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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. [ABSTRACT FROM AUTHOR]

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 open-up and shut-down operation causes cyclic rise and fall in the temperature of a pipeline, resulting in the periodic expansion and contraction of the pipeline. Due to the characteristics of heating-up operation, the expansion and contraction movement are symmetrical about different points, and it leads to the pipeline walking phenomenon. As a key factor, the axial soil resistance affects the walking displacement heavily. The shearing generated by expansion and contraction of a pipeline section causes intermitted consolidation of soil and cyclic increment in the equivalent soil resistance coefficient. This paper analysed pipeline walking of a free-end pipeline laid on even seabed with consideration of cyclic soil strength hardening. New driving mechanism for pipeline walking caused by thermal transient due to the influence of time-dependent soil resistance was proposed. A common theoretical method to calculate walking displacement was also established. [ABSTRACT FROM AUTHOR]

Offshore pipelines used for transporting hydrocarbons are cyclically loaded by great variations of pressure and temperature. These variations can induce axial instability in such pipelines. This instability may cause the pipelines to migrate globally along their length; an effect known as pipeline walking. Traditional models of pipeline walking have considered the axial soil response as rigid-plastic (RP); however, such behavior does not match observations from physical soil tests. It leads to inaccurate estimates of walking rate (WR) per cycle and over design. In this paper, a trilinear (3L) soil resistance model is used to represent seabed resistance to investigate the behavior of pipeline walking. Different parameters, i.e., shapes and properties of trilinearity (within the peaky soil model type), have been considered leading to a closed-form solution. This solution improves the understanding of the main properties involved in the peaky trilinear soil behavior by providing a set of analytical expressions for pipe walking, which were benchmarked and validated against a set of finite element analyses. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract Axial pipe-soil interaction is crucial for the prediction of pipeline walking, especially on a sloping seabed. In this study, a mechanical-actuator facility has been specially designed and utilized to simulate the axial pipe-soil interactions along a sloping sandy seabed. Based on dimensional analyses, an apparent axial anti-sliding coefficient is proposed to describe the anti-sliding capacity for pipeline walking on a sloping seabed under the fully-drained condition. Effects of the pipeline surface roughness, the submerged weight (G) of the pipeline and the slope angle (α) of the seabed on the apparent axial anti-sliding coefficient are investigated experimentally. The experimental results indicate that, for a fixed value of the non-dimensional submerged pipeline weight, both the ultimate anti-sliding capacity and the corresponding mobilization displacement increase with the increase of the slope angle of the seabed. There exist approximately linear correlations between the apparent anti-sliding coefficient and the examined seabed slope angle (− 9 o ≤ α ≤ 9 o ) or the non-dimensional submerged pipeline weight (0.34 ≤ G ≤ 0.81). An expression for the apparent axial anti-sliding coefficient incorporating an updated wedging factor is finally deduced and validated with the present experimental results. This axial pipe-soil interaction model may help assessing the pipeline walking along subsea continental slopes. Highlights • A mechanical-actuator facility to simulate the axial pipe-soil interactions along a sloping seabed. • An apparent axial anti-sliding coefficient is proposed to describe pipeline walking on the sloping sandy seabed. • Effects of the surface roughness, the submerged weight of pipelines and the seabed slope angle on axial anti-sliding coefficient. • An expression for the apparent axial anti-sliding coefficient incorporating an updated wedging factor. [ABSTRACT FROM AUTHOR]

Connection of floating production vessels to subsea pipelines requires careful consideration of the stresses placed on the steel catenary riser (SCR), subsea spool and pipeline end termination (PLET). Due to vessel motion, environmental conditions, flow conditions and pipeline temperature gradients during start-up/shut-down and operation, the forces on all sections of the subsea pipeline system may deviate from their static configurations. Pipeline risers, PLETs and spools have design limits that must not be exceeded in order to ensure the integrity of the pipeline/SCR system. The operational/dynamic loads on the pipeline/SCR system cause expansion and contraction of the pipeline at the riser and free end locations, and these also need to be kept within the pipeline system design limits. The most appropriate method to account for the pipeline system movement is to ensure the pipeline has sufficiently long run-out to accommodate the pipeline system loading or to provide anchoring locations for the pipeline section. This paper addresses, with examples and calculations, the criteria that must be considered during the design of the pipeline/SCR system to determine if hold-back anchors are needed and their optimum locations in the system. The criteria for the anchoring are valid for short pipelines with route bend and no lateral buckling. • Addresses the criteria to determine requirement for anchoring a short pipeline connected to a SCR in the absence of lateral buckling. • Discusses the potential methods available to anchor the pipeline and limit the axial feed-in towards the SCR. • Provides insights into the driving walking mechanisms of a pipeline connecting to a SCR. • Highlights the risks associated with the lateral ratcheting of a route bend. [ABSTRACT FROM AUTHOR]

