THE SHIP’S DYNAMIC UNDER KEEL
CLEARANCE AS AN ELEMENT OF PORT
SAFETY MANAGEMENT
Galor W.
Martitime University of Szczecin, ul. Wały Chrobrego 1/ 2, PL 70-500 Szczecin, Poland
Abstract: The safety of a ship manoeuvring within a port area mainly
de-pends on its under keel clearance. In many cases there is a need to accom-modate ships larger than those the port has been designed for. This can be achieved by modernizing some components of port water areas or by chang-ing operatchang-ing conditions. This article will examine the components of some methods of under keel clearance determination. Their verification has been proposed.
1. Introduction
The world fleet tends to expand in terms of total capacity, with vessels growing in size, while their number is maintained on a similar level. The building of new ports is restricted on the one hand by natural conditions of sea areas, and necessary large financial effort on the other hand. As economic and geopolitical conditions change, directions of cargo trans-port (bulk in particular) also change, sometimes in a cycle lasting a few years. This in turn, makes building new ports a risky enterprise for investors, as the invested capital re-turn amounts to at least twenty years. Therefore, a need arises to use the existing ports for handling ships larger than those the ports are designed for.
This objective can be achieved through changes in operating conditions withing ports and the modernization of certain components of port basins and areas. These measures should results in ports handling ships as large as possible on condition that specified safety level is maintained. Safe manoeuvring of a ship within a given area requires that the manoeu-vring area of a ship with a specific draft is comprised within available port water area hav-ing a required depth.
There are two undesired types of events that can lead to a navigational accident within a port area: impact on the shore (or another port structure), contact with the area bottom. In the former case the area depth is sufficient, whereas the horizontal dimension is too small. In the latter case the ship’s draft is to deep in comparison with the basin depth. This relation is defined by the distance of the lowest point of the ship bottom to the basin bot-tom, usually referred to as the under keel clearance or water depth under ship’s keel. The under keel clearance (UKC) is used for the description of the criterion of safe manoeu-vring in a port area. This criterion is most often expressed in this way:
H – T ≥ Rmin (1) where:
H - water area depth , T - ship’s maximum draft, Rmin - safe under keel clearance.
Rmin is the value of minimum under keel clearance of a ship manoeuvring within a given area that is to assure the ship safety, that is no contact of ship’s hull with the bottom should occur. This clearance is also called the required or safe water clearance.
2. Methods of the determination of under keel clearance
Conclusions from analyses of selected methods are that UKC is mostly determined by the coefficient method and method of summed components. The coefficient method consists in determining the value Rmin as part of ship’s draft:
Rmin = η Tc (2) where:
Tc - maximum draft of the hull, η - coefficient (0.05-0.4).
The values of coefficient η used in practice range from 0.04 do 0.4. In the other method the value Rmin is determined as an algebraic sum of component reserves [5], where in addi-tion errors of the particular components are taken into account [3, 4]:
n 1 i r i min R R (3) where:Ri - component reserves of depth,
Although similar components are used in methods of UKC determination, their definitions differ. Table 1 presents component reserves of UKC for the following methods of their determination: A – Mazurkiewicz B. [5], B– Gucma S., Jagniszczak I. [3], C – Jurdziński M. [4], D – OMC IMSD [2], E – PIANC [1]. It can be generally stated that the examined methods are not much different as the overall idea of taking UKC components into con-sideration is common for them. They only differ in the specific definitions of particular components.
Table 1. The comparison of under keel clearance determination methods
Lp. UKC components A BMethodC D E
1. Reserve for sounding errors + + + +
2. Reserve for dredging error +
3. Reserve for bottom silting up and changes in bottom shape +
4. Navigational reserve enabling ship sailing + + +
5. Navigational reserve (bottom type, non-continuous soundings) + +
6. Reserve for sediment + + +
7. Reserve for changes in water states and sea level alterations + + + +
8. Reserve for low water states +
9. Reserve for tide determination error + + +
10. Reserve for increased draft in fresh water +
11. Reserve for draft determination error and changes in water
salin-ity + + + +
12. Reserve for trim and heel + +
13. Reserve for list + + +
14. Reserve for waves + + + + +
15. Reserve for aft trim of a ship in motion +
16. Reserve for squatting + + + + +
These differences refer to:
reserve for sounding error, in method C extended with dredging error,
navigational reserve, which in methods A, D and E is defined as the minimum UKC component, accounting for the type of bottom, whereas in methods B and C it is ex-tended with errors of interpolation between soundings,
reserve of bottom silting up (shallower water), in method C extended with changes of
bottom shapes,
reserve for tide height determination, accounted for in methods B, D and F only, reserve for the determination of sea (water ?) state is included in all the methods.
However, method A calls it the reserve for low water states and its determination is based on multi-year records or the difference between mean sea level and mean low sea level recorded over the years. Also, in method C it is a correction determined, as reccommended, from observations carried out at pilot stations or harbour master’s of-fice. In method B this reserve is determined by an error of water level determination. Therefore, it is assumed that water level is predicted,
reserve for error of ship’s draft determination and changes in water salinity is
ac-counted for in methods B, C, D and E, while method A only takes into account an in-creased draft in fresh water,
reserve for rolling is included in all methods,
all methods take into account squatting of a moving ship, although in method A ship’s trim is additionally considered as a separate reserve.
