2.20 PREPARING THE LOADING PLAN

The loading plan for a tanker must address a multitude of requirements including carrying capacity, hull stresses, loading and discharge instructions, parcel separation, sequence of loading and discharge, vapour pressure, expansion and contraction. A properly prepared cargo loading plan begins with the most basic requirements and proceeds through a review of all other essential considerations. The objective is to adequately ensure the safety of the cargo at a minimum preparation expense in time, fuel and effort.
In sections 2.8 and 2.9 the process for selecting a preliminary cargo layout was examined. By the time the tank washing and tank cleaning operations have been completed, the tanker should have received the final loading orders and any necessary clarifications.
Loading orders may have been prepared in some detail by the owner's or charterer's shore staff and may already represent the optimum loading for the ship. Even detailed loading instructions must never be implemented without careful review by the chief officer and master. The responsibility for safe loading of the ship always rests with the master!

2.20.1 Loadline, loadline zones and loadline seasons
The International convention on loadlines, 1966, (with amendments of 1971, 1975 and 1979), had the objective of establishing a uniform set of requirements to prevent the overloading of ocean going vessels. The desired outcome was the assurance that a ship would always leave port with enough freeboard to provide the necessary margin of safety against the weather and accidents which may be encountered during the voyage.
The physical result of these agreements is the presence of the loadline engraved on the side of each tanker. The practical implementation is the chief officer's diligence in ensuring that the applicable loadline is never submerged (on either side of the ship), when departing for sea.
The chief officer must know exactly what draft is the limiting draft when loading at and departing any given port. The complications are the route of the ship, the season of the year and the salinity of the water in the loading berth.
If the vessel is departing a port in a more restrictive zone (Winter) and proceeding toward a less restrictive one (Summer) the ship cannot be loaded below its Winter loadline mark. An example would be a tanker loading at Valdez, Alaska and proceeding toward Panama. However, if it is departing in a less restrictive zone and proceeding toward a more restrictive one, it may depart with the limiting loadline immersed so long as it consumes enough fuel to offset the immersion before entering the more restrictive zone. An example would be a VLCC departing the Arabian Gulf in November, bound for Rotterdam. The tropical loadline applies at the time of departure, however the ship must have consumed enough fuel and water by the time it passes the Mozambique Channel (20 degrees south latitude) that its summer loadline is not immersed and enough additional fuel before passing Cape Torinana, Spain (44 degrees north latitude) that its winter loadline is not immersed.

Load-line markings on the starboard side of an oil tanker.
1 Summer to fresh - ship's fresh water allowance. 2 Fresh to Tropical fresh - one forty-eighth of summer draft above fresh. 3 Summer to tropical - one forty-eighth of summer draft. 4 Summer to winter - one forty-eighth of summer draft. 5 Winter to winter North Atlantic - for ships not more than 100 meters (328ft) in length this distance is 50 millimetres (21 inches). For other ships the winter north atlantic freeboard is the same as the winter freeboard and is indicated only by 'W'. All measurements are to be taken from the tops of the cross lines.

If the VLCC's tonnes per centimetre of immersion is 150 and the distance between tropical and summer loadlines is 30 centimetres, then the tanker would need to consume 4500 tonnes of fuel and water between the AG and the Madagascar Channel in order to load to her full tropical marks on departure. That is clearly impossible for even the thirstiest turbine tanker! Nor would most ships be able to burn that amount of fuel between the AG and Cape Torinana. The chief officer must therefore work backwards from Cape Torinana, calculating the fuel burned to that point and loading his ship to its Winter marks plus that amount. If the ship burns 60 tonnes per day and it is a twenty five day voyage to Cape Torinana, then he can load to 10 centimetres over his Winter marks on departure:

60 tonnes/day x 25 days/150 tonnes/cm = 10 cm

This calculation must be done correctly, since the immersion of the draft marks will be closely examined by port state control officers on arrival and the fines for arriving with the ship overloaded will be significant compared to the (approximately $3000), extra revenue gained by loading another 10 cm.

