8. General Arrangements in Ships

BASIC SHIP CONSTRUCTION8. General Arrangements in Ships

8.1 IntroductionShips are designed to carry specific types of cargoes or to carry out specific work. The cargo handling machinery and cargo space arrangement will differ depending on the type of cargo carried. Some vessels will require special strengthening and attention in their construction.

Figure 8.1. A product/chemical tanker

8.2 Tankers are designed to carry liquid cargoes. They are single deck vessels and a longitudinal system of framing is generally used. If the length of the vessel is less than 150 m then a combined system of framing may be used. The length of any one tank must not exceed 0.2 the length of the ship. If the length of the tank exceed 0.1 the length of the ship or 15 m, a transverse wash bulkhead is to be fitted at the mid length of the tank.

Large tankers such as VLCC’s will have more than one longitudinal bulkhead. Smaller tankers such as product tankers or chemical tankers may have only one longitudinal bulkhead. The cargo tanks are additionally separated by transverse bulkheads. Transverse oil tight bulkheads may be plane or with vertical or horizontal corrugation. Where frames are longitudinal the stiffeners on the longitudinal bulkheads are to be horizontal.

According to the Ship Construction Rules, cofferdams are to be fitted at the ends of the oil cargo spaces. The cofferdams are to have oil tight bulkheads. In some cases theses cofferdams are used as ballast tanks or as pump rooms. The cofferdams are to be at least 7.6 m in length.

_____________________________________________________________________ Basic Ship Construction

HPH/07

68

The cargo carried will be a liquid one so the ship must be equipped with pipe lines, valves and pumps to handle it. The number of pump rooms and the arrangement of the pipe line system will depend on the size of the ship, the type of cargo carried and the different types of cargoes which may be carried at any one time.

Oil tankers of 5000 tonnes deadweight or more are required to have a double hull construction in way of the cargo tanks. Segregated ballast tanks are to be used on these ships. This is to protect against oil pollution.

Small tankers may have 2 sets of cargo wing tanks (port and Starboard) separated by a single longitudinal bulkhead and a number of transverse bulkheads. Larger tankers may have a set of port cargo wing tanks, starboard cargo wing tanks and centre cargo tanks separated by two longitudinal bulkheads and a number of transverse bulkheads. The general arrangement for an oil tanker is shown in Figure 8.2.

The hatchway openings are to be kept as small as possible and the coamings are to be at least 600 mm high.

Figure 8.2. General arrangement in a double hull product tanker

_____________________________________________________________________ Basic Ship Construction

HPH/07

69

Figure 8.3. Section through a double hull tanker

Figure 8.4. A container ship

8.3 Container Ships.These ships are designed to carry unitized cargo consisting of containers. The containers are of standard dimensions coming as 20 feet or 40 feet in length. By standardizing the dimension of the cargo great efficiency in loading or discharging the cargo is achieved.

_____________________________________________________________________ Basic Ship Construction

HPH/07

70

Because of this much attention is paid to the form and structural arrangement for the cargo holds. The length of time these vessels will spend in port is very short.

Cargo handling gear on board the ship usually consists of cranes on deck. In most cases shore gantry cranes are used to handle the containers as they are more efficient.

A longitudinal system of framing is used in the main body of the vessel. A transverse system is used at the forward and after ends. High tensile steel is used to form box longitudinal girders at the port and starboard side of the decks and longitudinal bulkheads. High tensile steel is also used to form the decks and shell platings. The purpose of the box girders is to provide resistance to racking stresses and longitudinal stress. These ships are built with a cellular construction at the sides as shown in Figure 8.5.

The hatchways are fitted with 2 continuous hatch girders to resist bending and to compensate for the lost in strength caused by the hatchway opening. Containers are also carried on top of the hatch covers. Because of this the decks and hatch covers are to be adequately strengthened.

The cargo spaces are fitted with slotted cell guides and are designed to take standardized containers measuring 20 feet or 40 feet in length. The cell guides consist of angle bars 150 mm by 150 mm and may be welded to the tank top. Bulkheads are fitted at intervals as required by the Rules.

