Acoustic Fatigue Analysis of Weld on a Pressure Relief Line

Aker Solutions in Glasgow has recently completed a project in Acoustic Fatigue Analysis of a welded connection that failed the EI Guidelines [1]. A detailed finite element model of the connection was developed and an acoustic excitation equivalent to the acoustic power calculated from Carucci & Mueller empirical data was applied [2]. This was achieved by generating a stress field associated with acoustic wave whereby stresses resulting from excitation in the range 0.0-2.5 kHz, were summed up. The summation took into consideration the random nature of the acoustic noise field. A 2% hysteretic damping was assumed which is considered to be adequate. The analysis indicates that acoustic induced stresses are well below the allowable stress at the weld and that fatigue damage per safety relief valve operation is small enough to mitigate 25 years of life time.The analysis therefore quantifies the conservatism embedded in the Carucci & Mueller empirical data and demonstrates that risk assessment tools should not be applied to effect design changes without further investigation. The acoustic field generated is assumed to peak at 500Hz corresponding to Strouhal number of 0.2 and 5% attenuation is assumed allowing 2.5 kHz band width.