Buried Offshore Pipelines Research Articles

Propagation buckling failure of a buried offshore pipeline with integral arrestors under external overpressure and strike-slip faulting disturbance

In this study, the propagation buckling failure of a buried offshore pipeline under external overpressure and strike-slip faulting disturbances was investigated numerically. A nonlinear shell model, accounting for geometric, material, self-contact, and pipe-soil interaction nonlinearities, was developed using the vector form intrinsic element method. The arresting performances of the single- and double-integral arrestors were analyzed based on a series of simulations considering various combinations of arrestor diameter-to-thickness ratios, lengths, and sub-arrestor intervals. A comparative analysis was conducted between single- and double-integral arrestors, focusing on their arresting performance, material usage, and manufacturing complexity. The results revealed that pipelines under external overpressure are prone to local collapse and bidirectional buckling propagation under small fault displacements. Reverse ellipticities occur downstream of the buckling propagation and their development leads to flipping propagation. Buckling arrestors function by exhausting the upstream flattening propagation and inhibiting the development of reverse ellipticities downstream. The material consumption of each single-integral arrestor was significantly higher than that of a double-integral arrestor with the same diameter-to-thickness ratio. The weld count of the double-integral arrestors was lower than that of the single scheme, but this difference narrowed as the pipeline mileage increased. These results can serve as valuable guidelines for the design and construction of pipelines crossing seismic zones.

Numerical modelling of upheaval buckling of offshore pipelines with unstressed and stressed initial imperfections

Buried offshore pipelines transporting hydrocarbon at high temperatures and pressures might experience upheaval buckling. Although pipelines are designed to remain in place, field evidence shows that the buried pipeline might displace a significantly large distance in the upward direction. In the present study, the pre- and post-buckling behaviour of offshore pipelines buried in sand is investigated considering the degradation of uplift soil resistance with the upward displacement of the pipe. The soil resistance degradation models are implemented in a finite element program, and the analyses are performed for large upward displacements even when a buckled section moves above the seabed. The simulation results show that the capacity of the pipeline to carry the load generated from an increase in pressure and temperature reduces significantly if post-peak degradation of uplift soil resistance is considered. The effects of the internal pressure modelling approach, uplift soil resistance degradation model, burial depth and pipe dimensions (diameter and thickness) on upheaval buckling for varying initial imperfections are investigated. Based on a simplified model for developing initial stresses in a pipeline during installation, it has been shown that initial stresses primarily affect the upheaval buckling at lower ranges of upward displacement of the pipe.

Visualized Experimental Study of Soil Temperature Distribution around Submarine Buried Offshore Pipeline Based on Transparent Soil

The temperature distribution around the offshore burial pipeline is an important factor affecting its safety design and economic operation. The traditional test method cannot obtain the continuous temperature distribution of soil owing to the constraints of placing measurement sensors in soil. The transparent soil model test is an alternative method to realize the visualization research of soil temperature. In this paper, a relationship between the temperature of transparent soil and pixel intensity was first established. Then, the transparent soil test and numerical simulation, considering the natural convection, were carried out to study the temperature distribution around the submarine pipeline during start-up and stable operation. The influence of buried depth and pipeline diameter was analyzed. The results suggest that the continuous temperature distribution can be obtained visually by using a transparent soil test, and the observed heating zone of influence extended to a radial distance of 2.6 pipe diameters. The numerical analysis results show that the influence zone of the temperature of pipeline is a distance of four pipeline diameters at a temperature difference of 45 °C. The buried depth and pipeline diameter have little influence on the influence zone. In addition, the contour curves of soil temperature around the pipeline with different diameter are similar in shape. With the decrease in the buried depth of pipeline, the temperature gradient of soil around the pipeline decreases, which is caused by the natural convection.

Propagation and arrest of collapse failures in a buried offshore pipeline crossing reverse fault areas

Offshore pipelines crossing fault rupture areas suffer the risk of local buckling, which can in turn initiate propagating buckles. This study presents the results of evaluating several series of local collapses, subsequent propagating buckles, temporary buckling arrests, and crossover simulations of a deformed (due to reverse fault displacements) offshore pipeline under external overpressure. A numerical model of a buried pipeline with a single (twin) integral arrestor(s) was developed using a nonlinear vector form intrinsic finite element method. The residual external pressure capacity of a deformed pipeline after reverse fault displacements was assessed. Single integral arrestors at different installation sites and twin integral arrestors of different lengths were tested. Effects of fault dip angles on the propagation and arrest of collapse failures were investigated. The longitudinal and bending deformations caused by reverse fault displacements result in a significant decrease in the external pressure resistance of offshore pipelines. Four weak areas appeared around the bending regions, the size relationship of their collapse pressures determined the sequence of multiple local collapses and directions of the subsequent propagating buckles. Integral arrestors are effective designs with significant arresting efficiency in such scenarios. The flexure curve of the pipeline changed significantly only when an arrestor was installed in the left- or right-bending region. A large fault dip angle leads to both higher first collapse pressures and crossover pressures due to smaller bending deformations. A flipping crossover occurs when a downstream reverse ovality develops, which rotates the collapse direction by 90°. The material efficiency of integral arrestors will decrease with the occurrence of flipping crossovers.

