8 research outputs found
A novel investigation into the application of non-destructive evaluation for vibration assessment and analysis of in-service pipes
Flow induced vibrations that are close to resonance frequencies are a major
problem in all oil and gas processing industries, so all piping systems require
regular condition monitoring and inspection to assess changes in their dynamic
characteristics and structural integrity in order to prevent catastrophic failures.
One of the main causes of pipe failure is weak support causing low frequency
high amplitude flow-induced vibration. This causes wear and tear, especially
near joints due to their dissimilar stiffness resulting in fatigue failure of joints
caused by vibration-induced high cyclic stress. Other contributing factors in pipe
failure are poor or inadequate design, poor workmanship during installation or
maintenance and inadequate or weak and flexible support. These pipes are
usually required to work non-stop for 24 hours a day 7 days a week for weeks,
months or years at a time. Regular monitoring and in-service dynamic analysis
should ensure continuous and safe operation. A novel method of non-destructive
testing and evaluation of these pipes, while in service, is proposed in this paper.
This technique will enable early detection and identification of the root causes of
any impending failure due to excess vibration as a result of cyclic force induced
by the flow. The method pinpoints the location of the impending failure prior to
condition-based maintenance procedures. The technique relies on the combined
application of Operating Deflection Shapes (ODS) analysis and computational
mechanics utilizing Finite Element Analysis (FEA), i.e. linear elastic stress
analysis. Any structural modification to the pipes and their supports can then be
applied virtually and their effects on the system can be analysed. The effect on
vibration levels is assessed and verified. The effect of any change in the forces
corresponding to changes in the Differential Pressure (DP) at constant flow rate
through the pipes can then be estimated. It was concluded that maintaining the
differential pressure above some “critical” threshold ensures the pipe operates
under the allowable dynamic stress for a theoretically “indefinite” life cycle
Superluminal Radio Sources
Several compact radio sources studied with VLBI show apparent transverse velocities much greater than the speed of light. This review is a summary of VLBI observations of these “superluminal” sources as of August 1981. The physical models proposed to explain this phenomenon are also discussed