International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 6
Number 1
March 2012
63
1 INTRODUCTION
1.1 Criteria of the subject choice
It is a common knowledge that statistical, ground-
ings are on the top of the list of all reported ships’
accidents, incidents and near miss situations. Upon
UK Maritime Accident Investigation Bureau
(MAIB) reports, groundings represented 73.2% of
all noted ships accidents last year and over 64% for
the latest decade. Additionally, groundings are in
strict conjunction with secondary hazards and acci-
dents, appearing shortly after that, e.g. oil pollution,
lost of the stability, damage to the hull and finally
collision with other ships in extreme cases. The
above mentioned hazards classified in conjunction
with the groundings represent 7.5% of the total
number listed accidents. Significant numbers ( over
55% roughly precise estimations are still under the
count) of all groundings had happened on the re-
stricted waters, such as rivers, estuaries, bays and
creeks. The average tidal range for these cases was
above 5 meters.
Due to the careful study and analysis, it was con-
cluded that the main reasons of the majority of the
total groundings were insufficient under keel clear-
ance, errors in squat assumption and wrongly calcu-
lated tidal figures in connection to the passage plans.
The groundings are relatively easy to classify, so
any precautionary measures should be selected very
carefully using all available traditional methods with
modern software and sophisticated tools.
1.2 River Humber as an example of restricted tidal
area
The Humber Estuary is one of the busiest trade
routes in Britain and represents about 11% of overall
seaborne trade.
Numerous sandbanks, swift tidal currents, dense
traffic, dynamic changing of bathymetric data and
unprecedented elsewhere silltation, do the River
Humber very difficult for navigation and conservan-
cy. Since 1972 the VTS Humber likely others in
Great Britain continuously developing and self eval-
uating, under supervising Maritime Coastguard
Agency (MCA) and Maritime Accident Investiga-
tion Bureau (MAIB), fulfilling SOLAS Convention,
Chapter V Regulation 12, Maritime Safety Commit-
tee (MSC) Circular 1065, International Maritime
Organization (IMO) Resolution A.857(20) and cor-
responding with International Association of Marine
Aids to Navigation and Lighthouse ( IALA ) VTS
Manual 2008 placing emphasis on IALA Model
courses and Training Guidelines.
Despite of every effort was made by the local
Hydrographic Department Hull in the scope of sur-
vey and dredging, the dynamic and unpredicted
silltation of the river due to daily migration of the
huge masses of the mud and the other riverbed mate-
Comprehensive Methods of the Minimum Safe
Under Keel Clearance Valuation to the
Restricted Tidal Waters
G. Szyca
Associated British Ports, Humber Estuary Service, Pilotage Department, England
ABSTRACT: The main purpose of the paper is familiarization with the matter of most crucial aspects of the
minimum under keel reserve for the sea-going ships navigating on the restricted tidal waters. For the purpose
of this paper, river Humber was used as good example of high tidal range in conjunction with variety weather
conditions encountering in this area affecting the tides. The Author made use of his research and job experi-
ence as the pilot, closely co-operating with other authorities, e.g. Vessel Traffic Service ( VTS ) Humber, lo-
cal Hydrographic Department as well as Maritime Coastguard Agency ( MCA ) and Maritime Accident In-
vestigation Bureau ( MAIB ). The extensive statistics of groundings and near miss situations, in connection
with in-depth analysis will be due to presented with conclusions and proposals of sorting out some problems.
The new concept of Dynamic Under Keel Clearance DUKC® software with the trials assessments will be
widely put forward in compare with currently utilizing tools and software by VTS and Ships’ Traffic Centre.
