally, we perform a literature review on how mode and route choice can be modelled simultaneously (Chapter 7).
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.
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.
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 (ld), dent depth (dd), dent width (wd), dent rotation angle (θd) 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 ld and dd have a significant effect on the residual ultimate strength of dented metallic pipes, while the effect of wd is slight. Finally, an empirical formula with respect to ld and dd has been proposed for the prediction of bending moment, which can be deployed for practical purposes.
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.
The latest research progress on residual ultimate strength of metallic pipelines with structural damage is presented through literature survey. The investigated pipe diameter-to-thickness ratios majorly lie between 20 and 50, which are typically applicable in deep water. Influential parameters in terms of pipe load, installation process and material that affect the ultimate strength of pipes are categorised. Structural damage including dent, metal loss and crack is identified and efforts are made to summarise critical damage factors such as dent length and crack depth. Furthermore, research and prediction methods on pipe residual ultimate strength in terms of experimental tests, numerical simulations and analytical predictions are summarised and discussed. Specific details on how to introduce, simplify and simulate structural damage are presented and discussed. It is expected that the mechanism of residual ultimate strength of metallic pipes with structural damage can be clarified through this study so that guidance will be provided for researchers in this field.
The stability of thin plates has been investigated by many authors. However, in aeronautical structures, thin metal sheets are often used beyond the stability limits, and the load which can be carried by the structure is determined by the ultimate strength in compression. A recent series of experiments by the Bureau of Standards showed the ultimate load to be independent of width and length of the plate and approximately proportional to the square of the thickness. In the present paper an approximate theoretical analysis of this problem is developed, by which the “effective width” and the ultimate strength can be found. The result of this analysis shows the ultimate strength of a plate to be proportional to the square roots of the modulus of elasticity and the yield point of the material, and to the square of the thickness. This result gives a good check with the experiments mentioned.
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 (Mcr) and critical curvature (κcr). 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.
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 (Mcr) and critical curvature (κcr). 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 (dd) or metal loss depth (dm), 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.
Introduction Primary Characteristics Contributing Factors Mechanisms for Stress Corrosion Crack Initiation Mechanisms for Stress Corrosion Crack Propagation Models for the Prediction of a High-pH Stress Corrosion Crack Growth Rate