Cost–Risk Software for P91 Steam Piping and Hot Reheat Piping

In this section

Summary

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.

Background

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.

Objectives

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.

WeldCRO input-output framework

Main functions of WeldCRO

The automatic meshing and stress analysis process

Users may also access number of innovative functions in the system, such as auto-meshing design for stress analysis using Abaqus, and a universal POD (probability of detection) calculator for evaluating different NDT techniques.

Future developments

A programme of further work is suggested covering the following areas.

Firstly, there is a requirement to quantify the development of creep damage with consumed life fraction in the Type IV region of P91 weld HAZ. An intensive series of interrupted cross-weld multi-heat plain and notch creep tests is envisaged. The need for this work is emphasised by the powerful refinement to the failure probability that such damage detection and quantification can provide.

Moreover, to accurately utilise the results of the stress analysis it is imperative that the multi-axial components of the stress state which drive the Type IV failure process in P91 are known. A series of multi-axial tests, eg bi-axial (shear), tri-axial (Bridgeman notch) and uni-axial creep rupture tests are planned. The work should build upon previous multi-axial work on low-alloy ferritic steels.

Besides, the understanding of the driven forces of creep crack growth, damage mechanism and failure modes of P91 weldments under high temperature is crucial. In WeldCRO, this determines whether failure occurs by leakage or ruptures, which is important because leakage is a safer failure mechanism. Finite element analyses combined with creep testing on notched tensile specimens with different geometry is proposed for this work.

In addition, the algorithm of WeldCRO, originally designed as a framework, allows implementation to be extended to other applicable damage mechanisms and materials.

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