Analysis Of Interaction Between Steel Catenary Riser And Seabed

Steel Catenary Riser(SCR)is widely used in the exploration of deep offshore areas because of its low operating costs and high temperature and high pressure applications.The bending deformation of the pipe in the touchdown zone(TDZ)leads to large bending stress in this area.Meanwhile,under the influence of the operating platform and environmental loads,this position will constantly interact with the seabed.In the whole interaction process,the pipe in the touchdown area will not only bear the reaction force from the seabed,but also undergo the change of the stress distribution because form of touchdown zone changes,which is very complicated.Based on Lingshui 17-2 project,this paper analyzes and studies the pipesoil interaction of steel catenary riser in service.The main contents are as follows:(1)By considering the interaction between the riser and the seabed as a plane strain problem,based on the large deformation finite element method and the geotechnical material as an ideal elastic-plastic model,the resistance of the pipeline in the process of pipeline penetration and the stress distribution of the seabed after deformation are analyzed.On this basis,the effects of rock and soil strain rate effect,strain softening effect and their combined action on resistance to pipeline penetration,seabed stress distribution after deformation and seabed strength are analyzed respectively,and the results of strain softening when the firstorder and second-order softening models are used respectively are compared.At the same time,the sensitivity of parameters in strain rate model and strain softening model is analyzed.(2)A numerical model of the interaction between steel catenary riser and seabed was established.Based on the finite element method,the stress distribution characteristics along the pipe length,the stress characteristics of the touchdown area,the overall shape of the riser and the touchdown area were analyzed in the initial state of the steel catenary riser.The stiffness attenuation model was selected to describe the seabed cyclic softening in the touchdown zone,and then the suspension point of steel catenary riser was subjected to cyclic displacement.The displacement response,stress response and form of the riser in TDZ changes with time were obtained when considering the seabed cyclic softening,and the influence of different seabed cyclic softening conditions on the above results was analyzed.(3)Based on the numerical model of interaction between steel catenary riser and seabed,the initial state of the opposite pipe is analyzed after different riser design parameters,the displacement of the upper platform and the amplitude of the displacement load at the suspension point are selected.The design parameters of the riser include pipe diameter and wall thickness.After static analysis of initial state of riser,the displacement load is applied to the suspension point.The effects of the design parameters of the riser,the position of the upper platform and the amplitude of the displacement load at the suspension point on the dynamic response and fatigue life of the touchdown zone of the riser are analyzed.Based on the analysis,the influence of some seabed mechanical characteristics on the interaction between steel catenary riser and sea-bed is clarified in this paper.At the same time,the influence of some factors such as the position of the upper platform change on the dynamic response and fatigue life in the touchdown zone of the steel catenary riser is also pointed out,the study provides some guidance for the analysis of the interaction between SCR and seabed in Lingshui 17-2 project and the subsequent riser design.