Effect of Damping on VIV Response in Umbilicals and Flexible Risers

2015 ◽  
Author(s):  
Kay Hansen-Zahl ◽  
Veronica Henøen ◽  
Rolf Baarholm ◽  
Farzan Parsinejad ◽  
Yiannis Constantinides
Keyword(s):  
Fatigue Life ◽  
Internal Pressure ◽  
Contact Force ◽  
Damping Ratio ◽  
Life Assessment ◽  
Stick Slip ◽  
Modal Damping Ratio ◽  
Modal Damping ◽  
Vortex Induced Vibrations ◽  
Flexible Risers

The fatigue life assessment of umbilicals and flexible risers due to vortex-induced vibrations (VIV) require special attention to the structural properties of the flexible bundle due to stick/slip behavior of helical elements in bending. The VIV response on umbilicals and flexible risers is complex, and it is controlled by several parameters. Structural damping is one of these parameters, but this is typically calculated based on the material damping of the structure. Similar to flexible risers and umbilicals, hysteresis damping due to the stick/slip behavior in the helical layers may be significant for cable structures. The purpose of this study is to evaluate the effect of the hysteresis damping on VIV response, and to demonstrate a procedure for consistent VIV fatigue analysis of flexible risers and umbilicals. The stick/slip hysteresis varies with varying contact force, and the contact force is dependent on internal pressure and effective tension. The effect of varying these parameters is included in the study. Furthermore, the study includes different riser system configurations and current profiles. In this paper, the stick/slip hysteresis has been established for two umbilicals and two flexible risers, with varying internal pressure and effective tension. Combined with five different configurations (both shallow and deep water) and 16 different current profiles, the VIV modes have been calculated and the corresponding stick/slip modal damping ratio has been established. Finally, using fatigue current profiles, fatigue life estimates have been made based on different damping ratios, including the estimated stick/slip damping ratio. The study presented in this paper has been carried out as part of the Norwegian Deepwater Programme (NDP).

Nonlinear Span Assessment by Amplitude-Dependent Linearization

2020 ◽  
Vol 142 (5) ◽  
Author(s):  
Ralf Peek
Keyword(s):  
Eigenvalue Problem ◽  
Current Practice ◽  
Nonlinear Eigenvalue Problem ◽  
Damping Ratio ◽  
Mode Shapes ◽  
Inelastic Behavior ◽  
Modal Damping Ratio ◽  
Linear Behavior ◽  
Modal Damping ◽  
Vortex Induced Vibrations

Abstract Although it has long been recognized that vortex-induced vibrations of subsea pipeline spans involve nonlinear and inelastic behavior, the current practice to assess such spans for fatigue and ultimate loading conditions is based on the modal analysis assuming linear behavior. Nevertheless, nonlinearity can be captured approximately by making the linearization amplitude dependent. The eigenvalue problem to be solved for the natural frequencies and mode shapes then involves a stiffness matrix that depends on the mode shape and amplitude of vibration. An important part of the nonlinearity comes from the soil, which is generally represented by springs. This paper presents a simple and particularly effective algorithm to solve this nonlinear eigenvalue problem by using the same algorithm that serves to track the bifurcated solution branches in quasi-static structural stability (buckling) analyses. This method is applied to an example in which the nonlinearity comes from the soil springs. The results demonstrate the importance of the nonlinearity, even at relatively low vortex-induced vibrations (VIV) amplitudes typical of the pure inline response. The inelasticity of the soil springs is also used to calculate the associated contribution to the modal damping ratio.

scholarly journals Dynamics of a Pipeline Span on an Elastic Seabed

2021 ◽  
Author(s):  
Ralf Peek ◽  
Matt Witz ◽  
Knut Vedeld
Keyword(s):  
Axial Force ◽  
Exact Analytical Solution ◽  
Damping Ratio ◽  
Approximate Solutions ◽  
Mode Shapes ◽  
Element Analysis ◽  
Modal Damping Ratio ◽  
Modal Damping ◽  
Vortex Induced Vibrations ◽  
Arch Action

Natural frequencies, mode shapes and modal damping ratios must be estimated to assess subsea pipeline spans for vortex-induced vibrations (VIV) and response to direct wave loading. Several approximate solutions exist for a linearly elastic pipe under constant axial force supported by linearly elastic springs beyond the span’s shoulders. An exact analytical solution has only recently been published. That solution is used here in a Rayleigh-Ritz approximation to account for arch action arising from combined effects of sag under gravity loads and axial restraint at the shoulders. The method allows survey data to be used directly to quantify arch action. Its accuracy is confirmed by finite element analysis. Further, the modal damping ratio is estimated based on the fractions of the potential energy in bending, the axial force, and the soil springs, all of which are determined analytically. Thus, it is found that the effective modal damping ratio increases without a bound as the axial load approaches the buckling load in compression.

