1 2 2 2 - bulk carrier main particulars. 2 2 bulk carrier main particulars - 11/10/2007 bulk...

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Bulk Carrier Main Particulars - 11/10/2007

Bulk CarriersBulk Carriers

General Arrangement

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General purpose dry Bulk Carriers

• A single deck ship usually with engines aft with minimum allowed number of holds for carriage of dry cargo in bulk.

• Carries a variety of dry cargoes, low density sugar, coal, grain etc.; some are strengthened for high density cargoes e.g. iron ore.

• Some holds may be left empty depending on hull strength.

• Vessel size limited by port depths.

• Large hatchways to accelerate cargo handling.

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• Holds are self-trimming (sloping topside tank) to facilitate loading and prevent shifting.

• Number of holds determined by loading of various types of cargo and ensure full holds; also flooding requirements. Handysize may have 5 cargo holds whereas a Capesize can have 9 holds.

• Ballast capacity should ensure immersion of propeller (when empty of cargo) and raise the centre of gravity position to prevent excess of stability.


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• IACS/BV definition “…single deck, double bottom, hopper side tanks and topside tanks and with single or double side skin construction in cargo length area and intended primarily to carry dry cargoes in bulk”.

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The secret to success of bulk carrier concept are the slopes in holds:

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• Bulk carrier hold structure has been deliberately designed to provide a flush surface as much as practicable for cargo handling purposes.

• Where it is necessary to fit stiffeners e.g. side shell frames, sloped shedder plates are fitted to prevent cargo from collecting.

• Hold is purposely shaped to prevent cargo shift at sea and gather remaining cargo on tank top for easy removal.

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Ore Carriers

• Wing tanks are used for ballast when the ship is empty of cargo.

• Deep double bottom tanks provide more ballast capacity and raise the centre of gravity of the ore to prevent ships being “too stiff” in a seaway, i.e. prevent high GM values.

• Small cargo volume but cargo very dense: 2-3 T/m3.

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Ore Carriers

IACS/BV definition “…single deck, two longitudinal bulkheads and a double bottom throughout the cargo length area and intended primarily to carry ore cargoes in the centre holds only”.

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Combination Carriers (Combis)

• Ore/Oil or Ore/Bulk/Oil (OBO)

• Gives maximum flexibility, can switch cargo as availability or freight rates dictate.

• Can carry cargo not just one way but also on return voyage instead of in ballast.

• Oiltight hatches and bulkheads.

• Ore and oil not carried simultaneously – risk of oil or vapour penetrating to a space containing an ignition source.

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• Today, almost all combination carriers are of the OBO type.

• OBO ships never become as popular as dedicated bulk or oil carriers - their complexity increases building and operating costs.

Combination Carriers (Combis)

IACS/BV definition of Combination Carrier: “… general term applied to ships intended for the carriage of both oil and dry cargoes in bulk… cargoes not carried simultaneously… except oily mixture retained in slop tanks”

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Ore/Oil Carrier

IACS/BV definition: “…single deck, two longitudinal bulkheads and a double bottom throughout the cargo length area…carry ore cargoes in the centre holds or oil cargoes in centre holds and wing tanks”.

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Oil/Bulk/Ore (OBO) CARRIER

IACS/BV definition: “… single deck, double bottom, hopper side tanks and topside tanks, and with single or double side skin construction in the cargo length area… carry oil or dry cargoes, including ore, in bulk”.

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Stability Characteristics

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Stability Characteristics

• As large conventional vessels, the intact stability of bulk carriers is not normally critical. • In fact heavy cargoes such as iron ore, which may be carried with hold partially loaded, lowers the centre of gravity of the vessel; this improves the stability “too much” by increasing the GM value.

• This condition is undesirable as it causes a condition known as “stiff ship” where the rolling of the ship can become uncomfortable due to high righting lever values.

• The main stability aspects of interest for bulk carriers are:- Grain stability- Damage stability (and in particular the requirement to comply with cargo hold No.1 flooding).

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Grain stability• Bulk cargoes when loaded form a cone. The angle formed between the slope of the cone and the bottom of the hold is the “angle of repose” and it depends on the cargo. Dense cargoes, e.g. iron ore, form a steep cone whereas grain cargoes have shallower angle.

