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Submarine pipeline was widely used to transport oil and gas in offshore petroleum exploitation. In practice, deep-water pipeline experiences several times of maintenance every year. This periodic o...

Deep-sea pipelines exhibit lateral global buckling under high temperature and pressure differences. Numerical simulations are an effective and easy way to obtain the deformation and stress distribution of a post-buckling pipeline with a low cost. A relatively accurate and convenient simulation model using the explicit dynamic method was developed based on ABAQUS software to analyse pipeline lateral global buckling. In pipeline global buckling analysis, the dynamic variation of soil resistance as the pipeline moves laterally, the optimized computational length and the use of smooth initial imperfection profiles influence the accuracy of the simulation results. These three key factors were discussed here, and calculation methods were proposed. A CEL simulation model with improved boundary conditions that reduce the influence of wave reflection with a low calculation time cost was built to calculate the variation dynamics of soil resistance throughout the buckling process. Different computational lengths for pipelines with different parameters were simulated, and a fitting function to obtain an optimized computational length was proposed. A method to build the profiles of a pipeline with smooth imperfections was developed. Based on these analyses of the three impact factors, a relatively accurate and convenient simulation model for pipeline global lateral buckling analysis was established. Finally, an engineering case was evaluated to show the application of the simulation model and to test the reliability of this model.

Pipelines are important to offshore oil and gas development, but suffers from the pipeline walking phenomenon due to cyclic temperature variations—where large axial walking distances threaten the safety of pipeline systems. Current research indicates that pipeline walking is triggered by steel catenary riser (SCR) tension, seabed slopes, or thermal transients. This paper proposes a new driving mechanism for the pipeline walking phenomenon, involving cyclic hardening soil strength. The finite element analysis method was adopted to analyse the soil friction difference induced walking phenomenon, and the influence of key parameters on the gain in soil friction on walking distance was studied. Pipeline walking distances under different drainage conditions in the heating and cooling processes were also calculated, and the impact of the degree of drainage in the heating process was determined. To better understand the new pipeline walking mechanism, theoretical analysis of the walking behaviour under different cyclic soil friction conditions was carried out. Analytical solutions for estimating the pipeline walking distance were also provided, based on the simplified theoretical analysis.

Pipeline walking is a global axial movement caused by the asymmetric distribution of axial force along the pipeline during the repetitive operational loading cycle. The pipeline walking rate, increasing with the loading cycles, may lead to high stress thereby the failure of connectors at PLETs or PLEMs. Current research studies and design guidelines focused on free-end pipelines and neglected changes in the pipeline walking rate and axial resistance with effects of pipeline end structures. However, in practice, several pipelines are adjacent to PLETs or PLEMs, which potentially affect the pipeline walking and expansion behaviour thereby walking rate and axial displacement. This study analyses the walking phenomenon considering the effects of mobilisation of the resistance at the pipeline end structures and develops new analytical solutions for the present problem, in terms of different walking mechanism, maximum axial resistance Pmax, and positions of pipeline end structures. The analytical solutions are validated through numerical simulation results. The case study results indicate the axial resistance generated by PLETs/PLEMs can effectively mitigate pipeline walking rate compared with free-end pipeline walking. In particular, the influence of PLETs/PLEMs is investigated, highlighting the sensitivity of the pipeline walking to the end boundary conditions covering different design scenarios.

Pipelines are widely used for transporting oil resources in offshore oil exploitation. As burying an entire deep-water pipeline is not possible owing to the large water depth, an on-bottom pipeline inevitably exhibits lateral global buckling deformation under high temperature and pressure in practice. Triggering several controllable mitigatory global buckling deformations by installing sleepers under pipelines is a more effective alternative to prevent cross-sectional failure caused by excessive buckling. While sleepers may trigger vertical global buckling and this risk must be verified before they are installed in practice. This study analysed the feature of lifting deformation for a pipeline laid on a sleeper. Nine influential factors of the variation in the lifting displacement were analysed. Based on the nonlinear relationship between lifting displacement and temperature difference, three key points and four relevant key parameters for describing the lifting displacement curve were proposed and calculated for the conditions of a pipeline with different combinations of influential factors. A back propagation neural network was trained to model the relationship between the locations of the three key points and the values of the nine influential factors. The error analysis indicated that the trained network can effectively predict the vertical lifting deformation and is suitable for a pipeline that experiences no lifting deformation. Based on the feature points predicted by the trained network, the approximate profile of vertical lifting displacement during the heating process can be described, and the assessment of whether the lifting displacement with the loading conditions in practice is allowable can be conducted.

