Design of Cost-Effective Solution Against Acoustic Induced Vibration through Advanced FE Analysis
Authors: Karim Hafiz , Ancian Loïc 
Published: October 2017

Abstract
Acoustic Induced Vibration (AIV) refers to the high acoustic energy generated by pressure-reducing devices that excite pipe shell vibration modes, producing excessive dynamic stress. Analysis of this risk is an important part of Asset Integrity Management systems as AIV can cause catastrophic piping failure. Existing guidelines address this risk through an analytical assessment. However, these methodologies are not fully known and input parameters are limited. Some limits to the guidelines are pointed out with recommendations to improve them.
The approach presented for identifying AIV damage is based on a dynamic stress evaluation at pipe discontinuities (welded connections and supports). This evaluation is performed through a fluid-structure coupling Finite Element Analysis. Pressure fluctuations inside the pipe are predicted and coupled with a pipe structural analysis. This methodology is provided with its validation through measurement on an actual AIV field case, corresponding to a crack initiation due to AIV on a FPSO flare network tail pipe.
To conclude the paper, the method is then applied to quantitatively assess the mitigation actions' efficiency base on an actual case. Different solutions have been tested individually to end up with a final solution that reduces the damage to acceptable levels in the most cost-effective manner.
The sources of this high acoustic energy are pressure-reducing devices (valves, restricted orifices…) with high pressure drop and important mass flow rate. In such devices, the amount of energy dissipated is quite high, although most of the energy is converted to heat but there is still a significant part converted to sound or pressure waves that will excite the pipe wall. This broadband and high-frequency excitation propagates through the pipe, amplified by transverse acoustic pipe modes which later excite the pipe's shell vibration mode. While running along straight pipes, the impact of vibration is limited due to axisymmetry of the pipe shell mode shape. However, when the excitation comes to a non-axisymmetrical discontinuity (branch, small bore, support…), vibrations are amplified, leading to high dynamic stress that can cause pipe fatigue failure. As these vibrations occur at high frequencies, i.e. with a high fatigue cycle rate, fatigue failure occurs within a few minutes to a few hours.
Figure 1 (a) Fluid Acoustic mode; (b) Pipe shell mode.
For offshore plant, the major risk associated with this phenomenon is related to flare systems. Blowdown valves, restricted orifices and pressure safety valves in these systems usually encounter large pressure drops and important mass flow rates. As the acoustic energy generated by these devices propagates downstream with small attenuation, the whole flare network is affected by the risk of AIV failure. Flare systems are gas associated systems and they are safety-related, potential pipe failure could lead to catastrophic consequences. Therefore, assessing and controlling the AIV risk is an essential part of Asset Integrity Management.
AIV has been an on-going research subject since initial publications in the late 70s and methodologies have been developed to help engineers to assess this risk. The dominant methodology for the Oil & Gas industry is published by the Energy Institute. In Energy Institute guidelines address the AIV risk through an analytical assessment methodology. It is very efficient in performing a quick screening for large numbers of pipes. However, when it comes to mitigation measures, the limited number of input parameters used to quantify the Likelihood of Failure (LOF) reduces the range of possible mitigation measures. Since the efficiency of certain mitigation measures are not LOF calculation parameters and therefore cannot be assessed.
To overcome this limitation, a new detailed Finite Element methodology has been developed using the coupling between fluid and structure, making it possible to predict dynamic stress for complex piping models. This methodology will be introduced in the next chapter, including a validation through measurements on actual AIV field case.
Using this methodology, different AIV mitigation actions (such as: the use of sweepolets, forged tees, full encirclement supports, full encirclement wrap branch reinforcements) that not included in the scopes of Energy Institute guidelines are assessed. Comparison between computation results with and without mitigation measures makes it possible to quantify the impact of such modifications accurately and to establish a LOF adjustment coefficient when using these modifications.