TPT May 2010

A rticle

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):

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] .

Notations

E

P P D P i

buckling propagation pressure

material tangent modulus SMYS minimum characteristic yielding stress of pipeline steel t nom nominal wall thickness of pipeline S minimum characteristic yielding stress of steel pipeline D 0 outer diameter of pipeline q cr critical buckling pressure

pipeline diameter

initial buckling pressure

t

pipeline thickness

L L S y

rings spacing

specimen length

minimum yielding pressure

n

number of waves

(1)

yielding stress of pipeline steel

s y

105

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