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

Into the Deep: A Fundamental Improvement of the Analytical Formulation
for the Prediction of Collapse of Thick-walled Pipelines
Ruud Selker

Joost Brugmans

Intecsea
The Hague, The Netherlands

Intecsea
The Hague, The Netherlands

Ping Liu

Carlos Sicilia

Intecsea
The Hague, The Netherlands

TotalEnergies
Paris, France

ABSTRACTKEY WORDS: Pipelines; Thick Wall; Collapse; External Pressure;
Deep Water; First Principles; Mechanics.

Submarine pipelines can get damaged in various ways. An important
failure mode is collapse. In the deep sea, the hydrostatic pressure isvery large and the threat of said failure exists. There are prospects forpipelines in water depths of up to—at least—3,700 m. A pipeline
requires a very thick wall to cope with such, hostile environment. Thismeans that 𝐷⁄𝑡, the diameter-to-wall-thickness ratio, must be low.

INTRODUCTION
Background
One of the gaps identified in the research roadmap of the Design
Committee of the European Pipeline Research Group (EPRG) is thelack of a valid formulation to evaluate the local-buckling limit state
under combined loading for thick-walled pipe. The formulations inDNVGL-ST-F101 (2017), a prevailing standard for the design ofsubsea pipelines, were not valid for a diameter-to-wall-thickness ratio(𝐷⁄𝑡) less than 15.

The analytical limit-state equations included in DNV-ST-F101, aprevailing design standard for submarine pipeline systems, do not work
well for pipe with extremely thick walls. Collapse-pressure predictions
are increasingly conservative for lower 𝐷⁄𝑡. By following the standard,
pipelines operating in ultra-deep water may be over-dimensioned. Thiscan culminate in manufacturing requirements that adversely affectproject economics or threaten the technical feasibility altogether.

Intecsea has completed a desktop study for EPRG’s Design Committee
as part of EPRG Project 202/2017 (“Pipeline Design for Thick Pipe(𝐷⁄𝑡 ratio below 15)”). The study focused on the behaviour of line pipe
subjected to a combination of bending and external overpressure andwas summarised by Selker et al. (2022). The main conclusion was that,when behaviour is load controlled, the design equations in
DNVGL-ST-F101 (2017) lead to increasingly conservative results
when reducing 𝐷⁄𝑡. The most extreme underprediction of bendingmoment capacity was found for an external pressure close to thecollapse pressure. When 𝐷⁄𝑡 is below 15, the collapse pressure can be(conservatively) underestimated by more than 10%. This work has
ultimately resulted in DNV omitting the lower-bound limit on 𝐷⁄𝑡 forthe combined-loading criteria in the latest revision of its design
standard, DNV-ST-F101 (2021).

This paper proposes an improved analytical framework for calculating
the collapse pressure. Like the DNV model, the proposed model
reflects the fundamental underlying and interacting physics of material
yielding and geometric instability. It has been found that the collapse
resistance of pipe with extremely thick walls (roughly 𝐷⁄𝑡 ≤ 15) isunderestimated when using the DNV formulation. On the other hand,the capacity of pipe with slightly thinner walls (about 15 < 𝐷⁄𝑡 ≤ 40)
could be overpredicted. The latter may occur if cold-formed line-pipe
joints have not been subjected to some form of heat treatment after
forming—for instance, via a coating process.
To improve consistency between the predictive model and finiteelement analysis, the analytical model has been extended to account for
behaviours that were not yet included: triaxiality of stress and plasticbifurcation. The result is an elegant set of equations that can be used toevaluate the collapse pressure. Although the focus of this study wasthick-walled pipe, the improvements lead to better collapse-pressurepredictions for pipe with thinner walls, too.

Those findings have justified a study (EPRG Project 202a/2020,“Pipeline Design for Thick Pipe – Improvement of Collapse Equation”)
to establish an improved design formulation that will reduce thisconservatism. An improved formulation could result in a reduction of
the required wall thickness of thick-walled pipe loaded by high external

1662