scholarly journals Axial mechanical experiments of unbonded flexible pipes

2020 ◽  
Vol 17 (5) ◽  
pp. 1400-1410
Author(s):  
Jun-Peng Liu ◽  
Murilo Augusto Vaz ◽  
Rong-Qi Chen ◽  
Meng-Lan Duan ◽  
Irving Hernandez
Keyword(s):  
Global Analysis ◽  
Axial Stiffness ◽  
Loading Frequency ◽  
Structural Damping ◽  
Flexible Pipe ◽  
Friction Forces ◽  
Damping Behavior ◽  
Flexible Pipes ◽  
Unbonded Flexible Pipes ◽  
Tensile Experiment

Abstract Axial structural damping behavior induced by internal friction and viscoelastic properties of polymeric layers may have an inevitable influence on the global analysis of flexible pipes. In order to characterize this phenomenon and axial mechanical responses, a full-scale axial tensile experiment on a complex flexible pipe is conducted at room temperature, in which oscillation forces at different frequencies are applied on the sample. The parameters to be identified are axial strains which are measured by three kinds of instrumentations: linear variable differential transformer, strain gauge and camera united particle-tracking technology. The corresponding plots of axial force versus axial elongation exhibit obvious nonlinear hysteretic relationship. Consequently, the loss factor related to the axial structural damping behavior is found, which increases as the oscillation loading frequency grows. The axial strains from the three measurement systems in the mechanical experiment indicate good agreement, as well as the values of the equivalent axial stiffness. The damping generated by polymeric layers is relatively smaller than that caused by friction forces. Therefore, it can be concluded that friction forces maybe dominate the axial structural damping, especially on the conditions of high frequency.

An Analytical Model to Predict the Bending Behavior of Unbonded Flexible Pipes

2013 ◽  
Vol 57 (03) ◽  
pp. 171-177
Author(s):  
Leilei Dong ◽  
Yi Huang ◽  
Qi Zhang ◽  
Gang Liu
Keyword(s):  
Bending Moment ◽  
Bending Stiffness ◽  
Nonlinear Formulation ◽  
Flexible Pipe ◽  
Flexible Pipes ◽  
Bending Behavior ◽  
Interlayer Slip ◽  
Unbonded Flexible Pipes ◽  
Relationship Of ◽  
Theoretical Results

Analytical formulations are presented to determine the bending moment-curvature relationship of a helical layer in unbonded flexible pipes. Explicit expressions describing the variation of both bending stiffness and moment as a function of the applied curvature are given. The approach takes into account the nonlinearity of the response caused by the interlayer slip. The contribution of local bending and torsion of individual helical elements to the bending behavior of helical layers is included. Theoretical results for a typical unbonded flexible pipe using the nonlinear formulation for helical layers are compared with experimental data from the available literature. Encouraging correlations are found and the importance of the initial interlayer pressures is seen. The influence of local bending and torsion of individual helical elements on the bending behavior of the entire pipe is also evaluated. The results show that the inclusion of this local behavior significantly influences the full-slip bending stiffness.

Qualification Approach to Unbonded Flexible Pipes With Fibre Reinforced Armour Layer

2004 ◽  
Author(s):  
Jan Rytter
Keyword(s):  
Offshore Structure ◽  
Flexible Pipe ◽  
Load Bearing ◽  
Compression Load ◽  
Metal Parts ◽  
Analysis Tools ◽  
Industry Standards ◽  
Flexible Pipes ◽  
Unbonded Flexible Pipes ◽  
Load And Resistance Factor

