Terrain or severe slugging has been observed operationally in a 5-mile long subsea oil tie-back when certain wells or combination of wells are flowed back to the platform through the test line. The flowlines have a downward incline to the base of the riser that enhances terrain or severe slugging. Under operating conditions with slugging, the tuned flow management tool failed to predict any slugging. Additionally, according to multiphase flow analysis with a commercial transient simulator during design studies, slugging was not predicted except for significantly lower flowrates and higher water cuts. Recent rigorous modelling highlighted significant differences to the resulted slugging predictions depending on the modelling approach and different versions of the tested simulator, OLGA®. Also the commercial transient simulator LedaFlow® was tested yielding similar results to OLGA® with same input parameters. Some slugging mitigation methods shown through modelling to mitigate slugging have been tested in the field without success. While the more rigorous modelling achieved better agreement with operating data, still poor accuracy was achieved. The inability to properly capture multiphase flow characteristics during the design phase of the project has led to an under-designed system and significant process upsets.
The accurate prediction of multiphase flow phenomena has been the topic of research for many years. Even though steady state conditions can usually be predicted with good accuracy, significant efforts are still being expended to model transient flow phenomena, such as various slug types and transient operating conditions, such as shut-downs, blowdowns, restarts, and ramp-ups. Accurate steady state and transient flow assurance analyses during early design phases are paramount to the further engineering of a subsea oil or gas project. Slugging inherently creates significant issues in the topsides process train due to large fluctuations in operating conditions.
