Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Finite Element Investigation on Flexible Pipe Birdcaging with Damage on The Outer Layers
Xiaotian Li1, Murilo Augusto Vaz2
1, School of Marine Engineering and Technology, Sun Yat-sen University, Zhuhai, Guangdong, China
2, Ocean Engineering Program, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, Brazil.

radial buckling in the exposed area. Borges et al. (2017) investigatedthe birdcaging of a flexible pipe with rectangular damage on the outerlayers.

ABSTRACTIn deep-water applications of flexible pipes, particular attention needs
to be paid to the birdcaging failure, which occurs more likely when theouter layers are damaged. This paper presents a FE model for thebirdcaging analysis of a flexible pipe with damage on the outer layers.
The local pipe bending caused by the armour wires non-axisymmetricdeflections in the birdcaging section is identified and discussed. Theresults indicate that the ultimate axial compressive strength of a flexible
pipe with damage on the outer layers may be inherently related to theevent of combined radial-lateral buckling of the inner armour wires.

Note that the birdcaging risk level is intrinsically related to the damage
morphology and sizes. Due to the Capstan’s effect, the constraint
capacity of the helically wound anti-birdcage tape layer will bereestablished after a certain distance away from the damage. Therefore,
for damage of small size, the residual radial expansion resistance maystill be sufficient to prevent birdcaging. Thus, a good understanding of
the birdcaging mechanism under different damage conditions and its
consequence is essential for risk analysis and integrity management.
If the estimated risk of birdcaging is too high as compared to themaximum acceptable level, or the birdcage is already formed but
mildly, actions must be taken to mitigate the failure risk, which may berealized by repairing. A repair method with potential is winding highstrength filaments over the birdcaging section with large pretension.
This can be accompanied by covering a polymeric layer for watersealing and protection purposes. Thereby, the armour wires radial
expansions in the birdcaging section are eliminated and the radialresistance is rebuilt. Preliminary work has been performed on the repair
design for flexible pipe with damage but no birdcage formed (Liu et al.,2016, Li et al., 2019). For a damaged pipe with residual radial
deformation, the repair design is more complex as non-axisymmetricgeometry and complex elastoplastic deformation are involved.
Moreover, there is a need for qualifications to ensure that this repair
technology meets the specified requirements.

KEY WORDS: Flexible pipes; birdcaging; damaged outer layers.
INTRODUCTION
Flexible pipes are extensively used in offshore oil and gas explorationin all water depths. In deep-water applications, particular attention
needs to be paid to the tensile armour layers buckling. The failure ismost likely during pipe installation or for operating pipes with low borepressure. Due to the reversed end-cap effect, large axial compression
can be generated which are principally carried by the tensile armourlayers. Driven by this axial compression, the slender helical armour
wires may buckle in either lateral or radial directions, leading topermanent damage and possibly loss of containment.
The armour wires buckling in the radial direction, also known asbirdcaging, occurs when the radial resistance is not sufficient to preventthe armour wires radial expansion. The radial expansion resistance isgenerally guaranteed by the outer sheath and aramid or glass fiber
reinforced anti-birdcage tape. Models have been developed for thebirdcaging 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 willlead to an elevated risk of birdcaging failure as the local radial
expansion resistance is reduced. Sertãet al. (2012) and Ebrahimi et al.
(2016) studied the birdcaging in a flexible pipe with full circulardamage on the outer sheath. Lu et al. (2017) studied the armour wire

Aiming at assessing the need and feasibility of repair for damaged outerlayers, a research project is set up by the Ocean Structures Nucleus, alaboratory from the Federal University of Rio de Janeiro. As part of theproject, a finite element method is developed for the birdcaging
analysis of a flexible pipe with damage on the outer layers, which
serves as the foundation for the assessments of failure risk andconsequence and repair feasibility. One of the highlights of this modelis that it is applicable to simulate birdcaging in a non-straight pipe
condition. This has been achieved by applying distributed surfacepressure on the pipe ends and sealing barriers. The FEM development
is presented in this paper and some interesting results are discussed.

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