Structural Integrity’s roots lie in evaluating and preventing mechanical failures in the nuclear power industry. Fracture mechanics is the principal engineering discipline applied to support these evaluations, and has been an area of expertise and source of work for Structural Integrity since its founding in 1983. Through this work, we have become recognized as an industry expert in component evaluation, developing advanced software tools to aide in fracture mechanics analyses and participating in the development of ASME Code rules for evaluation of in-service inspections.
While fracture mechanics work remains a core engineering discipline for the work we perform in the nuclear and fossil industry, new regulations in the oil and gas industry targeting gas transmission pipelines and subsea components will require a significant increase in the application of fracture mechanics. Applying lessons learned from the nuclear and fossil industries, we are continuing to build and apply our Fracture Mechanics expertise to solve unique problems emerging in the oil and gas industries.
The U.S. Department of Transportation, Pipeline and Hazardous Materials Safety Administration (PHMSA) recently issued a Notice of Proposed Rulemaking (NPRM) titled Safety of Gas Transmission and Gathering Pipelines proposing extensive changes to 49 CFR Part 192. The proposed regulation includes new requirements for verification of pipeline material, Maximum Allowable Operating Pressure (MAOP) verification, pipeline assessments, integrity management and repairs. It also includes new detailed guidance on fracture mechanics modeling for failure stress and crack growth analysis.
Section 192.624 of the NPRM specifies new methods for verifying the MAOP of designated gas pipelines nearly all of which require a rigorous fracture mechanics analysis if there is reason to believe the pipeline segment contains or may be susceptible to cracks or crack-like defects. The analysis requires modeling for failure pressure considering both ductile and brittle failure modes, incorporating fatigue models to predict flaw growth, and including a sensitivity analysis to determine estimates of time to failure for cracks. The NPRM prescribes a very conservative approach of assuming minimum strength and toughness properties when determining failure pressure for a known defect size. For example, maximum properties, Charpy V-Notch (CVN) of 120 ft-lbs, are to be used for determining the largest flaw that could have survived a pressure test, while when analyzing in-line inspection assessments, default CVN values of 5 ft-lbs for base material and 1 ft-lb for ERW seam bond line defects are to be assumed when data are not available. These values are extremely conservative and operators will need to implement extensive data gathering and material testing in addition to analytical support to justify the use of conservative, but more realistic values.
Another area of key concern for the oil and gas industry, specifically in the subsea deepwater production market, is ensuring the integrity of High Pressure, High Temperature (HPHT) components using existing instrumentation. The fatigue life of HPHT subsea completion equipment and other thick section components becomes an increasing concern. ASME and API code guidance does not exist for components that now are faced with shut-in exposure above 15,000 psi class. Designers are having to develop an approach and technical basis for component performance when challenged by high temperatures and pressures (up to 350°F and 20,000 psi), a corrosive environment, and complex stress states induced by, among other things, thermal transients and pressure cycling. Stress cycles with sufficient frequency and severity in the HPHT environment could initiate a fatigue crack. If allowed to continue, a fatigue crack could propagate exponentially through the wall of the component and lead to a leak or even general structural failure. Further complicating this will be when Corrosion Resistant Alloy (CRA) liners are needed to combat sour service environmental conditions. Once a crack propagates through the liner, the failure threat changes to Sulfide Stress Cracking (SSC) and the fatigue crack growth rates will change
Draft regulations for HPHT equipment proposed by the Bureau of Safety and Environmental Enforcement (BSEE) state that “the lessee and operator must submit a summary of the proposed load monitoring methods and record keeping for each of the assemblies or components that are considered fatigue sensitive.”
Structural Integrity was a key contributor in API Technical Report 17TR8 (TR8), which was recently revised to serve as a guideline for designing HPHT subsea components subject to high pressure (>15 ksi) and high temperature (>350 °F). TR8 is intended to serve as a technical guidance document to augment other standards when operating above 15,000 psi or 350°F. . In the TR8, fracture mechanics is presented as a method for establishing a fatigue life. Coupling environmentally specific fatigue crack growth data with initial cracks sizes, identified by the resolution of the applied Non-Destructive Examination (NDE) technique, the total crack propagation life can be identified. In the TR8 report, maximum allowable life is defined as half of the number of cycles to predicted failure. Furthermore, in the unplanned event that design conditions are exceeded, known as a survival event, the TR8 recommends that a potential crack should remain stable.
Crack stability can be assessed in many ways, and the TR8 directly references the methodologies in ASME B&PVC VIII- 3, API 579-1/ASME FFS-1 and BS 7910. In conjunction with these standards, TR8 identified the highly dependent nature of fracture as environment, and appropriately warns an operator should be aware of potential detrimental environmental effects.
