Simulating High Mode Vortex Induced Vibration of a Riser in Linearly Sheared Current using an Empirical Time domain Model

2020 ◽  
pp. 1-15
Author(s):  
Sang Woo Kim ◽  
Svein Sævik ◽  
Jie Wu ◽  
Bernt J. Leira
Keyword(s):  
Time Domain ◽  
Deep Sea ◽  
Oil And Gas ◽  
Response Prediction ◽  
Domain Model ◽  
High Mode ◽  
Vortex Induced Vibration ◽  
Use Of Time ◽  
Offshore Oil And Gas ◽  
The Time Domain

Abstract As the development of offshore oil and gas continues to move into deeper waters, the risers connecting the seabed infrastructure to the offshore platform are getting longer. This means that Vortex-induced vibration (VIV) may occur at different frequencies along the riser due to current variation, making the VIV prediction challenging. To increase the VIV prediction accuracy, an empirical time domain VIV model has been developed. The advantage of the model is that it can account for structural non-linearities such as variable tension and time-varying flow. The robustness of the time domain VIV model has been verified with respect to the several tests, but the length/diameter (L/D) ratio of the risers used in the experiments has so far been relatively small, and the response mode number accordingly low. Therefore, additional validation is needed to understand the uncertainties for the prediction of VIV for deep-sea riser systems at high mode order. The purpose of the present study was to evaluate the use of time domain model for high mode VIV response prediction of deep-sea riser systems. This was done by a comparison study applying the Hanøytangen high mode VIV test data. The main comparison was made regarding the dominating frequency and fatigue damage estimation. For selected empirical parameters, the model demonstrated a good correlation with the experiments. However, the simplified riser model which was utilized in the simulations caused some discrepancies between numerical simulations and the experimental results, which needs to be further investigated.

Numerical Simulation on Mooring Performance of LNG-FPSO System in Realistic Seas

2005 ◽  
Author(s):  
Yoshiyuki Inoue ◽  
Md. Kamruzzaman
Keyword(s):  
Time Domain ◽  
Oil And Gas ◽  
Three Dimensional ◽  
Time Domain Analysis ◽  
Analysis Tool ◽  
Motion Response ◽  
Non Linear ◽  
Floating System ◽  
Exciting Forces ◽  
The Time Domain

The LNG-FPSO concept is receiving much attention in recent years, due to its active usage to exploit oil and gas resources. The FPSO offloads LNG to an LNG carrier that is located close to the FPSO, and during this transfer process two large vessels are in close proximity to each other for daylong periods of time. Due to the presence of neighboring vessel, the motion response of both the vessels will be affected significantly. Hydrodynamic interactions related to wave effects may result in unfavorable responses or the risk of collisions in a multi-body floating system. Not only the motion behavior but also the second order drift forces are influenced by the neighboring structures due to interactions of the waves among the structures. A study is made on the time domain analysis to assess the behavior and the operational capability of the FPSO system moored in the sea having an LNG carrier alongside under environmental conditions such as waves, wind and currents. This paper presents an analysis tool to predict the dynamic motion response and non-linear connecting and mooring forces on a parallel-connected LNG-FPSO system due to non-linear exciting forces of wave, wind and current. Simulation for the mooring performance is also investigated. The three-dimensional source-sink technique has been applied to obtain the radiation forces and the transfer function of wave exciting forces on floating multi-bodies. The hydrodynamic interaction effect between the FPSO and the LNG carrier is included to calculate the hydrodynamic forces. For the simulation of a random sea and also for the generation of time depended wind velocity, a fully probabilistic simulation technique has been applied. Wind and current loads are estimated according to OCIMF. The effects of variations in wave, wind and current loads and direction on the slowly varying oscillations of the LNG and FPSO are also investigated in this paper. Finally, some conclusions are drawn based on the numerical results obtained from the present time domain simulations.

Coupled Time-Domain Hydro-Elastic Simulation for Submerged Floating Tunnel Under Wave Excitations

2021 ◽  
Author(s):  
Chungkuk Jin ◽  
Sung-Jae Kim ◽  
MooHyun Kim
Keyword(s):  
Time Domain ◽  
Domain Model ◽  
Mooring Line ◽  
Wave Forces ◽  
Rod Theory ◽  
Discrete Module ◽  
Simulation Based ◽  
Submerged Floating Tunnel ◽  
The Time Domain ◽  
Elastic Simulation

Abstract We develop a fully-coupled time-domain hydro-elasticity model for the Submerged Floating Tunnel (SFT) based on the Discrete-Module-Beam (DMB) method. Frequency-domain simulation based on 3D potential theory results in multibody’s hydrodynamic coefficients and excitation forces for tunnel sections. Subsequently, we build the time-domain model with the multibody Cummins equation and external stiffness matrix from the Euler-Bernoulli and Saint-Venant torsion theories. We establish the mooring line model with rod theory and couple components with translational springs at their respective connection locations. We then compare the dynamic motions, wave forces, and mooring tensions between the present and Morison-equation-based elastic models under regular wave excitations at different submergence depths. The present model is especially important for the shallowly submerged tunnel in which the Morison model shows exaggerated motions, especially at high-frequency range.

