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Bulk carriers structural problems associated with corrosion, metal fatigues & other operational factors
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 . Failing to identify such deterioration may lead to sudden and unexpected accident. Bulk carrier crews may be unaware of the vulnerability of these vessel types. The consequential loss of a ship carrying heavy cargo can be expected to be very rapid, should a major failure occur.
The following structural problems are associated with bulk carriers:
Ships are built of steel, which in a marine environment exposed to water (both fresh and sea) and air is prone to the formation of rust. Contributing factors that accelerate the rate of corrosion include:
- Cargo damage this occurs when heavy bulk cargo is allowed to freefall from height onto the tank tops. The heavy impact of this cargo on the tank top causes damage and breakdown of the coatings on the ceiling of the double bottom tank underneath
- corrosive cargoes a number of bulk cargoes contain chemicals of a corrosive nature and this is particularly the case in newly mined coal. It is essential that the data sheet is inspected prior to loading the cargo. For example, in the case of a high sulphur contact coal cargo, severe pitting can result. To counter this, the hold floor can be coated in lime, but this does not protect the bilges or bilge lines
- equipment damage grab damage to the hold floor, frames and ladders can occur at most discharge ports. This not only causes material damage to the ship's structure, but can also break down the paint coatings exposing the base steel to the atmosphere. The deliberate hammering of the floor and sides of the hold by grabs and bulldozers to free cargo residues trapped between the frames will result in structural damage and the breakdown of the paint coatings
- seawater corrosion in the majority of cases, this will take place in the ballast tanks. Many companies now place sacrificial anodes in the ballast tanks, which considerably reduce the corrosive effect of air and saltwater
- under SOLAS Chapter II-1 double side skin spaces must be provided with a compliant protection coating.
Fig : Cargo hold construction of a typical bulk carrier
The weakening of the steel in a structure due to constant flexing, under the repeated cycles of stress may result in structural fatigue failure. The concern about fatigue failure is that it occurs without any apparent forewarning (eg deformation of a structure that results in a crack).
Fatigue usually begins at welded joints, notches, discontinuities in structures and areas of high rigidity in particular. However, variations in the size, shape and design of each component and the conditions that the ship operates mean this may not necessarily result in a structural failure. Areas where extra vigilant inspection is recommended include:
- The brackets at the connection of frames to the upper and lower wing tanks
- the upper and lower connection of corrugated transverse bulkheads
- corners of the hatch coamings where they are joined to the main deck.
Bulk carriers in particular become progressively weaker due to continuous corrosion. In addition, the repetitive cycles of changing loads and the resulting stresses due to hogging, sagging, panting, pounding and vibration all increase fatigue.
High tensile steel (which is stronger than mild steel) is used in all areas likely to experience high levels of stress. It means that scantlings can be reduced but the vessel will still have higher strength and resistance to stresses, eg slamming due to heavy pitching that may cause fatigue on the forward section of the hull.
It is recommended that, as soon as any cracks are seen, arrangements are made immediately to repair them. Where possible, a crack arrestor hole should be drilled at each end of the crack before any temporary repair is made. If the extent of the crack is not evident, a detector dye can be used to establish this. As soon as possible, Class should be called for a survey to make a permanent repair because a crack that is overlooked may become a central point for localised stress resulting in structural failure.
A crack may also damage protective coatings such as paintwork, creating an `open' area for corrosion. While cracks may not initially be apparent, corrosion in any area should be carefully checked for signs of minor cracks, particularly if there are dents in the structure.
Corrosion and fatigue will gradually weaken the hull over time. This can be increased by variations in loading patterns and particularly heavy density cargoes such as iron ore.
Another factor that gradually weakens a ship's structure is the abrasive and corrosive nature of bulk cargoes such as coal, which can cause unintentional damage to cargo hold coatings. Areas such as welded frame joints with tanktop or deck plating are very likely to develop corrosion and subsequently crack if the coatings are damaged.
Other factors include:
- Liquefaction of cargoes, caused by water ingress or moisture in the cargo, can cause cargo shift during the voyage
- movement of ballast water in partly filled ballast water tanks or holds can cause damage and create corrosion. To avoid this, tanks and holds should be completely filled.