Effect of non-symmetrical corrosion imperfection on the collapse pressure of subsea pipelines

The effects of non-symmetrical corrosion defects (about the major or minor axis of the ellipse) on the collapse modes and collapse pressures of subsea pipelines are studied using the Finite Element (FE) method. The corrosion defects are represented by a groove of a given length, width, and depth which is created by the “element death” technology. Parametric studies are conducted and the influences of corrosion location angle, length, width, and depth on the collapse pressure are discussed. Several significant and interesting results are achieved: (1) The collapse modes are mainly affected by the corrosion location angle, width, and depth; (2) The collapse pressure of a pipe may increase as the corrosion length, width, or depth increases when the corrosion location angle is small; (3) The longer the corrosion length, the larger the effect of corrosion location angle on the collapse pressure; (4) For collapses controlled by corrosion defect (0.3≤h/t ≤ 0.7), the relationship between the collapse pressure and corrosion location angle follows a simple cosine function. For collapses controlled by the ovality (h/t < 0.3), the relationship can be expressed by the combination of straight-line and cosine function.     

On the effect of the ply stacking sequence on the failure of composite pipes under external pressure

The use of high performance structural composites has become very important over the last decades, especially where weight is an essential factor. Particularly in the oil and gas industry, several designs of composite pipes for deep water applications have been recently proposed as competitive solutions against traditional steel pipes. Thus, it is important to assess the performance of composite pipes under high external pressure in order to avoid pipe failure or overconservative designs. In this paper, experimental tests of different composite pipe configurations are performed and then compared to analytical and numerical predictions. Unlike the case of internal pressure loads, the collapse pressure of composite pipes depends on the initial ovality and on the ply stacking sequence. The collapse resistance of different composite pipes is firstly studied through simplified analytical equations combined with different failure criteria. Then, a finite element model is developed using a progressive failure criterion [1]. Both analytical and numerical failure predictions were compared to experimental tests carried out on four composite pipes produced with different ply stacking sequence by the filament winding method [2]. An experimental-numerical-analytical comparison shows that numerical and analytical models provide results in good agreement with those obtained experimentally. Finally, a parametric analysis is carried out to show the effect of ovality and ply stacking sequence on the failure pressure of composite pipes.     

Study on dented hemispheres under external hydrostatic pressure

This paper is dedicated to the comprehensive investigation on dented hemispheres under external hydrostatic pressure. A total of 20 stainless steel hemispheres are fabricated, which have a nominal radius of 60 mm and a nominal wall thickness of 0.76 mm. These fabricated hemispheres are divided into four groups with five samples in each group. Wherein, three groups are dented using conical, spherical, and cylindrical indenters respectively, while the remaining group is designated as undented one. Both dented and undented hemispheres are hydrostatically tested into collapse to examine the effects of indention on the collapse characteristics of hemispheres. Additionally, all tested hemispheres are semi-analytically and numerically evaluated based on the measured geometric data and tested material properties. The experimental, semi-analytical, and numerical results are compared in figures and tables.     

Reliability analysis of ovalized deep-water pipelines with corrosion defects

The accurate assessment of the remaining strength of corroded pipes is a subject that has been increasingly investigated over the past decades. This is because of the risk of significant social, economic, and environmental effects that may be caused by an accident. The finite element method has been successfully used to predict the collapse pressure considering external load. It was also used in this study. The literature primarily focused on the corroded pipes subjected to internal pressure. In this study, the out-of-roundness (ovalization) of the pipe was considered to evaluate the collapse pressure. Uncertainties should be incorporated into a computational model to assess the reliability of corroded pipes. Three methods for evaluation of the probability of failure were used: the first-order reliability method (FORM), traditional Monte Carlo (MC), and a new proposed methodology that combines MC results with the kernel density estimation method (MCkde). The probability of failure of ovalized corroded pipes subject to external pressure was computed. The results exhibited a good agreement between FORM and MCkde method. The statistical importance of each random variable was observed and the results were compared with those from intact ovalized pipes. The computation cost of the MC method with numerical simulation limits its use to the application under study. Solutions using the FORM and MCkde methods exhibited good agreement with those of the full MC method. However, the computational effort of the latter was independent of the stochastic dimension, and it was a derivative-free method. As expected, in general, the solutions based on empirical methods were conservative.     

