2.2.1 Overview
Flexible pipeline are used in the offshore oil and gas industry to connect wellheads with subsea (e.g., flowline to manifolds) and surface (e.g., riser to floating platform) facilities, and interconnect subsea infrastructure (e.g., jumpers, PLEM, PLET). The flexible pipe comprises a cross-section with multiple layers having different materials (e.g., steel, thermoplastic) that are used to meet specific functional requirements (Figure 2-1). The cross-section geometry, number of layers, material selection and lay-up are prescribed and tailored to meet project specific design requirements.
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Figure 2-1. Cross section of unbounded flexible pipe.
There are a number of technical and economic advantages for the use of flexible pipe with respect to conventional rigid line pipe, particularly for fields located in deepwater that may have high pressure and high temperature requirements but not need long distance tie-backs or export transmission line [1]. In general, the pipe cross-section can be tailored to meet project specific requirements with inner pipe diameter ranging from 50 mm to 508 mm that can be wound on spools for rapid installation with rates as high as 5 km to 10 km per day.
The extruded thermoplastic layers (i.e., plastic sheath) provide corrosion resistance and thermal resistivity, and product containment with respect to mitigating leaks. The carcass and pressure armour provides circumferential resistance to hydrostatic loads exceeding 2,000 m water depth with recent development in flexible pipe technology being qualified for water depths exceeding 3,000 m. The helically wound armour wires provide longitudinal strength to axial tension loads
Carcas s Plastic sheath
Pressure armours Anti-wear tape
Tensile armour wires Anti-birdcaging tape
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and flexibility in bending response. These armour wires can have different cross-sectional geometric configurations with the pitch angle approximately 35 degrees. High strength tape is used to prevent radial expansion of these tensile armour wires. This composite integration provides advantageous mechanical performance characteristics with respect to section collapse, combined loads and strength, and vibration response with excellent fatigue and abrasion resistance.
2.2.2 Analytical, Experimental and Numerical Modelling Studies
The integrated composite structure hindered the early development of engineering models examining the mechanical response of flexible pipe where idealized analytical solutions and numerical modelling procedures were constrained by underlying assumptions and idealizations, and limitations in computational hardware and software.
Ostergaard et al. [2] presents an analytical solution for the lateral buckling of tensile armour layers due to cyclic bending and compression loads procedure, which was partially supported by experimental data. This local instability mechanism may occur during the installation process. The solution addressed the effect of initial imperfections, within the tensile armour wires, on the lateral buckling response; however, it is concluded that further study be conducted to assess the importance of interlayer friction on triggering lateral buckling events.
Braga et al. [3] conducted experimental studies simulating the effects of axial loading due to hydrostatic pressure, up to an equivalent 2000 m water depth, on the mechanical response of flexible pipe. This physical modelling approach was adopted due to the technical challenges and cost associated with deep-water hyperbaric chambers and connections. A flexible riser and flowline configuration was examined through a unique experimental program. Although the study was rigorous, the paper lacks detail that limits the value and leveraging by third parties.
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Tan et al. [4] presents an analytical solution using total strain energy approach for modeling the buckling response of tensile armour wires. The results from a series of field test known as deep immersion performance tests (DIP), implemented by Wellstream are presented. This study was carried out to test flexible pipe for qualification in depth more than 2000 m. The driver for this experimental test program was the qualification for risers in water depths greater than 2000 m to address future deep-water field development opportunities.
Another local instability mechanism, known as birdcaging or radial buckling, that may occur with flexible pipe is the radial outward deformation mechanism due to a loss of circumferential constraint (i.e. damage to the pressure sheath or tape) subject to hydrostatic pressure and axial load Figure 2-2.
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In a numerical modelling study, Vaz et al. [5] developed finite element modelling procedures to examine the effects of external pressure and interface friction on the potential for bird-caging mechanism to develop. Idealizations were incorporated in the modelling procedures where only two tensile armour wires, represented by spring elements without contact interaction, were used to represent the inward and outward radial deformation modes for the external and internal armour wires, respectively. A parameter study examined the effects of external pressure and interface friction on the potential for birdcaging mechanism to develop. The study highlighted key governing parameters, such as the effect of external and internal pressure, on radial instability. Experimental studies conducted by de Sousa [6] provide the basis for establishing confidence in the numerical modelling procedures developed in his study. Physical tests on a 2.5 m length of 4” flexible pipe subject to axial compression were conducted. Continuum finite element modelling procedures were also developed using ANSYS. The use of physical and numerical modelling techniques was a significant step for improved understanding of the birdcaging (radial buckling) mechanism in flexible pipe. Although, correspondence between the results from physical and numerical investigations was demonstrated, uncertainties still remain on technical details (e.g., lack of reporting on the characterization of initial imperfections, damage state, or contact conditions). Although it is known the presence of initial geometric imperfections or damage state influences the buckling response and propagation of instabilities, details on the amplitude or distribution of initial geometric imperfections in the physical or numerical model were not provided. In addition, the effect of other key parameters, such as external or internal pressure, was not examined.
Another study on the bird-caging mechanism, conducted by Serta et al. [7], compared numerical simulations, using Explicit finite element methods, with physical test results. For the simulation
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of birdcaging or lateral buckling mechanisms in the tensile armour wires of flexible pipe, where many layers may develop contact interaction, the use of Explicit methods is not recommended due to computational issues associated with large contact penetrations.
2.2.3 Motivation and Scope for this Study
In this study, the development of continuum FE modelling procedures simulating the mechanical response of flexible pipe for local instability associated with bird-caging mechanism is presented. The motivation was to develop more robust computational tool, by reducing idealizations used in previous studies, promote confidence in the numerical modelling procedures through verification with available public domain data and to conduct a parameter study examining the key factors influencing potential local instability and failure of flexible pipe associated with radial birdcaging and lateral buckling mechanisms. The processes and requirements needed, challenges encountered and solutions developed to address this objective are discussed. In addition, the technical requirements to optimize these numerical procedures in terms of solution run time and model accuracy are also discussed.