Understanding Design Of Bulk Carriers

One of the oldest types of ships plying at the seas are the ones that carry unpackaged bulk cargo like coal, grains, iron ore, sulphur, etc. The design of bulk carriers depends on a lot of factors. Over the time, the type of cargo carried by bulk carriers have increased, and economic forces have caused a growth in the number, types, and size of such ships. Today, SOLAS (International Convention for Safety of Life at Sea) define a bulk carrier as ‘a ship constructed with a single deck, top side tanks and hopper side tanks in cargo spaces and intended to primarily carry dry cargo in bulk; an ore carrier; or a combination carrier.’

Though the term bulk carrier encompasses all the ships that carry cargo in bulk, solid and liquid, both. But the ones that carry liquid cargo in bulk are further categorised as oil carriers and liquefied gas carriers. Hence, the term bulk carrier is generally used to refer to ships that carry dry cargo in bulk. In this article, we will look at all the different types of dry bulk carriers based on their size, type of cargo, and structural arrangements. We will study the general arrangement of a typical bulk carrier, and its unique design features. The article will support each design feature with the reason behind it, which is something a ship designer should be very well versed with.

Types of Bulk Carriers:

Dry bulk carriers are categorised on different aspects like size, type of cargo, and structural properties. While considering design of bulk carrier, it is important for a designer to know them well because a client would use these terminologies in the technical contract to specify the size, type of cargo. It is based on these inputs that the final particulars are fixed after various iterations and feasibility studies.

According to Size:

• Handysize: 10,000 DWT to 30,000 DWT
• Handymax: 35,000 DWT to 60,000 DWT
• Panamax: 60,000 DWT to 80,000 DWT
• Capesize: 80,000 DWT and over.

Handysize and Handymax size bulkers comprise of around 70 percent of the total dry bulker fleet. This size of bulk carriers are seeing the maximum growth rate in today’s market, as shipping companies prefer smaller sizes due to many restrictions put on ships of larger size. Ships of these size are also able to access most of the ports and canals, which increases their scope of trade-making abilities.

According to Structural Properties:

• Single Bottom Bulk Carrier: These type of bulk carrier ships do not have a double bottom, and the only barrier between the sea and the cargo is the outer bottom plate. Bulk carriers under 120 m length do not require a double bottom as per structural requirements, but today, ship designers still prefer to avoid single bottoms in order to prevent contact of cargo with sea water in case of structural damages.

• Double Bottom-Single Hull Bulk Carrier: These ships have a single hull, but are provided with a double bottom or a tank top throughout its length (from aft of the forward collision bulkhead to the aft peak bulkhead). The double bottom spaces are used for storage of ballast and duct keel for passage of pipelines.

• Double Hull Bulk Carrier: The use of double hull in bulk carrier designs have increased rapidly over the last ten years. The wing tanks at the sides are an added advantage, and provide more marginal ballast, and better control on the stability of the ship.

Based on the type of cargo carried and the size, there are some commercial terms used for nomenclature of bulk carriers, as discussed below:

OBO Carriers: Ore-Bulk-Oil Carriers have holds such arranged that they can carry ore, solid dry bulk, and oil in the same voyage, without interference between each cargo type. We will discuss the design of these bulk carriers in a later stage of this article.

O/O Carriers: Ore and Oil carriers can carry a combination of ore and oil in the same voyage. These vessels need to comply with special codes that are to be followed for containment and transportation of oil at sea.

VLOC: Very Large Ore Carrier.

VLBC: Very Large Bulk Carrier.

ULOC: Ultra Large Ore Carrier.

ULBC: Ultra Large Bulk Carrier.

We will now look into the design of bulk carriers which include the hullform, general arrangement of different types of bulk carriers based on their functionality, structural design and arrangement of bulk carriers and how they vary with the type of cargo. One unique feature we will look into, is the various types of loading sequences followed in order to maintain the structural loads within limits, and how these functional factors are incorporated in the design process.

Design Aspects of Bulk Carriers:

Hullform:

The characteristic feature of a bulk carrier’s hull geometry is its high coefficient of buoyancy. In other words, it has a full-form, which means when compared to finer form ships like container ships or naval warships, the volume of the hull at the forward and aft sections is higher in case of a bulk carrier.

The aforementioned is very clearly notable in figure above. As can be seen, a bulk carrier has a long parallel mid-body extended over most part of its length forward and abaft midship. This is to enable maximum volume within the hull for maximizing the usable cargo carrying volume within the holds. The blunt shape of the bow is actually a result of the long parallel mid body. The coefficient of buoyancy of bulk carriers range from 0.7 to 0.8.

Though bulk carriers fall under the category of Type A according to International Load Line Convention for Ships, they have low freeboard in loaded condition, that is, these are ships with high loaded draft. This makes it important for the ship designer to calculate the ballast capacity accordingly so that the ballast draft achieved in fully ballasted condition is not significantly different from the fully loaded draft. Attaining an optimised draft in all loading conditions is necessary because maximum hydrodynamic efficiency is achieved at these drafts. The ballast capacity of bulk carriers are usually high so as to achieve propeller immersion.

