Understanding Heavy Lift Vessels (HLVs): Design, Operation And Types
Ships are massive structures capable of carrying enormous loads. They move trillions of dollars’ worth of goods across the entire globe.
But what happens when you want to shift cargo that is considerably heavy, sometimes too heavy for even conventional ships?
Moving such cargo could be for a variety of reasons- from shipping heavy, dense goods that cannot be borne on normal carriers, to transporting other ships across the world. And this is where Heavy Lifting Vessels (HLVs) comes into place.
These gigantic vessels can lift and ship just about any load in the world, from massive carriers to large semi-submersible oil rigs.
What makes them different from conventional heavy-duty carriers is their ability to handle loads that would otherwise have compromised the structural integrity of a ship. They perform these difficult tasks by using innovative techniques in engineering and ship design.
In this article, we will delve into the world of heavy lifting vessels and cranes, and the science behind their enormous lifting prowess.
Types of Heavy Lift Vessels
Broadly categorizing heavy lifting vessels (HLVs), they are of four main classes –
- semi-submersible vessels,
- dock ships,
- open deck cargo ships,
- and project cargo carriers.
These different classes of vessels use widely contrasting techniques in lifting their large loads.
These are classes of ships that are able to use ballast water to increase their draft. This lowers the ship deeper into the water until the upper deck is submerged a few metres below the water’s surface. Hence the name “semi-submersible”.
The principle behind such ships is that floating cargo and goods can be loaded onto the submerged deck by floating them directly over the resting area. Once they are correctly positioned, the ballast water is pumped out until the deck of the vessel rises above the water. With the cargo now resting on the deck, the vessel is able to safely move this load.
This type of ships is common for a few specific types of cargo. This includes oil and drill rigs, oil processing plants, floating factories, equipment used for dredging purposes, offshore structures, floating drydocks, and other ships that can fit on board.
The construction, build and design of this class of vessels is considerably different from that of conventional ships. For instance, the superstructures present on the deck are located at the extreme fore and stern.
The bridge, living quarters, offices and other areas used by the crew are located at the fore. This provides the captain and officers in the bridge with a clear view of the surrounding area. This also proves useful during manoeuvres and loading operations.
At the other extreme end, the machinery used for the operations of the ship is located. For instance, some types of semi-submersibles have their ballast pumps located in this region, while others have their marine engines built into this area.
Each has its own advantage depending on the design idea and purpose of the ship. The deck of the ship that extends between both the superstructures is long, wide and perfectly flat. Since enormous amounts of load are exerted on it, this deck is not a single continuous sheet of metal.
Instead, several small panels are welded into place and are supported with an intricate webbing and framing system below the deck.
On average, the height of the hull from the keel to the upper deck is relatively low for these ships. This allows for a stronger design of the deck and ensures structural integrity.
Loading and Unloading Process
Now, let us analyse the loading and unloading process used to transfer floating cargo on to these HLVs. The first step in the process is the lowering of the upper deck below the water’s surface until it reaches a safe and pre-determined depth.
The basis on which this depth is calculated includes the depth of water in that particular region, the type and weight of cargo, the draft of the cargo, and the ballast levels of the HLV.
Once this depth has been calculated, the ballast tanks of the vessel are rapidly filled using a series of ballast pumps located on the lower side of the hull. The ballast tanks serve the purpose of loading seawater on to specific chambers in a ship that sinks it into the water.
On these semi-submersibles, the tanks are generally located under the deck, in between the overall framing and support members. Ballast tanks must be filled to the same levels during this process, such that the keel and upper deck of the HLV remain horizontal. If this is not taken care of, there is a chance that the cargo which is loaded may be lopsided, slide off, and cause damage to both itself and the vessel.
Once the ballast tanks have been filled to the required level, it is now time for the loading process to begin. Because of their design, only floating cargo can be carried on these vessels. For the most part, tug boats and barges are used to correctly position the cargo about the upper deck.
Once arranged above the supports, the connections to the tug boat are released, and the HLV gradually begins to pump out ballast water. It does not completely rise out of the water, as this places an enormous strain on the vessel. Instead, it merely raises itself above the waterline, such that it can safely transport the cargo without any water on the deck.
