International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 6

Number 1

March 2012

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.

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

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.

MAIB strictly recommends to apply by VTS

sophisticated and efficient software which allows to

assess a require under keel clearance and increase

Annual Maritime and Coastguard Agency Report, London 2009

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.

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]

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

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 ).

The Pilot No.296, United Kingdom Maritime Pilot’s Associa-

tion, January 2009

− 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

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

N.Bowditch, The American Practical Navigator, National

Imagery and Mapping Agency L., Publication No.9, 2002

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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Dynamic under keel clearance, Information Booklet, OMC In-

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clearance in decision making process of port captain. Proc

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SOLAS 74/78 Convention with further Amendments, London

2007.

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Harbour Master Humber Annual Report, Hull, January 2009.

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