Research On Vortex Induced Vibration Of Free Spanning Pipeline Under Pipe-Soil Interaction Condition

Free spans often become a challenge in the pipeline design and operation due to pipeline installation on uneven seabed or seabed scouring effects. Amplified responses due to resonance between the vortex shedding frequency and natural frequency of the free spans may cause fatigue damage. And a whole free span VIV system should include three parts:pipe, seabed and current. Hence, improving the knowledge about the effect of pipe-seabed interaction on VIV is quite important for free spanning pipelines, which are often highly nonlinear due to interaction with the seabed. A reasonable VIV analysis model based pipe-seabed interaction is helpful for VIV fatigue estimation, then provide the support for safe operation and cost reduction of submarine pipeline designIn the presented VIV response analysis of free span under pipe-soil interaction boundary, three prediction models were adopted respectively, including CFD model based on loose coupling algorithms, semi-empirical model and wake oscillator model. And a fatigue analysis method is established for multi-spans pipeline VIV.The main contents and contributions of this thesis may be summarized as follows:(1) Dynamic mesh technology wase adopted to achieve the simulation of moving wall boundary. And computational fluid dynamics (CFD) techniques, which analytically solve the viscous Navier-Stokes equations in order to obtain the hydrodynamic forces directly, were developed based on loose coupling algorithms in time domain. The codes can simulate successfully the VIV response of rigid and flexible rigid in 2-dimension and 3-dimension flow fluid domain. In the study, a research was implemented to analysis the reliability, validity and uncertainty of 2D VIV coupling code. the turbulence was presented by SST model, and fourth order Runge-Kutta algorithm used for cylinder movement resolution. The results show that time step is one of uncertain factors during CFD simulation, and amplified responses will come forth due to free in-line degree of freedom or high Reynolds number.(2) The vortex induced vibration response of a rigid cylinder supported by perfectly plastic nonlinear spring in a fluid flow was simulated for the first time. Wake-oscillator prediction model and 2D CFD method based on loose coupling algorithms were adopted. The results of above two methods all showed that, vibration amplitude jumps to a maximum value when cylinder movement exceeds the nonlinear spring limited displacement, then decreases with velocity increasing. The response peak value for nonlinear spring case is higher than linear spring, but at the lower flow velocity. As the same time, the simulation of CFD presented that vibration frequency for synchronization is close to vortex shedding frequency at the beginning, and then switch to above the natural frequency in air when the amplitude reaches a peak value, while the phase angle of displacement and lift force transfer from 0 to 180 degree.(3) An empirical hydrodynamic coefficient models based on the data from 2D rigid cylinder vibration experiments were used to predict the vortex-induced vibrations response of free spans under pipe-soil interaction boundary. The response frequencies were resolved from a eigenvalue problems of free spans with some certain added mass. And response amplitude could be received from a iteration process, which based on energy balance. After response frequency and hydrodynamic coefficients, were determined by use of above linear response model. This result is then used in a time domain model that can handle non-linear boundary conditions at the span shoulders. The results demonstrate that non-linear effects can be significant, in particular for pipe stress in the span shoulder. The significance of the new method is that displacements, and hence also stresses, in the pipe at the shoulders will be far better described by the non-linear method than what is possible from linear theory.(4) And a time domain analysis method for non-linear pipe-seabed-interaction analysis of free spanning pipelines under VIV conditions has been developed. The flow is modeled by a van-der-Pol wake oscillator developed by Facchinetti et al. And the stall parameter is taken into account in the general form of the VIV amplitude and the response frequency, which can successful model the VIV response amplitude comparing with a constant. After the free spanning pipeline discrtized into the finite elements, the average acceleration method is chosen to integrate the equations of motion. Three types of pipe-seabed-interaction (linear spring, perfectly plastic nonlinear spring, and tension cut-off nonlinear spring) are considered. The results show that the presented time domain method can capture the non-linear interaction between the pipeline and the seabed at the free span shoulders. And the maximum response amplitude of nonlinear case is lower than the one with linear spring. Hence, the response will be underestimated at low current velocity, or overestimated at high current velocity.(5) When the eigen-frequencies are close, several potential vibration modes of multi-spans can become active at a given low flow velocity, i.e. multi-mode vortex induced vibration. Considered for statistical distributions of current speed and VIV response of free spans in long-term operating condition, in the study, a fatigue assessment method for multi-spans was developed. It can provide a reference on the initial design of free spanning pipeline. At last, the fatigue damage due to multi-mode vortex induced vibration was investigated. It is to assume that the stress ranges are narrow-banded and Rayleigh-distributed, and a 3-parameter Weibull distribution is appropriate for modeling of the long-term statistics for the current velocity. The results based on the Response Model in DNV rules were shown that soil stiffness is sensitive. For reduction coefficient of multi-mode response, it is more conservative to use the DNV RP-105 than the weight of response amplitude. If the structure vibrates in a purely sinusoidal fashion with constant amplitude, then the calculation based on the Rayleigh distribution overestimates the damage rate.In conclusion, from both the numerical and the experimental empirical models, the research in this thesis sets out from the VIV of a two dimensional circular cylinder supported by linear or nonlinear spring, ends up with the prediction of VIV of slender pipelines under more complicated pipe-soil interaction boundary. Many experience and findings will be helpful for the future work. It can provide a reference on the initial design of free spanning pipeline.