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On the response of flexible risers to loads imposed by hydraulic collars
Abstract
Even though hydraulic collars are largely used to install flexible risers, neither the loads imposed by this equipment nor the response of the risers to these loads have been previously studied. Hence, this paper presents a three-dimensional nonlinear finite element (FE) model to predict the response of flexible risers to loads imposed by hydraulic collars and also provides a set of equations to predict these loads. The FE model relies on an analogy between helical tendons and orthotropic shells to simulate the inner carcass and the pressure armour of flexible risers. Shell elements are used to represent the polymeric layers and three-dimensional beam elements simulate the wires of the tensile armours. Material, geometric and contact nonlinearities are addressed. Contact interactions between the layers of the riser are handled by surface to surface contact elements with a contact detection algorithm based on the pinball technique and contact forces evaluated with the augmented Lagrangian method. A 9.5″ flexible riser is analyzed and the numerical results are compared to those from the experimental tests. The agreement between all results indicates that the proposed FE model is an efficient approach to predict the response of flexible risers to loads imposed by hydraulic collars and, moreover, may be used to analyze the response of such structures to other types of loads.
... As for the simulation for the contact between layers and steel wires in de Sousa's work, the 3D spar element was used to simulate the contact between steel wires of the tensile armor and the 3D beam element was used to simulate the contact between layers. After that, de Sousa et al. [3][4][5] improved the model. The contact between layers in the new model was simulated by contact element instead of the beam element, and the contact between steel wires was simulated by nonlinear spring instead of the spar element [3][4][5] . ...
... After that, de Sousa et al. [3][4][5] improved the model. The contact between layers in the new model was simulated by contact element instead of the beam element, and the contact between steel wires was simulated by nonlinear spring instead of the spar element [3][4][5] . At the same time, the influence of material nonlinearity was also considered. ...
... In Model 1, the fluid barrier, anti-wear layers and outer sheath are simulated by isotropic shell, the carcass and pressure armor are simulated by orthotropic shell, and the tensile armor is simulated by beam element. In Model 2, It should be emphasized that the parameters of orthotropic shells of the carcass and pressure ar-mor are calculated by the principle of equivalent stiffness [3] . The detailed calculation procedures to get the parameters of orthotropic shell are introduced in Ref. [11]. ...
Three kinds of models based on the same flexible pipe with 8 layers have been separately created to investigate the effects of different modeling approaches on numerical simulation results of finite element (FE) models for unbonded flexible pipes. Then the mechanical property of the unbonded flexible pipe under tension, torsion and bending load has been analyzed and compared via ABAQUS software on the basis of three created models. The research shows that different modeling methods of flexible pipes make a great difference in the results. Especially, modeling simplifications of the carcass and pressure armor have a great impact on the accuracy of the results. Model 3, in which the carcass is simulated by spiral isotropic shell and other layers are simulated by solid element, possesses good adaptability, which has been proved by comparing the experiment data and other models. This paper can offer a reference for the FE modeling methods’ selection and mechanical property analysis of unbonded flexible pipe.
... In all these models, a key aspect is to properly estimate the equivalent geometric parameters, such as the thickness of the layer and, if necessary, equivalent material properties. Several different methods to obtain these equivalent properties have already been proposed [5,10,[17][18][19][20][21][22], but these methods often lead to very different mechanical characteristics and, consequently, to different collapse pressures. Cuamatzi-Melendez et al. [6] employed different equations to [3], (b) "eight" [4], and (c) "heart" [3] obtain the equivalent thickness of a 4 in. ...
... In this work, the equivalent approach is employed in the three-dimensional numerical model proposed by de Sousa et al. [20]. The model represents the carcass, the inner sheath and the pressure armor of flexible pipes with shell elements and accounts for geometric and material nonlinearities as well as interlayer contact. ...
... Modeling. Aiming at modeling the inner carcass and the pressure armor, de Sousa et al. [20] propose the use of an analogy between helical beams and orthotropic shells. This analogy guarantees the equivalence between the stiffnesses of the helical beam (tendon) and the shell resulting in an equivalent shell thickness, Eq. (15), and material properties, Eq. (16) given by ...
The hydrostatic collapse strength of a flexible pipe is largely dependent on the ability of its carcass and/or pressure armor to resist radial loading and, therefore, its prediction involves an adequate modeling of these layers. Hence, initially, this work proposes a set of equations to estimate equivalent mechanical properties for these layers, which allows their modeling as equivalent orthotropic cylinders. Particularly, equations to predict the equivalent ring bend stiffness are obtained by simulating several two-point static ring tests with a three-dimensional finite element (FE) model based on beam elements and using these results to form datasets that are analyzed with a symbolic regression (SR) tool. The results of these analyses are the closed-form equations that best fit the provided datasets. After that, these equations are used in conjunction with a three-dimensional shell FE model and a previously presented analytical model to study the bisymmetric hydrostatic collapse mechanism of flexible pipes. The predictions of these models agreed well with the collapse pressures obtained with numerical models and in experimental tests thus indicating the potential use of this approach in the design of flexible pipes.
... In all these models, a key aspect is to properly estimate the equivalent geometric parameters, such as the thickness of the layer and, if necessary, equivalent material properties. Several different methods to obtain these equivalent properties have already been proposed [10,[15][16][17][18][19][20], but these methods often lead to very different mechanical characteristics and, consequently, to different collapse pressures. Cuamatzi-Melendez et al. [5] employed different equations to obtain the equivalent thickness of a 4" carcass and calculated the related collapse pressures with the Timoshenko [14] equation. ...
... In this work, the equivalent approach is employed in the three-dimensional numerical model proposed by de Sousa et al. [18]. The model represents the carcass, the inner sheath and the pressure armor of flexible pipes with shell elements and accounts for geometric and material nonlinearities as well as interlayer contact. ...
... Aiming at modeling the inner carcass and the pressure armor, de Sousa et al. [18] propose the use of an analogy between helical beams and orthotropic shells. This analogy guarantees the equivalence between the stiffnesses of the helical beam (tendon) and the shell resulting in an equivalent shell thickness, Eq. (15), and material properties, Eq. (16) given by: ...
The hydrostatic collapse strength of a flexible pipe is largely dependent on the ability of its carcass and pressure armor to resist radial loading and, therefore, its prediction involves an adequate modeling of these layers. Hence, initially, this work proposes a set of equations to estimate equivalent thicknesses and physical properties for these layers, which allows their modeling as equivalent orthotropic cylinders. These equations are obtained by simulating several two-point static ring tests with a three-dimensional finite element (FE) model based on beam elements and using these results to form datasets that are analyzed with a symbolic regression (SR) tool. The results of these analyses are the closed-form equations that best fit the provided datasets. After that, these equations are used in conjunction with a three-dimensional shell FE model and a previously presented analytical model to study the dry and wet hydrostatic collapse mechanisms of a flexible pipe. The predictions of these models agreed quite well with the collapse pressures obtained in experimental tests thus indicating that the use of the equivalent approach is promising.
... De Sousa et al. , however, employed the three-dimensional nonlinear finite element (FE) model proposed in de Sousa et al. (de Sousa et al., 2009) to analyze the mechanical response of a 6" flexible pipe with wires broken in its tensile armor layers and subjected to pure tension. This FE model is capable of representing each wire of these layers and localized defects, including total rupture, may be adequately addressed. ...
... The extensional-torsional response of flexible pipes is a key aspect in the design of equipments to detect the rupture of tensile armor wires in field (Camerini et al., 2008) and, moreover, to adequately simulate pull-out operations . A modified version of the FE model proposed by de Sousa et al. (de Sousa et al., 2009) is used to study a 9.13" flexible with wires broken in its outer tensile armor. Two different types of load cases are considered: pure tension; and torsion (clockwise and anticlockwise) combined with tension. ...
... Thus, an alternative approach capable of reducing the number of degrees of freedom and adequately modeling the inner carcass and the pressure armor of typical flexible pipes is necessary. De Sousa et al. (de Sousa et al., 2009) initially supposed that: ...