On Vessel Motion Induced Vortex-Induced Vibrations of a Steel Lazy Wave Riser

2021 ◽  
Author(s):  
Decao Yin
Keyword(s):  
Vortex Shedding ◽  
Time Domain ◽  
Oscillatory Flow ◽  
Domain Model ◽  
Time Varying ◽  
Structure Interaction ◽  
Response Characteristics ◽  
Vortex Induced Vibrations ◽  
The Time Domain ◽  
Vessel Motion

Abstract Deepwater steel lazy wave risers (SLWR) subject to vessel motion will be exposed to time-varying oscillatory flow, vortices could be generated and the cyclic vortex shedding force causes the structure vibrate, such fluid-structure interaction is called vortex-induced vibrations (VIV). To investigate VIV on a riser with non-linear structures under vessel motion and oscillatory flows, time domain approaches are needed. In this study, a time-domain approach is used to simulate a full-scale SLWR. Two cases with simplified riser top motions are simulated numerically. By using default input parameters to the time domain approach, the key oscillatory flow induced VIV response characteristics such as response frequency, curvature and displacements are examined and discussed. More accurate VIV prediction could be achieved by using realistic hydrodynamic inputs into the time domain model.

Simulating High Mode Vortex Induced Vibration of Riser in Linear Sheared Current Using an Empirical Time Domain Model

2020 ◽  
Author(s):  
Sang Woo Kim ◽  
Svein Sævik ◽  
Jie Wu
Keyword(s):  
Frequency Domain ◽  
Vortex Shedding ◽  
Time Domain ◽  
Structural Model ◽  
Cross Flow ◽  
Domain Model ◽  
Vortex Induced Vibrations ◽  
The One ◽  
The Time Domain ◽  
Empirical Coefficients

Abstract This paper addresses the performance evaluation of an empirical time domain Vortex Induced Vibrations (VIV) model which has been developed for several years at NTNU. Unlike the frequency domain which is the existing VIV analysis method, the time domain model introduces new vortex shedding force terms to the well known Morison equation. The extra load terms are based on the relative velocity, a synchronization model and additional empirical coefficients that describe the hydrodynamic forces due to cross-flow (CF) and In-line (IL) vortex shedding. These hydrodynamic coefficients have been tuned to fit experimental data and by considering the results from the one of existing frequency domain VIV programs, VIVANA, which is widely used for industrial design. The feature of the time domain model is that it enables to include the structural non-linearity, such as variable tension, and time-varying flow. The robustness of the new model’s features has been validated by comparing the test results in previous researches. However, the riser used in experiments has a relatively small length/diameter (L/D) ratio. It implies that there is a need for more validation to make it applicable to real riser design. In this study, the time domain VIV model is applied to perform correlation studies against the Hanøytangen experiment data for the case of linear sheared current at a large L/D ratio. The main comparison has been made with respect to the maximum fatigue damage and dominating frequency for each test condition. The results show the time domain model showed reasonable accuracy with respect to the experimental and VIVANA. The discrepancy with regard to experiment results needs to be further studied with a non-linear structural model.

Time-Domain Dynamic Analysis for Dynamic Positioning System of Deepwater Semi-Submersible

2012 ◽  
Vol 204-208 ◽  
pp. 4518-4522 ◽  
Author(s):  
Li Ping Sun ◽  
Shu Long Cai ◽  
Jing Chen
Keyword(s):  
Dynamic Analysis ◽  
Deep Water ◽  
Time Domain ◽  
Pid Control ◽  
Oil And Gas ◽  
Dynamic Positioning ◽  
Positioning System ◽  
Dynamic Positioning System ◽  
The Time Domain ◽  
Thrust Allocation

Semi-submersible plays an important role in ocean oil and gas exploitation. This paper carried out some researches for the dynamic positioning system (DPS) of a deep water semi- submersible. Mathematic modal was made, and a special program was created with M-language for the time-domain dynamic analysis of the dynamic positioning system of the deep water semi-submersible, on basis of the mathematic modal. PID control strategy, kalman filtering theory and optimal thrust allocation method were used in the analysis. Simulation result indicated the DPS of this platform is safe and efficient.

Investigation on Fluidelastic Instability Accompanied by Wake Shedding With a Time-Domain Model

2019 ◽  
Author(s):  
Kai Guo ◽  
Yipeng Wang ◽  
Tong Su ◽  
Liyan Liu ◽  
Zhanbin Jia ◽  
...  
Keyword(s):  
Vortex Shedding ◽  
Time Domain ◽  
Domain Model ◽  
Elastic Instability ◽  
Flow Induced Vibration ◽  
Fluid Forces ◽  
Double Peaks ◽  
Critical Instability ◽  
Periodic Characteristic ◽  
The Time Domain

Abstract As the most dangerous flow-induced vibration (FIV) mechanism, fluid-elastic instability is always accompanied by the wake shedding. If both of the two FIV mechanisms are considered, fluid forces in this condition can be quite complex. In this paper, a time-domain model based on unsteady flow theory was used to investigate the fluid-elastic instability in a rotated triangular tube array. The vortex shedding forces were simplified as harmonic forces. Computational fluid dynamics (CFD) was used to get the fluid force coefficients with vortex shedding. The model was established by a finite element code with MATLAB software and simulation results agreed with the experiment results. The results showed the critical instability velocity can be influenced by vortex shedding forces, and double peaks can be found in the frequency spectrum of displacements of tubes. The time-domain displacements showed the phases had been impacted by the shedding and periodic characteristic was found in the displacements results. The model can also be adopted in fluid-elastic instability analysis in other tube arrangements and flow conditions.

