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 failure analysis of flexible pipe carcass layers with erosion defects
Yanfei Chena,b , Zhiming Jianga, Hao LiuaaNational Engineering Laboratory for Pipeline Safety/MOE Key Laboratory of Petroleum Engineering/Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China University of Petroleum (Beijing).
Beijing, China
bState Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology.
Dalian, Liaoning, Chinamodel was later improved by Neto et al. (2012). Tang et al. (2016) usedthe equivalent strain energy method to calculate the equivalent thicknessof the corresponding thin ring. Li et al. (2018) refined the finite element
model to simulate and compute the carcass layer's strain energy,
demonstrating that the equivalent strain energy approach is a reliable anduseful tool for forecasting the carcass layer's collapse pressure. Edmans(2014) concluded that the predictions of several comparable approaches
are considerably diverse. These techniques need to be improved. The
carcass layer, being the flexible pipeline's deepest component, is in direct
touch with the conveying medium within the pipe. The conveying
medium traveling at high speeds under operating conditions may corrode
the carcass layer's steel belt due to defects on the inner surface of thecarcass layer. Helgaker et al. (2017) did an experimental investigation on
the erosion of flexible pipes, and the findings revealed that erosion isunevenly distributed throughout the steel strip of the carcass layer, withthe largest erosion occurring at the leading edge of each part.
A finite element analysis model for the non-bonded flexible pipeline
carcass layer with erosion defects is developed in this paper. The externalpressure load is employed as the main load to examine the pipeline'scapacity to withstand compression using the nonlinear analysis approach.
We can better understand the failure process of the non-bonded flexible
pipeline carcass layer with erosion defects as a result of this research,and provide a timely safety evaluation of the non-bonded flexible
pipeline carcass layer with erosion defects.

ABSTRACTThe carcass layer of flexible pipe adopts the structural form ofinterlocking wrap, which leads to its inner surface not being smooth andhaving gaps. The fluid transported at high speed in the pipe may containsand or solid impurities. These impurities will impact the gap between
close parts of the carcass layer causing erosion, leading to thinning of thesteel strip of the carcass layer and reducing the pressure collapse
resistance of the carcass layer. The carcass layer will have seriousconsequences in the event of a collapse failure. The collapse pressure offlexible pipes is investigated by modeling the carcass layer of flexible
pipes containing erosion defects. The proposed solution proved to beable to predict the collapse pressure of flexible pipe carcass layers
containing erosion defects.

KEY WORDS: flexible pipe; erosion defects; carcass layers;collapse.

INTRODUCTION
Non-bonded flexible pipes better adapt to the ocean's complex
environment. Local steel layers in the carcass layer get thinner when thecarcass layer erodes. Furthermore, with the action of external load, theerosion position of the carcass layer may have stress concentration. The
pipeline will collapse and fail as a result of the external pressure. Paumieret al. (2009) completed over 200 experimental verifications on thepipeline structure and determined the flexible pipeline collapse pressure.
Malta et al. (2012) did a study on pipe collapse modes, proposing thatwhen a three-layer pipe is destroyed, it will emerge in two collapse
modes. Souza et al. (2002) conducted experimental testing with flexible
pipes to illustrate the structural behavior of the pipe until it collapses
under the action of radial compression and external pressure. Clevelario
et al. (2010) did a similar investigation and discovered that the collapsepressure of curved pipes was lowered by more than 10% when compared
to straight pipes. Zhang et al. (2013) estimated the equivalent thicknessof the complicated cross-section to anticipate the collapse pressure of thecarcass layer , using the cross-sectional area equivalent technique toequate the complex cross-section carcass layer to a steel ring of uniform
thickness.
Based on the principle of virtual work, Bai et al. (2016) created asimplified analytical model of a multi-layer structure. Sousa et al. (2001)
used the equivalent rigidity of the carcass layer section to calculate theequivalent thickness of the carcass layer. Martins et al. (2003) equatedthe carcass layer to a thin-walled ring using the approach of equalbending stiffness per unit axial length of the carcass layer. The prediction

Flexible pipeline parameters(1) Structural parameters of the carcass layer
When calculating the collapse pressure of the non-bonded flexiblepipeline carcass with erosion, this work used typical circular construction,
as illustrated in Fig. 1. The carcass layer, which has an inner diameter of
50.8 mm and is created by 0.7 mm thick stainless steel into aninterlocking structure. Table 1 displays the section's detailed sizespecifications.

Fig. 1 Schematic diagram of carcass spiral section

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