Analysis Of VIV Dynamic Response And Fatigue Damage Reliability For Marine Riser

Marine riser is an important appertain equipment of ocean oil producing system. An investigation emphasizing on dynamic response including VIV of marine riser subjected to waves and ocean currents in 3-D space was presented in this paper in order to be used in the actual engineering.The key is to determine the hydrodynamic coefficients C_d and C_m in Morison equation for the force calculation of marine riser. The two coefficients were calculated by applying BP neural network which had one hidden layer, then used additional momentum method and auto-adjustive learning rate to modify the network. Results indicate that this method is reliable and it can calculate Morison equation's hydrodynamic coefficients according to different Re,KC and roughness number k.Based on the minimum potential energy principle, the marine riser model was developed containing the strain energy due to bending and effective tension, the kinetic energy due to both the riser and the internal fluid motions, the work due to external forces. Then the governing equation of marine riser was derived through functional variation method, and the forms of the equation at inline and transverse direction are similar. And, VIV calculation was based on Matteoluca's wake oscillator model. The dynamic response problem of marine riser is a highly nonlinear problem, including the square term of the velocity, the determination of the hydrodynamic coefficients, VIV, fluid-solid coupling between the riser and fluid and geometrical nonlinearities. All these nonlinear factors were considered in this paper, the governing equation is dispersed by using Hermite interpolation function. An iterative-incremental method based on the updated Lagrangian formulation of beam element, Newton-Raphson and Newmark method was used to solve the nonlinear problems. Finally, the natural frequency characteristics of the marine riser were given in the frequency domain. And in the time domain, the displacements in the direction of in-line and cross-flow were given under different currents, waves, pretensions, movement of the platform etc. The results demonstrate that the dynamic response of marine riser was described well by using these methods.The flow over a marine riser with and without helical strakes was modeled to explore the VIV phenomena through FLUENT software. The simulation model can be specified over a range of Res. The number of the strake, the strake width and the pitch were variable in order to find the better form to suppress VIV. Finally, the instantaneous vorticity and total pressure, drag and lift coefficients were displayed. The results indicate that the response characters of a bare riser can be quite distinct from that of a riser with helical strakes, and their performances were dependent on their geometry. Finally an optimization model was built and solved, thus the optimal form of the helical strake was obtained in this problem, and these results may provide a theoretical basis for the practical engineering application.Many fatigue damage of marine riser were only analyzed to VIV. Acturally, it is also important to in-line vibration especially for the small sea depth. The time history of marine riser's key point's stress can be got through Von Mises method including the effect of multiaxial stress and mean stress. Thus the fatigue life of the marine riser can be solved through rain-flow method, S-N curve and Miner linear cumulative law model and the reliability analysis can also be solved through Wirching method. The results demonstrate that these methods are valid and they can provide basises for the analysis of fatigue damage and reliability.