An Investigation on the Dynamic Compression Instability of Steel Lazy-Wave Risers

Abstract

Abstract. The steel lazy-wave riser (SLWR) is a good alternative for deepwater oil and gas exploitation, as its buoyancy section alleviates the top tension and TDP (touchdown point) movements. Although studies exist on dynamic compression instability in SCRs (steel catenary risers), no work has been found on dynamic compression instability in SLWRs. The differences in geometry and dynamic behavior between the two configurations do not enable extrapolation of results from SCRs to SLWRs. In SCRs, dynamic compression instability occurs near the touchdown zone; in SLWRs, the critical zone is near the top. This article investigates whether, despite this difference, the analytical formulation for dynamic instability in catenary risers available in the literature can be applied to SLWRs. To check and validate the analysis, the proposed methodology was implemented and verified for catenary risers. Once the results were checked against the literature, the same procedure was applied to a steel lazy-wave riser case study. Then, the influence of torsion was verified, as its presence can lead to out-of-plane buckling at a significantly lower load. The results showed that the model developed for catenary risers, disregarding torsion, did not always get accurate results for the lazy-wave configuration, but was always on the safe side. When torsion was present, as the instability occurred at the top, the observed critical load remained unchanged, but the instability always occurred out-of-plane. The article provides a project guideline for using the analytical critical load value to ensure no instability occurs.