Pipeline Stability Research Articles

Research on Stability Analysis of Bonded Flexible Submarine Pipeline

As an important part of underwater oil and gas transportation system, the in-situ stability of submarine pipelines is particularly important. Comparing with steel pipe, flexible pipe has the advantages of high strength, corrosion resistance, light weight, coil supply and laying. In this paper, the finite element software ABAQUS is used to calculate the initial in-situ stress balance of soil, pipe weight, internal pressure load analysis and horizontal buckling simulation. The transverse horizontal displacement and stress distribution under high temperature and high pressure in flexible pipeline operation are calculated. The results show that the temperature difference and pressure difference in pipeline operation are the main causes of pipeline failure. The results show that the relevant parameters of seabed soil have different effects on pipeline failure, and the friction coefficient of soil and pipeline is more obvious.

Vibration and dynamic stability of composite pipelines conveying a two-phase fluid flows

The paper deals with the vibration and dynamic stability of composite pipelines conveying slug flow on viscoelastic foundations. A viscoelastic model of the theory of beams and the Winkler base model are used in the study of pipeline oscillations with a gas-containing slug flowing inside. The Boltzmann-Volterra hereditary theory of the viscoelasticity is used to describe the viscoelastic properties of the pipeline material and earth bases. The effect of gas and liquid phases flow rates, influence of tensile forces in the longitudinal direction of the pipeline, parameters of Winkler bases, parameters of singularity in the heredity kernels and geometric parameters of the pipeline on the oscillations of structures with viscoelastic properties are numerically studied. It is revealed that an increase in the length of the gas slug zone leads to a decrease in the amplitude and oscillation frequency of the pipeline. The critical rates for a two-phase slug flow are determined. It is revealed that an increase in the soil density of the bases leads to an increase in the critical rate of gas flow. It is shown that an account of viscoelastic properties of structure material and earth bases leads to a decrease in the critical flow rate.

Lateral resistance of “rigid” pipelines and cables on rocky seabeds

Accurate assessment of lateral resistance is critical to ensure the on-bottom stability and integrity of subsea pipelines and cables in the oil–gas and marine renewable energy industries. However, on rocky seabeds recommended practices provide limited recommendations on pipe–seabed interaction, suggesting only a single value for the friction coefficient of 0.6. This paper reports on a programme of physical experiments and theoretical modelling investigating the lateral resistance of pipes on rocky seabeds. It is shown that the peak and mean effective friction can significantly exceed the interface (or Coulomb) friction coefficient when the pipe diameter (D) is similar to the median rock diameter (dn50). Only when the pipe diameter becomes large compared to the rock size does the mean effective friction approach the interface friction. The effective friction coefficient was found to vary with variability in rock size and shape, as well as the length of pipe relative to median rock diameter. Each of these findings is reproduced well using the theoretical model. Collectively, the results demonstrate that the effective lateral friction coefficient may be higher than 0.6 for mean friction, and significantly higher for peak friction. This implies that inaccuracy may exist in current design, which may be rectified using the theoretical model.

REFLECTOMETRY AT LINE MAIN PIPELINE INFRASTRUCTURE MONITORING

Background Currently, an important problem in the operation of the line main pipeline is the timely detection of stability loss, due to which the operational reliability of the pipeline is reduced, industrial safety indicators, environmental conditions, and uninterrupted supply of hydrocarbon materials are deteriorating. Aims and Objectives To develop a system for monitoring the pipeline stability state in real time. Methods To achieve the aim, empirical (in the form of experiment) and theoretical (analysis and synthesis) research methods were chosen. The following tasks were posed and solved: to select equipment, to conduct experiment planning, to perform bench tests; to identify patterns and propose mathematical models, based on which to develop an algorithm for pipeline stability assessment in real time. Results The results obtained indicate the feasibility and advisability of this approach applying to existing pipelines. Implementation of the proposed algorithm allows to achieve the aim: knowing the magnitude of the power loss of the scattering signal, one can determine the current (actual) pipeline curvature radius in real conditions and in any section using the proposed mathematical model, evaluate its stability and analyze the stress state of the pipeline section (calculate stress) without opening it at any time using a reflectometer, well-known techniques and the corresponding calculation program.

