Page 140 - Handbook Of Multiphase Flow Assurance

136 5. Flow restrictions and blockages in operations

Riser base valve was shut to isolate the flow line from the riser. Riser contents were treated
with methanol injected at the riser base.
Fluids displaced from the top of the riser during the injection of methanol showed that
there had been some oil at the top of the riser, with the rest of the riser filled by seawater.
After displacement of riser with methanol the riser base valve was opened but a “greater
than hydrostatic” pressure differential between the flow line low point and the riser base did
not disappear.
A differential pressure of up to 30 bar has been retained by the blockage.
It was determined that a combination of system failures allowed seawater to enter the
flowline through spare manifold slots, which were corrected.
Hydrate was dissociated by depressurizing through riser base gas lift system.
August 2008 Africa offshore topsides flare line
Prolonged flaring of uninhibited cold gas from glycol contactor into the wet gas flare line
simultaneously with wet gas from slugcatchers caused ice/hydrate blockage in the flare line.
Wet gas flare line became overpressured causing reverse rupture of safety bursting discs
and the flow of gas into the cooling water, heating medium and oil production systems.
The bulk of the gas got discharged overboard, resulting in the formation of a significant
gas cloud. Prior HAZOP did not identify blockage potential.
February 2008 Africa onshore process plant pipe
Six high pressure header main inlet valves were closed.
Each valve was fitted with a 1 in. bypass line and ball valve to facilitate opening the valves
when flow need to be redirected.
Operator used a spare manifold slot to control the low pressure compressor by recycling
gas from high pressure line to low pressure line by partially opening the equalization bypass
valve and the low pressure valve of the same unused slot.
The equalizing 1 in. bypass line valves were subjected to frequent temperature change by
opening the valves to recycle gas.
Hydrate formation and subsequent movement due to differential pressure resulted in the
manifold pipe work moving 3–5 in., causing excess stress on a weak point in the system (1 in.
equalizing valve).
Three of the 1 in. valves failed, two immediately and one a few moments later, the bodies
breaking apart at the screwed connection with the balls being ejected from the valves, result-
ing in a significant gas release.
February 2005 Europe onshore
In preparation for a planned regeneration of molecular sieve dryer the dryer has to be
emptied by removing the liquid propane butane fraction to a drum.
Control room operator found no flow to the drum after he had opened the two necessary
motor valves. Then of the two valves he closed one positioned near to the drum.
After a field operator had inspected the status of both valves he tried to check whether
flow to flare is possible or not by opening a 1 in. pipe to cold flare.
The origin of this pipe is between the two valves. Nearly instantaneously a leak occurred
at a 2 in. valve gasket of the closed valve near to the drum, followed by release of propane
butane.
Operator mentioned no flow to the drum. Field operator reported a bump just after cold
flare valve opened. Both findings are explainable by a plug.