This Blog is intended to provide Maritime Shipping community with guidance on Cargo operation of a Bulk carrier to prevent her hull from getting over-stressed during such period.
A review of the potential problems that could be encountered during cargo operations is discussed. Thus an attempt has been made to provide guidance on the measures that should be taken to monitor and control cargo and ballasting operations in order to reduce the possibility of over-stressing the ship’s structure.
As per various studies conducted by the International Association of Classification Societies (IACS), the principal factors contributing to the loss of the bulk carriers were:
- Corrosion and cracking of structure within the cargo spaces.
- Over-stressing of the hull structure due to incorrect loading of the cargo holds and
- Physical damage to the side structure during cargo discharging operations.
To counter such conditions, several measures were taken by the classification societies ,like Introduction of corrosion protecting requirements for ballast tanks, Implementation of Enhanced survey programme (ESP) for Bulk carriers, requirements for specific approved loading instruments and Publications of various Brochures and Manuals to assist the ship owners to avoid such incidents.
Particular concern is presented by potential problems which may result during operations such as the introduction of very high capacity loading systems, lack of communication between ship and terminal and inadequate planning of cargo operations. The issue that some seafarers, and ship and cargo operators do not have a clear understanding of the limitations imposed by the ship’s classification society regarding the strength capability of hull structure, is also a matter of concern.
Thus, this mega-blog has been structured into Five Sections to take the reader through a proper flow of the argument, namely:
- Typical Bulk Carrier Structure
- Cargo distributions along the ship’s length
- Shipboard Loading Guidance Information
- Planning and control of Cargo Loading and Unloading operations on a Bulk Carrier
- Potential Problems which may happen during Cargo operations in a Bulk Carrier
Typical bulk carrier structure
The most widely recognized structural arrangement identified with bulk carriers is a single deck ship with a double bottom, hopper tanks, single skin transverse framed side shell, topside tanks and deck hatchways, as illustrated in the three diagrams below.
In general, the plating comprising structural items such as the side shell, bottom shell, strength deck, transverse bulkheads, inner bottom and topside and hopper tank sloping plating provides local boundaries of the structure and carries static and dynamic pressure loads exerted by, for example, the cargo, bunkers, ballast and the sea.
This plating is supported by secondary stiffening members such as frames or longitudinals. These secondary members transfer the loads to primary structural members such as double bottom floors and girders or the transverse web frames in topside and Hopper tanks, etc.
If you refer the figure no. 3, it is evident that the transverse bulkhead structures, unclosing it’s upper and lower stools, together with the cross deck and the double bottom structures are the main structural members which provide the transverse strength of the ship to prevent the hull section from distorting. In addition, if ingress of water into any one hold has occurred, the transverse watertight bulkheads prevent progressive flooding of other holds.
Design limitations of Hull structure of a Bulk Carrier
All ships are designed with limitations imposed upon their operability to ensure that the structural integrity is maintained. Therefore, exceeding these limitations may result in over-stressing of the ship’s structure which may lead to catastrophic failure.
The ship’s loading manual provides a description of the operational loading conditions upon which the design of the hull structure has been based.
The loading instrument (loadicator software) provides a means to readily calculate the still water shear forces and bending moments, in any load or ballast condition, and assess these values against the design limits.
A ship’s structure is designed to withstand the static and dynamic loads likely to be experienced by the ship throughout its service life.
The loads acting on the hull structure when a ship is floating in calm water are static loads. These loads are imposed by the:
· Actual weight of the ship’s structure, outfitting, equipment and machinery.
· Cargo weight.
· Bunker and other consumable weights.
· Ballast weight.
· Hydrostatic pressure (sea water pressure acting on the hull)
Dynamic loads are those additional loads exerted on the ship’s hull structure through the action of the waves and the effects of the resultant ship motions namely:
· Acceleration forces
· Slamming loads- It is induced on the ship’s bottom shell structure forward due to emergence of the fore end of the ship from the sea in heavy weather.
· Sloshing loads - Sloshing loads may be induced on the ship’s internal structure through the movement of the fluids in the tanks/holds.
Cargo over-loading in individual hold spaces will increase the static stress levels in the ship’s structure and reduce the strength capability of the structure to sustain the dynamic loads exerted in adverse sea conditions.
Analysis of Shear Forces and Bending Moments on Hull Girder
All the Bulk carriers are assigned permissible still water shear forces (SWSF) and still water bending moments (SWBM) limits, by their Classification Society.
