Dynamic Analysis Of Deepwater Steel Catenary Risers And Fatigue Characteristic Assessment At The Touchdown Point

Nowadays, China has accelerated the exploitation in the South China Sea. The development of deep-sea engineering equipments with core technology has become a priority. The steel catenary riser (SCR) is the key apparatus in ultra-deep water work. Since the complex nonlinear dynamic behaviors, the SCR brings new challenges in design, manufacture, installation and security application.The touchdown point (TDP) is the characteristic point in structure analysis, also the combined point of sag bend and flow line. The most vulnerable location to fatigue damage exists at the TDP, where the riser first touches the seafloor. Furthermore, the SCR fatigue damage will endanger the whole oil production system. The SCR-Seabed interaction relates to the large deformation and nonlinear reaction. Parts of the riser interact with the soil significantly near the touchdown zone, which above the seabed at the beginning. Therefore, the TDP is the principal region to be focused in numerical analysis.Dynamic analysis of deepwater steel catenary risers and fatigue characteristic assessment at the touchdown point are studied in this work, the main tasks are given as follows:1. Based on nonlinear springs, a Lumped-Mass SCR coupling model is established in marine environment.On the basis of discussions in the SCR comprehensive development, numerical research and fatigue analysis, the study methodology is brought. The TDP is defined as the principal objective during the whole investigation. Due to P-y Curve and Morison Equation, three-dimensional seabed reactions and marine-ambient excitations are simulated respectively. Above all, the SCR model is proved successfully through reasonable analysis.2. Acquire nonlinear dynamic features of the SCR in different operating conditions, and focus on dynamic responses at the TDP.Comparing with frequency domain dynamic analysis, it's better and more accurate to solve the strong nonlinear problems of SCR in time domain. The overall structure research indicates: in fatigue assessment, top end is the key location of rigid riser in different cases, and the TDP is the characteristic point of SCR. By studying dynamic responses in multi-operating conditions, the impacts of various factors to the TDP are discussed: The vessel motion and construction material are significant influences. On the contrary, the wave load couldn't change dynamic features at the TDP directly. In addition, other environment loads and design parameters have some effects as well.3. The TDP is investigated as a key position in the SCR fatigue study, meanwhile, some fatigue life enhancements are stated.Through dynamic analysis and S-N Curve Method, the results of fatigue assessment are shown: The TDP appears the shortest fatigue life, and the fatigue accumulation can be alleviated by transforming the touchdown zone regularly. Selecting S-N curves carefully or optimizing manufacturing crafts are helpful to control the stress concentration in the feature point. The SCR fatigue life is highly sensitive to vessel heave, so an excellent vessel could relieve coupling movement notably. Use of the buoyancy device or hybrid form is an available approach to better the soil nonlinear reaction. Besides, the employment of protective coating and titanium in characteristic locations could expand the SCR application.