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Propagation pressure collapse in conventional pipelines and MLPs: Numerical investigation
Abstract
The collapse of submarine pipelines can have immense economic and environmental impacts, and understanding the mechanical behavior of such structures under extreme external pressure is vital for the design, fabrication, installation, and operation. The pipe's outer surface is susceptible to damage/impacts due to uncontrolled circumstances, possibly leading to structural changes. This work applies numerical tools to evaluate the collapse and buckle propagation in steel pipes, considering the effects of an initial known deformation caused by different puncher's shapes. Therefore, the structure assumes a new geometry leading to modification in the capacity to withstand external pressure. The analysis applies and compares two numerical methods: the arc-length (Riks) and dynamic-explicit approaches. This comparison allows establishing a direct association between the observed features of the two methodologies. In a general manner, the observation indicates that an increasing indentation causes a reduction in the collapse pressure but also significant changes in the propagation pressure. The similar results between the two methods applied in steel pipes permitted extending this concept to estimate the propagation pressure in Mechanically Lined Pipes (MLPs), where the Riks method is unstable, and the dynamic approach is suitable. In this scenario, the possibility of the internal liner buckling before the external backing steel and the appearance of new failure modes is part of a new complex scenario where the number of variables grows significantly, and the collapse circumstances must be reassessed. The boundary conditions, interface state, and other structural parameters increase the difficulty in this type of analysis, showing that in MLPs the number of variables associated with propagation pressure is much higher than in steel pipes.
Composite coatings are indispensable for protecting the offshore steel materials serviced in the splash zone. But the mechanical properties of conventional epoxy resin (EP) coatings are poor, which would lead to a short-term lifespan. The carbon fiber (CF) with excellent mechanical properties is an ideal filler to reinforce the EP coating, thus we chemically grafted Ti3C2Tx nanosheets onto CF via dopamine to enhance the interface adhesion and compatibility between CFs and EP, and improve the mechanical strength and erosion wear resistance of CF/Ti3C2Tx composite coating. The flexural property of CF/Ti3C2Tx@EP coating was evaluated by a three-point bending test, and its flexural strength was increased by 41.95% compared with pure epoxy resin coating. The tribological behaviors and erosion wear resistance of composite coatings were tested using a UMT-3 tribometer and erosion test rig. A UMT-3 tribometer and erosion test rig were used to evaluate the tribological performances and erosion wear resistance for composite coatings. When contrasted to EP coating, CF/Ti3C2Tx@EP ′ wear rate was lowered by 43.67%, and its erosion mass and volume were reduced, respectively, by 21.31% and 44.50%, this was ascribed to its enhanced interfacial combination strength with Ti3C2Tx/CF hybrids and EP. We investigated failure behavior and revealed the interfacial strengthening mechanism of the CF/Ti3C2Tx@EP composite coating, which would evoke widespread interest in developing high-performance and long-term protective composite coatings used in the splash zone of the marine environment.
The composite coatings are indispensable for protecting the offshore steel structures serviced in the splash zone. But the mechanical properties of conventional epoxy resin (EP) coatings are poor, which would lead to the short-term lifespan. The carbon fiber (CF) with excellent mechanical properties is an ideal filler to reinforce the EP coating, thus we chemically grafted Ti 3 C 2 T x nanosheets onto CF via dopamine to enhance the compatibility between CF and EP. While improving the interface adhesion between CF and EP, the mechanical strength and erosion wear resistance of the CF/Ti 3 C 2 T x @EP coating were also significantly enhanced. The flexural properties of CF/Ti 3 C 2 T x @EP composite coating was evaluated by three-point bending test, and the flexural strength of CF/Ti 3 C 2 T x @EP was increased by 41.95% compared with pure EP coating. The tribological behaviors and erosion wear resistance of those composite coatings were carried out by UMT-3 tribometer and erosion test rig. Comparing with the pure EP coating, the wear rate of CF/Ti 3 C 2 T x @EP was decreased by 43.67%, and the erosion mass and volume of CF/Ti 3 C 2 T x @EP were respectively reduced by 21.31% and 44.50%, this were ascribed to the increased interfacial combination strength between the Ti 3 C 2 T x /CF hybrids and EP matrix. We investigated the failure behavior and revealed the interfacial strengthening mechanism of the CF/Ti 3 C 2 T x @EP composite coating, which would evoke widespread interest in developing high performance and long-term protective composite coatings used in the splash zone of marine environment.
