Zhimin Tan

Time Domain Simulation of the 3D Bending Hysteresis Behavior of an Unbonded Flexible Riser

This paper presents a ‘state of art’ in the development of the time domain dynamic simulation of 3D bending hysteresis behaviour of a flexible riser under offshore environment loading. The main technical challenge is to understand and model the riser tensile armour behaviour under continuous changes in both the magnitude and direction of bending, and its subsequent impact on the riser’s bending hysteresis characteristics. Because of this technical obstacle, the current industry practice is to model the riser as a linear structure, with certain conservatism enforced, and then to extract the global dynamic loads to a detailed local model for stress and life assessment. This paper introduces two 3D flexible riser bending hysteresis models, developed by Wellstream and Orcina respectively, and their calibrations against the bending hysteresis loops measured in full scale tests. Both models are implemented using the analysis program OrcaFlex; the Wellstream model is a detailed model that calculates both the total bending moment and the stresses in the tensile armour; the Orcina model is a simpler model that only calculates the total bending moment. A study is presented to illustrate the difference in the riser dynamic responses with and without consideration of the bending hysteresis behaviour, and to assess the difference in the dynamic responses between the Wellstream and Orcina 3D bending hysteresis models. This development permits more realistic riser structural properties to be modeled in the dynamic simulation, and reports detailed time history stress or strain results of the strength components of the riser. This expands the current practice of riser fatigue analysis of only using the regular

Behavior of Tensile Wires in Unbonded Flexible Pipe Under Compression and Design Optimization for Prevention

In ultra deepwaters, helical tensile armour wires under certain loading conditions may exhibit localized deflection in either the radial (out-of-plane) or lateral (in-plane) direction when subjected to significant high axial compression and bending. This phenomenon is often referred to as birdcage buckling. This paper presents the development of a total strain energy approach for modeling the buckling and post-buckling behaviour of these wires and for illustrating the characteristics of such behaviors. The paper presents the summary of the full scale offshore (Deepwater Immersion) DIP tests performed by Wellstream to date, all of which have been successful in resisting the failure modes. In addition, the results of pressure chamber tests are also presented and discussed, especially some of the key test constraints which can have a significant influence on the final result as observed on a recent 10-inch structure chamber test. The presented model provides a tool to define and implement the design criterions against such failure modes. The further validation of the model through comprehensive tests is planned, which is critical to ensure the ability to economically design and optimize flexible pipe structures to prevent this phenomenon, and to facilitate more cost effective products that meet the ultra deepwater design challenges.Copyright

Higher Order Effects on Bending of Helical Armor Wire Inside an Unbonded Flexible Pipe

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.Copyright

Bent Collapse of an Unbonded Rough Bore Flexible Pipe

This paper investigates the collapse behaviour of an unbonded rough bore flexible pipe as it adopts a straight and bent configuration respectively. An unbonded flexible pipe typically consists of several layers, where each layer is designed to provide a specific structural function. The collapse strength of an unbonded rough bore flexible is provided mainly by its Flexbody layer, and enhanced by the Flexlok layer through the Flexbarrier layer in between. A 3D detailed FEA model was developed to simulate the most susceptible collapse situation, where the outermost Flexshield is assumed bleached and the external hydrostatic pressure acts directly on the outer surface of the Flexbarrier layer. This is often referred to as the wet collapse. The model was calibrated against the collapse test data performed on a 6 inch ID pipe in both straight and bent configuration. The equivalent test data shows about 5.0% reduction in the collapse strength under the bent configuration. The paper proposed a practical approach for estimating the collapse pressure for a pipe bent to various bend radii. The work presented is part of an ongoing research and development project.Copyright

The Stress Analysis and Residual Stress Evaluation of Pressure Armour Layers in Flexible Pipes Using 3D Finite Element Models

A pressure armour layer is an essential feature of un-bonded flexible pipes. The layer is made of an inter-locked helically wound metal wire of profiled section, whose primary use is to provide the circumferential strength of the pipe to resist internal pressure. The general design philosophy of the layer is defined in API 17J in terms of the stress “utilisation” factor that specifies the maximum allowable average hoop stress in the layer, which is conventionally produced by the elastic stress analysis. During pressure armour layer manufacturing (a cold forming process), the armour wire is however subjected to a sequence of cyclic bending and twisting deformations which take it beyond its material elastic limit. This paper presents FE structure models for investigating the detailed local and residual stress variation during the forming process, and the subsequent stress relaxation as a result of the factory acceptance test (FAT). A study case is presented for illustrating the typical stress and strain behaviour after FAT pressurization. The paper also introduces X-ray diffraction technology as a method for residual stress measurement on full scale samples.Copyright

Influence of Bore Pressure on the Creep Behaviour of Polymer Barrier Layer Inside an Unbonded Flexible Pipe