• When grain is loaded, there tends to be air gaps between individual grains. During the voyage, the grains settle by occupying the space taken by air. This is called sinkage.

• Sinkage is dangerous because it leaves a free cargo surface below the hatches and with the motions of the vessel the cargo can shift to one side.

• The centre of gravity moves and the vessel heels to one side or can even capsize.

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Grain stability

SOLAS definition of grain

“The term grain covers wheat, maize (corn), oats, rye, barley, rice, pulses, seeds and processed forms thereof, whose behaviour is similar to that of grain in its natural state”

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Grain stability• International Code for the Safe Carriage of Grain in Bulk (International Grain Code) was introduced to address this issue. Cargo ships intending to carry grain are to comply with this Code as stated in SOLAS Ch.VI Reg. 9.

• The Code sets out requirements for stability and stowage. Angle of heel due to grain shift is required to be < 12°. There are also strict trimming requirements.

The success of the bulk carrier cargo hold is that it is ideal for carriage of grain cargoes - generally self trimming with the sloped topside tanks and feeder ducts to fill void spaces fore and aft of cargo hatches.

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Damage stability• Bulk carriers are normally required to meet the damage stability requirements as specified in SOLAS Ch.II-1 for “cargo ships” over 80 metres.

• These requirements are based on a “probabilistic approach” to damage stability where calculations are based on risk. This approach determines a minimum standard of subdivision.

“Required subdivision index R” is calculated followed by an

“Attained subdivision index A”. A should not be less than R.

Attained subdivision index A : A = Σ pisi (ship safety in damaged

condition based on probability of survival after collision), where:pi : probability of compartment or group of compartments “i”

floodedsi : probability of survival after flooding of compartments “i”

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Damage stability• However in spite of bulk carrier casualties and studies showing that large bulk carriers are at high risk if no.1 cargo hold is flooded, a special requirement for damage stability was introduced in SOLAS (incorporated in URS 23). This in fact is a “deterministic approach”, i.e. extension of damage is specified in the regulations.

• SOLAS Ch. XII states that single skin bulk carriers over 150m require to meet the damage conditions specified in the Load Line Reg. 27 as amended (i.e. requirements for Type B ships with reduced freeboard):“…constructed on or after 1 July 1999 shall...withstand flooding of any one cargo hold…”

and a specific requirement (for existing ships at the time ), for withstanding flooding of hold no.1 when carrying heavy cargoes e.g. iron ore:“…carrying cargoes… density of 1780 kg/m3 and above, constructed before 1 July 1999 shall… withstand flooding of foremost cargo hold…”

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Loading

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• Loading is one of the most important issues for bulk carriers - exceeding loading limitations may result in over-stressing the structure which may lead to catastrophic failure.

• The loading manual provides a description of the operational loading conditions based on the design of hull structure. • The loading instrument can readily calculate the still watershear forces and bending moments, in any load or ballast condition, and assessthese values against the allowable limits.

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• The weight of the ship with cargo acting down combined with buoyancy forces acting upwards along the length creates shear forces (Qsw) and bending moments (Msw) in still water.

• Shear forces and bending moments are further increased at sea with wave action (Qwv and Mwv).

Weight Buoyancy

Shear Force

Bending Moment

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Alternate loading exerts high shearing forces on hull girder that creates high shear stresses on side shell .

Uneven loading causes “sagging” or “hogging”.

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Standardized design loading conditions (IACS)

BC-B: Homogeneous heavy cargo (>1.0 t/m3) - As required for BC-C, plus homogeneous cargo loaded condition with cargo density 3,0 t/m3, and the same filling ratio (cargo mass/hold cubic capacity) in all cargo holds at maximum draught with all ballast tanks empty.

BC-C: Homogeneous light cargo (<1.0 t/m3) - Cargo density corresponds to all cargo holds, including hatchways, being 100 % full at maximum draught with all ballast tanks empty.

BC-A: Heavy cargo (>1.0 t/m3) with specified holds empty - As requirement for BC-B plus at least one cargo loaded condition with specified holds empty (cargo density 3,0 t/m3), and the same filling ratio (cargo mass/hold cubic capacity) in all loaded cargo holds at maximum draught with all ballast tanks empty.