On the buckling propagation and its arrest in buried offshore pipeline crossing strike-slip fault

This study focused on buckling propagation and its arrest in buried offshore pipelines crossing strike-slip faults. A buried offshore pipeline with integral arrestors was analyzed using nonlinear vector-form-intrinsic-finite-element-method-based shell elements and modified soil springs. The effects of different external pressures on structural response were investigated. Integral arrestors with different design parameters were tested to prevent flattening and flipping propagation. The results showed seven or five fluctuations in the flattening parameters with alternating orthogonal directions along the pipeline. These fluctuations led to multi-failure modes, including S-shaped and Z-shaped flexures, two local collapses followed by flattening propagations, and a single local collapse followed by flipping propagation. Integral arrestors should be designed specifically to provide sufficient local bending rigidity in the circumferential direction. The arrest mode of flattening propagation is stopping the earliest fluctuations whilst that of flipping propagation is stopping the earliest fluctuations and depressing the downstream reverse ovalities. For the same arrestor thickness, the smallest effective arrestor length to stop flattening propagation was significantly less than that for flipping propagation. The arrest is temporary because of gross deformation of cross-sections at integral arrestors or the downstream pipes, and secondary failure accidents are likely to occur with any external perturbation.

Uplift Behavior of Pipelines Buried at Various Depths in Spatially Varying Clayey Seabed

The behavior of buried offshore pipelines subjected to upheaval buckling has attracted much attention in recent years. Numerous researchers have made great efforts to investigate the influence of different soil cover depth ratios, soil strengths and pipe-soil interfaces on failure mechanisms and bearing capacities during pipeline uplift. However, attention to soil spatial variability has been relatively limited. To address this gap, a random small-strain finite element analysis has been conducted and reported in this paper to evaluate the influence of the random distribution of soil strength on pipe uplift response. The validity of the numerical model was verified by comparison with the results presented in the previous literature. The spatial variation of soil strength was simulated by a random field. The effect of soil variability on the failure mechanism was determined by comparing the displacement contours of each random realization. Probabilistic analyses were performed on the random uplift capacity obtained by a series of Monte Carlo simulations, and the relationship between the failure probability and the safety factor was also determined. The findings of the present work might serve as a reference for the safety designs of pipelines.

Probabilistic seismic responses and failure analyses of free-spanning subsea pipelines under offshore spatial earthquake motions

A comprehensive literature review in seismic analysis of free-spanning offshore pipelines (FSOPs) reveals that two critical aspects, i.e. the offshore earthquake motions and various uncertainty sources, have been normally ignored in numerous previous studies. Specifically, the offshore pipelines are usually modeled by assuming all the structural parameters to be fixed and are excited using the onshore earthquake motions. Such analytical schemes may result in severe misestimates of seismic response predictions of FSOPs. In this paper, a probabilistic approach is developed for numerically investigating the seismic responses and failure mechanisms of FSOPs subjected to the offshore spatial earthquake motions (Off-SEMs). For this purpose, a buried offshore pipeline of API X65 with a free span of 30 m is selected and the corresponding three-dimensional finite element (3D FE) model is established in the ABAQUS software, where the soil–pipe and water–pipe interactions are modeled using the nonlinear soil springs and added mass and damping methods, respectively. Then, the three-dimensional Off-SEMs are stochastically synthesized by explicitly considering the effects of spatial variability and overlying seawater. Next, the general law of dynamic responses and failure modes of FSOPs subjected to the Off-SEMs is analyzed and summarized by the deterministic dynamic analysis. Defining the earthquake motions, structural modeling and soil properties as uncertain sources, the relative sensitivity of seismic behaviors to uncertain parameters involved in these uncertain sources is identified separately by the sensitivity analysis. Finally, uncertainty analysis is performed to examine and discuss the influence of different types of uncertainty sources on the seismic behaviors of FSOPs. Numerical results indicate that the pipeline diameter has the highest influence on the seismic behaviors of FSOPs, while the Poisson’s ratio of pipeline material has the smallest effect; uncertainties in earthquake motions, structural modeling and soil properties have significant contributions to the dynamic characteristics and seismic behaviors.

