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The interaction between the pipeline and the surrounding soil is a critical factor in the buckling analysis of the pipeline operating at high temperatures and pressures. The heating pipeline alters the temperature and induces excess pore pressure in the surrounding soil, which in turn affects the mechanical properties of the soil. Therefore, a comprehensive understanding of the thermo-mechanical response of the soil surrounding the pipeline during operation and shutdown is essential. To address this issue, this study developed a temperature-controlled pipe model test device that simulates the heating and cooling cycle of the pipe and the temperature field and pore pressure response of the surrounding kaolin clay. The test results showed that the soil temperature at the closest location to the pipe (0.1 D) reached a peak value of approximately 48.7°C when the pipe was heated from 12.5°C to 55°C. The peak temperature decreased with increasing distance, reaching 22.5°C at 0.875D. Heat transfer analysis using ABAQUS software revealed that the influence range of the pipeline on the soil temperature field during the heating cycle was about 2D. Additionally, a parametric study focused on the thermal conductivity (λ soil ) of kaolin, and the best-fit value was determined to be 0.6 W/(m·°C. The excess pore pressures were generated in the kaolin surrounding the pipes during the heating cycle, and negative pore pressures were observed after cooling cycle. Furthermore, a theoretical solution for calculating the thermally induced pore pressure was derived, and preliminary verification showed good agreement between the theoretically computed and experimentally measured results. These findings provide valuable insights into the thermo-mechanical response of the soil surrounding pipelines during operation and shutdown, which can improve the design and safety of oil and gas pipelines.

In nearshore regions, bidirectional tidal flow is the main hydrodynamic factor, which induces local scour around submarine pipelines. So far, most studies on scour around submarine pipelines only consider the action of unidirectional, steady currents and little attention has been paid to the situation of bidirectional tidal currents. To deeply understand scour characteristics and produce a more accurate prediction method in bidirectional tidal currents for engineering application, a series of laboratory scale experiments were conducted in a bidirectional current flume. The experiments were carried out at a length scale of 1:20 and the tidal currents were scaled with field measurements from Cezhen pipeline in Hangzhou Bay, China. The experimental results showed that under bidirectional tidal currents, the scour depth increased significantly during the first half of the tidal cycle and it only increased slightly when the flow of the tidal velocity was near maximum flood or ebb in the following tidal cycle. Compared with scour under a unidirectional steady current, the scour profile under a bidirectional tidal current was more symmetrical, and the scour depth in a bidirectional tidal current was on average 80% of that under a unidirectional, steady current based on maximum peak velocity. Based on previous research and the present experimental data, a more accurate fitted equation to predict the tidally induced live-bed scour depth around submarine pipelines was proposed and has been verified using field data from the Cezhen pipeline.

A fracture mechanics-based fatigue reliability analysis of a submarine pipeline is investigated using the Bayesian approach. The proposed framework enables the estimation of the reliability level of submarine pipelines based on limited experimental data. Bayesian updating method and Markov Chain Monte Carlo simulation are used to estimate the posterior distribution of the parameters of a fracture mechanics-based fatigue model regarding different sources of uncertainties. Failure load cycle distribution and the reliability-based performance assessment of API 5L X56 submarine pipelines as a case study are estimated for three different cases. In addition, the impact of different parameters, including the stress ratio, maximum load, uncertainties of stress range and initial crack size, corrosion-enhanced factor, and also the correlation between material parameters on the reliability of the investigated submarine pipeline has been indicated through a sensitivity study. The applied approach in this study may be used for uncertainty modelling and fatigue reliability-based performance assessment of different types of submarine pipelines for maintenance and periodic inspection planning.

Buckling initiation devices/techniques, including sleepers, distributed buoyancy, snake lay, and residual curvature method (RCM), have recently been widely applied in engineering. These initiated buckles may induce a long pipeline to transform into multiple short pipeline segments, which promote the occurrence of pipeline walking. Thus, a pipeline, which is designed to buckle laterally, may laterally and axially displace over time when subjected to repeated heating and cooling cycles. This study aims to reveal the coupling mechanism of pipeline walking and global lateral buckling. First, an analytic solution is proposed to estimate the walking of pipeline segments between two adjacent buckles. Then, the sensitivity of this method to heating and cooling cycles is analyzed. Results show the applicability of the proposed walking analytical solution of buckling pipelines. Subsequently, an influence analysis of walking on global buckling, including the capacity of buckling initiation, buckling amplitude, buckling mode, and failure assessment of the buckling pipeline, is performed. The results reveal that the effect of walking on the buckling axial force is negligible. However, pipeline walking will aggravate the asymmetry of the pipeline buckling and the failure parameters of the pipeline during the post-buckling.

The implementation of corrosion detection in submarine pipelines is difficult, and a combined PCA-MLP prediction model is proposed to improve the accuracy of corrosion prediction in submarine pipelines. Firstly, the corrosion rate of a submarine multiphase flow pipeline in the South China Sea is simulated by the De Waard 95 model in the multiphase flow transient simulation software OLGA and compared with the actual corrosion rate; then, according to the corrosion data simulated by OLGA, principal component analysis (PCA) is used to reduce the dimensionality of the corrosion factors in the pipeline, and the multiple linear regression model (MLR), multi-layer perceptron neural network (MLPNN), and radial basis function neural network (RBFNN) were optimized. The PCA-MLPNN model has an average relative error of 3.318%, an average absolute error of 0.0034, a root mean square error of 0.0082, a residual sum of squares of 0.0020, and a coefficient of determination of 0.8609. Compared with five models, including MLR, MLPNN, RBFNN, PCA-MLR, PCA-MLPNN, and PCA-RBFNN, PCA-MLPNN has higher prediction accuracy and better prediction performance. The above results indicate that the combined PCA-MLPNN model has a more reliable application capability in CO2 corrosion prediction of submarine pipelines.

