An investigation on flexible pipes birdcaging triggering

• Conjecture proposed and assessed: the instability of the external plastic layer.
• Experimental analysis on HDPE tubes under internal pressure and axial compression.
• Plastic analytical criterion for HDPE tubes subjected to monotonic loading.
• Assessment of previous cases of birdcaging experimentally observed.
• Strong evidences on conjecture validity.

An investigation on triggering mechanisms for the birdcaging failure mode of flexible pipes, used in offshore oil and gas production, is carried out. From previous experimental observations, a conjecture is made: the local axisymmetric instability of the external plastic layer, caused by the high radial loading which is internally applied due to the helical armor wires tendency to expand when the pipe is subjected to compression, would be this trigger. A simple instability onset criterion for the external plastic layer, namely, the polymer intrinsic yielding limit stress, is proposed and assessed, analytically, numerically and experimentally for HDPE tubes. Then, previous birdcaging experimental observations are investigated further, focusing on the flexible pipe external plastic layer, to assess the proposed criterion. Strong evidences of validity are obtained.