Pipeline Buckling

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Pipeline arrestor

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1 Copyright 2003 by ASMEProceedings of OMAE0322nd International Conference on Offshore Mechanics and Arctic EngineeringJune 8-13, 2003, Cancun, MexicoOMAE2003-37220BUCKLE PROPAGATION AND ITS ARREST: BUCKLE ARRESTOR DESIGNVERSUS NUMERICAL ANALYSES AND EXPERIMENTSEnrico TorsellettiSnamprogettiVia Toniolo 1, 61032 Fano, Italye-mail: enrico.torselletti@snamprogetti.eni.itRoberto BruschiSnamprogettiVia Toniolo 1, 61032 Fano, Italye-mail: roberto.bruschi@snamprogetti.eni.itFurio MarchesaniSnamprogettiVia Toniolo 1, 61032 Fano, Italye-mail: furio.marchesani@snamprogetti.eni.itLuigino VitaliSnamprogettiVia Toniolo 1, 61032 Fano, Italye-mail: luigino.vitali@snamprogetti.eni.itABSTRACTBuckle propagation under external pressure is a potentialhazard during offshore pipeline laying in deep waters. It isnormal design practice to install thicker pipe sections which, incase of buckle initiation and consequent propagation, can stopit so avoiding the lost of long pipe sections as well as threats tothe installation equipment and dedicated personnel.There is still a series of questions the designer needs toanswer when a new trunkline for very deep water applicationsis conceived:- What are the implications of the actual productiontechnology (U-ing, O-ing and Expansion or Compressione.g. UO, UOE and UOC) on the propagation and arrestcapacity of the line pipe,- How formulations for buckle arrestors design can be linkedto a safety objective as required in modern submarinepipeline applications.The answers influence any decision on thickness, length,material and spacing of buckle arrestors.This paper gives an overview of buckle propagation andarrest phenomena and proposes a new design equation,applicable for both short and long buckle arrestors, based onavailable literature information and independent numericalanalyses.Partial safety factors are recommended, based on acalibration process performed using structural reliabilitymethods. Calibration aimed at fulfilling the safety objectivesdefined in DNV Offshore Standards OS-F101 and OS-F201.INTRODUCTIONAn offshore pipeline installed in deep waters is oftencollapse-critical due to the ambient external pressure.Designing against collapse involves selecting the appropriatewall thickness for a given pipe diameter and line pipe material,as well as specifying appropriate geometric fabricationtolerances. Unfortunately incidental dents induced by impactingobjects, ovalisation induced by excessive bending duringinstallation, wall thickness reduction due incidental corrosionetc., may locally reduce the collapse strength of the pipeline.If the pipe is not sized against propagation when collapseor sectional ovalisation buckling occurs in the depths, thebuckle propagates and stops at a depth the required work forsectional plastification is larger than the one the externalpressure can do.The buckle propagation phenomenon can be considered tooccur in three phases (see Figure 1):- Buckle initiation,- Buckle propagation,- Buckle arrest at the arrestor or crossover of the arrestor.The buckle propagation pressure has been extensivelystudied in the last decades and design approaches have beendeveloped and experienced in a number of projects. Researchactivities, both experimental and analytical, have beendedicated to the development of the most suitable bucklearrestors shape for deep water applications: integral, external orinternal rings, spiral, etc. In this paper integral buckle arrestors(BA) are considered and analysed, as classified into two maincategories:- long arrestors are those for which the buckle crossoverthe arrestor after it has collapsed for its whole length, e.g.PIPE TOC2 Copyright 2003 by ASMEthe capacity to arrest propagation is ensured by suitablysized wall thickness, in accordance with the propagationpressure formula.- short arrestors, where the buckle crossover the arrestorthat remains integer in shape i.e. the arrestor capacity isensured by wall thickness far thicker than for longarrestors.As far as the transition between long and short bucklearrestor behaviour is considered, it can be affirmed that longarrestors are longer than about 4 to 6 pipe diameters. In designguidelines for offshore pipelines [1], there is no indication onhow to size short buckle arrestors while long arrestors arecovered. For both cases, it does not appear that a rationalcalibration of partial safety factors has been carried out.Figure 1 How Kyriakides describes the different load phasesto which a pipeline is subjected during a buckleinitiation, propagation and crossover [18].According