64
rials ( mainly on the upper part of the river ) is con-
sidered as a main reason of the massive ships’
groundings, posing highest percentage of overall
marine casualties on the River Humber. Grounding
incidents divided onto to two groups: first when
ships were re-floated on the same tide and the se-
cond, when ships not re-floated on the same tide (
means: re-floated on the next tide or under external
assistance). Generally speaking, it is quite obvious
that the measured two kinds of groundings showing
very similar figures. In presented hereunder results,
about 50% of them were caused by wrongly calcu-
lated tide values ( time and high ), or inadequate
prediction (tide fluctuation). In 40% of overall cases,
the measured accidents were brought out by pilots
with 2 years practice or less. Over 25% of total
groundings concerned sudden “tide cut” either
height or time, in the areas called “no point to re-
turn”, where was not enough space to turn around
the ship and abort the passage
1
.
Below diagram illustrates the figures of the total
groundings of sea-going ships on the river Humber
within the space of five years:
Diagram 1.1 The statistics of the vessels groundings with re-
floating
Although every single case of the grounding has
been detailed evaluated and analyzed both by
MAIB and Humber Estuary Service, at the present
stage, it is very difficult to find out about any links
between the casualty, the professionalism of the
VTS staff, ship’s crew negligence and pilot’s lack
of knowledge. MAIB’s grounding investigations re-
vealed that over 50% of all accidents were caused by
insufficient under keel clearance and in over 30%
cases the official soundings were not covered with
actual depths found while grounding or shortly after
that.
2
MAIB strictly recommends to apply by VTS
sophisticated and efficient software which allows to
assess a require under keel clearance and increase
1
Annual Maritime and Coastguard Agency Report, London 2009
2
Harbour Master Humber Annual Report, Hull 2009
ruling Under Keel Clearance ( UKC ) for restricted
waters, meeting assumed goals.
Execution of increasing the minimum under keel
clearance, in practice is very difficult, because the
period of flood tide in the rivers Trent and Ouse is 3
hours only, so high demands would lead to too fre-
quent cancellations of the ships and discontinuation
upper river traffic. The most reasonable solutions is
applying highly, advanced software, particularly de-
signed for specific waters, conformed with existing
systems and flexible collaborated with tide gauges in
many locations. Practically such a tool should con-
sist of two independent modules: one for the lower
river, second for the upper, therefore UKC require-
ments are quite different for those two parts of the
river. One additional module should be designated
for very large ships (VLS), which are subject to
completely different UKC and passage criteria. An-
other additional obstacle making difficult execution
for the required UKC, is the fact, that upper river is
not covered by radar stations and all monitoring of
the traffic is via Automatic Identification Systems
(AIS), Radiotelephones (VHF) and Closed-Circuit
Television (CCTV). One alternative option could be
using the Portable Operation Approaching and
Docking Support System (POADSS), by all transit-
ing ships. That conception will be widely presented
in the next chapters.
2 ACTUALLY APPLY UNDER KEEL
CLEARANCE STANDARDS TO THE RIVER
HUMBER
The precise determination of the clear-cut un-
der keel clearance figures still remains as a open
issue and the subject to the dispute between safe-
ty measures and commercial interests. The rela-
tively huge differences of the surveys in the short
intervals is still the main obstacle to work out
uniformed UKC figures.
65
Table 2.1 Illustration of the dynamic changes to the depths of
the River Ouse (upper Humber) for the period less than one
month
Hydrographic surveys presented above, for the
specific reaches of the river Ouse, in duration of the
23 days show average difference about 0.20 m and
in extreme cases even 0.40 and 0.50 m. The com-
monly established under keel clearance standards for
the rivers Ouse, Trent and upper Humber are 0.20m
during the day and 0.30m at night (not excluding
ships’ Companies higher standards), it means that
the difference between fault given depth is almost
twice more than actual under keel clearance and di-
rectly leads towards potential marine hazard. Anoth-
er idea to establish higher UKC standard may cause,
that required minimum would be (e.g. for value
0.50m )equivalent for a ship with the draught of
5.0m, for lower Humber ( UKC is 10% of max.
draught ). In the cases of less drafted ships, the UKC
for upper river would be greater than for lower.