Effect of the elastic hysteresis term formulation and response to non-harmonic periodic excitations of a non-linear SDOF dynamical model with weak frequency-dependency in the time domain

2021 ◽  
pp. 095440622110182
Author(s):  
Christos Spitas ◽  
Mahmoud S Dwaikat ◽  
Vasileios Spitas
Keyword(s):  
Correction Factor ◽  
Time Domain ◽  
Hysteresis Loops ◽  
Domain Model ◽  
Viscous Damping ◽  
State Variables ◽  
Hysteretic Damping ◽  
Vibration Responses ◽  
The Time Domain ◽  
Damping Model

We elaborate a SDOF time-domain model for elastic hysteretic damping, by modifying the viscous damping model to introduce an instantaneous correction factor that recursively depends on the state variables of the system, such that the response exhibits weak dependency on frequency, corresponding to a large array of engineering materials. The effect of different formulations for calculating the instantaneous correction factor on the predicted hysteresis loops and the potential manifestation of singularities is studied. Hysteresis loops anticipated by the model are plotted and forced vibration responses to harmonic and other periodic non-harmonic excitations are simulated and discussed, also in comparison to the conventional viscous and Reid’s damping models.

An Improved Time Domain Simulation for Dynamic Milling at Small Radial Immersions and Large Depths of Cut

2001 ◽  
Author(s):  
Marc L. Campomanes ◽  
Yusuf Altintas
Keyword(s):  
Surface Finish ◽  
Time Domain ◽  
Dynamic Models ◽  
Three Dimensional ◽  
Domain Model ◽  
Time Domain Simulation ◽  
Chatter Stability ◽  
Linear Effects ◽  
Deep Cuts ◽  
The Time Domain

Abstract This paper presents an improved time domain model for milling, which can simulate vibratory cutting conditions at very small radial widths of cut and large depths of cut. The improved kinematics model allows simulation of very small radial immersions. The varying dynamics modeled along the cutting depth allows milling with very flexible cutters and/or flexible workpieces at very deep cuts to be simulated. The model can predict forces, surface finish, and chatter stability, accurately accounting for non-linear effects that are difficult to model analytically. The discretized cutter and workpiece kinematics and dynamic models are used to represent the exact trochoidal motion of the cutter, and to investigate the effects of forced vibrations and changing radial immersion due to deflection and vibrations on chatter stability. Three dimensional surface finish profiles are predicted and are compared to measured results. Stability lobes generated from the time domain simulation are also shown for various cases.

Simulating Riser VIV in Current and Waves Using an Empirical Time Domain Model

2017 ◽  
Author(s):  
Mats J. Thorsen ◽  
Svein Sævik
Keyword(s):  
Frequency Domain ◽  
Vortex Shedding ◽  
Time Domain ◽  
Theoretical Background ◽  
Domain Model ◽  
Wave Period ◽  
Time Variability ◽  
Analysis Tool ◽  
Load Model ◽  
The Time Domain

The theoretical background of an empirical model for time domain simulation of VIV is reviewed. This model allows the surrounding flow to be time varying, which is in contrast to the traditional frequency domain tools. The hydrodynamic load model consists of Morison’s equation plus an additional term representing the oscillating effect of vortex shedding. The magnitude of the vortex shedding force is given by a dimensionless coefficient, and this force is assumed to act perpendicular to the relative velocity between the cylinder and the fluid. The time variability of the vortex shedding force is described by a synchronization model, which captures how the instantaneous frequency reacts to cylinder motion. The parameters in the time domain load model are calibrated against a commonly used frequency domain VIV analysis tool, VIVANA. To do this, a finite element model of a vertical tensioned riser is established, and the structure is exposed to a linearly sheared flow. Key results such as cross-flow displacements along the riser, frequency content, r.m.s. of bending stresses and mean in-line displacements are compared, and it is shown that the frequency and time domain methods are close to equivalent in this simple case with stationary flow. Finally, the time domain model is utilized to study VIV of a riser subjected to regular waves. The characteristics of wave induced VIV are discussed in light of the simulation results. It is seen that VIV is excited in the zone close to the surface, and the energy is transported downwards as traveling waves. The vibrations typically build up as the horizontal water particle velocity is high, and die out as the velocity decreases. The effect of varying the wave amplitude and period is investigated, and it is found that the dominating frequency, mode and r.m.s. stresses increase together with the wave height. The effect of the wave period is however more complicated. For example, reducing the wave period increases the dominating mode but decreases the displacements. Hence the stress may increase or decrease, depending on which of these effects are strongest.

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