One reason for not transporting cargo in a submerged mode is because it increases the waterplane area (an important hydrodynamic coefficient) that increases the resistance faced by the vessel.
Understand similar operation in this video:
Dock ships are similar in design and functionality to the semi-submersible class of HLVs. However, it differs in one main region- it has large side panels that act as a reservoir for water, on the port and starboard sides of the ship.
The fore is generally where the primary superstructure is located. This region houses the bridge, quarters, galley etc. The stern area is usually left open for the cargo to enter.
To picture how a Dock ship appears, imagine a floating open-top box that is able to lower itself in the water, and is missing one face which serves as a gate.
The operation of dock ships is divided into three phases similar to the semi-submersible class-
- lowering itself,
- floating on of cargo, and
- lifting back up.
The lowering process is relatively simple, where the ship fills its ballast tanks until it begins to sink into the water. However, unlike semi-submersibles that prevent the water from accumulating on the deck, the walls of the dock retain water once the deck sinks below the surface.
Once it reaches a sufficient depth, the cargo is floated on to the deck. The common types of cargo carried on dock ships mainly include other ships. These ships are carried either for transport or for repair purposes.
Once the cargo is towed into the containment region, the tug boat gradually reverses and exits the dock. The ship then begins to rapidly pump out its ballast water and remove water present on the deck. This continues until the vessel is at a sufficient height above the water.
The process of removing the ballast water is extremely important and must be carried out with the utmost care. This is because ships lose buoyancy as their draft decreases.
Unlike semi-submersibles on which ships can be anchored, docks can only implement such provisions once the water has been drained out. Thus, supports often made of large wooden blocks are placed against the hull at regular intervals. Once the water level decreases, the ship is attached to the walls and floor of the HLV.
Dock ships are commonly used for repair purposes. These ships pose an alternative to using dry docks built on the land. This is because they do not encounter the high costs required to maintain dry docks.
Common costs incurred on land include repair to the floor and walls, proofing of the dock gates, maintaining the keel blocks on which the keel of the ship rests, reinforcing the walls of the dock etc. In addition, ships are required to come to the dry dock, while dock ships can often be used to repair ships that cannot move. Also, they can conduct repairs and maintain equipment while the ship is still in transit.
The main repairs on ship docks are the replacement of the pumps and the keel blocks. These docks also often have derricks and spreader cranes located on the walls. These aid in the maintenance process by allowing large objects to be easily lifted and moved.
The advantages of using ship docks in place of conventional dockyards are that they can be deployed in any part of the world and are easily customizable. For instance, multiple docks can be floated longitudinally and then attached in place. This is used to accommodate ships of varying lengths in a safe manner.
In addition to these advantages, these docks are built so that they rely mainly on green water on the deck, rather than ballast tanks. This removes several hassles, such as the need to regularly service and clean ballast tanks. The only component that must be repaired in this regard is the pumps.
One important point to keep in mind is that these ships sail when they are emptied of any water present on the deck. This is because any additional weight on the deck can have adverse effects on the hydrostatics and resistance faced by the hull.
Open Deck Cargo Ships
Unlike semi-submersibles that require their cargo to be floated into position, open deck ships simply require the cargo to be shifted on, either using cranes present on the deck or driven up in a fashion similar to ro-ro carriers (roll on- roll off).
Open deck ships have a large flat deck that can span anywhere between 100 to 300 meters. The superstructure housing the bridge, galley and cabins is located at the extreme fore of the ship. There are no walls unlike the structure of dock ships, that allow for cargo wider than the ship to be loaded onboard. These cargo ships are different in design and build to the other heavy lifting ships that we have discussed here.
Open deck cargo ships are used to transport large, heavy structures such as cranes, large trucks, construction equipment, yachts and small boats. The main condition on the type of cargo that is shipped is that they can be driven onto the open deck, either under their own power or using cranes and tow trucks. Thus, large ships cannot be transported through this class of HLVs.
The structure of these cargo ships must be sufficiently reinforced to handle the enormous loads exerted on the deck. The main deck is supported using a metal framing web that runs along the entire length and width of the ship. The vessel has separate regions under this deck to house the ballast tanks and marine propeller shafts.
