Mechanical behaviour of steel lined pipes under monotonic and cyclic bending
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Date
27/11/2021Author
Gavriilidis, Ilias
Metadata
Abstract
Steel pipelines that transfer hydrocarbons may be exposed to several corrosive
elements. In order to ensure the structural integrity of the pipeline against corrosion,
double-walled pipes are produced, containing a thick-walled low-carbon steel (“outer
pipe”) providing strength, and a thin layer (“liner pipe”) from a corrosion resistant
alloy (CRA) material, which is fitted inside the outer pipe, resulting in a cost effective
solution instead of producing pipelines from stainless steel or nickel alloy.
First, the structural response of double-walled pipes, also called “lined pipes”, under
monotonic bending is investigated. In the first stage, two types of lined pipes are
examined, with and without mechanical bonding between liner and outer pipe referred
to as tight-fit pipe (TFP) and snug-fit pipe (SFP), respectively. It is shown that, upon
applying monotonic bending, the liner pipe gradually detaches from the outer pipe,
forming a uniform wrinkling at the compression zone leading to localized buckling
with further increase of the curvature. Subsequently, the bending response of lined
pipes under low or moderate levels of internal pressure is investigated, showcasing
its beneficial effect on the bending performance of lined pipes. Furthermore, the
influence of initial geometric imperfections on liner pipe buckling is examined, showing
the imperfection sensitivity of internally pressurized and non-pressurized bi-metallic
(double-walled) pipes.
Additionally, the effects of manufacturing process on the structural performance
of mechanically lined pipes are investigated. Alternative manufacturing processes are
considered, associated with either purely hydraulic or thermo-hydraulic expansion of
the pipes. A three-dimensional model is developed, which simulates the manufacturing
process in the first stage of the analysis, and subsequently, proceeds in the bending
analysis of the lined pipe. This integrated two-stage approach constitutes an important
contribution of this research to existing knowledge. Thermo-hydraulically expanded
lined pipes are examined, with special emphasis on the case of partially heated liners,
and reverse plastic loading in the liner pipe wall has been detected during depressurization.
Furthermore, the numerical results show that the thermo-mechanical process
results in higher mechanical bonding between the two pipes compared with the purely
mechanical process, and that this bonding is significantly influenced by the liner pipe
temperature level. It is also concluded that the value of initial gap between the two
pipes before fabrication has a rather small effect on the value of liner buckling curvature.
Numerical results on imperfection sensitivity are reported for different manufacturing
processes, and the beneficial effect of internal pressure on liner bending response is
verified.
Furthermore, the structural performance of a lined pipe under cyclic bending is investigated,
motivated by offshore reeling installation. Five bending cycles are considered,
representing the two installation cycles and three additional cycles of a failure/repair
scenario. The loading cycles impose a bending curvature range corresponding to the
strains developed during a typical reeling installation process. Different loading patterns
are considered and their effect on liner performance is investigated. The results show
that the application of reverse (negative) curvature during the loading cycles, representing
the straightener, has a significant influence on the wrinkle size of the liner developed
at the two critical generators and its rate of increase, compared with cyclic bending
patterns with non-negative curvature. Numerical results on imperfection sensitivity
are obtained, considering two types of imperfection of the liner pipe. In addition, the
structural performance of liners with different thickness is examined, and the results
show that there exists a minimum value of wall thickness, above which the liner does
not exhibit local buckling at the end of the cyclic loading history. The beneficial effect
of internal pressure on liner cyclic response is also verified, especially for thin-walled
liners, preventing the development of wrinkles. The effect of manufacturing process is
also examined, showing the superior structural performance of partially heated lined
pipes, with respect to fully heated lined pipes, and to lined pipes manufactured by purely
mechanical process.
Moreover, a three-dimensional numerical model of a mechanically bonded lined
pipe is also developed, simulating its structural response during the reeling installation
method. An installation failure/repair scenario is considered with five winding/
unwinding cycles, accounting for the straightening process. The cyclic deformation
of a lined pipe is presented, monitoring ovalization, local curvature and liner detachment
from the outer pipe during reeling. The influence of pipe contact with the reel and the
effects of back tension on the structural response of the lined pipe are examined. The
results indicate a residual curvature of the pipeline at the end of unspooling. Applying
different levels of back tension, the local curvature on the pipe decreases, affecting the
detachment of the liner pipe from the outer pipe. Numerical results on imperfection
sensitivity demonstrate the significant influence of geometric imperfections on liner
buckling. Comparison of the present results with those from a pure bending model
shows that a lower liner detachment rate is predicted by the present model. Furthermore,
the reeling performance of thicker liners is examined, showing the gradual reduction
of liner detachment with increasing wall thickness. Finally, the reeling process in the
presence of moderate levels of internal pressure is simulated, verifying its beneficial
effect on structural performance, preventing local buckling.