Hydrodynamic and Terrain Induced Slug Sizing

FEESA employ several engineers with PhDs in Multiphase flow and remain close to the cutting edge of slug flow modelling research by being members of Imperial College’s Transient Multiphase Flow (TMF) Research Programme.  This ensures we have an up to date understanding of the pro’s and con’s of the various slug flow modelling methods as well as a practical understanding of their relevance on projects.

Severe Slugging Avoidance

The severe slugging phenomenon is illustrated in the animation below.  The phenomenon is a cyclical production of liquid and gas coupled with cyclical flowline pressure fluctuations.  The first phase of the cycle is referred to as ‘slug formation’.  Here the base of the riser has become blocked with liquid preventing free passage of gas.  The pressure in the pipeline then increases as more liquid runs down to the base of the pipeline increasing the size of the liquid slug.  The system continues in this fashion until the pressure has built sufficiently to overcome the gravitational head associated with the liquid slug.  The system is then hydrodynamically unstable and the liquid slug is discharge rapidly up the riser followed immediately by a gas surge as the pipeline blows down.  The pressure in the pipeline then returns to a low value, leading to insufficient gas velocities to carry the liquids up the riser, and the process is repeated.

The animation below shows results of a transient multiphase flowline simulation in the slug flow regime.  The animation shows how the liquid periodically builds up in several dips near the riser base and are then purged, dropping the flowline pressure.

Hydrate Avoidance During Shutdown

The design of most production systems must include a provision for what to do in the event of an unplanned shutdown.  Most insulated systems require a reasonable cooldown time performance in order to give the Operator a reasonable “hold-time” before they must begin a hydrate remediation operation.  FEESA consultants have developed the hydrate avoidance strategies of dozens of different systems in collaboration with Operators and hardware vendors to ensure that they are robust and feasible.

Hydrate Avoidance and Slug Management During Restart

FEESA has designed Restart Strategies of several offshore developments to ensure that they manage within the limitations of tight slugcatcher surge, insulation performance and/or inhibitor injection system constraints.  It is essential to do this with consideration of the demand it puts on the Operators team, given that such events should be relatively rare operations.

Hydrate Avoidance and Slug Management Dead Oil Displacement

Dead Oil Displacement is one alternative to hydrate avoidance if a pipeline is cooling towards the hydrate region and a restart is not possible.  FEESA consultants have great experience at design such operations and sizing the associated equipment.  Our knowledge of the pro’s and con’s of the various simulator tools, the physical processes that go on during such operations and regular attendances at conferences where results from such operations are discussed ensure that a robust solution is found.

Reaching Steady State in a Long Oil Pipeline

FEESA has project experience at the design and operation of various long distance viscous crude pumping systems.  In this example, the crude in question had a viscosity that was a strong function of temperature and pressure at conditions close to the operating conditions.  Consequently thermal hydraulic issues such as heat loss to the environment and transient issues such as such as start-up were a big issue to the development as they meant that the time taken to reach the steady state design rate was considerable.

FEESA also has experience of the development of pigging strategies for waxy oils. For more information on the projects we have worked on, click here.