Method A is intended for the determination of water area depth, called designed found at marine hydro-technical structures. This depth makes up a basis for the determination of the admissible depth used in designing or verification of structures during their long-time operation. Method E also is used for designing waterways (approach channels, canals). Methods B, C and D, in turn, are intended for such applications as checking possible ma-noeuvres of a ship with specified draft in a given area. Methods B and C enable ing the UKC as a constant value for specific conditions (accounting for errors of determin-ing particular components). Method D, on the other hand, called the dynamic under keel clearance (DUKC) is used for computing UKC for a relatively short time interval (from a few to several dozens hours). The other methods determine a UKC for a longer period of time, shall we say a few years.
3. Components of an under keel clearance
From the analysis based on Table 1 it can be said that the UKC determination methods differ in the definitions of particular components, but this in fact does not change the gen-eral principle. Presented below is a proposal of verifying those divergencies by correcting the definitions of some components.
1. Reserve for errors. This is an error of water area depth measurement. This component results from:
inaccurate measurement of depth by various instruments,
inaccurate determination of the position of depth measurement point resulting from the accuracy of the fixed position of the vessel performing bathymetric survey,
inaccuracies in determining the mean sea level,
errors of measurement in muddy water dependent on measurement frequency and the
type of equipment used (echosounder operating frequency),
errors of interpolation between subsequent soundings,
knowledge of changes in bottom shape,
accuracy of the reduction of changed depth to the datum, bottomcleanness.
2. Navigational reserve (bottom clearance). The navigational reserve can be defined as the minimum component of UKC that enables navigation of a given ship, or as the minimum value of UKC that has to (should) ensure safe manoeuvring in the area at ship’s safe
speed, or as a minimum UKC relative to the area bottom, assuring minimum risk at ship‘s contact with the bottom, if that ship’s behaviour was acceptable [5].
It seems that the last definition truly renders the idea of navigational reserve in connection with the determination of risk of a ship manoeuvring in the area. The navigational reserve depends on:
nature of the bottom and it can affect possible ship’s hull-bottom contact,
ships size,
manner of ship-handling (own propulsion, tugs, thrusters).
3. Reserve for shallower water. The area gets shallower due to silting up or sanding up, or loose bulk cargo falling into water. This reserve is determined on the basis onmean yearly reduction in bottom depth. The value of this reserve depends on its quantity the period be-tween subsequent soundings and dredging work.
4. Reserve for a change in water level. This component refers to changes in water level other than those caused by tides and waves. These changes are caused by meteorological and hydrological conditions. The oscillations may have periods ranging from centuries to short periods. The latter are essential for ship manoeuvres in port areas. Permanent winds and pressure differences may lead to low water levels, particularly in bays, gulfs, river mouths etc.
5. Reserve for the ship draft determination. This reserve results from the varying ship draft due to changes in water salinity, changes in ship’s weight as a result of fuel and stores consumption, ballast operations and hull deformation caused by the hull deformation due to non-uniform distribution of cargo masses. If the ship’s draft is determined by measurement before entrance or departure from the port, the error will also depend on the accu -racy of that measurement. If, however, the draft is determined by calculations, the error will be dependent on the accuracy of the calculations.
6. Reserve for heeling. The error of maintaining the ship in a vertical position does not ex-ceed 10. An additional list of the ship is due to alteration of its course. It results from the moment between water pressure on the hull and the force of the gravity centre, the phe-nomena that occur when a ship turns. The ship’s list depends on ship’s draft, its beam, po-sition of the gravity centre, metacentric height, ship’s speed and angle of turn. The degree of list may be determined by various methods, but all of them are not free from an error. 7. Reserve for squatting of a ship in motion. This phenomenon takes place when a ship moves and is caused by changes in the distribution of water pressure around the underwa -ter part of the hull, due to an increased speed of wa-ter flowing around. This changes the position of the hull along its verical axis, i.e. ship’s draft increases. Besides, the ship may become trimmed by the stern or head. Squatting depends on:
ship’s speed, block coefficient,
vertical clearance (ratio of area depth to ship’s draft) for manoeuvres in shallow wa-ter,
channel navigability coefficient (ratio of ship’s midship cross-section to channel
cross-section) for a ship proceeding in the channel.
8. Reserve for waves. This reserve results from the fact that affect the ship’s hull whose motion is described by six components. As a result, the ship draft changes depending on wave parameters (height, length, period), type of water area (shallow water, deep water), angle of wave uprush, ship’s dimensions and speed.