ECONOMIC EFFICIENCY OF VACUUM ANCHOR DEVICES APPLICATION FOR FIXING SUBSURFACE PIPELINE IN SOFT SOIL CONDITION

Background Subsurface pipelines operating experience in swamps and the flooded area prove that water force (buoyancy in water) is a key factor, which causes pipeline stability loss. Under this force, steel pipeline shell receives larger movements in water-saturated soils, which leads to sharp increase in tension in a pipeline wall and to decrease in a capacity of the pipeline. That is why the providing of steady subsurface pipeline design position in a soft soil condition is a recent problem for Russian oil and gas industry. The main objectives are new materials and designs using for fixing pipelines on design marks, material carrying capacity decreasing, construction cost and cargo transportation decreasing, building speed increasing. Aims and Objectives The purpose of the article is an economic efficiency analysis of vacuum anchor devices application for fixing subsurface pipeline in a soft soil condition. The main contents of an economic analysis are: o an estimated cost calculation of pipeline construction with application of the covering type reinforced concrete weighting compounds, the ring type reinforced concrete weighting compounds, the screw anchor devices, the vacuum anchor devices; o a calculation of net present value, profitability index, internal rate of return, pay-back period. Methods А comparative analysis was used as the main method. The economic efficiency determination is estimate by comparing theу following indicators: net present value, profitability index, internal rate of return, pay-back period. Results It is defined that pipeline anchoring by vacuum anchor device is economically expedient way to achieve stable pipeline position on design marks.

Determining the conditions for stability of elevated pipelines on supports while pigging

The aim of the present study is to determine the maximum allowable pig-running speed which guarantees that the design position of above-ground sections of elevated pipelines is maintained during pigging or liquid cleaning. This paper presents the results of 3D computer modelling, showing the strain in an elevated expansion loop of a pipeline due to the impact of the inertia load during pigging or liquid slug running. The strain in the elevated expansion loop sections of the pipeline was modelled using the finite-element method in the LS-DYNA and ANSYS software packages. Calculations were made for four standard sections of pipeline with expansion loops. As a result, the extent of displacements could be determined for elevated pipelines. The modelling results were used to calculate the maximum pig-running speed at which the design position of the elevated pipeline is maintained. This article compares the results of calculations made using engineering techniques with those made using computer 3D modelling methods. It also presents a comparison of computer modelling performed using the ANSYS and LS-DYNA software packages.

Observed changes to the stability of a subsea pipeline caused by seabed mobility

High resolution bathymetry combined with structural modelling is used to estimate changes in the on-bottom stability of an offshore pipeline due to scour and sedimentation over an 11 year period. Detailed observations of post-lay embedment changes have been combined with the pipeline structural characteristics and an elastic-plastic model of soil resistance to estimate the vertical and horizontal stability of the pipeline using a finite difference solution to the beam bending equation. Application of the design approach indicates that post-lay increases to the critical (break-out) velocity of 1–2 m/s occur along the full 19 km of surveyed pipeline due to scour and sedimentation, which act to reduce load and increase soil resistance. The rate at which this increase in stability occurs with time is found to vary along the pipeline, and is dependent on the mechanism of pipeline lowering (i.e. whether the pipe lowered due to sagging into widely spaced scour holes, or by sinking into the shoulders between many closely spaced scour holes). By incorporating sediment transport into the pipeline design, the present results suggest potential for significant improvements in pipeline on-bottom stability and associated reductions in minimum required specific gravity and/or secondary stabilisation.

