The cargo industry is worth trillions of dollars. From transporting fresh produce to airplane parts, a large variety of goods are moved across the globe.
The worldwide shipping industry thrives on hundreds of vessels that ply the Earth’s oceans. They range all the way from small fishing trawlers and speed boats, to gigantic behemoths such as Heavy Lift Vessels (HLVs).
In addition to container vessels that are commonly used for shipping, there is another class of vessels known as bulk cargo carriers. These vessels carry loose items in bulk quantity without compartmentalizing them in a manner like container vessels.
One such type of cargo vessel is the Supramax class of ships.
They are considered to be on the relatively smaller side of bulk carriers but are still commonly found deployed around the world.
In this article, we will delve into the world of bulk cargo carriers, and in specific- the Supramax class.
We will analyze the design, construction, and safety issues of these vessels.
Bulk Cargo Carriers
Bulk cargo refers to goods and items transported in a loose and unpacked manner. To understand what this means, consider the container class of ships. Container vessels measure their goods in terms of units that are carried. This could be in crates, pallets, boxes etc.
A major specification with container transported goods is that they need to be maintained at specific conditions throughout the journey. For instance, a crate of beverage cans cannot be manhandled, as this may cause them to rapidly depressurize and eventually vent gas while in transit. So, they are stored in pallets inside containers.
Similarly, fresh foods and vegetables cannot be simply kept in a loose condition inside a container, as they will suffer from bruising and abrasions while being shipped. They are stored inside specialized boxes with a protective lining along the edges. Thus, container vessels transport goods that have to be stored in a stable condition.
So, the question arises, how do we transport an uncountable commodity- say rice? Rice cannot be measured in terms of the number of individual grains. Instead, we use a system of weights to compute the quantity being transported.
If we decided to transport rice in a container, loading and unloading would be a nightmare. Grains would lodge themselves in every corner and might shift during the journey. This would make opening the container doors at the destination a hazard.
Instead of wasting plastic to package rice into more manageable units, it is easier to use a class of vessels known as bulk carriers, that can store large quantities of loose goods. They are commonly used to transport materials such as loose grains, ore, cement etc. The goods are stored in hatches present below the deck. Operations are carried out using a system of cranes present on the upper deck.
Bulk cargo carriers are divided into many size classes depending on their size and routes. These include Aframax, Chinamax, Panamax, QMax (stands for Qatar-max size), and Suezmax that are named on the basis of the regions that they are allowed to travel through based on dimension restrictions.
On the basis of nominal sizes, there are types such as – Handymax and Supramax.
Supramax Cargo Vessels
The term Supramax and Handymax are often used in close relation, but both have key differences. The Handymax class constitutes vessels of tonnage 35,000 to 50,000 DWT, while the Supramax class ranges from 50,000 to 60,000 DWT.
To note, deadweight tonnage is a term used to describe the weight of the total load a ship can carry, which does not include the ship and machinery weight. Instead, it is made up of the cargo, fuel, ballast water etc.
The Supramax vessels are popular due to their relatively small size. They are favourable for docking at ports across the world and can range between 150 to 200 meters in length. These vessels are one of the most common ships of the size classification above 10,000 DWT. Their size allows them to cross through most canals and locks that would restrict other ships.
For instance, the Panama canal and Suez canal have very strict restrictions on the size of ships that can travel through them. This has in fact given rise to a class of vessels known as New-Panamax and Suezmax.
Supramax vessels face no difficulty in crossing these straits and canals. It is interesting to note that even the Handymax ships are being modified to carry up to 55,000 DWT of goods. This is due to the ever-popular demand for Supramax vessels. Although the demand exists for such vessels, they take a long time to manufacture.
Due to the multiple holds present on these vessels, different types of cargo can be carried. For instance, it is common to have 3 to 4 holds filled with ore, while the remaining holds contain grains.
Based on the route and logistical demands, some holds may be unloaded at a port along the journey. These empty holds are then filled with another commodity that needs to be unloaded at the final destination.
This versatility of Supramax vessels makes it favourable for shipping across the world. In fact, due to the high demand for such ships, the majority of Handymax vessels are modified to fit Supramax specifications. In fact, nearly 80% of the global Supramax fleet consists of modified Handymax carriers.
During the economic boom of 2008, Handymax vessels were increasingly manufactured in shipyards around Japan and Korea. The cost to manufacture such a vessel was nearly 40 million USD at that time.
Following the various economic collapses in the succeeding decades, the price has somewhat decreased, with the cost of a Supramax or Handymax vessel at 25-30 million USD currently.