This work investigates the extensional-torsional response of a flexible pipe with one up to ten wires broken in its outer tensile armor. Experimental tests were conducted, and the results obtained are compared to those from a finite element (FE) and a simplified analytical model. In these tests, the pipe was firstly subjected to pure tension and, then, to clockwise and anticlockwise torsion. The experimental results indicate a decrease in the stiffness of the pipe with the increasing number of broken wires as well as a significant impact on its extensional-torsional balance. High stress concentrations in the wires close to those damaged were observed. Furthermore, local changes in the curvature of the pressure armor and the inner carcass induced bending strains not only in these layers, but also in the inner tensile armor wires. The FE model captured these effects and agreed well with the experimental measurements, while the analytical model predicted the overall response of the pipe, but local effects could not be evaluated.
... De Sousa et al. [6][7][8] and Muñoz et al. [9] however, employed the three-dimensional nonlinear finite element (FE) model proposed in de Sousa et al. [10] to analyze the mechanical response of flexible pipes with wires broken in its outer tensile armor. This FE model is capable of representing each wire of the tensile armors and, consequently, localized defects, including total rupture, may be adequately represented. ...
... Copyright © 2014 by ASME Figure 2 presents a general view of the model proposed by de Sousa et al. [10]. ...
... The construction of a solid three-dimensional finite element model to direct represent the inner carcass and the pressure armor of flexible pipes is, by itself, an extremely onerous computational task [12,13], due to the high number of degrees of freedom involved. Thus, de Sousa et al. [10] employed an alternative approach in which the inner carcass and the pressure armor are represented as orthotropic shells. This approach relies on the following hypotheses: ...
This work focuses on the structural analysis of a damaged 9.13″ flexible pipe to pure and combined axisymmetric loads. A set of experimental tests was carried out considering one up to ten broken wires in the outer tensile armor of the pipe and the results obtained are compared to those provided by a previously presented finite element (FE) model and a traditional analytical model. In the experimental tests, the pipe was firstly subjected to pure tension and, then, the responses to clockwise and anticlockwise torsion superimposed with tension were investigated. In these tests, the induced strains in the outer armor were measured. Moreover, the axial elongation of the pipe was monitored when the pipe is subjected to tension, whilst the twist of the pipe was measured when torsion is imposed. The experimental results pointed to a slight decrease in the stiffness of the pipe with the increasing number of broken wires and, furthermore, a redistribution of forces among the intact wires of the damaged layer with high stress concentration in the wires close to the damaged ones. Both theoretical models captured these features, but, while the results obtained with the FE model agreed well with the experimental measurements, the traditional analytical model presented non-conservative results. Finally, the results obtained are employed to estimate the load capacity of the pipe.
... The polymeric layers work as sealing, anti-wear and/ or heat- insulated components while the metallic layers withstand the imposed loads, e.g. radial inward forces, internal pressure and axial tension [2,3]. The function and the most commonly used materials of each layer are listed in Table 1 [4,5]. ...
... This method is often used to study the responses of carcass layer subjected to axial loads [27][28][29] or crush [3,[30][31][32] due to the ortho- tropic mechanical properties of the equivalent shell. Since the carcass layer, however, takes responsibility for radial resistance only, the treatment of helical carcass wire as a homogenous ring by discarding its lay angle in collapse studies is more acceptable to academics. ...
Flexible riser is a key enabler for the oil and gas production in ultra-deep water which transports production fluids between floating production systems and subsea wells. As oil and production heads to water depths in excess of 3000 m, high hydrostatic pressure has been one primary challenge facing the riser operators. Excessive hydrostatic pressure may cause collapse failure of flexible risers and thus predicting the critical collapse pressure is of significant importance to their anti-collapse design. Collapse is a complex phenomenon related to the material properties, the geometry of the pipe and its overall surface topography and, therefore, makes the prediction of critical pressure challenging. Related prediction approaches of flexible risers have been developed for decades, yet a comprehensive review of their predictive capabilities, efficiency and drawbacks is lacking. This paper reviews the recent advances on collapse studies of flexible risers and highlights the gaps in existing prediction methods, aiming to facilitate the current anti-collapse design and be a baseline for future utilization of flexible risers in deeper water expansion.
... De Sousa et al. [12][13][14][15] and Merino et al. [16], however, employed the three-dimensional nonlinear finite element (FE) model proposed in de Sousa et al. [17] to analyze the mechanical response of flexible pipes with wires broken in their tensile armors. This FE model is capable of representing each wire of the tensile armor layers and, consequently, localized defects, including total rupture, may be adequately simulated. ...
... Figure 13 presents a general view of the FE mesh. More details on the FE model can be obtained in de Sousa et al. [15,16]. ...
In this work, the mechanical response of a damaged 2.5″ flexible pipe under combined tensile and bending loads is studied. A set of experimental tests was carried out either considering the pipe intact or with one up to four broken wires in its outer tensile armor. In these tests, the deflections along the pipe as well as the strains in its outer tensile armor wires were measured thus allowing estimating the bending stiffness of the pipe and the force distribution among the wires, respectively. The results obtained are compared to those provided by a previously presented finite element (FE) model and analytical models. The numerical and analytical predictions agreed well with the experimental measurements pointing to a negligible decrease in the stiffness of the pipe with the increasing number of broken wires and, furthermore, a redistribution of forces among the intact wires of the damaged layer with high stress concentration in the wires close to the damaged ones.
Copyright © 2015 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
... Numerical methods need to simulate the action of the layers of an unbonded flexible riser as much as possible, as well as the mutual contact between the layers; most of the early numerical models of unbonded flexible risers simplified the internal complex structure to some extent. Sousa et al. [72][73][74][75][76][77][78][79][80] carried out many studies on the mechanical properties of unbonded flexible risers based on numerical methods. They established a finite element model of unbonded flexible risers with a multilayered structure using ANSYS finite element software based on the study of Cruz [81], and investigated the structural responses under individual and combined axisymmetric loads, respectively. ...
Unbonded flexible risers consist of several helical and cylindrical layers, which can undergo large bending deformation and can be installed in different configurations to adapt to harsh marine environments; thus, they can be applied to transport oil and gas resources from ultra-deep waters (UDW). Due to their special geometric characteristics, they can ensure sufficient axial tensile stiffness while having small bending stiffness, which can undergo large deflection bending deformation. In recent years, the development of unbonded flexible risers has been moving in an intelligent, integrated direction. This paper presents a review of unbonded flexible risers. Firstly, the form and properties of each interlayer of an unbonded flexible riser are introduced, as well as the corresponding performance and configuration characteristics. In recent years, the development of unbonded flexible risers has been evolving, and the development of machine learning on unbonded flexible risers is discussed. Finally, with emphasis on exploring the design characteristics and working principles, three new types of unbonded flexible risers, an integrated production bundle, an unbonded flexible riser with an anti-H2S layer, and an unbonded flexible riser with a composite armor layer, are presented. The research results show that: (1) the analytical methods of cross-sectional properties of unbonded flexible risers are solved based on ideal assumptions, and the computational accuracy needs to be improved. (2) Numerical methods have evolved from equivalent simplified models to models that account for detailed geometric properties. (3) Compared with ordinary steel risers, the unbonded flexible riser is more suitable for deep-sea resource development, and the structure of each layer can be designed according to the requirements of the actual environment.
... Most of the earlier numerical models of unbonded flexible risers provide some simplification of the complex internal structure or create new elements through the secondary development method [17,18]. Sousa et al. [19,20] carried out many studies on the mechanical properties of unbonded flexible risers based on numerical methods; he simplified the carcass layer and pressure armor layer, which have complex cross-sectional properties, into orthogonal anisotropic shell units, and simulated the tensile armor layer by isotropic three-dimensional Euler beam unit. And the above numerical model can be used to analyze the cross-sectional mechanical properties of an unbonded flexible riser under axisymmetric loads such as axial force and external pressure. ...
Unbonded flexible risers consist of several helical and cylindrical layers, which can undergo large bending deformation and can be installed to different configurations to adapt to harsh marine environments, and is a key equipment in transporting oil and gas resources from Ultra Deep Waters (UDWs) to offshore platforms. The helical interlayer of an unbonded flexible riser makes the structural behavior difficult to predict. In this paper, the axial tensile behavior and the axial tensile ultimate strength of an unbonded flexible riser are studied based on a typical 2.5-inch eight-layer unbonded flexible riser model, and verified through a theoretical method considering the contact between adjacent layers. First, the balance equation of separate layers is deduced by a functional principle, and then the overall theoretical model of an unbonded flexible riser is established considering the geometric relationship between adjacent layers. Then, the numerical model considering the detailed geometric properties of an unbonded flexible riser is established to simulate the axial tensile behavior. Finally, after being verified through the experimental results, the axial tensile stiffness and axial tensile strength of an unboned flexible riser considering the elasticity of the tensile armor layer are studied using the proposed two methods. Additionally, the effect of frictional coefficients is conducted. The numerical and theoretical results show good agreement with the test results, and the friction between adjacent layers would increase the axial tensile stiffness of an unbonded flexible riser.