Analysis of the Structure Suffered Submarine Landslides Using SPH and CEL Methods

Study shows that the submarine landslides have greatly affect the stability and security of subsea production systems, subsea pipelines and other Marine structures. Landslides can also take place even though the slope Angle is less than 1 °. At present the study of onshore landslide is more, but the study of submarine landslides are very few.The landslides mainly deal with the issues of stability. The analysis of landslide stability mainly use the Galerkin method, discrete element method, SPH and so on.But there is less research on the influence of the subsea structures.When the landslide occurs, Fluid characteristics of the landslide are significant, the soils translate into mudflow or debris flow,which is not solid state but fluid.So both SPH and CEL can be used for simulation, this paper uses the CEL and SPH to study the submarine landslide, the comparative analysis of the advantages and disadvantages of both methods is taken on. Both motheds are introduced at fisrt, Then through calculation, compares the two kinds of calculation methods. It can be seen that the SPH method and the CEL method are basically consistent with the shape of the landslide mass.It shows that both methods are feasible. The results of CEL method is more accurate.When the number of elements of SPH and CEL is few, the calculation time of SPH is shorter than that of CEL, but with the increase of element number, the calculation of the CEL method is more faster. If stress value of Lagrangian structure is required,it is better to choose CEL method. It is useful to further study on submarine landslides on Subsea structures.

Wave-induced dynamics of marine pipelines in liquefiable seabed

This paper presents a simple but workable modeling method to simulate the wave-induced liquefaction scenarios around a marine pipeline within the framework of the Biot's theory, incorporating the main features such as relation for the consolidation describing the pore-volume reduction, hysteretic stress-strain behavior of soil skeleton and soil-pipe contact effect. In this context, special attention is paid to the implementation of a well-calibrated cyclic soil model for hysteretic and nonlinear stress-strain behavior (i.e. strain softening and cyclic degradation), associated with a semi-empirical shear-volume coupling equation for capturing the accumulative volumetric change, which links the increment of volumetric strain per cycle of wave with the shear strain occurring during that particular cycle. The proposed modeling framework is then incorporated into an explicit time matching finite difference analysis procedure, allowing a full non-linear dynamic analysis of the intensive interactions between the pipeline and the seabed undergoing buildup of pore pressure and residual liquefaction. Retrospective simulation of the wave flume test performed by Sumer et al. (2006c) using the proposed model shows good agreement, calibrating the reliability of the modeling method for the prediction of wave-induced liquefaction of sandy seabed and failure process of the buried pipelines. Finally, the liquefaction mechanism around a buried pipeline under a nonlinear wave loading is investigated by numerical examples. The obtained results interpret the cause of liquefaction and the resulting consequence for pipeline stability in wave environment.

ANALYSIS OF THE TOTAL RESISTANCE OF UNDERWATER PIPELINES IN LONGITUDINAL DIRECTION

Background The loss of stability of oil and gas pipelines is accompanied by significant deviations from the design position with the occurrence of excessive, often unacceptable mechanical stresses in the pipe wall and their damage. Damage to pipelines for pumping oil, oil products and gas is associated with high costs. The loss of the underwater pipelines overall stability in the longitudinal direction and their damage is associated with even higher costs and requires a separate analysis. In this case, it is necessary to take into account the possible erosion under the action of the water flow of the soil backfill over the pipeline laid in the trench, which affects the stability of the pipeline. Aims and Objectives Obtain analytical expressions for the determination of the longitudinal compressive critical force at which the loss of longitudinal stability of the underwater pipeline laid in a trench and covered with soil in conditions of erosion of backfill in a certain area occurs. Results The dependence of the critical force resulting in the loss of the longitudinal stability of the underwater pipeline, on the extent of the blurred section, the geometric characteristics of the pipes, its weight with the pumped product and the load, and the properties of the soil in contact with the pipeline is obtained.

Analysis of the Interaction of Main Pipelines with the Earth Basement in the Evaluation of Their Stability

We consider the main concepts of evaluation of the general stability of underground pipelines according to the currently existing norms of Russian Federation. We analyze the main dependences that describe the interaction of pipes with soils in the transverse and longitudinal directions and are used in the software complexes intended for the design of main pipelines. The proposed solutions make it possible to substantially increase the accuracy of checking the total stability of underground pipelines in the longitudinal direction.

