I have a subsea gas well with reasonable amount of water content. Tieback distance to the host platform is about 5 miles, and it is an 8" pipeline. Initial calculations  show that there'll be slugging. So my flow assurance engineer advises periodical pigging operation.

My question is, is there any other slug mitigation method as effective or even better than pigging? As much as possible I don't want to do subsea pigging, it is  costly.

>>>>>>>>>>>>>>>>>>>>>Safety valves

Pigging does not prevent slugging, pigging causes slugging as the pigs/spheres push accumulated liquids ahead of the them!  The only difference is that when you launch a pig, you have an idea of when the slug will arrive and can prepare for it.  Otherwise, slugs arrive when the physics of the pipeline decide its no longer possible to leave the accumulated liquids undisturbed and pick them up and blow them along downstream all at once.

You can try to keep flow velocities high enough such that liquids are sweept along and the slugging flow regime is avoided.  Slugging is more prevelant at low velocities.  The flow regeime also depends on ratio of gas to liquid flowrates and slope of the pipeline, so it still may not be possible to avoid, but you might be able to minimize it.  

Usually its a gas well, Natural Gas that also produces associated water as well as some quantities of Gas condensates.  Slugging would then be the water and the condensates that tend to collect in low points or at the base of a riser to the next platform until they reduce the gas flow to the point where pressure builds and eventually  increases pressure and velocity enough to sweep the liquids out at once.

Whether he needs a vessel "slug catcher" or an extended dead-end pipeline segment "drip", increase velocities or implement regular pigging depends on the ratio of the quantity of liquids produced to that of gas and the resultant flow regime.  The problem with large vessels located upstream and close to the well, so that pigging the pipelines can be avoided, is that offshore space is very costly for placing large mostly empty vessels that can weigh a lot if they do happen to get full.  Additionally, a means to empty the vessel to a boat must also be employed.  Again not cheap.  If velocities can't be keep high enough to continuously sweep the line, the standard solution is to pig the liquids all the way back to the beach in a 2-phase flow pipeline to where a large vessel can be economically positioned onshore.

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2011-01-25

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