Abstract Pipeline Walking is a phenomenon that occurs when high pressure and high temperature pipelines experience axial instability over their operational lifetime, and migrate globally in one direction. Existing analytical solutions treat the axial soil response as rigid-plastic but this does not match the response observed in physical model tests. In this paper, the authors develop a new analytical strategy using elastic-perfectly-plastic axial pipe-soil interaction, which leads to more realistic walking rate predictions. The new analytical methodology is benchmarked with a series of Finite Element Analyses (FEA), which constitutes a parametric study performed to test the proposed expressions and improve on the understanding of the influence of axial mobilisation distance. Highlights • Provides a pipeline walking analytical solution for elastic-perfectly-plastic pipe-soil response. • Shows the difference between non-moving sections and maximum effective axial force sections. • Explains the relationship between effective axial force area swept and the change in walking pattern. • Resolves how rigid-plastic fundamental solution requires expansion to allow for elasticity. • Establishes a correction factor to currently used rigid-plastic analytical approach. [ABSTRACT FROM AUTHOR]

Unburied subsea pipelines under cyclic thermal loading are often prone to axial walking. The axial movements accumulate over operational cycles, which can lead to potential failure at the mid-line or end connections. Anchoring is a commonly adopted method to -control pipeline walking, however, it requires accurate estimation of anchor force for efficient pipeline design. In view of this, the present study proposes a calculation method for anchor force of full mobilized lengths for different anchoring positions. Furthermore, the anchoring mechanism for partial mobilized lengths is analyzed. The anchor force is then determined using a strain diagram method for partial mobilized lengths. Finally, an analytical solution for anchor force based on the strain diagram method is derived and validated against finite element results. The results show that the mobilized length ratio of pipeline anchor force increases with the loading temperature and seabed slope but decreases with the pipeline length. The proposed analytical solution for anchor force which has advantages over traditional analytical solutions can comprehensively consider the effects of pipeline parameters, load parameters, and contact parameters. • A new analytical solution of the anchor force considering the anchoring position is derived. • The factors affecting the mobilized length of anchor force are studied. • The strain diagram method is proposed to analyze the anchor force mechanism for partial mobilized length. • A new analytical solution which can determine the mobilized length of the anchor force is proposed. [ABSTRACT FROM AUTHOR]

• The factors affecting the mobilized length of anchor force are studied. • The strain diagram method is proposed to analyse the anchor force mechanism for partial mobilized length. • A new analytical solution which can determine the occurrence of pipeline walking and the mobilized length of the anchor force, while considering a range of factors such as pipeline parameters, load parameters, and contact parameters is proposed. • To address the problem of large anchor force at the end anchor, a flexible anchor is proposed to reduce the anchor force, and a formula for predicting the flexible anchor force is fitted. The deep-sea pipeline exposed on the seabed is prone to pipeline walking under the influence of thermal loading cycles. Anchors are commonly used to prevent the pipeline walking from causing the end connection of the pipeline system to break. The development of a simple and practical method for calculating anchor force is essential for the preliminary design of the project. In this paper, a finite element model for calculating anchor force is established, and the factors affecting the accumulated length of effective axial force are analyzed. A strain diagram method is proposed for calculating the anchor force under partial accumulated length, and an analytical solution for the rigid anchor force is derived based on the strain diagram method. To address the problem of large anchor force at the end anchor, a flexible anchor is proposed to reduce the anchor force, and a formula for predicting the flexible anchor force is fitted. The study shows that the accumulated length proportion of anchor force increases with the increase of loading temperature and SCR tension, and decreases with the increase of pipeline length and friction coefficient. Flexible anchors can significantly reduce the anchor force. The proposed analytical solution for anchor force can comprehensively consider the influence of factors such as pipeline length, SCR tension, loading temperature, and friction coefficient, and has certain advantages over traditional analytical solutions. [ABSTRACT FROM AUTHOR]

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.