Where max. draught for Ouse and Trent is 5.5m in
highest spring tide and average draught is 3.80m,
such a solution would be absolutely pointless and
leads to nowhere. At the present stage it is seeking
for the compromise between demanded safe level of
navigation and keeping the waterways fully naviga-
ble working on the higher standards but that major,
essential problem is not still sorted out and it is the
subject to further consultations and advanced trials
both by Humber Estuary Service and MCA.
3 STATIC METHODS OF THE
DETERIMNATION UNDER KEEL RESERVE
Generally speaking, the main strategic assumption in
the calculating process of estimation a required un-
der keel clearance (UKC) is available water level at
the destination referred to the actual ship’s draught.
Applying this method, consists of the several varia-
tions and derivatives but mainly basis on the tide ta-
bles for the specific location, date and time upon
drew up harmonic curves and math algorithm. Un-
fortunately mentioned method does not take into
consideration changing hydro-met condition affect-
ing desired tide level and leading straight away to
apply additional corrections or decisive modifica-
tions current passage plan. All factors must be take
into the consideration while unexpected conditions
are being encountered to complete safe passage of
the ship, including sufficient water level when pilot-
ing act is aborted ( return passage ).
3.1 Analytic estimation of the demand height of the
tide.
The analytic calculation of the predicted tidal level
at the port of destination generally basis on the tide
tables worked out for the specific ports and it is the
part of preliminary process preparing ship’ passage
plan. Mentioned method may be recognized as a es-
timated only, because the all tides given in the tide
tables are referred to the High Water for the specific
location, not providing the heights for intermediate
periods. The manual height interpolation of the tide
gives the errors about 7 to 12%, there is 0.35 and
0.5m respectively for the height of the tide 5m,
which is unacceptable for 0.3m of the UKC. Be-
sides, relying on the recalculating figures only, given
in the Tide Tables without taking into consideration
seasonal changes and specifications of the river bed
increases the error to the additional 10%, in ex-
tremes. Only right, correct action should be applied
additional other support or/and alternative reliable
methods for double check.
3.2 Using remote gauges for the current tidal
valuation
The river Humber is fitted with several tide gauges
throughout the navigation traffic routes. The average
distance between the gauges is 5-7 Nm, which gives
to the navigator current information about tidal con-
dition for the specific location via VTS or internet
connection.
The tide prediction is not made of each gauge lo-
cation ( current tide height remotely reading only).
Presuming the ship’s average speed of 10 knots,
bearing mind changing of the datum and assessing
variation of the reading for the respective tide gaug-
es the navigator is able to extrapolate the demanded
UKC for the specific location, time and height of the
tide with necessary margin of the error using follow-
ing empirical obtained formulas:
UKC= [ Dr ( ( R1 R2 )/2 + dD ) + Dth ] + 10%
while sailing upriver on rising tide (1)
UKC = [ Dr (( R1 – R2 )/2 – dD) + Dth ] while
sailing downriver on rising tide (2)
UKC = [ Dr (( R1 – R2 )/2 – dD ) + Dth ] + 15%
while sailing down river on falling tide (3)
where: UKC – under keel clearance [m]
66
Dr ship’s draught [m]
R1 – reading from passing tide gauge [m]
R2 reading from the next nearest tide
gauge [m]
dD difference in the datum between the
gauges [m]
Dth – actual depth for specified point below
chart datum [m]
For the double check purpose of the UKC may be
used the below table, drew up in the over 15 years
period basis on statistical observation, referred to the
HW Albert Dock and recalculated for the significant
location. This table includes all observed seasonal
changes of the tide as well as other fluctuations and
there is verified and updated annually.
Table 3.1 Height of the tides referred to Albert Dock HW 8.0
metres
3.3 Comparing the tide fluctuation to the secondary
location
Seeking of the secondary ports, with similar charac-
teristic of the sea/river bed, the datum and not far
away located from defined area where is more likely
any fluctuation tide data may be used for the extrap-
olating tidal condition on the site in our interests.