Modal damping ratio analysis of dynamical system with non-stationary responses

2016 ◽  
Vol 59 ◽  
pp. 138-146 ◽  
Author(s):  
Da Tang ◽  
Ran Ju ◽  
Qianjin Yue ◽  
Shisheng Wang
Keyword(s):  
Dynamical System ◽  
Damping Ratio ◽  
Ratio Analysis ◽  
Modal Damping Ratio ◽  
Modal Damping

Consistent Free Span VIV Fatigue Analysis of Flexibles

2017 ◽  
Author(s):  
Mário Caruso ◽  
Xu Han ◽  
Nils Sødahl
Keyword(s):  
Fatigue Life ◽  
Cross Section ◽  
Fatigue Analysis ◽  
Life Assessment ◽  
Response Analysis ◽  
Stick Slip ◽  
Analysis Scheme ◽  
Industry Practice ◽  
Current Loading ◽  
Calculation Procedures

The fatigue life assessment of free spanning flexible products, such as subsea cables and umbilicals, due VIV requires special attention to the structural properties due to the stick/slip behaviour of helix elements in bending. Essential parameters for assessment of stick/slip effects in free span VIV response are the structural damping in the stick regime (i.e. umbilical behaves as a solid cross section due to friction between the helix elements) as well as the additional damping introduced by the hysteretic damping due to the stick/slip behaviour in bending. Furthermore, consistent fatigue stress recovery considering the stick/slip behaviour in bending is essential for fatigue life predictions. The consistent evaluation of stick/slip behaviour requires more sophisticated calculation procedures due to the non-linearity it introduces. Hence, industry practice has been to consider simplified, linear calculation procedures. However, future flexibles utilization may be much benefitted by a consistent stick/slip treatment in free spanning VIV fatigue assessments, as it may allow for longer allowable free span lengths or longer fatigue life. The overall objective of the paper is to establish a consistent free span VIV analysis methodology for flexibles in compliance with requirements given in ISO 13628-5 ‘Subsea Umbilicals’ and the overall philosophy of DNV-RP-F105 ‘Free Spanning Pipelines’. A consistent fatigue analysis scheme for VIV in free spans is outlined using commercially available state-of-the-art computer programs for free span VIV response analysis (FatFree) and cross section stress analysis (Helica). The performance of the calculation scheme is demonstrated by case studies in a complex long-term current loading environment. It is shown that consistent treatment of the mechanical properties of flexibles is essential for VIV fatigue life assessments of free spans.

scholarly journals Fatigue Life Assessment of Bourdon Tube against Cyclic Internal Pressure.

1996 ◽  
Vol 62 (599) ◽  
pp. 1535-1542
Author(s):  
Juichi FUKAKURA ◽  
Hiroshige ITOH ◽  
Masanori ISHIMATU ◽  
Kenji OHBA
Keyword(s):  
Fatigue Life ◽  
Internal Pressure ◽  
Life Assessment ◽  
Fatigue Life Assessment

scholarly journals Coupling of Flexural and Longitudinal Damped Vibration in a Two-Layered Beam

1998 ◽  
Vol 5 (5-6) ◽  
pp. 337-341
Author(s):  
F. Pourroy ◽  
S. Shakhesi ◽  
P. Trompette
Keyword(s):  
Damping Ratio ◽  
Modal Damping Ratio ◽  
Modal Damping ◽  
Resonance Frequencies ◽  
Layered Beam ◽  
Flexural Vibrations

In dynamics, the effect of varying the constitutive materials’ thickness of a two-layered beam is investigated. Resonance frequencies and damping variations are determined. It is shown that for specific thicknesses the coupling of longitudinal and flexural vibrations influences the global modal damping ratio significantly.

ODS Analysis on the Rack of Batching System Mixer

2013 ◽  
Vol 437 ◽  
pp. 257-260
Author(s):  
Li Zhang ◽  
Guang Yuan Nie
Keyword(s):  
Working Condition ◽  
Damping Ratio ◽  
Side Surface ◽  
Modal Parameters ◽  
Modal Shape ◽  
Modal Damping Ratio ◽  
Modal Damping ◽  
Before And After ◽  
Structure Vibration ◽  
Batching Machine

By using ODS (Operating Deflection Shapes) technology, the modal parameters of the rack of a batching system mixer under operating condition are identified and the modal shape and modal damping ratio of the rack in a few working frequencies are obtained. The results show that, the batching machine rack on working condition has a significant effect on some frequency and the work principal modes that appear as before and after exercise of two beams above the rack and swaying motion of the brackets of the two side surface. This paper provides a valuable reference for the structure vibration optimization of batching system mixer.

Free Vibration Analysis of Frame-Core Tube Structures Attached with Damped Outriggers

2012 ◽  
Vol 238 ◽  
pp. 648-651
Author(s):  
Zhi Hao Wang
Keyword(s):  
Damping Ratio ◽  
Free Vibration Analysis ◽  
Structural System ◽  
Viscous Dampers ◽  
Core Tube ◽  
Modal Damping Ratio ◽  
Modal Damping ◽  
New Energy ◽  
Energy Dissipation System ◽  
Simplified Calculation

The classical outrigger in frame-core tube structure cantilevering from the core tube or shear wall connected to the perimeter columns directly, which can effectively improve the lateral stiffness of the structure. A new energy-dissipation system for such structural system is studied, where the outrigger and perimeter columns are separate and vertical viscous dampers are equipped between the outrigger and perimeter columns to make full use of the relative big displacement of two components. The effectiveness of proposed system is evaluated by means of the modal damping ratio based on the proposed simplified model. The mathematic models of the structural system are obtained with both the assumed mode shape method and finite element method according to the simplified calculation diagram. Based on the modal damping ratio, the optimal damping coefficients of linear viscous dampers are determined, and effectiveness of proposed system is confirmed.

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