There are normally two sets of permissible SWSF and SWBM assigned to each ship, namely:
1. Seagoing (at sea) SWSF and SWBM limits.
2. Harbour (in port) SWSF and SWBM limits.
The SWSF and SWBM limits are not to be exceeded when the ship puts to sea or during any part of a seagoing voyage.
In harbour, where the ship is in sheltered water and is subjected to reduced dynamic loads, the hull girder (a ship’s hull is like a beam/girder which is non-uniformly loaded and non-uniformly supported) is permitted to carry a higher level of stress imposed by the static loads. The harbour SWSF and SWBM limits are not to be exceeded during any stage of harbour cargo operations.
When a ship is floating in still water, the ship’s lightweight (the weight of ship’s structure and its machinery) and deadweight (all other weights, such as the weight of the bunkers, ballast, provisions and cargo) are supported by the buoyancy force acting upwards on the exterior of the hull.
Along the ship’s length there will be differences in the vertical forces of buoyancy and ship’s weight. The unbalanced net vertical forces acting along the length of the ship will cause the hull girder to shear and to bend, as shown in figures 4,5 and 6, inducing a vertical still water shear force (SWSF) and still water bending moment (SWBM) at each section of the hull.
At Sea, the ship is subjected to cyclical shearing and bending actions induced by continuously changing wave pressures acting on the hull. These cyclical shearing and bending actions give rise to an additional component of dynamic, wave induced, shear force and bending moment in the hull girder. At any one time, the hull girder is subjected to a combination of still water and wave induced shear forces and bending moments.
The stresses in the hull section caused by these shearing forces and bending moments are carried by continuous longitudinal structural members. These structural members are the strength deck, side shell, bottom plating and longitudinal, inner bottom plating and longitudinal, double bottom girders and topside and hopper tank sloping plating and longitudinal, which all together are generally defined as the Hull girder.
Examples of permissible and calculated SWSF and SWBM are shown in figures 7 & 8 respectively.
Local Strength of Transverse Bulkhead, Double bottom and Cross Deck Structure
To further enhance safety and flexibility, some bulk carriers are provided with local loading criteria which define the maximum allowable cargo weight in each cargo hold, and each pair of adjacent cargo holds (i.e. block hold loading condition), for various ship draught conditions.
The local loading criteria are normally provided in tabular and diagrammatic form.
Over-loading will induce greater stresses in the double bottom, transverse bulkheads, hatch coamings, hatch corners, main frames and associated brackets of individual cargo holds, as depicted in figure 9.
The double bottom, cross deck and transverse bulkhead structures are designed for specific cargo loads and sailing draught conditions. These structures are sensitive to the net vertical load acting on the ship’s double bottom. (The net vertical load is the difference between the vertical downward weight of the cargo and water ballast in the double bottom and hopper ballast tanks in way of the cargo holds and the upward buoyancy force which is dependent on the ship’s local draught.)
Overloading of the cargo hold in association with insufficient draught will result in an excessive net vertical load on the double bottom which may distort the overall structural configuration in way of the hold, as depicted in figures 10 & 11.
A typical Local Loading Diagram for a cargo hold (strengthened hold) combined with the adjacent hold limits, of a bulk carrier is shown in figure 12.
The important trend to note from the local loading diagram is that there is a reduction in the cargo carrying capacity of a hold with a reduction in mean draught. Exceeding these limits will impose high stresses in the ship’s structure in way of the over-loaded cargo hold. There are two sets of local loading criteria depending upon the cargo load distribution namely, individual hold loading or two adjacent hold loading.
The allowable cargo loads for each holds or combined cargo loads in two adjacent holds are usually provided in association with empty double bottom and hopper wing tanks directly in way of the cargo hold. When water ballast is carried in the double bottom and hopper wing tanks, the maximum allowable cargo weight should be obtained by deducting the weight of water ballast being carried in the tanks in way of the cargo hold.
The maximum cargo loads given in the Local Loading Criteria should be considered in association with the mean draught in way of the Cargo holds.
Cargo distributions along the ship’s length
Bulk carriers are designed and approved to carry a variety of Cargoes. The distribution of cargo along the ship’s length has a direct influence on both the bending and shearing of the hull girder and on the stress in the localized hull structure.
