Characteristics and forces due to slugs in an 'S' shaped riser

The characteristics and forces due to slugs in an 'S' shaped riser have been investigated. A series of experiments were carried out using the Cranfield University Riser Test Facility, using a 9.9m high riser. Single phase (water) and two phase (air and water) tests were conducted at a system pressure of 2 bara. The two phase tests covered a range of flow regimes: severe slugging, transitional severe slugging, oscillation and normal slug flow. The two phase data was used to investigate the characteristics of severe slugging, to determine the liquid inventory in the riser downward limb and, to model the forces on a bend during the slug build-up, production and bubble penetration and gas blowdown stages of the severe slugging cycle. During the bubble penetration and gas blowdown stage, high velocities and large fluctuations in the force were observed. The resultant dynamic forces on the bend during the slug build-up and production stages were small compared to the bubble penetration and gas blowdown stage, and dominated by the hydrostatic forces due to liquid in the riser. Normal slug flow is potentially more problematic in terms of fatigue damage than severe slug flow, due to the higher velocity of the slugs coupled with large dynamic forces.