Hector Quintanilla

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 Scale Fatigue Performance of Pre-Strained SCR Girth Welds: Comparison of Different Reeling Frames

The current work aims to point out the influence of plastic strain history, due to reel-lay installation, on the fatigue resistance of welded SMLS (seamless) steel pipes used for fabrication of Steel Catenary Risers (SCRs) for oil and gas development. A C-Mn steel X65 pipe 10.75″ (273.1 mm) outside diameter (OD) and 25.4 mm wall thickness (WT) was chosen for this program. The Welding Procedure designed for girth welds manufacturing involved the use of Lincoln STT-GMAW™ (Surface Tension Transfer–Gas Metal Arc Welding) process for the root pass and SAW (Submerged Arc Welding) process with twin wire configuration for the fill and cap passes. This welding procedure presents a special post-weld finishing treatment, which consists in flapping the inner and outer weld overfills to produce a flush profile between weld metal and outer/inner pipe surfaces. The experimental approach was focused on quantifying the effect of accumulated plastic deformation using two different reeling frames simulating the same laying vessel: the Technip’s Apache. In this program, two reeling trials were performed at Heriot Watt University, Edinburgh, U.K., and two other trials at Stress Engineering Services, Houston, U.S.A. Then, the strained specimens were full scale fatigue tested at TenarisTamsa R&D facilities. Those results have been compared with fatigue results obtained on unstrained specimens. Post-tests fractographic investigations were systematically performed on all samples to identify the causes for fatigue initiation. The results were statistically analyzed to determine which standard fatigue design curves best represent the measured S-N fatigue endurance. Finally, the results were also compared with the available literature.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

Fatigue Qualification of Heavy Wall Line Pipe and Girth Weld for High Pressure Applications

As the offshore oil & gas industry moves into deeper waters, more and more offshore projects, specifically the subsea, riser and flowline designs, rely on heavy wall line pipe materials. These pipe materials must be capable of operating in stringent working conditions such as high pressures, high temperatures, large deformations, fatigue loading, sour environments, etc.Within this context, ExxonMobil Development Company and Tenaris have jointly conducted a detailed technical assessment of the mechanical and fatigue performance of the newly developed heavy wall X65 line pipe (LP) developed by Tenaris.The main goal of such project is to evaluate and demonstrate via full-scale testing the fatigue performance of this new generation of heavy wall line pipe materials and the associated girth weld also recently developed by Tenaris. Although conclusive fatigue performance results at ID are not included in the present paper, the comprehensive test/qualification plan also includes a fatigue performance comparison at ID and OD.The present work clearly demonstrates weldability of this Heavy Wall X65 (273 mm OD × 46 mm WT) line pipe. A narrow-groove bevel welding procedure utilizing the STT® process for the root pass and single torch GMAW process for hot-pass, fill, and cap has been successfully developed. Four full scale fatigue tests were also successfully conducted using a resonant fatigue test machine. The presented fatigue results help demonstrate a realistic level of fatigue performance achievable with this thick wall LP/girth weld technology which will help facilitate assessment of riser/flowline design feasibility in ultra-deep water applications and/or high pressures fields.Copyright

Assessment of Specimen Geometry for All-Weld Metal Tensile Test in Narrow Groove Welding

The development of adequate welding procedures for the offshore pipeline industry has required improvement of weld designs due to stringent field operations, e.g., deep and ultra-deep water application, strain-based design, weld overmatching, low temperature toughness, fatigue resistance, etc. Gas-metal arc welding (GMAW) procedures have required the optimization of the groove design, from conventional (wide opening) to narrow-groove (NG) solutions. The assessment of the weld metal tensile properties, based on international codes and specifications, is necessary to deliver full compliance and reliability of requirements for the girth weld performance. However; complexities arise in order to establish weld metal testing procedures, when narrow-groove design is used for welding a heavy-wall (HW) seamless pipe material. The latter requires the assessment of specimen geometry in HW line pipe in order to produce reliable measurements of all-weld metal tensile properties, which can be representative of the weld metal performance.The objective of the present work is to present the assessment in the differences on specimen geometry for all-weld metal tensile test (AWMTT). The work includes the welding of a HW seamless pipe material; 273.1 mm OD × 46 mm WT, X65 steel grade using STT®+GMAW, single-torch, as welding process, with narrow J-bevel, in 2° angle opening. Experiments considered the use of an ER80S-G solid wire as filler material for behavior analysis. The assessment methodology consisted in the machining of three different specimen geometries; strip, round and cross weld specimens were prepared for tensile tests. Efforts were dedicated to extracted tensile specimens avoiding heat affected zone areas in the specimen. Assessment of obtained results is based on tensile tests as well as the analysis of stress-strain data for the yielding behavior regarding the specimen geometry.Copyright

