WDDM field umbilical configuration after Phase IXa.
time, increasing the supply voltage would
not be possible as the umbilical cable rating
would be exceeded.
Another issue was the inrush current that
would be absorbed by the WGFMs during
start-up. Typically all sensors within the subsea control module (SCM) were powered up
on the application of system power, allowing
data to be displayed as soon as communication was established. However, data from
the WGFM is only of value once the well is
fowing, and does not need to be switched
on automatically when power is applied to
the SCM. Therefore, the software was modifed to have the WGFM “off” during power‐
up, requiring the operator to actively switch
the device on prior to well start. This allows
the Phase I and IV equipment to operate on
a single power channel by selecting only the
WGFMs that are really needed.
The Phase VI Sequoia development comprised six additional wells to the existing
Sapphire and Rosetta developments. The
Sequoia South wells are routed to the existing subsea Rosetta infrastructure and are
operated from the Rosetta control room.
These wells are independent of the WDDM
operation, although gas metering is crucial
to defne the production level.
Phase VIII originally was intended to be
the fnal WDDM development and was initially split into three installation campaigns,
“a”, “b”, and “c,” later reduced to “a” and “b,”
with “c” replaced by the current Phase IXa
One advantage of the original premise
was the requirement for simplicity and a
common design approach. This meant the
umbilical design and the platform-located
electrical power unit (EPU), were a common design for both the Simian and Sapphire hubs, even though Sapphire was half
the distance from the platform, at 24 km
( 15 mi), compared to Simian, at 53 km ( 33
mi). Routing back into the Scarab/Saffron
facilities would add another 22 km ( 13. 7 mi),
bringing the maximum step out to roughly
46 km ( 28. 5 mi), still less than the distance
An initial vendor review of the Simian and
Sapphire power system indicated scope for
further expansion to accommodate Phase
VIIIa requirements. This was confrmed by
detailed analysis, although the results revealed that the system was at the limits of
its existing design capacity, with the power
transformer running at 95% of its rating, the
system supply voltage approaching 98%, and
the maximum SCM voltage/minimum load
case being within 5% of the upper limit.
The VIIIb campaign eventually added
fve wells, one with a HIPPS, along with one
manifold on Simian; one well on Scarab/
Saffron; and two wells, one with a HIPPS,
on Sapphire. However, based on the work
for Phase VIIIa, it was apparent that in the
single-channel case the Sapphire hub was at
or potentially beyond its maximum capacity.
Analysis suggested that the conventional,
dual-channel supply would best to support
the VIIIb needs. In addition the subsea
confguration at the distribution units was
revised to better distribute the supply from
the incoming Sapphire umbilical.
The VIIIb expansion effectively exceeded
the rating of the power transformers, with a
worst-case demand of 130% of the rated output. In addition, for the single-channel case
several of the higher power requirement
sensors need to be switched off to maintain
a suitable margin.
At the completion of Phase VIII the system had effectively been expanded beyond
its practical design limit, with sensors needing to be “disconnected” to maintain operation using a single power channel on both
the Simian and Sapphire hubs. Also, the
earlier Phase IV expansion had exhausted
all available capacity of the shore-based
Aker Solutions control system. To further
expand the system to accommodate potentially another 16 wells, nine HIPPS, and one
manifold, all distributed across the Simian,
Scarab/Saffron, and Sapphire areas, a signifcant reassessment and system upgrade
would be needed.
The conceptual stage examined options
for expanding the control system, including
a control buoy; high voltage power supply (<
1kV AC); a new control system; and a new
umbilical to the platform or directly to shore.
A control buoy would provide the signifcant beneft of a relatively short length
of electrical umbilical by locating the buoy
directly above the system tie-in point. This
would minimize both the cost and voltage
Controls system capacity.
3 wells, 1 manifold
8 wells, 2 manifolds
7 wells, 2 manifolds
SAP- 1 SCA- 1