Numerical simulation of pig motion in gas and liquid pipelines using the Flux-Corrected Transport method

The transportation of fluids in long pipelines occurs very often in the oil and gas industries. To keep high efficiency and productivity in pipeline operations, a device called pig is routinely passed throughout the pipeline for the purpose of cleaning, inspection, sealing, corrosion evaluation, among others, despite the uncertainties and risks associated with its passage inside a long duct. For these reasons, it is important to use numerical simulations to follow the motion of a pig inside a pipeline and estimate the pig travelling time. This paper presents a one-dimensional mechanical model, along with a numerical scheme, to obtain approximate solutions for the pig movement and for the transient one-phase flow inside the pipeline, which are physically and mathematically connected. In this model the pig is treated as a moving singular boundary and its mathematical model reduces to an initial-boundary-value problem. This system of equations is coupled to a boundary value problem for the transient fluid flow inside the duct. Since the pig is modeled as a moving surface, the pipeline is divided in two sections, one upstream and one downstream of the pig, and these two separate fluid flows are coupled with the pig motion system. The mathematical domain is discretized within the finite-volume framework and is solved using the Flux-Corrected Transport (FCT) method, which is second-order in space, as our results attest. We present simulations for the pig velocity history and for the distributions of flow pressure and velocity along the pipe in air and water single-phase flows. The results show physically consistent results for no-hole and one-hole pigs, for a rapid transient valve closure problem and in a direct comparison with a commercial software.