Coupled Mechanical Analysis And Numerical Simulation Of Deepwater Drilling Riser

At present, deep-water drilling is one of the most advanced drilling technologies at home and abroad. The bad oceanic environment puts forward strict requirements for deep-sea drilling equipments. Marine riser system is the most critical component of offshore drilling operations and the weak link in the whole drilling system at the same time, The stress state of the riser column is more complicated due to deep water conditions, The fatigue and damage problems under the condition of ocean fluid loadings of the riser are becoming increasingly prominent, therefore, research on mechanical analysis of deepwater riser system is a vital topic in the design and use of the riser. A systematical study is described in this thesis by means of theoretical analysis and numerical simulation, which includes the stress and deformation of deepwater drilling riser, strength and fatigue, dynamic response, and the law of the fluid-structure interaction mode and fluid-structure interaction vortex-induced vibration.In the thesis, the research mainly focuses on the following aspects:(1) Deepwater drilling riser static analysisAccording to the characteristics of big deformation with small strain under outside loadings, a static mathematical model has been derived which considers both internal and external forces applied on the riser, and the finite-difference method is used for the numerical analysis of the riser system stress situation. The influences of the top tension, drilling platform drift and the length of the riser on the riser lateral deformation are studied. At the same time the coupled static analysis model of drill string and riser was established. The drill string is simplified as rigid model in this paper to calculate the influence on the riser bending deformation caused by drill string lateral force. A dynamic model of marine riser is established under the drilling operation condition and sea environment considering the interaction of environmental loads, riser and drill pipe, which is more in line with the actual drilling condition. On the basis of static analysis, the static strength and fatigue analysis of deepwater drilling riser are carried out under the wave, current and other environmental loads using ANSYS, the influences of riser top tension, wave height, wave period, current velocity and other factors on the strength of riser are also analyzed, which provides a scientific basis for the design and application of deepwater drilling riser.(2) Deepwater drilling riser dynamic response analysisThe dynamic response of deepwater drilling riser is analyzed by using ANSYS software, including pre-stress mode analysis, harmonic response analysis and transient dynamic analysis under seismic load. The effects of top tension and current velocity on the natural frequency are studied in the modal analysis. In addition, the natural frequency of the riser considering drill string is analyzed and compared with which do not consider drill string. The main resonance frequency of the riser is obtained in the harmonic response analysis, which has high similarity to the natural frequency of modal analysis, it provides the preliminary judgment for the resonance of the riser under wave load. In the analysis of seismic response of the riser, the riser deformation, stress, tension, shear force and bending moment changes with time are mainly evaluated.(3) The fluid-structure interaction mode and fluid-structure interaction vortex-induced vibration analysisThe wet mode according to fluid-structure interaction analysis is calculated, and the ANSYS acoustic fluid unit is used to simulate internal drilling fluid and external sea water of the deepwater drilling riser in the analysis. The natural frequency of the fluid-structure interaction mode and the vibration type of the riser are extracted and the impacts of internal drilling fluid density and viscosity on the natural frequency of the riser are also analyzed. According to the two-way fluid-structure interaction transient analysis for deepwater riser which is carried out based on Fluent and Mechanical, the mutual coupling response of the fluctuation of fluid load and the deformation of structure vibration are calculated, which provides a certain guiding significance for the design of the riser to reduce the vortex induced vibration. On the basis of structural analysis, the riser fatigue analysis is conducted and the fatigue life of the riser is calculated.