Stability and Frequency Response of Top-Tensioned Risers and SCR’s: The L/D Ratio in Perspective

Stability is closely linked to the dynamic behavior of risers and SCR’s. This paper reviews the L/D criterion as used to categorize offshore structural elements and places this criterion in perspective when used in VIV calculations to compare slender beams, beam-columns and cable behavior to anticipated results for top-tensioned risers with like L/D ratios. The stability of beam-columns with various end conditions and loadings are compared to that of an in-service 2400m top-tensioned riser. An analytic solution for the frequencies and mode shapes of top-tensioned risers at varying top-tension is given and factors which influence the dynamic behavior detailed. The analysis demonstrates that loads and representation of the load distribution are vital to obtaining valid results. Close-coupling of displacement and slope with loading are essential to the analysis. Results are shown of the effects of varying top-tension factor (TTF) on the stability and dynamic response of risers and SCR’s.