General Constructional Features
- Maximum utilization of space is generally ensured easily. The cargo’s SF (about 0.3 for ore & about 1.7 for grain) is the key factor.
- The overall cargo weight is the limiting factor in the design of an ore carrier, since the cargo is so dense. Coal, Grain, Rock Phosphate, wood chips, etc on the other hand, are limited by overall volume. For a given tonnage, the second factor which governs the ship’s dimensions is the size of the ports and waterways it will travel to.
- The ER on a bulker is usually near the stern, under the superstructure. In general, hatch covers are between 45% and 60% of the ship’s breadth, or beam, and 55% to 65% of the length of the holds.
- Large hatch covers have always been an area of concern, particularly post Derbishire report by professor Falkner of SNAMES. Extra strengthening of hatch covers was suggested then. Hatch covers appear to be the weak link in otherwise strong looking bulk carrier. For better efficiency hatches must be large, but large hatches present structural problems. Hull stress is concentrated around the edges of the hatches, hatch areas are reinforced by locally increasing the scantling or by adding structural members called stiffeners.
- These vessels have hydraulically operated metal hatch covers. The ‘Load Line 66’ imposes a requirement that hatch covers must withstand load of 1.74 tons/m² due to sea water and a minimum scantling of 6 mm for the tops of the hatch covers. Though, LL Convention provides a table to find out the scantling details for hatch cover. The IACS had also increased this strength standard requiring that the pressure due to sea water be calculated as a function of freeboard and speed, especially for hatch covers located on the forward portion of the ship.
- A B-100 bulker gets a freeboard equivalent to a tanker, which actually speaking can not be considered justified. This is because there is no substitute for discontinuity for hatch openings. Derbyshire was just a B-60 bulker.
- Bulkers have the upper and lower corners of the hold used as ballast tanks in addition to the DB area. The angle of the upper hopper tanks with horizontal is kept more than that of the angle of repose of the anticipated cargoes, thus making the holds self trimming.
- The double bottoms are made high enough to allow the passage of pipes and cables and also spacious enough to allow people, safe access to perform surveys and maintenance.
- Hulls are made usually of MS.
- High-tensile steel used on newer vessels reduces the light weight. However, the use of high-tensile steel for longitudinal and transverse reinforcements can reduce the hull’s rigidity and resistance to corrosion.
- Transverse partitions are normally corrugated bulkheads, reinforced at the bottom and at connections. Upper and lower stools provide such support.
Theory of Bulk Carrier Losses
Bulk carriers have one of the highest loss rates of the world merchant fleet. During the period 1990-95 the average age of bulk carriers lost through leaks or disappearance was 18 years. This hinted at a continual decay or overall decay occurring o bulk carriers. This was also foo for thought for the concept of goal based regulations. The age related vessel damage appeared as a prime contributor to bulk carrier losses. The findings of several research programs support this view by identifying both corrosion and fatigue as weakening the structures of bulk carriers.
1. Deterioration of ships hull / structure through corrosion, fatigue and damage is identified as a principal factor in the loss of many ships carrying cargo in bulk. The cargoes under scanner being the coal and iron ore. There are these two things which happen here:
(i) Failing to identify such deterioration; and
(ii) sudden and unexpected accident.
Bulk carrier crew may be unaware of the vulnerability of these vessel types. The consequential loss of a ship carrying heavy cargo can be very rapid and possibly mysterious.
2. High Tensile Steel (HTS) is commonly used in bulk carriers to reduce the thickness of the structure, increasing cargo carrying capacity. However, the drawback of HTS is that, when any structure is corroded, the loss of strength occurs far quicker than for mild steel. Therefore, high notch tough steel is used in crucial areas such as the keel, bilge, deck stringer, sheer strakes and top/bottom parts of bulkheads to provide continual strength throughout the vessel’s life.
3. Bulk carriers are known to be more susceptible to structural failure than other similar sized ships, particularly when a hull breach causes water ingress into the cargo holds. Monitoring the structure for any signs of deformation, fatigue or corrosion and applying preventive rather than reactive maintenance is so important. However, despite all these precautions, incidents on bulk carriers causing loss of both life and cargo have caused concern of late, leading to the development of new structural standards for bulk carriers.
4. Association of large machines causing instant, direct damage to various hold parts is like an inherent problem with bulk carriers. The direct damage initiates the process of further deterioration if neglected.
5. In 1990 the IMO issued a circular (MSC/Circ.531) which warned against the risks of shifting cargo and requested Member Governments to implement revised recommendations for trimming cargoes which were included in the 1989 edition of the Code and are intended to minimize sliding failures.
6. Excessive SF BM as the major cause of ship loss. The sudden increase in bulk carrier losses in 1990 and 1991 caused considerable alarm in the shipping industry. In response, the IMO Assembly adopted Resolution A.713(17) (“Safety of Ships Carrying Dry Bulk Cargoes”) which contains interim measures designed to improve the safety of ships carrying solid bulk cargoes. The resolution noted that the nature of cargo and ballast operations can subject bulk carriers to severe patterns of bending and sheer forces and also to significant wear. It referred to the dangers posed by some bulk cargoes through their high density and tendency to shift.
7. Spaces forward of the collision bulkhead are vulnerable in rough seas, in case of most ships in loaded condition, in the event of flooding. Any ingress forward will affect the trim of the ship and reduce freeboard at the bow. Green seas would impact on hatch covers and other fittings that provide weather tightness. Incidents of hatch cover and deck machinery getting uprooted are there on record.
Bulk carrier corrosion rates are highly variable
a. Coal and iron ore cargoes, frequency of cargo loading and ballasting, expected trade routes, ballast ratios and type of coal transported, are the main parameters affecting corrosion rates due to varying levels of chloride, sulphate and acidity produced from various coals.
b. Coal is more corrosive than iron ore due to the presence of chlorine and pyritic sulphur which provide sources for Cl2 and SO4 ions respectively.
c. The time of wetness (TOW) and wear of protective coatings varies for different locations within the vessel, depending on which cargo is loaded.
d. The oxygen content within loaded coal is approximately 20.5%, which is similar to open air.
e. Corrosion is also dependent on the pH of the electrolyte. Coal comprises 50-65% carbon. The electrolytic potential of carbon is 2.1V compared to -0.44V for mild steel. It is therefore a condition favouring the formation of a galvanic cell.
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