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Loading and local strength

• Net vertical load is the difference between the vertical downward weight of cargo in hold, ballast in tanks in way of cargo hold and the upward buoyancy force which is dependent on the draught.

• Overloading of cargo hold combined with insufficient draughtcauses excessive net vertical loadson double bottom which can distort the structure in way of hold.

• Cargo hold structures are designed for specific cargo loads and sailing draught conditions. Structure is sensitive to net vertical load acting on the double bottom.

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• Cargo carrying capacity of a hold is reduced with reduction in the mean draught. • Allowable cargo loads for each hold or combined cargo loads in two adjacent holds are provided assuming empty ballast tanks in way of the hold(s). If ballast is carried in the double bottom and hopper wing tanks, maximum allowable cargo weight is obtained by deducting the weight of ballast in the tanks.

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For all standardised design load conditions, any cargo hold should be capable of :

- Carrying cargo mass MFull (cargo with virtual density filled to the top of hatch coaming), FO tanks in DB 100 % full and ballast water tanks in DB empty, at maximum draught.

- Carrying minimum 50 % of cargo mass MH (actual cargo mass for homogeneously loaded condition), with double bottom tanks empty, at maximum draught.

- Being empty with all DB tanks empty, at the deepest draught.

Additional requirements apply for vessels loading and unloading in multiple ports and for type BC-A.

Loading and local strength

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Overloading creates high stresses in double bottom, bulkheads, hatch coamings, hatch corners, frames and brackets in cargo holds.

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Shearing of corrugated bhd. and compression of cross deck.

Excessive flexural deformation of double bottom structure.

Overloading the cargo hold

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Requirement for standardized ballast conditions - applicable to all notations (BC-A, BC-B and BC-C)

Normal ballast condition (no cargo):• ballast tanks may be full, partially full or empty • ballast cargo hold(s) are empty • propeller fully immersed, • trim by stern and not exceeding 0,015 L

Heavy ballast condition (no cargo):• ballast tanks may be full, partially full or empty • at least one ballast cargo hold is full • propeller immersion requirement • trim by stern and exceeding 0,015 L + min. draught forward requirement

Bulk carriers also load ballast whilst discharging cargo, into dedicated ballast tanks and holds. This is to maintain an “air” draught that allows continuous discharge of cargo, to control stresses on the ship’s structure and prepare ship for sailing.

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• Asymmetric distribution of water ballast induces torsional loads, causing twisting of the hull girder.

• This contributes to recurring cracking at the hatch corners and to problems associated with hatch cover alignment and fittings.

• In some cases it can lead to extensive buckling of the cross deck structure.

Asymmetric loading

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Loading/ unloading operations

• Correct loading and unloading sequences of bulk carriers are crucial. Many of the structural failures are the result of improper loading and unloading.

• IMO published Code of Practice for the Safe Loading and Unloading of Bulk Carriers (BLU Code).

• IACS Rec. 46 “Guidance and Information on Bulk Cargo Loading and Discharging to Reduce the Likelihood of Over-stressing the Hull Structure” lists problems as:

- Exceeding the limits given in the Loading Manual.- Loading cargo in shallow draught can impose high stresses in

hold structure.- High hold loading rates can cause overstressing due to sensitive

shear forces and bending moments, overloading the local structure and synchronisation of ballasting (pumps may not be able to keep pace with loading rate).

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IACS Rec. 46 (cont.)- Asymmetric cargo and ballast distribution.- Lack of effective ship/shore communication.- Exceeding assigned load line marks.- Partially filled ballast holds or ballast tanks – sloshing.- Inadequate cargo weight measurement.- Existing structural damage or mechanical damages during cargo

operations.

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Ultimate consequence of incorrect loading

1 2 2 2 - bulk carrier main particulars. 2 2 bulk carrier main particulars - 11/10/2007 bulk...

Documents

bulk carrier main particulars

remaining cargo

cargo shift

carriage of dry cargo

cargo length area

cargo handling purposes

small cargo volume

various types of cargo