... Typical unbonded flexible ( Figure 1) pipes have complex structural layers, making it difficult to perform a structural mechanical study of the entire pipe. Therefore, a large number of scholars use numerical simulation to analyze flexible risers [9][10][11][12][13][14][15][16][17][18][19]. Because the working environment of the flexible riser is located in the deep sea, it is difficult to conduct field experimental research, so there are only a few experimental studies [20][21][22][23]. ...
As oil and natural gas production continue to go deeper into the ocean, the flexible riser, as a connection to the surface of the marine oil and gas channel, will confront greater problems in its practical application. Composite materials are being considered to replace steel in the unbonded flexible pipe in order to successfully meet the lightweight and high-strength criteria of ultra-deep-water oil and gas production. The carbon-fiber-reinforced material substitutes the steel of the tensile armor layer with a greater strength-to-weight ratio. However, its performance in deep-water environments is less researched. To investigate the mechanical response of a carbon fiber composite flexible riser in the deep sea, this study establishes the ABAQUS quasi-static analysis model to predict the performance of the pipe. Considering the special constitutive relations of composite materials, the tensile stiffness of steel pipe and carbon fiber-reinforced composite flexible pipe are predicted. The results show that the replacement of steel strips with carbon fiber can provide 85.06% tensile stiffness while reducing the weight by 77.7%. Moreover, carbon-fiber-reinforced strips have a lower radial modulus, which may not be sufficient to cause buckling under axial compression, so the instability of the carbon fiber composite armor layer under axial compression is further studied in this paper; furthermore, the characteristics of axial stiffness are analyzed, and the effects of the friction coefficient and hydrostatic pressure are discussed.
... In current research, the skeleton layer structure and the compressive/tensile layer structure are regarded as mutually orthogonal cylindrical structures. The structure simulation method is effective to obtain the circumferential pressure load, and the stress response process of the flexible riser under a circumferential load can be proposed [5,6]. ...
Multilayer composite flexible risers have been widely used in engineering. However, this type of structure is complex, as there are influences between layers. Moreover, a range of uncertain factors need to be considered in fatigue analysis. Therefore, it is difficult to perform the fatigue analysis research of multilayer flexible risers. In this paper, the fatigue spectrum analysis and reliability analysis method of a nine-layer flexible riser structure are proposed, and a complete fatigue and reliability analysis process for multilayer structures is developed. The theoretical basis of the fatigue spectrum analysis method is introduced, and the calculation program is described. Finite element software is used to analyze the stress of the multilayer flexible riser under the influence of the upper platform structure movement and the ocean current. Moreover, the stress response of the riser structure of each layer is obtained. According to this, the irregular wave load is simulated by the method of random number simulation, and the stress response spectrum is formed. Then, the appropriate S-N curve is selected to calculate the fatigue damage degree of each layer, and the fatigue damage nephogram is displayed, so as to analyze the structural fatigue damage. Finally, the uncertainty in the process of fatigue damage calculation is analyzed. According to the results, the methods of multilayer riser analysis are summarized and the future research directions are put forward.
... The detailed FEM of the riser consists of the core, internal tensile armour wires, anti-wear tape 2, external tensile armour wires and outer sheath (from inside to outside). To decrease the calculation cost without compromising the analysis accuracy, the pressure armour is modelled by an equivalent orthotropic layer as proposed in Sousa et al. [25]. In addition, two "fake" orthotropic layers are introduced to complete the gaps originated in the model [26]. ...
The flexible riser top connection with floating production units presents additional tensile armour integrity assessment uncertainties associated to the bend stiffener contact interaction that leads to a non-uniform curvature and contact pressure distribution in a region of large dynamic forces and moments. In this paper, a full-scale 6″ flexible riser and bend stiffener bending-tension experimental campaign is carried out in a horizontal rig. The external tensile armour wires are instrumented with strain gauges for axial strain measurement in several riser cross-sections inside and outside the bend stiffener region. The rig assembly allows the riser rotation in order to measure strains at different circumferential angular positions. The bend stiffener polyurethane hyperelastic mechanical response is obtained by uniaxial tensile tests performed at room temperature. A nonlinear finite element model (FEM), including the riser/bend stiffener contact interaction and interlayer friction mechanisms, is employed to numerically investigate the mechanical behaviour of the flexible riser subjected to the experimental loading conditions. The FEM axial strains on the tensile armour wires are compared with the measured results for pure tension and tension-bending loads. Under axisymmetric loading a relevant experimental axial strain dispersion is observed when compared to the average values. Under tension-bending loading, the interlayer friction coefficient influences on the contact pressure and curvature distribution are initially assessed with the numerical model. The strain gauges located in the cross-sections with highest values of contact pressure and curvature are selected for a detailed numerical-experimental strains comparison. In addition, a parametric study is conducted to calibrate the friction coefficient that yields the minimum mean squared error for all measured data. Generally, good correlations are found between the experimental and numerical results.
... Thus is a typical nonbonded flexible pipe which comprises multiple layers. assembled as a unit structure that is commonly used in transporting or extracting oil, gas and water due to their mechanical and chemical properties [2]. The layers are free to Table 1. ...
Non-bonded flexible pipe in the oil and gas fields is often experiencing blockages as on one the major problem. Consequently, it is essential to control the spread. To uncover the issues, this paper therefor presents the behaviour of non-bonded flexible pipes with methane hydrate blockage under the influence of diverse load conditions. A Nonlinear tri-dimensional finite element models were carried out on a seven-layer blocked and unblocked flexible pipe, modelled, and simulated. It comprises two tensile armour layers, an external polymeric sheath, high strength tape, orthotropic equivalent carcass, and pressure armour layers with an internal polymetric sheath. Several studies were conducted to verify the influence of significant parameters on the instability phenomenon when the flexible pipe is under hydrate blockage. The internal pressure and compressive loads were mainly considered as the parameters, and their variation resulted into a significant change in the stability response of the pipe layers. This work includes a detailed description of the finite element model and a case study where the non-bonded flexible pipe is blocked by methane hydrate. The obtained results showed a significant influence of methane hydrate on Sample A (blocked), while Sample B (unblocked) behaves normally under various load conditions.
... Proper computations are required on hose behaviour for different hose-riser configurations, such as the Lazy-S (see Figure 14) and Chinese-lantern configurations (see Figure 15). Some applications with different configurations exist on thermoplastic tubes (Avery and Martins 2003;Picard et al. 2007;Yu et al. 2015Yu et al. , 2017, flexible pipes (Li and Kyriakides 1991;Martins et al. 2003;Lu et al. 2008;Paumier et al. 2009); LNG transfer hoses (Rong-Tai Ho 2008), offloading hoses for CO 2 (Brownsort 2015a(Brownsort , 2015b, slurry simulation in spooled hoses (van Rhee et al. 2013), seawater intake hoses (Antal et al. 2003(Antal et al. , 2012, ship-to-ship transfer hoses (Rong-Tai Ho 2008; Conti-Tech 2019), composite risers (Sobrinho et al. 2011;Wang et al., 2016;Amaechi and Ye 2017;Amaechi et al. 2019cAmaechi et al. , 2019dAmaechi et al. , 2021a, flexible risers (Sousa et al. 2009;Liu et al. 2013;Ramos 2016), moorings (Ja'e et al. 2022, ALi et al. 2020, and other types of pipelines have led to more advances on this area. ...