Calculation of departure draft and cargo quantity where a loadline is the limiting factor.

The above calculation determinSes the deepest permitted draft from the regulatory viewpoint. Other loading draft considerations include:

With the departure draft known, the chief officer can use the Deadweight Scale (or tables), to determine the equivalent deadweight tonnage allowed. That calculation determines the total tonnage capacity of the ship. The quantity of fuel water and stores and ROB cargo anticipated to be on board at departure is subtracted from the total deadweight to obtain the tonnage of cargo to be loaded. On loaded passage, the fuel, water and stores should always be the minimum required to safely complete the voyage. If the loading orders, do not provide adequate fuel and water allowance, owners/ charterers must be advised of the discrepancy.

2.20.2 Cargo allocation - draft, stress and trim
Cargo allocation may be indicated in the owner's/charterer's voyage cargo orders received, especially if parcels of different products or crude oils are to be carried. If the orders are for a straight cargo (all one grade), allocation by tank will probably not be indicated.
For a straight cargo, the chief officer will multiply the tonnes of cargo to be loaded by the cubic meters per tonne factor of the cargo to determine the cubic meters of capacity required. He must then allocate the cargo among the ship's tanks to provide the desired trim within permitted hull stress factors.
The task of allocating the cargo and calculating the resulting trim and stress is best done by a loading computer. The loading computer saves hundreds of hours of chief officer time and effort yearly and eliminates the arithmetic errors found in manual calculations.
Nevertheless, an officer must understand the basis of the calculations to use the loading computer efficiently and there is always the possibility that the computer will fail, leaving only the manual method available. The steps for completion of a manual calculation are:
1 Trial allocation of cargo volumes to each tank.
2 Calculation of cargo tonnages in each cargo tank.
3 Calculation of fuel, water and stores for the voyage.
4 Calculation of total displacement for the voyage.
5 Calculation of trimming moments for the proposed loaded condition.
6 Calculation of the longitudinal centre of gravity for the proposed loading.
7 Determine the longitudinal centre of buoyancy for the proposed loaded displacement (from hydrostatic tables).
8 Calculation of total trimming moments.
9 Determine the moment to trim one centimetre (or inch), from the hydrostatic tables.
10 Calculation of trim (forward or aft).
11 Calculation of cargo/ballast allocation change necessary to achieve the desired trim, using two or more cargo/ballast tanks.
12 Calculation of bending and shear stresses resulting from the desired loading.
13 Calculation of bending and shear stresses for the arrival condition at the discharge port.

14 If the stress factors are acceptable, determine the ullages for loading of trim tanks.
15 Independent verification of calculations by another officer.

The examples indicates that the initial allocation of cargo results in a trim of 7 cm forward (by the head). The trim can be adjusted to zero (even keel), by reducing the cargo quantity in the No.l cargo tank by 45 tonnes and adding 45 tonnes of ballast to the afterpeak tank, or by shifting 78 tonnes from the No.l cargo tank to the No.5 cargo tank. The first alternative reduces total bending stresses more than the second alternative. If the bending stresses for the first cargo allocation are near the maximum allowable for sea conditions, then the first alternative is the preferred option.

