Research On The Key Characteristics Of SPAR Vortex-induced Motions

Along with the further development of ocean engineering technology, more and more Spar platforms have been fabricated and applied in different regions of the world, and they have become efficient facilities in deepwater oil and gas exploitation. The step to deepwater exploitation of our country is on the horizon, and the most advanced techniques and equipments are urgently needed. As a competitive configuration in deepwater and ultra-deepwater region, the Spar platform will become one of the most important facilities in our deepwater exploitation career in the near future.The Spar platform has become an important design type for offshore deepwater operations due to many advantages compared to other type of platforms such as small waterline area, low center of gravity, excellent stability and small responses to waves. These excellent performances make the Spar platform accepted by the world soon after it appears, and now this design type is moving towards a period of flourish. However, due to the deep draft cylinder hull piercing into the water, all types of Spars, including Classic Spar, Truss Spar, Cell Spar, etc, could be subjected to Vortex- Induced Motions (VIM) in certain flow conditions such as strong currents like the hurricane and loop currents in the Gulf of Mexico. Since long time standing VIM can cause riser damage, mooring line fatigue, and even work stoppage, it's a major consideration both in the riser and mooring strength/fatigue design and the safeness assessment of the platform. VIM of the Spar platform, as an important phenomenon which affects the motion performances of a Spar in certain current conditions, has become a new subject in the ocean engineering field.VIM study of Spar platform is a hot frontier topic in worldwide ocean engineering field, and it is also an important subject in the deepwater oil and gas exploitation in South China Sea. This dissertation, which is financially supported by the National High Technology Research and Development Program (863 program) of China, focuses mainly on the key characteristics of Spar VIM performance. The vortex induced mechanism, the substaintial performances and the suppression methods of VIM response, are studied through theoretical analysis, CFD simulation and model test method, and many new explorations are carried out in lock-in phenomenon, vortex shedding characteristics with high Reynold number, nonlinear mooring system, strake design of the multi-surface hull, and so on.In Chapter 1, the development of Spar platforms are presented, and the configuration and hydrodynamic characteristics of the three generations of Spars are summarized. The phenomenon of Vortex-Induced Motions is introduced, and the general development and progress of the investigation on VIM is concluded. The main content and the research method of this thesis are generalized in the end.The phenomenon of VIM is related with several important issues in hydromechanics, such as the velocity and pressure distribution in the fluid field, the flow separation in the boundary layer, the vortex shedding, the lift and drag forces of viscous flow passing objects, and so on. The study of the mechanism and characteristics of VIM should be carried out on the base of hydromechanic theories. Chapter 2 is mainly about the physical property of the fluid, the hydromechanic theories, and the numerical calculation method involved in VIM study. Some classical conclusions on viscous flow around a circular cylinder are presented and discussed. This chapter sets up the theoretical basement of the successive investigations in the following chapters.VIM is a complex phenomenon essentially due to the complex physical mechanism behind the vortex shedding. Therefore, it is necessary to carry out a foundational investigation on the fluid field and vortex shedding characteristics around a low mass ratio cylinder, without the influence of the exterior factors such as the complex vertical profile of the current, the appurtenance on the hull surface and the deepwater mooring system, etc, before the study of VIM of Spars. This could be helpful to reveal the inherent characteristics of the vortex-induced motion mechanism. Chapter 3 studies the wake flow and the vortex-induced motions of a floating circular cylinder under uniform current conditions. The cylinder could be considered as a 1:100 scale model of the hard tank of a typical truss spar. In order to understand the underlying fundamental principles contributing to the complex VIM phenomenon, both experimental and Computational Fluid Dynamics (CFD) methods are adopted to study the wake field feature, the fluid exciting force and VIM performance of the cylinder. The flow field around the circular cylinder, the flow sepration, the vortex-shedding characteristics and the resulting vortex-induced motion performances under different current conditions are investigated. Fixed, forced-motion and elastically moored conditions of the circular cylinder are studied, and many important parameters and conclusions related to the vortex-induced motions of the cylinder are obtained, including the lift and drag exciting forces, the influence of the forced-motion to the fluid field and the forces on the cylinder, the reaction of the moving cylinder to the fluid field, the property of the fluid-structure coupled system, and the vortex induced responses of the cylinder under different reduced velocities, lock-in mechanism, the efficiency of the helical strakes, and so on. The effects of mooring stiffness and distributions, current velocities and directions, and the function of helical strakes are all taken into consideration. Particularly, by using a flow