Page 215 - Handbook Of Multiphase Flow Assurance

214 8. Flow assurance stability issues

thickness in subsea flowlines is 3 in., which provides sufficient time for the operator to take
preventive action to manage the risk of flow assurance blockages in the system.
Startup of production is a transient operation which can cause a surge of liquids settled in
the low spots of the production system to arrive in the separator and stop the production if
separator is not sufficiently large to hold the arrived liquids. Transient multiphase simulation
tools are available to estimate the volume of liquid surge at different well ramp-up rates during
a start-up.

Rate ramp-up and ramp-down

Production operator may increase or decrease wells' production rate according to the field
development plan.
During a flow rate ramp-up an event similar to a liquid surge during start-up may be
expected. Higher flow rate sweeps liquid holdup accumulated in the flow line; the liquid
travels to the separator and temporarily increases the liquid rate.
During a flow ramp-down, less liquid is expected to be produced to the process facilities.
At lower flow rate more liquid will accumulate in the flow line.

Slugging in gathering lines

Gathering lines carrying multiphase fluids may also experience slugging when slugs orig-
inate in a wellbore as the well starts to be loaded with liquids.
Choke opening or artificial lift methods may be used to reduce the liquid loading in wells
and slugging in the gathering lines.
Lowering production tubing into the Boycott range may also help stabilize wells produc-
tion and extend well life.
If no method works to mitigate the slugging, then flowline restraints or bracing for the
flowlines should be used as recommended by Hill and Wood (1994) to help with the loads on
the pipework.

Calculation of slug impact force on Tees and Elbows

Slugs traveling at high velocity through a production flow line carry a substantial momen-
tum M and impact the pipe locations with change in direction with a significant force. Slugs
are known to have knocked flow lines off their support stands and caused significant (greater
than 1 pipe diameter) movement.
Slugging has led to loss of integrity as in-field pipelines made of fiber epoxy got discon-
nected from the Tee at the location of slug impacts.
Liquid slug is pushed through a flow line by gas. Liquid slug travels at nearly the velocity
of gas which pushes the slug like a piston.
A simplified correlation for slug length based on pipe diameter was proposed in
Chapter 4.
[
L ft [] = ( Dinch]) 2