131,580 research outputs found
A new continuum based non-linear finite element formulation for modeling of dynamic response of deep water riser behavior
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The principal objective of this investigation is to develop a nonlinear continuum based finite element formulation to examine dynamic response of flexible riser structures with large displacement and large rotation. Updated Lagrangian incremental approach together with the 2nd Piola-Kirchhoff stress tensor and the Green-Lagrange strain tensor is employed to derive the nonlinear finite element formulation. The 2nd Piola-Kirchhoff stress and the Green-Lagrange strain tensors are energy conjugates. These two Lagrangian tensors are not affected by rigid body rotations. Thus, they are used to describe the equilibrium equation of the body independent of rigid rotations. While the current configuration in Updated Lagrangian incremental approach is unknown, the resulting equation becomes strongly nonlinear and has to be modified to a linearized form. The main contribution of this work is to obtain a modified linearization method during development of incremental Updated Lagrangian formulation for large displacement and large rotation analysis of riser structures. For this purpose, the Green-Lagrange strain and the 2nd Piola-Kirchhoff stress tensors are decomposed into two second-order six termed functions of through-thethickness parameters. This decomposition makes it possible to explicitly account for the nonlinearities in the direction along the riser thickness, as well. It is noted that using this linearization scheme avoids inaccuracies normally associated with other linearization
schemes. The effects of buoyancy force, riser-seabed interaction as well as steady-state current loading are considered in the finite element solution for riser structure response. An efficient riser problem fluid-solid interaction Algorithm is also developed to maintain the quality of the mesh in the vicinity of the riser surface during riser and fluid mesh movements. To avoid distortions in the fluid mesh two different approaches are proposed to modify fluid mesh movement governing elasticity equation matrices values; 1) taking the element volume into account 2) taking both element volume and distance between riser centre and element centre into account.
The formulation has been implemented in a nonlinear finite element code and the results
are compared with those obtained from other schemes reported in the literature
Vortex-induced vibration of catenary riser: reduced-order modeling and lock-in analysis using wake oscillator
No full textA new reduced-order model capable of analyzing the vortex-induced vibration of catenary riser in the ocean current has been developed. This semi analytical-numerical approach is versatile and allows for a significant reduction in computational effort for the analysis of fluid-riser interactions. The incoming current flow is assumed to be steady, uniform, unidirectional and perpendicular to the riser plane of initial equilibrium curvatures
Characteristics and forces due to slugs in an 'S' shaped riser
No full textThe characteristics and forces due to slugs in an 'S' shaped riser have been investigated.
A series of experiments were carried out using the Cranfield University Riser Test
Facility, using a 9.9m high riser. Single phase (water) and two phase (air and water)
tests were conducted at a system pressure of 2 bara. The two phase tests covered a
range of flow regimes: severe slugging, transitional severe slugging, oscillation and
normal slug flow.
The two phase data was used to investigate the characteristics of severe slugging, to
determine the liquid inventory in the riser downward limb and, to model the forces on a
bend during the slug build-up, production and bubble penetration and gas blowdown
stages of the severe slugging cycle.
During the bubble penetration and gas blowdown stage, high velocities and large
fluctuations in the force were observed. The resultant dynamic forces on the bend
during the slug build-up and production stages were small compared to the bubble
penetration and gas blowdown stage, and dominated by the hydrostatic forces due to
liquid in the riser. Normal slug flow is potentially more problematic in terms of fatigue
damage than severe slug flow, due to the higher velocity of the slugs coupled with large
dynamic forces
Reduced-order modelling of vortex-induced vibration of catenary riser
No full textA new reduced-order model capable of analyzing the vortex-induced vibration of catenary riser in the ocean current has been developed. This semi analytical-numerical approach is versatile and allows for a significant reduction in computational effort for the analysis of fluid-riser interactions. The incoming current flow is assumed to be steady, uniform, unidirectional and perpendicular to the riser plane of initial equilibrium curvatures. The equations of riser 3-D motion are based on a pinned-pinned, tensioned-beam or flexural cable, modelling which accounts for overall effects of riser bending, extensibility, sag, inclination and structural nonlinearities. The unsteady hydrodynamic forces associated with cross-flow and in-line vibrations are modelled as distributed van der Pol wake oscillators. This hydrodynamic model has been modified in order to capture the effect of varying initial curvatures of the inclined flexible cylinder and to describe the space-time fluctuation of lift and drag forces. Depending on the vortex-excited in-plane/out-of-plane modes and system fluid-structure parameters, the parametric studies are carried out to determine the maximum response amplitudes of catenary risers, along with the occurrence of uni-modal lock-in phenomenon. The obtained results highlight the effect of initial curvatures and geometric nonlinearities on the nonlinear dynamics of riser undergoing vortex-induced vibration
Preliminary instability-analysis of deepwater riser with fairings
No full textInstability of deepwater riser with fairings is investigated in this study. Despite the advantages over other devices for suppressing vortex-induced-vibration (VIV), fairings may be susceptible to flutter type instability. A two-body mathematical model is established for the coupled transverse-torsion motion of a top tensioned riser with fairings. The inner part (riser) can only move transversely while the outer part (fairing) has transverse-torsion motion. The effect of the transverse velocity on the angle of attack is taken into account and damping is considered for both degrees of freedom. An eigenvalue analysis is employed to examine the issue of stability. The emphasis is on identifying the critical current speed for a given riser and fairing configuration. The effects of key parameters are investigated and the results indicate that the section hydrodynamic characteristics of the fairings have a significant impact on the instability
Development of a constitutive model to simulate unbonded flexible riser pipe elements
No full textThis thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The principal objective of this investigation is to develop a constitutive model to simulate the hysteresis behaviour of unbonded flexible risers. A new constitutive model for flexible risers is proposed and a procedure for the identification of the related input parameters is developed using a multi-scale approach. The constitutive model is formulated in the framework of an Euler-Bernoulli beam model, with the addition of
suitable pressure terms to the generalised stresses to account for the internal and external pressures, and therefore can be efficiently used for large-scale analyses. The developed non-linear relationship between generalised stresses and strains in the beam is based on the analogy between frictional slipping between different layers of a flexible riser and frictional slipping between micro-planes of a continuum medium in nonassociative elasto-plasticity. Hence, a linear elastic relationship is used for the initial response in which no-slip occurs; an onset-slip function is introduced to define the ‘noslip’ domain, i.e. the set of generalised stresses for which no slip occurs; a nonassociative rule with linear kinematic hardening is used to model the full-slip phase.
