Page 114 - Handbook Of Multiphase Flow Assurance

110 5. Flow restrictions and blockages in operations

2. Build a multiphase flow model
Construct a multiphase model of a flow line to determine the areas of water holdup accu-
mulation. Determine the superficial liquid flow velocity for these areas. One or more of these
areas are prone to hydrate plugging. The key factor is whether or not the formed hydrate
particles are carried out by the flow or remain in the “low spot.”
If hydrate conditions are present throughout the flowline, one may need to rely on the
segments of increased liquid holdup for prediction of the likely blockage location. Segments
with liquid holdups higher than in other locations of the flow line may be the likely locations
of hydrate accumulations. Multiple blockages may be present.
3. Establish the pipeline segment where hydrate conditions are present
Determine the pipeline segment where hydrate conditions are present. In most cases the
hydrate conditions are present in 100% of a shut-in flow line. Exceptions may include warmer
climates where some sections of the flow line are exposed to an ambient temperature warmer
than the hydrate conditions.
4. Determine hydrate particle mobility
Use a solids transport model to estimate the flow velocities required to avoid solids settling
out in two phase pipeline flow. Beggs and Brill flow and Thomas' friction velocity correlations
may be used to determine the minimum transport condition at the transition to intermittent
boundary line.
The Thomas' correlation based on the Oak Ridge National Lab work (1961, 1962) can be
separated into two models. The upper model is used when the particle diameter exceeds the
laminar sub layer thickness, the lower model is used when the particle diameter is less than
that of the laminar sub layer. Unfortunately it has been shown that the upper model is only
suitable for use with high superficial gas velocities and that it greatly over predicts the pres-
sure drop at low superficial gas velocities. The new approach is that under conditions where
solids are larger than the laminar sub layer the maximum superficial liquid velocity predicted
by the lower model gives the highest superficial liquid velocity required to ensure particle
transport. This approach always gives conservatively high liquid velocities when compared
to all the values for three phase gas/liquid/solid flow.
3
The default value for the density of hydrate particles is 50 lb/ft . The minimum frictional
pressure drop required to avoid settling can then be calculated.
4
Thomas (1961) indicates that Reynolds numbers as high as 2.9–3.6 × l0 are required to
prevent solids deposition.
A recent model for gas hydrate deposition from water saturated vapor in deadlegs was
presented by Zhang (2017), which allows to estimate hydrate plug potential by a different
mechanism of hydrate deposition by condensation from vapor in vertical short pipe sections
without flow.

Prevention of hydrate formation

Hydrate easily plugs production systems and should be either avoided or managed both
in flowing and shut production and injection systems.
On production system shut-in, the fluid cooldown time should be no less than a sum of
time to safe out the system by normal operating procedure plus the no-touch time. No-touch
time should be no less than 2 hours to allow operator to respond, to reset control systems and