Safety margins in large vessels

ANKER&BtJLK CARRIER

ESSELS OF OVER 500 000 tons dwt, wing a length of sonic 1350ft, a beam over 200ft and a loaded draft of )out lOOft, are now on order and even rger vessels are contemplated. Apart orn the economies of size, it is con-Jered that such large vessels will )ntribute to safety at sea. This is partly oc so far as collisions are concerned,

the risk of collision increases as e number of vessels rises. From this

int of view, a shipbuUding policy hich neutralises the rise in the number vessels by increasing their size is a )licy of safety, since it makes it issible to reduce congestion and

con-quently the risk of collision.

However this safety factor may be T-set by the inability of such large ssels to comply fully with the collision gulations due to draft restrictions in allow seas such as the North Sea and

e Malacca Straits. For example, a

:ssel may be programmed to make a tssage through a shoal area at the top

high water to give her adequate iderkeel

clearance; should fog be

icountered when making such passage .e master may be placed in a dilemma; duce speed and take the risk of run-ng aground, or press on at full speed ith all its attendant dangers?

lt is also contended that another ajor risk, that of grounding, can be sessed in the same way. Grounding is nerally the result of human error, and

first sight it would appear that the ;k is halved by using one vessel of

)O 000 tons instead of two vessels of ;o_{000 tons, or reduced by one hundred} hen compared with vessels of 5000

ns. This would be correct if the related .fety margins applied equally to all

ssels, but this is not necessarily the se.

Although both large and small vessels e constructed to withstand the stress

ìposed by bad weather at sea, the nail vessel could run aground on a

nd bank and refloat without danger

hile a large vessel under the same rcumstances may well break her back.

Further, due to economic considera-Dns, large vessels may be sent to ports hich were originally designed for much nailer vessels and consequently man-mvring areas, channel widths and

pths may shrink to dangerously small

Stewart, DSC RD, Marine

margins. Frequently at such ports the navigational aids, especially buoys and leading lights originally designed for much smaller vessels, do not extend to the range required by very large vessels thus making the approach hazardous.

Sometimes large vessels are sent to such ports under ideal trial conditions,

and having been berthed in safety it

becomes the practice to nominate similar large vessels

as a matter of

course. This may have disastrous results hen the conditon arc less than ideal or where a particular vessel may not be acquainted with the peculiarities of the port.

From the navigational viewpoint the most crucial factor would appear to be

the ability of the master to ascertain

accurately the underkeel clearance of his vessel at all relevant times, and also to predict what the clearance will be at specific times and places en route.

To enable this to be done he must

know his actual draft fore and aft and

Co n sTJJISCHE UNIVERSITE3T Laboratoilum voor Scheepshydromechwilca Archief Mekelweg 2,2628 CD Deift Tel.: O15-7e6873.fax 015.781833

the actual depth of water; whilst, to

predict underkeel clearance, he must know the accuracy of the chart being used, the reliability, of any tidal predic-tion, the effect of rolling, pitching and

heaving due to wave action on his

vessel, also the effect of hull flexing, squat and trim on his draft.

At present no instruments exist to enable the master to ascertain the effect of all these variables, while the adequacy of some of the present equipment may be questioied. However, there are already instruments on the market

which give warning whenever vessels are in waler of a predetermined depth

and all large vessels should be fitted with such a device.

The requirement for accurate

under-keel clearance falls under two broad beadings, loading and navigation inside sheltered harbour waters where depths are accurately known and where clear-ances are measured in inches rather than in feet, and navigation in the open sea

Fig 1. (A) Vessel loaded down to her Plimsoll line and carrying her full deadweight. (B) Same vessel hogged. (C) Same vessel sagged.

NOVEMBER 1973

SafE t

Margins n Large Vesse!s

4

TANKER & BULK CARRIEj

where considerably greater clearauces

are required.

