bulk carriers structural arrangements

Structural Arrangement of Typical Bulk Carriers

CHALMERS University of Technology

Bulk Carriers

Structural arrangement of typical bulk carriers

Marine Structural Engineering – MMA 167

Autumn Semester 2012

Oscar Lindecrantz [email protected]

Nicolas Iris [email protected]

Jeremy Peter [email protected]



Table of Content

1 INTRODUCTION 3

2 STRUCTURAL ARRANGEMENT 4

2.1 BULKHEADS 4

2.1.1 COLLISION BULKHEAD 5

2.1.2 AFTER PEAK BULKHEAD 5

2.1.3 MACHINERY SPACE BULKHEAD 5

2.1.4 TANK BULKHEAD 6

2.2 FUNCTIONAL PARTS 6

2.2.1 DOUBLE BOTTOM TANKS 6

2.2.2 COFFERDAM 7

2.3 CARGO HOLDS 7

2.3.1 HATCH OPENINGS 8

2.3.2 TOPSIDE & HOPPER TANKS 9

3 STRUCTURAL DESIGN 11

3.1 MATERIALS 11

3.1.1 GRADES OF STEEL AND STRUCTURAL MEMBERS 11

3.1.2 HULL CONSTRUCTION 12

3.2 LIMIT STATES 13

3.2.1 SERVICEABILITY LIMIT STATE 13

3.2.2 ULTIMATE LIMIT STATES 13

3.2.3 FATIGUE LIMIT STATE 14

3.2.4 ACCIDENTAL LIMIT STATE 14

4 FUTURE TRENDS 14

5 BIBLIOGRAPHY 15



1 Introduction

In the shipping industry a bulk commodity is a substance that is traded in large quantities and has

physical character which makes it easy to handle and transport in bulk. Bulk Shipping refers to

carrying cargo by mass and by sea. Transporting cargo in shipload is a strategy that has been around

for millennia. The grain transport in the Roman Empire, the Dutch “fly boats” in the 16th

century or

the tea clippers in 19th

century are examples of the history of bulk shipping. However, the modern

bulk shipping industry has its root in the developing coal trade of the 18th

century between North of

England and London. At first, those bulk coal ships were wooden framed and wind powered. They

evolved in about half a century into iron-hulled, screw-propelled ships that could carry about 600

tons of coal which is more than double the amount of cargo as the previous generation of ships. The

following technical breakthroughs enable the bulk trade to develop more over the past two centuries

than during the past 4000 years. Nowadays, the bulk carriers’ capacity varies from 5,000dwt to

440,000dwt.

Bulk cargo can be divided in three main components which are the liquid bulk, the major dry bulk

and the minor bulks. We will treat here in this paper the dry bulk ships because we feel that the

liquid bulk cargoes are treated as tankers and are then dealt from another perspective. The main dry

bulk cargoes are iron ore, coal, grain, agricultural products, fertilizers, metals, minerals and steel

products. Those cargoes are transported in bulk because it is more economical to ship those goods in

large quantities.

The stake of bulk shipping is to minimize cost by maximizing cargo capacity and increasing the ship

efficiency. It is also very critical for the ship’s profitability to smooth out the cargo handling

operations as much as possible.

In this project we will overview the structural arrangement of a typical dry bulk carrier and we will

explore how the ship is laid out in order to maximize cargo capacity, ease cargo handling while

fulfilling the strength and design requirements.

We will first go over the structural arrangement of the bulk carrier ship by trying to understand the

role of the bulkheads and the functional parts of the ship. We will then go in detail over how the

cargo hold, the most important functional part of the ship, is constructed. There are many structural

elements that take parts into making cargo holds strong, safe and operable. The hatch and the hatch

opening is one more component of the cargo hold that we will look into in order to determine how

important the hatch opening is in regard to the ship’s overall strength.

In a second part, we will view over the structural design of the ship and we will then learn about the

choice of material of different parts of the ship, the four limit states on which the ship is designed

and a last minor part on how to determine the ship’s plates’ thickness accounted for omnipresent

corrosion.



Figure 1 Cape Hawk, a 161,425dwt ore bulk carrier

As a group, we base our research on the DNV’s common structural rules for bulk-carriers with length

90 meters and above dated from July 2012. We feel that this document is the stone base for

structural arrangement and design. We feel confident that the information provided in this

document is reliable.