All bulk carriers today have a transom stern and a bulb shaped aft below the waterline which allows undisturbed streamlined flow onto the propeller disc and maximises the propeller efficiency. The design speed of most bulkers range from 12 to 18 knots.

General Arrangement:

The following figures show the general arrangement (Profile view, Plan view, and Midship Plan) of a bulk carrier. A common dry bulk carrier has a clear main deck with the machinery room and superstructure. Hatches with unrestricted access to holds are designed on the main deck with steel hatch covers to facilitate easy loading and discharge of cargo.

Most bulk carriers have deck mounted cranes that have 360 degree access and can load and discharge cargo from the holds immediately forward and aft. We will look into the ballast spaces when we study the midship section of bulk carriers. The just forward of the forward collision bulkhead is used for chain locker. The fore peak tank in the bulbous bow is used to control and maintain the trim of the vessel and to ensure complete propeller immersion.

Usually, cylindrical type bulbous bows are used for bulk carriers. That is because most of the volume in these bulbs is concentrated at the center of the bulb, hence providing optimum efficiency at both, fully loaded and 100 percent ballast conditions.

Structural Design:

Midship Section:

Single Hull Double Bottom Bulk Carrier: The drawing that makes for unique identification for a ship’s type, functionality, and structural arrangement is the midship section drawing. In order to be able to design a midship section, a designer should be able to read a midship section drawing and understand why each component is present where it is, and how is the scantling of each component decided. We will now look into the midship sections of different types of bulk carriers and discuss the functional aspects along with.

The figure below shows the midship section of a typical single-hull bulk carrier with a double bottom. Ducted keels are generally preferred in such ships as they provide enough space for the passage of pipelines. Some designers also prefer to call it a pipe tunnel. But its structural purpose is to transmit the weight of the ship to the keel blocks when dry-docked. The thickness of the duct keel plates is higher than the adjacent plating by at least 5 mm.

Bulk carriers are longitudinally stiffened, that is the stiffeners on the plates run in a longitudinal direction. This is because the loads coming onto a bulk carrier (which we will discuss at a later stage) cause longitudinal bending moments. Hence the failure of the hull girder due to longitudinal bending becomes more probable.

The tank top plating and outer bottom plating are held together by a vertical plate structure called the plate floor. Note here that though the term uses the word ‘floor’, it is actually a vertical structure. The circular perforations on the plate floor are provided for reducing the weight of the structure. They are called lightening holes. The oval perforations are manholes that provide human access for maintenance operations.

The bottom corner of the section is provided with angular tanks running along the length of the ship. These are called hopper tanks, and the angular plating that covers them is called hopper tank plate or hopper tank sloping bulkhead. These are also stiffened longitudinally by steel profiles. The most common profile used in stiffening hull girder plates is bulb and L sections. Bulb sections are more preferable as they have more moment of inertia for the same cross-sectional area as compared to L sections. Hopper tanks are used for storage of water ballast.

The hopper tank section is stiffened by a heavy web as shown above. These web sections are provided at frames having plate floors, and these are repeated after every three to four frames. The stiffeners at the bilge and the hopper plate are welded to the web frame. The web plate is further stiffened by flat bar stiffeners to prevent distortion.

The topside corner is also provided with a sloping bulkhead, and space is used for either storing another type of cargo or for storage of water ballast. These tanks are called upper wing tanks. The main purpose of providing the wing tank sloping bulkhead is however different. Bulk cargo when stored always forms a slope on the surface. The angle of this slope is called the angle of repose. If the angle of the surface of the heap increases, the material shifts until the angle of repose is attained. In order to prevent the angle of the surface from exceeding the angle of repost, the wing tank sloping bulkhead is provided at such an angle so that the above phenomenon is prevented. Hence, in case of bulk carriers carrying more than a single type of cargo in multiple voyages, the angle of the topside sloping bulkhead is determined by the cargo having the minimum angle of repose. This prevents cargo from shifting in the transverse direction and does not result in listing during transit.

The side shell plate is stiffened transversely instead of longitudinally, by a side frame which is usually an L section. The side frame is connected to the hopper tank plating and wing tank plating by brackets. The brackets are also flanged at free ends and are provided for proper stress flow from the plates to the web frames.

Sheer strake is the plate that connects the deck plating and the side shell. The thickness of sheer strakes is usually at least 3 mm more than the deck plate or side shell (whichever is more). This is because a lot of stress concentration occurs in this region. Hence higher plate thickness is required to keep the stress levels within design limits.

A hatch coaming is provided around the hatch opening. The height and thickness of the hatch opening above the main deck is determined by the rules prescribed by classification societies. The coaming is usually supported by brackets that maintain stress flow from the coaming to the deck plate.

Double Hull Bulk Carrier: Many companies prefer double-hull designs today, not only because of the increased safety of cargo containment but also due to increased stability of these designs. Figure 4 shows the midship section of a double hull or double skin bulk carrier.

The space within the outer and inner skin is strengthened by transverse frames and longitudinal stiffeners running along the ship’s length. Stringers (Shown as Stringers 1, 2 and 3 in the figure) is used to decrease the span and depth of the transverse frames. They are also used to divide the double hull space into multiple wing tanks.