With the development of submarine oil and gas resources, research on global pipeline buckling caused by high temperature and pressure in the process of oil and gas transport is becoming an important issue. Owing to human factors or to uneven seabed conditions, local bends are produced in the process of pipeline manufacturing and laying. Those bends are called geometric initial imperfections whose deformation forms can be divided into a single arch symmetric deformation and a double arch antisymmetric deformation. In this paper, the energy method is introduced to calculate the analytical solution of pipeline lateral buckling with a single arch initial imperfection, and the cause of snap buckling phenomenon is discussed. A FEA model is established, and the difference between the analytical solution and numerical solution is also presented.

Submarine pipelines are the primary component of an offshore oil transportation system. Under operating conditions, a pipeline is subjected to high temperatures and pressures to improve oil mobility. As a result, additional stress accumulates in pipeline sections, which causes global buckling. For an exposed deep-water pipeline, lateral buckling is the major form of this global buckling. Large lateral displacement causes a very high bending moment which may lead to a local buckling failure in the pipe cross-section. This paper proposes a lateral global buckling failure envelope for deep-water HT/HP pipelines using a numerical simulation analysis. It analyzes the factors influencing the envelope, including the thickness t, diameter D, soil resistance coefficient μ, calculating length Lf, imperfection length L and imperfection amplitude V. Equations to calculate the failure envelope are established to make future post-buckling pipeline failure assessment more convenient. The results show that (1) the limit pressure difference pmax (the failure pressure difference for a post-buckling pipeline when it suffers no difference in temperature) is usually below the burst pressure difference pb (which is the largest pressure difference a pipeline can bear and is determined from the strength and sectional dimensions of the pipeline) and is approximately 0.62–0.75 times the value of pb and (2) thickness t has little influence on the normalized envelopes, but affects pmax. The diameter D, soil resistance coefficient μ, and calculating length Lf influence the maximum failure temperature difference Tmax (the failure temperature difference for a pipeline suffering no pressure difference). The diameter D also significantly affects the form of the normalized envelope.

Subsea pipelines are important facilities in offshore oil and gas industry to transport High-Pressure and High-Temperature (HPHT) hydrocarbon. They are often exerted by cyclic thermal loading through the whole operational life, which may trigger asymmetry in the effective axial force (EAF) profile, leading to a global axial movement, defined as ‘pipeline walking’. It may cause the downtime and structural risks, since this directional accumulation in axial movement results in the overstressing of end connection, loss of tension in a steel catenary riser (SCR) and other issues in the field, therefore subsea pipeline design requires a reliable estimate of the global pipeline walking rate. Current design methods adopt a constant soil friction coefficient and ignore variations in soil-pipeline interaction with time during the heating up and cooling down processes. In reality, the overlooked changing soil friction is important, and may alter the pipeline walking behaviours. To achieve an accurate assessment of pipeline walking, this paper advances the conventional design practice, by introducing a time-dependent axial soil friction function, to examine the pipeline walking behaviour over the operational thermal cycles. A suite of time-dependent matrixes is provided to reveal the development of the expansion/contraction of the pipeline, the corresponding mobilised soil frictions and the EAF profile. The significant nonlinear EAF profile in response to the time-dependent soil friction is examined, and is used to well assess the accumulation in pipeline walking rate. The proposed analytical framework is applied into case studies to demonstrate its validity and applicability in practice.

Subsea pipelines are commonly employed in the offshore oil and gas industry to transport high-pressure and high-temperature (HPHT) hydrocarbons. The phenomenon of pipeline walking is a topic that has drawn a great deal of attention, and is related to the on-bottom stability of the pipeline, such as directional accumulation with respect to axial movement, which can threaten the security of the entire pipeline system. An accurate assessment of pipeline walking is therefore necessary for offshore pipeline design. This paper reports a comprehensive suite of numerical analyses investigating the performance of pipeline walking, with a focus on the effect of increasing axial soil resistance on walking rates. Three walking-driven modes (steel catenary riser (SCR) tension, downslope, and thermal transient) are considered, covering a wide range of influential parameters. The variation in walking rate with respect to the effect of increased soil friction is well reflected in the development of the effective axial force (EAF) profile. A method based on the previous analytical solution is proposed for predicting the accumulated walking rates throughout the entire service life, where the concept of equivalent soil friction is adopted.

With the increase in pipeline operating depth, research on pipeline global buckling during the process of oil and gas transport is drawing considerable attention. Numerical simulation is an important method that is used to analyze pipeline buckling, which is immediately caused by the combined action of high temperature and high pressure. Two important problems must be solved before simulating submarine pipeline global buckling. Because the finite-element (FE) model length greatly affects analysis of the buckling results, finding a reasonable model length is the first problem in finite-element analysis (FEA). The second is how to trigger global buckling in the pipeline because the ideal pipeline would not buckle in FEA. Previous studies only state that geometric initial imperfection and interference force could trigger pipeline global buckling. Therefore, simulating pipeline global buckling in FEA becomes a problem. In this paper, an effective method for calculating the reasonable model length (als...