Hydrocarbon pipelines constructed in geohazards areas, are subjected to ground-induced actions, associated with the development of severe strains in the pipeline and constitute major threats for their structural integrity. In the course of pipeline design, calculation of those strains is necessary for safeguarding pipeline integrity, and the development of reliable analytical/numerical design tools that account for soil-pipe interaction is required. In the present paper, soil-pipe interaction models for buried steel pipelines subjected to severe ground-induced actions are presented. First, two numerical methodologies, (simplified and rigorous) and one analytical are presented and compared, followed by an experimental verification; transversal soil-pipe interaction is examined through full-scale experimental testing, and comparisons of numerical simulations with rigorous finite element models are reported. Furthermore, the rigorous model is compared with the results from a special-purpose full-scale “landslide/fault” experimental test in order to examine the soil-pipe interaction in a complex loading conditions. Finally, the verified rigorous model is compared with both the simplified models and the analytical methodology.

Among NDE techniques for monitoring damages on pipelines in service, the Acoustic Emission (AE) technique is effective to check initiation and propagation of critical defects under stress and exposed to aggressive environment. To investigate both reliability and applicability of the AE technique to pipelines used in Oil&Gas industry, different full scale tests have been designed and performed as significant items. A pilot line (48″ diameter and length 194m), formed from pipes made of both steels grade API 5L X80 and X100, has been buried and monitored for about 16 months by using AE technique. External defects (dent & gouge type) have been executed on each pipe. A pressure cycling test has been provided giving pressure in the range 135–150 bar for simulating gas transport operation over many years. By means of AE technique the source localization of defects and correlation between the damage growing and the acoustic transient energy have been analyzed. The growth in the crack areas has been also periodically controlled using NDT inspections (US and magnetic particles). The results provide good perspective for using AE technique as advanced methodology to improve the reliability of pipeline.

The present work examines the behavior of pipe elbows subjected to strong cyclic in-plane bending loading in the presence of internal pressure. In the first part of this work the experimental procedure is presented in detail. The tests are conducted in a constant amplitude displacement-controlled mode resulting to failures in the low-cycle fatigue range. The overall behavior of each tested specimen, as well as the evolution and concentration of local strains are monitored throughout the testing procedure. Different internal pressure levels are used in order to examine their effect on the fatigue life of the specimens. The above experimental investigation is supported by rigorous finite element analysis. Using detailed dimensional measurements and material testing obtained prior to specimen testing, detailed numerical models are developed to simulate the conducted experiments. An advanced cyclic plasticity material model is employed for the simulation of the tests. Emphasis is given on the local strain development at the critical part of the elbow where cracking occurs. Finally, the results of the present investigation are compared with available design provisions in terms of both ultimate capacity and low-cycle fatigue.

Temporary ground deformations produced by strong seismic activity can result in severe cyclic loading applied to piping, fittings and components such as flanges, elbows, tee joints etc. The integrity of the piping system in such condition is of critical importance for the safety of petro-chemical plants or refineries. Among various reasons of failures under earthquakes, the accumulation of plastic strains due to cyclic bending loading of pressurized piping sections containing bolted flanged joints, have to be carefully considered. This paper reports the results of the experimental full scale tests performed within the RFCS INDUSE Project [1] on PN40 and PN63 piping sections containing bolted flanged joints subjected to monotonic and cyclic bending load, in presence of internal pressure. On the basis of the experimental results, a FE model adopting Lemaitre-Chaboche nonlinear kinematic hardening rule for the pipe material has been developed, allowing to extend the results of the tests by performing a study on the main parameters affecting resistance of the joint.

Tenaris and Centro Sviluppo Materiali (CSM) are carrying out a Joint Industrial Project aimed at developing heavy wall line pipes. The suitability for very severe applications, involving high service pressures and temperatures, the latter causing large strain fluctuations, in presence of an aggressive sour environment, is analyzed both theoretically and experimentally, including small and full pipe models and tests. The full project program aims at developing a new generation heavy wall product, supported by a comprehensive laboratory analysis of the material response under severe mechanical loading in aggressive environment and a full scale testing program, including both pipe and girth weld. Both investigations are mainly addressed to basic understanding of impact on design criteria of interaction between severe loading and aggressive environment. Four papers, [2], [3], [4] and [5], have been already presented, in previous OMAE conferences, on this project. The present paper focusses on the full scale testing performed on strings of pipeline consisting of different pieces of pipe. In particular, two full scale testing have been performed applying the severe straining sequence defined as extreme in term of resistance against ratcheting, also involving plastic straining, as deduced in a previous work inside the project. The loading sequence was applied in global strain control, averaged on the whole string length, but necessarily the local distributions of strain differs in the three pieces of pipe. Analytical considerations are done about the expected straining behaviour and experimental results confirmed the theoretical considerations. The high strain hardening capability of the X65 steel pipes, metallurgically designed for strain based applications, guaranteed a good recovery of any non-homogeneity in straining, both during cycling that in larger axial deformation of the string. During severe cycling in elastic-plastic regime in presence of pipe internal pressure, the strings demonstrated good resistance to ratcheting. In fact, due to the material capability to redistribute the cycling strains along the whole sample length, any section experienced limited ratcheting with unreversed circumferential expansion, during cycling, well within limits of tolerability. It is worth noting that, even in presence of severe cycling conditions, both on-shore type girth welds (SAW for double joint) and off-shore type (GMAW in Narrow Groove Bevel Preparation) preserved their integrity with no cracking or other damage.

