The maximum allowable still water bending moments and shear forces will be stated in the loading manual.
The in-port values do not apply when the ship goes to sea; the at-sea values must be used. Where two alter- native values are provided, one set for alternate hatch loading and the other for ballast or uniform loading, the appropriate set must be used. In the longhand calculations of longitudinal strength for some vessels a separate 'bulkhead factor' is used to reduce the calculated values of shear forces. This correction should be applied strictly in accordance with the instructions in the stability booklet. The bulkhead factor takes account of the fact that some of the load caused by cargo in adjacent holds is transferred to the transverse bulkhead through the double-bottom structure instead of the side shell plating. Computer- ised loading programs offer the operator the opportu- nity to make this correction. If he chooses not to apply the correction the resulting error will be a safe one; the shear forces will appear higher than they really are.
7. Check that the tonnage allotted to each hold is not greater than the classification society permits. This check is important to ensure that the ship is not over- stressed, but often the requirement is not clearly stated in the loading manual, particularly on older ships. The maximum tonnage, when plainly stated, may be a single sum, or the permitted tonnage may vary with the draft of the ship. In some loading manuals the permitted tonnage is not stated. In such cases the ship may be put at risk if any hold is loaded with more cargo than is shown in any of the standard conditions of loading contained in the loading manual. When such a loading is proposed, masters are advised to consult the classification society, through their owners. This matter is fully discussed in Appendix 9.2. The unintended overloading of a hold is most likely to occur when loading a high-density cargo in alternate holds and when loading to tropical marks. Unintended over- loading may also occur when extra cargo is carried because only a small quantity of bunkers has been loaded, or when several parcels of cargo are loaded, requiring uneven distribution between holds.
8. Check that the hold tank top, or double-bottom, loadings are not excessive when the cargo is one such as steel coils or slabs. Permissible tanktop loadings are imposed to prevent damage to the internal structure of the double-bottom tanks. They are normally expressed in tonnes per square metre (t/m2). If the maximum tonnage of steel coils or slabs per hold is calculated from tanktop area x permissible loading the maximum tonnage will normally be less than that allowed for alternate hatch loading. In other words, the tonnage of ore which can be carried in a strengthened hold is greater than the tonnage of steel which can be carried, unless a special assessment has been made by the classification society, taking account of the ship's structure. Provided that a cargo of steel is distributed throughout all holds, and not placed only in the strengthened holds, there should be no difficulty in loading a full cargo.
9. If the ship is to be block loaded ensure that the loadings for individual holds remains within the special block loading limits set by the classification society. Block loading is the name given to a loading in which adjacent holds are heavily loaded whilst one or two remaining holds are empty. Block loading is likely to be proposed when several parcels of closeweight cargo are to be loaded or discharged at different berths, or in different ports. It may also be proposed as a means
of reducing the longitudinal stresses which occur when a vessel is jump loaded. The problems associated with block loading are described later in this chapter, and full details appear in Appendix 9.4. If block loading is planned and the ship has not been provided with a statement of maximum tonnages for use with block loadings they must be requested from the classification society, through the owners.
10. Work through the voyage stage by stage, adjusting bunker quantities to reflect bunkers consumed and taken, and repeating the calculations at steps 3, 4 and 5 for arrival at and departure from each port.
11. If any of the above steps gives an unacceptable result the cargo tonnages or other weights must be redistri- buted, and the calculation must then be repeated.
12. When an acceptable cargo distribution has been produced it must be carefully reconsidered to see whether any errors have been introduced. For example, is there room in the hold for the amended ton- nage allotted to it? Can the tonnage distribution between holds be further amended in any way that will help to speed loading or discharge?
Reasons to amend the tonnage distribution:
Results that would be unacceptable include insuffi- cient positive stability, excessive shear forces or bend- ing moments, or excessive tanktop or hold loadings.
Also unacceptable would be a large trim by the stern which makes the ship's draft too deep to enter or leave a port, or a large trim by the head which is likely to make the ship steer badly and which usually makes it difficult to draw fuel from forward bunker tanks to the engineroom.
In the case of a bulk carrier which is normally sagged when fully loaded, it is worth examining the possibility of putting increased tonnages of cargo in the end holds and reduced tonnages in the centre holds to reduce the sag, provided that this does not take bending moment and shear force values close to the limits.
It is sensible to respect the wishes of the chief engineer, as far as possible, with regard to the most convenient bunker tanks in which to carry the bunkers. Such tanks will usually be close to the engine- room. The voyage plan should not involve the transfer of bunkers, except from bunker tank to engineroom, since any other transfer of fuel increases the risk of spil- lage and pollution.
Guidelines for amending the tonnage distribu- tion: It is most unlikely that the first attempt at distri- buting the cargo between the holds will satisfy all the requirements. It is much more likely that the calcula- tions will show that the ship's trim, stability or stresses will be unacceptable at some stage in the voyage.
When this occurs there are several useful guidelines to keep in mind when redistributing the weights through- out the ship:
1. Wherever possible an unacceptable trim should be cor- rected by repositioning cargo, not bunkers. If the cargo can be positioned so that the draft and trim throughout the voyage are acceptable, then the possibility of repositioning bunkers can be kept in reserve for emergencies.