Since the wing tanks at various heights are used as ballast spaces, these designs do not require hopper tanks, therefore increasing the space for cargo containment. However, topside sloping tanks are provided to prevent cargo shifting. The left side of the above figure shows an ordinary frame without plate floors. At intervals of every three to four such frames, web frames (right half of Figure 4) are provided with plate floors or solid floors. Pipe ducts are provided within the duct keel and uppermost part of the double hull space for carrying oils and ballast water.

Ore Bulk Oil (OBO) and Ore Oil Carriers: Hybrid bulk carrier designs have been developed to carry oil, dry bulk cargo, and ores in a single voyage. The midship section of an Ore/Oil carrier is shown in Figure 5. The ship is transversely divided into sections, where the section at the centre is used to carry ore, and the tanks at the sides and bottom are used to carry oil. If you, however, look at the arrangement of the ore hold, you would notice it is positioned at the top. Why do you think this is done?

It is because, most ore carriers carry iron ore, which has high density compared to most other dry bulk cargo. Due to this, the weight to volume ratio of ore is much higher than other dry bulk cargo. Hence, placing the ore hold at a lower position would lower the vertical centre of gravity of the ship to such an extent that the ship would become too stable or stiff. This would cause the ship to experience rapid rolling motions, causing cargo shift and motion sickness to the crew.

Figure 6 shows the midship section of an ore carrier. The notable feature of an ore carrier is its high double bottom. Now that we have discussed why the ore holds in ore/oil carriers are placed at higher vertical levels, it would be easy for you to understand the reason behind providing high double bottoms for these ships.

Alternate Hold Loading:

Alternate hold loading is a common technique practised by bulk carriers in case of iron ore transportation or in cases when the amount of cargo to be transported is not sufficient to fill all the cargo holds to their full capacity. Such a loading arrangement is shown in Figure 7.

The advantage of this loading system is that it enables equal distribution of cargo weight in the forward and aft holds of the ship, hence preventing any unwanted trim due to longitudinally concentrated cargo. It also allows the cargo holds to be either fully empty or filled to full capacity so as to prevent any shifting of cargo due to ship motions.

The only disadvantage is that the alternate loading pattern leads to increased shear forces at the transverse bulkheads. In order to prevent the shear forces from exceeding safe limits of the bulkhead material, suitable alternate ballasting is done in the wing tanks of the empty holds. Ships that are designed for alternate hold loading are also provided with increased bulkhead plating thicknesses to maintain shear stresses below ultimate levels.

Bulkheads:

All bulk carriers are provided with transverse watertight bulkheads between holds that divide the ship into watertight compartments. The secondary purpose of these bulkheads is to provide additional transverse strength to the ship structure.

The most common type of bulkhead used today is the corrugated transverse bulkhead, as illustrated in Figure 8. Corrugations eliminate the need for the bulkhead plating to be additionally stiffened and also have a higher strength to weight ratio than typical stiffened bulkhead panels. The base of the bulkhead is provided with a sloping plate called the shredder plate (shown in Figure 9), and the corrugated bulkhead is mounted on a bulkhead stool which transfers the weight onto the solid plate floors below. The shredder plate prevents the accumulation of cargo at the base of the corrugations.

Types of Hatch Covers:

There are different types of watertight hatch covers used in bulk carriers, and a general knowledge of functionality and usability of each type is necessary for a designer to make the right choice of hatch cover for a particular bulk carrier.

• Fore and Aft Single Pull Hatch Cover: These are the most common type of hatch covers used in small scale bulkers. They consist of a series of plates comprising the hatch cover. The plates at the center of the hatch opening (leading plate) is driven by a chain pulley or by hydraulic motors, such that the cover plates behind the leading plate are vertically stowed open.
• Folding Hatch Cover: These hatch covers consist of a series of cover plates that can be lifted into folded condition by on board deck cranes, or the leading plate can be pushed back by hydraulic actuators and the remaining plates are hence folded into vertical position. These are provided on bulk carriers with less deck space or in cases when the spacing between two holds is minimum. Figure 10 shows a folding hatch cover being opened.

• Piggy Back Hatch Cover: Piggy back hatch covers allow horizontal stowage of the hatch cover forward and aft of the hatch opening. These are provided in longer bulk carriers, where spacing between holds are sufficient. It consists of a panel driven by motor that enables the bottom panel to roll in a piggy-back style with the dumb panel stowed on top of it. Figure 11 shows the schematic diagram of a piggy back hatch cover.

Recent development in bulk carrier designs have focused on improvement of hull geometry to attain better hydrodynamic characteristics. Many design firms have developed models of single hull and double skin bulk carriers keeping in mind advanced cargo containment and waste treatment facilities. The International Association of Classification Societies (IACS) have now developed Common Structural Rules for design of bulk carriers, that are being followed by all classification societies to design the structure of single hull and double hull bulkers. Advancement in design of bulkers are now more focused on size optimization and achieving maximum cargo carrying capacity under given space constraints, which gives a lot of scope not only for naval architects but also for structural engineers and fluid dynamic specialists.

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