For a mixed cargo loading, the chief officer should begin with the recommended cargo allocation contained in the loading orders, completing the deadweight, trim and stress calculations accordingly. If the plan works and makes best use of the vessel's capacity, then owners/charterers can be advised that it is acceptable and will be followed. If the plan does not work, ie. it causes excessive stress or trim, does not permit proper segregation of cargo parcels, or if it causes deadfreight which could be reduced by an alternative arrangement, then the master should advise owners of the discrepancy and advise a suggested alternative. The alternative may be rejected or accepted (due to reasons other than the best use of the ship) and the master will proceed with the cargo plan as agreed, provided it is safe to do so.
When cargo calculations are being performed manually, the independent verification by a second officer is essential. A junior officer should be assigned to independently do the verification. The verification task gets the junior officer more involved in the cargo operation and aids the development of his skills.
On conventional tankers, transverse stability is not a serious concern. With most or all of the cargo tanks full, the transverse stability (as represented by the metacentric height or 'GM'), is more than adequate. On some combination carriers (ore oilers, bulk oilers, or OBO's), with wide centre tanks, the number of slack tanks must be limited at loaded draft to ensure adequate transverse stability. For those ships, transverse stability must be calculated along with the trim and stress in the loaded condition. Transverse stability in bulk carriers must be calculated at several steps during the loading to ensure that the free surface effects of the tanks being loaded do not create and unstable (negative GM), condition. The calculation of transverse stability is an assumed skill of all ships officers and will not be discussed here.

If LCB would be aft of LCG on an even keel, then the ship will trim forward 'by the bow' until LCB moves under LGB.

The calculation of the cargo load should include a review of the arrival condition of the ship at the discharge port. On some VLCC's it is necessary to compensate for the consumption of bunkers to keep stresses within safe limits. If loadline regulations allow, the required compensation may be achieved by ballasting the after permanent ballast tanks or the afterpeak. On other ships it may be necessary (with charterer's prior authorisation), to transfer cargo so that the ship will arrive at an even-keel trim. The stress conditions before and after the transfer of cargo must be evaluated.

2.20.3 Separation, segregation and compatibility
The term 'compatible grades' means that two cargo parcels are similar to the degree that they can be handled by the same pump and pipeline system, after draining, without risk of contamination of either parcel. Some cargo grades do not even require draining of pumps/pipelines between parcels. When evaluating compatibility of cargoes, a number of critical factors must be taken into account, including colour, flash point, pour point, viscosity, specific gravity, sulphur content, lead content, gum, water separation index, initial boiling point and vapour pressure (see section 2.9.1). In addition, oil cargoes are classified as 'clean' or 'dirty' according to their composition and handling characteristics. Ships are normally restricted to carrying either clean or dirty cargoes because of the risk of contaminating a clean cargo on a normally dirty ship and the cost of trying to get a normally dirty ship clean enough to carry a clean cargo. Between clean cargoes, flash point is the most critical factor in terms of compatibility and special precautions must be taken to prevent mixing between gasolines and other grades.
With dirty or black product cargoes, the question of flash point becomes critical only when fuel oils are being carried with or after crudes. Crude oil parcels must always be segregated from fuel oil parcels by at least two valves, ie. 'double valve' protection.
Compatible grades can be loaded with single valve separation, provided that the valve has been tested for tightness. This test is achieved by applying liquid or air pressure on either side of the valve and observing for leakage on the other side. The results of the test should be logged and recorded in the maintenance journal/ program. Cargo operation should be planned so that the separation valve(s) are not opened before the contamination risk is passed at the discharge port.
Non-compatible cargoes must be completely segregated and cargo parcels of different classes (ie. gasoline and heating oil), must be loaded and discharged by separate cargo systems. Cargo parcels of the same class, but of a different type (ie. two crude oils), may be handled within a segregated cargo system.
On vessels with un-coated tanks, different grades must always be separated by two closed valves (a master valve and a tank suction valve), due to the possibility of the valve leaking from rust scale in the valve seat. Ships which carry mixed parcels should pay special attention to scale build-up underneath cargo valve discs. Scale accumulation may prevent a valve from closing properly and result in a cargo contamination. The problem of scale build-up is essentially eliminated in fully coated tanks and single valve separation (with a master valve or a crossover valve), may be permitted, provided there is program in place to regularly verify valve tightness.
When planning the disposition of the various cargo parcels, care should be taken to minimise the risk of contamination by undetected bulkhead or pipeline leaks. If valve or bulkhead leakage is suspected, then the adjacent grades must be loaded so that the more delicate product is loaded to a smaller ullage (higher innage), than its neighbour. If the more delicate product is a lower specific gravity (often the case), then the difference in ullage must be large enough so that at the bottom of the tank the delicate product still has a higher hydrostatic pressure than the adjacent grade. If any leakage does occur, it will be from the more delicate cargo into the less delicate cargo with less possibility of that product being put 'off test'. The difference in ullage required to provide for equal pressures at the tank bottom is:

U = D (H - L)/H

where:
U = ullage difference
D = depth of lighter parcel
H = heavier parcel sp. gr.
L = lighter product sp. gr.

