|TABLE OF CONTENTS|
- - ABSTRACT
2. MAIN CHARACTERISTIC FEATURES
3. PROCEDURE FOR PRIMARY CIRCUIT PIPING
4. SOME RESULTS OF Augur APPLICATIONS
5. SOFTWARE FOR STRENGTH SUPPORTING OF
- - REFERENCES
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
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).
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
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|
Comparative analysis for both types repaired weldment zones showed the following:
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:
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.
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 . 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 . 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.