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Knowledge of tide height as well
as speed and direction of tidal currents is vital to navigation of water ports
and inland waterways. There are two considerations of tidal predictions, one
the astronomical tide which is the tidal levels resulting from the gravitational
effects on the oceans, and the residual which is a difference in the
astronomical tide and the observed water level. The residual difference can be
extratidal due to storm surges and a sea level anomaly. Challenges for ships entering
and departing ports can be alleviated by monitoring predicted tides and
currents, and maintaining proper communication with harbor masters. Ship
captains and shipping agents must have a clear understanding of the ship’s
Under Keel Clearance (UKC) whether transiting light or under load. Most ports
have a minimum UKC clearance in order to berth or depart during a tide schedule.
As ships continue to increase in size and marine traffic within ports increase,
monitoring tidal predictions is more important as depths and widths of channels
are slow to accommodate the rapid growth of ship size. Major ports are now
implementing real-time water level, current, and weather measurements.

Port of Weipa and Port of Cairns
in Queensland will be used in this analysis of the challenges to port
navigation with regards to port tides. Port of Weipa and Cairns, use a Dynamic
Under Keel Clearance program (DUKC). The program can require deep draft vessels
to submit a DUKC report from the shipping agent prior to arrival or departure
from the berth. The DUKC reports are used to determine the tidal window for
ship departures and the max draft that a vessel may sail at a particular tide (Queensland
Government). Most ports have a minimum UKC requirement for transit through the
port and while alongside any berth. Draft restrictions can be established for
loaded vessels and loading operations may be adjusted to UKC conditions.

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Risk of groundings can be
mitigated using the Dynamic-Net regime. This regime uses fixed safety limit
with variable allowances of environmental and vessel conditions (Pearce). Variables
considered are the tidal residual and astronomical tide, the channel depth,
static draft, and the gross UKA which includes variability of tidal residual
fall, squat, heel, water density, draft tolerance, siltation, wave response,
survey tolerance, and the calculated bottom clearance.  The risk assessment is a constant as opposed
to the traditional gross static rules which provide no allowance for change.
The traditional static rules consist of predicted tide height, channel depth,
and static draft with UKC calculated as a percentage of the static draft. This
percentage of static draft is to allow for tidal residual chance, squat, wave
response, and safety allowances. The traditional static rules have a variable
risk (Pearce).

PIANC Guidelines are an improved
methodology for assessing under keel clearances. The new guidelines establish
three factors to consider when establishing UKC. Water Level Factors include
the tidal offset during transit and maneuvering and allowance for unfavorable
conditions. The other two factors considered are Ship Related Factors
including: Static draught including trim and list, allowance for static draught
uncertainties, change in water density, squat and dynamic trim, dynamic heel
due to wind and turning, wave response allowance, Net UKC; and the third, Bottom
Related Factors including: Allowance for bed level uncertainties, allowance for
bottom changes between dredging, and dredging execution tolerance (Pearce).

            Berthing and sailing at Weipa, as
example, is determined to allow multiple vessels to use the same tide schedule.
Entry times for vessels over 200m LOA at Lorim Point Berths is 1 hour prior to
low water, 2.5 hours prior to high water or 1 hour before the first vessel
departs Lorim Point. Ebb tide berthings are permitted based on current meter data
reads. The number of tug required for berthing is based on current speeds and
wind directions. Current 2 knots wind 30 knot W, safe operation with 2 ASD
tugs, Current in excess of 2 knots wind 30knts W, 3 ASD tugs required
(Queensland Government). There is no restriction on Ebb tide berthing of either
W or E Wharf is occupied (Queensland Government). Departing vessels, in excess
of 150m LOA, from Lorim Point are only permitted on flood tide only. Ships over
200m LOA must follow the multiple ship movement matrix and will utilize the
South channel and specific berthing designations are applied. Ships less than
200m LOA do not have to use the designated South channel, but still require
contact with the Regional Harbor Master for passing and berthing (Queensland

            Ship movement matrices are
established to delegate the number of ships allowed for arrivals and departures
at a given tide, the order in which the ships will arrive into or depart the
port, and establishes a time schedule for how long it should take the ship to
departure or arrive at various points within the port. The matrix will also
establish the required number of tugs needed for arrival and departure and
where these tugs will meet the ship. Environmental factors which could deviate
the normal tidal levels and visibility will be assessed and alternate plans may
be employed in the event of higher risk due to these circumstances.

Port of Cairns has established
less specific guidelines for tide arrivals and departures. Vessels less than
150m are allowed to depart at any stage of the ebb tide as long as UKC is
maintained. Vessels over 150m can depart on ebb tide as long as the hourly rate
of tide change is less than or equal to 40cm per hour (Queensland Government).
Maximum drafts are established for tide departures and vessels departing with
max drafts must depart 1 hour or 1 ¼ hour depending on their facing direction to
allow additional time for maneuvering. Ships are required to stem the tide,
head its bow into the current, when berthing and additional tugs will be
required if the tide is astern. Both ports, Weipa and Cairns, and most ports
with tidal restrictions, will require permission from the Harbor Master for all
arrivals and departures of the port. 

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