Sensitivity Studies on the Evaluations of Hydrogen Effects on Linepipe Integrity Challenges
Authors: Hill Lance , Bagnoli Ken , Twombly Elizabeth , Punch Ed , Gao Xiaosheng , Wilkowski Gery 
Published: December 2024

As part of DOT/PHMSA project efforts being conducted at Emc2, a number of sensitivity studies are being conducted on the effect of hydrogen on the integrity of pragmatic pipeline integrity challenges. These sensitivity studies will provide the basis for recommendations provided to DOT/PHMSA on what hydrogen challenges are more important for additional future experimental/analytical efforts.

Three different types of analysis evaluations were conducted, and some are still in progress. The first set of sensitivity analyses are for evaluating the hydrogen concentrations that will locally develop at different integrity locations of concern with plastic-strain history effects and elastic residual stresses. The second type of analyses involves simple assessments of critical flaw size changes with existing methods. The third set of analyses involved unique and advanced numerical modelling for flaws for burst pressure predictions, for instance a crack in a pipeline hardspot. A fourth type of analysis underway involves predictions of hydrogen cracking by phase field/crack growth finite element (FE) analyses, and will be the subject of a future paper.

Hydrogen concentration evaluations for pipeline-integrity challenge assessments involved the following cases. (a) The effects of the plastic straining on hydrogen concentration at an axial surface crack were examined with and without a prior hydrotest before going into hydrogen service. (b) The hydrogen concentration at weld residual stress locations such as the fillet weld of a repair sleeve is examined. The FE models in this case included welding simulations to develop the plastic strain history and the residual stresses. (c) The hydrogen concentration at a corroded local thin area in a pipe was also explored where there was significant plastic straining during hydrotesting, although the plastic strains and elastic residual stresses vary through the thickness from tension to compression.

For evaluations using existing fracture mechanics models, evaluations of hydrogen effects on the burst pressure of axial surface flaws in pipes examined two cases. (a) The first case involves an axial surface crack in the base metal of a pipe with and without hydrogen using existing J-estimation analyses and constraint adjustments to the C(T) specimen J-R curve of the vintage linepipe steel. (b) The second involves evaluation of axial surface cracks in a hardspot where the strength and toughness changes with different hardness levels. The crack-driving force in these evaluations comes only from the pressure, and the analyses assume homogeneous strength properties in the pipe.

Evaluations using more advanced FE modelling are undertaken for two cases as well. (a) For a crack in a hardspot, advanced FE modelling includes the pipe fabrication stresses from plate to pipe, the pressure-induced bending stresses through the thickness due to the flat region of the hardspot included in the FE mesh, as well as the pressure loading. The thermal plastic/phase transformation residual stresses are being added in additional future analyses, which is the fourth contribution to the crack-driving force. The simple burst pressure models only account for the pressure and assume the material is homogeneous and the pipe is perfectly circular. (b) For an axial surface crack in a harder electric resistance weld (ERW) seam weld, rigorous FE modelling was performed. In this FE modelling, the hard region of an ERW weld is included with the axial surface crack in the center of that ERW weld region, while the rest of the pipe has base metal strength. A variety of crack lengths and depths are evaluated to systematically see the difference due to including the hardness of the weld metal. The toughness difference for an inert environment (air or methane) and in hydrogen on the weld and base metals is included in the critical crack evaluations.