Factors affecting clearance

when loading

Subject to certain known allowances depending on the season of the year, the density of the vater in which the vessel is loading and the route to be followed, vessels are permitted to load down to a

specific freeboard which is measured

amidships from a marked deck line to

the sea level. Should there be no restric-tions such as lack of adequate depth of

water en route or at the terminal port, owners require their masters to load down to the permitted freeboard to

ensure maximum profitability. While the earning capability of any vessel depends on freight conditions, a sum of £100 000 per annum per foot of draft for a vessel of 500 000 tons is a reasonable figure to assume; consequently it will be seen that large financial considerations are

involved.

lt is desirable to eniphasise that

free-board calculations are based on the

density of water as opposed to specific

gravity. Although comparable, density and specific gravity are not the same

thing; the density of water in ounces per

cubic foot must not be confused with

specific gravity. Sorne countries do use specific gravity but the requirements set by the Department of Trade and Industry for freeboard calculations assume that salt water has a density of I 025oz/ft3.

The actual density of the water in

which a vessel is _{floating should be}

obtained by using the special glass

hydrometer, graduated in density of oz/ft3 at 60 deg F and certified by the National Physical Laboratory to the

standard set by the department of Trade

and Industry. If the temperature is

other than 60 deg F a correction hts to be applied.

As the top of the horizontal line of the

loadline disc must not be submerged

when a vessel in the Summer Zone is

floating in sea water with a corrected

density of ¡025 oz/ft), it is essential ;hen floating in water other than with a

density of 1025 that an appropriate correction be made. In some parts of the world, the density of sea water is more than 1025 or/ft3 and it may be as high as ¡033 oz/ft3 and _{consequently a} vessel has to be loaded _{light' of the}

appropriate salt water loadline. In the case of a vessel of 500 000 tons she

soukj have to be loaded _{'light' by}

about 8m.

When ascertaining the freeboard of a loaded vessel it is therefore essential to know accurately the_{density of the water}

in _{hich the vessel is floating, and in} areas where the water is brackish or

where it may vary at different states of the tide, it is essential that the sample of

Fig 2. A 250 000 tons dwt vessel of 11 SOft length and i 601t beam loaded to a draft of 64ft with an underkeel clearance of 3ft.

water used to ascertain the density be representative.

This is easier said than done in the

case of a vessel which when fully loaded

may displace some 630 000 tons of water. Even if an accurate sample is obtained it is important that the actual

density, corrected as previously men-tioned, should be read before the temperature of the sample has time to change, in other words that the density used in calculations is the density in

which the vessel is floating.

Failure to observe this precaution

may result in error as the change in

density of water varies between 00 and

02 oz/ft3/deg F. An error due to any

cause in the reading of the density of only 0001 can in the case of a large

vessel make a difference of about 500 tons in Fer aeadweight calculations or about one inch in her draft.

As it is not practicable except under ideal conditions to read the freeboard

direct, it is generally ascertained by reading the draft fore and aft and mak-ing a small calculation. If a vessel could be treated as a rigid beam such a

method would be correct, but

unfor-tunately vessels may either sagor hog. In any event, in the case of very large

vessels, it is physically impossible

visually to read the draft marks within

an accuracy of six inches, especially if the vessel is _{loading at an exposed} berth. Since the weight to sink a vessel

one inch (called TPI or tons per inch

immersion) is, in the case of a vessel of 500 000 tons about 590 tons, it will be seen that quite large quantities are

involved.

As adequate instrumentation to read

the draft is not generally available,

vessels are usually loaded to deadweight;

in other words the actual weight of

cargo required to bring the vesse! to the correct freeboard is calculated, after

allowing for other weights such as

bunkers, stores and water already on board.

In the case of liquid cargoes this in itself may be open to error_{as it may}

depend on the accurate reading of the specific gravity and temperature of the cargo; an error of only one deg F may make a difference of 2500 tons in the case of large vessels, _{i.e. about 4m.}

in the draft.

Hogging and sagging will affect the

draft of a vessel and the amount of

cargo carried. Figure I(A) illustrated a vessel loaded down to her Plimsoll line and carrying her full deadweight, while figure IB) shows the same vessel

hogged. The additional cargo carried

is equal to the weight of water displaced by the wedges shown in black; it should be noted that the draft fore and aft has been increased. Figure 1(C) shows the vessel sagged, but in this case cargo is shut out which is equal to the weight of

the volume of water not displaced by

the wedges; in this case the draft

amidships is greater than the draft at

the bow and stern.

Overall, a figure of one foot would

appear to be a reasonable amount tO allow for all errors above described.