The more commonly adopted cargo distributions are:
1. Homogeneous hold loading condition.
2. Alternate hold loading condition.
3. Block hold loading condition.
4. Part hold loading condition.
Homogeneous hold loading condition (Fully loaded)
This condition refers to the carriage of cargo, evenly distributed in all cargo holds as shown in figure 13. This type of cargo load distribution is, generally, permitted for all bulk carriers and is usually adopted for the carriage of light (low density) cargoes, such as coal and grain. However, high density cargoes such as iron ore may be carried homogeneously.
Alternate hold loading conditions (Fully loaded)
Heavy cargo, such as iron ore, is often carried in alternate cargo holds on bulk carriers, as depicted in figure 14.
It is common for large bulk carriers to stow high density cargo in odd numbered holds with the remaining holds empty. This type of cargo distribution will raise the ship’s centre of gravity, which eases the ship’s rolling motion (ship rolling period is inversely proportional to the square root of GM and as the cargo is stowed up to more height in the holds, this raises the position of Centre of Gravity, G, thus reducing the value of distance GM, resulting in high rolling period, which in turn means comfortable rolling of the ship).
When high density cargo is stowed in alternate holds, the weight of cargo carried in each hold is approximately double that carried in a homogeneous load distribution.
To support the loading of the heavy cargo in the holds, the local structure needs to be specifically designed and reinforced (by adding extra frames in the holds). It is important to note that the holds which remain empty, with this type of cargo distribution, have not been reinforced for the carriage of heavy cargoes with a non-homogeneous distribution.
Ships not approved for the carriage of heavy cargoes in alternate holds by their classification society must not adopt this cargo load distribution.
Block Hold loading and Part loaded Conditions
A Block hold loading condition refers to the stowage of cargo in a block of two or more adjoining cargo holds with the cargo holds adjacent to the block of loaded cargo holds being empty, as shown in figure 15.
This load distribution is adopted when the ship is partly loaded.
Part loaded and block hold loading conditions are not usually described in the ship’s loading manual unless they are specifically requested to be considered in the design of the ship.
When adopting a part loaded condition, to avoid over-stressing of the hull structure, careful consideration needs to be given to the amount of cargo carried in each cargo hold and the anticipated sailing draught.
When a ship is partly loaded, the cargo transported is less than the full cargo carrying capacity of the ship. Hence, the sailing draught of the ship is likely to be less than its maximum design draught.
The weight of cargo in each hold must be adequately supported by the buoyancy upthrust acting on the bottom shell. A reduction in the ship’s draught causes a reduction in the buoyancy upthrust on the bottom shell to counteract the downward force exerted by the cargo in the hold. Therefore, when a ship is partly loaded with a reduced draught, it may be necessary to reduce the amount of cargo carried in any hold.
To enable the cargoes to be carried in blocks, the cross deck and double bottom structure needs to be specially designed and reinforced.
Block loading results in higher stresses in the localized structure in way of the cross deck and double bottom structures and higher shear stress in the transverse bulkheads between the block loaded holds. The weight of cargo that can be carried in the block of cargo holds needs to be specially considered against the ship’s sailing draught and the capability of the structure.
In general, the cargo load that can be carried in blocks is much less than the sum of the full cargo capacity of individual holds at the maximum draught condition.
Part loaded and block hold loading conditions should only be adopted in either of the following situations:
1. The loading distributions are described in the ship’s loading manual. In this case, the ship’s structure has been approved for the carriage of cargo in the specified loading condition and the loading conditions described in the ship’s loading manual should be adhered to, or,
2. The ship is provided with a set of approved local loading criteria which define the maximum cargo weight limits as a function of ship’s mean draught for each cargo hold and block of cargo holds. In this case, it is necessary to ensure that the amount of cargo carried in each hold satisfies the cargo weight and draught limits specified by local loading criteria and the hull girder SWSF and SWBM values are within their permissible limits.
Shipboard Loading Guidance Information
It is a statutory requirement of International Load line Convention (with some exemptions though) that, “ The Master of every new vessel be supplied with sufficient information, in an approved format, to enable him to arrange for the loading and ballasting of his ship in such a way as to avoid the creation of any unacceptable stresses in the ship’s structure.”
The ship’s approved loading manual is an essential onboard document for the planning of cargo stowage, loading and discharging operations. The manual describes:
1. The loading conditions on which the design of the ship has been based, including permissible limits of still water shear force and bending moments.
