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3.25 LOADING WHERE DRAFT OBSERVATIONS ARE NOT AVAILBLE

Completion of loading may take place in a number of circumstances where the draft of the vessel cannot be observed, or cannot be reliably observed. Normally such situations apply to straight cargoes of crude oil only and the following recommended procedures assume this to be the case. In these cases it is essential that the following be determined to the greatest possible accuracy:

  • The specific gravity of the cargo being loaded.

  • The temperature of the cargo loaded into the tanks.

  • The quantity of fuel and water on board.
  • The ship will be loaded by calculation, that is, the weight of cargo allowed on board must be very closely calculated, with appropriate allowance for the vessel's experience factor and for the sag normally observed when fully loaded. When loading by calculation the salinity differential is not considered. The salinity differential allowance can only be used where the draft can be observed and the vessel is fully afloat.
    Loading by calculation includes the following steps:

  • Determine the controlling/limiting draft mark.

  • Deduct the anticipated sag to determine the calculated mean draft.

  • Determine the corresponding deadweight tonnage for the calculated mean draft.

  • Deduct tonnages of fuel, water and stores on board to determine the cargo tonnage available.

  • Multiply the available cargo tonnage by the factor for cubic meters per tonne corresponding to the cargo gravity; the result is the net volume of cargo to be loaded.

  • Divide the net volume by the cargo temperature correction factor normally used to convert from observed loading temperature to net volumes at 15 C (60 F); this gives the anticipated gross volume to be loaded.

  • Multiply the gross volume by the ship's experience factor to obtain the volume to be loaded as measured in the ship's cargo tanks.
  • As the vessel loads, samples of the cargo should be drawn and the specific gravity measured and corrected for observed temperature. Two or three samples should be taken over the period of the bulk of the loading period to verify a uniform gravity or obtain an accurate average. If the observed gravity differs from that used to calculate the load, revise the calculation using the observed gravity.
    Cargo temperatures should be measured for each tank when they have been topped off and the net tonnage in each tank calculated separately. The actual loaded temperature should be used to revise the earlier calculation. When the vessel is loading trim tanks, the calculations of all completed tanks are verified and the volume to be loaded in the trim tanks is computed. The ship loads until the trim tanks have reached the calculated ullage level. If the ship is provided with automatic draft sensors, the chief officer may use them, provided he is confident of any correction factor to be applied to their indications and provided he stops the loading at the desired draft when indicated by the sensors, or on reaching the calculated cargo volume, whichever occurs first!

    3.25.1 Loading 'over the tide'
    Loading over the tide is a procedure employed where ships load at a berth where there is adequate water depth for the ship fully loaded only at certain stages of the tide. Such berths normally have a significant tidal range and a relative freedom from weather induced tidal variations. In loading over the tide timing is everything. The ship and shore terminal must be ready in all respects to conduct the loading operation when it arrives at the terminal and all formalities and preparations must be suitably manned and streamlined to minimise the time between all fast and commence loading. Preparation of a time-bar diagram of the loading procedure is essential to determining the time the ship will need to complete the necessary operations. (See section 2.20.5).
    Where the tide is diurnal the ship is berthed at a time when it can safely load and depart on the next flood tide.
    Where tides are semi-diurnal, there will rarely be enough time to permit the ship to load and sail on a single tide. In those cases one of two alternatives must be used. The first occurs when the ship attempts to complete loading before the next fall of the tide, but some delay or interruption interferes. The ship is now too deeply loaded to lie in the berth safely afloat through the next low tide. There is no choice then but to disconnect and unmoor the ship, to proceed to a safe anchorage and await the next rise of the tide. When the berth again has adequate water, the vessel returns, reconnects and completes loading.
    An alternative procedure is to accept that the vessel will not be able to sail on the high tide following the start of loading and to stop the loading while the ship still has adequate under-keel clearance to lie safely afloat through the following low tide. The ship waits until the low tide has passed, plus a reasonable interval and then resumes loading. In this case the chief officer must closely calculate the rate at which his draft will increase with the normal shore loading rate and compare this against the rate of increase of the water level in the berth following low tide. It is possible for some terminals to load a tanker faster than the tide will rise and the ship may thereby be put on bottom if loading is resumed immediately after the time of low water. The chief officer should wait until he has a positive indication that the tide is flooding before agreeing to resume loading. He should personally observe the tide board/gauge in the berth to confirm the start of the flood before opening his loading manifold. The tanker completes loading in the normal way and endeavours to depart on the last of the flood tide.
    Because any errors made in this procedure could result in stranding the vessel with pollution consequences, the practice of loading over the tide is generally not accepted by major oil companies or concerned charterers. 'Safe berths' are now considered to be those which the vessel may safely proceed to, lie in and proceed from, always afloat.

    3.25.2 Loading on bottom
    The first part of this section described a procedure which may be used to load a tanker where the draft cannot be directly or accurately read at all. In some berths, a vessel may be directed to load cargo to a draft exceeding the available berth depth. This is seldom done with chartered vessels as voyage or time charters call for the vessel to be operated 'always afloat' and to call only at docks in safe ports where they may 'proceed to, lie at, and proceed from always afloat'.
    However, the regulatory and environmental impediments to dredging, particularly in the United States have resulted in many berths being silted to less then normal tanker drafts at the face of the dock. The sediment is soft and uniform and is harmless to the hull if the ship is loaded to only a few centimetres deeper then the berth face depth. In the US Gulf coast, there is usually too little tidal range to either strand the vessel hard, or to permit loading 'over the tide'.
    To complete loading at such a berth, the chief officer must accurately determine the water depth that will be available at the berth face about the time he is completing the loading operation. It is advisable to take soundings along the inboard side of the ship after berthing to confirm any information supplied by the terminal.
    The ship is loaded in the normal way until its draft is slightly less than the berth depth. The chief officer observes the forward, aft and amidship drafts closely, measures the salinity of the berth water and verifies the cargo gravity and temperature as indicated in section 3.25. The chief officer then calculates the quantity of cargo needed to put the ship down to its marks and trimmed to an even keel. Loading is completed without further inspection of the draft marks, for when the ship takes the bottom and begins to list slightly the draft readings will become useless.
    Tank gauging is complicated by the slight list and appropriate corrections must be made when calculating the quantity on board. When inspection formalities are complete, the tugs are used to pull the vessel off the berth. Immediately she is afloat and before she begins making way, the chief officer will have an opportunity to accurately read his amidship draft and verify the accuracy of his loading calculations.

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