The future water depth capabilities for unbonded flexible pipes is being pushed by NKT Flexibles I/S through the development of an innovative flexible pipe structure, taking full advantage of the material characteristics of metallic, polymeric and fibre reinforced materials. The fluid tight liner and possible insulation of this pipe structure are supported by an inner armour, capable of carrying the external hydrostatic pressure, clamp and crushing loads, as well as axial compression load, and an outer armour, consisting of two cross wound layers of carbon/epoxy composites, carrying the internal pressure as well as end cap forces and applied tension. A permeable and radially flexible outer layer protects the composite armour. Combining known and well-proven flexible pipe technologies and new solutions for materials, structure and functionality of the flexible pipe, positions this future product outside the present industry standards for flexible pipes, e.g. API-17J. The analysis tools used for the conventional flexible pipes are validated by NKT according to the API-17J specification. The API-17J describes load cases and corresponding allowable utilization ratios, stated as design criteria. However, this approach is not directly applicable to the composite pipe, where the same analysis tools will be used, but the material in one of the two primary load bearing layers is made of fibre reinforced polymer, a material class not covered by the API allowable utilization factors. The DNV offshore standard DNV-OS-C501 considers any offshore structure in which the load bearing material is a composite. An accompanying Recommended Practice DNV-RP-F202 for composite risers has also been issued, but is not applicable to the composite flexible pipe. The design equations of the DNV standard are formulated in the so-called Load and Resistance Factor Design (LRFD) format, where partial safety factors are applied to the load effects and to the resistance variables that enter the design equations. The DNV standard DNV-OS-C501 covers composite materials and composite metal interfaces of a structure, metal parts should be designed according to other relevant standards. The API standard can therefore be used for the metal parts. One of the challenges in using this combined approach is the different ways loads are defined in the two standards. In short, this will result in a conventional API design check of the inner armour, the polymer layers, and the secondary layers, whereas the composite tensile armour, special intermediate layers and the interfaces will be analyzed with composite specific tools based on the criteria derived from the DNV standard. The qualification procedure is described and exemplified in the following.

On the Structural Response of a Flexible Pipe With Damaged Tensile Armor Wires

2011 ◽  
Author(s):  
Jose´ Renato M. de Sousa ◽  
George C. Campello ◽  
Antoˆnio Fernando B. Bueno ◽  
Eduardo Vardaro ◽  
Gilberto B. Ellwanger ◽  
...  
Keyword(s):  
Mechanical Response ◽  
High Stress ◽  
Three Dimensional ◽  
Structural Response ◽  
Experimental Tests ◽  
Element Model ◽  
Axial Stiffness ◽  
Stress Concentrations ◽  
Flexible Pipe ◽  
Flexible Pipes

This paper studies the structural response of a 6.0″ flexible pipe under pure tension considering two different situations: the pipe is intact or has five wires broken in its outer tensile armor. A three-dimensional nonlinear finite element model devoted to analyze the local mechanical response of flexible pipes is employed in this study. This model is capable of representing each wire of the tensile armors and, therefore, localized defects, including total rupture, may be adequately represented. Results from experimental tests are also presented in order to validate the theoretical estimations. The theoretical and experimental results indicate that the imposed damage reduced the axial stiffness of the pipe. High stress concentrations in the wires near the damaged ones were also observed and, furthermore, the stresses in the inner carcass and the pressure armor are affected by the imposed damage, but, on the other hand, the normal stresses in the wires of the inner tensile armor are not.

Evaluation of the Temperature Effect on the Viscoelastic Responses of Flexible Risers

2019 ◽  
Author(s):  
Junpeng Liu ◽  
Jinsheng Ma ◽  
Murilo Augusto Vaz ◽  
Menglan Duan
Keyword(s):  
Material Properties ◽  
Global Analysis ◽  
Viscoelastic Behavior ◽  
Step Test ◽  
Nonlinear Material ◽  
Polymer Layer ◽  
Axial Stiffness ◽  
Loading Frequency ◽  
Flexible Risers ◽  
Improved Model

Abstract Mechanical behavior of flexible risers can be challenging due largely to its complex design generating strong nonlinear problems. Nonlinear material properties, as one of them, from polymer layers dominate the overall viscoelastic responses of flexible risers which may play an inevitable role on the global analysis in deepwater application. An alternative to predict the viscoelastic behavior comprising of the time domain and the frequency domain has been proposed recently by the authors (Liu and Vaz, 2016). Given the fact that polymeric material properties are temperature-dependent and that the temperature profiles in flexile risers vary continuously in both axial and radial direction, the temperature of the internal hydrocarbons must affect the viscoelastic responses. However, such phenomenon dose not draw much attention in previous studies. This paper presents an improved model for overcoming some drawbacks in the proposed model involving assumption of steady temperature distribution in polymer layer and no gap appearance between the adjacent layers. The computing method of model is developed by using a step by step test approach. Consequently, some important parameters like equivalent axial stiffness, contact pressure or gap between the near layers, and force-deformation relationship can be observed. Parametric studies are conducted on the axisymmetric viscoelastic behavior of flexible risers to study the role of input temperature and loading frequency. Results show that equivalent axial stiffness given by the improved model is smaller than before. It can also be found that the gap between metal layer and polymer layer appear easily and increases as time goes on.