钢带软管内压和弯曲研究

复合材料管已成为管道发展的热点和趋势。文中对钢带软管在同时受到内压和弯曲组合载荷作用下的力学性能进行了研究。基于建立的钢带软管的环向和径向力学平衡方程,提出了不同弯曲半径下该种类型管道的爆破压力理论计算方法。文中还进行了有限元数值模拟,并与理论计算结果对比分析,分析结果表明文中提出的爆破压力计算方法可作为此类管在同时受到内压及弯曲组合载荷作用下的通用算法,为解决此类工程问题提供重要参考。     

Finite element analyses of corroded pipeline with single defect subjected to internal pressure and axial compressive stress

This paper describes the application of finite element method (FEM) and the development of equations to predict the failure pressure of single corrosion affected pipes subjected to internal pressure and axial compressive stress. The finite element analysis (FEA) results were verified against full-scale burst tests and theoretical calculations. Material non-linearity, which allow for large strains and displacements, were considered. In addition, true UTS instead of engineering UTS was used to determine the point of failure. The pipes used in the FEA was modelled based on API 5L X52 modified steel with a length of 2000 mm, a nominal outer diameter of 300 mm, and a nominal wall thickness of 10 mm. The results obtained from FEA were compared to that of existing comprehensive corrosion assessment method, known as DNV-RP-F101. Six equations, utilizing the Buckingham's π theorem and multivariate non-linear regression techniques, were developed for predicting the failure pressure of corroded pipeline with single defect subjected to both internal pressure and axial compressive stress. These equations provide improved failure pressure predictions with good margins of errors (less than 10%).     

FEM-based evaluation of friction and initial imperfections effects on sandwich pipes local buckling

Sandwich pipes have been studied as one option to overcome the high pressure problems in deep and ultra-deep waters. They have become a possible alternative solution for submarine infrastructure due to its thermal insulation capacity. This contribute to preventing the pipeline from clogging due to the difference in temperature between reservoir fluids and water at the bottom of the sea. The pipelines in ultra-deepwater are continually exposed to severe operating conditions, such as the effect of high levels of external pressure that can cause local deformation or even collapse of the pipe. Thus, a greater understanding of the mechanical behavior of sandwich pipes is required. This paper presents a FEM-based evaluation of friction and initial imperfection effects on sandwich pipes local buckling. The non-linear evaluation was carried out in FEM of local buckling of two sandwich pipes, with polypropylene and cement as filled annular material. The influence of initial imperfections and the degree of friction, between the annular material and the steel pipes, as well as geometric variations of the pipe were considered. The numerical simulations results indicate a capacity to withstand ultra-deep waters collapsing pressures, around 3000 m, either for polypropylene or cement filled annular material model. In addition, the results indicate that the collapse pressure is inversely proportional to the increase in annular thickness and directly proportional to the decrease in friction which have an impact and contribution on the carrying capacity of the sandwich pipe. Further research will consider a design of experiments analysis of reported effects for different diameter-to-thickness ratios.     

Accuracy of nonlinear finite element collapse predictions for submarine pressure hulls with and without artificial corrosion damage

Nonlinear finite element (FE) collapse pressure predictions are compared to experimental results for submarine pressure hull test specimens with and without artificial corrosion and tested to collapse under external hydrostatic pressure. The accuracy of FE models, and their sensitivity to modeling and solution procedures, are investigated by comparing FE simulations of the experiments using two different model generators and three solvers. The standard FE methodology includes the use of quadrilateral shell elements, nonlinear mapping of measured geometric imperfections, and quasi-static incremental analyses including nonlinear material and geometry. The FE models are found to be accurate to approximately 11%, with 95% confidence, regardless of the model generator and solver that is used. Collapse pressure predictions for identical FE models obtained using each of the three solvers agree within 2.8%, indicating that the choice of FE solver does not significantly affect the predicted collapse pressure. The FE predictions are found to be more accurate for corroded than for undamaged models, and neglecting the shell eccentricity that arises due to one-sided shell thinning is found to significantly decrease the resulting accuracy of the FE model.     