Free spans of submarine pipelines are prone to be subjected to vortex-induced vibration (VIV) under the action of currents, leading to fatigue damage of submarine pipelines. In the traditional method, the fatigue damage is predicted assuming that the length of free span is a constant. However, the free-span length may vary in time and space due to local scour and sand wave migration in engineering practice. This study proposed probabilistic methods to predict the fatigue life of the free spans by considering the effect of variant span length and span position. Truncated Gaussian, Raileigh and Uniform distributions of span length due to local scour, and a sinusoidal pattern with a constant migration rate is assumed for the sand wave due to the lack of field scan data. The fatigue life of a 120 m long span under a constant current-induced flow with the velocity of 0.7 m/s has been assessed. Results show that comparing with the fatigue life of a fixed span, the present method leads to an increase in the fatigue life by about ten times.

Submarine pipelines are the main transport carriers of marine resources. In order to protect these pipelines, geotextile and stone covering measures are adopted in this paper and the protective effect is studied. A sequence of physical model tests was conducted to carry out the research. The hydrodynamic characteristics and seabed oscillation response of the seabed surrounding the pipeline were analyzed with or without geotextile and stone cover protection, and it was found that they were affected by waves (and currents). The experimental results show the following: (1) comparing the regular wave and current with the regular wave alone, it is found that forward current promotes wave propagation and reverse current inhibits wave propagation; (2) the protective effect of geotextile and stone covering measures on different positions of the pipeline (the front, the bottom, and the back of the pipe) is basically same; (3) in the case of waves with large wave heights and long wave periods superimposed with ocean currents, the protective effect of geotextile and stone coverings on the hydrodynamic and seabed pore pressure around the pipeline is more significant.

Sand waves are large-scale bed forms commonly occurring on the continental shelf seabed and can result in free spans of submarine pipelines, which may have an influence on the stability of the pipelines. Existing span rectification procedures have primarily focused on local rectification methods for free spans caused by local scour or individual spans resulting from seabed unevenness. This paper aims to present a span rectification design applicable to the pipeline crossing sand wave region, and to offer practical guidance on sand wave intervention strategies. A large-scale approach is necessary for the rectification of multiple spans across the field, which may involve the use of either a mass flow excavator (MFE) or a remotely operated vehicle (ROV) jetting tool. A comparative analysis of the estimated durations for post-lay trenching using the MFE and ROV jetting tools is also provided. In instances where the large-scale method fails to achieve span lengths suitable for long-term operation, a localized approach is necessary to address individual spans. The desired trench depth can be attained through a combination of pre-lay and/or post-lay trenching techniques. The analysis of on-bottom roughness and free span has demonstrated that, given the natural seabed profile without trenching, there are no spans surpassing the ultimate limit state (ULS) or fatigue limit state (FLS) criteria for the temporary installation scenario. Consequently, pre-lay rectification is not necessary. However, the analysis indicates that post-lay rectification is essential to meet ULS and FLS criteria under operating conditions. All spans that exceed the ULS and FLS criteria can be effectively rectified by trenching to a depth of 1 m.

The instability of a partially embedded pipeline under ocean currents involves complex fluid–pipe–soil interactions, which may induce two typical instability modes; i.e., the lateral instability of the pipe and the tunnel erosion of the underlying soil. In previous studies, such two instability modes were widely investigated, but separately. To reveal the competition mechanism between the lateral instability and the tunnel erosion, a coupled flow-seepage-elastoplastic modeling approach was proposed that could realize the synchronous simulation of the pipe hydrodynamics, the seepage flow, and elastoplastic behavior of the seabed soil beneath the pipe. The coupling algorithm was provided for flow-seepage-elastoplastic simulations. The proposed model was verified through experimental and numerical results. Based on the instability criteria for the lateral instability and tunnel erosion, the two instability modes and their corresponding critical flow velocities could be determined. The instability envelope for the flow–pipe–soil interaction was established eventually, and could be described by three parameters; i.e., the critical flow velocity (Ucr), the embedment-to-diameter ratio (e/D), and the non-dimensional submerged weight of the pipe (G). There existed a transition line on the envelope when switching from one instability mode to the other. If the flow velocity of ocean currents gets beyond the instability envelope, either tunnel erosion or lateral instability could be triggered. With increasing e/D or concurrently decreasing G, the lateral instability was more prone to being triggered than the tunnel erosion. The present analyses may provide a physical insight into the dual-mode competition mechanism for the current-induced instability of submarine pipelines.

To investigate the different influencing characteristics of local scour around submarine pipelines, hydrodynamic and sediment transport two-dimensional models based on Flow-3D are used to numerically simulate the local scour around the pipeline under steady currents. An RNG k-ε turbulence model is applied to simulate the turbulent flow field around the pipeline. The instantaneous shear stress of the bed surface is taken as the starting and transporting conditions of the sediment. The simulation results of the equilibrium scour depth and terrain around the pipeline are verified with the previous experimental results, which perform with good agreement. Then, the numerical simulation method is applied to investigate the local scour process around the pipeline. The results show that shear stress is the main driving force of scour around a pipeline. The velocity, sediment grain size, pipeline diameter, and the initial gap between the pipeline and the seabed, significantly affects submarine pipeline equilibrium scour depth and terrain in varying degrees.