Research On Fatigue And Monitoring Theory Of Girth Weld Of Deepwater Steel Catenary Riser

In the deep water environment,the steel catenary riser,as a slender flexible structure without design tension,is prone to fatigue failure at key parts of the platform under the excitation of platform heave motion and seabed contact load.Compared with the base material,the position of the riser girth weld is more prone to cracking under cyclic loading.Based on the structural stress method developed in recent years,this paper studies the fatigue problem of steel catenary riser girth welds,and further develops this type of riser monitoring theory and monitoring method based on the new fatigue evaluation theory.The thesis first uses the Orca Flex finite element model to study the load-bearing characteristics of the steel catenary riser girth welds at typical locations.The results show that the steel catenary riser pipe is compared to the girth welds in the middle section and bottom-touching section of the riser.The tension of the girth weld of the suspension section is significantly greater,and its value is almost twice that of the girth welding section of the middle section,and four times of the girth welding section of the bottom section;compared with the girth section of the suspension section and the middle section,the girth section of the bottom section The shearing force and bending moment are more significant,and the fatigue problem needs more attention.Aiming at the uniaxial fatigue problem of steel catenary riser suspension section and middle section controlled by tension,the thesis obtains the equivalent structural stress by defining the welding wire unit in Fe-safe software,and then uses the main SN curve interpolation to obtain the relevant fatigue A comparative study of the methods of life.The calculation results show that for the example riser,the structural stress of the girth weld of the suspension section is almost twice that of the girth weld of the middle section.According to the fatigue life obtained by interpolation of the equivalent structural stress,the girth of the suspension section The seam is only one fifth of the girth weld in the middle.Aiming at the problem of multi-axial fatigue of the circular weld of the bottoming section of the steel catenary riser,the paper combined the tensile and bending structural stress and the shearing structural stress to redefine the equivalent structural stress range,and obtained the bottoming section of the circular weld Fatigue life of circumferential nodes.Aiming at the crack propagation mode where the toe cracks and penetrates the pipe wall,the paper introduces the dimensionless load bending ratio function in the definition of equivalent structural stress,and obtains the residual fatigue life of the ring weld with different initial cracks.The comparative study shows that the presence or absence of initial cracks has a great influence on the residual fatigue life of the girth weld.With the increase of the initial crack depth,the equivalent structural stress of the riser girth weld gradually increases,and the residual fatigue life gradually decreases.For the research of steel catenary riser fatigue monitoring,it is proposed to determine the location of the acceleration sensor based on the modal analysis results and the effective independent coefficients of each node.Three evaluation criteria of modal confidence,total displacement and matrix trace are selected,and the layout schemes of measuring points based on EI method and MKE method are compared and studied.The results show that MKE method considers the influence of structural energy and has better optimization The effect can be used as an optimization method for the layout design of the riser monitoring sensors.Finally,the possibility of using the acceleration monitoring signal to realize riser damage identification was discussed.The finite element results of the riser vibration analysis were selected as the simulated monitoring signal to preliminarily verify the damage identification process based on the FDDM method.