An experimental and numerical investigation on the mechanical response of a lined pipe (compound pipe) under a dynamic impact is presented. The influence of the impact energy has been studied in terms of the depth of the dent formed, and of the strains and residual stresses. To this end, a three-dimensional explicit dynamic non-linear finite element model has been developed and successfully validated against the results of impact-test experiments conducted on pipes made of AISI 10305 steel, with and without the AISI304 stainless steel liner. The validation was made by comparing numerically computed strains with those measured by strain gauges, as well as in terms of permanent deformation. The model is then utilized to evaluate the residual stresses, the amount of energy dissipation and the velocity of impact process as a function of different pipes (i.e. with or without liner) and of the free drop heights.
In this article, it aims to propose effective approaches for hydro-forming process of bi-metallic composite pipe by assuming plane strain and elastic-perfectly plastic material model. It derives expressions for predicting hydro-forming pressure and residual stress of the forming process of bi-metallic composite pipe. Test data from available experiments is employed to check the model and formulas. It shows the reliability of the proposed model and formulas impersonally.
The structural behavior of mechanically lined pipes (MLPs) during the spooling-on phase is investigated in this paper, motivated by their promising offshore applications relying on reel installation. By applying quasi 2D models, we first investigated the gripping stresses preserved in the MLP after the hydraulic expansion manufacturing process and the detachment of the liner under spooling-on curvatures. Furthermore, a comparative 3D finite element (FE) analysis for the liner wrinkling behaviors of MLP with different wall thicknesses of outer layers was performed and indicated that when the wall thickness of outer layer increases from 14.3 mm to 17.9 mm, MLP's critical spooling-on curvature increased more than 47%, reaching 0.1432 rad/m.
Submarine-pipeline developments are moving into deeper waters. As a result, thicker pipe walls are needed and hence the quotient of the pipe diameter and its wall thickness (i.e., D/t) reduces. On the other hand, there is a push to optimise properties and requirements for moderate water depths of several hundred metres. Design criteria in the prevailing submarine-pipeline design standard DNVGL-ST-F101 (2017) were developed for straight pipes with 15 ≤ D/t ≤ 60 but applicability is limited to D/t ≤ 45. There can be a desire to go beyond this limit when designing for moderate depths.
When pipe walls are becoming thinner, this makes the pipeline more susceptible to collapse. It has been found that, in some cases, the DNVGL-ST-F101 formulations that aim at predicting the collapse pressure of relatively thin-walled pipelines can lead to nonconservative results. This is a concern when the line-pipe manufacturing technique employs cold forming—such as the cross-section expansion step in the UOE and JCOE methods—and the adverse contribution of the Bauschinger effect is not mitigated in a suitable manner.
Dents have been identified as among the most common deformation defects in oil and gas pipelines. In this study, the multidimensional mechanical responses, springback, and rerounding behaviors of dents as well as the interaction of multiple dents on pipelines and their effect on pipe integrity were investigated through the three-dimensional finite element method and full-scale burst tests. The burst test demonstrates that a plain dent has a minimal effect on the pipeline burst pressure, and the burst failure position is not located in the dented area. Generally, when a dented pipeline that is subjected to monotonically increasing internal pressure bursts, the dent does not fully reround, and a convex protuberance forms in the axial shoulders on both sides of the dent. During pressurization, the maximum equivalent plastic strain in the dented pipe always occurs on the inner surface near the dent center in which no necking is found in the radial direction of the wall thickness. However, a larger equivalent stress is observed around the edge of the dent center, whereas the stress at the center is relatively small, resulting in the formation of a plastic hinge in the dented area. If internal pressure fluctuations occur, this can easily lead to fatigue failure. Finally, the interaction among multiple longitudinally aligned dents was investigated, and the critical spacing affecting the integrity of the pipeline was determined. The investigation reveals that when the dent spacing exceeds the pipe diameter, the interaction between two dents may be ignored, and the dent effect on the pipe integrity may be separately evaluated. However, if the dent spacing is less than 0.5 times the pipe diameter, then the dents must be treated as a combined dent. Further, if the spacing is 0.5–1.0 times the pipe diameter, then dent interaction must be considered.
