An Approach to Engineering Critical Assessment of Assets That Cannot Be Inline Inspected

Hydrostatic pressure testing is the most widely accepted approach to verify the integrity of assets used for the transportation of natural gas. It is required by Federal Regulations 49 CFR §192 to substantiate the intended maximum allowable operating pressure (MAOP) of new gas transmission pipelines. The Pipeline and Hazardous Materials Safety Administration (PHMSA) Notice of Proposed Rulemaking (NPRM) with Docket No. PHMSA-2011-0023 [1], proposes an additional requirement for MAOP verification of existing pipelines that: i) do not have reliable, traceable, verifiable, or complete records of a pressure test; or ii) were grandfathered into present service via 49 CFR §192.619(c). To meet this requirement, the NPRM proposes that an Engineering Critical Assessment (ECA) can be considered as an alternative to pressure testing if the operator establishes and develops an inline inspection (ILI) program. The ECA must analyze cracks or crack-like defects remaining or that could remain in the pipe, and must perform both predicted failure pressure (PFP) and crack growth calculations using established fracture mechanics techniques. For assets that cannot be assessed by ILI, however, the implementation of an ECA is hindered by the lack of defect size information. This work documents a statistical approach to determine the most probable PFP and remaining life for assets that cannot be assessed by ILI. The first step is to infer a distribution of initial defect size accumulated through multiple ILI and in-ditch programs. The initial defect size distribution is established according to the as-identified seam type, e.g. low-frequency electric resistance weld (LF-ERW), high-frequency electric resistance weld (HF-ERW), flash weld (FW), single submerged arc weld (SSAW), or seamless (SMLS). The second step is to perform fracture mechanics assessment to generate a probabilistic distribution of PFPs for the asset. In conjunction with the defect size distribution, inputs into the calculation also include the variations of mechanical strength and toughness properties informed by the operator’s materials verification program. Corresponding to a target reliability level, a nominal PFP is selected through its statistical distribution. Subsequently applying the appropriate class location factor to the nominal PFP gives the operator a basis to verify their current MAOP. The last step is to perform probabilistic fatigue life calculations to derive the remaining life distribution, which drives reassessment intervals and integrity management decisions for the asset. This paper will present some case studies as a demonstration of the methodology developed and details of calculation and establishment of database.

Diffuse X-ray scattering from ion-irradiated materials: a parallel-computing approach

A computational method for the evaluation of the two-dimensional diffuse X-ray scattering distribution from irradiated single crystals is presented. A Monte Carlo approach is used to generate the displacement field in the damaged crystal. This step makes use of vector programming and multiprocessing to accelerate the computation. Reciprocal space maps are then computed using GPU-accelerated fast Fourier transforms. It is shown that this procedure speeds up the calculation by a factor of ∼190 for a crystal containing 109unit cells. The potential of the method is illustrated with two examples: the diffuse scattering from a single crystal containing (i) a non-uniform defect depth distribution (with a potentially bimodal defect size distribution) and (ii) spatially correlated defects exhibiting either long-range or short-range ordering with varying positional disorder.

A Methodology for Generation of Best-Estimate Pipeweld Failure Probabilities and Leak Rates for Pressurised Water (PWR) Reactor Plant

This paper presents an overview of the work undertaken by Rolls-Royce to develop a methodology for the generation of best-estimate Pressurised Water Reactor (PWR) pipeweld failure probabilities and leak rates using the WinPRAISE software code. WinPRAISE (the code) uses fracture mechanics principles to generate best-estimate pipeweld Probabilities of Failure (P(f)) by performing a monte-carlo statistical crack growth analysis. Calculation inputs include material properties and a detailed model of service loadings. Since the WinPRAISE default initial weld defect size distribution and frequency database is for MMA type welds, factors can be applied to the output to take credit for improved welding techniques such as automated TIG. The leak before break philosophy for ductile PWR pipe materials has also been used to develop a criterion to determine an appropriate best-estimate initial through-wall crack geometry. The applied bending stress at the pipeweld and fluid mechanics principles can then be utilised to provide the corresponding initial leak rate. This model of initial through-wall crack geometry has resulted in a reduction in pipeweld initial leak rate estimates which benefits the probabilistic safety assessment for the applicable plant. This paper presents the Rolls-Royce methodology for generation of best-estimate probabilities of failure and leak rates for PWR pipewelds based upon fatigue crack growth of welding defects.

Determination of strain within Si1-yCy layers grown by CVD on a Si Substrate

In this work, we performed quantitative measurements of strain in structures consisting of a 30 nm-thick Si1-yCy layer grown by chemical vapour deposition (CVD) on a Si (001) substrate at 550 or 600°C. The total C concentration varies from 0.67 to 1.97% that was measured by SIMS. Geometric phase analysis (GPA) of high resolution transmission electron microscopy (HR TEM) cross-section images and convergent beam electron diffraction (CBED) were used to deduce the strain within these Si1-yCy layers. Finite-element simulations were carried out to estimate the impact of strain relaxation in thin areas of a specimen. These results were compared with the data obtained by high resolution X-ray diffraction and Raman spectroscopy and with the predictions of elasticity theory. Particular interest is paid to the formation of the structural defects within Si1-yCy layers as a function of a C concentration, growth temperature and incorporated strain. Both cross-sectional and plan-view TEM specimen configurations were used to obtain quantitative information on the defect size distribution, their density and structure.