Pneumatic Testing of Piping Assemblies: Criteria for Stored Energy and Pinhole Leak Detectability

Pneumatic testing is beneficial as an alternative to hydrotesting particularly in remote areas where access to hydrotest fluids becomes logistically difficult or impossible. The present work is aimed at addressing two salient questions often face pneumatic testing. First is related to the appropriate piping volume to consider for calculating the stored energy in use with ASME PCC-2 calculation for determining the safe exclusion distance for a given piping geometry and test conditions. It was found that the 8D criteria specified in ASME PCC-2 cannot be generalized for all pipe sizes, different material toughness, grades, wall thicknesses and test conditions. A criterion is developed based on the ductile fracture arrest length that considers all these factors combined. The second criterion is related to the ability to detect pinhole leak from the pneumatic test data, again for a given geometry and test conditions, and what constitutes the minimum pinhole effective area in relation to the system total volume, measured uncertainties in the test pressure and temperature over the duration of the test. A semi-normalized physics-based parameter is suggested that can be applied to determine the effective pinhole leak size in relation to the volume of the piping assembly and conditions for detectability limit. A methodology is developed and applied to a pneumatic filed test on DN200, 12.2 km pipeline lateral.

Crack Propagation Calculation for Axial Cracks in Hollow Cylinders Subjected to Thermal Shock

The core region of the RPV can be considered a hollow circular cylinder disregarding the geometrical details due to nozzles. This contribution investigates the prediction capabilities for crack initiation, crack growth and arrest by means of a rather simple method based on the closed-weight function formula for the stress intensity factor (SIF) for axial cracks in hollow cylinders subjected to thermal shock. The method is explained together with some illustrative examples for real low allow steel used in nuclear applications. In order to obtain the temperature and stress distribution in the cylinder during the thermal shock, a finite element (FE) model is defined to obtain the uncoupled solution of these two fields needed for the closed-weight function. Since the material exhibits a ductile-brittle transition fracture behavior, the temperature-dependent fracture toughness for initiation and for arrest are described using the ASME model. The solution for the SIF is based on linear elastic fracture mechanics (LEFM) and therefore only elastic material is assumed and the crack can propagate in brittle manner. The crack initiates propagation if the SIF value at the crack tip reaches the fracture toughness (for initiation) and propagates unstably in mode I unless the fracture arrest toughness is reached. The quality of the solution is checked by comparing the obtained solution for a “stationary” crack with the calculated extended finite element method (XFEM) solution for the same loading transient. The results show that for some geometries of the cylinder, the crack stops and in some other cases the crack propagates until the cylinder fails. The combined closed-weight function-initiation-growth-arrest (WFF-IGA) algorithm does not require expensive computational resources and gives fast reliable results. The WFF-IGA method provides a powerful and economical way to predict the crack propagation and arrest of the initial crack. This is an advantage when an optimization of the structure is needed.

Recent Progress in Development of Ductile Fracture Arrest Methodology Based on CTOA: Test Standard, Transferability and Methodology

Significant progress has been made in development of a new fracture arrest methodology based on a toughness parameter designed to characterize propagation — the crack-tip opening angle (CTOA). A CTOA test procedure using lab-scale DWTT-type specimens has been standardized by ASTM, and recently published experimental work has demonstrated transferability of CTOA from DWTT to full-scale pipe. This paper will present the basic methodology for determination of CTOA using DWTT-type specimens (i.e., ASTM E3039) and other specimens such as modified double-cantilever-beam (MDCB). Recent numerical studies using cohesive zone models (CZM) and others based on damage mechanics will be discussed, including models of full-scale pipe fracture. The effects on CTOA of loading rate, specimen flattening and constraint (bending vs. tension) will be reviewed. The effect on CTOA of loading rate between quasi-static and impact (covering five orders of magnitude) is small or negligible, being within experimental scatter. Observed differences between surface and mid-thickness CTOA values will be discussed. Models of DWTT specimens using damage mechanics have shown that the CTOA for tensile loading is the same at the surface and mid-thickness and equal to the mid-thickness value for bend loading, but that the surface CTOA is significantly larger than the mid-thickness CTOA in bending. Model calculations have revealed the dependence of crack velocity on stress for a given CTOA, enabling construction of fracture resistance curves (pressure required to propagate fracture as a function of crack velocity). These first-principles curves based on CTOA can then be used in the Battelle two-curve model (BTCM) to replace empirical resistance curves based on Charpy absorbed energy (Cv). It has been known for some time that Cv over-represents the propagation resistance for high-strength high-toughness steels, requiring empirical “correction factors” to Cv in the BTCM. Experiments have shown that there is a non-linear correlation between Cv and CTOA, explaining the need for correction factors to Cv and supporting the use of CTOA as a more appropriate propagation toughness.

