Gianluca Mannucci

Techniques for Fracture Toughness Testing of Offshore Pipelines

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.Copyright

Full History Burst Test Through Finite Element Analysis

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.Copyright

Numerical Simulation Analysis of High Strength UOE Steel Pipes

Large diameter longitudinally welded linepipes have to fulfil increasing technical requirements in order to guarantee best performance during construction and service. The increase in natural gas demand in European, North American, South American and Asian countries, foreseen for the immediate future, necessitates the development of cost effective transportation solutions to economically exploit gas fields located in remote area. A competitive option of gas to market is represented by the use of high-pressure natural gas transmission pipelines. In particular, for natural gas transportation over long distances, the use of high grade steel (X80, X100 or even higher) large diameter (36″ to 56″ of outer diameter), gas pipelines is found to be very attractive and economical. With respect to SAW pipes attention is focused on seam weld consumables and forming tools. In particular, forming tools must be designed in order to manage the large spring back effect that high grade plates, such as those for X100 pipes, exhibits when the pipes go from the U-press to the O-press. The objective of this paper is to present the evaluation of X100 pipes inside the UOE process from TenarisConfab mill with a mathematic modeling to get the best parameters. The X100 production process has been analyzed via Finite Element Model to evaluate goodness of tools geometry and pipe mill capability to produce higher grades pipes.Copyright

Ultra Heavy Wall Linepipe X65: Ratcheting in Severe Cyclic Straining

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.Copyright

Ultra Heavy Wall Linepipe X65: Material Performances for Severe Applications

Tenaris together with Centro Sviluppo Materiali launched a Joint Industrial Project aimed at developing heavy wall linepipes and evaluating their suitability for very severe applications possibly involving high service pressures and temperatures, large strains applied to the line, aggressive sour environment. The full project programme includes development of a the new generation of heavy wall products, laboratory scale evaluation of the material response when subjected to severe mechanical and environmental loading, evaluation of full component, pipe and girth weld, behaviour by means of full scale testing. Another technical publication in this conference (OMAE2009-79153) reports the activities of development of the new generation of heavy wall seamless pipes. In the present paper indeed, main outcomes of laboratory testing activities of the above programme on pipe material (grade X65, outer diameter 10 3/4″ , wall thickness 46 mm) are reported as far as pipe body material properties are concerned. A fitted for purpose special testing programme, including mechanical and SSC laboratory scale testing, has been executed. Full thickness longitudinal specimens were extracted from the pipe body to apply severe strain cycling (1% and 2% maximum strain for various numbers of cycles, up to 200 cycles). Material showed a very encouraging behaviour, exhibiting an important reserve of strength even after application of severe strain cycling. Both mechanical, tensile compressive and toughness, properties and stress corrosion properties resulted to be suitable for the envisaged applications. Furthermore the pipe material showed suitable mechanical and stress-corrosion properties even after the severe cycling as well as after severe cycling and subsequent ageing. The influence of different straining conditions was also investigated, showing no significant difference in material properties after strain–ageing, due to different straining histories.© 2009 ASME

Development of a Reliable Model for Evaluating the Ductile Fracture Propagation Resistance for High Grade Steel Pipelines

The recent experience on ductile fracture propagation control on gas pipelines has shown that the applicability of the Battelle Two Curve Method (based on Charpy-V energy) to high grade steel pipes from API5L-X80 to X120 (ISO3183-L555M to L830M) operated at very high hoop stress values (≥500 MPa) is highly questionable.The reduced geometry of the specimen, the intrinsic low value of ductility of very high strength steels, as low work-hardening and low value of the strain at maximum load are pointed out as the main causes of the mismatch.Starting from these assumptions a new EPRG (European Pipeline Research Group) project has been launched with the aim to develop, with reference to the ductile fracture propagation resistance, a suitable fracture parameter(s) with an associated laboratory methodology based on a simple sample which would be able to take into account the role of the ductility of the material on this specific fracture event.The present paper shows the approach adopted in this EPRG Project: an innovative approach based on “plastic damage model” which allows to describe the stable ductile crack propagation by means of stress-state parameters (named triaxiality and deviatoric parameters).Moreover the proposed “damage model” has been implemented inside a commercial finite element code and used to predict the fracture crack propagation behaviour of Single Edge Notch Bend (SENB) tests in terms of load-displacement diagram and residual plastic deformation.One of the main topics of this project was the application of this method to six selected grade steels (with grades in the range of API X65 – X100) many of them coming from experimental full scale burst tests. The comparisons between experimental results and numerical simulations are substantially good; besides the results confirm that Charpy-V specimens, during the fracture propagation, work in different “constraint” conditions with respect to pipe and that DWTT specimen is in the middle between the two. Finally the “damage model approach” seems also able to discriminate between low and high grade steels in terms of failure deformation at rupture. So it resulted very promising to quantify the role of both ductile of the steel and geometrical constraint of the specimen in the ductile fracture propagation event.Copyright

