Structural stability of flexible lines in catenary configuration under torsion

Catenary risers can present during installation a very low tension close to seabed, which combined with torsion moment can lead to a structural instability, resulting in a loop. This is undesirable once it is possible that the loop turns into a kink, creating damage. This work presents a numerical methodology to analyze the conditions of loop formation in catenary risers. Stability criteria were applied to finite element models, including geometric nonlinearities and contact constraint due to riser-seabed interaction. The classical Greenhill's formula was used to predict the phenomenon and parametric analysis shows a “universal plot” able to predict instability in catenaries using a simple equation that can be applied for typical risers installation conditions and, generically, for catenary lines under torsion.

The authors acknowledge FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) for the support under the grants 2006/06277-0 (PhD), 2012/09912-0 and 2012/21167-8 (PostDoc). The second author acknowledges the support by CNPq (Conselho Nacional de Desenvolvimento Cientí fi co e Tec- nológico) under the grant 310105/2009-9.