This calculation yields the ullage difference required to produce equal hydrostatic pressure at the bottom of adjoining tanks.
To provide a positive pressure from the lighter product tank into the heavier product tank the ullage difference should be increased. Valves/hammerblinds separating cargo parcels of different class should be sealed in the loading port. These seals should not be removed until the cargo tanks have been sampled at the discharge port and not until the first grade of cargo has been discharged and the valve must be opened to complete the cargo discharge.

To prevent leakage of fuel oil to the clean product cargo depth of fuel oil must be less than 80/92 x the cargo depth of the clean product.

Most tankers are configured to provide some segregation capability, with the normal number of parcel divisions corresponding to the number of cargo pumps and main cargo lines on board. A tanker with three pumps should have the capability of segregating three parcels, while one with four pumps may be able to segregate four. The volume of each of these segregations is fixed by existing bulkhead, valve and pipeline configurations. For carrying mixed parcels on one voyage, the objective of charterers or owners is to arrange the volumes of the parcels so that they most closely fit the natural segregation capabilities of the ship. When an exact match cannot be achieved, it is better to arrange the parcels so that a tank is left empty in one section, than to risk a contamination by loading one or more tanks beyond the natural segregation barriers of the ship.
Another source of contamination is common or collected vent systems. When non-compatible cargoes are loaded into segregated tanks which share a common vent system, the cargo ullages must be large enough to prevent any cargo from sloshing into the vent system while rolling, and flowing into another tank.
If tanks are only partly filled, the movement of the vessel will cause the cargo to move, or slosh, in the tanks. In some circumstances, such as harmonic rolling, the sloshing can be violent.
This results in a breathing or pulsing inside the ullage space and will cause the P/V valve to alternately relieve on pressure and vacuum. This breathing action produces vapour losses or 'boil off of the lighter ends of the cargo. Therefore, the number of partially filled tanks should be minimised as a means of avoiding breathing losses and possible tank structure or heating coil damage.
Recent developments in cargo tank stress analysis have indicated that some cargo tank bulkheads constructed according to older class rules contain less strength than required to withstand severe stresses imposed by harmonic sloshing. This is another reason to minimise the number of slack tanks, particularly during winter voyages.
When a vessel is directed to carry both heated products and volatile (high vapour pressure), parcels on the same voyage, the cargo should be arranged so that they are not in adjacent tanks. If this cannot be avoided, the stowage should be arranged so that the products have only one adjacent bulkhead and the temperature of the heated product must not exceed 65 °C.
Some product carriers have sufficient pumps and pipelines to dedicate one to each parcel of cargo carried. When other tankers carry multi-grade cargoes, it is necessary to load or discharge similar grades of the same class via the same pipeline/pump system, accepting some down-grading (of the more critical product into the less critical product), in the loading and discharge operations. Where it is essential that no co-mingling or downgrading occur, a pump pipeline and tank system will be dedicated exclusively to the parcel, at least until it has all been pumped ashore safely at the discharge port.
A useful precaution when handling parcels requiring two-valve separation and strict sequencing of loading/discharging is to apply temporary labels to all of the valve actuators. If the valves are operated by hand on deck, they can be labelled with 5 cm masking tape stretched across the handwheel and marked with an indelible marker. If the valves are operated from the cargo control room, small, clear plastic boxes should be placed over the critical valve controls (fastened with tape if necessary) and a 'white board' marker used to label the box with an appropriate notice. For valves which must absolutely remain closed, a small sketch of a severed hand is usually an effective deterrent!