This paper investigates the influence of bore pressure, combined with the nonlinear behaviour of the polymer material, on the creep behaviour of the polymer barrier layer inside an unbonded flexible pipe. Creep behaviour in the barrier layer may result in its reduction in thickness and is therefore an important design consideration in ensuring the structural integrity of this layer. It is meaningful to study the variation in creep behaviour in an unbonded flexible pipe under different bore pressures and temperatures, especially in high pressure pipelines for deep or ultra-deep sea applications. Creep behaviour in polymer material is complex, as it is governed by a number of variables such as the stress/strain state, temperature, and pressure for example. It is generally time-dependent and often associated with larger strains or states of deformation. Owing to the complexity of polymer material creep, an implicit time hardening creep model, based on the Maxwell viscoelastic model, has been selected to represent the creep behaviour in polymer materials and implemented into the Gap Span model, which is an in-house ANSYS based finite element model. The coefficients of this creep model were initially calibrated according to standard creep tests performed on polymer materials. The study presented in this paper focuses on the influence of bore pressure and high temperature on the creep behaviour of the polymer barrier layer. Comparisons between the simulation results of the calibrated Gap Span creep model and the corresponding small-scale creep tests demonstrate that these model predictions are overly conservative for the polymer material of the barrier layer inside an unbonded flexible pipe. Comparisons between the experimental test results and the finite element modelling results show good correlation.Copyright

Effective Bending Stiffness of an Unbonded Flexible Riser

A simplified model is proposed for describing the hysteresis behaviour of an unbonded flexible pipe during bending. The model has been developed based on available bending test data and numerical studies. Using the model, a simplified hysteresis loop for a given flexible pipe under specified loading conditions can be evaluated. With the simplified hysteresis loop, the global dynamic analysis and the fatigue life analysis of a flexible riser can be conducted more efficiently without loss of accuracy.Copyright

Time Domain Simulation of the 3D Bending Hysteresis Behaviour of an Unbonded Flexible Riser

This paper presents a ‘state of art’ in the development of the time domain dynamic simulation of 3D bending hysteresis behaviour of a flexible riser under offshore environment loading. The main technical challenge is to understand and model the riser tensile armour behaviour under continuous changes in both the magnitude and direction of bending, and its subsequent impact on the riser’s bending hysteresis characteristics. Because of this technical obstacle, the current industry practice is to model the riser as a linear structure, with certain conservatism enforced, and then to extract the global dynamic loads to a detailed local model for stress and life assessment. This paper introduces two 3D flexible riser bending hysteresis models, developed by Wellstream and Orcina respectively, and their calibrations against the bending hysteresis loops measured in full scale tests. Both models are implemented using the analysis program OrcaFlex; the Wellstream model is a detailed model that calculates both the total bending moment and the stresses in the tensile armour; the Orcina model is a simpler model that only calculates the total bending moment. A study is presented to illustrate the difference in the riser dynamic responses with and without consideration of the bending hysteresis behaviour, and to assess the difference in the dynamic responses between the Wellstream and Orcina 3D bending hysteresis models. This development permits more realistic riser structural properties to be modeled in the dynamic simulation, and reports detailed time history stress or strain results of the strength components of the riser. This expands the current practice of riser fatigue analysis of only using the regular wave approach, to using an irregular wave approach employing the rainflow counting method.Copyright

Analysis of Flow Induced Vibrations Effect on Service Life of Unbonded Flexible Pipe

High frequency vibrations have been reported on flexible pipes for gas export risers installed on platforms offshore in the North Sea. The vibrations are believed to be caused by internal flow vortex shedding as pressured gas passes through the bore at a certain speed. The vibration frequency is typically above 150Hz resulting in approximately 100 billion cycles over the service life. This paper presents analytical models developed to assess the alternating stresses of the strength components induced by the flow-induced carcass vibration, and the consequent influence over their fatigue damage and overall pipe structure integrity. The studied strength layers include the interlocked carcass layer, interlocked hoop strength (pressure armor) and axial tension layers. The models illustrate the behavior of these components inside both the end fitting structure and the free suspended section. The study of a large 15 inch inner diameter gas export riser is presented with test measurements reported in literature.Copyright

Direct Hang-Off Model to Evaluate Fatigue Damage at Riser Hang-Off

Fatigue damage assessment at the flexible riser hang-off location, where the pipe frequently endures maximum tension and curvature variations, is key to verify design integrity for service life.Traditionally, the fatigue analysis is performed in a separate local structure model, as the commercial global analysis software lacks the capability of handling the local behavior of the riser structural component, which is dependent on materials and manufacturing processes. During global fatigue analysis, the riser configuration is built with pinned connected at the hang-off point. The resulting tension and angle responses at the hang-off location, are then input to a local model to perform the stress and fatigue analysis, where the detailed pipe layer structure and bend stiffener are modeled. This traditional approach is conservative, time costly and is often limited to regular wave approach.Wellstream developed an external function to work with specialized commercial riser dynamic analysis software. The external function simulates the detailed behaviour of flexible pipe structure components and the resulting bending hysteresis during dynamic simulation in the time domain. Therefore, the stress time history of the tensile armour becomes available at the end of global simulation in the time domain and is ready for fatigue damage assessment by rain flow counting.This paper presents a study case where the fatigue assessment is performed directly at the hang-off region within the riser global dynamic simulation. The riser hang-off is situated at the top of the I-tube and a bend stiffener is fixed at the bottom of the I-tube. I-tube and pipe section are precisely modeled as pipe-in-pipe facility, where the interaction of riser/I-tube can be captured.Copyright