PLUCK TESTS ON OPERATING DEEPWATER PIPELINE SPANS

Key uncertainties in the assessment of subsea pipeline spans for fatigue due to vortex-induced vibrations (VIV) are the effective axial force, the soil spring stiffness, and the soil damping. To reduce these uncertainties, pluck tests have been carried out, to determine the natural frequency and damping of single and multiple spans. These are carried out by pulling the span laterally at midspan with the ROV, until a 6mm or 8mm PP rope that serves as a weak link in the connection from the pipeline and the ROV breaks. The free vibrations resulting from this pluck are measured with accelerometers attached to the pipeline. The paper presents selected results from these tests and their interpretation in terms modal frequencies and damping ratios. Already at the achieved amplitudes of vibration of up to about 0.01D, the results already show considerable nonlinearity and inelasticity that is thought to come from the soil supporting the pipe at the shoulders of the span, and can be captured in FE models by making the soil springs nonlinear and inelastic.