Marine bonded hoses are conduit-tubular structures used for loading, discharging, transferring and transporting fluid products like oil, gas, and water. These marine conduits are applied in the offshore industry by utilising novel marine materials and sustainable technologies. Based on sustainability, there are advances made as solutions for challenging environments. These challenges include scouring gases, deep water regions, changing sea water temperatures, platform loads and vessel motions. These environments also require sustainable materials like marine composites. This paper reviews historical timeline and patent development of hoses in the marine environment. It highlights key developments on marine hoses and their configurations. These configurations include FPSO-FSO with hose attachments in catenary configurations and CALM buoy-PLEM in Lazy-S configurations. The review also discusses the evolutions in the hose designs, potentials of the hoses, and recent state-of-the-art developments in the industry. Comprehensive discussions with necessary recommendations are made for fluid applications in the offshore industry.
... A calculated sample of carcass and pressure armour by following the highlighted steps above has been detailed as attached. Contrary to my initial application of lay angles in place of the orientation angle of the carcass and pressure armour materials, the lay angle is designed to resist hoops stress due to internal and external pressures [46]. ...
The concept of blockages in a non-bounded flexible pipe during oil and gas mining operations have over the years shown to be a persistent problem, and the need for a standardized remediation approach is paramount. This research work studies the behaviour of non-bonded flexible pipes with methane hydrate blockage under the influence of diverse loading conditions. Nonlinear tri-dimensional finite element models were used on two (2) scenarios, blocked and unblocked conditions. These models recreate a seven-layer flexible pipe with two tensile armour layers, an external polymeric sheath, high strength tape, orthotropic equivalent carcass, and pressure armour layers with an internal polymetric sheath. With these models, several studies were conducted to verify the influence of key parameters on the instability phenomenon when the flexible pipe is under hydrate blockage. The internal pressure and compressive loads can be considered one of these parameters, and their variation causes a significant change in the stability response of the pipe layers. This work includes a detailed description of the finite element model and a case study where the non-bonded flexible pipe is blocked by methane hydrate. The procedure of this analysis is here described, along with the results.
For in-depth knowledge of hydrate formation and its consequences in flexible pipes, this thesis used ABAQUS, a standard finite element (FE), in modelling, simulating, and investigating a hydrate blocked and unblocked non-bonded flexible pipe. It is divided into two Samples, A and B, respectively, under the influence of various load conditions. FE model was adopted to investigate the effects of hydrate on the layers as were not detailed in America Petroleum Institute codes [1]. This was carried out under various conditions such as pipe with and without blockage at various pressure, forces (longitudinal and compressive) values, different hydrate lengths, coefficients of friction and stiffness constants.
In addition to the FE analysis, an experimental investigation was carried out on the samples and where necessary mathematical analyses were undertaken to reverify results. The studies carried out were to determine the non-bonded flexible pipes responses under certain load conditions. This determines the deformation, stress concentration on individual layers, making sure the results are within the recommended API standards, hoop, axial and radial stresses, reactive force, and contact pressure between the layers.
A simplified model was employed and a finer mesh to resolve the issue with the FE model. And progress the effect of the hydrate on the pipe layers.
Importantly, this present work considered and investigated a 7-layers 6” diameter non-bonded flexible pipe as a case study. The results were obtained from the numerical and experimental investigations, analyzed, and presented accordingly. Obtained results showed a significant influence of methane hydrate on Sample A, while Sample B behaves normally under various load conditions. The detailed outcome and further research works are presented in this thesis.
... Based on the equivalence of membrane, bend-255 ing and torsional stiffness between the carcass and the orthotropic tubular shell, the equivalent properties for the latter one were determined. (Ribeiro et al., 2003) This method is often used to study the responses of carcass layer subjected to axial loads (Ribeiro et al., 2003;Yue et al., 2013;Provasi et al., 2016) or crush (de Sousa et al., 2009(de Sousa et al., , 2001Soki et al., 2015;Caleyron et al., 2014) due to its orthotropic mechanical properties. Since the external pressure is mainly resisted by the radial stiffness of the carcass, the treatment of the carcass as a homogeneous ring by discarding its large lay angle in the collapse studies is more acceptable to academics. ...
... Contact mechanics is a considerable challenge when creating numerical models. The distribution of normal and tangential force, penetration and local constraint conditions, and variation in material and mechanical properties of each layer present difficulties in obtaining solution convergence and successful modeling outcomes [28,29]. ...
As oil and gas exploration moves to deeper areas of the ocean, the weight of flexible risers becomes an important factor in design. To reduce the weight of flexible risers and ease the load on the offshore platform, this paper present a cylindrical tensile armor layer made of composite materials that can replace the helical tensile armor layer made of carbon steel. The ACP (pre) of the workbench is used to model the composite tension armor. Firstly, the composite lamination of the tensile armor is discussed. Then, considering the progressive damage theory of composite material, the whole flexible riser is analyzed mechanically and compared with the original flexible riser. The weight of the flexible riser decreases by 9.73 kg/m, and the axial tensile stiffness decreases by 17.1%, while the axial tensile strength increases by 130%. At the same time, the flexible riser can meet the design strength requirements of torsion and bending.
... However, the ultimate strength of unbonded flexible risers under torsion has not been calculated, only torsional stiffness can be attained. Sousa et al. [11] and Merino et al. [12] improved the previous model by considering more details like contact, geometric and material nonlinearities, thus the results are more realistic. Merino [13] built a three-dimensional nonlinear finite element model subjected to torsion based on their previous work [12]. ...
... Other installation aspects related to flexible risers and offshore umbilical systems can be found in the paper by Johnston (1989). The mechanical behavior of flexible risers and engineering problems regarding such offshore components can be found in (de Sousa et al., 2009;Morandeau et al., 2013). Finally, a recent review paper on pipelines, risers and umbilicals features must be mentioned (Drumond et al., 2018), where several references on these topics can found for extra reading on the subject. ...
Nowadays, in order to be competitive on the market, it is fundamental to design optimized devices. This is especially true in the Oil&Gas sector where the product's weight has a material influence on all operations. Reducing this parameter implies a significant decrease in the project's cost. This paper wants to propose an innovative approach to the study of the kinematic rotation of a prototype machine required to lay down an umbilical product, needed for the distribution of electricity among platforms. The aim of this paper is to resolve whether an optimization in terms of size and weight of the main piston that activates the chute rotation might be found, and if so what might be the magnitude of the possible optimization. The project has been carried out in collaboration with F.lli Righini, a worldwide leader in designing and manufacturing offshore equipment. A comparison between analytical and actual components leads to a real design approach which can be used by other companies and research centers in the field.
... The Carcass layer is shown in Fig. 1. According to the ideological stiffness equivalent of Jose et al. 14 ...
Flexible pipes, as a typical composite structure, are generally applied to the ocean oil engineering. The Carcass layer of flexible pipes, with its particular structural formation, demonstrates orthotropic properties and the moduli of the Carcass layer are not the material moduli. Just the moduli of the Carcass layer are calculated, the whole structure will be analyzed. In the present paper, the equivalent modulus equations of the Carcass layer are deduced with the use of the ideology that under the same loading, the similar material and the original material have the same displacement. This analytical method of equivalent moduli shows good capacity in the prediction of the behaviour of the flexible pipes, which provides practical and technical support for the application of unbonded flexible pipes.
... Relying on previous experimental tests [20,21], the equivalent moment of inertia of the wire, I eq , proposed by Souza [22] is employed ...
In this paper, the coupled extensional-torsional behavior of a 4 in. flexible pipe is studied. The pipe is subjected to pure tension and two different boundary conditions are considered: ends free and prevented from axially rotating. The response of the pipe is predicted with a three-dimensional nonlinear finite element (FE) model. Some aspects of the obtained results are discussed, such as the effect of restraining the axial rotation at the extreme sections of the model; the effect of friction or adhesion between the layers of the pipe on the induced axial rotation (or torque) and elongation; and the reduction to simple plane behavior usually assumed by analytical models. The numerical results are compared to the ones measured in experimental tests. Reasonable agreement is observed between all results pointing out that the analyzed pipe is torque balanced and that friction mainly affects the axial twist induced by the applied tension. Moreover, the cross sections of the pipe remain straight with the imposed load, but different axial rotations are found in each layer.
