## 1 Introduction  
Nowadays the status of complex (especially dangerous) industrial structures is widely assessed by structural health monitoring (SHM), that is the continuous evaluation of their structural health [1]. The main reasons to apply SHM are:  
* access to the structure during operation is difficult or impossible;
* periodic nondestructive testing (NDT) is not possible or is time-consuming;
* temporary stoppage of equipment for its testing induces large financial losses;
* rapid development of operational defects and, as a result, low damage tolerance of the structure;
* the consequences of structural failure, which can lead to large material losses and a danger for maintenance personnel;
* the presence of known defects when it is necessary to extend the lifetime of the equipment.  
SHM of equipment allows the early detection of most of the failures and alerts maintenance personnel about developing malfunction that already exists but is not yet dangerous and does not disrupt the operability of the equipment. Among various NDT methods, the acoustic emission (AE) method is the most effective one in the monitoring mode. This method due to the possibility of remote testing and high sensitivity in the crack detection allows real-time monitoring of the structures with a length of up to several hundred meters. It is important to emphasize that the detectability of a defect is not affected by its orientation. AE method makes it possible to detect the following types of defects: cracks, microcracks, various types of corrosion damage and leaks [2]. Despite all the advantages of AE method, other types of testing should be used to provide a more accurate and objective assessment of the structural health [3; 4; 5; 6; 7]. This is caused by several reasons: firstly, in the case of usual AE testing, a temporarily decommissioned structure must be additionally loaded to stimulate the growth of the defects, while in the monitoring mode this need disappears, since testing is performed continually and AE sources emit under the influence of variations in operational loads, the value of which is not precisely known. Therefore, it must be ensured that the almost unlimited time of the AE testing can compensate the reduced magnitude of the load changes. Secondly, AE testing in the monitoring mode is complicated by increased process acoustic noise of the equipment. Experience in the use of monitoring systems shows that under such conditions defects are reliably detected only at the final stages of destruction, when the number of AE signals and their amplitudes increase by more than an order of magnitude. All this leads to the need of complex monitoring, that is the use of AE method with other NDT methods, as well as with the tracking of operational parameters.  
Also, the design and implementation of structural health monitoring system (SHM-system) should always take into account the specificity of the monitored structures. It is necessary first to study the design features of the structure and the results of previous investigations, analyze the types of loads and factors that generate defects [8]. For the determination of the zones in the structure, in which the occurrence of cracks is most likely, the stress-strain state of the structure can be numerically simulated. Then a complete investigation of the structure is carried out, usually with trial AE testing. As a result, the acoustic parameters of the structure are determined, with the types of possible defects, their nature of developing, reasons of the formation, their most probable positions and their influence on the lifetime of the structure are revealed. Sufficiency of load changes for the stimulation of AE sources is estimated. After that, the selection of methods and instruments of NDT and the positions of the sensors are determined. In addition, in the course of preliminary work, noise filtering and data analysis procedures are developed for each specific structure (due to the large volume of primary data).