Fatigue Assessment Of Steel Catenary Riser

Steel catenary riser is a new type of riser developed in recent years.It has the advantages of low cost,strong adaptability to floating body movement,simple and fast construction operation,and operate in high temperature and high pressure working environment.Therefore,steel catenary riser has been paid more and more attention by engineering circles.One part of the steel catenary riser is suspended in the water,and the other part is buried in the soil.The connecting part of the two parts is called "Touchdown point".The pipe soil interaction of the steel catenary riser in the touchdown zone is the key to predict the fatigue life of the pipeline.At present,there are three main models of pipe soil interaction: RQ model proposed by Randolph and Quiggin,AB model proposed by Aubeny and Biscontin and ABY model proposed by You,which are P-y curves representing the relationship between force and displacement.In this paper,the numerical simulation method is used to study the state and stress of three pipe soil interaction models in soil,which provides reference and guidance for engineering application.In the first part of the paper,the characteristics of the three models are compared through the analysis of the three models.The "Winkler foundation beam" model is used to solve the equation by the central difference method.The numerical iteration method is used to study the shape and stress of pipe in soil under cyclic displacement.The influence of three models on fatigue damage is compared.The results show that the fatigue damage of RQ model is the largest,followed by AB model and ABY model.The main reason is that the trench of seabed will reduce the fatigue damage and the suction will increase the fatigue damage.Because the trench effect is not considered in RQ model,the range of suction will increase and the fatigue damage will increase.The trench effect is considered in AB model,followed by fatigue damage.With the development of the cyclic motion of the pipeline,the suction of ABY model decreases and the fatigue damage is minimum.In the second part of the paper,the coupling analysis is carried out by connecting the pipeline in water and the pipeline in soil.The calculation model of the pipeline in water is analyzed,and the numerical iteration method is used to solve the problem.The pipeline in soil is still calculated by the central difference method.The whole pipeline is analyzed and solved according to the continuity conditions of displacement,rotation angle,tension and bending moment of the two pipelines,the paper discusses the influence of different mudline shear strength and shear strength gradient on the pipeline,and analyzes the influence of different soil shear strength on the "spoon" shape of the pipeline in the touchdown zone,so as to affect the stress of the pipeline.