Fatigue analyses of flexible pipes through the application of long-term response statistics, when considering bimodal seas, is particularly challenging due to the excessive computational costs involved, since it requires, generally, to solve numerically a four-dimensional integral associated with time-domain finite-element-based analyses of the flexible pipe. Thus, the search for effective approaches to solve this integral is the target of this paper, which investigates the efficacy of some Dimension-reduction methods (DRMs), which had never been applied to the problem posted, according to the authors' knowledge, as an option to compute the fatigue damage of these structures. The performance of the DRMs is compared with the Monte Carlo simulation method (MCSM). Two case studies are presented: a 4in, and a 7in, flexible riser connected to a Semi-submersible, and a spread-moored FPSO platform, respectively. The studies demonstrate that some DRMs presented very accurate results, exhibiting high efficacy with errors below 3% and that the Bivariate dimension-reduction method (BDRM) required only 18% of the computational cost compared to the Standard Gauss-Hermite Quadrature (SGHQ) in R^4 or much less when compared to the MCSM.
As key components connecting offshore floating production platforms and subsea imports, offshore flexible pipes play significant roles in oil, natural gas, and water injection. It is found that torsional failure is one of the failure modes of flexible pipes during transportation and laying. In this paper, a theoretical model (TM) of a flexible pipe section mechanics is established, in which the carcass layer and the pressure armor layer are equivalent to the orthogonal anisotropic layers due to its complex cross-section structure. The calculation results of the TM are consistent with those of a finite element model (FEM), which can accurately describe the torsional response of the flexible pipe. Subsequently, the TM and FEM are used to discuss the influence of boundary conditions on the torsional response. The structure of the flexible pipe is stable when twisted counterclockwise. However, limiting the top axial displacement can improve the axial and radial instability of the tensile armor layer when twisted clockwise. Finally, it is recommended that the flexible pipe can be kept under top fixation during service or installation to avoid torsional failure.
Ovalization, as an important factor affecting the performance of flexible pipes, is unavoidable in the manufacturing process. In this paper, a model of a flexible pipe with initial ovalization is established numerically. The numerical model is verified by a case analysis, and the influence of ovalization on the torsional response of a flexible pipe is discussed. It is found that the ovalization always has a negative effect on the torsional stiffness of the flexible pipe. Then, the effective numerical model is used to calculate the torsional response of a flexible pipe with initial ovalization under dry and wet annulus pressure. It is found that the dry annulus pressure improves the torsional stiffness of the flexible pipe by advancing the interaction between the layers and hindering the radial expansion of the outer tensile armor layer. However, the dry annulus pressure also increases the stress of the tensile armor layers, which reduces the withstanding capacity of the flexible pipe. Especially with the increase of ovalization, the loading capacity decreases more obviously. The wet annulus pressure has little effect on the torsional response of the flexible pipe, but it extends the risk of radial collapse of the carcass layer.
This work proposes closed-form equations to calculate the bisymmetric dry and wet hydrostatic collapse strengths of flexible pipes with imperfections. Initially, the bisymmetric hydrostatic collapse is discussed, relying on the classic Timoshenko's equation. Then, the main parameters involved in the collapse mechanism are indicated. Particularly, approaches to account for the effect of the carcass’ cold work and the pressure layers’ residual manufacturing stresses are suggested. After that, an equivalent three-dimensional shell finite element (FE) model is revisited and employed to predict the flexible pipes' wet and dry collapse strengths. Several FE collapse analyses were performed addressing different material properties, initial ovalities, and interlayer gaps. The obtained responses showed that the hypotheses assumed by the modified Timoshenko's equation only hold for the dry collapse condition with no interlayer gaps. However, the calculated collapse pressures with this equation deviate from the FE values as higher ovalities are considered. Hence, an alternative approach was employed to overcome these limitations. In this approach, the FE collapse pressures and the flexible pipes’ characteristics constituted datasets analyzed with a symbolic regression (SR) tool. These analyses led to the closed-form equations, named semi-empirical equations, that best fitted the datasets. Sensitivity analyses of these equations showed that the bisymmetric collapse is significantly affected by the yield strength of the pressure layers’ materials and imperfections. Furthermore, under wet annulus conditions, the confinement provided by the pressure armor also has a substantial impact. Finally, comparisons with experimental tests verified the applicability of the proposed equations.
As offshore oil and gas exploration takes place at greater depths, so more flexible pipes are failing by collapsing. Here, based on finite-element analysis, full and equivalent models of the dry and flooded annulus collapse of flexible pipes are established and compared. The equivalent models that retain the cross-sectional detail of the main resistance metal layer are highly consistent with the full models, thereby validating the equivalent models. The critical load for dry collapse increases with increasing yield strength of the pressure armor layer, but with dry collapse gradually becoming elastic instability. The critical load for flooded annulus collapse is affected strongly by the yield strength of the carcass layer, and the different yield strengths of the pressure armor layer and carcass layer may cause a different mode of flooded annulus collapse. The results indicate that the yield strength of the carcass layer should be emphasized when selecting the metal-layer materials to improve the critical collapse under flooded annulus conditions, while the yield strength of the pressure armor layer should provide the necessary resistance against plastic buckling in dry collapse.
This paper presents a method to predict the bending hysteretic behavior of unbonded flexible pipes using full layered numerical general finite element models with an implicit solver. A predefined stress fields method is proposed to simulate the initial contact pressure effect introduced by the fabrication process. Detailed finite element modeling techniques are illustrated based on system balance principles of a cantilever beam system. The models can capture the interactions (normal contact and tangential friction behavior) between and among layers adequately, thus giving reasonable stress predictions of tensile armor tendons in both longitudinal and transverse (radial and lateral) directions. The results show that the proposed method through prescribing a fixed initial hoop stress value in the outer sheath could give consistent bending moment-curvature relations for all test pressure levels and corresponding numerical results. The stress-curvature relations reveal that the tensile armor tendons slide in an interval between the loxodromic and geodesic curves as proved in the open literature. In addition, sensitivity studies including element types, boundary conditions, material properties, normal contact stiffness and friction coefficients are performed to validate the feasibility of the proposed models. Moreover, if plastic layers are modeled by solid elements, the friction moment will be larger than that of using shell elements due to Poisson’s effect on pipe elongation.
Collapse bucking is a typical failure mode of flexible pipes during deepwater service. It is essential to investigate the failure characteristics and obtain the accurate critical collapse pressure to improve the design of flexible pipes and make them more durable in a deepwater environment. Firstly, an equivalent layer method based equivalent hoop stiffness is presented in the paper. Then, the proposed method is verified by corresponding comparative analysis. Next, two-dimensional (2D) collapse numerical models are created based on the verified method. Finally, the parametric analysis of collapse influence factors is carried out by the arc-length method. The results show that the equivalent method is valid and convenient for obtaining the equivalent thickness of the carcass layer. The parametric analysis results under different collapse influence factors can provide some useful advice for collapse-resistant design of flexible pipes.
Unbonded flexible pipes are widely utilized in the exploitation of offshore oil and gas resources. They are connected to two of the most critical types of system: floating production platforms and underwater production systems. However, if some tensile armor wires are substituted by cables or broken, the tensile armor layer will be incomplete, which seriously reduces the safety and reliability of the flexible pipe. In the present study, models of a flexible pipe with a complete tensile layer and with the tensile layer partially missing were established. The error for the tensile stiffness obtained by the finite element model of an intact flexible pipe was only 1% compared with experimental results. Because the load borne by the inner tensile armor layer is larger under tension than that borne by the outer tensile armor layer, the loss of inner tensile armor wires has a greater impact on the tensile properties. The maximum axial elongation of the flexible pipe increases with the number of missing inner tensile armor wires as a cubic polynomial. If the distribution of the missing armor wires is too dense, a stress concentration and local bending may occur, which will reduce the tensile strength of the flexible pipe.
Flexible pipes play an important role in the transportation of oil and gas from the seabed to floating production units. They are also employed to inject water or gas into offshore wells. In deep-sea oil and gas extraction, flexible pipes must be able to withstand very high tensile and hydrostatic pressures. In this paper, an equivalent method is used to represent the complex carcass layer and the pressure armor layer as a regular cylinder, and then a numerical model of a 2.5" flexible pipe was developed and verified by experiment. A tensile analysis of flexible pipes with different degrees of initial ovalization indicated that ovalization of 0–1% does not affect the tensile rigidity, but increasing the ovalization creates an uneven load and stress concentration on the tensile armor layer. When a flexible pipe is subjected to hydrostatic pressure, the outer pressure increases the tensile rigidity of the flexible pipe more than the annular pressure, but the reduction of the ultimate tensile load is clear. A high annular pressure can cause the collapse of the carcass layer.