Girth Welding Development of Heavy Wall Materials for Severe Applications

The needs for oil and gas exploration in deep water (DW) and ultra-deep water (UDW) severe environments involve critical requirements of heavy wall materials. Offshore DW and UDW impose demanding service conditions of sour environment, mechanical properties, fatigue performance, gas service, high pressure and wide temperature ranges not only for heavy wall seamless line pipe materials but also for the girth weld performance. Thus, the development of heavy wall materials for severe applications is essential for DW and UDW, where complex material requirements are sought. Additionally the girth welding of heavy wall materials has imposed particularities typical of large wall thickness materials’ welding. The latter requires the development of particular solutions for pre-production and GMAW narrow groove offshore welding procedures.The present work presents the development of two welding processes of a heavy wall seamless pipe material: 273.1 mm OD × 46 mm WT, X65 steel grade. Pre-production welding involves STT®+SAW using a dual slope V-bevel, filler material for root processing was an AWS ER80S-G, while welding deposition for fill and cap passes was made using twin-wire technique, with two different electrodes (ENi1K and EG AWS designations), in combination with a neutral flux. On the other hand, narrow groove welding procedure considered a J-bevel, 3° angle, applying STT®+GMAW; filler material for GMAW was as well an ER80S-G AWS designation. Both welding procedures are aimed to deliver adequate mechanical properties to meet sour-service requirements (<250HV10), weld metal overmatching (120 MPa minimum) and toughness (CVN 45JAVE/38JIND) at low temperature. Mechanical characterization included hardness Vickers measurements using a 10 kgf load, tensile tests in all-weld metal and transverse impact fracture Charpy V-notch tests and CTOD tests.Copyright

In-Service Fatigue Performance of SCR Girth Welds Installed by Reeling

Interest arises on verifying the SCR girth welds fatigue response to a more representative loading spectrum of the actual in-service conditions and after reel-lay deformation. It is important to determine if the actual riser component’s qualification, without pre-straining and under constant amplitude loading, evidences discrepancies with in-service conditions, in terms of fatigue strength.This situation has motivated the full scale S-N fatigue performance evaluation of SCR girth welds under constant and variable amplitude loading, and after reel-lay simulations. A CMn steel X65 pipe 10.75” outside diameter (OD) and 25.4 mm wall thickness (WT) was chosen for this program. The Welding Procedure developed for girth welds manufacturing involved the use of the Lincoln STT® process for the root pass and the GMAW process for the fill and cap passes. Reeling trials were performed at Stress Engineering Services, Houston, U.S.A.. A dedicated commercial software was used to simulate the variable amplitude loading spectrum, which is representative of a SCR Touch Down Point (TDP) in West of Africa at a water depth of 1,200 m (3937 ft) and a FPSO as production platform.The experimental approach was focused on estimating the damage introduced by reeling and by loading cycles of various magnitudes in the riser service time history. Results of strained and unstrained specimens, tested at constant and variable amplitude, have been compared, and the cumulative damage rule typically used by Riser fatigue designers has been evaluated (i.e. Miner’s linear cumulative damage rule). Systematic fractographic investigations were performed on all the samples after testing to identify their fatigue failure initiation causes.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

FATIGUE PERFORMANCE OF SMLS SCR GIRTH WELDS - COMPARISON OF PREFABRICATION-TYPE WPS

The objective of the present research work is focused on evaluating the fatigue performance of different prefabrication welding procedures and determining the best compromise between manufacturing specifications, productivity and fatigue strength. Therefore, a large full scale fatigue campaign was launched at Tenaris to comply with this objective. SMLS SCR pipe (grade X65, outer diameter 10 ¾’ ’, wall thickness 25.4 mm) was selected and manufactured according with the current most challenging offshore specifications and girth joints representative of prefabrication welding were manufactured in 1G position. Two different bevel geometries, two different welding techniques for the root pass and two different welding techniques for fill and cap passes were studied and compared. Finally, a post weld finishing technique has been implemented, aiming to improve the fatigue strength by removal of the weld root and cap reinforcements. Misalignment measurements, with stress concentration factor calculations, and post-tests fractographic investigations have been systematically performed on all the samples after testing. This activity was of paramount importance in determining the causes for fatigue initiation.