Design and Layout Of Supramax Vessels
Starting from the upper deck, a superstructure is commonly located at the stern. This structure houses crew quarters, machinery, equipment, and the bridge. Conventional superstructures on Supramax vessels range between 40 to 50 feet in terms of height. The remaining area of the upper deck has hatch covers and deck cranes.
Generally, there are 5 watertight hatches present on conventional ships. The hatch covers are operated using hydraulic systems that run along the edge of the upper deck. To improve strength along with the hatch opening, high scantling members are often added for support.
There are usually four cranes present on the upper deck. They help in the loading and unloading of certain types of goods. This is very useful when a Supramax vessel berths at a port without fully developed crane facilities.
In such cases, the vessel has a faster turnaround time. The average lifting capacity of cranes on Supramax vessels is nearly 30 tons. The cranes used are often of the double-articulating type.
Regarding dimensions, Supramax carriers are around 150-200 meters long and have a beam (a term for breadth) of nearly 30 meters. The total depth of the hull ranges between 18 to 20 meters. In most cases, there are restrictions on the maximum permissible draft values for these vessels.
On larger vessels, it can be around 13 meters, while smaller variants cap the draft at 10 to 12 meters. The current specification of a modern Supramax vessel is as follows- overall length at 199 meters, beam at 32 meters, draft at 12.2 meters, and depth at 18.8 meters.
The machinery section is mainly placed in the engine compartment located below the primary superstructure. The engine and propeller shafts are housed in here and are placed on specialized platforms built to absorb shock and vibrations. The most common engine is a twin-stroke marine diesel engine that works in tandem with a fixed-pitch propeller.
Modern variants of smaller ships use two four-stroke engines that can operate variable pitch propellers. Various other auxiliary machines are placed in and around this region. Hydraulics and other systems generally run along the length of the ship, within the hull side shell. At the bow, provisions are made for the anchor machinery.
Let us now analyze the structure of the holds and the hull. The vessel features a double bottom that increases the torsional stiffness of the entire ship. It also prevents leakage of gaseous and liquid cargo in the event of an accident. Most vessels have only a single hull that is supported by the bulkhead framing.
However, on vessels with fewer holds, it is advisable to have a side shell, which increases both the stiffness of the vessel. The easiest form of construction is a single hull structure. Similarly, the bulbous bow at the fore is often replaced with a vertical region that cuts through the water.
To reduce wave resistance, the remaining area of the hull is modelled effectively. The block coefficient that is used to measure the fullness of a vessel, is very high for bulk carriers.
Although this forces them to be slow while sailing, they carry a large amount of cargo that makes up in terms of efficiency.
Another key feature of bulk carriers is the inwardly sloped hulls present near the bilge. This prevents the accumulation of substances and also allows for ballast water storage in the void present near the bilge.
Operation of Supramax Vessels
The operation of Supramax vessels can vary depending on their primary route and cargo type. For instance, the crew of such ships can number anywhere from 10 to nearly 30 individuals.
On the larger variants, maintenance and repair work often require a large number of working people. On the other hand, small ships can function with just 10 crew members. The number of officers is usually kept under 5, including the chief engineer, captain, and deck officers.
The route followed by most Supramax vessels depends on supply-demand economics. For instance, a vessel carrying ore in the forward journey might transport grain during the return voyage. Since these vessels can pass through most locks and canals, they have no restriction on the basis of the main path to traverse.
To cross from the Atlantic to the Pacific Ocean, it would be feasible to use the Panama Canal as opposed to sailing around South America. Similarly, a Supramax vessel would take the Suez Canal route across Europe and Asia, instead of travelling around the Cape of Good Hope.
Another important factor in deciding ship routes is the turnaround time. Bulk carriers are notorious for spending a large amount of time in the loading and unloading process. Compared to the 12-hour turnaround times for container vessels, a Supramax vessel can take up to 60 hours at a port. With deck cranes pressed into operation, the time does reduce slightly but is still considered to be among the highest in the shipping industry.
The most important operation of any bulk vessel is the loading and unloading operations. Particularly for such vessels, these operations can be dangerous, as there is always a probability of the vessel capsizing or breaking in half. Depending on the type of cargo, a number of loading methods can be implemented. In the case of grain, a large gantry bin or bucket is suspended over each individual hatch opening.
The bottom of the bin is hinged such that is can swing open and release its load. Once the bucket is empty, it heads back to the pier to be reloaded. For other types of cargo, gantry and deck cranes place the goods inside the hold. The cargo is then positioned using bulldozers lowered into the holds. Once placed correctly, the cargo is restrained so that it does not shift during the voyage.
