Philippe Darcis

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

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

Free-Span Flowlines Resistance to In-Service Fatigue Loading

Offshore petroleum fields frequently pass over areas with uneven seafloor. In such cases the pipeline may have free spans due to depressions crossing and are subjected to complex loading spectra. A major source for dynamic stresses in free span pipelines is vortex induced vibrations (VIV) caused by steady current since wave induced velocities and accelerations will decay with increasing water depth.The complex loading spectrum differs sensibly from the constant amplitude loading commonly adopted for qualification of the product (line pipe and its girth welds). Centro Sviluppo Materiali and Tenaris for some years are involved in the study of in-service variable amplitude fatigue loading of risers trough numerical calculations and comparison of the actual fatigue loading experienced by free span system with constant amplitude qualification typical loading. Two papers [1, 2] have been already presented in previous OMAE conferences. The present work reports a study dedicated to the free-span flow lines.The flow line analyzed is composed by OD 273.05 mm × WT 25.4 mm pipe lying on the uneven seabed. In particular the attention is focused on the analysis of VIV and its effect on fatigue life of the line. The Ormen Lange field, located at 120 km northwest of the Mid-Norway, was selected as reference scenario for the study.One of the most important factors influencing the pipeline response to the VIV is the free-span length. A sensitivity analysis about the influence of different parameters (free span length and fluid velocity) on system response and consequent fatigue damage has been performed.A case study has been selected among the cases considered in the sensitivity analysis, to produce the loading spectra to be considered in a laboratory fatigue testing campaign on strip specimens.The fatigue performance of these samples has been compared to analogous samples subjected to constant and variable amplitude loading available from previous works [1, 2] on riser systems (Steel catenary riser and Hybrid riser).Copyright

Heavy Wall Bends for Sour and Arctic-Alike Environment

During last decade, customers’ requirements of Line Pipes and accessories became more and more stringent. This process is led by the exploitation of fields with more severe conditions of pressure, environment and temperature. For this reason, heavy wall products (both straight pipes and bends) need to be developed.A development program was carried out in order to satisfy more stringent requirements and higher wall thickness. The metallurgical approach to steel design aimed to improve the combination of strength and toughness and increase the control of hardness after quenching and tempering, preserving adequate weldability.Industrial trials were performed to manufacture 60 mm thick hot induction bends in grade X65. Characterization was carried out after various post bending heat treatments (quenching, tempering and post welding).The new low-C steel showed promising results. The full characterization of off-line Q&T bends at various locations (tangent length, transition zone, and bend body) confirmed the achievement of X65 grade, with hardness HV10 0.7 mm at −45 °C, and very good HIC and SSC resistance.Copyright

Hybrid Riser Resistance to In-Service Fatigue Loading

Riser systems are subjected to complex loading spectra. A wide range of amplitude loading are induced by naturally complex sea states. The complex loading spectrum differs sensibly from the constant amplitude loading commonly adopted for qualification of the product (riser pipe and its girth welds or threaded connections).The present paper reports numerical calculations and a comparison of the actual fatigue loading experienced by different riser systems, Steel Catenary Risers (SCR), Lazy Wave Riser (LWR), Hybrid Risers (HR). For the Hybrid riser system, a good fatigue resistance, due to the vessel and wave first motion decoupling, is obtained. On the other hand, the phenomenon of cross flow vibration induced by Vortex Induced Vibrations (VIV), could significantly affect the hybrid riser fatigue performance. In this case, a sensitivity analysis has been performed to evaluate the influence of different parameters, on the applied fatigue loading, like: riser tension, hydro diameter (i.e. external pipe diameter including coating), riser wall thickness, current velocity.The HR variable amplitude loading spectrum, derived from in-service conditions, referred to Gulf of Mexico scenario, has been calculated and applied in laboratory tests on girth welds of X65, 10.75″ OD, 25.4 mm WT riser. Furthermore, the fatigue performance of these tests has been compared to analogous tests on samples subjected to constant and variable amplitude loading available from previous work on SCR system[1].© 2014 ASME

Riser Resistance to In-Service Fatigue Loading

Riser systems are subjected to complex loading spectra induced by sea states. The complex loading spectrum differs sensibly from the constant amplitude loading commonly adopted for qualification of the product (riser pipe and its girth welds).The present work deals with numerical calculations of the actual loading spectrum experienced by a Steel Catenary Riser when adopted in different scenarios, Gulf of Mexico (GoM) and West of Africa (WoA). Sensitivity analyses have been performed to evaluate the influence on applied fatigue loading of different parameters, like: water depth, seabed stiffness, hang-off angle.The variable amplitude loading, derived from in-service conditions, has been applied to laboratory scale, full thickness specimens from X65 SCR girth welds. The fatigue performance of these samples has been compared to analogous samples subjected to constant amplitude loading. The samples subjected to in-service loading conditions exhibited a lower fatigue performance than in constant amplitude.To allow a comparison with common qualification procedure under constant amplitude, full scale testing of SCR girth welds have been performed on resonance machine by in-service loading spectra rearranged in constant amplitude blocks. Laboratory full thickness specimens have been adopted to compare the girth weld fatigue performance when subjected to the original cycle-by-cycle sequence and the in-blocks rearranged sequence.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