In the 1990s, TWI developed a limit state equation to estimate the remaining life of P91 weldments. This, in conjunction with a risk-based management approach, has enabled the introduction of cost-risk optimisation tool WeldCRO, for P91 boiler piping lifetime management.
WeldCRO, a two-year project funded by the Electric Power Research Institute (EPRI) and completed by TWI in 2014, is the successful application of EPRI's knowledge base from previous work on P91 steel within an integrated user-oriented cost-risk optimisation (CRO) tool for lifecycle management of P91 steam lines. The new software is being rolled out through direct-host utility demonstration and subsequent implementation.
Improvement in power plant thermal efficiency requires development of creep strength enhanced ferritic steels, such as P91. Like all dispersion-strengthened low-alloy ferritic steels, P91 suffers substantial strength reductions in the welded condition, especially at elevated temperature - the so-called type IV cracking in the fine-grained heat-affected zone (HAZ).
Engineers faced with run-repair-replace decisions will be able to define risks and introduce risk mitigation measures through CRO. WeldCRO will enable these engineers to schedule inspection and maintenance plans and obtain maximum value from budgets.
The project team's objective was to develop user-friendly software to enable plant operators to plan the maintenance of P91 main steam piping and hot reheat piping through a CRO process.
The work covered:
- Main steam and hot reheat piping
- Fully quantitative probabilistic remaining life analyses
- Financial analysis targeted on CRO
- Software design and production
Programme of work
The main areas of assessment treated by the WeldCRO software are estimating remaining life based on the limit state equation, assessing the risk associated with creep damage at type IV regions of welded P91and making recommendations on the most economically effective mitigation plan.
The software estimates remaining life through encountering the operating conditions of the equipment. It will then conduct the risk assessment, combining the effect of probability of failure and consequence of failure. The system provides a sensitivity analysis for the user to assess the influence of different parameters, as well as the evaluation of the effect of different mitigation actions (ie non-destructive testing [NDT] inspection, repair/replacement or changing operating conditions). Finally, WeldCRO presents a cost-risk optimised mitigation schedule.