Sandwich pipe (SP) combining high-strength performance and thermal insulation has been considered an effective solution for oil and gas transportation in ultra-deepwater. Strain hardening cementitious composite (SHCC) is well known for its capacity to withstand both tensile load and external hydrostatic pressure. The sandwich pipe considered in the research is constituted of concentric steel pipes with SHCC annular layer. In the present research work, the SHCC was manufactured, and full scale sandwich pipes were assembled. Intact and damaged specimens were submitted to controlled external pressure in a hyperbaric chamber to obtain the collapse and propagation pressures, respectively. Modeling and simulation of the buckle propagation of the SPs were correlated with the experimental results. The results show that sandwich pipe with SHCC core has an excellent structural strength under high external pressure in both intact and damaged conditions. Moreover, the results also show that the interaction between the annular and the inner/outer pipes provides a significant contribution to the buckling resistance under propagation pressure.
Lining internally a thin corrosion resistance alloy (CRA) layer within an ordinary carbon steel pipe provides one economical approach, when transmitting highly corrosive offshore hydrocarbons. The lined pipe is expected to make optimize usage of the two types of materials, providing significant corrosion and structural resistance. Existing studies mostly covered the lined pipe under bending, corresponding to the load case during Reel-lay procedure. The behaviour of the lined pipe under axial compression, which can be induced by thermal field when transporting hot hydrocarbons, however, has received much less attention. The present paper describes a series of laboratory tests performed on lined pipes, including tensile coupon tests of the outer and the liner materials, initial geometric imperfection measurements and lined pipes tested under axial compression. Results of the column tests were used to validate numerical models and employed for subsequent parametric investigations. Based on the results, design provisions DNV–OS–F101 and EN 1993-1-1 were assessed for lined pipes under axial compression and a buckling curve was proposed for lined pipes for the first time.
Collars are devices similar to thick-walled rings which are welded to adjacent strings of pipe during installation through the J-lay method. They are designed to support the hanged portion of the pipeline between the seafloor and the vessel while the downstream segment of pipe is welded to the line. Because they locally increase the rigidity of the pipe, these devices also have the potential to arrest an eventual propagating buckle and thus, in such event, limit the damage to a small region. In this paper, the effectiveness of this type of local reinforcement as a buckle arrestor is studied through a nonlinear finite element model capable to reproduce the buckle propagation along pipes and its engagement to collars. The model is based on finite deformation kinematics and properly treats the contact that develops in the collapsed pipe behind the propagating buckle. Pipe and collar materials are modeled as J2 flow theory solids with isotropic hardening. Numerical procedure and results are summarized here and then used to propose a simplified method to evaluate the effectiveness of collars as buckle arrestors.
During offshore installation, steel lined pipes are subjected to severe plastic bending, resulting in detachment of the thin-walled liner pipe from the outer pipe and eventually, the formation of local buckling in the form of shortwave wrinkles that menace the structural integrity of the pipeline. The paper focuses on the mechanical behaviour of mechanically lined pipes subjected to monotonic bending, considering for the presence of low and moderate levels of internal pressure, aimed at preventing or delaying wrinkle formation, and improving structural performance. The problem is solved numerically, accounting for geometric non-linearities, local buckling phenomena and elastic-plastic material behaviour for both the liner and the outer pipe. Two types of lined pipes are examined, with and without mechanical bonding between liner and outer pipe referred to as tight-fit and snug-fit lined pipe, respectively. The results demonstrate that the bending performance of lined pipes, under low or moderate internal pressure levels, is significantly improved. The influence of initial geometric imperfections on liner pipe buckling is also examined, showing the imperfection sensitivity of bi-material pipe bending behaviour.
The amount of residual contact stress between an inner corrosion-resistant alloy pipe and its external carbon steel counterpart is the main challenge when manufacturing lined pipes. This study outlines the experimental and numerical evaluation of the manufacturing parameters of mechanically bonded double-walled pipes, produced by the thermo hydraulic shrink fit process. The measurements indicate that the gripping force between the outer 4-inch carbon steel and inner 3-inch stainless steel pipes was 33.94 MPa when processed at 350 °C with a hydraulic pressure of 30 MPa. The experimental results correlate with those of finite element method simulations for gripping force. The magnitudes of the measured gripping forces in the experiment are sufficiently high for most scenarios. The cross-sections of the final lined pipes were inspected for possible defects with the most common being the lamination-type defect for this production process.
