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Bulk carriers have become larger, carrying ever increasing tonnage of various cargoes. They are required by law to have a hull inspection carried out yearly.
Due to the structural damage to the hull that can be sustained during cargo loading and discharge, ships officers are being trained in hull inspections.
The following sections examine the structure of the hull highlighting the areas where structural damage can take place, the first section providing an overview of the history of bulkers.
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Bulk Carriers - A Short History
The old sailing ships were the first to carry bulk in the 1800's, in this case corn and wheat, they gave way to the first steamships that carried coal.
The first motor ships carrying bulk cargos came into service in early 1900’s; these incorporating double bottoms, and triangular hopper ballast tanks.
However, it wasn’t until after the Second World War that world trade in bulk cargoes picked up and bulk carriers were designed to carry increasing amounts of cargo.
1970’s saw the design of the OBO bulkers; they were designed to carry oil, bulks and ore; but were plagued with structural defects that led to a few disasters.
The main one was the sinking of the MV Derbyshire; she went down with all hands; subsequent inquiries ruled structural defects to be the cause of her sinking.
The eighties and nineties heralded numerous very large bulkers, but because of restrictive design requirements instigated by various international marine organizations on these gargantuan ships, the smaller bulker was preferred. However even these still produced many casualties through sinking, mostly with all hands as they sank so fast especially the iron ore bulkers.
Nowadays modern bulkers are designed to carry iron ore, bauxite (for alumina smelting) phosphates, coal and grain; the bulkers making up almost 40% of world shipping fleet.
An image of the World's largest bulker is shown below.
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Cargo Handling, Hatches and Hull Design
There are number of methods of loading;
- Modern cranes can load 1000-2000 T/hour.
- Conveyor systems can load between 100 and 700T/hour with the modern systems capable of 16000T/ hour; imagine the stresses imposed on the hull structure at these rates!
- Depending on the cargo, cranes are normally used and the rate of discharge is dependent on the size of the bucket and capacity of the crane averaging around 800T/hour.
Once most of the cargo has been discharged, front loaders and bulldozers are lowered into the holds to direct the residue into sizeable mounds for collection by the crane buckets; then manual labor is used to brush out the holds; a dirty and thankless job.
I had just joined a cargo ship in Dublin as fourth engineer in the late 1960’s and she carried molten latex in a couple of tanks, that as far as I can remember was sucked out through pipes to a shore side hopper. Anyway there was a leak in the steam coils and a only the Chief and I were aboard, he volunteered me to go down into the tank and repair the leaking flange. In those days there was no entry permits or breathing sets; so fortified with a large dram of the Chief’s black navy rum, a rope tied around my waist and a roll of ‘clingerite’ jointing, spanners and a hammer I climbed down into the murky interior. Every so often the Chief would holler ‘are you all right there fourth’ that would echo around and make me jump and drop spanners into the goo at my feet. I found the leaking flanges (there were two) and after repairing them I climbed back up to the deck and had a few beers with the Chief. He gave me the night off and I went ashore to a local hop in the Dublin dockside. There I was, dancing away on the floor when I happened to look down at my feet; yep, I had worn my good shoes down the latex tank and the floor was now being covered in sticky elastic strips coming off them! That was my only experience of tanks and once was enough.
Anyway, enough of my ramblings, they will all be in my book; if I ever finish it. The men and, often women who go down the bulker holds to sweep out, must endure terrible conditions; as do the mates when they enter for inspection of the structural members.
We will come to that in a minute, but first let's look at the hatches. Once again my experience of these on cargo ships were limited to ‘McGregor Hatches’ these were on rollers and, when being opened up accordian style against each other, gave access to the cargo hold.
The bulker hatches work on more modern hydraulic actions and are large, ¾ of the width of the ship and about ½ the length of the holds, so you can imagine their size. They have to be large to facilitate the cargo loading and discharge, they also have to be watertight.
The hull comprises of the engine room space, accommodation, bridge, main deck, cargo holds and fo'c'sle.
Basically, the hull is divided into cargo holds by watertight bulkheads, covered as we have seen by the watertight hatches. The holds are ventilated by manual ventilators and fans to prevent dampness and build up of fumes. Below the holds are the double bottoms, and at the lower and upper corners of the holds are triangular ballast tanks known as hoppers; also help prevent cargo shifting.
The bulkers normally have a vertical bulbous bow, above which is the fo’c’sle being raised from the main deck. The windlass mechanism, anchor locker and fore peak tanks are arranged below the fo’c’sle head. Images of hatches, ballast tanks, and hull are shown below.
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Causes of Structural Damage Through Cargo Handling - Critical Inspection
Such is the seriousness that the maritime authorities view cargo handling in relation to structural damage, that they have made the following maritime regulations;
- A loading and discharge plan must be prepared and agreed to by the master, 1st mate and the terminal chief cargo handling engineer. These are based on hull stress calculations where uncontrolled loading can cause longitudinal bending stress and shear force. Notwithstanding this, ships still break in half or capsize at loading terminals due to miscalculations, or stevedores ignoring the discharge plan.
- Inspections by trained deck officers of cargo space structural members before loading and after cargo discharge.
Because of their size they present stress loads on the deck and coamings, these areas must be reinforced using extra scantlings or stiffeners; these areas forming part of the critical inspection.
As we have seen the rate of loading is very fast therefore it is imperative that the loading plan must be followed; the ship’s trim being maintained level using ballast and uniform level during loading or unloading the cargo.
Bulldozers are used to assist unloading and remove the last of the cargo; however these can inflict damage on the shipside plates, structural beams and hold bottom plates; the holds also being critical inspection areas.
If the plan is not adhered to, high torsional stress levels will be put on the hull structural cross and longitudinal girders, bulkheads and welding. These are all areas of critical inspection where any damage will be seen as cracks or fretting of the members and attachment welding.
All of these areas are classed as critical and must be inspected before leaving port or loading the next cargo. Most senior deck officers should be now trained in the inspection of the critical areas, and through time will gain experience, passing this onto junior officers. A record of inspections should be kept in the ships inspection logbook where the dates of inspections along with locations of any defects and photographic evidence of the extent of defects.
I hope through reading these guidelines, they will go some way towards reducing the number of losses of bulkers and crews.