Axisymmetric Mechanical Behavior of Flexible Risers

2015 ◽  
Author(s):  
Liu Junpeng ◽  
Murilo A. Vaz ◽  
Menglan Duan ◽  
Xiaotian Li
Keyword(s):  
Correspondence Principle ◽  
Mechanical Analysis ◽  
Axial Stiffness ◽  
Prony Series ◽  
Structural Damping ◽  
Flexible Riser ◽  
Damping Behavior ◽  
Polymer Properties ◽  
Flexible Risers

Polymer properties, i.e. viscoelastic characteristics, may pose a non-negligible impact on the axisymmetric analysis, especially on the axial structural damping behavior. An available mechanical model taking viscoelastic into account is established for capturing such parameters as axial strains and forces on each layer, axial stiffness and so on. In this paper, the flexible riser is divided into helical and cylindrical elements to carry out the mechanical analysis due to the complexity of structure and material property. Prony Series is used to describe the creep and relaxation behavior of material viscoelasticity. A viscoelastic solution is obtained using the correspondence principle. Finally, a case study is performed and some interesting results are presented.

Controlling Factors of Carcass Fatigue in Unbonded Flexible Pipes

2019 ◽  
Author(s):  
Upul S. Fernando ◽  
Andrew P. Roberts ◽  
Michelle Davidson
Keyword(s):  
Fatigue Failure ◽  
Failure Modes ◽  
Dynamic Stress ◽  
External Pressure ◽  
Controlling Factors ◽  
Flexible Pipe ◽  
Flexible Pipes ◽  
Innermost Layer ◽  
Key Features ◽  
Unbonded Flexible Pipes

Abstract Carcass, the innermost layer of a flexible pipe structure is designed to prevent the collapse of the pressure sheath due to external pressure. Weakness, damage or failure of the carcass layer can result in collapse with associated loss of production and potentially serious risk to pipe integrity and hydrocarbon leakage to the environment. Avoiding carcass failure in service is therefore an essential consideration during the design of unbonded flexible pipes. Carcass failure is rare in service. This paper highlights the three possible failure modes and presents further analysis on the fatigue failure mode, relevant to dynamic service. Two key features of carcass manufacture are identified as causes for dynamic stress; locking of the carcass profile due to extended pitch and polymer ingress within the carcass cavity. Guidelines for the design of carcass profiles, setting safe pitch limits and appropriate barrier profile controls to mitigate carcass fatigue failure in dynamic service are presented.

Analysis of Structural Parameters for Unbonded Flexible Pipes

2013 ◽  
Vol 652-654 ◽  
pp. 1514-1519
Author(s):  
Zhi Bo Li ◽  
Hui Xu ◽  
Gui Zhen Zhang
Keyword(s):  
Structural Parameters ◽  
Steel Strip ◽  
Complex Structure ◽  
Flexible Pipe ◽  
Complex Deformation ◽  
Flexible Pipes ◽  
Bending Behavior ◽  
Unbonded Flexible Pipes ◽  
Walled Cylinder ◽  
Nonuniform Material

Due to the complex structure and nonuniform material of unbonded flexible pipes, an elastic thin-walled cylinder model and a helical steel strip model were established respectively to simulate different layers based on the specific structure form and parameters. Quasi-static incremental load was adopted to identify the structural parameters which had significant effects on the axial, radial and bending behavior of the pipes during the complex deformation. Sensitivity of these parameters were also analysed. The conclusion in this paper could provide guaidance for the design of unbonded flexible pipe.

