The residual stress pattern surrounding gouges is complex and, to date, has not been accurately modeled using stress modeling software. Thus measurement of these stress distributions is necessary. Neutron diffraction is the only experimental method with the capability of directly evaluating residual strain throughout the entire thickness of a pipe wall, in and around dent or gouged regions. Neutron diffraction measurements were conducted at the NIST reactor on three gouged dents in X52 pipeline sections. These were part of a larger sample set examined as part of the comprehensive MD4-1 PRCI/DOT PHMSA project. Gouges contained in pipeline sections were termed BEA161 (primarily a gouge with little denting), and BEA178 (mild gouging, very large dent). Measurements were also conducted on a coupon sample – P22, that was created as part of an earlier study. For the moderate gouges with little or no associated denting (BEA161 and P22) the residual stress field was highly localized around the immediate gouge vicinity (except where there was some denting present). The through wall stress distributions were similar at most locations — characterized by neutral or moderate hoop and axial stresses (50–100MPa) at the outer wall surface (i.e. at the gouge itself) gradually becoming highly compressive (up to −600MPa) at the inner wall surface. The other sample (BEA178) exhibited a very mild gouge with significant denting, and the results were very different. The denting process associated with this kind of gouge+dent dominated the residual stresses, making the residual stress distribution very complex. In addition, rather than having a residual stress field that is localized in the immediate gouge vicinity, the varying stress distribution extends to the edge of the dented region..

External corrosion growth rate is an essential parameter to establish the time interval between successive pipe integrity evaluations. Actual corrosion rates are difficult to measure or predict. NACE Standard RP0502 [1] recommends several methods including comparison with historical data, buried coupons, electrical resistance (ER), and Linear Polarization Resistance (LPR) measurements. This paper presents a testing program and procedure to validate the use of the LPR and ER methods to enhance the estimation of corrosion growth rates and improve the selection of reassessment intervals of gas transmission pipelines. Laboratory and field tests were performed using the LPR and ER technologies. The evaluation of soil parameters that affect localized corrosion included its type, moisture content, pH, resistivity, drainage characteristics, chloride and sulfite levels, and soil Redox potential. The results show that the LPR device provides instantaneous measurement of corrosion potential and it may be used to reflect the variations of corrosion rates with the changes of soil conditions, moisture, and temperature. However, LPR measurements are more efficient in saturated soils with uncertainty about its validity in partially and totally dry soils. Consequently, seasonal changes in soil conditions make it difficult to estimate total corrosion growth rate. On the other hand, the measurements using the ER method provided consistent estimates for long-term corrosion growth rates. Corrosion growth rates were also evaluated from a previous study by the National Institute of Standards (NIST) [2]. A procedure was developed to correlate soil properties to corrosion rates from the ER measurements and NIST data. The procedure was implemented in a computer program to provide an estimate of corrosion rate based on the soil input data and allows the operator to use the ER probes to improve the reliability of corrosion rate estimates.

A suite of tests characterizing X100 pipeline steels was initiated at the National Institute of Standards and Technology (NIST) in Boulder. Part of the test matrix included testing the toughness of the base metal, welds, and heat-affected zones (HAZ) by use of modified double cantilever beam specimens for crack tip opening angle (CTOA) testing. The thickness of the test section was either 3 mm or 8 mm. Girth welds perpendicular to the growing crack, and seam welds and their HAZ parallel with the crack, were tested with a crosshead displacement rate of 0.02 mm/s (with the exception of one girth weld specimen for each thickness, which were tested at 0.002 mm/s). Analysis of the data revealed some general differences among the weld specimens. The tests where the crack ran perpendicular to the girth weld demonstrated changes in CTOA and crack growth rate as the crack moved through the base metal, HAZ, and weld material. We observed the values for CTOA increasing and the crack propagation slowing as the crack moved through the weld and approached the fusion line. The stress field appeared to be strongly influenced by the thin HAZ, the fusion line, and the tougher base material. Consequently, the CTOA of the HAZ associated with the girth weld was larger than that of the seam-weld HAZ. It was not possible to obtain CTOA data for the seam weld, with the crack parallel within the weld, because the crack immediately diverted out of the stronger weld material into the weaker HAZ. CTOA values from both girth welds and seam-weld HAZ were smaller than those of the base material. The 8 mm thick specimens consistently produced larger CTOA values than their 3 mm counterparts, introducing the possibility that there may be limitations to CTOA as a material property. Further tests are needed to determine whether a threshold thickness exists below which the constraints and stress field are sufficiently changed to affect the CTOA value.