Bulk Carrier Guide Online
Bulk Carrier Guide Online
Home ||| Bulk Cargo ||| Planning ||| Care ||| Safety||| Self unloaders

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 Corrosion

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:
  1. 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

  2. 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

  3. 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

  4. 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

  5. under SOLAS Chapter II-1 double side skin spaces must be provided with a compliant protection coating.

A Bulk carrier cargo hold structure

Fig : Cargo hold construction of a typical bulk carrier

Metal fatigue

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:
  1. The brackets at the connection of frames to the upper and lower wing tanks
  2. the upper and lower connection of corrugated transverse bulkheads
  3. 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.

Operational Factors

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:
Bulk cargo hold heavily pitted with rust scale
Fig: These holds are unlikely to pass a grain survey, as they are heavily pitted with rust scale and embedded with coal staining

Cathodic protection

Cathodic protection is a system of preventing corrosion by forcing all surfaces of a structure (e.g. hull) to be cathodes by providing external anodes. It can be achieved by superimposing on the hull an impressed current provided by a remote power source through a small number of inert anodes (impressed current cathodic protection). Also accomplished by fitting aluminium, magnesium or zinc anodes in tanks or underwater portion of a ship, which waste away by galvanic action (sacrificial anode cathodic protection).

Top articles

  1. Indication of unusual motion or attitude of bulk carriers and risk management / evacuation

  2. Deterioration of ships structure and consequences of forward flooding

  3. Handling water ingress problems in bulk carrier, investigation and countermeasures

  4. Survival and safety procedure for bulk carriers

  5. Hatch cover strength requirement for a seagoing bulk carrier

  6. Ships longitudinal subdivisions - use of transverse watertight bulkheads

  7. Suitability of Shore Terminals for handling bulk cargo

  8. Preparation for ships carrying bulk cargo & standard loading condition

Countermeasures against all above structural problems are discussed here:

Steel hatch cover maintaining watertightness - Classification society guideline

Why bulk carriers are highly susceptible to corrosion ?

Structural standards & strengthening of bulk carriers

Hull stress monitoring system for bulk carriers

Our detail pages illustrated many safety aspects of Bulk carrier

Home page |||Bulk carrier types ||| Handling of bulk coal |||Cargo planning ||| Carriage of grain |||Risk of iron ores |||Self unloading bulk carriers |||Care of cargo & vessel |||Cargoes that may liquefy |||Suitability of ships |||Terminal guideline |||Hold cleaning |||Cargo cranes |||Ballast handling procedure |||Bulk carrier safety |||Fire fighting systems |||Bulk carrier General arrangement

Operation of sea going bulk carriers involved numerous hazards . Careful planning and exercising due caution for all critical shipboard matters are important . This site is a quick reference to international shipping community with guidance and information on the loading and discharging of modern bulk carriers so as to remain within the limitations as specified by the classification society.
It is vital to reduce the likelihood of over-stressing the ship's structure and also complying with all essential safety measures for a safe passage at sea. Our detail pages contain various bulk carrier related topics that might be useful for people working on board and those who working ashore in the terminal. For any remarks please Contact us

Copyright © 2010 bulkcarrierguide.com All rights reserved.

Although every effort have been taken to improve the accuracy of content provided the publisher of this website cannot take responsibility for errors. Disclaimer Privacy policy Home page