Separation Characteristics of an X65 Linepipe Steel From Laboratory-Scale to Full-Scale Fracture Tests

Separations are small fissures that form along the rolling-plane of some steels when sufficient stresses are created to open planes of weakness in the material. In the pipeline industry, separations have been observed on the fracture surfaces of tensile, Charpy, and drop-weight tear tests — the key tests for determining the fracture arrest capabilities of line pipe steels. When compared, the separation appearance between lab-scale tests and full-scale fracture test are noticeably dissimilar. Therefore, the influence separations have on the fracture behaviour may not clearly scale between lab-scale and full-scale tests. In this study, the separation severity of Charpy, DWTT, and full-fracture propagation test fracture surfaces was measured and compared. Two full-scale burst tests were carried out with pipes containing a CO2/N2 mixture. Fracture surfaces were observed along the length of the pipe and captured when the separation appearance changed. For each pipe section, the corresponding lab-scale test surfaces were compared. With the separations measured across all fracture faces, the separation appearance of the full-scale test surfaces did not provide the same values as the lab-scale tests. However, the lab-scale tests did capture the trend in separation severity for each pipe section. Only the lab-scale test surfaces showed a correlation in separation severity.

Failure through expanding voids in bacterial streamers

We investigate the failure of thick bacterial floc-mediated streamers in a microfluidic device with micro-pillars. We found that streamers could fail due to the growth of voids in the biomass that originate near the pillar walls. The quantification of void growth was made possible by the use of 200 nm fluorescent polystyrene beads. The beads get trapped in the extra-cellular matrix of the streamer biomass and act as tracers. Void growth time-scales could be characterized into short-time scales and long time-scales and the crack/void propagation showed several instances of fracture-arrest ultimately leading to a catastrophic failure of the entire streamer structure. This mode of fracture stands in strong contrast to necking-type instability observed before in streamers.

Modified West Jefferson Burst Test for Assessment of Brittle Fracture Arrest in Thick-Wall TMCP Line-Pipe Steel With High Charpy Energy

Three partial gas pipe burst tests were conducted to assess the brittle-to-ductile transition temperature and brittle fracture arrestability of a heavy-walled TMCP line-pipe steel. This steel had a very high Charpy energy (400 J) which is typical of many modern line-pipe steels. In standard pressed-notch DWTT specimen tests this material exhibited abnormal fracture appearance (ductile fracture from the pressed notch prior to brittle fracture starting) that occurs with many high Charpy energy steels. Such behavior gives an invalid test by API RP 5L3, which makes the transition temperature difficult to determine. The first burst test was conducted in a manner that is typical of a traditional West Jefferson (partial gas vessel) burst tests. The crack was initiated in the center of the cooled vessel (with a partial air gap), but an unusual result occurred. In this test a ductile fracture just barely started from each crack tip, but one of the endcaps blew off. The pipe rocketed into the wall of a containment building. The opposite endcap impacted the wall of the building and brittle fractures started there with one coming back to the center of the vessel. The implication from this test was that perhaps initiation of the brittle fracture in the base metal gives different results than if the initial crack came from a brittle location. The second burst test used a modified West-Jefferson Burst Test procedure. The modification involved cutting a short length of pipe at the center of the vessel and rotating the seam weld to the line of crack propagation. The HAZ of the axial seam weld had a higher dynamic transition temperature. The initiation flaw was across one of the center girth welds so that one side of the initial through-wall crack had the crack tip in the base metal while the other side initiated in the seam weld HAZ. On the base metal side, the crack had about 220 mm of crack growth before reaching steady-state shear area, i.e., the shear area gradually decreased as the fracture speed was increasing. On the other side, a brittle fracture was started in the HAZ as expected, and once it crossed the other central girth weld into the base metal, the fracture immediately transformed to a lower shear area percent. These results along with those from the first burst test suggest that the DWTT specimen should have a brittle weld metal in the starter notch region to ensure the arrestability of the material. The final burst test was at a warmer temperature. There was a short length of crack propagation with higher shear area percent, which quickly turned to ductile fracture and arrested. In addition various modified DWTTs were conducted and results were analyzed using an alternative brittle fracture arrest criterion to predict pipe brittle fracture arrestability.

