Sunday, 19 April 2015

Bulk Carriers (A Detailed Synopsis)

Ore Carrier Berge Stahl (Copyright: BW Fleet Management Pte. Ltd. Singapore)

Whenever the word “ship” comes to our mind, we may invariably think of lavish yachts and passenger ferries like the Titanic, the robust fighter ships and destroyers used for defence purposes or pleasure crafts used for recreation. But we must also note that a commendable share of our fleet traveling around the globe is comprised of the ships known as ‘Bulk Carriers’ which are also termed as “workhorses of maritime trade”. Over 15-17% of our merchant vessels are comprised of these amazing bulk carriers.

As of 1999, the International Convention for the Safety of Life at Sea defines a bulk carrier as a ship constructed with a single deck, top side tanks and hopper side tanks in cargo spaces and intended to primarily carry dry cargo in bulk; an ore carrier; or a combination carrier. But, let us not get into the technical lingo first, a bulk carrier as a general purpose cargo-carrying ship which is employed to carry enormous amounts of bulk unpackaged (note: they are not like container vessels ) cargo in its single-deck structure. Broadly there may be 2 types of cargo:
  • Liquid bulk cargo  transported by chemical tankers, crude oil carriers, product tankers, petroleum tankers.
  • Dry bulk carriers carrying ore, grains, raw materials, coal, steel etc.
  • Another special type of carrier called OBO carriers are found which carry all the three in combinations (Ore-Bulk-Oil) and that too in a single voyage.
Different types of bulk carrier based on cargo arrangement.
(Copyright: Ship Construction, D.J. Eyres)

Profile view of a bulk carrier.

Plan view of a bulk carrier (Main Deck Plan)

Now, let me come to the point. A simple bulk carrier is a single deck, high capacity cargo ship mainly intended for carrying unpackaged bulk cargo. It normally has a complex internal hull structure designed to meet its efficiency, capacity and storage, strength as well as safety.

Types of Bulk Carriers Depending on Size and Capacity


There are various types of bulk carriers based on their containment capacity or deadweight, sizes and dimensions and sometimes business and corporal standards. Some of the common types of bulk carriers are:
  • MINI BULKERS: Deadweight (dwt) capacity< 10000 tons.
  • SMALL HANDYSIZE CARRIERS: 20000-28000 tons DWT.
  • HANDYSIZE CARRIERS: 28000-40000 tons DWT.
  • HANDYMAX: 40000-50000 tons DWT.
  • SEAWAYMAX: It is a design-specified type made to cross the St. Lawrence Seaway and has the beam restricted to within 23.16 m.
  • AFRAMAX: 75000-115000 tons DWT.
  • SUEZMAX: Specially designed to pass the Suez Canal and has the load capacity up to 150000 tons DWT.
  • PANAMAX: Designed specifically to traverse the Panama Canal with breadth/beam within 32.2m and a capacity of 65000-80000 tons
  • CAPESIZE: It is designed specifically to move through the Cape of Good Hope and Cape Horn.
  • VLBC or Very Large Bulk Carriers: These are huge sized, with tonnage capacity of 80000-120000 tons DWT.


Hullform



Now a bulk carrier has the main concern of carrying large amounts of loaded bulk cargo “economically and safely” from one place to another in stipulated time over varying distances. So, think about it, the prime concern driving all the vessels should be capacity and cargo-friendliness and not speed or luxury. So, in all the ships which are essentially of displacement-type (slow speed), it has a broader and fuller hull form.

A broader beam has a fuller bow as well as a stern to accommodate large amounts of cargo, reducing its concerns on speed. In terms of the body plan or the lines plan, the buttock lines or the curvature lines of the hull both fore and aft are far spaced accounting for its fullness.

Now the hull form of a bulker is generally cell-guided to account for its longitudinal strength (for larger ships) and is web framed (in shorter ships) to account for its transverse strength. The basic design of the hull form of bulk carriers is mainly comprised of a thick double shell plating and girded by tanks in the sideways, bottom, and top zones. 

Strength of The Hull Girder


The double bottom structure not only adds to strength of the hull girder and protect the bulk cargo inside from any kind of oceanic disturbance. It also provides a protective layer against accidental flooding, breakage, leakage or grounding of a ship.

Double bottom structure (Solid floor and Bracket floor arrangement)
(Copyright: Ship Construction, D.J. Eyres)

The elaborate arrangement of tanks with the double bottom tank below, lower hopper side tank or bilge tank at the bilge or the upper hopper side tank underneath the corners of the upper weather deck mainly account for the ballasting systems in ships. Ballasting is an operation done by the intake of some amount of freshwater or seawater in the tanks for the purpose of maintaining the stability and buoyancy of the ship (hence maintaining its centre of gravity in diverse sea conditions). The tanks also have manholes for the purpose of surveying, discharging, repairing and maintenance. 