Denting, caused by working condition and by rocks or external objects during the laying process, is an important failure mode of steel pipes. To study the failure mechanism of a steel pipe, steel pipe behavior with longitudinal, transverse, and tilted dents were investigated. Effects of the diameter-thickness ratio and indenter displacement on the pipe mechanical behavior were studied. The results showed that plastic deformation occurred mainly at the dent, where there was an obvious stress concentration. The plastic deformation of the pipe continued to increase during the unloading process. The axial strain at the dent location was compressive strain. Dent's rebound rate (depth ratio of the dent after loading and unloading) of the pipe increased with increasing the diameter-thickness ratio and decreased with increasing the indenter displacement. With transverse denting, both sides of the dent were more dangerous than the dent location. The force-displacement curve for the pipe subjected to a tilted dent was different than those for longitudinal and transverse denting. There is no raised effect for the tilted dent.
In this present study, an improved theoretical model is developed to analyze the manufacturing process of a mechanically lined corrosion resistant alloy (CRA) pipe by a hydraulic expansion. The formula of the relationship between the applied hydraulic pressure and the resulting residual interfacial pressure between the inner liner and outer pipes for the mechanically lined CRA pipe is obtained. The minimum and maximum critical hydraulic pressures are also investigated and an effective forming pressure ranges is found. A 2D axisymmetric finite-element model is built in ABAQUSTM to simulate the mechanically lined CRA pipe during the hydraulic expansion manufacturing process. The analytical and simulation results are compared with the experimental results found in the literature, which reveals that the theoretically calculated residual contact pressure and the finite-element computed results are in good accord with the experimental results. Therefore, the models built in this paper can be applied in actual manufacturing process of mechanically lined CRA pipes.
First-gas production from BP Amoco's Bruce Phase II development in the U.K. North Sea flowed in October 1998 through subsea flow lines that represent the largest manufacture of bi-metal-lined pipe to date. Significant cost savings on capital expenditure for flow lines were achieved by the use of corrosion-resistant alloy (CRA) lined pipe, instead of solid CRA and metallurgically clad pipe options. The bi-metal, CRA-lined pipe was supplied by United Pipelines (U.K.) and H Butting (Germany). The flow line was fabricated and the bundle installed by Rockwater Ltd. by controlled depth tow method.
Offshore oil and gas production was conducted throughout the entire 20th century, but the industry's modern importance and vibrancy did not start until the early 1970s, when the North Sea became a major producer. Since then, the expansion of the offshore oil industry has been continuous and rapid. Pipelines, and more generally long tubular structures, are major oil and gas industry tools used in exploration, drilling, production, and transmission. Installing and operating tubular structures in deep waters places unique demands on them. Technical challenges within the field have spawned significant research and development efforts in a broad range of areas. Volume I addresses problems of buckling and collapse of long inelastic cylinders under various loads encountered in the offshore arena. Several of the solutions are also directly applicable to land pipelines. The approach of Mechanics of Offshore Pipelines is problem oriented. The background of each problem and scenario are first outlined and each discussion finishes with design recommendations. * New and classical problems addressed - investigated through a combination of experiments and analysis * Each chapter deals with a specific mechanical problem that is analyzed independently * The fundamental nature of the problems makes them also applicable to other fields, including tubular components in nuclear reactors and power plants, aerospace structures, automotive and civil engineering structures, naval vehicles and structures.
Accidental damage of offshore pipelines in the form of local buckles induced by excessive bending deformation during deep-water installation may severely lead to local collapse of the tube and consequent buckle propagation along the pipeline. The present paper describes experimental and numerical research conducted to predict the magnitude of buckle propagation pressure of offshore pipelines under external pressure. The experiments of buckle propagation for pipe specimens with different initial geometric imperfections using 316 grade stainless steel tubes are carried out under quasi-static steady-state conditions in a sealed hyperbaric chamber. The stress-strain characteristics in the axial tensile test are measured for the tube material, and then used to numerically calculate the buckle propagation pressure of the pipe. The comparisons between experimental and numerical results are conducted to establish the precise numerical simulation technique. Based upon experimental and extensive numerical results, a more reasonable empirical formula for buckle propagation pressure of offshore pipeline with various values of diameter-to-thickness ratio as well as different strain hardening modulus and yield stress is proposed.
