Structural stability of flexible lines in catenary configuration under torsion

•Catenary risers stability analysis were performed.•Torsion load induced the instability of the structure.•A nonlinear finite element model was used including contact constraint and large rotations.•Greenhill's formula was compared to numerical results.•A “universal plot” for catenary riser stability under torsion load was developed.

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.