This paper presents a new mechanism, observed directly for the first time, to explain low carbide fractions in Ni-WC overlays produced with GMAW. In this loss mechanism, a significant amount of powder loss is a consequence of the non-wetting behaviour of tungsten carbide. High speed videography and quantitative metallography of weld deposits are used to identify this mechanism. The non-wetting mechanism found acts simultaneously with the carbide dissolution mechanism, which until now was the only suggested cause of low carbide fraction in GMAW Ni-WC overlays. The non-wetting behaviour is observed in both short circuit and free flight metal transfer, accounting for carbide losses between 20 and 70% in the experiments performed. Low carbide fraction has prevented the mainstream use of GMAW for Ni-WC overlays, despite the advantages of simplicity, capability of in situ repair, and low capital costs. The findings presented here have a potential large impact for further consumable and process development.
Economic and environmental incentives encourage operators to maintain pipeline operation during repair and maintenance procedures including hot-tap branch fitting installation onto pipelines. Welding onto a liquid-filled pipeline induces accelerated cooling of the weld and heat affected zone (HAZ), increasing the propensity for cracking. In-service welding codes and due diligence requires that several key factors be considered during the design of an in-service welding procedure specification for its intended purpose. The level of restraint (LoR) imposed by the geometry, material, or dimensional differences of the branch compared to the run pipe has also been shown to be a significant contributor to cracking. Finite element analysis (FEA) was utilized to investigate the geometric effects of an in-service weld procedure to approximate the LoR of hot-tap branch installation. The LoR was quantified and compared by simulating multi-pass weld sequences on two configurations: a branch-on-pipe (BoP) configuration of various dimensions and a configuration using perpendicular plates (PP) that has been used as an alternative to the branch-on-pipe configuration. The highest LoR, as measured by transverse tensile stress at the fillet weld toe, was the branch-on-pipe configuration with the largest branch wall thickness, the smallest branch diameter, the largest run pipe diameter, and the largest run pipe wall thickness. FEA modeling revealed that the PP configuration has lower LoR, thus it is not recommended to use for simulating in-service branch weld procedures.
For continued safe operation of pipelines, thousands of integrity digs are conducted every year to repair ILI detected defects. Integrity-driven pipeline excavations can be quite costly, present significant scheduling challenges with landowner consultation and seasonal access limitations, and an unmitigated defect may have required a pressure reduction or service outage, resulting in a loss of revenue from the asset. Dents are known to be one of the drivers for many integrity excavations, especially for liquid pipelines. A pipeline with a minimal mechanical deformation is not expected to fail immediately, however, severe pressure cycles combined with the geometric distortion can cause fatigue crack initiation and growth that can lead to failure. To account for the possibility of fatigue failure, recent changes to pipeline codes, such as CSA Z662, are requiring pipeline operators to repair any dent susceptible to fatigue failure unless an engineering assessment proves it is fit for service. A commonly used dent fatigue assessment methodology is outlined in API RP 579, also known as the EPRG-2000 model. The assessment methodology uses an S-N curve from DIN 2413 part 1 with a safety factor of 10, which has been derived from undamaged pressurized pipe sections experiencing pressure cycles with stress ratios of zero, and separate stress enhancement factors for dents and gouges which take into account the shape of dents and gouges. To account for the effect of mean stress, Gerber mean stress correction, which has been developed for pressure cycles with stress ratios of −1 (i.e., for fatigue bar specimens), is also applied on pressure cycles. According to the literature, API 579 Level 2 fatigue assessment methodology results in very conservative estimates of fatigue lives compared to experimental data. This paper will discuss the potential factors resulting in conservative assessments and propose refinements in the methodology. This will include the safety factor used for pipes with known operating pressure fluctuations and the mean stress correction model suitable for a pipeline with pressure cycles that have R ratios greater than zero. The acceptable number of cycles obtained using the proposed refinements were compared to experimental data and EPRG-1995 model’s predictions — the comparison revealed that the proposed methodology results in a more realistic safety margin for dented pipelines. The proposed methodology can be used as a part of engineering assessments in mechanical damage integrity management programs to improve the pipeline operator’s understanding of a dent’s remaining life and enable a more appropriate repair timeline.
While the uncertainties associated with actual pipeline asset condition demand the use of probabilistic methodologies to assess the integrity of pipelines, a realistic and validated probabilistic method to demonstrate post-hydrostatic test (PHT) integrity has eluded the pipeline industry. Traditionally, deterministic methods grow a “just-surviving flaw” (JSF) under worst-case pressure cycling to predict the remaining life of the most severe imperfection which could have survived a high-pressure event, such as hydrostatic test. The deterministic analysis results in a JSF fatigue life but does not identify the likelihood that the flaw exists. Furthermore, identifying the most severe flaw is not intuitive and attempts to probabilistically model material variabilities have failed to match known historical PHT reliability. A pipeline operator has now developed a novel approach to the task of quantifying marginal pipeline reliability after hydrostatic tests. Rather than limiting random values to only material properties, potential defects are assigned sizes and pressure cycling values, randomly sampled from validated distributions of defect size and pressure cycling severity (equivalent to downstream location). The number of generated defects is determined by a validated defect density, and defect size remains limited to what could have physically survived the hydrostatic test. The question posed is no longer “what are possible sizes of JSF close to discharge pressure surviving to a specific time under known load conditions?”, but rather “what proportion of the pipeline segments with similar defect populations would survive to a specific time under known load conditions?”. This represents a fundamental paradigm shift away from considering only a worst-case scenario to the quantification of plausible pipeline health conditions. Monte Carlo simulation time is kept practical by using an equivalent load integral method to project crack growth. This proposed methodology was validated by applying it to a selection of pipeline segments with known historical fatigue failures following hydrostatic tests in order to quantify the predictability of each section’s reliability at the failure time. The initial validation of the method was found to reasonably predict the past incidents. This paper will discuss the methodology, input parameters including their distributions, methods for assigning defect size distributions and densities based on extrapolations of field nondestructive examination (NDE) and in-line inspection (ILI) data, and a minimum defect density floor established based on the PHT fatigue failure of a newly constructed pipeline. While this method originally targets PHT pipeline segments, the development of a similar method for pipelines managed exclusively by ILI data is under development. The largest potential flaw for ILI-managed assets is then dictated by what could have evaded ILI tool detection rather than what could have survived a hydrostatic test. Herein, the progress on this development and future suggested research will be provided.
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