In deep-water applications of flexible pipes, particular attention needs to be paid to the birdcaging failure, which occurs more likely when the outer layers are damaged. This paper presents a FE model for the birdcaging analysis of a flexible pipe with damage on the outer layers. The local pipe bending caused by the armour wires non-axisymmetric deflections in the birdcaging section is identified and discussed. The results indicate that the ultimate axial compressive strength of a flexible pipe with damage on the outer layers may be inherently related to the event of combined radial-lateral buckling of the inner armour wires.
Flexible pipes are extensively used in offshore oil and gas exploration in all water depths. In deep-water applications, particular attention needs to be paid to the tensile armour layers buckling. The failure is most likely during pipe installation or for operating pipes with low bore pressure. Due to the reversed end-cap effect, large axial compression can be generated which are principally carried by the tensile armour layers. Driven by this axial compression, the slender helical armour wires may buckle in either lateral or radial directions, leading to permanent damage and possibly loss of containment.
The armour wires buckling in the radial direction, also known as birdcaging, occurs when the radial resistance is not sufficient to prevent the armour wires radial expansion. The radial expansion resistance is generally guaranteed by the outer sheath and aramid or glass fiber reinforced anti-birdcage tape. Models have been developed for the birdcaging analysis of an intact flexible pipe (Vaz and Rizzo, 2011, Sousa et al., 2012, Malta and Martins, 2014, Sævik and Thorsen, 2017). However, damage is often caused on the outer protective layers principally during installation. The damage may be generated by impact, abrasion, cutting by sharp edges, etc. For on-bottom pipes, this will lead to an elevated risk of birdcaging failure as the local radial expansion resistance is reduced. Sertã et al. (2012) and Ebrahimi et al. (2016) studied the birdcaging in a flexible pipe with full circular damage on the outer sheath. Lu et al. (2017) studied the armour wire radial buckling in the exposed area. Borges et al. (2017) investigated the birdcaging of a flexible pipe with rectangular damage on the outer layers.