Ultra Heavy Wall Linepipe X65: Double Joint Girth Weld Performances for Severe Applications

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.© 2011 ASME

Modello coesivo per l’avanzamento di fratture mediante rilascio nodale di strutture discretizzate con elementi finiti

La simulazione numerica della propagazione di una frattura in MODO I, viaggiante ad elevata velocita in un acciaio a comportamento duttile e realizzata attraverso un modello coesivo che governa la distribuzione delle forze di rilascio nodale. Come noto, la ricerca di un valore di tensione all’apice non ha alcun senso nel caso elastico; infatti, la tensione puo essere valutata solo mediante fattori di campo. Nel caso elasto- plastico, incrudimento e softening di origine geometrica o legato al progressivo danneggiamento influiscono decisamente sull’andamento esponenziale del campo tensionale. E possibile comunque individuare un valore di riferimento, di entita finita, mediante estrapolazione delle tensioni elasto-plastiche nella zona di inizio softening della frattura. Tale grandezza puo essere presa come fattore di riferimento per il calcolo delle forze di rilascio coesive e quindi dell’energia dissipata. Nel lavoro viene discusso come determinare, dal campo di tensione elasto-plastico locale, il valore che governa la zona coesiva al variare del T-stress.

Ultra Heavy Wall Linepipe X65 for the Most Stringent Applications: Metallurgical Design and Industrial Development

Offshore industry has evolved to meet numerous challenges, e.g. deep water, high currents, high pressure and high temperature (HPHT), and sour reservoirs, facing deepwater exploration. The trend in flowline specifications for deepwater offshore fields is a consequence of complex oil-gas field conditions, such as HPHT and developments in design criteria (i.e. limit state design), welding and laying technologies. The technological evolution exhibits a trend towards an increasing wall thickness (WT) to provide sufficient resistance for the very high operating pressures. Furthermore, the pipelay operations, especially when linepipes are installed by means of the reel laying method, cause repeated plastic bending and straightening deformation cycles. These cyclic loads affect final material stress-strain properties. Reeling is currently applied to an increasing range of pipe geometries, being the present limit given by pipes with 16″ outer diameter (OD) and 30 mm wall thickness (WT). Other pipeline installation techniques, for example, J-lay, S-lay and steep S-lay also introduce plastic strain. All previous factors mentioned before and adding one more variable when exploring and producing in regions alike to the Artic where low temperatures implied several material challenges calls for high performance seamless pipes tailored to the specific application required by the oil and gas industry. In this paper, a description is given of the results of latest fundamental studies on high-strength heavy-wall steel materials manufactured by Q&T processing. This work is part of an on-going development program on high performance heavy wall seamless pipes for special applications such as HPHT, low temperature design criteria, sour requirements and studying the material under the strain based design criteria involving metallurgical modeling, laboratory tests, industrial trials and advanced metallographic examinations. The most recent findings and overall conclusions are reported hereafter, these results have been exploited by Tenaris to manufacture a limited production seamless pipes in a wall thickness range from 40 mm to 48 mm in steel grade X65 Sour Service.© 2009 ASME

Development of High Steel Grade Seamless X100 Weldable

The technological evolution in the offshore sector points out a trend towards an increasing use of high strength steels (grade 80ksi and higher), for both pipelines and risers. Pipeline specifications for deepwater offshore fields demand developments in design criteria (i.e. limit state design), welding, installation, and laying technologies. As long as the market goes deeper in offshore exploration and production, the market trend is to use heavier pipes in steel grade X65/X70 and some technological limits from several fronts are faced and more attractive becomes for the market to have a lighter high strength 100ksi seamless steel grade. The joint industrial program (JIP), termed “Seamless 100 ksi weldable” launched by Tenaris in order to address the complex design issues of high strength Q&T seamless pipes for ultra deep water applications has been finalized. The 100ksi steel grade has been achieved in two wall thickness 16 mm and 25 mm. The main results from both phase I devoted to the development and production of seamless pipes with minimum 100ksi and phase II devoted to evaluate the high strength seamless pipe weldability will be addressed in this paper. Main microstructural features promoting the best strength-toughness results obtained from phase I and the results from phase II, where the heat affected zone (HAZ) characterization made using stringent qualifying configuration such as API RP2Z and the promising results after qualifying the girth welds simulating a typical offshore operation and the Engineering Critical Assessment for installation will be addressed. The results from this development are of interest of all oil & gas companies who are facing as an output from the design project analysis the need to have high strength seamless pipes.© 2008 ASME