Pressure Propagation and Flow Restart in the Subsea Pipeline Network

Abstract

Waxy crude oil transportation at cold subsea condition encounters a severe flow assurance problem. Occasional maintenance and emergency shutdown requirement of the crude oil pipeline may profound gelation process. The gelation of crude oil may result in pipeline blockage. In order to restart flow in a gelled pipeline, a high axial pressure gradient is applied, across the gel plug, to breakdown the gel structure. During shutdown, waxy crude gelation leads to shrinkage in the gel structure as seen in dead oil (gas de-saturated oil) and subsequently releases free gases. Free gases result in void formation and a multi-plugged gel in the industrial pipeline. The gel separated by gas pocket is termed as multi-plugged gel, which may form due to uneven earth surface, especially on sea-bed. In this work, flow restart in multi-plug gelled pipeline is investigated by solving mass and momentum balance equations together with strain-dependent rheological equation. The volume of fluid (VOF) method is utilized to distinguish bulk phases and the advection equation of volume fraction traces the motion of gel–gas interface. The time evolution of applied pressure propagation in the gelled pipeline has been compared with earlier works. It shows that the present profile exactly matches with the pressure profile in the single-phase flow before it encounters the gas pocket. The effect of multi-plug in restart operation may result in early gel degradation and flow restart. The result obtained in this work can be used to reduce the margin of safety commonly adopted due to overestimated pressure requirement.