News & Events
- July 15th, 2008Flowasta released
Flow Model
Flowasta assumes the total fluid composition to be in equilibrium at the inlet by performing a hydrate PT flash on the total composition at inlet conditions. From that point on the hydrocarbon and aqueous phases are treated as two separate sub-systems as described in the Driving Force section. The enthalpy of the fluid is determined from heat exchange with the surroundings and pipeline elevations, and the pressure drop from a pressure drop model. The fluid temperature at the outlet from a section is determined by a PH flash calculation that accounts for phase transitions including heat released when hydrates are formed. In some situations this heat of formation can be so high that it is limiting for further hydrate growth.
The energy balance calculations are sketched below.
Q is the heat transferred to the fluid through the pipeline walls and W the work performed on the fluid due to changes in elevation. The work term, which becomes significant for instance in a riser, is calculated from
In this equation ρbulk is the average bulk fluid density, g is the gravitational acceleration and h the elevation change.
The heat loss is calculated as
A is the pipe wall area, Tamb is the ambient temperature, and Tbulk is the mean bulk temperature in the actual pipe section. Utot is the overall heat transfer coefficient.
The liquid hold-up, pressure drop and flow regime are calculated using either the Mukherjee Brill (1985) or the OLGAS pressure drop model used in the dynamic flow simulator OLGA from SPT Group (Bendiksen et al., 1991).
Knowing pressure (P) and enthalpy (H) at the outlet from a pipe section, a hydrate kinetics PH flash determines the outlet temperature considering diffusion of components between the hydrocarbon and the aqueous sub-system. An implicit solver scheme is used, which ensures numerical robustness and a calculation speed that is relatively fast despite the complexity of the equations to being solved.
