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has always had a number of potential
uncertainties associated with it:
1. As distance increases from the tide
gauge, the recorded water level
increases in uncertainty.
2. Vessel squat can be measured and
modelled, but is a combination of
many factors including speed, current
and water depth, meaning the squat
model will not always be accurate.
3. Long-period swell is difficult to measure
and heave sensors do not always
capture it accurately, due to the settling
of heave over time within the sensor.
All the above factors increase the
uncertainty of the final depth. Considering
survey uncertainty must be factored in to
the calculated under-keel clearance of a
bulk carrier operating under the DUKC
system, it is of critical importance that
these uncertainties are minimised.
In order to minimise the uncertainties
associated with the above method, a real-
time kinematic (RTK) or post-processed
kinematic (PPK) solution was used to
measure position with centimetre-level
accuracy in easting northing and height.
This process is a blended solution using
accurate GPS height and vessel motion
measured by high-accuracy inertial
motion unit (IMU), this solution provides
a very accurate position of each depth in
all three planes connected to the ellipsoid.
The measured depth can then be
corrected either online or in post-processing
for the ellipsoid to LAT separation model,
giving an accurate depth regardless of
distance from the tide gauge.
To facilitate this survey methodology,
GPS base stations were located in three
locations along the channel to minimise
any base line errors when surveying away
from the base station.
A very important factor to navigational
safety is the existence of objects on
the seabed. Detection of small objects
requires a rigorous and closely monitored
acquisition and processing methodology.
All survey data is examined by experienced
surveyors, not only to assess quality, but to
identify small objects on the seabed.
Objects detected in the survey data
are very costly to remove and must be
confirmed by at least 200% coverage,
to ensure they are in fact an object and
not an erroneous detection of noise or fish.
Objects within the harbour or channel
can represent the controlling depth for
that section and removal can result in
significant increase in available depth.
To make use of the tidal model and
accurate survey data, it is essential that
the tide gauges remain operational and
the data is used to determine the allowable
draft for vessels.
The hydrographic survey results and
the tide gauge data was introduced into
the Port's DUKC system on the 12th of
December 2013, and the effects on the
ships' draft and available tidal windows
became immediately evident. OMC
International conducted a study on the
effects of the implementation of the
'Hydroid' in combination with the annual
maintenance dredging that was carried
out that year. Their study concluded that
the tidal window to sail draft restricted
vessels from their berths to the Indian
Ocean had increased, on average, by 57
minutes, whilst the average draft for draft
restricted bulk carriers had increased, on
average, by 71 centimetres.
The activities conducted in Port Hedland
over that last two years have highlighted
the importance of hydrography not only
to the port, but to the Australian economy.
Since the implementation of the new
Hydroid model into the survey data
and DUKC, the port of Port Hedland
has achieved record drafts and export
tonnage of iron ore.
"The largest single shipment of iron
ore has left the port of Port Hedland with
263,989 tonnes on board the vessel Abigail
N. The BHP-loaded iron ore carrier also
departed the port (Tuesday 24 February
2015) on a record sailing draft of 19.67
metres... The port achieved a record annual
tonnage throughput of 372.3 million tonnes
(Mt), an increase of 29% from the previous
year." (Source: www.pilbaraports.com.au.)
Hydrography has played a critical role
throughout the process, not only with
the installation of tide gauges and survey
for dynamic under-keel clearance, but
ongoing survey for maintenance dredging
programs to ensure depth are maintained.
Figure 5. Ellipsoidally referenced surveying.
Figure 6. Detected object removed from seabed.
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