Cheryl Anderson submitted a report on a synthetic mud spill from the Henry Goodrich which is operating offshore Newfoundland.
An estimated 26,400 litres (approximately 166 bbl) synthetic based mud spill occurred Monday east of St. John’s from Suncor Energy Henry Goodrich rig while drilling an exploration well on Monday but not reported until today – a three day delay. Preliminary reports indicate the cause may have been a valve left open on the mud pit. vocm.com
Bud's Offshore Energy (BOE) 
It brings up the whole issue of gas in a marine riser with SOBM in use. This would be the case where a small amount of gas is in the riser above the BOP. This gas has been recognized and the BOP has been shut and the well below the BOP has been killed and circulated.
What to do next?
There may be 2000 bbl of mud in the riser. Overboard? In any case if this is done, it may get quite exciting when the (small) gas pocket reaches surface. Do we always send this mud overboard? Whenever there is a doubt?
The alternative of circulating through a “riser gas handler” (if the rig is equipped with one) is quite safe as long as some back-pressure (perhaps 200 psi as a minimum) is held on a choke in order to prevent what was termed “explosive unloading” from happening. Without this back-pressure, the events that follow are disastrous.
It is time to take a step back and re-look at this whole situation. To do so does require the realistic modeling software….OLGA, Drillbench etc.. Simple gas calculations (P1V1=P2V2…..or even allowing for Z) don’t include the dynamic effects that you will see and simply must not be used. The gas rates that come from these simple calculations are mis-leadingly low.
Certainly this is an area where the industry needs some focused effort.
@Colin, there was a dump valve left open after a pit was cleaned and the SBM was accidentally returned to the wrong pit. End of story !!