Stability of onshore pipelines in liquefied soils: Overview of computational methods

One of the significant problems in the design of onshore pipelines in seismic areas is their stability in case of liquefaction. Several model tests and numerical analyses allow investigating the behavior of pipelines when the phenomenon of liquefaction occurs. While experimental tests contribute significantly toward understanding the liquefaction mechanism, they are costly to perform compared to numerical analyses; on the other hand, numerical analyses are difficult to execute, because of the complexity of the soil behavior in case of liquefaction. This paper reports an overview of the existing computational methods to evaluate the stability of onshore pipelines in liquefied soils, with particular attention to the development of excess pore water pressures and the floatation of buried structures. The review includes the illustration of the mechanism of floating and the description of the available calculation methods that are classified in static and dynamic approaches. We also highlighted recent trends in numerical analyses. Moreover, for the static condition, referring to the American Petroleum Institute (API) Specification, we computed and compared the uplift safety factors in different cases that might have a relevant practical use.

Flow-pipe-soil coupling mechanisms and predictions for submarine pipeline instability

The stability of a submarine pipeline on the seabed concerns the flow-pipe-soil coupling, with influential factors related to the ocean waves and/or currents, the pipeline and the surrounding soils. A flow-pipe-soil coupling system generally has various instability modes, including the vertical and lateral on-bottom instabilities, the tunnel-erosion of the underlying soil and the subsequent vortex-induced vibrations (VIVs) of free-spanning pipelines. This paper reviews the recent advances of the slip-line field solutions to the bearing capacity, the flow-pipe-soil coupling mechanism and the prediction for the lateral instability, the multi-physical coupling analysis of the tunnel-erosion, and the coupling mechanics between the VIVs and the local scour. It is revealed that the mechanism competition always exists among various instability modes, e.g., the competition between the lateral-instability and the tunnel-erosion. Finally, the prospects and scientific challenges for predicting the instability of a long-distance submarine pipeline are discussed in the context of the deep-water oil and gas exploitations.

Interaction between submarine landslides and suspended pipelines with a streamlined contour

ABSTRACTRecently, the security and stability of submarine pipelines have attracted much attention in ocean engineering. In this paper, pipelines with a streamlined contour (wedge, airfoil, double-ellipse, and arc-angle hexagon) are designed in hopes of defending against the impact of submarine landslides, and the computational fluid dynamics (CFD) approach is used to investigate the interaction between submarine landslides and streamlined pipelines. The results show that the peak interactional force is more representative of the hazard level of pipelines imposed by submarine landslides. It is also found that the streamlined pipelines possess a significant advantage in reducing the drag force and lift force of landslide–pipeline interaction with a maximum lessening percentage of 66.32 and 40.17%, compared with a conventional circular pipeline. In addition, the influence of applying streamlined pipelines to engineering is briefly discussed, and the empirical equation for estimating the drag force and lift force of streamlined pipelines induced by landslides is recommended based on the numerical test results.

Upheaval buckling of surface-laid offshore pipeline

Offshore pipelines operating under high pressure and temperature are subjected to upheaval buckling. Pipeline behaviour in upheaval buckling depends on a number of factors including the shape of pipeline imperfection, installation stresses, loading types, seabed sediment behaviour and the flexural stiffness of the pipe. Current method of predicting upheaval buckling is based on simplified shapes of pipeline imperfection developed for idealized seabed conditions. To account for the effect of internal pressure, the pressure load is represented using an equivalent temperature. However, the applicability of these idealizations on the prediction of upheaval buckling has not been well-investigated. In this paper, the three-dimensional finite element modelling technique is used to investigate the applicability of idealized shapes and their effects on the upheaval buckling of pipeline for a seabed condition at offshore Newfoundland in Canada. The finite element model is then used to conduct a parametric study to investigate the effects of installation stress, loading types, seabed parameters and the flexural stiffness of the pipe. Finally, a design chart is developed to determine the optimum height of seabed features to manage pipeline stability against upheaval buckling under different temperature and pressure loadings.

Steady-state seismic vibrations of a buried pipeline in a viscoelastic soil

Steady-state coupled longitudinal vibrations of a buried pipeline and a viscoelastic soil during an earthquake are studied. Using specific examples of soils described by the Kelvin–Voigt model and by the viscous liquid model, the main qualitative and quantitative effects of viscosity on the behavior and seismic stability of metallic (steel) and concrete pipelines are clarified.