There are several basic assumptions should be
made:
the occurrence of the High Water must be not ear-
lier than 3 hours and later than 7,
no significant seasonal changes should be affect-
ing the observations,
mutual changes with the datum not exceeding 1
meter,
the secondary location has to be situated in the
north of defined area,
The North Shields was chosen as a secondary lo-
cation for the River Humber upon the years of care-
ful observations and wide-ranging analysis. So far
there is not any math algorithm allowing precisely
described mutual correlation between such a huge
main and secondary locations. The average accuracy
applying above method are vary and oscillating be-
tween 65 and 72%.
4 DYNAMIC EVALUATION OF THE
AVAILABLE UNDER KEEL CLEARANCE
Dynamic changes of the shipping conditions, such as
tidal stream sets and rates, weather conditions, avail-
able depths, ships’ traffic density or common tech-
nical difficulties with the service of the locks or
berths, leading towards arising the potential threats
and near miss situations for shipping safety in rela-
tion to the execution of the original passage plans.
VTS operators make every efforts to update any es-
sential data, affecting shipping, either currently or at
the periodical broadcasts, but it contents only major
information and not included any minor and dynam-
ic developing potential endangers for the specific ar-
ea. All navigators
( pilots, masters, pilot exemption certificate (PEC)
holders ) should have the access to any online nauti-
cal, hydro-meteorological, bathymetric and traffic
information, covering their passage areas. Such pos-
sibilities offers newly working out into the practice,
Portable Operational and Docking Support System,
commonly known as POADSS.
4.1 Usage of the Portable Operational and
Docking Support System (POADSS).
The POADSS project successfully culminated in
live demonstration in Lisbon, last October 2008
proving its complete suitability
3
.
The POADSS unit consists of three main ele-
ments, two onboard units and the shore one. One
onboard unit is an Instrument Unit and the other is a
laptop displaying all relevant information for receiv-
ing and transmitting data to and from the shore
based unit by means of mobile broadband. That in-
formation exchange ashore by POADSS Ground
Server Station, which sources data from VTS, tide
gauges and AIS transmitters. Such own stored data
gives to the navigator overview ship’s static and dy-
namic information details as well as surrounding
traffic image and environmental conditions in com-
prehensive overview of all necessary parameters of
the particular ship on her passage. Distinct from
mostly applied pilotage units the POADSS monitors
vertical position ( 3D ) and all dynamic motions.
There are four main new applications:
Internal Measurement Unit with Global Naviga-
tion Satellite System ( GNSS ) for determination
all dynamic ship’s movement,
Wireless broadband to exchange information in
real time ( Web or local map service ).
3
The Pilot No.296, United Kingdom Maritime Pilot’s Associa-
tion, January 2009
67
Dynamic high density bathymetric and survey da-
ta displayed on electronic chart including true dy-
namic safety contour.
Dynamic Under Keel Clearance (DUKC®) soft-
ware.
The above mentioned applications efficiently re-
duce voice radio communications and maximizes the
usability of fairway and enhances the efficiency of
the traffic flow. Interoperability with VTS centre is a
key element and by using Web Map Service the
overall VTS traffic image can be overlaid on the
POADSS Electronic Navigation Chart. If the broad-
band connection lost then AIS information remains
available by pilot’s plug connection.
However benefits of usage the POADSS are ob-
vious some restrictions and inconveniences still ex-
ist:
if specialized docking system is deployed, this
might take up to 15 minutes to set up it,
still some vessels, such barges or yachts are not
fitted with AIS causing POADSS not effective as
expected,
not approved operating and training standards,
by using the POADSS in conjunction with Dy-
namic Passage Planning DPP the maximum draft
could be considerably increased and the tidal
windows widened without compromising the
safety or efficiency of other traffic,
development of Fibre Optic Gyro’s and Micro
Electronic Motion Sensors MEMS
presently are not advanced enough and not offer
sufficient accuracy and reliability. It is expected
to reach those goal in the next five years.
the coverage of wireless broadband is still unsat-
isfactory.
present stage of development of E-Navigation is
not fully capable to be integrated with all
POADSS applications.
