via the pipeline end termination (PLET)
Subsea tiebacks involve the transportation of flowing hydrocarbons collected
at the manifold to an offloading location
through a subsea pipeline (Figure 3). The
FPSO previously cited is one example of an
offloading facility. Other tieback locations
include existing static structures of older
offshore production facilities and back to
shore. Tieback distances of over 50 miles
are not uncommon with ultra-long tiebacks
approaching 200 miles on the immediate
horizon. Subsea pipelines are critical to the
success of a subsea tieback. Subsea pipeline
design is a multi-domain design problem
concerned with multiphase flow, heat transfer, material erosion and corrosion, and
many a misplaced ship’s anchor. Pipeline
designs are further complicated by irregular
terrain of the ocean floor traveling over hills
and valleys, and external vortex-induced
vibrations (fatigue). The design, installation
and maintenance of subsea pipelines are
paramount for subsea production profitability and environmental protection.
The growing ultra-deepwater depths
have begun to experience new challenges,
viscous flow losses and hydrostatic head.
Despite the high pressures of the reservoir,
the pressure losses along the pipeline and
hydrostatic head of reaching the surface
have made artificial lift a necessity in many
subsea applications. There are two basic artificial lift methods, gas injection and compression systems. Gas injection is the most
common form of artificial lift. The injected
gas is used to reduce the hydrostatic head in
the riser to allow the reservoir to produce.
Compression systems are multistage com-
pressors driven by electrical motors. It is not uncommon
for the electrical motors to be rated to 8 megawatts and
stand about 20 feet tall. The multistage compressor comes
in two varieties, single-phase and multiphase compression
systems. Single-phase compressors have a subsea separa-
tor upstream which segregates the gas, oil, water and sand
using gravity and baffles. Multiphase compressors do
not require the upstream separator, directly boosting the
pressure of the multiphase flow as it leaves the compres-
sor. There are unfinished engineering challenges with
both compression-based systems. The subsea separator
has a foaming problem that occurs during the separation
process. The multiphase compression system faces the
stall phenomena similar to gas turbine engines. Both solu-
tions face blade wear (due to sand) and corrosion.
If the tieback terminates to an existing platform or an FPSO, the pipe-
line must interface with a riser, a vertical pipe reaching the water’s sur-
face. Risers are subject to dynamic loading conditions caused by the water
currents [ 4]. Risers are vulnerable to undesirable weather conditions such
as hurricanes. Essentially, risers accumulate metal fatigue and present a
completely different multiphase flow challenge than do pipelines (horizontal
flow). Newly-designed risers are now terminating 300 ft. below the water’s
surface, thereby isolating them from storms. These risers have redundant
buoys (the size of school buses) and cabling systems that provide riser sup-
port, stability and buoy separations.
The topside of subsea production is where the subsea production system
is managed and the power necessary to run the subsea facility is supplied.
Subsea production facilities are powered by two basic power forms, electrical power and fluid power. The electrical power supports sensors, subsea
control modules (SCMs), and communication (including acoustic communication). The fluid power is the muscle controlling subsea production. The
subsea gate valves (high pressure isolation) and subsea chokes (flow control)
are actuated using hydraulics. Both forms of power have redundant sources
for emergency shutdowns. Electrical power and hydraulic power are also
stored subsea to reduce the source-to-consumption path. Also located on the
topside are well intervention chemicals such as methanol, the master control
system and emergency shutdown (ESD) system.
FIGURE 3 Subsea tieback
with slug flow.
FIGURE 4 Subsea
risers with buoys.