AIV Mitigation Considerations
(By Ian Goble, WKC Group)
The final step in an Acoustic Induced Vibration (AIV) assessment is the implementation of mitigation measures at locations with Likelihood of Failure (LOF) values of between 0.5 and 1, these mitigation measures can range from changes to process parameters, such as splitting the flow by adding additional valves, to changing fitting types and pipe specification. These mitigation measures attempt to reduce the LOF to a value below 0.5 in order to ensure there is limited risk of failure of the system, however care must be taken not to view these numbers in isolation and as a ‘pass / fail’ criteria, as the formula used to generate these numbers does not take certain design aspects into consideration and therefore a more nuanced approach must be taken when considering the appropriate mitigation measures required.
When considering the LOF values and mitigations which are required for these connections many considerations need to be made. The LOF value in itself is not to be seen as a pass / fail criteria but rather an indication of risk which is the primary function of the AIV screening assessment, as stated in the EI Guidelines (2008):
“The likelihood of failure (LOF) is a form of scoring to be used for screening purposes. The likelihood of failure is not an absolute probability of failure nor an absolute measure of failure.”
The LOF score should therefore be looked at not in isolation but also in the context of the type of connection, pipe thickness etc. these factors will be discussed below.
Factors to Consider
One of the factors that needs to be considered is the diameter/thickness (D/t) ratio of the mainline pipe. Based on a review conducted by Carucci and Mueller it can be determined that a D/t ratio of less than 64 is desirable for most high energy systems as no failures should occur above this point. Typically, recommendations should therefore be given such that the D/t ratios are below this value.
One of the primary mitigating measures to reduce the LOF score is to increase the thickness of the mainline pipe, this is generally increased such that the LOF is reduced to below 0.5 or an upper limit of 19mm is reached at which case it is not normal to increase past this point as there have been no documented failures of pipes above this wall thickness. Consideration should be given also to not excessively increase the thickness due to significantly increased weight of the piping system (increased static stress) and cost, this is why the thickness needs the be increased conservatively with consideration given to the connection type and D/t ratio and not just until a LOF of below 0.5 is reached.
A further limitation of the LOF scoring is that the score does not reflect some of the mitigation measures, for example replacing a Tee connection with a Forged Tee or Reinforced Fabricated Tee would not show a reduction in the LOF score, even though the forged Tee would eliminate the welds at the discontinuity essentially removing the risk of fatigue at the weld entirely whilst the Reinforced Fabricated Tee would have a lower stress concentration factor due to the reinforcement at the welds. This is also true for 2-plane bracing not effecting the LOF score.
Therefore, when reviewing the LOF scores consideration must be given to all these factors, not just a pass/fail at 0.5, to ensure that the mitigations recommended are both sufficient to reduce the LOF to a reasonable level but not to be over designed to a point where the mitigations become impractical (too heavy or outside standard thickness specifications) or prohibitively expensive.
Mitigation Measures to Consider to Reduce Likelihood of Failure
In certain cases, however, a Finite Element Analysis (FEA) assessment may still be necessary for certain discontinuity points. If all of the above factors have been considered and the LOF is still high with low confidence in the applied mitigation measures an FEA assessment should be conducted on the discontinuity point in order to verify if the recommendations are sufficient.