All in all, it is important to remember that the ship’s design and structural arrangements serve one

purpose and that is to maximize the profitability of the ship while ensuring the seaworthiness and

the safety for all.

2 Structural Arrangement

In this part, we will describe the different bulkheads which will compose the structure of a bulk

carrier. We will explain where they are on the ship and their functional interest.

At the end of this part, we will present the cargo hold and all the different structural elements of a

bulk carrier.

2.1 Bulkheads

Transverse watertight bulkheads are very important to maintain the transverse form of the ship.

They are very strong and acting on the transverse strength of a ship.

All the ships are to have a number of watertight bulkheads to protect it. Bulk carriers are to have at

least the following watertight bulkheads to respect the classification:

- Collision bulkhead

- After peak bulkhead

- Bulkheads forming the boundaries of the machinery



In function of the length of the ship, it can also have additional bulkheads like representing in the

following table.

Table 1

Most of the bulkheads are plates of steel joined and stiffened by vertical and horizontal stiffeners.

But it also exist a new technologies which use corrugated bulkheads, and which avoid to put some

stiffeners on the plate. The aim of these technologies is to have a good resistance with a light weight.

2.1.1 Collision bulkhead

Collision bulkhead is the most strong and forward bulkhead of the ship, which has a very important

safety feature. Its aim is to keep the water out of the hull in case of a collision.

This bulkhead is positioned vertically and transversely. To fulfill its role, this watertight bulkhead has

to be located near the forefoot (between 0,05 and 0,075% of the length of the vessel, with a

maximum of 10 meters from the forward oh the ship), between the bottom of the hull to the

underside of the deck. Moreover, it is important for the watertight to have a collision bulkhead

without door, hole or any other opening.

Figure 2 Fore Peak

The space between the forward of the ship and the collision bulkhead will created the fore peak tank

(where we can found some water ballast), and after it, there is the first hold.

2.1.2 After peak bulkhead

The after peak bulkhead is situated at the bottom of the ship. It is also a watertight bulkhead, which

aim is to assure a watertight compartment to the stern tube and the rudder trunk. To the security,

and to respect the classification, the after peak bulkhead is to extend to the first watertight deck

above the waterline.

2.1.3 Machinery space bulkhead

The aim of the machinery bulkhead is to create a watertight compartment around all the machinery

compartment of the ship.



2.1.4 Tank bulkhead

The tank bulkheads have to separate the different holds of the vessel. They have to be watertight to

protect the cargo, but also to prevent the mixing of cargo. They also have to resist to the pressure

apply by the cargo and its movement.

Between two holds, there is always some space (between the bulkhead of 2 different holds), in case

of default of one bulkhead. Moreover this space is use to lead inspection and survey of the ship (we

will talk to this space later)

It is also possible to have some bulkheads in the longitudinal direction inside the hold. The aim is to

separate the cargo to reduce the influence of their movement. It is truer when it is liquid cargo.

The movement of the liquid or dry cargo created a free surface effect, which can be dangerous for

the stability and the structure resistance of the ship.

2.2 Functional parts

The structural elements are making different functional parts for the ship to be operated safely,

effectively.

2.2.1 Double bottom tanks

Double bottom tanks have to be enough high and wide to let enter one person for inspection or

maintenance. The aim of double bottom tank is to create some tanks for ballast with water or fuel. It

is also use for the passage of pipes and cable.

The second interest is to put the most important part of the structural element on the other side of

the tank, like that the tank is just compose of smooth surface. It increases the volume of the hold,

and simplifies the internal structure. It is easier to load and discharge the ship, but also to clean the

hold.

All these advantages are obtained without a big increase of the weight, and without loss of the

volume of the hold.

Most of the time, this configuration is discontinued in the fore peak tank and in the after peak tank.

¨

Figure 3 Double Bottom

DOUBLE BOTTOM



2.2.2 Cofferdam

Cofferdam is watertight space between two watertight bulkhead, which can be empty or use for

ballast.

The aim is to protect two different watertight compartments, to avoid a mix of cargo, or than water,

oils or fuels go to the machinery compartment.

