This study reports the results of an investigation relevant to the formation of comminglement volumes at the interface of different liquid fuels, to be transported in the Genova-Ferrera pipeline, owned by Eni and to be operated as a multiproduct system. This pipeline, operated in the past as an oil transport system, cannot be considered as a standard multiproduct pipeline, being a large size pipe with an elevation profile characterized by significant elevation differences.
The contamination volumes have been evaluated considering or not considering the use of interface separation pigs. The analysis without separation pigs has been carried out developing a new calculation model based on the Austin-Palfrey correlation, estimating the interface comminglement volumes for almost horizontal pipelines, modified by adding a gravity spreading model applied to tilted pipeline segments. In fact, density differences in inclined sections can cause an increased mixing volume due to the gravity induced spreading. Evaluations for both stable flow and accidental shutdown conditions have been carried out. In all cases without separation pigs the calculated contamination volumes exceed the allowable limits for this system.
Then the transport has been simulated considering the use of separation pigs (one or more). This is the only possible configuration that could avoid large fluid mixing effects, especially during flow stoppage with interface in inclined pipeline segments. Simulation models have been built using the commercial software OLGA and its pig model, in particular through the relevant slippage sub-model. This theory predicts that, due to slip between the pig and the surrounding fluid, some fluid ahead of the pig will not be displaced but rather leaks between the pig and the pipe wall, flowing back within the boundary layer. This picture is a well recognized driver of the comminglement mechanism in this kind of transport systems. A reliable fluid back leakage estimation depends on the evaluation of the physical gap among the pig sealing discs and the pipe wall. Multiple sealing discs are usually installed in the pig structure, normally four; their combined effects can be represented by an equivalent gap in the simulation model. Sensitivities have been performed to assess the expected leakage amount.
A final field-testing activity was performed to verify and tune the OLGA flow model with separation pigs. The pipeline was equipped with four sensing elements installed at different locations along the route and measuring the fluid speed of sound. Measurements have demonstrated that most of the mixing occurred at pipeline inlet due to a pig misplacing with respect to the correct interface position, while the back leakage amount was found minimal, corresponding to small disc to wall gaps as estimated in the previous simulation phase.
The models developed in this study can then be used to predict in a reliable way the behaviour of a multiproduct pipeline characterized by a significant hilly routing, and they confirm that the use of properly sized separation pigs can be an effective method to minimize interface contamination, provided that a proper pig launching control procedure is developed.