Helical Wire Behavior of Unbonded Flexible Pipes Under Bending

2015 ◽  
Author(s):  
Yutian Lu ◽  
Huibin Yan ◽  
Yong Bai ◽  
Peng Cheng
Keyword(s):  
Finite Element ◽  
Finite Element Model ◽  
Axial Strain ◽  
Bending Moment ◽  
Element Model ◽  
Flexible Pipe ◽  
Slip Mechanism ◽  
Flexible Pipes ◽  
Bending Behavior ◽  
Unbonded Flexible Pipes

The bending behavior of unbonded flexible pipe is governed by the response of the helical wires in the tensile armor to bending. The behavior of the helical wire, especially the axial strain, is influenced by the slip mechanism as a result of an increasing curvature under bending. In the present paper, two limit curves are considered with a certain curvature. A 3-D finite element model using ABAQUS is developed to simulate the practical behavior of the helical wires under bending. By comparing the FEA and theoretical results, a basic conclusion about the real slip path of the helical wire between two limit curves is introduced. A hysteretic bending moment-curvature relationship induced by the slip mechanism is obtained from the finite element model as well.

Assessment of the Integrity of the PVDF Barrier in Unbonded Flexible Pipes

2014 ◽  
Author(s):  
Michelle Davidson ◽  
Upul S. Fernando ◽  
John Hall ◽  
Brendon O’Donnell ◽  
James Latto ◽  
...  
Keyword(s):  
Polyvinylidene Fluoride ◽  
Single Layer ◽  
Flexible Pipe ◽  
Barrier Material ◽  
Premature Failure ◽  
Environmental Consequences ◽  
Flexible Pipes ◽  
Hard Polymer ◽  
Unbonded Flexible Pipes ◽  
Critical Consideration

The polymer barrier is the most important component in unbonded flexible pipe, providing the leak-tight boundary for transporting hydrocarbon medium. Premature failure of the barrier during service can be costly and may lead to disastrous environmental consequences. Design of the barrier for 25 years’ service integrity is therefore a major requirement in the flexible pipe design process. However, the API design code does not give a specific procedure for the design of the barrier and is mainly concerned with the design of other layers in the pipe which are intended to provide integrity to the polymer barrier. The selection of barrier material depends on many factors including the service temperature/pressure range and pipe bending requirements. Polyvinylidene fluoride (PVDF) is used as a barrier material in cases where high pressure and relatively high temperature applications are involved. However, a hard polymer such as PVDF can be susceptible to crazing and cracking under specific conditions and therefore the use of PVDF in flexible pipe barriers requires critical consideration of the above issues. This paper discusses the general design requirements of a single layer barrier, and different barriers in relation to static and dynamic applications. The details of a qualification test program performed to establish service integrity of single layer Solef 60512 PVDF barriers is discussed. The unique testing facilities developed to test the integrity of the barrier are presented.

Verification Scheme for Unbonded Flexible Pipes: Definition, Implementation and Reflection of API 17J

2017 ◽  
Author(s):  
Fabien Conti ◽  
François Migeon ◽  
Aymeric David
Keyword(s):  
Manufacturing Processes ◽  
High Complexity ◽  
Flexible Pipe ◽  
Verification Scheme ◽  
Type Approval ◽  
Flexible Pipes ◽  
Normative Requirements ◽  
Qualification Testing ◽  
Unbonded Flexible Pipes ◽  
Verification Methodology

The present paper is related to the verification of unbonded flexible pipes designed and manufactured as per API 17J [1]. Back in the early nineties, in response to the needs of the industry for better quality, reliability and safety in unbonded flexible pipe products, Bureau Veritas established an innovative verification scheme relying not only on qualification testing, but also on the assessment of design methods, materials as well as manufacturing processes. This approach has proven particularly relevant considering several specificities of the flexible pipe industry: no ‘on-the-shelf’ design (each pipe being designed for project specific conditions), manufacturer specific local design methodologies, specific materials and manufacturing methods, high complexity and cost of testing. This paper presents the verification methodology, its application as well as its ability to embody the latest normative requirements given in [1]. The specificity of the verification scheme, which consists in a breakdown between a Type Approval phase and a project phase, is also presented.

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