In bulk carrier vessels, we have to find cofferdams between compartments of liquid hydrocarbon like

fuel oil, lubricating oil; but also for those of fresh water like drink water, water for propelling

machinery, water for fire extinguishing …

Figure 4 Cofferdam

2.3 Cargo holds

A typical cargo hold for a Bulk carrier can be seen in figure 5. The structural arrangement is designed

as to being able to carry large volumes of dry cargo (Iron ore, coal, grain, etc.). The cargo has to be

loaded and discharged with cranes or suction pipes which requires the availability of open hatches

during the cargo handling and will have an impact on the section properties of the hull girder.

Because of the heavy cargo load the structure has to be designed to withstand high static loads

acting on the hull, such as the ship’s structural weight, cargo & ballast load and the hydrostatic

pressure from the sea water.

Figure 5



As mentioned earlier the double bottom enables the use of ballast system, as well as safety reasons

in case of grounding or other underwater damage. Also this double bottom can be seen as a safety

reason should the tank top get damage from the cargo due to high pressure over a long period of

time or forces occurring when loading.

In general all bulk carriers are designed with limitations imposed upon their operability to ensure

that the structural integrity is maintained according to classification societies. Exceeding these

limitations may result in over-stressing of the ship's structure which leads to failure in the hull. Each

ship has a restricted operational loading condition upon which the design of the hull structure has

been based.

2.3.1 Hatch openings

Because bulk carriers have these hatch openings, see figure 6, the transvers hull girder will not be

continuously longitudinal throughout the hull. Having these open sections will have an impact on the

shear stresses, which integrated over the vertical parts of the transvers section will give the hull

girder shear force. In other words the hatches will have an impact on the forces that will distort the

side shells and the longitudinal bulkheads in shear deformation.

Figure 6 Open Hatch

In order to sustain the forces in these open sections the hull structure needs additionally support.

This is achieved by strengthening of the ribs (supporting members defined as structural elements) in

the structure, together with the coaming which surrounds the hatch. These elements are divided into

two types, primary and secondary members. Some of these primary elements are extra important on

bulk carriers and the area around the hatches; such as the hatch/deck beams and the transverse

girders.

The transvers girders act as support to the ship’s hull and there function is to transmit forces acting

on the ship across the total hull girder, thus distributing the loads on a larger surface rather than



have a single area absorbing the total force. Since there are high pressure areas around the hatches

and section openings, additional transverse girders are here needed to maintain the shape of the

hatch.

Secondary elements, such as stiffeners and longitudinals, are then placed continuously over the

breath and length of the hull and bulkheads. Also inside the hatch coaming, stiffeners are being

placed. The purpose is to prevent the plate areas of the ship from distorting under the influence of

the shearing loads, bending moments and local lateral loads. This is no unique matter only

concerning bulk ships, they can be found on all ships, what differs is the amount and locations of the

structural elements as well as the dimensions because bulk carriers can be very large.

Hatch openings do not only need to be strengthened against shear forces, but also against impacts

caused by grabs during loading and discharging.

Figure 7 Longitudinals, girders and stiffeners

2.3.2 Topside & Hopper tanks

Other structural properties seen in bulk carriers are the topside and hopper tanks. The topside tanks

have a triangular shape and there purpose is to carry ballast water. Bulk carriers often have to travel

without cargo and thus need large amount of ballast water space. The tank consists of a transverse

ring made from frames in the transverse direction supporting the deck and side plating, strengthened

by longitudinals. The Hopper tanks are located at the side of the vessel within the bottom wing of

each cargo hold and have the same structural properties as the topside tanks. They are the

continuation of the double bottom tanks and thus also contribute to the ships ballast system. As

mentioned earlier the tank plating needs to be able to sustain static and dynamic loads due to the

cargo.

The reason that bulk carriers contain these side tanks is because the structural layout has to be

arranged in this way because of the cargo. The cargo hold is shaped in a way to reduce the shift in

cargo during voyage and avoiding the free surface impacts it will have on the stability of the ship.

Also the sloping angle of the tanktop sides collects the cargo in central part of the hold and makes

discharging easier.



Figure 8 Transverse Web

Figure 9 Cargo hold Cross-section



3 Structural Design

Structural design deals with the analysis of the structure in order to be able to support external and

internal forces acting on the structure. On ships, the water resistance, the waves, the viscous flow

and the weight of the ship and cargo are all examples of forces acting on the designed structure.

