At constant inflow conditions, large-amplitude pressure and flow rate fluctuations may occur in a pipeline-riser system operating at relatively low liquid and gas flow rates. This periodic flow instability has been referred to as severe slugging. In this experimental study, three different orientation angles of the pipeline upstream of the riser base were investigated. The experiments were carried out in a downward inclined pipeline, in a horizontal pipeline and in a hilly-terrain pipeline followed by a vertical riser. Air and water were used as the experimental fluids. For each pipeline-riser configuration, different types of flow instability were found.
Pipeline-riser configurations in an offshore oil and gas production facility are required to transport multiphase hydrocarbons from a subsurface oil and gas reservoir to a central production platform. The diameter of the pipeline and the riser ranges from typically 0.1 to 0.8 m. The length of the pipeline can vary from a few kilometres to more than hundred kilometres. The height of the riser depends on the water depth, which can be more than two kilometres (in deepwater areas). At relatively low flow rates, liquid accumulates at the bottom of the riser, creating a blockage for the gas, until sufficient upstream pressure has been built up to flush the liquid slug out of the riser. After this liquid surge, and subsequent gas surge, part of the liquid in the riser falls back to the riser base to create a new blockage. This transient cyclic phenomenon is called severe slugging. Severe slugging can significantly reduce the production from the reservoir (due to an increased back pressure) and also can damage or even lead to a shut down of the platform facilities, downstream of the riser, like separators, pumps, and compressors.
A Lagrangian three-phase slug tracking model is demonstrated for a severe slugging case with gas, oil and water in an S-shaped riser. The model is an extension of a hybrid twophase flow scheme in which a two-fluid model formulation is used in the stratified flow region. In the slug body region, an incompressible flow model is applied. Mass and momentum conservation equations are solved for open and moving control volumes. This moving grid formulation allows for tracking of discontinuities within the flow, without numerical diffusion. A sub-grid two-fluid model in the bubble region allows for slug initiation directly from the two-fluid model in combination with a fine grid. Alternatively, a mechanistic slug initiation model may be used in combination with a coarse grid. The model is presented and applied to a severe slugging case in which oilwater separation occurs during slug build-up. The third phase is modeled with a mass conservation equation and an oil/water slip model in a mixture liquid momentum equation formulation.
The introduction of nuclear power generation in the mid 1950s increased the need for accurate and reliable methods to predict two-phase steam and water flows in pipe networks. This led to the development of several commercial codes for one-dimensional two-phase pipe simulation. Almost two decades later, a new field of application of the multiphase technology arose with the emergence of oil and gas production from deep water offshore fields. Several codes specifically designed for such systems were developed. These tools were to a large extent based on the modelling and simulation principles originally introduced with the nuclear technology, and were mainly suited for simulation of two-phase oil and gas flows. Here, an algebraic slip model for the relative velocity between the oil and the water was utilized.