The usage of the POADSS is pretty limited at its
functions in very narrow river channels and re-
stricted fairways ( upper Humber, rivers Ouse
and Trent for instance ).
5 METEOROLOGICAL EFFECTS ON TIDES
All meteorological conditions more or less change
the tide figures, next affecting available depth at the
port of destination closely linked with under keel
clearance on the ship’s passage. Meteorological
condition which differ from the average will cause
corresponding differences between predicted and ac-
tual tide. Some of the effects are discussed below.
5.1 The effect of the wind
There was observed that winds blowing longer than
24 hours with the force above 7B from the north di-
rections causing drop of the tide about 30-40cm,
paradoxically strong southerly winds don not affect-
ing significantly the height of the tide. After long
wind blowing periods ( probability more than 50%)
there is more likely that the tide will be above pre-
diction in proportion to the windy period. The new
algorithm about wind effect to the tide for river
Humber is under the progress by the Author of here-
by Paper .
5.2 Barometric pressure
The tide tables are computed for average barometric
pressure, any significant changes in the atmospheric
pressure immediately are reflected in the tidal data.
A difference from the average of 34 hPa can cause a
difference in height of about 0.3m. That aspect is
frequently passed over but is still so essential for
navigation on the margin UKC. During predominant
of low pressure, for the stationary low, the increase
in elevation can be found by the formula
4
:
R = [0.01(1010-P)] x 0.3; [m]
R increase in elevation [m]
P actual atmospheric pressure [hPa]
For the moving low, the increase in elevation is
given by the formula:
R1 = {R / [ 1- ( C / gh )]} x 0.3 [m]
C rate of low motion [ m/s]
g – acceleration to the gravity [ 32.2 m/s]
h – depth of the water [ m]
The number of the British VTS are equipped with
modern software applying changes in barometric
4
N.Bowditch, The American Practical Navigator, National
Imagery and Mapping Agency L., Publication No.9, 2002
68
pressure for tide predictions. So, far VTS Humber is
not fitted with such a software.
5.3 Icing
The rivers Ouse and Trent were frozen since 1962.
During 2 months period relatively thick ice on the
both rivers made difficulties with the shipping as
well as seriously affected the tidal conditions. In the
extreme cases the congested ice caused decreasing
the height of the tide to figure of 60cm.
5.4 The Aegir
The “Aegir” is the local word means the head of the
significant tidal wave breaking through the river bars
and creeks. The aegir occurs while high spring tides
around the time of high water ( about half an hour
before ). That phenomena seriously interferes pre-
dicted height of the tide causing numerous eddies,
top runs and significantly gaining tidal streams. Ear-
ly indication and proper monitoring of the aegir al-
low to avoid unnecessary risks to navigation. Unfor-
tunately, only visual observations and usage of
probability methods can made at the present stage.
Gathering all information can allow in future to de-
velop more effective statistical methods.
6 SUMMARY
The huge numbers of groundings as a result of insuf-
ficient under keel clearance are still at unacceptable
high level for last several years. Every effort should
made to seek out a reasonable compromise between
securing minimum under keel clearance require-
ments for keeping the waterways navigable and rig-
id standards to the ships’ safety. There is no doubt
that disturbing statistics should prompt for imple-
mentation of the new solutions and different ap-
proach the subject of the minimum safe UKC taking
into consideration that the human factor still plays
major role. The matter of semi liquid sea/river bed
being the pattern of any measures and surveys re-
mains still open shall be without any delay sorted
out by relevant authorities as well as by the local
Safety Navigation River Committees ( SNRC).
So far there is no one defined universal formula for
UKC determination for the such extensive area as
Humber estuary in the wide space of time. Presently,
the main existing “tool” is the probabilistic method,
commonly known as dynamic prediction using with
co-relation with specialized software. The optimistic
prospects after series of advanced trials are placed in
the POADSS. At the present stage, the other meth-
ods as a deterministic or a stochastic can be used as
a support utilities only.
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