The Acoustic Emission (AE) technique allows taking under control the damage as superficial flaws (S-Flaws) occurred during service operation of remarkable zones of steel components, monitoring the initiation and the propagation of critical defects, submitted to static or variable stresses and aggressive environment exposure. In the framework of the eni gas&power research project oriented to the development of a “AE methodology” for monitoring critical sections of gas transmission pipelines, a study has been carried out jointly with Centro Sviluppo Materiali (CSM), aimed to investigate reliability and applicability of the AE technique to steels used in the Oil&Gas industry. Steel grades API 5L X65, X80 and X100, representative of traditional and new gas pipelines, have been selected. The project was scheduled investigating the potential of these steels to release elastic waves generated by sources of damage related to ductile or brittle fracture mechanisms. Hydraulic tests until failure were carried out on single pipes using steel grade API 5L X65 for monitoring the growing of the damage on the tip of artificial surface notches (S-flaw), machined on the wall thickness [1]. Water was used as internal fluid and temperature effect was considered as well. The capability of the AE technique to discriminate ductile and brittle fractures, the essential information to approach an integrity assessment procedure, was achieved. This paper, respect to the previous ones [1], extends the AE methodology for monitoring pipelines supplying both oil and gas. In fact a third burst test was carried out filling the pipe by air and the results on this item are presented.

Tenaris and Centro Sviluppo Materiali (CSM) launched a Joint Industrial Project aimed at developing heavy wall line pipes. The suitability for very severe applications, involving high service pressures and temperatures, the latter causing large strain fluctuations, in presence of an aggressive sour environment, is analyzed both theoretically and experimentally, including small and full scale pipe models. The full project program aims at developing a new generation heavy wall product, supported by: comprehensive laboratory analysis of the material response under severe mechanical loading in aggressive environment; and full scale testing program, including both pipe and girth weld. Both investigations are mainly addressed to the basic understanding of impact on design criteria from interaction between severe loading and aggressive environment. Three papers have been already presented, in previous OMAE conferences, on this project. In the present paper, main outcomes of laboratory testing activities of the above program on girth welds for double jointing (fill passes by Submerged Arc Welding) are reported. A fitted for purpose special testing program, including mechanical and SSC laboratory scale testing, has been executed. Full thickness longitudinal specimens were extracted crossing girth weld to apply severe strain cycling. The strain cycling has been defined as extreme in terms of resistance against ratcheting for the pressurized pipeline, as deduced in a work reported in one of the previously mentioned papers. The girth welds exhibited very satisfactory performance during severe cyclic straining. Furthermore, mechanical and stress-corrosion properties of the As-Weld girth joint have been compared with the corresponding properties after severe straining and ageing. This comparison highlighted high level of mechanical and SSC resistance, even after the application of severe straining and ageing.

Tenaris and Centro Sviluppo Materiali (CSM) launched a Joint Industrial Project aimed at developing heavy wall line pipes. The suitability for very severe applications, involving high service pressures and temperatures, the latter causing large strain fluctuations, in presence of an aggressive sour environment, is analyzed both theoretically and experimentally, including small and full pipe models. The full project program aims at developing a new generation heavy wall product, supported by: a comprehensive laboratory analysis of the material response under severe mechanical loading in aggressive environment; and full scale testing program, including both pipe and girth weld. Both investigations are mainly addressed to basic understanding of impact on design criteria from interaction between severe loading and aggressive environment. Two papers have been already presented on this project, [2] and [3]. The present paper deals with the study, carried out in cooperation with Saipem Energy Services, aimed at setting up a tool for the prediction of ratcheting extent for the pipeline in pressure subjected to axial cyclic, even plastic, straining. In such conditions, ratcheting may develop in the circumferential direction, as a consequence of both material cyclic performance and bi-axial plastic flow. So, detailed characterization of material is required, as well as calibration of plastic performance parameters, particularly in relation to relevant modeling. The final objective of the study is to establish a threshold for the plastic strain development at peak load, beyond which circumferential ratcheting may develop. A numerical model was set up, on-purpose developed and implemented on commercial software, where reverse yielding is modeled by kinematic hardening referring to Von-Mises yield criterion. Use of relevant parameters describing/approximating the actual material response has been made, based on laboratory Multi Plastic Straining Cycling (MPSC) of pipe full thickness samples. Full scale testing of pressurized X65, 10 3/4 ″ OD × 46 mm WT linepipe has been performed including plastic axial and cyclic straining. A huge measurement campaign allowed to establish the relevant parameters that characterize the response from numerical modeling, facilitating the validation of the set up by comparing the actual ratcheting exhibited by the heavy wall pipe with predictions obtained by the model. Limits of current tools for numerical modeling are also shown, with some degree of dependence on applied straining sequence. Possible paths of numerical modeling improvement are then envisaged.

High strength linepipe steels are being considered for several upcoming projects due to their potential to offer substantial savings through reduced wall thickness, particularly for high pressure designs. It is well known that mechanical damage from excavation activities is one of the most common causes of pipeline failure, however, little test data are available for higher grade steels. This paper presents the results of simulated excavator impacts on X120 pipe, and compares the results to those obtained on lower grade materials and to those predicted by computer simulations.