Numerical investigation of the collapse pressure of concentric tubes with frictionless and tied interface

The understanding and study of the mechanical behavior of submarine pipes have significant relevance in ocean exploration, allowing the application of these structures in adverse conditions. In this sense, protecting the tube's internal surface is vital for the transportation of corrosive materials, threatening structural integrity. Usually, the external surface is at constant hydrostatic pressure, leading to possible structural failure if the project does not consider all failure modes. Within the framework of Metallurgically Cladded Pipes (MCP) and Mechanically Lined Pipes (MLP), Corrosion-Resistant Alloys (CRAs) are inserted in the internal surface of pipelines. However, they are not typically deemed in the structural analysis as an integrant part of the mechanical resistance for the external load. This work presents an initial analytical proposal to calculate the collapse pressure of concentric tubes incorporating the rigidity provided by the CRA. Tied or frictionless numerical models are assumed to describe the interaction between the two bodies at the interface region. These two scenarios establish the upper and lower boundaries for cases where friction is part of the problem. The methodology applies a least-square minimization function based on nonlinear finite element simulations to extract analytical expressions that estimate the collapse pressure. An effort is made to reduce the number of sensitive parameters involved in the analytical proposals and minimize the complexity of the formulation. This process allows the analyst to visualize which parameters are more relevant in various scenarios. Nevertheless, the main goal is to evaluate how the variables are coupled and develop a methodology that can be adapted to reproduce the analyst's necessities.     

压力载荷下非粘结柔性立管应变影响因素分析

基于Abaqus/Explicit准静态和质量放大方法研究了一类典型非粘结柔性立管在压力载荷作用下应变响应特性,对影响立管整体轴向延伸率和绕轴向扭转角度的因素进行了分析。数值模型计入金属层实际截面形状、铺设角度以及几何、接触、材料非线性。计算结果表明：数值解与理论值吻合较好;立管端部边界条件对轴向延伸率影响不大但对绕轴向扭转角度影响较大;抗压铠装层为径向压力主要受力构件,其铺设角度虽然对压溃性能不大,但在应变分析中不可忽略;拉伸铠装层铺设角度对应变影响同样较大。文中数值方法可弥补理论方法限定在小位移、小变形范围,无法计入层间摩擦、材料非线性及初始制造椭圆率等缺陷。     

A new look at the external pressure capacity of sandwich pipes

Sandwich Pipes (SPs) have been developed to overcome the required flow assurance and pressure capacity issues in deep and ultra-deep waters. This research aims at studying the influence of certain structural parameters on the pressure capacity (also referred to as the plastic buckling pressure) of Sandwich pipelines. The use of high grade steel pipes, as the internal or external pipes, has also been considered as one of the design parameters in this study. Moreover, a comprehensive parametric study, considering a practical range of the parameters that influence the response of SPs (and considering 3840 SP configurations) was conducted. The results from this large array of pipes were used to formulate a practical equation, capable of estimating the plastic buckling pressure of SPs. The accuracy of the proposed equation was evaluated by comparing the results with the experimental and numerical results available in the literature. The comparative results demonstrated that the proposed equation could predict the buckling capacity of such pipes with a reasonable accuracy. Furthermore, the proposed equation was used, along with a general optimization procedure, to establish the most optimum and cost-effective combination of structural parameters for SPs suitable for use in various water depths.     