A Combined Experimental-Computational Confirmation of the Seismic Stability of the Branched Pipelines of Systems of Nuclear Power Stations

The increase in the extent and degree of complication of the geometry of the branched pipelines of nuclear power stations has led to the appearance of natural forms of vibrations with frequencies that cannot be determined by the traditional combined experimental-computational method. This has made it necessary to develop new approaches to the experimental determination of the dynamic characteristics and to the selection of the boundaries of the design plans. Through use of the proposed correction to the technique and algorithm used in the estimation of the seismic stability of pipelines it has been possible to increase the precision with which the lowest frequencies of the natural vibrations are determined and, in certain cases, to also reduce the volume of operations through “targeted” testing performed at sites of expected maximal amplitudes of the lowest forms of vibrations.

Quantitative risk assessment of submarine pipeline instability

Ensuring the on-bottom stability of the submarine pipeline is very important for safety concern during the operational stage of submarine pipeline. Due to the action of the external factors, e.g. strong current, wave and soil liquefaction, the submarine pipeline may have lateral displacement, vertical floatation or sinking. Although the submerged weight of submarine pipeline is designed to meet the requirement of on-bottom stability, the loss risk of pipeline on-bottom stability still exists due to the change of ocean environment or seabed. This paper presents a reliability-based assessment methodology for submarine pipeline instability. Firstly, a mechanical model of pipeline on-bottom state is built considering the hydrodynamic loads and pipe-soil interaction, which is the static forces equilibrium equations essentially, and a detailed analysis of loading condition of pipeline on seabed is also conducted. Then, based on the reliability theory, the limit state equations of pipeline instability are developed through converting the forces equilibrium equations. Because the parameters in limit state functions possess random uncertainty, such as wave and current loads, etc., the specific probability distributions are employed to present the random uncertainty of the parameters in limit state functions. The Monte Carlo method is then employed to solve the limit state equations for assessing the pipeline instability probability. The risk level of pipeline instability is judged using the risk ranking in DNV-RP-F107. The case study indicates that the proposed risk assessment methodology for pipeline instability possesses a good application performance. In essence, the assessment results could provide a powerful support for risk management or decision-making of submarine pipeline instability.

Pipeline–Seabed Interaction

A review of the existing research on the interaction between a pipeline and an erodible bed exposed to waves and/or currents is presented. The review covers three topics: scour, liquefaction, and lateral stability of pipelines. The basic mechanism that leads to scour in two-dimensional (2D) and three-dimensional (3D) cases is first described, as deduced from small-scale laboratory experiments. The onset of scour from piping and the developing tunnel erosion are among the processes described. The lateral expansion of the scour hole along the pipe is described, also based primarily on small-scale laboratory experiments. The state of the art of the mathematical/numerical modeling of the scour processes is presented. The associated self-burial of the pipe is described and compared to field observations. In addition to scour, liquefaction may also constitute a risk for pipeline stability. The cause of liquefaction and the resulting consequence for pipeline stability in a natural environment are discussed. Finally, the lateral stability of pipelines placed on an erodible bed and exposed to waves is briefly described.

A Case Study of Sediment Transport on Trenched Backfill Granular and Cohesive Material due to Wave and Current

In this study, after the installation of a subsea pipeline, backfilling was performed in the trenched area. During these operations, a stability problem in the subsea pipeline occurred. The pipeline was directly impacted by environmental loading such as waves and currents that were caused by backfill material when scouring or sediment transport and siltation was carried out. Therefore, this study reviewed whether trenching was necessary, and conducted research into an indigenous seabed property that contains granular soil. A study of cohesive soil was also conducted in order to cross-correlate after calculating the values of the critical Shields parameter relevant to elements of the external environment such as waves and current, and the shear Shields parameter that depends on the actual shearing stress. In case of 1), sedimentation or erosion does not occur. In the case of 2), partial sedimentation or erosion occurs. If the case is 3), full sedimentation or erosion occurs. Therefore, in the cases of 1) or 2), problems in structural subsea pipeline stability will not occur even if partial sedimentation or erosion occurs. This should be reflected particularly in cases with granular and cohesive soil when a reduction in shear strength occurs by cyclic currents and waves. In addition, since backfilling material does not affect the original seabed shear strength, a set-up factor should be considered to use a reduced of the shear strength in the original seabed.