visualization system, the unsteady flow around the cylinder and the vortices in the wake are captured and recorded during the model test procedure, and then compared with the CFD simulation. This study sets up a basis for the successive researches on VIM of real offshore platforms such as spars and semi-submersibles.Chapter 4 mainly concentrates on the VIM performance of Spar platforms. VIM of Spar platforms, which operate in deep sea, is influenced by many environment conditions such as sea state and current profile, the configuration and appurtenance of the hull, the mooring system distribution, and so on. In this chapter, a new concept of Spar platform called Cell-Truss Spar, which has been put forward recently, is studied using both CFD and model test method to research on its VIM performances under different reduced velocities in shear current conditions. The Cell-Truss Spar combines some good qualities of the Cell Spar and Truss Spar designs, aiming to bring in the lighter truss section and heave plate damping feature of the Truss Spar to obtain satisfactory heave motion performances, while reduce manufacture and installation difficulties by means of cell concept. In this study, the unique character of the Cell-Truss Spar is carefully considered in CFD simulations and the model tests, and the inertial parameters are modeled accurately. The actual distribution and the nonlinear stiffness of the mooring system are carefully considered in the study. The simulation method of different current profiles such as hurricane inertial current and the loop current are considered and suggested. Many important parameters in the vortex-induced motions of the Cell-Truss Spar are obtained, and the characteristics of vortex-induced-motion responses of the Cell-Truss Spar are analyzed and concluded. This is very important for the successive researches on VIM suppression of the Cell-Truss Spar. The suppression of vortex-induced motions is an important aspect of VIM study. To mitigate VIM, helical strakes are used on Spar hulls, and they have been proved to be effective and utility. For the Cell-Truss Spar, which is still on concept design stage at the present time, the design and optimization of the helical strakes is very important to control the VIM response and improve the hydrodynamic performance. In Chapter 5, the strake of the Cell-Truss Spar is designed and optimized, and the suppression of VIM is studied. As a result of the unique characters on the hull, the outer surface of the Cell-Truss Spar does not form a regular cylinder, and the strakes used on the existing Spars might not be suitable for the Cell-Truss Spar. In addition, it is more difficult to fabricate the conventional Spar strake members to fit on the multi-surfaced exterior of the Cell-Truss Spar. For the sake of suppression study on the VIM of the Cell-Truss Spar, four different strake configurations are put forward to be applied on the Cell-Truss Spar. The fluid field around the hull, the vortex disturbance near the strakes, and the forces acting on the Spar hull with different strakes are simulated by CFD method, and the strake efficiency is assessed through model test combining with CFD computation. The optimized strake configuration is finally chosen, and the VIM performance of the strake-equipped Spar is studied.In real ocean environment, the platforms are usually affected not only by current, but also wind and waves. The vortex-induced motions due to the current are always come forth together with other hydrodynamic responses, and they might be influenced by each other. The interaction of current and wave is especially important to VIM. The circular motion of the fluid particles in the wave makes the vortex shedding appear back and forth. This is a more complex problem. Field measurements of the classic Neptune Spar after the passage of hurricane Georges in 2001 suggest that VIM response in"hurricane inertial current"conditions in the presence of waves need to be considered in addition to VIM response in uniform loop/eddy current conditions. Hence, a comprehensive test program was undertaken to study the effect of waves on Spar VIM response in Chapter 6. The interactions of the current with regular and irregular waves are studied respectively. For the regular wave effect, three regular waves with different wave height and period are tested together with current, and the effect of different wave parameters to VIM is studied. While for the irregular wave effect, two classical sea state of current combining with irregular wave in Gulf of Mexico and South China Sea is considered to study the VIM responses. The mooring line forces are also measured and analyzed. The study of Spar VIM in wave and current is very important to understand VIM ulteriorly.Finally, in Chapter 7, the main content of the thesis and the corresponding results are concluded, and the developing trend for further studies in this field is put forward. Some suggestions for the related engineering practices are proposed in the end. To sum up, this thesis launches on several factors which could affect the Spar VIM response characteristics, including the current conditions, the Reynolds number, the hull configuration, the mooring system, the strake design, and so on. Numerical simulations and model tests are carried out to study the prediction and suppression of VIM. The vortex shedding exciting mechanism is studied, many important parameters of VIM are obtained, the mechanism and rules of lock-in phenomenon are analyzed, and the adoption of helical strakes are considered. The inherent characteristics of VIM are revealed, and several important conclusions are drawn. These will be very useful to the deepwater oil and gas exploitation in the future.