The results of several numerical simulations for a riser of small-length, obtained with a very detailed (small-scale) non-linear finite-element model, are used to identify the parameters of the constitutive law, bridging in this way the small scale of the detailed finite-element simulations with the large scale of the beam model. The effectiveness of the proposed method is validated by the satisfactory agreement between the results of various detailed finite-element simulations for a short riser, subject to internal and
external uniform pressures and cyclic bending and tensile loadings, with those given by the proposed constitutive law. The merit of the present constitutive law lies in the capturing of many important aspects of risers structural response, including the energy dissipation due to frictional slip between layers and the hysteretic response. This privilege allows one to accurately study the cyclic behavior of unbonded flexible risers subject to axial tension, bending moment, internal and external pressures
Experimental study on severe slugging mitigation by applying wavy pipes
No full textWavy pipes were installed in the pipeline for mitigating severe slugging in pipeline/riser systems. Experimental results have revealed that: with a wavy pipe applied, the operating region of severe slugging is reduced; the severity of severe slugging and oscillation flow is mitigated; the wavy pipe performs better with its outlet located upstream of the riser base. The wavy pipe is essentially reducing the slug length. For severe slugging the wavy pipe works by accelerating the movement of the gas in the pipeline to the riser base; for the oscillation flow it works by mixing the gas/liquid two phases
Analysis of process variables via CFD to evaluate the performance of a FCC riser
No full textFeedstock conversion and yield products are studied through a 3D model simulating the main reactor of the fluid catalytic cracking (FCC) process. Computational fluid dynamic (CFD) is used with Eulerian-Eulerian approach to predict the fluid catalytic cracking behavior. The model considers 12 lumps with catalyst deactivation by coke and poisoning by alkaline nitrides and polycyclic aromatic adsorption to estimate the kinetic behavior which, starting from a given feedstock, produces several cracking products. Different feedstock compositions are considered. The model is compared with sampling data at industrial operation conditions. The simulation model is able to represent accurately the products behavior for the different operating conditions considered. All the conditions considered were solved using a solver ANSYS CFX 14.0. The different operation process variables and hydrodynamic effects of the industrial riser of a fluid catalytic cracking (FCC) are evaluated. Predictions from the model are shown and comparison with experimental conversion and yields products are presented; recommendations are drawn to establish the conditions to obtain higher product yields in the industrial process.</p
Numerical and experimental comparisons of vortex-induced vibrations of marine risers in uniform/sheared currents
No full textThis paper presents a general theoretical reduced-order model capable of evaluating the multi-mode nonlinear dynamics of marine risers subject to uniform and sheared currents. The main objectives are to predict the vortex-induced vibration responses and parametrically compare between numerical and experimental results. The emphasis is placed on the analysis of cross-flow vibrations due to unsteady lift forces. The nonlinear equations governing riser axial/transversal motions are derived based on a top-tensioned beam model with typical pinned-pinned boundary conditions. The riser geometric nonlinearities owing to possible large dynamic displacements and multi-mode interactions are accounted for. To approximate the space-time varying lift force, the empirical hydrodynamic model, based on a nonlinear van der Pol wake oscillator with a distributed diffusive term, is used. A low-dimensional dynamic model and computationally-robust time-domain tool are then developed to evaluate the multi-mode fluid-riser interactions. These are very useful in dealing with large parametric studies involving varying system parameters
Severe slugging and unstable flows in an S-shaped riser
No full textSevere slugging and unstable flow in an S-Shaped riser has been investigated in three
research themes - experiments, criterion modelling and transient code modelling.
A series of experiments were carried out on the Cranfield University Three Phase
Facility and Riser Rig using a 10 m high riser over the pressure range 2,4 and 7
bar(a). The collected information was used to characterise the unstable flows in terms
of pressure cycling, riser liquid inventory and fluid production characteristics. From
analysis of the flow behaviour, it was found that in terms of slug characteristics,
transitional severe slugging and oscillation flows are as potentially problematic as
`classical' severe slugging. This is due to the magnitude of peak flow in excess of the
average fluid throughput in the riser and the size of the liquid slugs generated.
A criterion for the occurrence of unstable flows in an S-Shaped riser was developed
based upon considerations of bubble penetration at the riser base. This proved to be
successful at predicting the experimental results from this work and an independent
source.
Comparisons were made between the experimental results and a transient code. The
code could predict the occurrence of `classical' severe slugging however the detailed
characteristics of the experiments were not predicted by the code. The results for
transitional severe slugging and oscillation flow showed further differences between
the code prediction and the experiments. Drawing all simulation results together it
was concluded that the prediction of pipeline behaviour and the propagation of flow
regimes local to the riser base and in the curved riser pipe were significant sources of
error in the simulation
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