High-pressure gas release from subsea pipelines: Multiphase modelling and CFD simulation for consequences analysis in risk assessment

The aim of this work is to assess the time evolution of natural gas release from high-pressure subsea pipelines (117 and 137 barg) to both assess the consequences of it on the sea surface and delimit the hazard areas for offshore structures, vessels, ships and workers. Such a hazard analysis is applied for the first time to risk assessment by a quantitative determination of the area of the different hazard zones on the sea surface as a function of the following parameters: i) water depth of release, ii) release flow rate, iii) sea current and iv) wind velocity. In this way, safety maps are developed from the obtained results based on different scenarios of acci-dental events. In detail, when a fire scenario is developed, the hazard area is delimited by the inflammable limits of natural gas in air. To perform this investigation, multiphase computational fluid dynamic simulations involving three phases - i.e., released gas, seawater and air - are carried out using the open-source software OpenFOAM (R). The methodology introduced in this work goes beyond the current state-of-the-art simulations, as it takes into account the mutual interactions among all the three phases present in the system, for which there is no analytical model able to describe the behaviour of the three phases with a certain degree of precision. In particular, a multiphase hybrid model pairing Euler-Euler and VOF methods is implemented. As main results, the shape of the release gas subsea cone and superficial plume are precisely evaluated as a function of the above -mentioned operating conditions, determining the evolution of the gas dispersion also in the seawater, which is crucial in the description of accidental scenarios resulting from the consequences analysis. In particular, it is found that, at a water depth of release of 55 m, the cone diameter at the sea level is larger than that at 15 m due to dispersion of gas in the sea. On the other side, wind velocity does not affect significantly the cone diameter.