An un-bonded, multi-layered assembly of heterogeneous materials in the cross-section of flexible risers enables riser systems to withstand a large radius of curvature while preventing structural damage or buckling of many critical areas such as a touch down zone. The contact surfaces between layers exhibit a continuous sliding of metallic components after a certain level of bending, which lowers the bending stiffness of flexible risers. Such interactions are beneficial in order to reduce the stress of tensile wires, but complicate prediction of the limit state and fatigue damage of riser systems in that the sliding of metal layers involves a complex contact phenomenon that is difficult to solve numerically. This series of papers deals with the development of an improved analysis method for flexible risers using theoretical approaches. Part I, as presented in these pages, develops an analytical model that is capable of estimating the cross-sectional stiffness of flexible risers with consideration of external loads and inter-layer interaction. In Part II, the proposed analytical model is applied to a large-scale riser model to achieve an efficient global dynamic analysis of flexible risers.
This paper, the first part of a two-paper series, proposes an analytical model that formulates the stress of tensile armor layers through equilibrium equations that take into account the shear and radial deformation of polymer layers. The equilibrium equations are derived under a linear differential equation for each component of flexible risers, so that not only the shear interaction forces but also the contact pressure originating from the bending of layers is considered in the analytical model. The shear interaction forces between two tensile armor layers are formulated through an equivalent shear stiffness by which the shear stiffness of layers is linearly modeled as a series of springs. The proposed model is verified by comparison with the nonlinear bending stiffness from existing bending models and finite element (FE) analysis. For additional verification, the axial stresses of inner and outer tensile armor also are compared. And for further, more in-depth understanding, a case study of various combined load cases is carried out.
The local mechanical response of a 2.5" unbonded flexible riser with damaged wires is studied by a nonlinear finite element (FE) model. Firstly, the riser was supposed to be under pure tension and pure torsion. Stiffness and force distribution of the intact riser predicted by the analytical and numerical models are compared, and deviations between the two solutions are discussed. Then, two up to eight wires in the outer tensile armor layer are assumed to be damaged. Numerical results indicate that uneven deformations caused by the broken wires result in a clear nonlinear force redistribution among the outer wires. The maximum and minimum forces supported by the remaining intact outer wires are closely related to the number of damaged wires, boundary conditions and the load direction. Finally, the effect of internal frictions on the load recovery of damaged wires is studied by increasing the FE model length and applying the pre-pressure. It was found that damaged wires can progressively recover their load-carrying capacities with the increase of internal frictions. In addition to the boundary effects, axial stress of the damaged wire is linear with the length of the riser, which indicates that results obtained from the short model can be employed to estimate the load recovery characteristics of the long model.
Flexible pipes connect the two critical systems of the floating production platform and underwater production, which play an important role in the transportation of oil and gas as well as water injection. However, flexible pipes are often subjected to multiple loads that threaten their stability in the service process. In this study, a model of a flexible pipe containing practical cross section in all layers is established to study the effects of axial compression and wet collapse loads on clockwise and anticlockwise torsional responses. The results of calculation show that the axial compression and radial expansion of the flexible pipe are caused by the axial compression load, which reduces clockwise torsional stiffness. The wet collapse load induces the bending deformation of the flexible pipes in clockwise and anticlockwise torsion, where the bending deformation of clockwise torsion is significantly affected by the change in the Young’s modulus of the tensile armor layers. This study shows that clockwise torsional stiffness is small and exhibits a trend of axial compression. Clockwise torsion has poor stability in combination with axial compression and wet collapse loads, respectively. Therefore, to prevent instability in flexible pipes, clockwise torsion load, or which coupled with axial compression and wet collapse load, should be avoided.
Purpose
The internal force is more complicated in a combined load case than in a single load case, and the influence of the combined load on the stress cannot be neglected. The purpose of this paper is to study the mechanical behavior of the flexible riser under combined load conditions of tension and internal pressure or external pressure.
Design/methodology/approach
The mechanical behavior of the flexible riser under combined load conditions is studied by numerical simulation with a nine-layer detailed finite element model. The layers of flexible riser are modeled separately, and the interactions between layers have been taken into consideration in numerical simulation.
Findings
Under tension and internal pressure or external pressure, the pressure armor will bear extra external pressure because of the squeezing actions between layers caused by tension, and the extra external pressure will increase proportionately with the increase of the tension. Under internal pressure and tension, the internal stress for tension armor was nearly unchanged compared to that under unique tension load, whereas under external pressure and tension, the change of internal stress for tension armor was significant. Prediction methods of internal force for pressure armor and tension armor under pressure and tension are given, and the result from the formula agrees well with the simulation results.
Originality/value
The prediction methods on the internal force of flexible riser proposed in this study are proven accurate, with numerical simulation results, and the prediction methods are convenient for engineering applications.
A flexible pipe connects offshore platforms to the flowlines and transport gas and oil. It may experience axial compression due to the reversed end cap effect during installation. This can trigger radial buckling or lateral buckling of tensile armor layers. The ultimate strength assessment of the flexible pipe is complicated and time-consuming due to material nonlinearity, large deformation, and nonlinear contact mechanism. These difficulties make the nonlinear analyses difficult to converge. This paper proposes a simplified 5-layered model which can improve the convergence without deteriorating the accuracy. Analytical methods are suggested to determine an equivalent layer to replace inner four layers. In addition, the factor of penetration tolerance (FTOL) of shell element layers needs to reflect the thinning of polymer layers, which makes the axial stiffness equal to the solid element model. Analytical methods are used to determine the factor and a stepwise increasing approach is applied in a numerical analysis.
The 5-layered model with the stepwise FTOL application is verified by comparing with 8-layered model, analytical model and experiment results with respect to axial and bending stiffness. The model is used for an ultimate strength assessment, the failure mechanism and the interaction between layers are investigated in detail with incremental loading.
This study presents an analytical model of flexible riser and implements it into finiteelement software ABAQUS to investigate the fatigue damage of helical wires near touchdown point (TDP). In the analytical model, the interlayer contact pressure is simulated by setting up springs between adjacent interlayers. The spring stiffness is iteratively updated based on the interlayer penetration and separation conditions in the axisymmetric analysis. During the bending behavior, the axial stress of helical wire along the circumferential direction is traced to determine whether the axial force overcomes the interlayer friction force and thus lead to sliding. Based on the experimental data in the literature, the model is verified. The present study implements this model into ABAQUS to carry out the global analysis of the catenary flexible riser. In the global analysis, the riser-seabed interaction is simulated by using a hysteretic seabed model in the literature. The effect of the seabed stiffness and interlayer friction on the fatigue damage of helical wire near touchdown point is parametrically studied, and the results indicate that these two aspects significantly affect the helical wire fatigue damage, and the sliding of helical wires should be taken into account in the global analysis for accurate prediction of fatigue damage. Meanwhile, different from the steel catenary riser, high seabed stiffness may not correspond to high fatigue damage of helical wires.
Designs of flexible pipe utilized in offshore dynamic riser applications need to be subjected to strength assessment over the entire length in relation to the dynamic motion of the connected floater. Such assessment includes many uncertain factors including soil properties, internal pressure, ocean metadata and others. Failure of flexible pipe, moreover, leads to serious financial loss and environmental degradation. Thus, flexible pipe failure modes and safety need to be evaluated in terms of ultimate strength under various loads.
Ultimate strength assessment of flexible pipe is quite complicated and time-consuming compared with that of a steel catenary riser, due to the composite materials, geometric complexity and the contact mechanism between layers The material nonlinearity and large deformation, moreover, render stable convergence of nonlinear analysis to a solution problematic. This paper proposes practical and stable methods of ultimate-strength assessment using 8-layered and 5-layered 3D FE models subjected to axial tensile and compressive loads, respectively. In the 5-layered model, four inner layers (carcass, pressure sheath, pressure armour, and anti-friction tape) are replaced by one equivalent pressure layer which plays a role of withstanding pressure applied together with the axial compression. It aims to improve the convergence of nonlinear analysis by simplifying the interactions between layers. For each analysis, the failure mechanisms and the interactions between layers are investigated in detail with incremental loading. The effect of initial imperfection, i.e. ovalizations are also examined. In the compressive strength analysis, the influence of various external pressure are additionally studied.