Flotation allowance

Before one can make any calculation as to the various allowances that have to be applied it is necessary to assume that the vessel is afloat in the first place.

Consequently a minimum underked

clearance has to be allowed. Two feet

under the keel would appear to be a reasonable figure to accept and ofl

which would apply to berths and sea

areas where the bottom was level sand or mud; on rocky bottoms and where the sea bed is uneven a larger margin will have to be allowed. Other factors, such as errors in calculations mentioned above should be added to this amount to obtain the minimum underkeel clearance when loading and while under way. Other factors, such as allowance for chart errors, tidal predictions, squat

and roll will have to be added where relevant.

Factors affecting clearance

when under way

Accuracy ofCharts. In the open sea the greatest accuracy that can be expected of navigational charts between the 10 and 15 fathom sounding contour

is plus or minus one foot. but a great

deal depends on the date or age of the last survey as it has to be appreciated

that until recent years the six-fathom line was considered the danger line and consequently soundings below that depth were regarded more as naviga-tional aids when using the echo sounder. The date of the last survey is especially important in areas where the channel is unstable and liable to constant change, and also to areas in the North Sea and the Malacca Straits, to name but two,

where sand waves may considerably

alter the depth. Due to strong tidal

currents, large uniform sand ripples are formed, the height of these ripples or sand waves, which form at right angles to the current, may be as great as 3Oft from trough to crest, with a wave length varying between 800 to lSOOft.

Tidal Predictions. These fall into two categories:

I predictions for ports and approaches. In all but the most exceptional con-ditions these can generally be relied

upon within plus or minus 2ft; 2 _{predictions for open waters such as} the North Sea. These can at present only be relied upon within plus or minus 6ft, but extensive surveys are at present being undertaken which it is expected will reduce the above figure. Tn addition to the above normal predictions, a

method of forecasting storm surges in

the North Sea,

like the one which

caused the disastrous floods of 1955, would appear to be required. Besides

the positive surge which can cause floods, a negative surge may be experi-enced which will decrease the depth of water.

Considerable theoretical data exists concerning the various factors that may influence a ship's draft. To relate theory to practice and to be able to obtain the sum total of these factors, research has been undertaken in selected vessels to ascertain the value of the various factors Involved in underkeel clearance under

Varying conditions.

NOVEMBER 1973

Two approaches have been made to this problem. First, special echo-sounders, capable of providing accurate clearance measurements down to about two feet were fitted forward, aft and amidships on the port and starboard

sides, so that the minimum clearance over the whole ship's bottom could be ascertained. The findings are of value where the datum and tidal conditions are accurately known. For prediction purposes however, it is necessary to identify the various components, such as rolling and pitching and squat and ultimately to

correlate them to the

condition of a particular area or sea state. In cases where the sea bed is

rocky or uneven it was found impos-sible to correlate one passage with

another, and this severely limited the value of the trials.

To overcome this difficulty a second

approach was to fit a ship motion

recorder which was capable of measur-ing the various ship motions and deriving the various factors which increase the draft.

Rolling, Pitching and Heaving. When a ship is affected by wave action she may

roll, pitch and heave, and all

these motions will alter the underkeel clear-ance at different points of the vessel's hull. When pitching, the bow and stern are the areas where decrease in clear-ances will be experienced, while when

rolling the greatest decrease will be experienced along the turn of the bilge.

With a small underkeel clearance,

there is evidence to indicate that the

ship's motion may be dampened by the cushioning effect of the water trapped below the ship's bottom. But, while this in itself may increase the clearance, it would indicate that possibly excessive

stress is being placed on the hull of the vessel.

Fig 2 illustrates an outline of a vessel of 250 000 tons dwt with a kngth of

llSOft and a beam of I6Oft, loaded to a draft of 64ft with an underkeel clear-ance of 3ft. The bottom of the vessel is shown as flat, although ¡n some

instances vessels may have a rise in floor of about 05 deg. It will be apparent that even a slight list or roll will increase the draft and thus reduce undcrkeel clearance. In the case of the vessel

illustrated, a roll

of 35 deg would

increase the draft about Sft, while a roll of 11 deg would increase the draft by over I6ft.

