Analysis of Welding Technology about Bimetallic Clad Pipelines

Butt girth welding was a knotty problem for future application of bimetallic clad pipelines. At present, there were two kinds of problems: 1) To decide whether to use a variety of alloy welding procedure or to use corrosion resistant alloy full welding procedure; 2) After selecting the procedure, what kind of welding material should be equipped. In view of the above problems, taking 316L SS or 2205 DSS clad pipe as an example, welding process design and experimental analysis were conduted in this paper. Analysis of welding process from theory, standard and practice pointed out the control of welding hardness under different welding materials and procedure and directional suggestions of welding for bimetallic clad pipelines were provided. Futher the hardness distribution and CVN absorbed Energy test results of different welding processes showed welding quality could be guaranteed only when ENiCrMo-3 welding material was chosen for the whole weld.

Qualification of Mechanically Lined Pipe MLP with High Frequency Welded HFW Host Pipe for Subsea Applications with Reeling Installation

Known as HFW-MLP, Mechanically Lined Pipe (MLP) with High Frequency Welded (HFW) host pipes are potentially the most cost-effective bi-metallic pipes for subsea pipelines when corrosion resistant alloys (CRA) are required. However, HFW-MLP has a very limited track record for subsea applications. This paper details a recent programme to qualify MLP with HFW host pipes. The qualification programme has been performed in accordance with DNVGL-ST-F101 (2017) and internal supplementary requirements for reelability and weldability. It considers material testing of HFW-MLP at each manufacturing stage and product qualification including full-scale reeling simulation, anti-corrosion coating simulation and girth welding. Qualification is supplemented with a detailed evaluation of the manufacturing process HFW-MLP is compared to traditional MLP which is supplied with seamless carbon steel as the host pipe or backing steel. This novel product lowers the supply cost, reducing the Capital Expenditure (CAPEX) for subsea pipeline projects. Detailed evaluation of mechanical test results and dimensional inspections using laser profiling assess the impact of the HFW host pipe longitudinal seam weld and conclude that there is no detrimental effect on the performance of the completed MLP. A comprehensive review of full-scale reeling simulations, coating simulations and welding trials is completed, with the conclusion that an HFW host pipe does not adversely affect the ability of the MLP product to be girth welded and to withstand the plastic deformation exerted upon the product during reel-lay installation. It is concluded that HFW-MLP is qualified for subsea pipeline static applications via the reel-lay, S-Lay or J-lay installation methods. The qualification of HFW-MLP provides a more cost-effective solution for the development of corrosive subsea fields by reducing overall product supply and installation costs.

J and CTOD Based Tensile Strain Capacity Prediction for Pipelines with a Surface Crack in Girth Weld

Long-distance pipelines for the transport of oil and natural gas to onshore facilities are mainly fabricated by girth welding, which has been considered as a weak location for cracking. Pipeline rupture due to crack initiation and propagation in girth welding is one of the main issues of structural integrity for a stable supply of energy resources. The crack assessment should be performed by comparing the crack driving force with fracture toughness to determine the critical point of fracture. For this reason, accurate estimation of the crack driving force for pipelines with a crack in girth weld is highly required. This paper gives the newly developed J-integral and crack-tip opening displacement (CTOD) estimation in a strain-based scheme for pipelines with an internal surface crack in girth weld under axial displacement and internal pressure. For this purpose, parametric finite element analyses have been systematically carried out for a set of pipe thicknesses, crack sizes, strain hardening, overmatch and internal pressure conditions. Using the proposed solutions, tensile strain capacities (TSCs) were quantified by performing crack assessment based on crack initiation and ductile instability and compared with TSCs from curved wide plate tests to confirm their validity.

Optimisation of process parameters for M.A.G welding of API X70m material to predict tensile strength using Taguchi method

Girth welded replica of API X70M material have been produced on NG-GMAW welding technique. The particular area of interest is to develop suitable girth welding process parameter using NGGMAW. The major aim of the work was to replicate welds having tensile strength between 650 and 680 MPa. Design of Experiment (DoE) method by Taguchi design, using some selected welding processes was adopted. Two process parameters (factors) – arc voltage and wire feed rate, (the variables), and three levels were used. The resultant joint property on tensile strength of X70M pipeline was examined. The targeted mechanical property was achieved by selecting the best process parameters. Their effects on ultimate tensile strength – UTS was analysed using statistical technique – analysis of variance - ANOVA and Signal to Noise - S/N ratio with ‘thebigger-the–better’ value. Validation was done using MIDAS NFX (an FEA) mechanical engineering software. In conclusion, process parameters that affects or influences the girth welded properties of API X70M under field conditions were identified. Guidance for the specifications and selection of processes that could be used in field-welding for optimum performance has been recommended. Keywords: Optimization, Girth-Weld, Process Parameters, Tensile Strength, NG-GMAW