The objective of this study was to investigate the effect of the dent magnitude on the collapse behavior of a dented pipe subjected to a combined internal pressure and in-plane bending. The plastic collapse behavior and bending moment of the dented pipe containing several dent dimensions were evaluated using elastic–plastic finite element (FE) analyses. The indenters used to manufacture the dents on the API 5L X65 pipe were hemispherical rods with diameters of 40, 80, 160 and 320 mm. Dent depths of 19, 38, 76, 114 and 152 mm were introduced to the pipe with a diameter of 762 mm and a wall thickness of 17.5 mm. A closing or opening in-plane bending load was applied to the dented pipes pressurized under an internal pressure equivalent to atmospheric pressure as well as pressures of 4, 8, and 16 MPa. The FE analyses results showed that the plastic collapse behavior of the dented pipes was significantly governed by the bending mode and the dent geometry. Moment-bending angle curves for the dented pipe were obtained from computer simulations and evaluated with a variety of factors in the FE analyses. The load bearing capacity of the dented pipes under the combined load was evaluated by TES (Twice Elastic Slope) moments. The load bearing capacity of the pipe containing up to a 5% dent depth of the outer diameter was not reduced in comparison to that of the plain pipe. The opening bending mode had a higher load bearing capacity than the closing bending mode under the combined load regardless of dent depth. The TES moment decreased with increasing dent depth and internal pressure regardless of the bending modes.
The stability of a ring confined by a rigid boundary, subjected to circumferential end loads is investigated by an experimental approach. Dimensional analysis was used to establish the basic relationship of the critical buckling stress with material properties, geometric dimensions and initial geometric imperfections. The coefficients in the relationship were obtained experimentally. Compared to other theoretical methods, this approach is very simple, straightforward and accurately fits the experimental data. Therefore, it is suitable for practical applications.
The present paper investigates the structural stability of thin-walled steel cylinders surrounded by an elastic medium, subjected to uniform external pressure. A two-dimensional model is developed, assuming no variation of load and deformation along the cylinder axis. The cylinder and the surrounding medium are simulated with nonlinear finite elements that account for both geometric and material nonlinearities. Cylinders of elastic material within a rigid boundary are considered first, and the numerical results are compared successfully with available closed-form analytical predictions. Subsequently, the external pressure response of confined thin-walled steel cylinders is examined, in terms of the initial out-of-roundness of the cylinder, the initial gap between the cylinder and the medium, and the stiffness of the surrounding medium. Numerical results are presented in the form of pressure-deformation equilibrium paths, and show a rapid drop of pressure after reaching the maximum pressure level, as well as a significant imperfection sensitivity. A plastic-hinge mechanism is developed that results in a closed-form expression and illustrates the post-buckling response of the cylinder in an approximate manner. The distributions of plastic deformation, as well as the variation of cylinder-medium contact pressure around the cylinder cross-section are also depicted and discussed. Furthermore, the effects of uniform vertical preloading on the maximum pressure sustained by the cylinder are examined. Finally, the numerical results show good comparison with a simplified closed-form expression, proposed elsewhere, which could be used for design purposes.
In this paper the reduction in the collapse pressure of long cylinders which have local dents is evaluated through a combination of experiment and analysis. A number of stainless steel tubes, with diameter-to-thickness ratios of approximately 33,24 and 19, were indented to various degrees with spherical indentors of two diameters. The geometry of each dent was recorded using an imperfection scanning system and the cylinders were subsequently collapsed under external pressure. Denting reduces the local collapse resistance of the cylinder. For larger dents the collapse pressure was found to approach the propagation pressure of the tube. Collapse was found to be relatively insensitive to the detailed geometry of a dent but to be critically dependent on the maximum ovalization of its most deformed cross section (Δ0d). The collapse pressures of tubes with dents produced by indentors of different diameters could be well correlated through this measure of the dent geometry.The denting and collapse processes were simulated numerically using appropriately nonlinear elastoplastic shell analyses. Both steps of such simulations were shown to be in good agreement with experimental results for a broad variation of the parameters of the problem. The key role of the geometric parameter Δ0d was exploited in order to generate a Universal Collapse Resistance Curve for dented cylinders. It was possible to show that the post-limit load response (P — Δ) of a cylinder, with a small but axially uniform initial ovality, provides a very good lower bound to the collapse pressures of the dented cylinders plotted against the dent parameter Δ0d. The significance of this curve is that it can be used to estimate the reduction in the collapse pressure of a cylinder with any dent geometry from only one relatively simple measurement of the geometry of the dented section.
