Precommissioning flowline project
meets objectives offshore Angola
Multiple subsea deployments help minimize delay
Flooding, cleaning, and gauging 11 production flowlines and nine water injection flowlines at water depths of 1,050-1,410 m ( 3,445-4,626 ft) presented a wide range of challenges for a predevelopment project offshore Angola. In addition to the technical demands of executing the deepwater project, multiple changes in the already-challenging mobilization schedule required matching a pre-commissioning equipment spread capable
of providing the contracted flow rate and pressures to the available
deck space of the various vessels under consideration. The success
of the precommissioning project required a coordinated effort that
involved multiple executions of deepwater remote subsea flooding,
testing, and dewatering operations, as well as strong in-country assets, planning, and management.
Flooding, cleaning, and gauging
The deepwater development project produces four discoveries
from an FPSO. Production began in June 2014, with capacity set at
160,000 b/d and with proven reserves of 500 MMbbl. As of May
2015, the development had produced 2 MMbbl.
The scope of precommissioning responsibilities included subsea
pigging of production and water injection flowlines; surface precom-
missioning on the construction vessel and FPSO; dewatering and
monoethylene glycol (MEG) conditioning of the gas export line; and
umbilical testing and monitoring from the umbilical installation ves-
sel and FPSO locations.
Eleven production flowlines and nine water injection flowlines required flooding, cleaning, and gauging (FCG). The FCG operations
extending between the two flowline end terminations (FLET) were
remote subsea applications, performed without the need for a large
surface precommissioning spread by using the DENIZEN remote
subsea flooding and testing unit.
The FCG operation used seawater to propel single, multipurpose
pigs through each of the flowlines. The water was filtered to 50 microns and treated with a standard chemical mix of corrosion inhibitor and fluorescent tracer dye.
The multipurpose pig was launched using the precommissioning
skid’s free flooding system. Free flooding continued until the flow
rate was reduced to near zero, at which point the flooding system
was isolated and the receiving FLET valve was opened to free-flood
the remainder of the line with raw seawater. Following flooding, the
receiving FLET flooding cap was replaced with the pig cage. The
boost pump at the launching FLET pushed the pig the remaining
distance to the cage. Following pig and cage recovery, the cap was
re-installed on the receiving FLET.
Export line operations
FCG operations also were performed on the gas export line prior
to hydro-test operations. The line was installed full of air at atmospheric pressure. Two multipurpose pigs were propelled through
the line with seawater filtered to 50 microns and chemically treated
with the same mixture used in the production and injection lines.
Prior to launching the FCG pig, a slug of filtered seawater (1%
of the full line volume) was injected into the gas export line. The
pig was launched via the free flooding system, and flooding continued until the flow rate was near zero. The line was then flooded at
approximately 99% of full volume. The precommissioning system
was isolated, and the receiving FLET valve was opened to vent air
trapped in front of the pig to enable cage installation.
The filtered and treated seawater was pumped from the launching
FLET to push the pig to the cage. Pig and cage were recovered, and
the cap was installed on the receiving FLET. A contingency foam pig
was installed in the gas export line, in case the FCG pig required
To execute these operations, the pumping skid was enhanced
with a positive displacement flow meter for high-viscosity chemicals,
hydraulic flow and pressure measurement to monitor remotely operated vehicle (ROV) performance, and increased battery capacity
and an ROV-operable on/off switch for improved subsea logger run
The LP filling pump fitted to the DENIZEN flooding skid
to reduce deployments and improve diagnosis.
(All images courtesy Baker Hughes)