Influence of Steel Chemistry and Field Girth Welding Procedure on Performance of API X70 Line Pipe Steels

Owing to recent concerns regarding pipeline field girth weld performance, particularly heat affected zone (HAZ) softening and toughness, EVRAZ North America has initiated a research program to evaluate the response of API grade line pipe to the current field girth welding practices. In particular, this study aims to elucidate the role of steel alloy design as well as the welding procedure on field girth weld and HAZ properties. This understanding is critical to balance the detrimental effects of HAZ softening on the overall joint strength against factors affecting HAZ toughness. A selection of several different steels with different levels of alloying elements, Ceq and Pcm have been subjected to welding trials to assess the effects of chemistry on joint performance. Furthermore, an analysis on the effect of welding process parameters on the joint properties has been made. The welds, fabricated via a manual shielded metal arc welding (M-SMAW) process, were evaluated in terms of toughness, local vs global strain distribution during tensile testing using digital image correlation (DIC) technique, and hardness contour mapping of the weld and HAZ regions. The results explicitly show that the extent of HAZ softening decreased as the amount of Mo, Mn, Ti/N and Ceq increased. However, this alloying addition resulted in a detrimental effect on the HAZ toughness, particularly towards the cap and fill passes. The HAZ softening increased as the inter-pass temperature and the welding heat input increased. In addition, the strain analysis confirmed the weld passes towards the root/hot passes are more prone to HAZ softening compared with the upper cap and fill passes.

Austenite Grain Size Model for the Coarse Grain Heat Affected Zone in Line Pipe Steels

Pipelines are the safest and most cost-effective method of oil and gas transportation to storage and processing facilities. Large diameter welded pipes fabricated by submerged arc welding (SAW) are the preferred product in many cases for pipeline construction. Furthermore, pipelines are constructed by welding segments of pipe, typically by single or dual torch Gas Metal Arc Welding (GMAW). During welding, both during pipe fabrication and girth welding, the Heat Affected Zone (HAZ) experiences rapid thermal cycles with peak temperatures up to the melting temperature of the base metal. Controlling the microstructure evolution in the HAZ during welding of line pipe steels is critical to ensure that these products meet the Charpy impact testing and CTOD requirements imposed by clients and specifications. In particular, the Coarse Grain Heat Affected Zone (CGHAZ) is of concern. Here, austenite grain growth occurs readily due to the combination of high temperature and precipitate dissolution. Controlling the CGHAZ austenite grain size is critical to obtain final microstructures with acceptable impact properties. In this study, austenite grain growth has been measured and modeled for thermal conditions relevant for the CGHAZ in 27 steels, including industrial as well as laboratory steels with systematic variations of alloying element content. Austenite grain size was measured using a Laser Ultrasonics for Metallurgy (LUMet) sensor attached to a Gleeble 3500 Thermomechanical Simulator, which enables high-throughput in-situ monitoring of austenite grain growth. A classical grain growth model has been developed based on a standard test. The grain growth kinetics are described by combining curvature driven grain growth with pinning due to TiN precipitates. A phenomenological relationship has been developed for the grain boundary mobility that decreases with C, Nb and Mo alloying which is consistent with their expected grain boundary segregation. The pinning parameter is rationalized in terms of volume fraction and size of TiN particles. The proposed model has been validated for CGHAZ heat treatment cycles including an industrial welding trial. The results of this study provide a model to predict the austenite grain size in the CGHAZ as a function of steel chemistries and heat treatment paths, i.e. welding parameters. Austenite grain size maps have been constructed as a function of peak local temperature and line pipe steel chemistry. The model can be used both for steel chemistry design and for optimizing welding of steels with known chemical composition to minimize the CGHAZ austenite grain size both during pipe fabrication and girth welding.

Surface Crack Growth in Offshore Metallic Pipes under Cyclic Loads: A Literature Review

The surface crack, also known as the partly through-thickness crack, is a serious threat to the structural integrity of offshore metallic pipes. In this paper, we review the research progress in regard to surface crack growth in metallic pipes subjected to cyclic loads from the fracture mechanics perspective. The purpose is to provide state-of-the-art investigations, as well as indicate the remaining challenges. First, the available studies on surface cracked metallic pipes are overviewed from experimental, numerical, and analytical perspectives, respectively. Then, we analyse state-of-the-art research and discuss the insufficiencies of the available literature from different perspectives, such as surface cracks and pipe configurations, environmental influential parameters, the girth welding effect, and numerical and analytical evaluation methods. Building on these surveys and discussions, we identify various remaining challenges and possible further research topics that are anticipated to be of significant value both for academics and practitioners.