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.