4. Static and dynamic components of the under keel clearance
The under keel clearance is divided into a static and dynamic component. This division reflects the dynamics of particular reserves. The static component includes corrections that change little in time. This refers to a ship lying on calm waters. The dynamic compo -nent consists of the reserve for the squatting of a moving ship and wave action [methods B and C]. It should be noted that in this division the dynamic component should also in-clude the reserve for listing caused when a ship turns. Therefore, the UKC can be:
Rmin = RS + RD + δr and
6 1 i i S R R
9 7 i i D R R (4) where: Rs - static component, Rd - dynamic component,r - errors of component determination. R1 - reserve for sounding error,
R2 - navigational reserve, R3 - reserve for silting up,
R4 - reserve for water level change, R5 - reserve for tide determination error, R6 - reserve for ship draft determination, R7 - reserve for listing,
R8 - reserve for squatting of a moving ship, R9 - reserve for waves.
The determination of all the components is burdened with calculating or estimating errors. Considered as incidental errors, they can be defined as in [4]:
2 1 9 1 i ri r (
) (5)5. The determination of UKC
The size of UKC in ports is defined by: maritime administration (maritime offices, harbour master’s offices),
port authorities, ship masters.
The interests in this field are contradictory. Maritime administration responsible for the safety of navigation wants the UKC to be relatively high. This, in turn, reduces the possi-ble use of ships’ capacity to the full, which for both ship owners and charterers is far from advantageous. In extreme cases a ship’s owner or charterer may give up using port’s ser-vices. The determination of permanent value of UKC was connected with decade-long observations and restrictions in sufficiently accurate determination of its components. However, advances in the field, i.e. scientific methods enable the optimization of the UKC value. The objective function can be written as:
UKC= Rmin → min (6)
with the restrictions
R ≤ Rdop where:
R - risk of manoeuvring in an area,
Rdop - admissible navigational risk defined at an acceptable loss level, where:
Rdop= P[Zc(t) ≤ Rmin / 0 ≤ t ≤ tp] dla C ≤ cmin (7) where:
Zc(t) - the listdistance beetwin ships hull and bottom during manoeuvring Rmin - under keel clearance
C - losses,
cmin - accetable level of losses.
There are cases where a ship’s hull harmlessly penetrates the bottom up to 40 cm. Obvi-ously, it is possible only if the bottom ground is properly loose (sand, mud etc.). This kind of bottom is found at approach channels to Polish ports. Therefore, it is possible to predict the minimum value of UKC for a certain risk level. It should be emphasized here that the adoption of such assumptions can have another effect, namely certain ships will not be al-lowed to enter the port due to environmental conditions (mainly water level and waves). The ship will have to wait for the conditions to improve. In tidal ports whether a ship may enter or leave a port depends on the so called tidal window with waiting time amounting to several hours. The losses arising from the fact that a ship hits the ground while moving, such as hull damage or, possibly, loss of cargo (particularly liquid cargo, which may pol-lute the marine environment) depend on a number of factors which can be expressed by a variety of measures. The one of these is maximum ship hull load less than admissible value caused damage of its. The maximum ship hull load FK for such a case can be de-fined in dependent of probability as:
FK = f [P(Qsgr > ZG)] (8) where:
Qsgr - admissible pressure on ship’s hull, ZG - passive ground pressure
In non-tidal ports the waiting time may be longer due to the nature of changes in water level and waves. Bulk carriers may wait for a considerably long period, but this will be compensated by deeper admissible draft. There is a need for an examination of relations between the possible maximum draft (minimum UKC) and a risk connected with ship’s waiting for entry/leaving the port caused by the required under keel clearance.
6. Summary
The under keel clearance should ensure ship’s safe manoeuvring in a port area on the one hand, and the maximum ship’s draft on the other hand, particularly in port areas. This can be achieved through the minimization of UKC value while risk is kept at an acceptable minimum. One should expect that certain ships will have to wait for port entrance or de-parture due to insufficient UKC. The risk of such waiting period should be determined. The dynamic component of UKC should comprise, apart from the reserves for squatting and waves, a reserve for ship’s list. It can be claimed that the UKC may sometimes as-sume negative values (the bottom can be penetrated) in the case of fine loose bottoms. The basic condition of such a contact is that the ship’s hull does not get damaged during its contact with the bottom. Therefore, it seems necessary to do research aiming at the devel-opment of a method allowing determining the minimum keel clearance with the safety level being maintained. Then decisions whether a ship can manoeuvre within port waters will be taken at the acceptable risk. It permits to manage of ships safety. Additional taking into account the possibility of serious damage of ship hull the method can be used to choice of optimal ships draft.
References
1. Approach Channels. A Guide to Design – Final Report of the Joint PIANC – 11, 1, Brussels, 1997.
2. Dynamic under keel clearance – Information Booklet. OMC International Marine Ser-vice Department, Australia.
3. Gucma S., Jagniszczak I.: Nawigacja morska dla kapitanów. Wyd. Foka, Szczecin, 1999.
4. Jurdziński M.: Planowanie nawigacji w obszarach ograniczonych. Wyd. FWSM, Gdynia, 1999.
5. Mazurkiewicz B.: Zalecenia do projektowania morskich konstrukcji hydrotechnicznych – Z31. Wyd. KBMPG, Gdańsk, 1997.
