PSI - Issue 5

Chmelko Vladimír et al. / Procedia Structural Integrity 5 (2017) 614–619

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Garan, M., Chmelko, V./ Structural Integrity Procedia 00 (2017) 000 – 000

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assumption may change in localities of compressor stations because the dynamics of compressor work can induce unexpected vibrating of some adjacent pipelines as it was presented by Wachal, J. C., Smith, D. R., (1991), Radavich, (2001), Jungowski (1989), Rogers (1992). In the operation of line piping systems and their parts, the changing of the soil support and disrupting of the pipeline protection, can lead to the unwanted corrosion effect, whose result is a local loss of the pipeline wall thickness and corresponding increase of local stress. An increasingly frequent phenomenon, especially due to heavy rains, is the drop or slippage of the subsoil. As a result, there is a rise of additional bending stress, that is superimposed with the loading of internal stress. The most accidents of the pipeline systems arise as a result of the simultaneous interaction (system synergy) of these changes during operation like induced vibrations with additional bending stress from the drop of the subsoil i. e. corrosion decrease of the pipeline wall thickness at the point of that additional bending stress, etc. The creation of such non-standard or unprojected situation in real operation is difficult to predict. The only one possible solution, how to catch those state, is the development and implementation of monitoring systems that monitor not only the operating parameters like pressure and temperature of the transported media, but also the parameters of the construction that determine its reliable and safe operation. In the next, we introduce two monitoring systems which are able to monitor parameters like vibration of structure and its evaluation, fatigue damage or corrosion decrease of the pipeline wall. Their objective is to monitor the safety of the operation and also the integrity of these piping systems during transport of gas or oil.

Nomenclature  at

amplitude of total strain number of cycles to fracture depth of corrosion defect

N f

h 

time

2. Monitoring of vibrations and fatigue damage accumulation Nowadays, the measuring or monitoring of vibrations is a commonly used diagnosis of structures like base frames, shaft etc., where the effective accelerations or vibration velocities as well as corresponding frequencies of oscillating constructs are usually monitored. This diagnostic can be also used for monitoring of pipelines systems where we expect vibrations e.g. nearby the gas compressors. However, there is no relation between the magnitude of acceleration and velocity, or displacement of oscillation and corresponding fatigue damage at the appropriate pipeline cross-section e.g., there is no relationship between the velocity of vibration and the stress amplitude at that critical monitoring cross section place of the structure if we know the fatigue properties of the material. When evaluating the presence of the oscillation, it is necessary to realize that the oscillation itself does not automatically reduce the fatigue life of the critical cross-section. This requires the presence of a variable amplitude of the strain in the monitored cross-section that results only from the relative displacement of the two parts of the pipeline opposite to each other. In case, if the base tube and another T-shape tube, that is perpendicular connected to it oscillate with the same phase and with the same frequency, then the tension in the common cross-section may not even occur. Therefore, it is very difficult, based on the measurement of an oscillation parameter on the pipeline, like (acceleration, velocity or displacement), to consider about the admissibility or inadmissibility for operation. In fact, you need to know e.g. the deflection of the pipeline oscillation as well as the deflection of its detour. The difference of these deflections is a deformation at the cross-section of their joints and the corresponding amplitude of the stress. Measurements have shown that individual pipe sections oscillate not only in the direction of gas flow but also in the perpendicular direction, so finding the direction of the maximum vector of relative displacement between the two pipes is practically impossible in real time. A suitable method of vibration monitoring and assessing its permissibility can only be achieved by direct measure of strains (stresses) in critical cross-sections (e.g. T-shape weld joints) in a way that makes it possible to evaluate their maximum values in the cross-section (see Chmelko and Garan, 2016). These measured strains with respect of time must be evaluated in the operational safety point of view in relation to the fatigue properties of given material or construction node. For instant (in-time) evaluation of oscillation permissibility, it is appropriate to use dependence describing by following formula