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

Collapse Mechanisms of Composite Pipelines under External Pressure
Ms. Zichen Wei a, A/Prof. Hassan Karampour a, Prof. Hong Guan a, Prof. Hayder Rasheed b and A/Prof. Hong Lin ca Griffith School of Engineering and Built Environment, Griffith University.

Gold Coast, QLD, Australia.
b Department of Civil Engineering, Kansas State University, Manhattan, KS, USA.
c College of Pipeline and Civil Engineering, China University of Petroleum (East China), Qingdao, China

ABSTRACT

Subsea pipelines are slender structures that can experience several
structural instabilities. Global buckling can occur through lateral or
upheaval buckling modes, however, these two buckling modes are notessentially failure modes (Karampour et al., 2013). Propagation bucklingis the most critical one that increases the chance of pipeline failure,especially in the ultra-deep waters (Albermani et al., 2011; Karampouret al., 2017; Stephan et al., 2016; Alrsai et al., 2018a; Alrsai et al.,2018b). Hence, in deep waters, the failure mode associated with thebuckle initiation and its propagation is the main consideration in thepipeline design.

This study investigates the effect of wrapping carbon fiber reinforced
polymer (CFRP) over conventional steel offshore pipelines on improving
their capacity under external hydrostatic pressure. Buckling performanceof bare steel pipelines (conventional) and composite pipes (CFRPstrengthened) subjected to external pressure are examined analyticallyand numerically. In composite pipelines, the cylindrical steel tubes arewrapped with CFRP in 0/90° and ±45° fiber orientations. The finite
element analysis (FEA) results are validated against analytical solutions
on buckling of bare and composite pipelines. A parametric study is also
carried out to investigate the effect of D/t ratio, the ratio of the thicknessof the FRP to the steel-wall thickness, and the fiber orientation on thebuckling capacity of composite pipelines. A good agreement between thecurrent FEA and theoretical results are observed for pipelines with D/t >30. Results also show that wrapping 0/90° cross-ply CFRP on theconventional steel pipes is more efficient in bucking resistance than the±45° one. Using 0/90° orientation wrapping, the collapse pressure of athin pipelines (D/t=100) can be enhanced by a factor of 15.

Propagation buckling is a snap-through phenomenon that can betriggered by a local damage. Such a buckle may be initiated by local
dents or impacts and can quickly transform the pipe cross-section into adumb-bell shape that travels along the pipeline if the external pressure ishigh enough to maintain propagation (Albermani et al., 2011). Offshore
pipelines normally experience high service external pressure; therefore,
the buckle propagated through the length, will force the flow line toclose. The collapse pressure of a single pipeline (𝑃𝑐𝑟 ), with perfectly
circular cross-section can be approximated by the classical expression
for buckling of elastic tubes under uniform external pressure(Timoshenko and Gere, 1961):

KEY WORDS: Offshore pipelines; external pressure; CFRP;
composite pipelines; shell buckling; collapse under external pressure;cylindrical tube.

𝑃𝑐𝑟 =INTRODUCTION

2𝐸𝑡 3( )(1−𝑣 2 ) 𝐷(1)where E and  are the modulus of elasticity and Poisson’s ratio,
respectively, of a pipe with a mean diameter of D and a wall-thicknessof t. Eq. (1) can predict the elastic collapse due to external pressure in a
tube, and is mostly representative of the initiation pressure in a thinpipeline (large D/t ratio). However, in deep and ultra-deep waters thickpipelines with smaller D/t (as low as 10) are often employed. Buckle
initiation pressure in such thick pipelines is known to be elastic-plastic(Alrsai et al., 2018b).

Exploration of hydrocarbons has shifted to deep and ultra-deep waters
due to the scarcity of near-shore resources and recent advancements indeep water drilling technologies. Pipelines are long tubular structures
used to transport hydrocarbons over long distances, and are designed toresist installation, operation and environment loads safely during theirdesign lives. During installation and shutdown, an empty pipeline
undergoes large external pressure due to the hydrostatic load, which may
cause collapse in the tube wall. In addition, high-pressure and hightemperature (HP/HT) pipelines may collapse under combined loading
due to the interaction between external pressure and thermal loads(Karampour et al., 2015; Karampour, 2018) or seismic loads (Mina et al.,2020).

The lowest pressure that maintains propagation is known as thepropagation pressure (𝑃𝑃 ) and is much smaller than the collapse pressure(𝑃𝑐𝑟 ). The level of pressure has an important influence on the rate ofpropagation buckling. Propagation does not occur if the external pressure(𝑃) is less than the propagation pressure (𝑃 < 𝑃𝑃 ). However, at external

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