Study On Method For Nonlinear Analysis Of Deepwater SCR Based On Inertial Coupling

When offshore oil and gas activities move to deeper and deeper oceans, various innovative floating structures are proposed and developed to comply with severe deep-water environments and to reduce the cost. Steel catenary risers (SCRs) for deepwater development have become a viable option for oil and gas export from floating production facilities to shore, shallow water platforms, or to subsea pipeline hubs. SCR is considered over both traditional flexible riser and TTRs in deepwater development and a cost effective solution to deepwater oil and gas production. The concept of SCR is inherently simple and is often thought of an extension of the flow line or pipeline. However, the dynamic movements experienced by the SCR from vessel motions and hydrodynamic loading result in a more complex behavior of the structure compared with flow lines and pipelines. So, dynamic response analysis and nonlinear investigation of SCR are of both great scientific significance and extensive application value.The cable dynamic analysis program CABLE3D, which is 3-D nonlinear analysis program developed by Chen (2002) is introduced in this paper. Cable static and dynamic problems of the two structures including BAR (with bending, large elongation) and BEAM (with bending, small elongation) elements can be solved using this program. Based on the theory of a slender rod with bending stiffness, SCR is modeled as a small extensible beam considering large curve at Touch Down Point (TDP) of SCR. The situation of TDP changes along with motion of SCR. Moreover, the length of element at TDZ is not limited. The slender beam model with large curve used in SCR analysis have an important role on the study of pipe/soil interaction and fatigue analysis of SCR.The thesis, with its marked innovation of this subject, gives the integrated model of SCR-Spar and its mooring lines/TTRs considering inertial coupling, and the original CABLE3D is improved. Based on previous works by Garrett (1982), Ma, Webster (1994) and Chen (2002), a dynamic equation for integrated model of SCR-Spar is derived and the integrated analysis program V-CABLE3D is obtained. In the program, Spar moored by a spread mooring system is modeled as a rigid body, and mooring lines/TTRs are modeled by springs which act on the top of SCR. Both response of Spar induced by wave loads and SCR/soil interaction model (a spring-damping system) are considered in code. In the integrated analysis, the Spar can be approximated by a rigid body of three degrees of freedom (surge, heave and pitch), known as a 3-DOF body. First-order is only considered in the calculation of wave loads by diffraction method. The wave forces are calculated using approximation of diffraction method. The results of dynamic analysis of SCR by CABLE3D and V-CABLE3D show that large inertial force induced by mass of Spar has a large effect on dynamic response and it is not ignored. The method of integrated analysis considering inertial coupling factually reflects dynamic effect between SCR and Spar platform.The state of the art of vertical pipe/soil interaction models developed for use in SCR analysis is also described in detail. These model pipe move vertically downwards (soil stiffness) and vertically upwards (soil suction). The soil suction curve presented in this paper was the upper bound curve based on the previous STRIDE 2D pipe/soil interaction work by Willis (2001). The case studies investigated the effect of soil suction on SCR dynamic response. CABLE3D and V-CABLE3D are modified by SCR/soil interaction model with soil suction. Dynamic response of SCR with soil suction in time domain is analyzed by the improved program. The time histories of dynamic response and its amplitude spectrums by FFT at key zone of SCR are plotted in figures. The results show that soil suction has a large effect on extreme stress and induces small-amplitude and high-frequency oscillation of time histories of SCR dynamic response when the riser is pulled away from the seabed.The new research idea and method of deepwater SCR are proposed in this thesis through analyzing research method at home and abroad. The conclusion of this paper supplies the design and nonlinear analysis of deepwater SCR with theoretical-based calculation as well as the applications of SCR in China with the guiding significance and the reference.