Acoustic Induced Vibration, more commonly called AIV, is the broadband excitation of pipework when high-pressure gas is let down through pressure-reducing devices. The most widely adopted screening method for AIV assessment is the Energy Institute [EI] guidelines; however, the guidelines primarily employ a risk-based assessment method and lacks an approach to measuring the resistance level of the pipework against AIV. A fatigue-based approach to measuring the resistance of pipework against AIV is presented in this paper.
Previous studies have shown that correlations exist between acoustic modes in the pipe and structural mode frequencies. This paper takes this notion further, translating the structural modes into a predictable fatigue life using S-N curves. A typical AIV broadband excitation level can range anywhere between 100Hz – 3000Hz. Modal analysis of the pipework near discontinuities is performed, and frequencies are screened at levels nearly up to 2700Hz. The maximum modal stresses are taken at the elements for each mode, and the total stress cycles are counted to estimate the fatigue damage. A damage sum of unity is then used as the basis for calculating the acoustic fatigue life.
In addition to acoustic cycles, AIV-affected lines on the offshore floating facilities such as FPSO's and FLNG's are also subjected to functional (pressure and temperature) and environmental (vessel motion and acceleration) cyclical loads. This often leads to insufficient fatigue damage allocation resulting in low acoustic fatigue life. Most of the AIV-related failures can be attributed to high cycle fatigue failures at the geometrical discontinuities in the pipework. Small-bore discontinuities on the main header tend to display lower acoustic fatigue resistance than large diameter connections. Early identification and mitigation of sensitive discontinuities in pipework under cyclical loads and maximizing the damage allocation can improve acoustic fatigue life.
Concerns surrounding the risk of AIV failures on offshore facilities have become more prevalent in recent times due to higher plant output and increased gas flow rates and velocities in the pipework. When the screening method predicts concern for AIV, a fatigue-based approach can be used to establish acceptance criteria and help prevent expensive redesign of the piping system during execution.