The need to evaluate the significance of flaws in welded pipelines for gas transportation requires the knowledge of the material resistance to ductile tearing. In particular, the fracture resistance of pipe girth welds should be evaluated since they may potentially be critical for structural integrity. Standard toughness Three Point Bending tests (SENB) are too conservative since they are more constrained than actual pipeline. In this case, the adoption of a reduced notch depth, which is considered to reproduce well actual stress-strain conditions at the crack tip of a weld flaw, increases critical toughness values when compared to standard specimen configuration. Alternative solutions may be applied, even if not yet included in toughness standards. In particular, the Single Edge Notch Tensile (SENT) test is a possible solution reducing conservatism. A matter of concern for toughness characterization of weld joint is also represented by the notch orientation, since the weld microstructure is inhomogeneous in nature. The L–R oriented specimen (notch at the pipe inner surface) typically shows CTOD values strongly lower than the ones of L–T oriented specimens (through thickness notch) for both weld metal and heat affected zone. All these issues are discussed within this paper, while an advanced approach is presented to determine the resistance curve by using a single SENT specimen with the compliance method for crack growth evaluation. A relationship between the specimen elastic compliance and actual crack growth was determined through Finite Element Analysis and a Fracture Mechanics model. Such a relationship is presented and compared to other solutions available in scientific literature.

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In order to increase the knowledge necessary for the utilisation of grade X100 steel pipes, and to consolidate preliminary indications regarding the safe level of toughness required to control the ductile fracture propagation event within X100 gas pipeline, an ECSC-Demonstration Project, (DemoPipe), partially sponsored by EPRG, has been performed (2001–2004) using TMCP X100 pipes with a diameter of 36”. The project examines the problems of building a new high grade steel on-shore gas pipeline, with special emphasis given to the issues of the field welding technologies and selection of consumables, girth weld defect tolerance, field cold bending, and the fracture propagation behaviour in a high-pressure natural gas pipeline. In order to achieve these stated aims, a dedicated programme of laboratory and full scale tests was included in the project. This paper presents a summary of some of the results obtained, together with a discussion regarding their applicability to future X100 pipelines.

The work deals with the development of a finite element code, named PICPRO (PIpe Crack PROpagation), for the analysis of ductile fracture propagation in buried gas pipelines. Driving force estimate is given in terms of CTOA and computed during simulations; its value is then compared with the material parameter CTOAc, inferred by small specimen tests, to evaluate the toughness of a given line pipe. Some relevant aspects are considered in the modelling with the aim to simulate the real phenomenon, namely ductile fracture mechanism, gas decompression behaviour and soil backfill constraint. The gas decompression law is calculated outside the finite element code by means of experimental data from full-scale burst tests coupled with classical shock tube solution. The validation is performed on the basis of full-scale propagation experiments, carried out on typical pipeline layouts, and includes verification of global plastic displacements and strains, CTOA values and soil-pipe interaction pressures.

Ductile Fracture propagation phenomena have been widely investigated by researchers in the last years, with particular regard to large metallic structures such as pressurized vessels or gas pipelines. A large number of burst tests have been carried out by Centro Sviluppo Materiali S.p.A. (C.S.M.) in the last decades to identify a set of significant parameters characterizing fracture propagation conditions; the aim is to foresee the behavior (speed and its derivatives) of longitudinal running cracks. The optimal choice of these parameters is strongly helped by appropriate use of Finite Element analysis. To this goal a Finite Element software has been developed, it allows the correct computing of some particular aspects of fracture propagation and the behavior of pressured real gases during decompression. In the present paper a pipeline burst test, carried out on a X100 grade pipeline, and all laboratory tests and data manipulations necessary to build up the whole procedure have been discussed. One of the main objectives is the setting of a procedure able to identify the fracture parameters, when a ductile propagation occurs, avoiding any scatter due to transient effects.