Modified Two-Curve Model for Predicting Fracture Arrest Toughness and Arrest Distance of Full-Size Burst Tests

Running fracture control is a very important technology for gas transmission pipelines with large diameter and high pressure. The Battelle two-curve (BTC) model developed in the early 1970s has been widely used in pipeline industry to determine arrest toughness in terms of the Charpy energy. Because of its semi-empirical nature and calibration with test data only for grades up to X65, the BTC does not work for higher grades. Simple corrections were thus proposed to extend the BTC model to higher grades, but limited to those grades considered. Moreover, the BTC model only predicts the minimum arrest toughness, but not arrest distance. To fill the technical gaps, this paper proposes a modified two-curve (MTC) model and a fracture arrest distance model in reference to the Charpy energy. The MTC model coupling with an arrest distance algorithm can predict fracture arrest toughness and arrest distance in one simulation of numerical integration for a single pipe or a set of multiple pipes with given toughness. Two sets of full-scale burst test data for X70 and X80 are used to validate the proposed model, and the results show good agreements between the predictions and full-scale test data of arrest toughness and arrest distance as well. The MTC model is then applied to optimize a design of pipe segment arrangements for a mockup full-scale burst test on a high-strength pipeline steel. The MTC simulation results confirm the experimental observation that different pipe arrangements determine different arrest toughness and arrest distance for the same grade pipes.

An Operator’s Perspective on Fracture Control in Dense Phase CO2 Pipelines

Carbon Capture and Storage (CCS) is an approach to mitigate global warming by capturing and storing carbon dioxide (CO2) from large industrial emitters. Pipelines will play a significant role in the transportation of CO2 in CCS projects. National Grid has an interest in this, and has carried out research to investigate the requirements for the safe design and operation of CO2 pipelines. CO2 pipelines are susceptible to long running fractures which are prevented by specifying an adequate pipe body toughness to arrest the fracture. There is no existing, validated methodology for setting pipe body toughness for pipelines transporting dense phase CO2 with impurities. The methods for estimating the pipe body toughness are semi-empirical so full scale fracture propagation tests are required to validate and extend these methods. As part of a major research programme into pipeline transportation of dense phase CO2, National Grid conducted two full scale fracture propagation tests using 900 mm diameter pipe in 2012. The tests demonstrated that the current natural gas practices for setting pipe body toughness was incorrect and non-conservative for dense phase CO2 pipelines. National Grid recognises the importance of understanding fracture arrest as it required to ensure design code compliance, impacts on pipeline design and provides reassurance to stakeholders. As the results of the two tests cannot be used directly to set the toughness requirements for a specific project pipeline, a third full scale test was necessary to confirm the fracture arrest capability of the pipe for the proposed pipelines. A third full scale fracture propagation test was conducted in July 2015. A propagating ductile fracture was initiated and successfully arrested in linepipe representative of that to be used on the proposed project.