TPT May 2010

107 / 116

May 2010 105 ›
Article
Experimental and FEM investigation
on influence of ring stiffeners
on buckling behaviour of subsea
pipelines under hydrostatic pressure
H Showkati – Civil Eng. Department, Urmia University, Urmia, Iran
R Shahandeh – Civil Eng. Department, Sama organization (affiliated with Islamic Azad university), Khoy branch, Khoy, Iran
Abstract
Submarine pipeline is considered as a thin walled structure and,
therefore, buckling is an important consideration at design stage.
Initial buckling is created on pipeline because of problems such
as additional force on a point of pipeline, existing imperfections
on it or because of forces increasing and because of this buckling
propagation is started on the pipeline. These occurrences cause the
estimated strength of pipelines to decrease. For the prevention of
this problem the application of ring-stiffeners is a good solution. For
this purpose experimental and FEM analysis programs are arranged
for investigating the influences of ring stiffeners on buckling strength
of pipelines.
Pipelines are subjected to many forces such as hydrodynamic
and hydrostatic pressure, the dynamic effect of waves, free span
and so on. Because of the importance of hydrostatic pressure and
experimental limitations, this type of force is modelled and studied
in this paper. All models in experimental and FEM methods are
geometrically corresponded together. The modes of initial buckling,
buckling propagation and post-buckling of pipeline are evaluated
and compared in both methods. It was found in this research that
these quantities are highly influenced by attached light weight ring
stiffeners. Some new phenomena appeared with increasing the
number of rings such as torsion effects and lateral displacement
on pipeline. Keywords: pipeline, initial buckling, post-buckling, ring
stiffener.
Introduction
Pipelines include the most valuable of the oil and gas industries
and because of this, investigation of its characters its very
important. Important problems are included, pressure of initial
buckling, buckling propagation pressure and modes of collapsing.
Until now, many researchers studied these problems. According
to these studies most of these researchers and designers and
reputable standards suggested some relations for designing. Shell
Development Company lab studied about initial buckling pressure
and presented relation (1):
(1)
And some standards such as BSI, API, ABS, DNV [1, 2, 3, 4], presented
for buckling propagation following relations, respectively (2,3,4,5):
(2)
(3)
(4)
(5)
“Palmer” and "Martin”, according to their experimental and theoretical
studies, presented first theoretic relation for buckling propagation
pressure. “Mesloh” et al (1976), “Kyriakides” and “Babcock” (1981),
studied this problem experimentally and presented their relations [5, 6].
“Hutchinson” and “Charter” (1984) according to the principle of
virtual work studied pipeline’s elastic-plastic collapsing under
external pressure. “Kamalarasa” and “Calladine” (1988) developed
Palmer’s manner to a 3-dimensional model which had good
corresponding with experimental results [7]. “Murray” and “Zhou”
(1994) according to theory of shells investigated on local buckling
behaviour of pipelines under complex loads [8]. “Pasqualino” and
“Estefen” (2001) theoretically studied about buckling propagation
of pipelines [9]. “Kyriakides” and “Netto” (1999) experimentally and
theoretically investigated on dynamic propagating of buckling of
pipelines under external pressure [10, 11].
P P buckling propagation pressure
D pipeline diameter
P i initial buckling pressure
t pipeline thickness
L rings spacing
L specimen length
S y minimum yielding pressure
n number of waves
s y yielding stress of pipeline steel
E material tangent modulus
SMYS minimum characteristic yielding
stress of pipeline steel
tnom nominal wall thickness of pipeline
S minimum characteristic yielding
stress of steel pipeline
D0 outer diameter of pipeline
qcr critical buckling pressure
Notations