WHEN AN UNDERWATER PIPELINE IS BUCKLED IN THE PRESENCE OF SUFFICIENT EXTERNAL PRESSURE, A PROPAGATING BUCKLE IS INITIATED, AND THE BUCKLE FRONT WILL PROPAGATE ALONG THE PIPELINE UNTIL A REGION OF MUCH LESS EXTERNAL PRESSURE IS REACHED.THIS PAPER DESCRIBES THE RESULTS OF EXPERIMENTAL RESEARCH ON METHODS OF ARRESTING THE ADVANCEMENT OF A PROPAGATING BUCKLE ALONG THE PIPELINE.THE ARRESTING CAPACITIES OF THREE TYPES OF BUCKLE ARRESTORS CONCEIVED DURING THIS PROGRAM, FREE-RING, WELDED-RING, AND HEAVY-WALLED SECTION ARRESTORS, ARE DESCRIBED IN DETAIL.RESULTS OF PROPAGATING PRESSURE EXPERIMENTS ARE ALSO PRESENTED.(A).
Corrosion resistant alloy clad steel has been available in various forms for over 40 years and is being used increasingly in the oil and gas production industries. In the context of the specific requirements of this industrial sector, methods of manufacturing clad plate, pipe and fittings are given along with welding details and information on existing field applications of clad products.
1. INTRODUCTION
The use of corrosion resistant alloys (CRAs) for the control of corrosion in oil and gas systems has a number of benefits. Production systems constructed of correctly selected CRAs (based on appropriate laboratory testing or previous field experience in similar enVironments) will provide a safe, leak-free system for the full duration of a project. Whilst the inital capital expenditure often appears to be higher for a CRA system there are great savings to be made in reduced processing requirements through the safe handling of wet corrosive fluids. If well fluids can be transported to shore or to an existing platform without drying then the cost savings in equipment, weight andspace can be more than sufficient to cover the expense involved in the CRAs needed to handle the fluid from the well to the processing facility.
Additional savings can be made in operating costs since corrosion inhibitor injection is not required and inspection and maintenance costs are greatly reduced compared to a carbon steel production system (with or without internal organic linings). Thus if a life-time costing approach is taken CRAs can often be show to be an economic corrosion control option for oil and gas production systems.
Nevertheless, CRAs contain expensive alloying elements, particularly the more highly alloyed materials required for corrosive sour production systems. Clad steel is a composite product developed to provide effective and economic utilisation of expensive materials. The cladding layer which will be in contact with the corrosive fluids is made of the corrosion resistant alloy whilst the less expensive backing steel providesthe strength and toughness required to maintain the mechanical integrity. Because high strength backing steel can be utilised, wall thicknesses can be reduced relative to solid CRAs thus reducing fabrication time and costs.
The cost saving from using clad steel rather than solid CRA is particularly valid when the total thickness increases or when the cladding grade becomes more complex and hence expensive. An indication of the sort of savings which can be made is illustrated for integrally bonded clad plates in Figure 1. Clad steel plates have been utilised with great success in processing vessels, heat exchangers, tanks and a variety of material handling and storage facilities as well as for making longitudinally welded clad pipe. Clad steel has been available in various forms for more than 40 years and has been widely used in the chemical, oil refining and chemical transport industries and more recently in oil and gas production. Yet despite this there are still concerns regarding the use of clad steel..
An investigation has been carried out in an effort to understand the dynamics of a propagating buckle and to find a basis for designing efficient and effective arresting devices. The velocity of propagation was measured as a function of diameter to thickness ratio and pressure, Tests were carried out in different pressure media. It was found that the propagation velocity is affected by the pressurizing medium used as well as the pressure. It was also found that for buckles "initiated" near the buckling pressure a unique "flip-flop" mode occurs. Quasistatic buckle arrest experiments were carried out to determine the parametric dependence of the pressure at which "slowly" propagating buckles penetrate the arrestors.
