743 research outputs found
New model for vortex-induced vibration of catenary riser
This paper presents a new theoretical model capable of predicting the vortex-induced vibration response of a steel catenary riser subject to a steady uniform current. The equations governing riser in-plane/out-ofplane (cross-flow/in-line) motion are based on a pinned beam-cable model accounting for overall effects of bending, extensibility, sag, inclination and structural nonlinearities. The empirically hydrodynamic model is based on nonlinear wake oscillators describing the fluctuating lift/drag forces. Depending on the potentially vortex-induced modes and system parameters, a reduced-order fluid-structure interaction model is derived which entails a significantly reduced computational time effort. Parametric results reveal maximum response amplitudes of risers, along with the occurrence of uni-modal lock-in phenomenon
Optimal control of the heave motion of marine cable subsea-unit systems
One of the key problems associated with subsea operations involving tethered subsea units is the motions of support vessels on the ocean surface which can be transmitted to the subsea unit through the cable and increase the tension. In this paper, a theoretical approach for heave compensation is developed. After proper modelling of each element of the system, which includes the cable/subsea-unit, the onboard winch, control theory is applied to design an optimal control law. Numerical simulations are carried out, and it is found that the proposed active control scheme appears to be a promising solution to the problem of heave compensation
Reduced-order modelling of vortex-induced vibration of catenary riser
A 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
Vibration attenuation control of ocean marine risers with axial-transverse couplings
The target of this paper is designing a boundary controller for vibration suppression of marine risers with coupling mechanisms under environmental loads. Based on energy approach and the equations of axial and transverse motions of the risers are derived. The Lyapunov direct method is employed to formulated the control placed at the riser top-end. Proof of existence and uniqueness of the solutions of the closed-loop system is provided. Stability analysis of the closed-loop system is also included
Boundary control of vibration in coupled nonlinear three dimensional marine risers
This paper presents a design of boundary controllers implemented at the top end for global stabilization of a marine riser in three dimensional space under environmental loadings. Based on the energy approach, nonlinear partial differential equations of motion including bending-bending and longitudinal-bending couplings for the risers are derived. These couplings cause mutual effects between the three independent directions in the riser’s motions and make it difficult to minimize its vibrations. The Lyapunov direct method is employed to design the boundary controller. It is shown that the proposed boundary controllers can effectively reduce the riser’s vibration. Stability analysis of the closed-loop system is performed using the Lyapunov direct method. Numerical simulations illustrate the results
Multi-modes approach to modelling of vortex-induced vibration
Acknowledgements A.P. would like to acknowledge the support of the National Subsea Research Institute (NSRI) UK. E.P. and M.W. are grateful for partial support provided by the Italian Ministry of Education, University and Research (MIUR) by the PRIN funded program 2010/11 N.2010MBJK5BPeer reviewedPostprin
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A new continuum based non-linear finite element formulation for modeling of dynamic response of deep water riser behavior
This 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
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Development of a constitutive model to simulate unbonded flexible riser pipe elements
This 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
Improvisation of deepwater weight distributed steel catenary riser
Master's thesis in Offshore technology : subsea technologyNowadays, oil and gas sources are found in deeper water depths and in more hostile
environments. This results in the need for more advance technologies. Riser system is
a key element in providing safety. Riser failure results in spillage or pollution and
could endanger lives. Hence, it is important to establish a high degree of reliability for
riser design.
Steel catenary risers (SCRs) have been a preferred riser solution for deep-water field
developments due to its simple engineering concept, cost effective, flexibility in using
different host platform and flexibility in geographical and environmental conditions.
Flexible riser, on the other hand, is limited by technical and economical reasons when
it comes to deep water field. Larger diameter is required in deep water to increase
collapse resistance due to high hydrostatic pressure. Consequently, increase in cost
and limit the option of host platform. Alternatively, Hybrid riser is a robust design for
deepwater and harsh environments. It is insensitive to motion induced fatigue.
However, hybrid riser is considered to be an expensive solution because it comprises
a number of complex components (buoyancy can, riser bundle, flex joint, etc).
A number of SCRs have been installed worldwide over the past years and more to
come in the future oil and gas explorations. However, there is no SCR that has been
installed in deepwater with harsh environments to date. It is mainly because SCRs in
harsh environments experience a great challenge due to large motions from host
platform such as semi-submersibles and FPSOs. Therefore, significant design effort is
required to prove that the SCRs could safely withstand environmental loads in harsh
environments and the effects of deep water.
The study investigates the feasibility of 10 inch production SCR for Offshore Norway
in a 1000m water depth with SCR attached to a semi-submersible vessel.
Conventional SCR was analyzed and found difficulty in meeting strength design
criteria at the touch down point (TDP) and at the riser hang off location. From
previous industry work, the weight variation along the riser length has demonstrated a
remarkable improvement to SCR response, particularly at TDP.
This study concentrates on fundamental aspects related to improvement from
conventional SCR to weight distributed SCR. A number of insightful sensitivity
analyses were performed in order to understand the correlation between the peak
response and some fundamental parameters such as displacement, velocity and
acceleration. Feasibility enhancement of present weight distributed SCR concept was
also studied to provide more applicable SCR configuration solution. The study
addresses global design considerations including analysis of strength and fatigue.
Deepwater SCR Installation scheme was also discussed.
The study concludes that there is significant improvement in SCR response from
conventional SCR to weight distributed SCR concept. It also proves that even though
the design of SCR in harsh environments and deep water is technically challenging,
innovative solutions can be developed
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