Deep-sea pipelines are important facilities for transporting hydrocarbon resources in offshore oil development projects. Due to the weak axial soil resistance and the cyclic variation of temperature, the deep-sea pipeline walks axially after several loading cycles, threatening the integer of the connecters. The walking phenomenon is well studied in recent years, while the walking with consideration of the influence of buckling deformation is barely considered. This paper analysed this topic with theoretical method, and the analytical solutions for predicting walking distances of each pipeline sections are proposed. Then, the finite element (FE) method is used and a serious of FE models are established to verify the proposed analytical solutions. Meanwhile, the related three issues are discussed based on the analytical and FE method. The coupled walking and buckling deformation is studied, and the interaction between the walking and buckling is revealed. • Proposing analytical solutions for pipeline walking influenced by lateral buckling. • Revealing the deformation mode of the coupled axial walking and lateral buckling. • Showing interaction between pipeline walking and buckling, especially the increment in compressive strain and bending moment. [ABSTRACT FROM AUTHOR]

A novel approach for modelling axial pipe–soil interaction, consisting of bespoke finite elements, is proposed. The purpose is to have a model that represents a two-dimensional slice of soil perpendicular to the pipe which is computationally cheap enough to be incorporated in global analysis of subsea pipelines, whilst capable of capturing detailed time-dependent soil response, which involves partial drainage and cyclic plasticity. This is achieved by handling the circumferential dimension analytically, reducing the behaviour of the two-dimensional soil slice to a one-dimensional case. Coupled consolidation analysis along a vertical sequence of one-dimensional elements beneath each pipeline node, tailored to represent the axial–vertical (or -radial) plane across the seabed semi-space, is supplemented by an analytical solution for the circumferential drainage. The paper presents the model development, its implementation through symbolic programming and validation against previously published continuum finite element analysis results. [ABSTRACT FROM AUTHOR]

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. • Proposing a new driven mechanism of a free pipeline laid on an even seabed and suffering no thermal transients. • Revealing how the cyclic increasing soil friction and the degree of drainage affects the walking distance. • Providing a simplified analytical solution for estimating soil friction difference-induced walking distance. [ABSTRACT FROM AUTHOR]

• This paper outlines the significant effects of the pipe-soil interactions on pipeline walking. • A set of large-scale model tests are conducted on the axial soil resistances of pipelines placed on Bohai sand. • Based on the test results and the influence mechanisms of the pipe-soil interactions on pipeline walking, a bi-linear pipe-soil interaction model for pipelines placed on sandy soils is proposed. • This paper presents an alternative method for the identification of pipe-soil interaction during pipeline-walking analysis for a certain site-specific soil. Pipeline walking induced by heat-up and cool-down cyclic loadings during shutdown and restart cycles is a challenge in the design of relatively short pipelines exposed on seabeds. A major source of uncertainty in the analysis of pipeline walking is the pipe-soil response, which has a significant influence on pipeline walking. To investigate the pipe-soil response during pipeline walking, a set of large-scale model tests are conducted on Bohai sand, and they involve axially moving pipelines with different speeds and cycles. Based on these test results, bi-linear pipe-soil interaction models with normalization of the pipe weight and diameter are proposed for Bohai Bay sand. Subsequently, a comparison of the model test data to existing pipe-soil interaction models is performed, and the existing pipe-soil interaction models are calibrated with site-specific soil properties of Bohai Bay sand. Finally, the application of the site-specific pipe-soil interaction models for a pipeline-walking analysis of pipeline placed on Bohai Bay sand is performed with the finite element method to study how pipeline walking is influenced by these pipe-soil interaction models. Combined with the influence mechanisms, a design model is proposed that considers the potential risks of pipeline walking in pipeline design to reduce costs without compromising reliability. [ABSTRACT FROM AUTHOR]

The hemiball penetrometer is a geotechnical device developed to measure the property of surficial seabed sediments (<0.5 m), which is of most significance for pipeline design and for situations in which traditional site investigation methods face challenges in accurately characterizing the soil properties. This study developed a novel method to derive the axial frictional resistance along the pipeline through a simple scaling relationship from the torsional resistance measured by the hemiball penetrometer in situ. To achieve this, the undrained and drained torsional resistances of the hemiball for idealized conditions were theoretically explored using the plasticity theory and principles of critical state soil mechanics. The link between undrained and drained torsional resistances for the idealized conditions then was established. Calibrated against those theoretical solutions, coupled finite-element analyses were conducted to explore solutions for more-realistic real-life conditions and to study the effects of hemiball embedment:diameter ratio, initial strength profile, and consolidation after hemiball or pipe installation. Based on these analyses, a simple model using two scaling factors, the effective radius of the hemiball and the equivalent length of the pipe, was proposed to derive the pipe–soil interface friction from the measured torsional resistance of the hemiball. The method presented in this paper provides a simple yet powerful approach for interpretation of hemiball testing results, and is an important step toward the eventual application of the hemiball in deep-water pipeline engineering. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]