Carl E. Jaske

Fatigue-Strength-Reduction Factors for Welds in Pressure Vessels and Piping

Fatigue-strength-reduction factors (FSRFs) are used in the design of pressure vessels and piping subjected to cyclic loading. This paper reviews the background and basis of FSRFs that are used in the ASME Boiler and Pressure Vessel Code, focusing on weld joints in Class 1 nuclear pressure vessels and piping. The ASME Code definition of FSRF is presented. Use of the stress concentration factor (SCF) and stress indices are discussed. The types of welds used in ASME Code construction are reviewed. The effects of joint configuration, welding process, cyclic plasticity, dissimilar metal joints, residual stress, post-weld heat treatment, the nondestructive inspection performed, and metallurgical factors are discussed. The current status of weld FSRFs, including their development and application, are presented. Typical fatigue data for weldments are presented and compared with the ASME Code fatigue curves and used to illustrate the development of FSRF values from experimental information. Finally, a generic procedure for determining FSRFs is proposed and future work is recommended. The five objectives of this study were as follows: 1) to clarify the current procedures for determining values of fatigue-strength-reduction factors (FSRFs); 2) to collect relevant published data on weld-joint FSRFs; 3) to interpret existing data on weld-joint FSRFs; 4) to facilitate the development of a future database of FSRFs for weld joints; and 5) to facilitate the development of a standard procedure for determining the values of FSRFs for weld joints. The main focus is on weld joints in Class 1 nuclear pressure vessels and piping.

EFFECTIVE MODELING OF FATIGUE CRACK GROWTH IN PIPELINES

Pipeline operators must rely on fatigue crack growth models to evaluate the effects of operating pressure acting on flaws within the longitudinal seam to set re-assessment intervals. In most cases, many of the critical parameters in these models are unknown and must be assumed. As such, estimated remaining lives can be overly conservative, potentially leading to unrealistic and short reassessment intervals. This paper describes the fatigue crack growth methodology utilized by Det Norske Veritas (USA), Inc. (DNV), which is based on established fracture mechanics principles. DNV uses the fracture mechanics model in CorLAS™ to calculate stress intensity factors using the elastic portion of the J-integral for either an elliptically or rectangularly shaped surface crack profile. Various correction factors are used to account for key variables, such as strain hardening rate and bulging. The validity of the stress intensity factor calculations utilized and the effect of modifying some key parameters are discussed and demonstrated against available data from the published literature.Copyright

Inelastic Fracture Mechanics Model for Assessment of Crack-Like Flaws

Fitness-for-service assessment of pressure vessels and piping often involves the evaluation of existing or potential crack-like flaws to guard against fracture or leaks that could be caused by the presence of such flaws. This paper presents an inelastic fracture mechanics model that has been developed to evaluate longitudinal surface cracks in pipelines, piping and pressure vessels subjected to internal pressure loading. The model uses the J-integral parameter to predict toughness-dependent failure and an effective flaw concept to predict flow-strength dependent failure. The concepts of the model are reviewed. Then, the model is used to evaluate the results of in-service failures and full-scale burst testing of steel pipe and pressure vessel samples. Application of the model to remaining life assessment based on inspection data and hydrostatic testing results is illustrated. Stress-corrosion cracking (SCC) and fatigue are considered as possible crack-growth mechanisms. Examples of typical remaining crack-growth life calculations are presented using both deterministic and probabilistic methods. The benefits of each method are discussed. Finally, planned future additions to the model are presented.Copyright

Facility Integrity Management and Assessment of Associated Risk Conditions

This paper reviews the basic elements of a facility integrity management program and describes the process used to assess risk conditions related to a facility. The policies, goals and objectives of the program should be defined before implementing it. The location and details of the facility and all its equipment must be described and the information should be recorded in a computerized database. Important triggers for change management and the minimum features of change management are reviewed. Ensuring the competency and training of personnel responsible for integrity management is essential. The integrity management team must identify hazards associated and ways of controlling them. Once hazards are identified, risk assessment is performed and options for reducing risk are considered. Results of the risk assessment are then used to plan and execute activities of the integrity management program. Needed repairs or replacements are identified, planned and completed. Finally, the integrity management program should incorporate a continuous improvement process and information from investigations of incidents at the facility, at other company locations, and within the industry.Copyright

Probabilistic Assessment of Axial Crack-Like Flaws and Remaining Life in Onshore Pipelines

The predicted failure pressure and estimated remaining life of axial crack-like flaws are two key parameters pipeline operators use to develop excavation programs and set re-assessment intervals following an assessment (in-line inspection or hydrostatic pressure test, for example). Deterministic approaches routinely use conservative input values, such as specified minimum or worst-case conditions, leading to potentially overly conservative conclusions. Probabilistic approaches, on the other hand, can account for inherent variability and provide probabilities of failure; however, there are no current approaches to define an acceptance threshold in the onshore pipeline industry. This paper discusses the probabilistic approach Det Norske Veritas (USA), Inc., (DNV) uses to assess axial crack-like flaws. DNV’s approach incorporates an inelastic fracture mechanics model in combination with Monte Carlo simulations and Paris Law fatigue crack growth to estimate the cumulative probability of failure over time. Topics include the application of this methodology for two primary cases: (1) the defect population can be described with a certain degree of confidence (in-line inspection) and (2) the defect population cannot (hydrostatic pressure test). The potential for using the methodology for determining a case-by-case acceptance threshold will also be explored.Copyright

Fitness-for-Service of Steam-Methane Reformer Hot Outlet Manifolds

A majority of steam-methane reformers (SMRs) have hot outlet manifold systems that operate at temperatures in the range of 750 to 900 °C (1382 to 1652 °F) and internal pressures in the range of 2000 to 3500 kPag (290 to 510 psig). Under these operating conditions, the materials used in the manifold systems, including weld joints, are subject to degradation from high-temperature creep. Thus, fitness-for-service (FFS) assessment of the manifold systems requires evaluation of material degradation as a result of creep damage (voids, micro-fissures and cracks) and creep deformation (strain). This paper reviews a general approach to FFS assessment of SMR hot outlet manifold systems. Using data on operating temperature, pressure, and time in service stresses and strains in the manifold sections are calculated by the finite-element method. The calculated stress-strain data are then used to compute creep damage accumulation as a function of time in service for both base metal and weld joints using available creep-rupture data for the materials. These results are then used to predict remaining creep life and establish recommended inspection protocols.Copyright

Pressure Cycle Fatigue: A Statistical Assessment Approach

Operational pressure cycle fatigue (PCF) is one of the integrity threats managed by pipeline operators. Usually, hazardous liquid pipeline operators are most interested in the effects of pressure cycles since these pipelines inherently experience more significant pressure cycles than natural gas pipelines. The parameters considered in the assessment of operational pressure cycles include pipe geometry (diameter and wall thickness), mechanical properties of the pipe, distribution of hypothetical defect sizes that may exist in the pipeline, and pressure cycles. In performing these assessments, the most conservative value for each parameter is commonly used for predicting a time to failure. As such, the results are inherently overly conservative. A statistical assessment method, PCFStat, has been developed to more appropriately model the input parameters used in the assessment of operational pressure cycle fatigue; especially for cases where the deterministic approach identifies relatively short remaining fatigue lives. A distribution of each of the input parameters is developed, and then a Monte Carlo simulation of these parameters is performed. The results produced by this analysis are then used to evaluate the probability of a failure (leak or rupture) for a defined time interval.Copyright