Nonlinear Span Assessment by Amplitude-Dependent Linearization

Although it has long been recognized that vortex-induced vibrations of subsea pipeline spans involve nonlinear and inelastic behavior, the current practice to assess such spans for fatigue and ultimate loading conditions is based on the modal analysis assuming linear behavior. Nevertheless, nonlinearity can be captured approximately by making the linearization amplitude dependent. The eigenvalue problem to be solved for the natural frequencies and mode shapes then involves a stiffness matrix that depends on the mode shape and amplitude of vibration. An important part of the nonlinearity comes from the soil, which is generally represented by springs. This paper presents a simple and particularly effective algorithm to solve this nonlinear eigenvalue problem by using the same algorithm that serves to track the bifurcated solution branches in quasi-static structural stability (buckling) analyses. This method is applied to an example in which the nonlinearity comes from the soil springs. The results demonstrate the importance of the nonlinearity, even at relatively low vortex-induced vibrations (VIV) amplitudes typical of the pure inline response. The inelasticity of the soil springs is also used to calculate the associated contribution to the modal damping ratio.