To get an accurate representation of the number cycles and their magnitudes that contribute to crack growth, TR8 offers load monitoring as a method of accounting for operating cycles. Load monitoring ensures “in-service” loading falls within the design criteria, and since generally service loads are less than the design loads, load monitoring provides a more accurate representation of load cycles and subsequently fatigue life.
To date, TR8 serves as a foundation of the HPHT deep water equipment design and provides a basis on which the deep water industry can expand in the future.
We have completed a number of recent fracture mechanics projects for the oil and gas industry. Some recent examples of project work completed include:
Modeling and evaluation of cracks at hard spots
In a recent project for a major pipeline operator, a deterministic analysis using Linear Elastic Fracture Mechanics (LEFM) was completed for pipelines with metallurgical hard spots and hydrogen induced cracking. Applied stress intensity factor calculations were performed using Structural Integrity proprietary software (pc-Crack) – software that analyzes and predicts flaw behavior, including calculation of crack growth rates and critical crack sizes for pressure vessels and piping. The analysis used CTOD (crack-tip opening displacement) and Charpy V-Notch data to estimate fracture toughness for various hardness values. The analysis was validated with respect to field failure data and a series of full scale burst tests performed circa 1967 on similar pipes containing cracks of various lengths. Critical crack size and safety factors as a function of temperature and pressure were determined from this analysis and used to guide repair and replacement activities.
SCC Analysis and APTITUDE
Due to significant and extensive Stress Corrosion Cracking (SCC) discovered for another major pipeline operator in the Northeastern United States, Structural Integrity was retained to help refine their SCC Management Plan, categorize SCC identified (in accordance with ASME B31.8S), and help define re-assessment intervals using guidance provided in the NPRM. As part of the evaluation, Structural Integrity developed a software tool, APTITUDE™, that employed multiple methodologies to evaluate crack-like defects covering the full spectrum from low (brittle) to high (ductile) toughness regimes. The tool was developed to analyze axially oriented cracks present in pressurized steel cylinders with a wide range of material properties (yield strength, flow stress, fracture toughness) and pipe/flaw geometry (diameter, wall thickness, through-wall or surface crack, crack length and depth) using proven methodologies. The tool was developed to calculate the predicted failure pressure (from reference standards), determine crack categories, and establish re-assessment intervals. The following methodologies for evaluating crack-like defects were included:
Modified Ln Secant
API 579 Level 2 – Failure Assessment Diagram (FAD) Approach
Finite Element Based Limit Load Approach (Limit Load)
Structural Integrity also incorporated our advanced fracture mechanics expertise to incorporate logic that identifies a recommended method to ensure an accurate yet conservative result based on applicable toughness regimes (low, intermediate, and high toughness) and pipe/flaw geometries.
Fracture Mechanics Training for Pipeline Operators
Dr. Peter Riccardella recently provided on-site training for a key client on “Fracture Mechanics for the Pipeline Industry”. The training covered topics ranging from introductory concepts and principles to fracture mechanics as applied in the NPRM. Real example problems were covered with corresponding analytical approach and results.
NPRM Recommended Default Toughness Values
Working with a major gas association, Structural Integrity was retained to complete a statistical analysis of known pipe materials in published and proprietary sources. The sources included material with known defects that were burst tested and further analyzed along with lab data that was collected. The aim of this study was to identify bounding toughness values for fracture mechanics evaluations when material properties are not known or adequately documented.
A statistical analysis was performed of gas pipeline materials toughness data from a number of sources, including both ERW seam weld and pipe body base materials. The analysis revealed that the default toughness values proposed for fracture mechanics modeling in the NPRM, represent overly conservative values. SI’s statistical analysis concluded that more reasonable default toughness values should be used when evaluating crack defects, consisting of 13.0 ft-lb for pipe body toughness and 4.0 ft-lb for the long seam welds of vintage pipe (such as pre-1970 ERW seam welds). SI believes The use of these values is more appropriate in most cases (representing the 90th percentile) when analyzing crack-like defects with unknown toughness.
1 Kiefner, J.F., “Modified Equation Helps Integrity Management”, Oil and Gas Journal, Oct 6, 2008, pp 76-82 and “Modified Ln-Secant Equation Improves Failure Prediction”. Oct 13, 2008, pp 64-66.
2 Fitness-For-Service, API 579-1/ASME FFS-1, June 5, 2007
3 Kim, Tae-Kwang Song, Net-Section Limit Pressure and Engineering J Estimates for Axial Part-Through Surface Cracked Pipes, PVP 2007-26220