Page 214 - Handbook Of Multiphase Flow Assurance

Transient operation 213

The author indicates the stability criterion is that in order to prevent a bubble penetrating
the riser base, the inlet gas velocity must be lower than some critical gas velocity, which de-
pends on the ratio of the hydrostatic head in the pipeline and riser.
The Boe criterion for the occurrence of severe slugging is commonly used in the industry.
The Boe criterion provides an estimate of the minimum liquid velocity at which pressure
increase due to liquid accumulation in vertical part of the multiphase flow system is higher
than pressure increase due to gas compression in horizontal part of the system.
S
U L S ≥ P U G (ρ L g(1 − ε L L ) sinα )
/
P
S
U L = liquid superficial velocity.
P P = pipe pressure.
S
U G = gas superficial velocity.
ρ L = liquid density.
g = gravity acceleration.
ε L = pipeline or tubing liquid holdup.
L = length of downhill pipe upstream of the lowest point.
α = inclination of pipe upstream of the lowest point.
S
S
Boe estimated the holdup based on no-slip condition: ε = U L ( U L S + U G )
/
L
S
S
The Boe criterion is a straight line in U L vs. U G coordinates. Unstable flow is at values
above the line.
Additional work which provides stability criterion on horizontal-vertical flow systems in-
cludes Zakarian (2000). Author developed a stability criterion and validated it with labora-
tory analysis.
Suppression techniques
Slug mitigation options (Montgomery, 2002) include: gas lift, choke control and separation.
Choke control, separator pressure and gas lift are named as slug mitigation options in
(Sancho, 2015). The author also provides a severe slug classification into four categories.
A novel method for slug suppression by dividing large liquid slugs into smaller parts
which would be more easily accommodated by the separator was proposed (Makogon et al.,
2011) which was discussed in Chapter 4.

Transient operation

Shut-in and start-up

On production shut-in, liquids redistribute according to gravity in the production sys-
tem. This affects the cooldown of the produced fluids. Water tends to retain more heat. As
water drains to and accumulates in the low spots, this may provide additional time before
hydrates begin to form. However, system insulation is usually designed to provide sufficient
cooldown time in the gas-filled sections of the flowline. Gas has the least heat capacity of the
produced fluids. Sections of flowline filled with gas cool down the fastest. Typical insulation