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

Torsional Friction in Finite Element Modelling and Its Application in Pipelines with Residual Curvature
Yi Yu, Kristian Norland
Subsea 7Stavanger, Norway

ABSTRACTINTRODUCTION

Traditionally when the pipeline lateral buckling behaviour was analysedusing Finite Element (FE) method, pipelines were normally modelledwith beam elements without incorporation of torsional friction in the FEmodels. The neglection of torsional friction has limited impact on FE
analysis results if snake lay is used as the lateral buckling initiation
method, since the pipeline rotation in such a case is normally very
limited. However, it could be different for a pipeline with Residual
Curvature Method (RCM) used to trigger lateral buckling. If a ResidualCurvature (RC) section has not been laid horizontally on seabed, its
behaviour will be different from what has been expected during pipeline
design, because the pipeline will have axial rotation besides lateraldisplacement, as a result of eccentricity caused by the suspended RC. In
such a case, the effect of torsional friction between pipeline and seabedcannot be neglected.

Most subsea pipelines operated with High Pressure and High
Temperature (HP/HT) are subject to axial compressive forces andexpansion, which may cause the pipeline to move laterally, vertically, orin a combined direction. Such movement of subsea pipelines under axial
compression are so called global pipe buckling. Uncontrolled pipeline
global buckling may cause high stress/strain during operation, which canbe a safety hazard to subsea flowlines. To minimize the likelihood of
pipeline failure due to global buckling, rogue buckles should be avoided
in pipeline design and all potential buckles should be well planned andcontrolled. Therefore, global buckling triggers/initiators are normallyemployed in pipeline design to guide the pipeline to move at planned
locations, preferably laterally. Conventional lateral buckling triggering
methods such as snake lay and adding sleepers, normally have highinstallation cost and introduce considerable uncertainty in the initial
imperfection formation particularly for HP/HT pipelines on theundulating seabed, which normally means high buckling force and highpost-buckling bending moment.

In this paper, the torsional friction for pipelines with RC sections arediscussed. The incorporation of torsional friction in pipeline FE models
is presented and its effect on pipeline lateral buckling behaviour withresidual curvatures is illustrated. To facilitate lateral buckling ofpipelines with under-rotated RC sections, mitigation measures for areproposed and the post-laid residual curvature mitigation loading steps areincluded in the FE models. The effect of the torsional friction isdemonstrated by introducing it in FE models with under-rotated RCsections. In the end, the validity of this method is verified by comparing
the FE results with the site measurement for pipelines with under-rotated
RC sections mitigated after installation on seabed. The effectiveness andthe reliability of the proposed method for incorporation of torsional
friction in FE analyses for pipelines with RC sections are concluded. By
implementing connector elements with torsional friction properties in theFE model, a good match was observed between the predicted behaviour
and the observed field measurements. This FE model can be used
potentially in future projects with RC sections or with considerable
pipeline rotations.

Residual Curvature Method (RCM) is an innovative buckle triggering
method introduced by Endal et al (1995) and patented by Statoil (2002).
The basic ideal of RCM is to introduce pipeline initial imperfection by
under-straightening pipelines during reel lay and a RC section (a sectionof curved pipeline) will be laid on seabed as illustrated in Figure 1. A
typical RC section has initial imperfection with 0.15%-0.25% residual
plastic strain induced in a 40 m to 60 m long section. A 15 m section oneither side of the residual curvature section is provided with elastic
curvature joining the straight section of the pipe as shown in the figure
below. The RC configuration used in Pipe-in-Pipe (PiP) system is similar
to that of single pipeline system except only the initial plastic strain in
the outer pipe is in control during reel lay and the associated plastic strain
in the inner pipe is nearly zero in the RC segment as presented by Yu andNorland (2020).
If the whole RC section can touch seabed after installation, which means
the rotation of RC section is 90 degree on a flat seabed, lateral buckling
will be triggered with increased reliability and reduced uncertainty

KEY WORDS: subsea pipeline; residual curvature; finite element;

torsional; pipe-soil interaction; reel lay installation; lateral buckling.

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