|
 |
Located 70 km
(120 miles) southeast of Hong Kong in 370 meters (1,210) ft)
of water, the Liuhua project is a state-of-the-art production
system comprised of twenty five sub sea trees. The Liuhua
field is the largest oil reservoir to date in the China Sea.
Drilling and technical challenges at the site required the use
of cutting-edge technologies to develop the field. Sonardyne's
Compatt and Paroscientific's Broadband Depth Sensors are two
of the advanced technology instruments used.
Established in
1971, Sonardyne is an international group of companies
manufacturing subsea instrumentation. Sonardyne specializes in
the use of sound for underwater navigation, positioning, data
communication and control. Sonardyne's Compatt, the
Computing
and Telemetry
Transponder, has been the market-leader for
subsea positioning for over two decades. The latest generation
of this transponder is Compatt 5.
Compatt 5 is
based on a new electro-mechanical design and employs the
latest acoustic signal processing technology. This new
platform is designed to take Compatt forward for many years
with regular upgrading of capability, easily installed by the
user.
Sonardyne has
offered Digiquartz Broadband Depth Sensors
as an option in Compatt for over 20 years. The transponders
are a component in Sonardyne's Long Baseline (LBL) system
which is used world-wide in subsea survey and construction
work. |
|
|
LBL systems are
typically used:
- As tidal references to make periodic corrections to water depth
- For measurement of relative heights on piles and structures
- In spool piece metrology
This application note describes the acoustic positioning
techniques used during a multi-well sub sea development at 310
meters water depth in the South China Sea. Acoustic
positioning played an important and comprehensive role in every
aspect of the seabed engineering work, significantly contributing
to the overall success of the installation. The project included
the accurate setting of the conductors for 20 wells, manifold
piles and pipeline bases, production jumpers between each well,
and the installation of clump weights for the flexible risers
attached to the Floating Production System.
The 'as-built’ well and pile locations were surveyed with a
precision of +/- 0.15 meters. A sub sea gyro measured the
conductor orientation to +/-0.5 degrees accuracy. A +/-0.3 meters
elevation tolerance proved the most difficult to measure, as the
best available pressure sensors could provide an absolute depth
accuracy no better than +/-0.6 meters; however, use of a
differential technique established relative heights to better than
+/-0.1 meters. The riser clump weights were installed to within
the required positioning tolerances of +/-1.2 meters from an
anchor-handling vessel.
Jetting: Jetting consisted of circulating seawater through a mud
pump assembly inside the conductor, whilst slowly lowering the
pipe on its own weight and the weight of the drill collars inside
the conductor. From time to time the conductor was raised to
assist soil penetration. The acoustic positioning system was used
to monitor the following parameters as the conductor was being
jetted in:
(1) Orientation: The positioning system displayed the real time
gyro orientation of the piles on the drill floor, permitting the
drillers to orient the conductor to a high degree of accuracy.
This allowed all the guide bases to be built the same, as each
conductor could be orientated to the same azimuth, which in turn
simplified the fabrication of the rigid well-to-well pipe jumpers.
(2) Elevation: It was essential that each conductor be placed
within a ±0.3 meter relative height tolerance. This was necessary
for fabricating the jumpers between the guide bases, as the
specialized mechanical tool used to measure the hub-to-hub
distances and orientations was set up using the relative heights
and distances determined using the acoustic positioning system.
The relative heights were measured using 2000 psi-scaled
Digiquartz depth sensors included in the 'target' Compatts.
These depth sensors use quartz crystals to sense the
temperature-compensated ambient
pressure. The quoted
measurement accuracy of the depth sensors is 0.02 % of the full
scale value, giving an absolute measurement accuracy of ±0.14
meters. This equals the required tolerance, without taking into
account unknown effects of tides and water density, and to a
lesser extent, swell. A differential technique was, therefore,
devised to monitor the difference in the depths between a
reference and a target quartz pressure sensor. The four depth
sensors (one included in the RovNav transceiver and three in the
target Compatts), were calibrated during factory acceptance tests
in an environmental chamber to check their linearity and
repeatability over the pressure and temperature variations
expected near the seabed during jetting operations. The
differential measurement corrections established for each sensor
during these tests, allowed height differences to be monitored
off-shore to better than ± 0.1 meters.
For the first manifold pile, a reference transponder was set on
the seabed and the depth difference to the target transponders
mounted on the pile used to establish the height of the pile above
the seabed. This information was presented in both a digital and
analogue form, giving the drill floor the remaining distance to
drill. Once the pile was set, the ROV re-positioned the reference
transponder in the top of the pile to become the elevation datum
for all the other piles and conductors.
(3) Attitude: Measurement of tilt was important to establish the
relationship between adjacent conductors. Tilt was measured using
precision dual axis inclinometers mounted on the piles. Two sets of
inclinometers were used, one connected to a target transponder and
the second in one of the under water gyros. The data was
transmitted to the surface via acoustic telemetry and the gyro
cable respectively. The selected results were displayed by the
acoustic positioning system in conjunction with the orientation
and elevation information. All the conductors were spudded with
tilts within 0.5 degrees of vertical.
(4) 'As-Laid' Positions: After reaching the correct depth, the
conductors were left to soak for up to two hours, allowing the
surrounding soil to rebuild strength. During this period, the
acoustic system was used to determine the 'as-laid' position.
Immediately prior to releasing the piles, any conductor settlement
was measured and the 'as-laid' elevation established relative to
the reference pile.
This application note was submitted by Sonardyne. It is
based on a paper by John D Hughes (Amoco Orient Pet. Co.) and
Nigel C Kelland (Sonardyne Int. Ltd.) presented at the 11th
Offshore South East Asia Conference in September 1996. |