Next, let us analyze the process of unloading cargo from a Supramax vessel. For this, a combination of cranes and other machinery are used. Cranes are able to lift certain types of cargo and then transfer them to the pier. For other types of goods, conveyor belt systems are available. These belts can transport anywhere between 500 to 700 tons of cargo in an hour. However, advances in shipping technology have allowed for rates of up to 16,000 tons per hour to be achieved.
Most ships use a self-discharging mechanism with their own conveyor belts. Incorrectly unloading a vessel can cause damage to both the vessel and the pier. A major problem that is encountered is the shifting of cargo when the hold is partially empty. This can cause the vessel to tip over and capsize.
If the load shifts between the fore and stern, the vessel may even split in the middle. To prevent this from happening, ballasting must be correctly completed. Also, longitudinal bulkheads reduce the amount of shift that can occur.
Once the ship has been emptied, it is imperative to clean the holds thoroughly. This prevents any form of contamination and corrosion from occurring. For this purpose, high power water jets are generally used.
Chemicals are also dissolved in the water to sterilize the holds. Due to the sheer size of the holds and bulkheads, this process can be time-consuming. However, there is no alternative technology at present to shorten the turnaround time.
Safety Issues Faced By Supramax Carrier
Supramax carriers face an issue of cargo shifting during the course of the journey. This causes a hydrostatic problem known as the Free Surface Effect (FSE). When goods move towards one particular side during a voyage, it shifts the centre of gravity away from the centerline of the vessel.
This forces the vessel to dangerously heel to one direction, resulting in the vessel taking on water and eventually capsizing. This effect was the cause of several ship disasters in the late 1990s. To reduce the relative severity of the FSE, longitudinal bulkheads are built into the vessel.
These are in addition to the transverse bulkheads that are used to separate holds on a ship. Longitudinal separations are shown to reduce the FSE in the transverse direction by an inverse square relation. Transverse bulkheads are useful in dividing the holds into compartments and plays a role in flooding control. It also serves to prevent cross-contamination between the entire shipment load in case of an issue in one particular hold.
Another major problem with Supramax carriers is the structural integrity of the hull. In an attempt to reduce the weight of the vessel, single hull structures are often used, which cannot withstand the combined effects of corrosion and torsion. Similarly, hatches that are not watertight can result in flooding.
Lastly, longer Supramax vessels face the problem of hogging and sagging that rapidly flexes the hull. In case the structural integrity is compromised, water may be allowed to enter a hold at the fore. As this water builds up, it exerts enormous pressure on the wall to the neighbouring holds.
With time, this compromises the metal strength and may result in a leak. As the holds rapidly fill with water, the vessel gradually loses buoyancy and sinks. To prevent this from happening, safety precautions must be kept in mind during the design and operation phases. In addition, inferior quality metals must not be used, as they accelerate the corrosion process.
Supramax Cargo Vessels are a class of bulk cargo carriers that are used for the transport of unpacked and loose goods such as ores, cement, grains, steel pipes etc. Although they are considered to be one of the smallest bulk cargo carriers in service, they remain extremely popular. This in part due to their small size that allows them to berth at nearly every port around the world. In addition, their design with deck cranes makes the loading and unloading process considerably quicker.
These vessels divide their holds using both longitudinal and transverse bulkheads. The transverse bulkheads aid in restricting flooding, contamination etc. while longitudinal bulkheads mainly aid in reducing the Free Surface Effect (FSE) that is known for making a vessel unstable in poor weather conditions.
For the loading and unloading process, deck cranes aid in lowering and lifting large structures such as pipes, crates etc. For loose goods such as grains, gantry bins or suction pumps are used for this operation. Large deck hatches are present to access the cargo. During the voyage, they remain closed to prevent any sort of contamination. To prevent accumulation of loose goods in any inaccessible location inside the holds, the side shell is sloped near the bilge. In addition, the holds are provided with restraints to ensure that goods do not roll over during the journey.
The Supramax vessels have supplemented the fleets of several renowned shipping companies. They ply all across the globe, but are mainly manufactured in and around Japan and Korea. Despite their small size, they continue to dominate the shipping industry mainly owing to their versatility.
Disclaimer: The authors’ views expressed in this article do not necessarily reflect the views of Marine Insight. Data and charts, if used, in the article have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or recommendation on any course of action to be followed by the reader.
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Ajay Menon is a graduate of the Indian Institute of Technology, Kharagpur, with an integrated major in Ocean Engineering and Naval Architecture. Besides writing, he balances chess and works out tunes on his keyboard during his free time.