Several criteria come into play when designing a ship’s structure. The choice in materials is a crucial

turning point since each material has definite properties in terms of strength, flexibility, stiffness,

resistance to choc, etc. Over the years, classification societies have come up with elaborate grades of

materials, which become rules and regulations that ship builders must comply with. Another

important point that we will be treating in this paper is the limit states that the ship builders base

their structural analysis on. The limit states are the calculation methods used to determine the higher

or lower limit of design choices. The last topic, we are discussing in this paper is the thickness of the

plates and beams and how the corrosion must be taken into account when choosing the plates

thickness.

3.1 Materials

The materials used for shipbuilding is subjected to verification and standardization from the

classification societies. For shipbuilding materials, the major classification societies registered in IACS

(DNV, ABS, etc.) use the Society Rules of Materials as stepping stone for steel grades and metal

grades. ABS is one classification society that graded the structural steel, which are standardized for

use in shipbuilding.

3.1.1 Grades of steel and structural members

The American Bureau of Shipping (ABS) classified the steel grades into several categories. The

categories have different grades (A, B, D, E, F), two strength levels (normal-strength and higher-

strength), and finally classes ranging from I to III. Each category has different strength properties.

Each part of the ship is subjected to rules on which material to use. To illustrate that, the table below

sums up the material grades to be used for the structural parts of the ship.



Table 2 Material Classes and Grades

3.1.2 Hull construction

For bulk carriers, the higher-strength steel is to be used for hull construction. All ABS steels are

standard carbon steels. There are two types of ABS Steels, ordinary-strength and higher-strength

steels.

Below, there is a table summing up the minimum yield stress and ultimate tensile strength

requirements for each of the higher-strength steel used for the hull construction

Steel Grades for plates with

t≤100mm

Minimum Yield stress ReH, in

N/mm2

Ultimate tensile strength Rm, in

N/mm2

A-B-D-E 235 400 – 520

AH32-DH32-EH32-FH32 315 440 – 570

AH36-DH36-EH36-FH36 355 490 – 630

AH40-DH40-EH40-FH40 390 510 – 660

Table 3

Material Classes and Grades

Material class/grade

Longitudinal Bulkhead strakes Class I within 0.4L amidships

Deck plating exposed to weather

Side plating

Bottom plating, incl. Keel plate

Strength deck plating

Continuous longitudinal members above

strength deck

Uppermost strake in longitudinal bulkhead

Vertical strake (hatch side girder)

Uppermost sloped strake in top wing tank

Sheer strake at strength deck Class III within 0.4L amidships

Stringer plate in strength deck Class II outside 0.4L amidships

Deck strake at longitudinal bulkhead Class I outside 0.6L amidships

Class III within 0.6L amidships

Class II within the rest of cargo region

Class II within 0.6L amidships

Class I elsewhere

Class III within 0.4L amidships

Class II outside 0.4L amidships

Class I outside 0.6L amidships

Not to be less than Grade D/DH

SECONDARY MEMBERS

PRIMARY MEMBERS

SPECIAL MEMBERS

OTHER structural members

Grade A/AH outside 0.4L amidships

Strength deck plating at corners of cargo hatch

openings

Bilge strake

Longitudinal hatch coamings of length greater

than 0.15L

Structural Member Category

Grade A/AH outside 0.4L amidships

Class II within 0.4L amidships



We can see in the table above that the steel grades vary in minimum yield stress and ultimate tensile

strength. There exists several numbered grades such 32, 36 and 40, as seen above. For example, the

“32” grades have yield strength of 315 MPa, and ultimate tensile strength of 440-570 MPa.

It is important to note that it is possible to use other materials such as alloys or aluminum for certain

part of the structures. In such cases, the classification society must inspect and approve on a case-by-

case basis.

3.2 Limit States

Limit states plays a crucial role in designing the structure because it is where the engineers are

computing the possible design in order to fulfill all requirements in terms of safety, fatigue, strength,

etc. A limit state is a condition of a structure beyond which it no longer fulfills the relevant design

criteria.