A helical wire is a critical component of an unbonded flexible riser prone to fatigue failure. The helical wire has been the focus of much research work in recent years because of the complex multilayer construction of the flexible riser. The present study establishes an analytical model for the axisymmetric and bending analyzes of an unbonded flexible riser. The interlayer contact under axisymmetric loads in this model is modeled by setting radial dummy springs between adjacent layers. The contact pressure is constant during the bending response and applied to determine the slipping friction force per unit helical wire. The model tracks the axial stress around the angular position at each time step to calculate the axial force gradient, then compares the axial force gradient with the slipping friction force to judge the helical wire slipping region, which would be applied to determine the bending stiffness for the next time step. The proposed model is verified against the experimental data in the literature. The bending moment–curvature relationship under an irregular response is also qualitatively discussed. The stress at the critical point of the helical wire is investigated based on the model by considering the local flexure. The results indicate that the present model can well simulate the bending stiffness variation during irregular response, which has significant effect on the stress of helical wire.
Helical wire is a key component of unbonded flexible riser, and is vulnerable to fatigue failure. The present study simplifies the flexible riser into a beam element with constant axial and bending stiffness, and then investigates the vortex-induced vibration (VIV)-induced fatigue damage of the helical wire in a catenary flexible riser using a time domain VIV approach. The simplification of the flexible riser is based on the equivalence of axial and bending stiffness. The former can be iteratively calculated based on the integrated axisymmetric formulation in which the interlayer contact and separation are taken into account, while the latter is simply taken as the combination of all layers’ bending stiffness and keeps constant since helical wire may not slide (i.e. full-sticking) under intact outer sheath due to high external pressure. Based on the simplification, this study compares the VIV characteristics of the flexible riser under full-sticking and full-sliding conditions of helical wires since the latter associated with conservative results is often applied, and then parametrically investigates the fatigue damage of the helical wire. The results indicate that the critical position is located near touchdown point, and seabed stiffness, helical wire lay angle and the top end position have a significant effect on the fatigue damage.
This paper presents analytical and numerical models to predict the behavior of unbonded flexible risers under torsion. The analytical model takes local bending and torsion of tensile armor wires into consideration, and equilibrium equations of forces and displacements of layers are deduced. The numerical model includes lay angle, cross-sectional profiles of carcass, pressure armor layer and contact between layers. Abaqus/Explicit quasi-static simulation and mass scaling are adopted to avoid convergence problem and excessive computation time caused by geometric and contact nonlinearities. Results show that local bending and torsion of helical strips may have great influence on torsional stiffness, but stress related to bending and torsion is negligible; the presentation of anti-friction tapes may have great influence both on torsional stiffness and stress; hysteresis of torsion-twist relationship under cyclic loading is obtained by numerical model, which cannot be predicted by analytical model because of the ignorance of friction between layers.
Flexible pipes can be used as risers, jumpers, and flowlines that may be subject to axial forces and out-of-plane bending motion due to operational and environmental loading conditions. The tensile armor wires provide axial stiffness to resist these loads. Antibirdcaging tape is used to provide circumferential support and prevent a loss of stability for the tension armor wires, in the radial direction. The antibirdcaging tape may be damaged where a condition known as "wet annulus" occurs that may result in the radial buckling (i.e., birdcaging mechanism) of the tensile armor wires. A three-dimensional continuum finite element (FE) model of a 4 in. flexible pipe is developed using ABAQUS/IMPLICIT software package. As a verification case, the radial buckling response is compared with similar but limited experimental work available in the public domain. The modeling procedures represent an improvement over past studies through the increased number of layers and elements to model contact interactions and failure mechanisms. A limited parameter study highlighted the importance of key factors influencing the radial buckling mechanism that includes external pressure, internal pressure, and damage, related to the percentage of wet annulus. The importance of radial contact pressure and shear stress between layers was also identified. The outcomes may be used to improve guidance in the engineering analysis and design of flexible pipelines and to support the improvement of recommended practices.
In this article, we present the numerical analysis of a Free Standing Riser. The numerical simulation was carried out using a commercial riser analysis software suit. The numerical model's dimensions were the same of a 1/70 reduced scale model deployed in a previous experiment. The numerical results were compared with experimental results presented in a previous article [1]. Discussion about the model and limitations of the numerical analysis is included.
To effectively investigate the collapsing failure for the interlocked carcass layer of an un-bonded flexible pipe caused by a high value of external hydrostatic pressure, a three dimensional finite element model was developed based on the finite element analysis method. The model was developed to simulate the collapse behavior of carcass under hydrostatic external pressure, in which solid elements are used. Analysis results indicate that a good correlation between analytical and numerical calculation models was achieved for critical pressure, without considering geometrical imperfections and material nonlinearity. The pressure-displacement curve with considering initial elliptical defect approaches the critical pressure rapidly after a linear developing process, and the critical pressure value goes down as the initial ovality increases. When material nonlinearity is introduced to the model, the critical pressure of the carcass reduces significantly and is nearly half of the original perfect one. The numerical model and results provide an efficient method and basis for design and analysis of the un-onded flexible pipe collapse resistance in industry.
The present paper addresses a new method for calculating stresses in flexible pipe armour tendons. The method is based on sandwich beam theory where each tendon interacts with the supporting pipe structure by means of shear interaction springs. The differential equation for the problem is transferred into weak form by using the principle of Virtual Displacements, and solved by the finite element method. The finite element equations have been implemented into a computer code termed BFLEX. The computer code includes a sophisticated 3D graphical user interface for generating input data and efficient result presentation. Numerical testing has been carried out both against analytical solutions and full-scale strain measurements. Good correlations have been found between computed results and analytical solutions as well as experimental tests. The paper describes the theory, finite element implementation, numerical procedure as well as test examples.
This paper deals with the behavior of high-pressure unbonded flexible steel pipes which can be used as risers in offshore applications. It concerns primarily the behavior of the internal structure of the pipe. A theoretical approach allows to establish simple formulas for evaluating: the stresses, and the contact pressures between layers due to axisymmetrical loads; the stresses due to bending; the relative slip betwen layers due to bending. This is a first step towards the evaluation of the life expectancy of flexible pipes. It must be completed by the determination of friction and wear factors through test results.
This paper investigates the higher order geometrical effects on the deflections of a helical armour wire when the pipe is subjected to uniform bending and is away from any end effects from its remaining fittings (end fittings), and the effect on the subsequent bending stresses. Due to its complexity in both geometrical shape and loading conditions, the approaches found in the literature are often to assume either a bent helix or geodesic as its deflected configuration with linearized mathematical expressions for simplification. The bending stresses are then calculated based on the geometrical difference between the assumed deflection and the original helical shape. The effect of the wire cross-section characteristics, for example the width and thickness ratio, over its deflection are often ignored. This paper presents an analytical strain energy model to quantify the influence of the wire width and thickness ratio over its final deflection. The higher order geometrical effects are fully considered in determining the wire deflection by using the exact mathematical expressions, and in the subsequent wire deformation and stress calculations. The paper also discusses briefly the structural coupling behavior between the pipe axial and bending deformations raised by using the exact expressions. The analytical results are validated by finite element simulation of an identical structure. The results are shown in good agreement in both deflection and the bi-normal bending stress. The results also show desirable conservatism in the normal and the total bending stresses. The presented analytical approach is demonstrated as an efficient and conservative way for investigating the behaviour of such a helical wire.
Several types of laying equipments are used to install flexible pipelines for offshore field developments. These pipelines installation tools apply generally compressive radial forces to hold the pipe suspended weight. The deeper the pipeline has to be laid, the higher is its suspended weight and therefore the higher these radial loads need to be. As each flexible pipe construction is optimized for each field application, a design methodology is necessary to be able to evaluate the radial load acceptable by the flexible pipe. The failure mode associated with this type of loads is an instability thought to be similar to the hydrostatic collapse mode. Therefore an adequate design safety factor has to be considered. The water depth of offshore field developments becoming ever deeper, the resistance of the flexible pipe to installation loads becomes often a driving design criterion. A finite element model to address this type of loading has been developed and improved over the past years. To avoid over-sizing the flexible pipe with current design approach, this finite element model needed to be improved for the latest materials used and for higher crushing loads. To this effect, a new powerful test bench with a crushing load capacity of 1200 tons over one meter has been designed, procured and is now operational. It can handle pipes from 4" to 19" internal diameter. Many types of flexible pipe samples have been tested up to permanent deformation using bi-, tri- and quadri-caterpillar tensioners. The results of these tests have been used to validate a new finite element model using in particular non-linear elasto-plastic material laws. In this paper, several test results will be presented and compared with the calculations. The relevance of different possible design criteria depending on the type of loading regime, the slenderness of the pipe and the number of radially resistant layers, will also be discussed. This new model is operational and allows to optimize the flexible pipe design in particular for ultra-deep water applications down to 2500m or more.