2. A ship on passage often has spare lifting capacity, except at the limiting point of the voyage. At such times it may be possible to take ballast, ideally in the forepeak or BULK CARRIER PRACTICE 113
afterpeak tank, to improve the trim or stability without exceeding the permitted draft.
3. Transferring bunkers within the ship should be avoided as far as possible to reduce the risk of a spillage. If bunkers must be transferred, the transfer should be by gravitation if possible. Preplanning which ensures that the bunkers are loaded into the most suitable tanks and consumed in the optimum sequence is better than the transferring of bunkers.
4. Insufficient positive stability in seaworthy bulk carriers is a problem which is normally met only in vessels carrying forest products. In brief this problem is coun- tered by keeping the number of slack tanks to a minimum, keeping bunkers (both water and fuel) as low as possible in the ship, keeping ballast tanks filled, and if necessary carrying additional bunkers to act as ballast.
5. When rearranging the cargo aboard a vessel to alter the ship's trim or stresses a few simple rules may be useful.
• To reduce trim by the head, or increase trim by the stern, move a weight aft.
• To reduce hogging stresses move weights from forward and aft towards midships.
• Aboard Panamax and Cape-sized bulkers where alternate holds are the same length, it is usually possible to reduce longitudinal stresses whilst main- taining the same trim by moving two equal weights in opposite directions at opposite ends of the ship.
For example, to reduce a shear force at No. 9 hold, move 200 tonnes of cargo from No. 9 to No. 7 hold, and balance that by moving 200 tonnes from No. 1 to No.3 hold. This may have unexpected effects upon bending moments so they must always be rechecked after weights have been moved.
Loading/deballasting programme
The Nautical Institute's Cargo Operations Control Form: When a satisfactory distribution of cargo has been obtained, a programme must be devised for loading the cargo whilst keeping stresses within the permitted limits throughout the process and always maintaining a stern trim to assist efficient deballasting. Limited depth of water and height below the loading arm may restrict the draft and air draft which can be accepted.
The programme should provide all the information required by deck officers and loading personnel, presented in a clear and logical manner. The Nautical Institute has devised a Cargo Operations Control Form (Appendix 9.3) for this purpose, and has recom- mended that the form should always be completed, and a copy passed to the authorities ashore, before commencement of loading. Copies should be available as working documents ashore and aboard, and a copy should be filed aboard ship as an actual record of the cargo and/or ballasting operation. (A blank form is provided on the inside back cover.)
The loading programme lists each step in the debal- lasting, and the corresponding cargo pour. (A pour is the quantity of cargo poured into one hold as one step in the loading programme. Other expressions some- times used for a pour are a 'run', a 'shot' and a 'drop'.) A pass is composed of a pour into each of the holds to be loaded. Thus a ship loading five holds with 30,000 tonnes of cargo might load with a first pour of 3,000 tonnes in each hold. When the first pass was completed 15,000 tonnes would be distributed between the five holds. The second pass would be
114 THE NAUTICAL INSTITUTE
composed of a second pour of 3,000 tonnes into each of the five holds.
Information required for preparation of load- ing/deballasting programme: If a realistic loading/de- ballasting programme is to be devised, the following information is required:
• Maximum safe draft in berth.
• Minimum depth in the approach to the berth.
• Tidal range.
• Maximum permitted sailing draft.
• The minimum air draft beneath the ship loader.
• Characteristics of loading equipment.
• Limits of movement of the loading equipment.
• The maximum theoretical loading rate.
• The number of ship loaders to be used.
(These requirements are discussed in greater detail in Chapter 11.)
Guidelines for preparation of a loading/debal- lasting programme: As a starting point it is normal to assume that each pour will consist of about half the total tonnage to be loaded into the compartment in question. The loading sequence (or loading rotation) depends upon the size of ship and the number of holds to be loaded, but some guidelines can be offered.
1. The first pour should where possible be into a midship or after hold to provide or maintain a reasonable trim by the stern for ballast stripping purposes.
2. If the air draft is restricted it will be necessary to make the first pour into a hold which causes some increase in forward draft to ensure that the loading spout can continue to clear the heath coamings of the forward holds.
3. If the air draft is restricted the effect of a rising tide must be considered and deballasting cannot continue when the clearance is small.
4. Successive pours should alternate between forward and after holds to maintain a reasonable trim by the stern.
5. The end holds (i.e., the foremost and aftermost holds) have the biggest effect upon trim. Where possible they should receive the last pours of the first pass, and the first pours of the second pass, because the resulting large changes in the trim and maximum draft are likely to be least inconvenient at that point.
6. The ballast which is likely to present most problems should be discharged first, the normal sequence commencing with ballast holds, continuing with double bottom tanks and wing tanks and concluding with peak tanks.
7. Ballast should normally be discharged from a position close to the one where the cargo is being loaded at that time. For example, No.3 double bottom should be dis- charged whilst No. 3 hold is being filled, if No.3 double bottom is below No.3 hold.