Uplift and lateral buckling failure mechanisms of offshore pipes buried in normally consolidated clay

Buried offshore pipelines are crucial for the swift transportation of oils and similar fluidized materials from the oil rigs to the processing center or vice versa. In this study, a 2-D finite element analysis is performed to understand the pipeline-soil interaction, the failure mechanism of soil and the variation of capacity factors with different normalised soil properties during upward and lateral buckling of a buried offshore pipeline for no tension (NT) and full tension (FT) condition. The pipeline is installed in a normally consolidated clay bed with linearly varying undrained shear strength. The effect of embedment depth (H), unit weight of soil (γ'), undrained shear strength (Su), and roughness and tensile capacity of the pipe-soil interface on the failure mechanism and capacity factors of pipes are also studied. The current numerical model is validated first against a past numerical study, which showed a good agreement between the result obtained from the current study and the results obtained from the past study (Maitra et al., 2016). Then the failure mechanism and capacity factors are obtained, and their variation for different combinations of embedment depth and unit weight of soil are demonstrated. Both the uplift and lateral capacity factors are observed to be influenced by embedment depth and unit weight of soil. It is observed that, depending on the influencing parameters, both the uplift and lateral capacity factors can be critical for the buckling of the pipeline. Several displacement diagrams are presented to show the failure mechanisms clearly, while, the stress contours are given to observe the behavior of principal effective stresses during different failure mechanisms.

The influence of pipeline–trenchbed interaction intensity on lateral soil resistance and failure mechanisms

ABSTRACT Buried offshore pipelines may be subjected to large lateral displacements caused by ground movements, landslides, etc. A proper estimation of lateral soil resistance against the moving pipeline requires a deep understanding of pipeline–backfill–trench interaction mechanisms. Recent studies show that pipeline–trenchbed interaction can significantly affect the failure mechanisms and the resultant lateral soil resistance, an important aspect that is currently neglected by pipeline design codes. In this paper, the effect of pipeline–trenchbed interaction intensity on lateral soil resistance was investigated by performing centrifuge tests. The soil deformations and failure mechanisms were obtained by Particle Image Velocimetry (PIV) analysis. Two experiments with horizontal and downward inclined pulling directions were conducted to simulate different intensities of bed interaction. It was observed that increasing the intensity of pipeline–trenchbed interaction results in a faster and full development of shear bands in the trench wall and reduction of the mobilized lateral soil resistance.

Analysis of buried pipelines laid under the pipelines operation, is subjected to operational loads

Today’s successful operation within the oil and gas industry is based on the triangle “Safety – Reliability – Profitability (Efficiency)”. It is of high importance to properly balance these different and sometimes opposite positions. The article describes the characteristics of the strength of the buried offshore pipeline. Pipe geometric imperfections as the cross section ovality, combined load effects as axial and bending loads superimposed to the external pressure, material properties as compressive yield strength in the circumferential direction and across the wall thickness etc., significantly interfere in the definition of the demanding, in such projects, minimum wall thickness requirements.

Probabilistic analysis of the uplift resistance of buried pipelines in clay

Uplift resistance provided by soil cover is a key aspect in the design of a buried offshore pipeline. The capacity must be sufficient to avoid upheaval buckling but prediction of the uplift resistance is complicated by disturbance of the soil structure during installation. Previous numerical investigations of pipeline uplift capacity in undrained clays have employed simple elasto-plastic models and consider a homogeneous clay. These simplifications neglect critical features of soil-pipeline interaction and may not describe the real mechanical behaviour. In this paper, an advanced kinematic hardening model implemented in a finite element code is used to capture the degradation of structure as a pipeline buried in a natural clay is lifted upwards. The spatial variability of clay structure is represented by a random field and Monte Carlo simulation used to characterise the response. This novel framework shows that clay structure has a significant effect on the failure mechanism and uplift capacity of a buried pipeline. The probabilistic approach accounts for the uncertainty in the condition of the clay backfill and reveals that the spatial distribution of intact and remoulded material can change the mode of failure, emphasising the importance of considering clay disturbance in design.

Nonlinear finite element analysis of upheaval buckling of buried offshore pipelines in medium dense sand with fines

Offshore pipelines that transport oil and gas in different areas of the world are often buried in trenches to provide stability and protection against upheaval buckling. Hydrocarbons in the pipeline are transported at high temperatures and pressures to facilitate flow and prevent potential solidification. Such conditions of transport, however, generate increases in axial compressive forces in the pipeline, which may lead to upward buckling (in the direction of the least soil resistance). Upheaval buckling can result in pipeline failure causing severe environmental and economic losses. The potential for upheaval buckling is mitigated by the resistance of the soil overlying the pipeline. This resistance is a function of several parameters. In this study, a series of 3D displacement-controlled finite element analyses were conducted using AbaqusTM to investigate the effects of pipeline diameter, pipeline embedment depth, and fines content on the soil resistance against uplift buckling. In the analyses, the offshore pipeline was assumed to be buried in medium dense sand with fines and was pulled upward along its entire length to simulate plane-strain conditions. The response of the pipeline was: (1) studied through the variation of the normalized uplift soil resistance with respect to normalized pipeline displacement, and (2) compared to the predicted response from available analytical resistance models. The results show that the uplift resistance, along with the normalized mobilization distance, depends on the pipeline diameter, embedment soil depth and fines content. Moreover, analytical design methods which assume mobilized soil blocks with inclined slip surfaces better capture the behavior observed in the numerical models, when compared to methods in which vertical slip surfaces are considered.