2. The results of calculations of SWSF & SWBM for each included loading conditions.
3. The allowable local loading of the structure.
4. Operational limits.
The ship’s loading manual is a ship specific document, the data contained therein is only applicable to the ship for which it has been approved.
Often called loadicator and in modern era of computers, it is more than often available in the form of Software, which can be run on any (designated, though) computer (of generic configuration) onboard the Ship.
This is an invaluable shipboard calculation tool which assists the ship’s cargo officer in:
1. Planning and controlling cargo and ballasting operations.
2. Rapidly calculating SWSF and SWBM for any load condition.
3. Identifying the imposed structural limits which are not to be exceeded.
It is important to note that the loading Instrument is not a substitute for the ship’s loading manual. Therefore, the loading manual should also be referred when planning or controlling cargo operations.
Planning and control of Cargo Loading and Unloading operations on a Bulk Carrier
PREPERATION FOR CARGO OPERATIONS
1. Cargo and Port Information
To make it possible to plan the cargo stowage, loading and unloading sequences, the ship should be provided with the following information well in advance, by the Shipper/Charterer/Cargo Terminal:
· Cargo characteristics: stowage factor, angle of repose, amounts and special properties.
· Cargo availability and any special requirements for the sequencing of cargo operations.
· Characteristics of the loading or unloading equipment including numbers of loaders or unloaders to be used, their ranges of movement, and the terminal’s nominal and maximum loading and unloading rates, where applicable.
· Minimum depth of water alongside the berth and in the fairway channels.
· Water density at the berth.
· Air draught restrictions at the berth & Channels.
· Maximum sailing draught and minimum draught for safe maneuvering permitted by the port authority.
· The amount of cargo remaining on the conveyor belt which will be loaded onboard the ship after a cargo stoppage signal has been given by the ship.
· Terminal requirements/procedures for shifting ship.
· Local port restrictions, for example bunkering and deballasting requirements, etc.
Cargo trimming is a mandatory requirement for some cargoes, especially where the risk of the cargo shifting or where liquefaction could take place.
The ship’s master should also be aware of the harmful effects of corrosive and high temperature cargoes and any special cargo transportation requirements.
Ship masters, deck officers, charterers and stevedores should be familiar with the relevant IMO Codes (for example, IMO Code of Safe Practice for Solid Bulk Cargoes, IMO Code of Practice for the Safe Loading and Unloading of Dry Bulk Carriers and SOLAS Convention)
2. Making of a Cargo Stowage Plan and Loading/Unloading Plan
The amount and type of cargo to be transported and the intended voyage will have final say on the proposed departure cargo and/or ballast stowage plan.
The Chief Mate should always refer to the loading manual to ascertain an appropriate cargo load distribution, satisfying the imposed limits on structural loading.
There are two stages in the development of a safe plan for cargo loading or loading:
Stage 1: Given the intended voyage, the amount of cargo and/or water ballast to be carried and imposed structural and operational limits, make a plan for safe departure condition, known as Stowage Plan.
Stage 2: Given the arrival condition of the ship and knowing the departure condition (stowage plan) to be attained, make a safe loading or unloading plan that satisfies the imposed and operational limits.
In the event that the cargo needs to be distributed differently from the described in the ship’s loading manual, stress and displacement calculations are always to be carried out to ascertain, for any part of the intended voyage, so that :
· The still water shear forces and bending moments along the ship’s length are within the permissible Seagoing limits.
· If applicable, the weight of cargo in each hold, and, when block loading is adopted, the weights of cargo in two successive holds are within the allowable Seagoing limits for the draught of the Ship. These weights include the amount of water ballast carried in the hopper and double bottom tanks in way of the holds.
· The load limit on the tanktop and other relevant limits, if applicable, on local loading are not exceeded.
The Consumption of Ship’s bunkers during the voyage should be taken into account when carrying out these stress and displacement calculations.
Whilst making a plan for cargo operations, the Chief Mate must consider ballasting operation to ensure:
· Correct synchronization with cargo operation.
· That the ballasting/deballasting rate is specially considered against the loading rate and the imposed structural and operational limits.
· That the ballasting and deballasting of each pair of symmetrical port and starboard tanks is carried out simultaneously.
During the planning stage of cargo operations, stress and displacement calculations should be carried out at incremental steps commensurate with the number of pours and loading sequence of the proposed operation to ensure that:
· The SWSF and SWBM along the ship’s length are within the permissible Harbour Limits.