8. The time required for a deballasting step should be matched with the time required for a loading pour. A pour of 3,000 tonnes at a loading rate of 1,500 tonnes/hour will take two hours. This should be prog- rammed with a deballasting step which will take less than two hours, so as to reduce the likelihood that the deballasting will overrun, and become out of step with the loading.
9. The ballasting should be programmed to be completed several hours, at least, before completion of loading, and at a time when the vessel still has a stern trim, to assist the deballasting and stripping.
10. On many bulk carriers trim can be quickly and con- veniently changed by pumping ballast directly from forepeak to afterpeak, or vice versa.
11. Rules imposed by the Classification Society and quoted in the loading manual may restrict the sequence of loading: they must be strictly observed.
For example the manual may state that no hold can be completely filled until the mean draft is at least two thirds of the intended sailing draft.
12. In exposed berths the ship should be maintained at a draft and trim at which she can put to sea at short notice if required. This precaution is particularly recommended in areas where ports must be evacuated on the approach of a tropical storm.
Calculation of the loading/deballasting prog- ramme: From a starting point with the ship in ballas- ted condition and ready to commence loading, calculations must be undertaken for each step in the loading programme. These calculations are similar to those already undertaken for each stage in the loaded voyage, and are intended to find the ship's draft, trim, stability and longitudinal stresses at each stage in the loading. They are essential to ensure that the ship is not subjected to excessive bending moments and shear forces during the course of the loading, and their importance cannot be overstated.
Whenever the calculations show that the draft, trim, stability or stresses at the end of a stage are unacceptable, the programme must be changed by changing the loading or deballasting sequence or quantities. Unfortunately it is sometimes necessary to amend a number of earlier stages to remove a problem which arises in the later stages. When this occurs the data for all the stages which have been amended must be recalculated.
When rearranging the ballast and the cargo pours aboard a vessel to alter her trim or stresses the rules quoted earlier are still applicable:
• To reduce trim by the head, or increase trim by the stern, load cargo aft or discharge ballast from forward.
• To reduce hogging stresses load cargo amidships or discharge ballast from the forward and after ends of the ship.
• To reduce shear forces and bending moments whilst maintaining the same trim move two equal weights in opposite directions at opposite ends of the ship.
Ways of adjusting the loading/deballasting programme: Sometimes it will be found very difficult to devise a loading/deballasting programme which remains within the stress limits. This is most likely to occur when loading a high density cargo in alternate holds (jump loading), or when planning the loading of a segregated part cargo which is not to be distributed between all holds. Difficulties are more likely if the ship is observing the at-sea stress limits whilst in port to increase the safety margin and reduce the danger of structural damage to the ship during the loading process.
Several steps can be recommended to reduce the calculated stress values and improve the programme.
• The pour sizes can be varied. Better results may be obtained if 60 per cent, say, of the tonnage is loaded in the first pour, and 40 per cent in the second, or vice versa.
• The number of pours can be increased, using three pours in holds where two give difficulties. Since each shift of the loading spout will take 10 minutes or so, this will slightly increase the time required for loading, making this option less attractive than safer loading achieved by varying the size or the sequence of the pours.
Loading when the ship is too long for the berth:
If the ship is too long for the berth, so that the loading spout cannot reach all holds without the vessel shifting, the plan should require the ship to shift along the quay as seldom as possible. This can probably be best achieved by commencing loading in an after hold, the forward hold being beyond the end of the berth.
Then, after about one-third of the cargo has been distributed between holds except No.l, the vessel should be moved astern to allow a first pour into No. 1, followed perhaps by a second pour into an after hold
— though not the aftermost one, which would be beyond the end of the berth. This could be followed by a second pour into No.l hold, after which the vessel would be moved ahead and loading would be completed in the remaining holds. Loading the forward hold is likely to cause a bigger change of trim than loading the after hold, so it is better for the former to be loaded midway through the programme.
A possible loading sequence for a nine-hold bulk carrier loading alternate holds is:
3 , 9 , 5 , 7 , move astern 1,7, 1, move ahead, 9, 3, 5, trim.
Planning two-port or two-grade loadings It is quite common for bulk carriers of all sizes to be instructed to load several different grades of cargo, to be stowed in separate holds. Such different grades may be for loading and discharge at separate berths or even in separate ports. The loading orders quoted at the start of this chapter refer to such cargoes.
This sort of requirement can usually be satisfied by using a distribution with grade A shared between forward and after holds, and grade B treated similarly.
If there is a bigger quantity of one grade than of the other, that grade can be allotted to an additional hold, amidships. Possible distributions include:
Hold Option 1 Option 2 Option 3
9 8 7 6 5 4 3 2 1
A B A B A
A B A B A B A B A
A B C A C A C B A
These and any other distributions are acceptable provided that draft, trim, tanktop loadings, hold load- ings, shear forces and bending moments are within acceptable limits throughout the loading, the voyage and the discharge. There is another condition which must be satisfied: if block loading is used it must only be used in a manner which is safe.
BULK CARRIER PRACTICE 115