Experimental and theoretical studies on the lateral collapse of mechanically lined pipes

The mechanically lined pipe, which consists of a thin non-corrosive liner and a carbon steel pipe, is an economical way to transport acidic fluid in offshore applications. However, it is prone to lateral loading and potentially develops plastic collapse and liner separation during the operational stage. The present study examines its crushing performance using small-scale 2-inch lined pipes manufactured by a customized hydroforming facility. The lined pipes comprised a seamless GB45 carbon steel tube and a grade T2 copper liner. Four ring specimens extracted from lined pipes and two single-layer rings were crushed on a universal testing machine in a quasi-static manner. The tube was found to develop an ovalized shape initially and then changed into a "∞" shape in the end. The carrier and liner were found to stay together throughout the crushing process, with slight liner separation taking place. The manufacture and lateral collapse processes were also reproduced with finite element models and nonlinear analytical formulation. The analytical model was established based on the principle of virtual work and nonlinear ring theory, assuming small strain and finite rotation. It showed that the analytical and numerical simulation results agreed with the experimental results. The influence of major parameters of the problem was also studied based on the full-scale lined pipe. Amongst other findings, manufacture-related plastic hardening is shown to affect the load-carrying capacity and the growth of separation during lateral collapse.     

Buckle propagation of damaged SHCC sandwich pipes: Experimental tests and numerical simulation

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.     

Compressive tests on short continuous panels

Results of eight tests on stiffened panels under axial compression until collapse and beyond are presented. The tests consider panels with different combinations of mechanical material properties and geometric configurations for the stiffeners including the use of ‘U’-shaped stiffeners. The specimens are three bay panels with associated plate made of high tensile steel S690. Four different configurations are considered for the stiffeners that are made of mild or high tensile steel for bar stiffeners and mild steel for ‘L’ and ‘U’ stiffeners. The influence of the stiffener's geometry on the ultimate strength of the stiffened panels under compression is analyzed.     

Propagating buckles in corroded pipelines

Rigid–plastic solutions for the steady-state, quasi-static buckle propagation pressure in corroded pipelines are derived and compared to finite element predictions (ABAQUS). The corroded pipeline is modeled as an infinitely long, cylindrical shell with a section of reduced thickness that is used to describe the corrosion. A five plastic hinge mechanism is used to describe plastic collapse of the corroded pipeline. Closed-form expressions are given for the buckle propagation pressure as a function of the amount of corrosion in an X77 steel pipeline. Buckles that propagate down the pipeline are caused by either global or snap-through buckling, depending on the amount of corrosion. Global buckling occurs when the angular extent of the corrosion is greater than 90°. When the angular extent is less than 90° and the corrosion is severe, snap-through buckling takes place. The buckle propagation pressure and the corresponding collapse modes also compare well to finite element predictions.     

Exact and simplified estimations for elastic buckling pressures of structural pipe-in-pipe cross sections under external hydrostatic pressure

Structural pipe-in-pipe cross sections have significant potential for application in offshore oil and gas production systems because they combine thermal insulation performance with structural strength and self weight in an integrated way. Such cross sections comprise inner and outer thin-walled pipes with the annulus between them fully filled by a selectable filler material to impart an appropriate combination of properties. Structural pipe-in-pipe cross sections can exhibit several different collapse mechanisms, and the basis of the preferential occurrence of one over the others is of interest. This article presents an exact analysis for predicting the elastic buckling behaviours of a structural pipe-in-pipe cross section when subjected to external hydrostatic pressure. Simplified approximations are also investigated for elastic buckling pressure and mode when the outer pipe and its contact with the filler material is considered as a pipe on an elastic foundation. Results are presented to show the variation of elastic buckling pressure with the relative elastic modulus of the filler and pipe materials, the filler thickness, and the thicknesses of the inner and outer pipes. Case studies based on realistic application scenarios are used to show that the simplified approximations are sufficiently accurate for practical structural design purposes.     

