Data Driven Solutions for the Most Difficult Problems

In recent years, Structural Integrity has observed an increasing trend in the use of specialty instrumentation to solve “impossible” problems or answer “indecipherable” questions. This shift was particularly apparent within commercial nuclear, where data-driven solutions have long been perceived as challenging due to short outage windows, personnel dose concerns, and a significant paperwork burden, among other factors. Widespread adoption of instrumentation-based solutions creates new paths to tackling difficult/persistent problems, and shifts the industry focus for critical assets from reactionary to more of a predictive approach. In 2017,Structural Integrity assisted numerous clients with deployment of specialty instrumentation in this fashion, comprising two general scenarios: 1) new designs/modifications, and 2) repeat failures. Each application requires different sensors and varying analytical methods, but the approach used to leverage the resultant data to solve the problem is generically applicable throughout the energy sector. The text below details important considerations for both scenarios and highlights a successful application of the underlying process for management of thermal fatigue in reactor coolant system branch piping.

New Designs/Modifications 

Whether developing a new reactor or modifying an existing system, most engineers are intimately familiar with the process of qualifying structures/ components to meet ASME/regulatory criteria through analytical methods. However, Structural Integrity has observed a tendency toward over-reliance on computer models where empirical testing/validation could be used as an alternate means of qualification. Instrumentation/data is leveraged where available (i.e. from benchmarking tests and/or Operating Experience) but is often a low priority compared to analytical work. We have a different perspective, having observed that integration of testing/instrumentation into the design process provides advance insight on potential challenges. Early identification of unexpected conditions and/or concerns helps to avoid rework and regulatory scrutiny, and in this vein integration of design and instrumentation can have significant fiscal benefits.

Structural Integrity has worked with numerous OEMs and AEs to provide confirmatory instrumentation and testing services in parallel with new designs and/or modifications. As such, we have a unique perspective on the nuances of achieving the appropriate balance between uncertainty/confidence, technical rigor, and cost of testing. For new designs, the number of unknowns can be significant, as complex geometries and innovative manufacturing techniques contribute nonlinearities to modeling efforts.

Guidance and procedures for current- generation plants are based upon decades of operating experience; new designs are expected to achieve a similar level of confidence, the burden of which is borne through analytical models and prototypical tests. Full-scale testing is expensive, and thus typically limited in scope/quantity; however, such tests provide critical feedback on analytical assumptions such as boundary conditions, application of loads, and presence of complex phenomena (i.e. flow-induced vibration, thermal binding, etc.).

Performance of in-situ testing in parallel with design/analysis efforts helps to validate modelling assumptions, reducing rework and helping optimize the workflow of analytical tasks. Such testing can also help to quantify specific loadings and/ or eliminate the inclusion of complex phenomena, which can, in turn, be used to demonstrate additional margin and/or reduce iterations. SI has successfully leveraged our instrumentation and cause evaluation expertise to provide in-situ testing and evaluation of numerous components/systems under design and/or during construction.

Our results have subsequently been used to validate model parameters (i.e. boundary conditions, loading amplitudes/cases, mode shapes, damping, etc.) and inform future testing activities (i.e. updated test matrices, reduced full-scale test scope, etc.). We also routinely support in-field final qualification of as-built modifications, providing confidence that the design functionality/acceptability will be maintained throughout a component’s lifecycle.

Repeat Failures

When “unexpected” problems become “persistent” issues, obtaining the right data can mean the difference between recurrence and permanent mitigation. Through measured evaluation of the failure history and apparent cause evaluation(s), SI can suggest a targeted instrumentation approach to stop the failure cycle. Recent examples where SI employed this approach to successfully address problems for our Nuclear clients include:

  • Cause determination and remaining life assessment for cracking observed in a main steam pipe support; (Figure 1)
  • Modal characterization and subsequent piping redesign/tuning following small-bore socket weld failures (numerous examples);
  • Failure analysis, phenomena identification, and redesign support following a main steam condenser failure;
  • Source determination and evaluation of mitigation strategies for RHR check valve leakage;
  • Root cause and design support in response to failure of an MSR bellows assembly;
  • Response characterization for significant events during shipment of nuclear fuel rods on a tractor/ trailer bed (Figure 2).

The trend toward data-informed solutions is not Nuclear specific; rather, multiple industries are observing fiscal benefit in employing this problem focused, technology-driven approach. SI’s recent projects range from evaluating the impact/effectiveness of impulse cleaning on heat recovery steam generator (HRSG) tubing arrays, to cause identification/mitigation for premature wicket gate bushing failures at hydroelectric stations.  For the referenced cases, utility personnel could not ascertain whether design, maintenance, or operational issues were the primary contributor degradation and/ or poor performance; instrumentation quantified these relations and facilitated informed resolutions.

When faced with a recurring issue or repeat failure, a consistent problem- solving framework must be employed.

The first step entails thorough documentation of the issue – what is known/unknown, what has changed, and what gaps exist in the available information. Instrumentation can be used to address any identified gaps; Structural Integrity has specific expertise developing targeted instrumentation plans to supplement existing knowledge. These plans typically specify operational regimes of interest, sensor types and locations (including existing plant instrumentation), data collection approach/format, test repeatability/ confidence, and any analysis/post- processing routines necessary to provide correct context during data interpretation.

 

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