When approaching port through a

long dredged channel at right angles to wind and sea (for example Europort, in a strong Northerly wind and sea), a roll of 35 deg is not unusual. Pitching may be combined with roll, in which case a pitch of 05 deg would increase

the draft by 4 or 5ft. Heaving may

also occur which will tend to increase the draft and consequently reduce the underkeel clearance.

Hull Flexing. A ship is not a rigid structure and clue to a variety of causes the bending or flexing of a vessel will tend to increase the draft.

Squat and

Triai. When a

vessel previously stopped gets under way, two factors come into play; she tends to 'squat' i.e. increase her draft, due to the depression of the water level in her immediate vicinity caused by the for. ward motion; she may also change her trim fore and aft. While these factors are of little concern in the open sea with deep water, they may have considerable effect when navigating dredged channels and shallow seas.

Fig 3. Allowance for each individual factor in typical and occasional passages.

i

¡ NOTE - Octa abo brokn Lines refer te occasional cQnditjOnL

2tT

t5'

I

.1

Flotation allowance

Errors ClarIs Tidal Rolling Pitching HeaoinQ Null Squol L

prtiicton tie'ir'g tflrn

8

These two factors vary with the speed of the ship and static underkeel

clear-ance, and to a lesser extent on her

original trim when stopped. Hull form has also an effect. A vessel with a bluff bow will react differently from a vessel with fine lines forward. Figs 3 and 4 illustrate in diagrammatic form the

allowance for each individual factor in typical and occasional passages and also the combined totals. Examination of the diagrams indicates that the sum total of the various factors may appear excessively large, which if applied in full would seriously affect the economic running of many vessels. Consequently one has to be selective.

Few vessels are fitted with instru-ments to indicate each of the various factors involved under the particular

conditions of the moment and, even if they were, they may be of little use to a master if, once committed to a passage, they told hirn conditions were

danger-ous. While not expected to run his

vessel by the seat of his pants,

consid-erable assistance could be given by supplying him with information, in easily digestible form, of the effect of the various factors which would allow him to select the ones that apply to his particular case.

For example, for a passage down the

Malacca Strait, little or no rolling or

pitching is to be expected, while on the other hand a larger than normal allowance may have to be made for

squat and for the possible inaccuracy of charts, coupled with the difficulty of obtaining accurate position fixes in certain parts of the run.

More could be done in the field of

instrumentation which will enable the

master to ascertain his actual draft fore and aft, also amidships when in port. In addition, he should have an

instrument that will enable him accu-rately to ascertain the trim of his vessel

duri ng cargo operationsof special

importance when the underkeel clear-ance may be small. This will enable him at least to start his voyage with reliable

data on which he can base his

cal-culations.

Unfortunately, no instrument exists that can measure the draft of a vessel when under way nor, due to technical problems, is such an instrument likely to be available in the near future.

Although a vessel can very accurately ascertain her list when in port, the great majority of vessels have no means of

accurately finding the degree of roll when under way. Many have to depend

on the spirit

level on the standard

compass which is generally only accu-rate to plus or minus 2 deg.

An instrument to measure the speed of the vessel is required. When in open waters, speed is only required in the

forward direction with an accuracy to

the nearest 025k. _{The ordinary}

sub-FI oot ion

Typi cut

merged log which has a pitot tube or

probe extending some three feet clear of the turn of the bilge or bottom is only satisfactory in deep water.

In shallow water the pitot tube is generally retracted and in any *vent

speed indications are unreliable owing to the extension of the boundary layer of turbulence. For channel navigation

and docking, the master requires to know, within very narrow limits, the

vessel's -soced and rate of change of

speed, in th forward, astern and lateral direction, also the rate of turn.

Fortunately this situation has been

anticipated by a number of equipment manufacturers, and several different doppler log equipments are available which almost meet these requirements. Unfortunately, as is often the case with many instruments supplied for use ori board ship, most have failed to appreci-ate the importance of displaying the information in a way most amenable to the shipmaster.