Effect of Irregular Seabed Profile on Upheaval Buckling of Buried Offshore Pipelines

AbstractOffshore pipelines are commonly buried in seabeds for protection against damage, for better insulation, and to prevent upheaval buckling induced by thermal and pressure loadings. The seabed...

Generalized framework to predict undrained uplift capacity of buried offshore pipelines

Estimation of undrained uplift capacity is essential for the determination of optimal burial depth of buried offshore pipelines. However, a generalized prediction model that incorporates various factors influencing this capacity is scarce in the literature. In this paper, results from a series of small-strain finite element analyses are presented that explore the effects of pipe embedment, pipe–soil interface roughness, interface tensile capacity, soil shear strength, and unit weight on pipe uplift response. From the study, a simple method to predict the undrained upheaval resistance of buried pipelines for any practical range of pipeline and soil parameters is proposed. Factors associated with transition in failure mechanism with embedment are also examined. The numerical model is validated by comparing the results with available analytical and experimental data. Large-deformation finite element analyses have also been performed independently for a few cases to justify the applicability of small-strain methods in modelling pipe upheaval. Accuracy of the model for generalized shear strength profile is then examined by considering practical values of parameters over broad ranges. The proposed methodology gives results with maximum error less than 8% for all ranges of parameters and hence can be adopted in design practices.

Two-Dimensional Model for Pore Pressure Accumulations in the Vicinity of a Buried Pipeline

In this paper, we presented an integrated numerical model for the wave-induced residual liquefaction around a buried offshore pipeline. In the present model, unlike previous investigations, two new features were added in the present model: (i) new definition of the source term for the residual pore pressure generations was proposed and extended from 1D to 2D; (ii) preconsolidation due to self-weight of the pipeline was considered. The present model was validated by comparing with the previous experimental data for the cases without a pipeline and with a buried pipeline. Based on the numerical model, first, we examined the effects of seabed, wave and pipeline characteristics on the pore pressure accumulations and residual liquefaction. The numerical results indicated a pipe with a deeper buried depth within the seabed with larger consolidation coefficient and relative density can reduce the risk of liquefaction around a pipeline. Second, we investigated the effects of a trench layer on the wave-induced seabed response. It is found that the geometry of the trench layer (thickness and width), as well as the backfill materials (permeability K and relative density Dr) have significant effect on the development of liquefaction zone around the buried pipeline. Furthermore, under certain conditions, partially backfill the trench layer up to one pipeline diameter is sufficient to protect the pipelines from the wave-induced liquefaction.

Upheaval Buckling Analysis of Buried Offshore Pipelines under High Temperature and High Pressure

Nonlinear finite element upheaval buckling model of buried offshore pipelines under HT/HP is built using ABAQUS. The petroleum is defined as uniform flow; temperature field of offshore pipelines produced in the process of petroleum transportation is obtained by heat transfer analysis; offshore pipelines are buried in trench of sandy seabed, interaction between seabed and offshore pipelines is defined as friction, seabed interaction with offshore pipelines will limit the movement of offshore pipelines; coupled fluid-structure analysis for three phase model of oil-pipe-soil is conducted to obtain stress under HT/HP. Initial imperfection of pipeline is introduced to calculate upheaval buckling of buried offshore pipeline under HT/HP. Through numerical analysis, the axial force of pipelines under HT/HP is obtained and thus resulted in upheaval buckling.

Uplift of shallowly buried pipe sections in saturated very loose sand

Offshore pipelines are often buried to protect them from damage, and to provide additional thermal insulation. In sandy soils the pipes are trenched using jet-trenching or ploughing. In both cases the nature of the trenching operation means that the backfill material can be placed over the pipe in an extremely loose state. If the pipe then undergoes a small displacement or vibration, liquefaction of the backfill material may occur, and the resistance to upward movement of the pipe can be reduced. To explore this experimentally, an instrumented model pipe section was pulled vertically upwards at different rates in very loose, saturated, fine, uniform sand (representative of a North Sea sand). The instrumentation allowed for the measurement of the force on the pipe section as well as the excess pore water pressure regime around the pipe. The results show that, for sand at a relative density of zero, there is a reduction of capacity at the faster uplift rates. A simple analytical model, using the vertical slip model (for uplift resistance) modified to account for the development and dissipation of excess pore water pressures around the pipe, is used to predict the results from the experiments. Implications for the design of buried offshore pipelines in sand are discussed.