Research On The Analysis Method Of Vortex Induced Motion Of Spar Platform Considering Radiation Damping

With the offshore oil development grandually shift from offshore to the deep, platforms designed for deep water are widely studied. Spar platform is a new form of platform used for deep water development of oil at the end of the 20th century which is stable, convenient, and economy, and so it has a broad application prospect. Spar platforms will oscillate under certain velocity because of the alternate pressure generated by the alternate wake vortex which is called vortex induced motion(VIM). This phenomenon is bad for the fatigue life of risers and mooring system, and so it is significant to do research on vortex induced motion.Currently many studies of vortex induced motion of Spar platform are based on scaled model test in which Froude number similarity is used, and the dynamic characteristics of the prototype are deduced from the scaled model test which is based on this similarity. It is defective using the Froude number similarity in the scaled model test, because the vortex induced motion is closely related to Reynolds number and Reduced velocity not gravity. If the Reynolds number of the scaled model is different from that of the prototype, the dynamic characteristics of scale model may not be the same as that of the prototype. Especially to the scaled model test of Spar platform, if Froude number similarity is used, because of its small scale, the Reynolds number will become very different from that of the prototype in magnitude which means that the scaled model test may not be so reliable. Therefore, in Chapter 3, the application of similarity criterion in scaled model test of vortex induced vibration(vortex induced motion) is studied, and three scaled models, the both Froude number and Reynolds number similarity model, the Reynolds number similarity model and the Froude number similarity model have been studied. It can be concluded from the model test and numerical simulation that the Reynolds number similarity should be considered first in the scaled model test, and the dynamic characteristics between the scaled model and prototype are not the same when only Froude number similarity is used.Considering the existing deficiency of scaled model test of vortex induced motion of Spar, in Chapter 4, a program written by MATLAB is used to analyze vortex induced motion of Spar platform. In most Computational Fluid Dynamics softwares, the reactive force deduced by velocity and acceleration of the structure itself may not be considered when vortex induced vibration is simulated, and however, this factor is considerd in the program. Moreover, the damping coefficient in most research is an integration of structural damping and radiation damping, but in this chapter, the structural damping and radiation damping are calculated separately. Dynamic model of vortex induced motion of Spar platform considering radiation damping is established, and the coupling effect of surge, sway, roll and pitch is analyzed. In order to validate the program, model test is done in Chapter 5. It is shown that the result calculated by the program corresponds to the result of the model test.In Chapter 6, vortex induced motion of Spar platform under different velocity is simulated on real scale based on ANSYS-CFX. By comparing the result under different velocity, it can be concluded that although the cross section is different along its length, the vortex shedding is accordant which shows the obvious three-dimensional effect. By comparing the result of the numerical simulation to the result obtained from the program calculation, it is shown that the amplitude of surge get from program calculation is smaller than that get from numerical simulation which is due to the consideration of radiation damping in program calculation, but the average response of surge is almost the same. Moreover, the amplitude of sway get from program calculation is accordant with that get from numerical simulation outside the lock-in area. Because sway is the dominant motion of Spar platform when vortex induced motion is occurred, so the program written for VIM is also validated by the result of the numerical simulation.Mechanism research of helical strakes on suppression of vortex shedding is done in Chapter 7. The bare cylinder and cylinder with helical strakes are tested based on CFD method. By comparing the vortex shedding, drag coefficients and lift coefficients of the two models, it is shown that the lift coefficient becomes small but the average drag coefficient bcemces greater when helical strakes are installed. Moreover, the period of vortex shedding maybe influnced by the helical strakes. By comparing the drag coefficients and lift coefficients of the cylinder with helical strakes in different parts, it is shown that the period of vortex shedding is accordant along the length, and the lift coefficients in different parts are all decreased. So it can be concluded that the total lift coefficient is decreased due to its decreasing in each part not due to the adding of the lift coefficient in each part, and this conclusion is not contrary to the common research.To sum up, in this thesis the vortex induced motion of Spar platform is studied based on program calculation, numerical simulation and model test. A series of results are presented, which are useful for application of actual engineering. However, the research of vortex induced motion of Spar platform is a relatively new subject in the world. There are many problems left need to be researched.