American Oil and Gas Reporter - February 2019 - 88

SpecialReport: Offshore & Subsea Technology
The Norwegian Petroleum Directorate
commissioned a report on UWHPs in
2016 that concluded "unmanned wellhead
platforms can be cost- and productioneffective development concepts in the
shallower portions of the Norwegian Continental Shelf and that the regulatory
framework is open to the concept." This
same report estimated more than 1,000
unmanned platforms had been installed
in the U.S. Gulf and fewer than 600 were
installed throughout the rest of the world.
With this backdrop of renewed interest
in unmanned platforms, the application
of innovative subsea technologies to move
facilities from the deck to seafloor-and
in the process, enabling the demanning
of offshore surface facilities-is attractive
from an operational management perspective, and also offers significant potential cost savings.
Subsea Transfer Barge
Figure 1 illustrates a subsea transfer
barge with usable storage for 3,000 barrels
of production chemicals in three large
storage tanks built into the barge's hull.
Smaller (~200 barrel) storage tanks for
production chemicals utilized in lowdose injection can be included on the
barge's deck. The barge also includes the
complete subsea chemical injection unit
(SCIU) to inject all the typical production
chemicals. The barge's net payload of
facilities and production chemicals totals
more than 600 tons, enabling significant
deck space and load savings, as well as
platform demanning.
The need for subsea chemical storage
and injection initially was identified as

part of joint industry research in the DeepStar™ consortium group. The technology
development requirements were driven
by the need for longer and more cost-effective deepwater subsea tiebacks. The
subsea transfer barge illustrated in Figure
1 was developed as part of a Research
Partnership to Secure Energy for America
project supported by both industry and
the U.S. Department of Energy.
The current design includes:
· Conventional chemical barge technology, where applicable;
· Double-hull design to protect the
environment from potential accidental
chemical spills caused by vessel collisions
or barge grounding;
· A total of 3,000 barrels of usable
chemical storage in three or more separate
volumes;
· Dual-barrier containment for the
production chemicals;
· Innovative use of engineered fabric
as the primary containment;
· The ability to fill the chemical tanks
at quayside with optional in situ subsea
refill capabilities; and
· Innovative and cost-effective installation and retrieval using two anchor
handling tugs.
During the RPSEA project, the subsea
transfer barge was subjected to a number
of reviews, including industry subject
matter experts from operators, service
companies and the U.S. Bureau of Safety
& Environmental Enforcement. The American Bureau of Shipping performed a review and issued an approval in principle,
and a Gulf Coast shipbuilder performed a

FIGURE 1
Subsea Shuttle with 3,000 Barrels of Chemical in Tow to Production Site

constructability analysis and issued a
fixed-price proposal. The barge deployment/recovery process was modeled extensively with computational fluid dynamics. The modeling results and further
dynamic simulations demonstrated very
stable and predictable behavior for a twoline installation technique using two anchor
handling tugs. The RPSEA project included
a complete SCIU on the barge deck.
Following the original design in the
RPSEA project, the subsea transfer barge's
in-water behavior has been demonstrated
and validated further in a cooperative
effort between Woodside Energy Ltd.,
the University of Western Australia and
Safe Marine Transfer (SMT). The work
with UWA included both a near-shore
demonstration that used a scale model
and dynamic simulation studies. This
work highlighted the potential to use the
barge for cost-effective installation of
subsea facilities without requiring large
offshore crane vessels.
Moreover, the technology has been
advanced by developing adjustable buoyancy in a cooperative effort between
Trelleborg Offshore and SMT. The barge's
adjustable buoyancy offers several advantages over fixed buoyancy, including:
· Reusing the buoyancy for multiple
installations;
· The ability to adjust buoyancy to
use the same barge design for a wide
range of payloads;
· Buoyancy adjustment as a contingency for buoyancy damage or deterioration with time;
· Buoyancy adjustment to recover
subsea facilities where the weight on the
seabed may have changed because of operations, maintenance, marine growth or
damage; and
· The reduction or elimination of the
need to maintain ballast tanks while the
barge is on the seabed.
Small Subsea System
The smaller 200-barrel chemical storage and injection system was developed
in response to industry needs for lowercost subsea tiebacks, including single
well tiebacks. As shown in Figure 2, the
system includes much of the technology
developed for the barge, but the smaller
size and weight simplifies offshore installation. The 200-barrel unit is designed
with the footprint of a standard 40-foot

88 THE AMERICAN OIL & GAS REPORTER
American Oil and Gas Reporter - February 2019

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