One group of tanker owners has

recently developed a display unit to

amalgamate with the two-axis doppler log turning rate meter equipment which does meet operational requirements. The information, being displayed in analogue form, is instantly recogriisable. There are two distinct uses for such an instrumentchannel navigation and berthing. In the former, when a vessel is navigating in areas with cross

$ _{FoIcti nn} Occasional

Fig 4. Individual factors ¡n typical and occasional passages together with combined totals.

currents, it is essential to ascertain the drift of the vessel. The ability to do so accurately and quickly may have con-siderable bearing on the route selected and hence the profitability of the vessel. Take the case of a vessel bound from the Atlantic via the North of Scotland to a North Sea port. In ordinary circum-stances, due to the strong and varying current directions in the Pentland Firih a very large vessel would be inclined ta pass north of the Orkney Islands, thus adding a considerable distance to her passage. With an analogue display of log information, indicating every varia-tion of set, a large vessel has passed

through the Firth with no practical

deviation from her true course line as previously laid down on the chart.

Another example of the value of such an indicator is when approaching po1

along a restricted channel with strong cross currents. Taking the port 0 Zeebrugge, at present serviced by

vessels over 200000 tons dwt on a

restricted draft, vessels approach the port along a five mile channel by keeping leading marks in line. As such vessels have to berth al high water, at which time the tidal stream outside is flowing strongly al right angles to the axis of the channel it is necessary to proceed crab',',I Further, as the stopping distafl1

concluded on page 15

TANKER&BULKCARRIEI

35.

Sqot & Trim

30 NoII F1.axirr3 u _{Ho ring} 2 S

I

20

5qut & Tri ra

I

Pit chi ng and Polling Holl Fai,,j IS

_t

Pi tchi ng

I

I

10 Rolling Ti d3I Prraiclion Tidal Pradjcti, S

I

I

thons s Errors Errors

oniinued from page 8

nside the port is small, the speed of irrival off the breakwater is critical. )nly with an instrument showing for-yard and lateral movement can ports )f this nature be entered in reasc,nable

afety.

A final use of the instrument above lescribed is the actual berthing opera-ion, but in this case it is not just enough nforrning the master that his vessel is

pproaching a jetty too fast; means

nust be available to enable appropriate

ounter action to prevent damage. In

inc case involving serious damage it vas afterwards ascertained that the

ccident was due to failure in communi-ations. On further questioning it was ound that the pea in the pilotjs whistle

ad got lost and he was unable to signal he tug to hold off the vessel.

It has to be appreciated that large

essels are very fragile structures when ut of their natural element and should hey strike a jetty at the wrong angle or

t _{speed (which speed may be}

con-idered very slow for a smaller vessel)

xtensive damage may occur to the

hip, or if the berth is of the dolphin

-type, the even greater financial

con-sequence of damage to, or destruction of, the jetty.

Steering may present problems it very large vessels. Although auto-pilots can

be made to wke care of any special characteristics of a vessel, it is the normal procedure, when approaching

port and during berthing, to change

over to manual steering. In these cir-cumstances it has to be appreciated that the human eye can only detect

move-ment over a small range and

con-sequently, when attempting to steer very large vessels by gyro, slow movements of the compass may remain undetected

for some time,

and when noticed, excessive counter helm may be applied and kept on too long, resulting in the ship yawing from side to side.

In the matter of navigation in the

shallow seas, an instrument that could

monitor the depth of water up to a

distance of three miles ahead of the vessel over a width of some l000ft, would provide a very useful facility.

Present indications are that no such

development is likely in the immediate future. Consequently, recourse will have to be made to the practice of buoying a

suitable channel and carrying out frequent check surveys. \Vhere these

channels are narrow it may also be necessary to have sorne measure of

traffic control such as one way traffic.

A final problem is the navgation of very deep draft vessels in narrow channels. \Vhere such channels are inside national limits local authorities

can, and frequently do, make

regula-tions for the control of traffic. Unfor-tunately many deep channels now

extend into areas within international waters where the only form of control is the Collision Regulations; these cover traffic separation schemes,

hich at

the moment are fairly elastic, and the definition of 'hampered vessels' to which other vessels must give way.

Vessels constrained by their draft in their ability to manoeuvre are not included in this list although mention is made of them in the list of General

Definitions, and consequently a certain amount of ambiguity may occur. Further, the signals required to be

dis-played visually would appear to be

totally inadequate taken in relation to the length of \ery large vessels.