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The Hoover/Diana fields
are located in the Gulf of Mexico, 160 miles (258km) south of
Galveston. Diana covers several lease blocks, but is largely contained
within East Breaks Blocks 945 and 989 [also covers EB 946 and
EB 988]. Hoover is located in Alaminos Canyon Blocks 25 and 26.
The field was discovered in 1990 and successfully appraised in
1997. The Diana appraisal well established a water depth record
of 4657ft for testing from a moored vessel. This well recorded
maximum flows of 4800 barrels.
Hoover was discovered in
January 1997, 15 miles east of Diana. The two fields contain estimated
recoverable reserves exceeding 300 million oil-equivalent barrels.
RIG
For these subsea development
wells (and for subsequent drilling programmes), Exxon will use
a newbuild Marine 700 semisubmersible drilling rig from Marine
Drilling Companies (MDCO). The MARINE 700 is a fourth generation
semisubmersible drilling unit of the Bingo 8000 design. It is
classed by DNV as a Column-Stabilised drilling unit and is Panamanian
flagged. It is capable of operating in water depths ranging from
230ft to 5000ft.
The semisubmersible is designed
for a maximum wave height of 32m (105ft) and a wave period of
11 to 15sec. Box type elements and transitions provide a minimum
of stress concentration at critical connections and an adequate
fatigue life for 20 years based on North Sea criteria.
The semisubmersible has
a transit draught of 9.75 m (32ft), an operating draught of 23.75m
(77.9ft) and a survival draught of 21m (68.9ft). It has an overall
length of 119m (390.4ft) and an overall width of 71.4 m (234.25ft).
DEVELOPMENT
The development scenario
envisages the Diana reservoir being drained from a six wells subsea
development which will be tied back tied back to a Deep Draft
Caisson Vessel (DDCV) located over Hoover in 4800ft (1463m) of
water.
DEEP DRAFT CAISSON VESSEL
The Hoover Deep Draft Caisson
Vessel consists of a cylindrical hull that is buoyed with air-filled
compartments in the upper portion of the hull and ballasted with
seawater and fixed ballast in the bottom compartments.
On top of the steel hull,
the DDCV production facilities are designed to handle 100,000
barrels per day of crude and 325M ft3 /day of gas and
up to 60,000 barrels produced of water per day. A three level
deck will house separation, dehydration and treatment facilities
as well as a drilling rig.
Surface trees on the DDCV
are similar to trees found on traditional platforms. All valves
and flowlines are at the surface and are located in the wellbay
area of the DDCV. In this particular application, the trees have
tensioned risers that connect the surface trees to the well at
the seafloor. Surface trees allow for wellbore intervention using
conventional workover technology.
MOORING
The Deep Draft Caisson Vessel
is moored via 12 anchor lines that extend in a radial pattern
from the hull. The mooring lines will be 7100ft long and will
connect to piles driven into the sea floor approximately 6900ft
from the DDCV (lateral distance about 5100ft). The mooring lines
are taut, but will have some flexibility and form a catenary between
the hull and ocean floor. Each mooring line consists of chain
(top and bottom sections) and spiral strand wire rope (middle
section).
INSTALLATION
The installation of the
DDCV was carried out in 1999.
A gas pipeline approximately
85-miles long will be constructed from the DDCV to the High Island
Offshore System, an existing gas transmission system off the Louisiana
coast. In addition, an oil pipeline will be built and extend some
150 miles to Freeport, Texas.
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