PSI - Issue 5

Chmelko Vladimír et al. / Procedia Structural Integrity 5 (2017) 614–619 Garan, M., Chmelko, V./ Structural Integrity Procedia 00 (2017) 000 – 000

3

616



 f

at f N 2  

,

(1)

which is closer justified and described by Chmelko et all. (2015). The question of dangerous oscillation assessment remains undenounced. It is obvious, that the dangerous vibrations are significant in the view of possibility of creation the fatigue crack, also they are accompanying with generated unwanted noise. Piping with internal media pressure generates vibrations that depend on pipeline type and also the shape. In general, the pipelines oscillate in a wide spectrum of frequencies. The integrity of pipelines only threatens the frequencies that cause significant magnitude of strain amplitudes. From comparing of the frequency composition of the oscillation shown in Fig. 1, obtained by accelerometers and strain gauges on the identical section of the pipeline, it is obvious that significant deformation amplitudes occur only at frequencies up to 3 Hz. Because of the high mean stresses, due to internal pressure, it is not possible to easily compare the measured amplitudes of the strain from vibration with their safe limit against the fatigue crack. It is necessary to separate the individual oscillation cycles and take into account the mean value of the cycle. The real-time monitoring of dangerous vibrations can be accomplished by processing the measured time process of strains in time batches, e.g. in 30 or 60 seconds. The obtained amplitude of the strains after separation from their time process with the considering mean value of the cycle, has to be compared with the safe cyclic deformation limit. From equation (1) it is according to the value Chmelko (2014), 6 5 10 0 8 . at .  (2) for given probability of survival. The deformation amplitudes exceeding this value must be labeled as dangerous oscillations and recorded by the monitoring system. If the monitoring system monitors in-time also the fatigue damage progress, it is necessary to pay attention on amplitude separation using the Rain-flow algorithm by Amzallag (1994), and also choose the suitable hypothesis of fatigue damage accumulation. More specifically, this methodology is described by Chmelko et all. (2015).

Fig. 1 FFT analysis of typical processes of pipeline vibrations (left) and stresses (right)