桩土作用下缺陷大管桩单桩极限承载力理论计算

码头大管桩出现的不同类型缺陷,如混凝土脱落和钢筋锈蚀,会造成码头承载能力下降。基于完整桩-土体系的荷载传递理论,推导获得缺陷桩剩余极限承载力的计算公式。依托工程实践,考察混凝土剥落和钢筋锈蚀这两种缺陷类型对单桩极限承载力的影响,并得出相应结论：混凝土剥落位置对单桩竖向极限承载力和单桩抗拔极限承载力有影响,混凝土剥落位置位于土层内部会减小单桩竖向极限承载力和单桩抗拔极限承载力,单桩竖向极限承载力减小0.07%,单桩抗拔极限承载力减小1.72%;但对桩身竖向承载力却不同,混凝土缺损对桩身轴心受压承载力减小25.65%,钢筋损失对桩身竖向承载力减小20.95%。混凝土缺损比钢筋缺损对桩身各项承载力的影响要大得多。     

Predicting the wet collapse pressure for flexible risers with initial ovalization and gap: An analytical solution

As offshore hydrocarbon production moves towards ultra-deep water, flexible risers have to withstand the huge hydro-static pressure without collapse. They are designed with strong collapse capacities, allowing them to operate under the condition where their annuli are flooded by the seawater. However, initial imperfections can weaken the collapse capacity under such a flooded condition, triggering the so-called “wet collapse”. Two common initial imperfections, the carcass ovality and the radial gap between the carcass and pressure armor, would reduce the collapse strength of flexible risers significantly. Mostly, collapse analyses are performed through numerical simulations, which are less feasible for the design stage of flexible risers comparing with analytical models. To date, there are few analytical models available in public literature to predict the wet collapse pressure of flexible risers accounting for initial ovality and gap. To meet this demand, an analytical model is established in this paper to address these issues. This model is developed as a spring-supported arch, solving the collapse pressure with stability theories of ring and arched structures. This analytical model is verified by numerical simulations, which gives prediction results that correlate well with the numerical ones.     

Experimental investigation of the strength and stability of submarine pressure hulls with and without artificial corrosion damage

A submarine may have to operate for a period of time with local corrosion damage in the pressure hull if a suitable repair method is unavailable or too expensive for implementation. This paper describes collapse tests on twenty ring-stiffened aluminium cylinders, which were conducted to study the effect of corrosion damage on hull strength and stability. Artificial hull thinning was found to reduce the collapse strength of experimental models through high local stresses in the corroded region, leading to early onset of yielding and inelastic buckling. Bending associated with the eccentricity due to one-sided thinning was found to further increase the local stresses in the hull. Overall collapse pressures were more severely affected by corrosion damage than interframe collapse pressures. The percentage reduction in overall collapse pressure, compared with intact experimental models, was found to be closely related to the percentage depth of thinning. The accuracy of conventional collapse pressure predictions for the experimental models was significantly better for intact than for corroded cylinders.     

Mechanical model and mechanical property analysis of fibre-reinforced hybrid composite pipes

Compared to conventional fibre-reinforced composite pipes, fibre-reinforced hybrid composite pipes are more complex and are characterised by the use of hybrid fibres, hybrid matrices, and multiple fibre winding angles. In this study, based on the mechanical model of conventional fibre-reinforced composite pipes, the cross-section division method, the radial pressure on the adjacent layer by spiral wound rope structures, and the calculation method of axial force in each layer were improved. Furthermore, the von Mises stresses in each layer were calculated to discriminate the failure to establish a mechanical model of fibre-reinforced hybrid composite pipes with any number of reinforced layers under axial tension, internal pressure, and external pressure. Experimental data and the finite element method (FEM) were used to verify the reliability of the established model, with the axial tensile mechanical properties analysed based on the established model. The results showed that the large-angle fibres no longer withstood the axial tensile load when the winding angle of the large-angle fibres was greater than 45°. The matrices yielding was much earlier than the fibre breakage. The matrices hybrid methods have a large influence on the axial tensile properties of fibre-reinforced hybrid composite pipes, and improving the material properties of the inner and outer liners can significantly improve the axial tensile properties of fibre-reinforced hybrid composite pipes.