The deck and hull elements have an elaborate arrangement of girders and stiffeners for providing longitudinal strength.These may be welded or riveted depending on their location both in transverse and longitudinal direction.



Midship Section of a single skin bulk carrier. (Copyright: Ship Construction, D.J. Eyres)
Midship Section of double skinned bulk carrier.
(Copyright: D.J. Eyres)

Cargo Handling


Bulk amounts of cargo in bulk carriers may be loaded and unloaded by the virtue of large openings in the deck known as hatch openings. These openings are generally less than half of the beam (< B/2), generally one-third in single hatch ships and 0.75 in double or more hatches. These hatches are covered by hatch covers and have coamings which protect the cargo from flooding and damage in high seas and also accounts for compensation of loss of strength of the deck due to the openings.

The hatch covers may slide, fold, roll or be guided by hydraulic lifting systems. Some special techniques as in the pontoon decks may be adopted. All such designations are in congruence with the load outlines and the structural necessities of a ship.

Folding, Single Pull and Direct Pull hatch covers.
 (Copyright: Ship Construction, D.J. Eyres)


Rolling hatch covers.
 (Copyright: Ship Construction, D.J. Eyres)

Specialized cranes and derricks may be used for loading and unloading.  A crane's discharge rate is limited by the bucket's capacity (from 6 to 40 tons) and by the speed at which the crane can take a load, deposit it at the terminal, and to return to take the next. For modern gantry cranes, the total time of the grab-deposit-return cycle is about 50 seconds. It may also be of self-loading or self-discharging type where the process of loading and unloading may be by the use of conveyor belts where the loading and unloading rates may range from 100 to 700 tons per hour where the most advanced ports have a range of up to 16,000 tons.


Cargo handling arrangement on a bulk carrier with 4 cargo holds.
(Note how one derrick is used to operate on two holds)

Cargo handling arrangement of the same bulk carrier above, in plan view.
(Note how one derrick is used to operate on two holds)


Proper surveillance and checking methodologies are adopted for the cargo both ensuring its quality and the stability and safety factors of a ship. Grain shifting is an awkward and often dangerous problem in case of dry bulk, where the unpackaged or loosened cargo pose the problem of shifting when exceeding the angle of repose mostly due to lack of levelling or the heavy sea conditions. It leads to the loss of stability and precarious rolling motion.

If you observe the slope of the wing tank plating, it has a reason. This angle differs in different bulk carriers, matching the angle of repose of the cargo that is to be carried. If the angle of the wing tank plating matches the angle of repose of the cargo, then cargo shifting is considerably prevented.

Machinery (In Brief)


The machinery and the engine room is aft near the stern for proper control and also for maintaining the trim. Most of the common bulkers like the Panamax or Handymax have 2-stroke heavy duty diesel engine attached to a fixed-pitch propeller. Smaller vessels usually have one or two 4-stroke engines attached to the fixed or controllable pitch propeller via a reduction gearbox.



Engine Room Arrangement at three floor levels above bottom line.

Recent Developments


Bulk Carriers, being the workhorses of the maritime economy have come a long way through disasters, losses, and hazards. So, with the passage of time, especially after the tragic loss of MV Derbyshire, the IMO, and the other International Safety Organizations have become more cautious about safety measures to be taken in a bulk carrier. Though, it lists out to be huge, some of them are:

  • A stronger double-bottom accounting for more stable structure.
  • At least two or more holds watertight are necessary to avoid heavy trim in case of accidental flooding or leakage.
  • Improving hatch structures and coamings, so that water does not enter the holds even in heavy seas.
  • Generally it is difficult to assess loading conditions and heavy lifting operations are usually slow (it can take over an hour just to halt the operation), occasionally resulting in overloading the ship. Sometimes, unexpected shocks, over time, can damage the hull's structural integrity. 
  • Much more care is taken and advanced techniques are adopted for maintaining proper angle of repose and preventing grain shifting and also to keep the cargo evened out. 
  • Corrosion, due to a lack of maintenance, affected the seals of the hatch covers and the strength of the bulkheads which separate holds. The corrosion is difficult to detect due to the immense size of the surfaces involved. So more care is taken to prevent corrosion and prevent life span of the ships. 
  • Improved ballasting technologies. LSD


Article By: Subhodeep Ghosh     



1 comment:

  1. Interesting sysnopsis! it shows that you have done your research and put in lots of efforts into the making of this synopsis. I hope you get it approved.

    ReplyDelete