When designing the structure of the ship, a structural strength assessment must be run throughout

the major construction members. A yielding check, a buckling check, a ultimate strength check and

finally a fatigue check are the four checks to be run in the structural strength assessment. The

construction members that are concerned by this assessment are the plating, the ordinary stiffeners,

the hull girders and the primary structural members.

There is four limit states onto which the shipbuilders can rely: the serviceability limit state, the

ultimate limit state, the fatigue limit state and the accidental limit state. All four of them are

concerned by a specific use of the ship and they each have specific strength criteria. The global

design of the ship is then based on all four of them.

3.2.1 Serviceability limit state

To satisfy the serviceability limit state criterion, a structure must remain functional for its intended

everyday loading conditions, but also not cause discomfort to the occupants under routine

conditions. Serviceability limit state concerns the normal use of the ship including local damages or

unacceptable damages, which may reduce the working life of the structure or affect the efficiency or

appearance of the structural members.

The serviceability limit state has strength criteria regarding the hull girder, the plating and the

ordinary stiffener. For the yielding check of the hull girder, the stress corresponds to a load at 10-8

probability level. The same probability level applies for the yielding check and the buckling check of

the plating. Again the same value of 10-8

probability level applies for the yielding check of an ordinary

stiffener.

3.2.2 Ultimate limit states

Ultimate limit states correspond to the maximum load-carrying capacity. To satisfy the ultimate limit

state, the structure must not collapse when subjected to the peak design load for which it was

designed. It takes into account the attainment of the maximum resistance capacity of sections,

members or connections by rupture or excessive deformations. It also includes the instability of the

structure.

The ultimate strength of the hull girder is to withstand the maximum vertical longitudinal bending

moment obtained by multiplying the partial safety factor and the vertical longitudinal bending



moment at 10-8

probability level.

The ultimate strength of the plating between ordinary stiffeners and primary supporting members is

to withstand the load at 10-8

probability level.

The ultimate strength of the ordinary stiffener is to withstand the load at 10-8

probability level.

3.2.3 Fatigue limit state

Fatigue limit state relate to the possibility of failure due to cyclic loads. The fatigue life of

representative structural details such as connections of ordinary stiffeners and primary supporting

members is obtained from reference pressures at 10-4

probability level.

3.2.4 Accidental limit state

Accidental limit state considers the flooding of any one cargo hold without progression of the

flooding to the other compartments and includes the maximum load-carrying capacity of hull girder,

the maximum load-carrying capacity of double bottom structure and the maximum load-carrying

capacity of bulkhead structure.

In accidental limit state design, it is necessary to achieve a design such that the main safety functions

of the structure must not be impaired during any accidental event or within a certain time period

after the accident. The structural design criteria for the ALS are based on limiting accidental

consequences such as structural damage and environmental pollution.

Since the structural damage characteristics are strongly depends on the type of accident, there is no

straightforward method to apply to the calculations. Typically, for a bulk carrier ship, accidental

scenarios must be decided along with the risk assessment. Accidental scenarios include collisions,

fire, flooding of comportments, excessive loads, etc.

4 Future trends

The stake of carrying merchandise in bulk is to maximize profit and minimize risks. In these terms,

the future of bulk carriers lays in the risk reduction by strengthening hot spots and studying the

effect of flexibility of the structure. Another aspect to be looking at is the maximization of the cargo

capacity. It could be beneficial to lower the light weight of the ship by having thinner and lighter

materials that would have the same or better strength characteristics as the current steel. Moreover,

both the cargo hold openings and structures can be developed further in order to reduce difficulties

and smooth out the cargo handling. Both are involved in maximizing profit for the ship owner.

Nowadays, the maximum cargo carrying capacity of the bulk carriers no longer increases significantly

because of economies of scale matters. Therefore, the ships are not likely to grow much bigger. The

biggest current challenge is to reduce the ship’s resistance and weight in order to increase fuel

efficiency. The future developments will most likely act toward this aim.



5 Bibliography

American Bureau of Shipping. ”Rules for Materials and Welding.” 2012.

Bulk Carriers guide. 2012.

Det Norske Veritas. ”Common Structural Rules for bulk carriers with length 90m and above.” July

2012.

IACS. ”Guidance and Information on Bulk Cargo Loading and Discharging to Reduce the Likelihood of

Over-stressing the Hull Structure.” 1997.

bulk carriers structural arrangements

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