The ship’s engines are located either under this deck or at the base of the superstructure located at the fore. The issue with having the marine engines at the bow is that the marine shafts have to be extremely long. This can result in damages caused due to the catenary action of the shaft, or due to the high-speed vibrations of the engine.
Thus, it is preferable to locate the engines as close to the propellers as possible. The upper deck is constructed of several sheets of metal welded together to form plate-like structures. Thus, when the need arises, various sections of the ship can be easily accessed without having to remove the entire deck.
The deck usually has provisions for docking and tethering, so that the cargo can be transported safely. Since there are no protective sidewalls, these features play an important role. Pad-eyes are common, as they allow multiple structures to be linked directly to the deck.
The stern of the ship features a large ramp that spans the beam of the ship and is constructed of reinforced material. This ramp allows for the movement of vehicles and goods to and from the main deck. Occasionally, large ships may also have smaller ramps on the port and starboard sides that allows for multiple cargo loading. These HLVs are commonly classified as Ro-Ro carriers for heavy-duty cargo.
Watch now ferries are transported using Heavy Lift Vessels:
Project Cargo Carriers
Project cargo is a specific term referring to large and heavy goods, usually high valued or critical to some industry. They are unwieldy and cannot be carried in intermodal containers nor can they float or drive themselves up ramps. For such a scenario, cranes have to be used to help load and unload the goods.
Unfortunately, most cranes at the port are not able to properly move these goods due to their position. It would be considerably easier if the ship had its own set of cranes that could lift these types of cargo. And that is the function of project cargo carriers. They serve to transport large goods by lifting them up with cranes built on to the deck of the ship.
The design of these ships often depends on the type of goods that they intend to carry. For instance, there are often supports provided for balancing pipeline and bridge sections on the deck.
For the most part, the deck is flat with no protrusions. These ships sail at a considerably lower draft than most HLVs. This is because they usually have to lower their cargo into holds beneath the deck. To aid in the loading and unloading processes, two large marine cranes are usually present. These are positioned at the fore and stern, and usually on either the port or starboard side.
This tends to make the ship more unstable as the weight is concentrated on a particular side. However, unless the cranes are together, it will be impossible to lift long cargo that requires support from both ends. To balance the weights of the cranes, the ballasting process is very important and must be executed properly to ensure that the vessel is stable.
The side hull of the vessel is reinforced so that the cargo cranes are properly supported, and to provide torsional stiffness to the vessel. Some types of project cargo carriers also have a flat continuous deck on which the cargo must be stored. Such vessels keep a high draft so as to ensure proper stability.
However, in the event of some accident, there is a high chance of the cargo attempting to roll over the deck. This can cause a sudden imbalance that may capsize the vessel. Thus, proper care has to be taken while securing the cargo to the deck. Heavy-duty pad-eyes are commonly used, while blocks present at both sides prevent the cargo from rolling over during turbulent weather conditions.
Project cargo ships are generally smaller than other HLVs, spanning only a few hundred metres in length. They make up for this by being to carry extremely heavy cargo that may be difficult to load on other HLVs. Usually, project cargo includes large automobile parts, sections of industrial units, bridge and crane sections, and pipelines.
The cranes work in tandem, being able to lift enormous loads with nearly four times the pulling power of conventional deck cranes. However, the ballasting process plays a crucial role in stabilizing the entire vessel. These ships play an important role in the transport of heavy-duty project cargo across the globe.
The Bottom Line
Heavy Lifting Vessels (HLVs) are enormous vessels capable of carrying other ships, large industrial units, floating plants etc. They carry cargo that other types of ships cannot transport, and this is accomplished in four main ways.
The important thing to keep in mind is that proper balance and ballasting is extremely crucial in ensuring the stability and balance of these types of ship. They work together with port facilities to carefully load ad unload their high-value cargo.
They represent some of the most heavy-duty structures to be used on the face of the earth and act as an integral part of trade and commerce. Without these vessels, it would be near impossible to supplement countries with important and essential commodities and goods.
And now, over to you. Do you have any interesting observations about these HLVs? Do let us know in the comments section below…
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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.