2.20.4 Loading and discharging sequence
For straight cargoes, the loading sequence needs to consider only the stresses placed on the hull by the cargo and the sequence in which the cargo is likely to be discharged. Cargo tanks are normally loaded in 'sets'. The first set is selected to minimise hogging stresses on the ship while bringing the ship close to an even keel condition. The second set loads the ship evenly and bodily down to near the maximum loaded condition. The final set, called 'trim tanks' are used to complete the loading of the cargo to the vessel's marks and even keel (or if adequate water depth exists in the loading port, to a departure trim which will provide an even-keel arrival trim at the discharge port). On a vessel with wing and centre tank arrangement, the centre tanks should be loaded first, since the extra framing in the wing tanks makes them better able to support the outward pressure on the longitudinal bulkheads. 'Checkerboard' loading sequences should be avoided because such arrangements stress the bulkhead intersections unnecessarily.

Alternating wing and centre ranks in the loading pattern places tank bulkheads intersections under unnecessary stress 'Checkerboard' loading patterns should be avoided.

If the ship's designer has provided a recommended loading sequence it should be followed whenever other cargo requirements permit.
Because of their slower state of decrease in ullage when topping off, some chief officers prefer to use a centre tank rather than a wing tank as the final tank to be filled. In that case, the centre tank to be used for trim should be filled to 1/2 of its final ullage while loading the other centre tanks, then stopped and reserved to receive the excess flow while topping off the wing tanks. The loading can then be finished in the desired centre or 'trim' tank. SBT ships require a closely integrated loading and de-ballasting plan which minimises hull stresses during loading.
Where multiple parcels are being carried in separate ship systems, the primary considerations during loading are timing and hull stress. The same stress criteria should be observed as with a straight cargo. However, priority should be given to starting the grade of cargo which will take the longest to load and to continuous loading of that grade (safety permitting), until it is finished. Knowing which cargo will take the longest requires information on shore loading rates which should be requested by the master before vessel's arrival. This information may be available on board from previous loadings at the same terminal. (See section 2.20.5.)
Occasionally it will be necessary to carry two grades of cargo within the same ship system. Such a procedure is never done without an element of risk and the charterer should be advised of the master's objections to doing so. Cargo parcels of different classes (i.e. heating fuel and gasolines), should never be handled in the same system. Cargo parcels of the same class but different type/grade (i.e. regular and premium gasolines), may be handled in a single segregated system with proper precautions.
Despite any objections recorded by the master, once orders have been received to share a cargo system between grades, the vessel is still responsible for making every effort to safeguard the cargo's quality. For the loading operation, it is important to know which of the parcels is the most delicate. With all the ship's pipelines clean and empty before loading, the delicate cargo parcel should be loaded first and blocked off as completely as possible in its tanks. The less delicate parcel is loaded second. At the discharge port, the less delicate parcel is discharged first. Then the pipelines are stripped dry and the more delicate parcel is used to flush the lines into the shore line for the first cargo. After the flush is completed, the shore lines are changed and the delicate parcel can be safely discharged.
An alternative may be available if the two parcels are to share the aft cargo system on the ship. With this arrangement, the less delicate parcel is loaded first through the deck drop line and bottom lines, into the forward tanks of the aft system. The bottom line block valves between the two parcels are closed before loading begins. After the first parcel has loaded the deck lines are carefully drained down to the cargo tank, the deck drop valve is closed and the deck line block valve is opened. The more delicate cargo is loaded second, through the pumproom into the aft tanks of the system. At the discharge port, the delicate parcel is discharged first, but not stripped. Any residue in the tanks or pipelines is downgraded into the second parcel.
In dealing with loading sequences for shared systems, it is critical to know the limiting cargo specification which is most subject to contamination. If the limiting factor is colour, then the lightest parcel is the 'delicate' one; if flash point, then the highest flash point product is delicate; if lead, then the lowest lead content takes priority. Masters should request guidance whenever they are in doubt regarding the sequence of loading and/or discharging parcels in shared systems.