· If applicable, the weight of cargo in each hold, and when block loading is adopted, the weights of cargo in two adjacent holds are within the allowable Harbour limits for the draught of the ship. These weights include the amount of water ballast carried in the hopper and double bottom tanks in way of the holds.
· The load limit on the tanktop and other relevant limits, if applicable, on local loading are not exceeded.
· At the final departure condition, the SWSF and SWBM along the ship’s length are within the permissible Seagoing stress limits.
A cargo loading/unloading operation plan should be laid out in such a way that for each step of the cargo operation there is a clear indication of following:
· The quantity of cargo and the corresponding hold numbers to be loaded/unloaded.
· The amount of water ballast and the corresponding tank/hold number(s) to be discharged/loaded.
· The ship’s draught and trim at the completion of each step in the cargo operation.
· The calculated values of the still water shear forces and bending moments at the completion of each step in the cargo operation.
· Estimated time for completion of each step in the cargo operation.
· Assumed rate(s) of loading and unloading equipment.
· Assumed ballasting rate(s).
The loading/unloading plan should indicate any allowances for cargo stoppage (which may be necessary to allow the ship to deballast when the loading rate is high), shifting ship, bunkering, draught checks and cargo trimming.
3. Ship/shore Communication Prior to the commencement of Cargo operations
Effective means of communication are to be established between the ship’s deck officers and the cargo terminal which shall remain effective throughout the cargo operation. This communication link should establish:
· An agreed procedure to STOP cargo operations.
· Personnel responsible for terminal cargo operations.
· The ship’s officer responsible for cargo loading/unloading plan and the officer in charge responsible for the on-deck cargo operation.
· Confirmation of Information received in advance.
· An agreed procedure for the terminal to provide the officer in charge with the loaded cargo weight, at frequent intervals and at the end of each pour.
· An agreed procedure for draught checking.
· The reporting of any damage to the ship from the cargo operations.
The ship’s officer responsible for cargo operation plan should submit the proposed loading/unloading plan to the cargo terminal at the earliest opportunity to allow sufficient time for any subsequent modifications and to enable the terminal to prepare accordingly. The ship’s officers should be familiar with the ISM’s Ship/Shore Safety Checklist.
4. Before Commencing the Cargo operation
The cargo terminal should not commence any cargo operations until the loading/unloading plan and all relevant procedures have been agreed and the ship’s Master, where necessary, received a Certificate of Readiness issued by the respective maritime authorities.
MONITORING AND CONTROLLING CARGO OPERATION
1. Monitoring of Stevedoring Operation
The officer in charge has responsibility for the monitoring of the stevedoring operation and should ensure that:
· The agreed loading/unloading sequence is being followed by the terminal.
· Any damage to the ship is reported.
· The cargo is loaded, where possible, symmetrically in each hold and, where necessary, trimmed.
· Effective communication with the terminal is maintained.
· The terminal staff advise of pour completions and movement of shore-side equipment in accordance with the agreed plan.
· The loading rate does not increase beyond the agreed rate for the loading plan.
If there is likely to be a change by the terminal to either loading/unloading sequences or the Cargo loading/unloading rate, the officer in charge is to be informed with sufficient notice. Changes to the agreed loading/unloading plan are to be implemented with the mutual agreement of both the ship and terminal.
If a deviation from the agreed loading/unloading plan is observed, the officer in charge should advise the cargo terminal immediately so that necessary corrective actions are implemented without delay. If considered necessary, cargo and ballasting operations must stop.
2. Monitoring the ship’s Loaded Condition
The officer incharge should closely monitor the ship’s condition during cargo operations to ensure that if a significant deviation from the agreed loading/unloading plan is detected all cargo and ballast operation must STOP.
The officer incharge must ensure that:
· The cargo operation and intended ballast procedure are synchronized.
· Draught surveys are conducted at appropriate steps of the loading plan to verify the ship’s loading condition. The draught readings, usually taken at amidships and the fore and aft perpendiculars should be in good agreement with values calculated in the loading plan.
· Ballast tanks are sounded to verify their contents and rate of ballasting/deballasting.
· The cargo load is in agreement with the figures provided by the terminal.
· The SWSF,SWBM and, where appropriate, hold cargo weight versus draught calculations are performed at intermediate stages of the cargo operation. These results should be logged, for recording purposes, against the appropriate position in the loading plan.