INTRODUCTION
The propagating buckle is one of the new problems that has appeared as a result of the increased interest in offshore natural resources. The problem can occur during offshore pipelaying operations when the pipe is under combinations of loads such as external pressure and bending moment. A local buckling initiated by these loads can propagate down the pipeline if the external pressure is above a critical pressure known as the propagation pressure. This phenomenon was first discovered and studied by Mesloh, Johns and Sorenson at Battelle. Their studies resulted in an empirical expression for the propagation pressure as a function of the pipe material and dimensions as well as tests on various buckle arrestor designs. Work on buckling interaction was also carried out.
From these results it became clear that the design of deepwater pipelines could be seriously affected by the propagating buckle phenomenon since the propagation pressure is much lower than the buckling pressure of the pipe (Pc). The buckling pressure is proportional to Young's Modulus, E, and the propagation pressure is proportional to yield stress. The ratio of these two pressures depends on the yield strain. This is shown in Fig. 1 as a function of the diameter to thickness ratio.
The propagating buckle profile takes the form of a transition from a circle to a dog bone type shape. This is depicted along with the parameters of the problem in Fig, 2 and an actual buckle is shown in Fig. 3. The final collapse shape depends upon the pressure and the length of the transition section, L, depends upon the velocity of propagation, U, as will be discussed in subsequent sections. The dependence of velocity on the pressurizing medium was the primary motivation for the present work. This dependence was ignored in previous investigations and it will be shown that this leads to considerable errors in velocity.
Once the consequences of a propagating buckle were fully understood, the focus of attention shifted to the problem of arrest. Buckle arrestors similar to previously devised crack arrestors were proposed and studied by Battelle. This was done on an empirical basis and design parameters were not clearly defined. In an effort to elucidate these parameters, one of the arrestor designs proposed by Batelle has been studied experimentally in greater detail.
Commonly used procedures for installing offshore pipelines can induce substantial bending to the line in the presence of external pressure. These combined loads can lead to catastrophic collaspe of the structure. For typically used steels and higher diameter-to-thickness ratios (D/t), bending is limited by a bifurcation type of instability; for lower D/t values, the structures exhibit limit load types of instabilities. Similar distinctions can be made for bending in the presence of external pressure. Previous work (Corona and Kyriakides, 1988) addressed primarily the limit load types of instabilities. In this paper, this analysis is extended to include bifurcation instabilities. The predictions from the two types of analyses are critically compared to experimental results and recommendations about the regime of applicability of each are made.
Damaged sub-sea pipelines may experience propagating buckling provided the water depth is sufficient. Theoretical results are given for propagation pressures and arrestor capacities with strain hardened material.
Slip-on buckle arrestors are tight-fitting rings placed over the pipe at intervals of several hundred meters. They locally increase the pipe resistance to collapse by providing additional circumferential rigidity, and thus impede downstream propagation of collapse. This type of arrestor offers important advantages over other designs as it does not require welding. Alternatively, when split in two and by the addition of flanges, it can be clamped onto a continuous pipe. This is an essential characteristic for lines installed by the reeling method.It has long been known that such devices often cannot reach higher levels of arresting efficiency. The somewhat deficient performance is due to the fact that a propagating buckle can penetrate such devices via a folded-up U-mode at pressures that are lower than the collapse pressure of the pipe. Previous work on the subject from the 1970s dealt with relatively thin-walled pipes used in shallow waters. The subject has recently been revisited and bounds for this deficiency in performance have been established. The deficiency depends on the pipe D/t and on the steel grade. This paper presents the results of a more detailed study of such arrestors for pipes with lower D/t values (18–35) suitable for moderately deep and deep waters. The arresting efficiency has been studied parametrically through experiments and full scale numerical simulations. The results have also shown the previously developed efficiency bounds to be viable. A new empirical design formula has been developed. Used in conjunction with the efficiency bounds, it provides a reliable method for an expanded use of slip-on type buckle arrestors in many deepwater applications.
Theoretical and experimental studies on line-pipe bearing capacity and collapse pressures indicate that bending the pipe during deepwater pipe laying operations can initiate pipe collapse; stringent curvature control of pipelines is therefore of extreme importance. During installation, a pipe section can become unstable and collapse or buckle because of high hydrostatic pressures and certain combinations of axial forces, bending moments, and line-pipe imperfections (such as out-of-roundness). Researchers have studied these parameters, determining their qualitative and quantitative influence on the formation of a collapse mechanism.