Stress-strain relations are given for an initially isotropic material, ; which is macroscopically homogeneous, volume is considered to be composed of ; various portions, which can be represented by subelements showing secondary creep ; and isotropic work handening in plastic deformation. If the condition is imposed ; that all subelements of an element of volume are subjected to the same total ; strain, it is demonstrated that the inelastic stress-strain relations of the ; material show anisotropic strain-hardening, creep recovery, and primary and ; secondary creep due to the nonuniform energy dissipation in deformation of the ; subelements. Only quasi-static deformations under isothermal conditions are ; considered. The theory is restricted to small total strains. (auth);
The exploration and production (E&P) segment of the petroleum industry has progressively been led to act deeper. In Brazil, where the deep water (400 to 1,000m WD) and ultra-deep water (1,000 to 2,000m WD) have been the segments responsible for the major portion of the overall country petroleum production (approx. 1.3 MMBPD), one can find in Campos Basin the main arena for the pioneer deployment of new technologies. CSO has been working in this environment as a strong partner of Petrobras, helping them to reach their main objectives, from engineering to product manufacturing and supply, installation and commissioning.
This paper will describe:
The technical solutions incorporated in the design of flexible lines of up to 9.2" in 1,500m WD and 6" in 2,000m WD, well in excess of any previous design.
The installation procedures developed and the up to date installation experience in the field, the challenges encountered and lessons learned.
It will also mention the strategies that are being adopted by Petrobras and CSO to overcome the main technological ultradeepwater challenges, to go beyond the 2,000m WD mark.
Introduction
Petrobras history in Campos Basin is discerned by a permanent challenge to overcome deeper water depths. The discovery of giant fields, like Marlim, Albacora and more recently, Roncador, in water depths ranging from 1,500m up to 2,000m, has demanded the development of new technologies, mainly the optimization of subsea hardware, installation and operational procedures.
Flexible pipes have played an important role in the history of oil production in Brazil, gradually building an outstanding reputation of reliability and cost-effectiveness.
No other oil province worldwide has applied flexible pipes so intensively as Campos Basin, where up to the present date there are about 2223km of risers and flowlines installed. For Petrobras, flexibles is the best way to make feasible production of oil fields in a short time, even before the reservoir delimitation and subsea layout consolidation, as they can be easily recovered, inspected, repaired, re-laid and connected in new sites.
Flexible pipes are composed of unbonded layers of plastic and steel, each layer being designed to resist to a specific loading, such as: crushing loads, internal pressure, collapse pressure, axial loads, torsion and so on and may be designed both for dynamic (risers) and for static (flowlines) applications.
The recent advances on the application of flexible pipes in deeper waters associated with a significant CSO R&D effort on the improvement of current structure design has shown that the adoption of flexible will continue to have a large demand in Campos Basin.
The main goal of this paper is to describe how Petrobras has dealt with the necessity of starting oil and gas production on Roncador ultra-deep waters in a short period of time, and what has been done by CSO and Petrobras to overcome the main challenges of this field.
The main characteristics of an analytical model for realistic multi-layered structural strands and some new developments are presented. The results include easily used graphs for determining the pattern of interwire/interlayer contact forces and the associated relative displacements in a large spiral strand with its ends fixed against rotation. Because of interwire frictional phenomena, the effective axial stiffness of a strand is a function of the load perturbation applied. Simplified methods for obtaining upper and lower limits to the stiffness are presented. The lower bound is the "full-slip" large-disturbance modulus, which is independent of the friction coefficient and is obtained in a conventional load-extension test. The small-disturbance "no-slip" modulus can be significantly larger. Very encouraging agreement is found between the predicted full-slip moduli for several real constructions with widely varying strand and wire diameters and lay angles, and experimental data for long specimens. Numerical examples are presented to facilitate the use of the present paper to forecast various aspects of the behavior of other spiral strands.
This paper presents a finite element formulation for predicting the behaviour of complex umbilical cross-sections exposed to loading from tension, torque, internal and external pressure and external contact bodies. Helically wound armours and tubulars are formulated by a combination of curved beam kinematics, thin shell theory and the principle of virtual displacements to obtain finite element equations. Interaction between structural elements is handled by a penalty parameter contact formulation. The model takes into account a number of features, such as material non-linearity, gap between individual bodies, hoop response of tubulars due to contact effects and it may be applied to both cross-section design and verification. Experimental studies are presented to validate the axisymmetric performance of the model.
An analytical model for predicting the axial-torsional structural behaviour of flexible pipes, umbilicals and marine cables is presented. The analytical model is based on the interaction between the component layers of a generic flexible structure. The flexible structure is analysed as a combination of helical and cylindrical components placed in arbitrary order. A governing equation for the flexible structure is derived, the solution of which provides the axial-torsional load-displacement relationship. The model also provides additional information, such as the construction of individual component layers, interfacial contact pressures and interlayer gap amplitudes. Results are presented for a typical marine cable, an umbilical and a flexible pipe. Tensile tests have been carried out on the marine cable and the umbilical and good agreement between theory and experiment is demonstrated.
Stress-strain relations are given for an initially isotropic material,
which is macroscopically homogeneous, volume is considered to be composed of
various portions, which can be represented by subelements showing secondary creep
and isotropic work handening in plastic deformation. If the condition is imposed
that all subelements of an element of volume are subjected to the same total
strain, it is demonstrated that the inelastic stress-strain relations of the
material show anisotropic strain-hardening, creep recovery, and primary and
secondary creep due to the nonuniform energy dissipation in deformation of the
subelements. Only quasi-static deformations under isothermal conditions are
considered. The theory is restricted to small total strains. (auth)
Contact-impact algorithms, which are sometimes called slideline algorithms, are a computationally time-consuming part of many explicit simulations of non-linear problems because they involve many branches, so they are not amenable to vectorization, which is essential for speed on supercomputers. The pinball algorithm is a simplified slideline algorithm which is readily vectorized. Its major idea is to embed pinballs in surface elements and to enforce the impenetrability condition only to pinballs. It can be implemented in either a Lagrange multiplier or penalty method. It is shown that, in any Lagrange multiplier method, no iterations are needed to define the contact surface. Examples of solutions and running times are given.
In some of our applications we are interested in how a structure behaves after it buckles. When a structure collapses completely, a single surface may buckle enough that it comes into contact with itself. The traditional approach of defining master and slave contact surfaces will not work because we do not know a priori how to partition the surface of the structure. This paper presents a contact algorithm that requires only a single surface definition for its input.
This paper presents formulations and a solution for the response of umbilical cables and flexible pipes subjected to monotonic loading of tension, torque, internal and external pressures. The homogeneous and helical wound layers are, respectively, described by Lamé's and Clebsch–Kirchhoff's formulations hence yielding nonlinear algebraic equations which are solved by an iterative algorithm. The model takes into account a number of features, such as material nonlinearity, gap formation and interwire contact and it may be applied to cross-section design and verification. The case studies illustrate the benefits of a nonlinear model.
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Analysis of the coupled axial/torsional behaviour of spiral strands, wire ropes, and locked coil cables. B.Sc. dissertation, School of Architecture and Civil Engineering
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Kraincanic I. Analysis of the coupled axial/torsional behaviour of spiral strands,
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Updated method for the determination of the service life of flexible risers
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Flexible pipe’s collapse under external pressure. D.Sc. dissertation in portuguese
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in portuguese. COPPE, Federal University of Rio de Janeiro; 2002.
Local mechanical analysis of flexible risers through the finite element method. DSc dissertation in Portuguese
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Flexible pipe installation in deep and very deep waters
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Coutarel A. Flexible pipe installation in deep and very deep waters. In:
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