Following a deviation from the loading plan, the officer incharge should take all necessary corrective actions to:
· Restore the ship to the original loading/unloading plan, if possible, or:
· Replan the rest of the loading/unloading operation, ensuring that the stress and operational limits of the ship are not exceeded at any intermediate stages.
The modified loading/unloading plan should be agreed by both the officer responsible for the loading plan and the cargo terminal representative. Cargo operations should not resume until the officer in charge gives a clear indication to the terminal of his readiness to proceed with the cargo operation.
3. Hull damage caused by cargo operations
All damages should be reported to the ship’s Master. Where hull damage is identified, which may affect the integrity of the hull structure and the seaworthiness of the ship, the ship’s owner and the classification society must be informed.
A general inspection of cargo spaces, hatch covers and deck is always recommended to identify any physical damage of the hull structure.
Potential Problems which may happen during Cargo operations in a Bulk Carrier
1. Deviation from the limitations given in the approved loading Manual
Exceeding the permissible limits specified in the ship’s approved loading manual will lead to over-stressing of the ship’s structure and may result in catastrophic failure of hull structure. When deviating from the cargo load conditions contained in the ship’s approved loading manual, it is necessary to ensure that both the global and local structural limits are not exceeded. It is important to be aware that over-stressing of local structural members can occur even when the hull girder still water shear forces (SWSF) and bending moments (SWBM) are within their permissible limits.
Exceeding the maximum permissible cargo load in any hold will lead will lead to over-stressing of local structure. Over-stressing of the local structure will occur when:-
· The weight of cargo loaded into a hold exceeds the maximum permissible value specified at full draught.
· The weight of cargo loaded into adjacent holds exceeds the maximum combined value at full or reduced draught.
Overstressing of the local structure may also occur when the weight of cargo loaded into an individual hold has sufficient support of upward buoyancy force; this circumstance can occur when cargo is transported by the ship in a shallow draught condition (for example, partial load condition with some holds full and remaining holds empty)
2. Loading Cargo in a Shallow draught condition
To minimize the risks of over-stressing the local structure, the largest possible number of non-successive should be used for each cargo hold.
Loading cargo in shallow draught condition can impose high stresses in the double bottom, cross deck and transverse bulkhead structures if the cargo in the hold is not adequately supported by the buoyancy upthrust. If applicable, the cargo weight limits for each cargo hold, and two adjacent cargo holds, as a function of draught, (the local loading criteria) are not to be exceeded.
3. High Cargo loading rates in the Bulker.
High loading rates may cause significant overloading within a very short space of time. The officer incharge should be prepared to STOP cargo operations if the loading operation deviates from the agreed loading plan.
There are three main problems associated with high loading rates which may result in over-stressing the ship’s structure, namely:-
· The sensitivity of global (overall) hull girder SWSF and SWBM. To illustrate the point, see the example below
· Overloading the local structure.
· Synchronization of the ballasting operations.
EXAMPLE OF THE SENSITIVITY OF THE HULL GIRDER TO CARGO DISTRIBUTION OF A BULK CARRIER WITH 7 HOLDS
As shown in table below, the inadvertent loading of 900 tones into each of the holds numbered 1 and 7 took 5.4 minutes if two loaders were in operation. The re-distribution of cargo causes SWSF and SWBM to exceed the allowable limits by 17 and 33 percent respectively.
Hold 1 (tones)
see note 1
Hold 3 (tones)
Hold 5 (tones)
Maximum SWSF (tones)
Maximum SWBM (tones-m)
Approved ore load condition
10% of No.5 hold evenly loaded to holds 1&7
1. The time taken to load the additional cargo is presented in the brackets under the respective hold cargo weight, assuming a loading rate of 10000 tone/hour.
2. Figures in the bracket in SWSF and SWBM columns are the respective percentages of permissible.
High cargo loading rates may create problems with the ballasting operation as the pumping capacity of the ship may be relatively low compared to cargo loading rate. In such cases the cargo operation must be stopped to ensure synchronization with the ballasting operation is maintained. When necessary, the loading rate must be adjusted to synchronize with the ship’s pumping capacity.
4. Asymmetric Cargo and Ballast Distribution
It is recommended that high density cargo be stowed uniformly over the cargo spaces and trimming be applied to level the cargo, as far as practicable, to minimize the risk of damage to the hull structure and cargo shift in heavy weather.
