Ultimate Strength and Fatigue Life Analysis of Dented Riser

Abstract: Riser is one of the most important components in modern ocean engineering structural systems, but also a weak component which will occur collision, fatigue damage and fracture failure under the influence of internal fluid and external load. Those may cause serious disaster. Therefore, studying on ultimate strength and fatigue life of dented riser is of great significant. In this paper, finite element method was used to study the ultimate strength and fatigue life of dented riser.The ultimate strength of the wedge dented riser under combined axial tensile force and longitudinal bending, combined external pressure and longitudinal moment was analyzed respectively, and the tensile-bending and pressure-moment collapse envelope were determined; the results showed that, as long as the axial tensile force within a suitable region, the impact of the dent on the ultimate strength of the riser under combined axial tensile force and longitudinal bending was very small; Suitable pressure can improve the ultimate moment of the dented riser under combined external pressure and ultimate moment.To study the effect of axial tensile on the ultimate strength of the wedge dented riser, the riser under certain axial tensile, longitudinal moment and external pressure was analyzed, and the pressure-moment collapse envelope was determined. The results showed that,if the axial tensile exist, the ultimate strength can be improved when the external pressure is small. The axial tensile had greater influence on the ultimate strength of the dented riser as the depth was deeper.The parameter of the maximum depth of the dent ,the length of the dented region , the thickness of the riser are considered, to study there influence on the ultimate strength of the dented riser under the axial tensile force or longitudinal bending alone; The results showed that: the ultimate strength significantly reduced as the maximum depth of dent increase; the change of the length of the dented region have little effect on ultimate strength; increase in the thickness of the riser will greatly improve the ultimate strength, but the thicker was the wall, the greater impact did the dent have on ultimate strength.In this paper, contact analysis was used to form local dent on riser under spherical indenter. Without considering the impact of residual stress, the ultimate strength of the dented riser under axial tensile force, positive bending moment or the external pressure was analyzed respectively. The results showed that: similar to the wedge dented riser, ultimate strength reduced significantly as the maximum depth of the spherical dent increased; collapse pressure greatly reduced with the increase of the ovality of most deformed cross section.Finally, the effect of different maximum depth and length of the dent on the fatigue life of dented riser under the constant amplitude fatigue loads was studied. The results showed that:fatigue life significantly reduced as the maximum depth of dent increase; the changes of the length of dented region had obvious impact on the fatigue life…
Key words: riser; dent; ultimate strength; fatigue life; ABAQUS; MSC.Fatigue

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