Cumulative Creep Damage Using Advanced Predictive Analysis
Authors: Cohn Marvin J. , Cronin Michael T. , Liu Michael C. , Kumar Nikhil , Gascon Martin 
Published: October 2021

Abstract
Advanced techniques of evaluating cumulative material damage for power plant components operating in the creep range can enhance life management strategies, especially as power plants transition to more flexible operations. Power piping creep damage is strongly sensitive to the operating temperatures, operating pressures, and applied stresses.
Several empirical examples reveal large ranges of operating temperatures and pressures throughout the operating years of power piping systems. Multiple examples reveal that the operating conditions may be significantly different over the years. Time and pressure histories for main steam (MS) and hot reheat (HRH) piping systems are provided to illustrate these operational changes, including mean and median annual and monthly plant information data. Operating temperature and pressure histograms can reveal single modal and bi-modal plant operations. Examples of mean, median, and upper 95% confidence operating temperature and pressure values are included in this study.
There may be some instances of operating above design temperatures and design pressures for short periods of time. Advanced evaluations have revealed that some above design temperature events have occurred with low operating pressures, so coincident operating pressure and operating temperature evaluations are also discussed to determine more accurate estimates of cumulative creep damage. Accelerated creep-fatigue damage can occur from high-temperature ramp rates obtained from the detailed temperature profiles.
Evaluation of the applicable applied stresses in piping system weldments should consider the simulation as-found stresses, redistributed through-wall axial and circumferential stresses, weld residual stresses, and redistributed multiaxial stresses. The hot and cold piping system walkdowns should compare the actual piping displacements to the predicted design displacements. Piping walkdown evaluations should identify and evaluate the observed significant field anomalies. Several empirical examples illustrate that the simulation as-found stress analysis (considering adverse field conditions) can result in piping stresses significantly different than those predicted in the as-designed piping stress analysis.
In the past, the selection of weldment reexamination intervals has been based on standard industry practice. Advanced predictive analytic evaluations can be used to provide a technical justification for continued plant operation until critical components are reexamined.
This study provides several examples of using the actual operating temperatures, operating pressures, field stresses, and cumulative operating hours to estimate the cumulative creep/fatigue damage at critical system components. Assuming future piping system operations the same as in the past, remaining useful lives (RULs) for the critical system components can be estimated. The piping system RUL contour isometric illustrates the most critical girth weldment creep damage rankings (these are the lead-the-fleet nondestructive examination (NDE) locations). If there are no significant fabrication defects in the other weldments (which can be confirmed by a one-time examination), the evaluation can be used to justify that many of the remaining weldments do not have to be examined for more than a decade if future operation is the same as in the past.