2.20.5 Critical path loading

Bar chart loading plan for a multi-grade clean cargo, loading Regular should be started as soon as possible and Unleaded shortly after. Gasoil and PGO should be started as convenient but so that they do not finish at the same time as 'B' or 'C' grades.

Reductions in cargo loading times are possible through a careful examination of all of the activities associated with the loading operation and identification of the connected series of actions which will take the longest to accomplish. When the longest series has been identified, effort is focused on starting the first action as early as possible and subsequently reducing the 'delay time' between each critical action to a minimum. The technique is most useful on vessels, such as parcel tankers, which are scheduled to load a series of products. The critical path may be determined by the order in which the cargoes must use the ship's piping system, or it may result from the order they will be received through the shore piping. Identification of the critical path requires full information concerning:

This information can be converted to bar chart form, with a horizontal time axis and a vertical ship/shore piping axis and the loading of each parcel plotted as a time bar against the piping system to be used. Once the time bars have been plotted, it is relatively easy to identify the sequence which will take the longest: the 'critical path'. When the critical path has been identified, the ship's officers must focus on ensuring that the tank, piping and manifold to be used for the first parcel of the critical path series are fully prepared when the ship arrives at the loading berth. The corresponding hose/loading arm must be connected first and immediately after the tank is passed inspection for loading, the first parcel of the critical path can be started. When the first parcel is nearly finished loading, all preparations must be complete to start the second parcel. With the critical path under control, the remainder of the cargo can be attended to in an organised, but less urgent fashion.

2.20.6 Ullage allowance
Company policy should dictate the minimum ullage to which the cargo tanks may be filled. This should be a level at or below the 98% filled level, or the level of the 'hi-hi' level alarm setting (if fitted). To intelligently apply this guidance, the cargo watch officer must visualise the location of the level sensors and level alarms in the tank and compare this with the list or trim of the ship.
A very capable third officer once overfilled a centre tank in Houston. The ship had a large port list. The mechanical gauge was located on the starboard side of the centre tank and the tank lid, with open ullage hatch, was located on the port side. Several centimetres before the cargo reached to stop ullage on the tank gauge, a geyser of kerosene erupted from the ullage hatch!
The cargo plan must allow for any expansion of the cargo to be expected during the voyage. If there is insufficient ullage space, cargo expansion due to increasing temperature can cause overflow with consequent oil pollution, as well as possible structural damage.

If tank gauge is located at the high side of a listed vessel an overflow may occur before final ullage is reached. The same can occur if the ship has a large trim aft when topping off and tank gauge is located forward in the tank.

A tank filling limit of 98% of capacity is generally considered adequate for heated and non-heated grades alike. It allows room for cargo expansion resulting from raising the product temperature by about 20 °C. However, there are some occasions when an even greater ullage allowance may be appropriate. The standard expansion allowance could be exceeded given any one of the following conditions:

An example would be a product tanker loading product at 10 °C. The sea temperature en route may approach 28 °C. If the tank volume is 3200 cubic meters, what is the required ullage at the loading port to ensure a 12 cm minimum under-deck ullage at the discharge port?

Where cargo temperature may increase during a voyage the chief officer must calculate the necessary allowance for cargo expansion.

In some cases, contraction of the cargo must be considered. Combination carriers must maintain a minimum ullage in all tanks to prevent excessive free surface and resulting unacceptable loss of transverse stability. A combination carrier loading a warm cargo for delivery in a cold port must fill each tank to a small enough ullage that all tanks remain above the approved maximum ullage level when the cargo cools.

2.20.7 Written loading plan
When the chief officer has completed his review of all necessary factors and checked his calculations, he prepares a written loading plan which must include the following details:

2.20.8 Preparations for H2S cargo

When the vessel is ordered to load sour crudes, the master must ensure that: