ABSTRACT

The modern ultrasonic computer-based nondestructive testing system Augur 4.2 with coherent data processing is presented. First results of its practical on-site application to expert in-service inspection of primary circuit piping weldments are discussed. In brief, methodology of strength assessments using Augur 4.2 data is considered

1. INTRODUCTION

Insurance in pressure boundary integrity of NPP unit is strongly influenced by technical capabilities and efficiency of metal examination system. Ordinary ultrasonic examination tools and procedures have limitations in flaw sizing and positioning. The problems arise for welds and repair zones of welds made by filler materials of austenitic type.

Up-to-day reliability requirements as applied to NPP strength maintenance govern development of new generation of computerized systems for in-service inspection (ISI). These systems in parallel with capabilities of ordinary ultrasonic techniques allow to reconstruct high resolution image of inner flaw and increase available amount of information

2. Augur 4.2 MAIN CHARACTERISTIC FEATURES

Recently, series of computerized defectoscopes Augur for inspected area image reconstruction have been developed in Russia in the framework of RF Minatom program. One of them is device with coherent data processing for expert inspection named Augur 4.2. In the end of 1995 it have passed appropriate testing under supervision of Russian Regulatory Authorities and was recommended by them to be applied on-site during expert ISI. One of the main characteristic features of this device is the capability of precise flaw image reconstruction which enables to discover flaw real geometric parameters. Capabilities of good revealing planar type defects make Augur 4.2 an advantageous tool for inspection performance. Defectoscope Augur is a complex device including Notebook PC with additional electronic boards, small-size automatic two-coordinate scanner, set of piezoelectric transducers, up to 25 m communication line.

Augur 4.2 software is used to obtain two- and three-dimensional images of defects using various coherent data processing methods (angular spectra method, method of projection in spectrum space, reference hologram method), determine the sizes of defects in different sections, execute service and report preparation operations.

The Augur 4.2 system fundamental difference from a conventional flaw detector consists in the providing of high-quality images of defects by the use of coherent processing of the echo-signals. As a result of the processing on the basis of longitudinal 2.5 MHz wave in steel the lateral resolution of the flaw images is equal to 2.5 mm and does not depend on the depth (Fig. 1). The high resolution of Augur 4.2 defines precise determination of defect coordinates, dimensions and orientation.

Fig. 1. Demonstration of Augur 4.2 lateral resolution: a) testing scheme; b) conventional non-coherent image of the defects; c) Augur 4.2 coherent image; d) processed signal distribution.

Expert judgement on flaw sizes is obtained by the means of Augur 4.2 data processing. Based on this information strength and lifetime assessment could be updated. Augur 4.2 inspection records are stored for ten years in full amount and could be retrieved and re-analysed, for example, for the purpose of comparative analysis of consecutive inspections data.

Validation of Augur 4.2 according to internationally recognized procedure has been performed in 1996 in Risley NDE Validation Center (AEA Technology, UK). According to the program of validation tests there were performed inspections of a set of certified samples (pipes and parts of pressure vessels) with different type flaws (real fatigue cracks as well as artificial defects imitating crack-like flaws, pores, incomplete fusion zones).

3. Augur 4.2 PROCEDURE FOR PRIMARY CIRCUIT PIPING

A separate works have been carried out in order to develop Augur 4.2 procedure for expert ISI of primary circuit piping welds. A series of testing have been executed on full-scale parts of the weldments with defects (Fig. 2).

Fig. 2. Defect indications in the tested piping welds revealed by Augur 4.2

Fig. 3. Zoomed isometric view of the weld defect revealed by Augur 4.2

The main purpose of Augur 4.2 expert ISI is the application to zones where some flaws (indications) had been revealed by scheduled manual ultrasonic inspections. Information provided by Augur 4.2 is used for decision-making: further surveillance of these zones or corrective maintenance.

Before inspection, calibration test is carried out using test-blocks with artificial flaw models. Inspection is performed by shear-wave transducers with main frequency of 2.5 MHz and inclination angle of 42 and 62 degrees. Measurements of two types are applied in compliance with Augur 4.2 ISI procedure to provide high level of resolution of flaw images both along and transverse weld. Measurements from both sides of weld (where available) are carried out to obtain maximum information. Augur records, copied from acquisition computer to ZlP-diskette, are processed on another computer outside contaminated zone. During record processing flaw images are restored, analysed and described in protocol of expert inspection by the parameters as length, upper and lower edges distance from pipe surface. The images of a flaws detected by Augur in primary circuit piping welds are shown in Fig. 3, 4.

Fig. 4. Two images of the defect in the primary circuit weld detected by Augur 4.2

4. SOME RESULTS OF Augur APPLICATIONS

The procedure of Augur 4.2 examination is included into the approved general scheme of ISI [1]. Results of Augur 4.2>> ISI for primary circulation piping weldments at RBMK NPP Unit are given below. Representative set of weldments were examined: site welds (with pearlitic type weld metal) and two types of site weld repair zones with different welding technologies: using Cr-Ni filler metals and Ni-based alloy. Results of Augur 4.2 inspection were compared with manual ultrasonic inspection ones in terms of defect detection, characterization, positioning and sizing in order to estimate correlation between these data.

Comparative analysis for both types repaired weldment zones showed the following:

1. All defects detected by manual inspection were detected by Augur 4.2 system too. But Augur 4.2 indicated (by upper edge of defect) that the defects were located closer to the internal surface of pipe in comparison with manual inspection data.
2. Augur 4.2 revealed a number of <> defects with the height of about 2 mm did not detected by manual inspection.
3. Augur 4.2 provided necessary data for precise fracture analysis as applied to several near-by-located defects.
4. There were several complex geometry cases for which one defect was detected by manual inspection, but two defects located at the same distance from outer surface in parallel planes were detected by Augur 4.2.

Flaw dimensions in the direction of wall thickness in major cases were about 2 mm, and the flaws were located near the boundaries of cut-off zone. Finite element analysis showed that these defects likely had originated after first passes of repair welding and are caused by high level residual tensile stresses.

Due to the fact of small flaw dimensions discovered their possible growth to critical sizes can be surely excluded. Nevertheless surveillance of these flawed zones by means of Augur 4.2 is foreseen during further ISI to reveal flaw growth.

Using the results of the identification analysis recommendations on Augur 4.2 application on the basis of manual inspection records were formulated: first of all, for defects with indications less than 28-30 mm, or if group of near-located defects was detected.

As applied to site pearlitic welds the following conclusions have been done:

1. All defects detected by manual inspection were detected by the Augur 4.2 system too. As a rule, good correlation was observed between Augur 4.2 coordinate of defect upper edge and manual inspection data.
2. Correlation between amplitude or equivalent square data of manual control and defect height showed by Augur 4.2>> was poor.
3. There were cases in which the extensive defect, detected by manual inspection, was cut into two or three isolated defects with small length by Augur 4.2.
4. As applied to long defects detected in the middle part of wall thickness Augur control data had no effect on the defect acceptance.

A number of defects with manual control indications clarified by Augur 4.2 records have been accepted for further operation with prescription of next year Augur 4.2>> inspection.

5. SOFTWARE FOR STRENGTH SUPPORTING OF Augur 4.2 ISI

Information supplied by Augur 4.2 on precision defect dimensions has decisive influence on fracture assessment of the defect. Results of Augur 4.2 ultrasonic examination of main circuit pipeline weldments show that in order to take advantage of Augur 4.2 potentialities in full measure advanced methods of defect assessment should be applied using computer modelling and software developments [2].

Now computer system of defect assessments as applied to primary circuit piping is under development as the tool of Augur 4.2>> ISI strength supporting. Results of defect assessments are governed by the use in the proper way adequate data on stress distributions and defect geometry.

In order to describe inherited stress state of weldment the finite element modelling results are used. A series of finite element calculations were conducted to model step-by-step residual stresses as well as its redistribution due to heat treatment and operation [3]. The solutions for the reference weldment geometries are collected in the data base. If necessary (some variants of repair) the modelling is executed for this specific case.

The Augur 4.2 information on defect configuration is used to develop the three-dimensional solid model of damaged pipeline weldment by the use of geometry editor. The editor options provide by easy way creation and changing of the solid model. This model is used for fracture analysis by boundary element method with appropriate cross-section stress distribution and external loads.

The computational process of analysis is hidden from the user, and visually the analysis is conducted in terms of R6-assessment procedure [4]. On the basis of data of stress state and defect configuration the necessary assessment parameters (limit load, stress intensity factor variation along the crack-like defect edge) are determined. Special attention is devoted to realization of sensitivity analysis. Effect of variations in calculated stress distribution and defect configuration provided by Augur are estimated by built-in way.

The first results of computer-based assessment system application show that the benefits are obvious for repaired (without heat treatment) welds and complex defect configurations: defect with height local increasing, group of defects, case analysis of defects interference and possible joining.

6. CONCLUSIONS

1. Application of Augur 4.2 to different type of weld zones have showed it practical capabilities in revealing real dimensions of flaws both in pearlitic and austenitic type weld materials.
2. Augur 4.2 information on inspected zones reduces the level of ISI results uncertainty and therefore gives additional insurance of pressure boundary integrity.
3. The benefits of Augur 4.2 application are supported by strength maintenance software, based on computer modelling and realizing assessment procedure.

REFERENCES