to the limit state based approach drawn in [1]the buckle arrestor must be sized in order to fulfill the specifiedsafety targets. In particular, the failure probability of a bucklearrestor can be expressed as:(1)Stop F opagation F Initiation F F P P P P, Pr , , PF is the total failure probability that has to be comparedwith the specified target (see [1] for reference values to be usedfor offshore pipeline systems), PF,Initiation is the probability tohave a buckle, PF,Propagation is the probability that a given bucklewill propagate, PF,Stop is the probability that a given propagatingbuckle will crossover the buckle arrestor length, so continuingpropagation (the capacity of the buckle arrestor is exceeded).When pipes are sized to avoid propagation, the product ofthe probability of initiation by the probability of propagationgives the total failure probability (PF). For a pipeline systemwith buckle arrestors PF is given by the product of PF,initiation byPF,Stop, and PF,propagation is equal to 1.Scope of this paper is to introduce a new design formulaincluding partial safety factors that meet the safety objective ofDNV-OS-F101.PROPAGATION PRESSUREThe problem of propagating buckles was recognized in theearly 1970s [2]. Palmer and Martin made the first theoreticalanalysis in 1975 [3]. They recognized that the work done by theexternal pressure, as the buckle moves forward by unit distanceis mainly absorbed by plastic deformation associated to thechange in shape of unit length of pipe, from its original circularform to final "dog bone" conformation. Assuming a simplemechanism of plane strain collapse for the ring, involving fourconcentrated "plastic hinges" (Figure 2), the following energybalance equation for unit length of pipe, was defined:(2) M2 = A p pe Here pe is the external pressure, A is the change incross-sectional area and Mp is the full plastic moment per unitlength of the pipe wall.Figure 2 Sequence of collapse configurations of a long tubeunder external pressure.Being the above formulas considered as lower bound,researchers tried to introduce new buckle propagation equationsthat give a better prediction of the critical propagation pressurethan the formula from Palmer, see Kamalarasa and Calladine in1987 [4].While it has never been contested that the shortcomings ofequation (2) lies in its neglect of both surface stretching andstrain hardening, most researchers have chosen to overlook thestretching effects and concentrate entirely on "ring-bending"investigations.The concentrated plastic hinges, which are animportant feature of the analysis of Palmer and Martin, are onlylegitimate in the context of a perfectly plastic, non-hardeningmaterial. In the presence of strain hardening, we must expect tofind hinges of finite length which can travel around theUndeformed pipeBuckle initiationBuckle propagationBuckle arrestorengagementBuckle arrestorcrossover3 Copyright 2003 by ASMEcircumference of the ring. Several attempts to improve theanalysis of Palmer and Martin, by including strain hardening inthe study of the irreversible circumferential bending of rings,have been made. Wierzbicki and Bhat [5], Steel and Spence [6],Croll [7] and Kyriakides et al. [8] have analysed the bending ofstrain-hardening rings using different schemes, and proposeddifferent expressions for the critical pressure.In recent years, several tests were performed to evaluatethe formulation of the propagation pressure design format, andare reported in literature (Kyriakides et al. [2], Langner et al.[9] and Estefen et al. [10]).In 1996 a tentative reliability based calibration of designequations available in literature, for the evaluation of thepropagation pressure, was performed in the framework of theSUPERB project [11]. The equation reported in DNV 96 [12]reads:(3)((,\,,(jDt SMYS 26 =Pestnom2.5c p,This equation is based on a conditional target failureprobability PF,Stop of 10-2 per pipe joint.02040608010012010 15 20 25 30 35D/tPp * 1000 / SMYSExperiments X65Experiments X42Experiments Estefen [10]DNV96 [12], SUPERB [11]BS80110 [13]Langner [9]Battelle [14]Palmer [3]Kyriakides [15]Kyriakides [16]Kyriakides [16]AGA FowlerFigure 3 Propagation pressure formulas versus D/t andexperimental values [11].Figure 3 compares the propagation pressure calculated withequation (3) with the experiments by Kyriakides [2] and byEstefen et al. [10], respectively. Both BS8010 [13] and Langnerapproach [9] are considered conservative, while Battelle [14]and Kyriakides [2, 15, 16] are considered good mean valuepredictors. The formulation from DNV96 was a step forwardin terms of reducing excessive conservatism, fulfilling in a waypre-defined safety requirements.DNV-OS-F101 modified t