DURING its construction a submarine pipeline is at the same time bent and loaded by the external pressure of the sea. If it is bent too far, it buckles. If the local external pressure is small, the buckle is limited to a short kink on the compressed side of the pipe. But if the external pressure is large enough, the local bending buckle can transform itself into a buckle of a different form, which once initiated can propagate along the pipe, driven by external pressure alone, even into regions free of bending. Figure 1 shows a pipe along which a buckle has propagated in this way. The mode can be observed by grasping a tube of toothpaste firmly between thumb and forefinger, and then moving the fingers along the tube. In a short transition region, between the fingers, the pipe bends from a circular into a dumb-bell cross section.
Local imperfections induced in long tubes subjected to high external pressures can lead to local collapse, from which a propagating buckle can be initiated. This can result in catastrophic collapse of large sections of the structure. The propagation pressure is the lowest pressure at which such a buckle will propagate. For common structural metal tubes with diameter-to-thickness ratios of less than 100, the propagation pressure is typically half an order of magnitude lower than the collapse pressure of the intact tubes. As a result, the design of several tubular structures with external pressure loading requires that the collapse and propagation pressures be accurately known. This paper deals with the experimental and analytical challenges of establishing the propagation pressure. A special purpose three-dimensional analysis, in combination with experimental observations and results, is used to demonstrate a mechanism of initiation of propagating buckles in long tubes, to study the parametric dependence of the propagation pressure and to illustrate the effect of axial tension on the propagation pressure. Propagation pressures predicted with this analysis are used to evaluate the strengths and weaknesses of simpler models developed in the past.
This paper is concerned with the numerical solution of systems of equations of discrete variables, which represent the nonlinear behaviour of elastic systems under conservative loading conditions. In particular, an incremental approach to the solution of buckling and snapping problems is explored.The topics that are covered can be summarized as follows:—The computation of nonlinear equilibrium paths with continuation through limit points and bifurcation points.—The determination of critical equilibrium states.Characteristic to the procedures employed is the use of the length of the equilibrium path as control parameter. This feature, together with the second order iteration method of Newton, offers a reliable basis for the procedures described. Actual computations, carried out on a finite element model of a shallow circular arch, illustrate the effectiveness of the methods proposed.
If it would be possible to install Tight Fit Pipe by means of reeling, it would be an attractive new option for the exploitation of offshore oil and gas fields containing corrosive hydrocarbons. Tight Fit Pipe is a mechanically bonded double walled pipe where a corrosion resistant alloy liner pipe is mechanically fitted inside a carbon steel outer pipe through a thermo-hydraulic manufacturing process. Reeling is a fast method of offshore pipeline installation where a pipe is spooled on a reel, which is positioned on a vessel. The vessel subsequently sails to the offshore location where the pipe is unwound, straightened and deployed to the seabed. However, reeling of Tight Fit Pipe is not yet proven technology. The reeling process imposes high plastic strains (due to bending) in the pipe, which may cause unacceptable liner pipe wrinkling and Tight Fit Pipe ovalisation. This PhD project aimed to make a contribution to the possible development of the installation of Tight Fit Pipe by means of the reeling method. The focus of this research was on the initiation and the degree of liner pipe wrinkling as well as the degree of ovalisation occurring during the spooling-on phase of the reeling process, both theoretically and experimentally; the latter by performing full scale bending tests on 12.75 inch outer diameter Tight Fit Pipe. One of the test results indicates that a higher mechanical bending strength decreases liner pipe wrinkling and makes the Tight Fit Pipe more suitable for reeling. However, test results also show that the mechanical bonding strength for the Tight Fit Pipe tested was significantly reduced, irrespective of whether a high or a low initial mechanical bonding strength had been used prior to spooling-on. These findings justify further research into this phenomenon as the eventual mechanical bonding strength after reeling installation may be vital for its anticipated application during operation.
Finite element modeling and experimental validation of rectangular pin buckle arrestors for offshore pipelines
- Mantovano
Post-critical behavior of a rididly confined circular tube under external water pressure and temperature caused extension
- Glock
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