The distribution of cargo in a hold, and water ballast distribution, have an important influence on the resultant stress in the hull structure. The double bottom and cross deck structure are designed based upon a trimmed cargo distributed symmetrically in a hold space.
Still water shear forces and bending moments given in the ship’s loading manual and the corresponding calculations from onboard loading instruments are based on an even distribution of cargo in a hold space, unless otherwise indicated.
Still water shear force and bending moments calculated with an onboard loading instrument do not consider the torsional loads acting on the hull girder resulting from asymmetrical cargo or ballasting loading.
When heavy cargo is poured into a cargo space at one end of the cargo hold, the lateral cargo pressure acting on the transverse bulkhead, as a result of the cargo piling up at one end of the space, as depicted in figure 16, will increase the loads carried by the transverse bulkhead structure and the magnitude of transverse compressive stresses in the cross deck.
When the same loading pattern is also adopted for the adjacent cargo hold , as shown in figure 17, the lateral cargo pressure acting on the transverse bulkhead will be largely cancel out. However, in this case, a large proportion of the vertical forces on the double bottom are transferred to the bulkhead between the two loaded holds which could lead to shear buckling of the transverse bulkhead structure, compression buckling of cross deck and increased SWBM in way of the transverse bulkhead. Cargo should always be stowed symmetrically in the longitudinal direction, and trimmed, as far as practical.
Stowing cargo symmetrically about the ship’s centre line in a cargo space, as depicted in figure 18, induces torsional loads into the structure which causes twisting of the hull girder. When the hull girder is subjected to torsion, warping of the hull section occurs which gives rise to shearing and bending of the cross deck structure.
Water ballast should always be carried out symmetrically in port and starboard tanks with equal levels of filling, the final level of all water ballast tanks and holds must satisfy the requirements specified in the ship’s approved loading manual to avoid damage to the internal structure due to sloshing effects.
The ballasting and deballasting of port and starboard ballast tanks should be carried out simultaneously, so that the amount of water ballast in each corresponding pair of port and starboard ballast tanks remains the same throughout ballasting or deballasting operations, as depicted in figure 19 and 20. Asymmetrical distribution of water ballast induces torsional loads, causing twisting of the hull girder.
Torsional loading of the hull girder is considered to be an important contributory factor to recurring cracking at the hatch corners and to problems associated with hatch cover alignment and fittings. In extreme cases, this can lead to extensive buckling of the cross deck structure between the hatch openings.
5. Lack of effective Ship/Shore communication
The lack of effective ship/shore communication may increase the risk of inadvertent overloading of the ship’s structure. It is important that there is an agreed procedure between the ship’s officers and terminal operators to STOP cargo operations. The communication link established between the ship and the terminal should be maintained throughout the cargo operation.
6. Exceeding the Assigned load line marks
All ships engaged on international voyages are assigned with load line marks in accordance with the provisions of the International Load Line Convention 1966.
The appropriate lines marked on the ship’s side shall not be submerged at any time during the seagoing voyage.
To allow for the difference between the dock water density and the sea water density, the ship may be loaded beyond the appropriate mark by the dock water allowance. The dock water allowance is only applicable in a port environment. It is a statutory requirement that the ship is not to be loaded beyond the limits specified in the Load line Certificate.
The practice of inducing a hogging deflection to the hull girder by the end holds trimming to maximize the cargo carrying capacity of the ship to the appropriate marks is to be avoided as this may result in the over-loading of the end holds beyond the allowable limit and an increase in both overall and local stresses.
7. Partially filled Ballast Holds or Tanks
Sailing with partially ballast holds is prohibited unless the approved loading manual approves of such a practice. Cargo holds designed for partially filled in harbour for the purpose of reducing the ship’s air draught are not to contain any water ballast while at sea.
Where ballast holds, and in some instances ballast tanks, are partially filled, there is likelihood of sloshing. Sloshing is the violent movement of the fluid’s surface in partially filled tanks or holds resulting from ship’s motions in a seaway.
Sloshing will result in the magnification of dynamic internal pressures acting on the Hold/Tank boundaries. For any tank design, dimensions, internal stiffening and filling level, a natural period (frequency) of the fluid exists, which, if excited by the ship’s motions, can result in very high pressure magnification (resonance) which can result in damage to the tank/hold’s internal structure.
To minimize the effects of sloshing, the liquid’s motion needs to be controlled by ensuring that tanks are either pressed up or empty (sloshing can occur at low filling levels).