_____________________________________________________________________ Basic Ship Construction

HPH/07

71

Figure 8.5. General arrangement for a container ship

8.4 Ore CarriersOre carriers are designed to carry high density cargo in bulk. Because of this the vessel must be built with suitable strength to with stand the heavy loading. The centre of gravity of the ship must be high enough so as not to make the ship too stiff when the vessel is fully loaded with cargo low in the ship.

Ore carriers will have cargo handling gear suitable for the cargo to be discharged or loaded in an efficient manner.

_____________________________________________________________________ Basic Ship Construction

HPH/07

72

Figure 8.6. Section through an ore carrier.

An ore carrier may have self trimming hatches. Figure 8.6 shows the cross section of the hatch of an ore carrier with self trimming hatches. With this type of cargo space it is not necessary to erect shifting boards to prevent the cargo from shifting during the voyage. The hatches are generally very wide and the covers are hydraulically operated.

Figure 8.6 also illustrate the location of topside wing tanks as the vessel must have adequate ballast capacity.

_____________________________________________________________________ Basic Ship Construction

HPH/07

73

Figure 8.7. A bulk carrier

8.5 Bulk Carriers

Bulk Carriers are designed to carry cargo in bulk. Cargoes include grain, cement, bauxite and alumina.

Figure 8.8. General arrangement for a bulk carrier

Bulk carriers may be constructed using the combined system of framing. The double bottoms, the bottom of the wing tanks and the decks will have the longitudinal framing while the transverse frames may be used at the sides.

_____________________________________________________________________ Basic Ship Construction

HPH/07

74

The wing tanks will have transverse webs at intervals of 3.4 m. Figure 8.8 shows a bulk carrier with 2 longitudinal bulkheads.

Figure 8.9. Section of a bulk carrier.

OBO CarriersOre/Bulk/ Oil (OBO) carriers are designed to carry solid and or liquid cargo in bulk. The vessel will have a double skin with a longitudinal system of framing in the narrow wing tanks.

The double skin serves the purpose of reducing the free surface effect in the cargo hold when liquid cargo is carried. The frames are not located in the cargo space so the double skin also allow for the easier cleaning of the cargo space. Should in case the hull is breached there is less chance of an oil spill with a double hull. The wing tanks allow for segregated ballast.

The cargo handling arrangement on an OBO must be such that it can switch easily from dealing with solid cargo to liquid cargo.

The hatch covers are usually of the roller type and they usually open side ways and must be gas tight.

_____________________________________________________________________ Basic Ship Construction

HPH/07

75

Figure 8.10. General arrangement for an OBO

The bulkheads usually consist of cofferdams with the frames inside the cofferdams (and not in the cargo spaces). This arrangement also allows for the easy cleaning of the hatches.

The inner bottom will have a rise of floor to allow for the drainage of liquid cargo to the centerline

_____________________________________________________________________ Basic Ship Construction

HPH/07

76

Figure 8.11. Section of an OBO

_____________________________________________________________________ Basic Ship Construction

HPH/07

77

BASIC SHIP CONSTRUCTION

ASSIGNMENT 1GENERAL ARRANGEMENTS IN SHIPS

This assignment is to be submitted on or before June 7, 2007.Late assignments will have 20% deducted from the final mark

The sketches are to be done on plain letter size paper.Students must attempt Question 1a or 1b and all other questions.

Marks are awarded as follows:Presentation – 20 %Content – 70%Originality – 10%

1. (a) Sketch the elevation view of a general cargo vessel showing 5 cargo holds, engine room, peak tanks, double bottom tanks, hatchways, bulkheads and cargo

handling gears.Or

(b) Sketch the elevation view of a container vessel showing 7 cargo holds, engine room, peak tanks, double bottom tanks, ballast tanks, hatchways and bulkheads.

2. With the aid of sketches compare the constructional features of an ore carrier, a bulk carrier and an OBO and explain the differences in the double bottom construction.

3. Briefly describe the constructional features of a product oil tanker.

4. Sketch the elevation and plan views of an oil tanker showing 8 sets of cargo tanks, other tanks, engine room, bulkheads and other spaces.

_____________________________________________________________________ Basic Ship Construction

HPH/07

78

Single-hull oil tanker phase-outUnder the phase-out schedule, "Category 1" single-hull oil tankers will not be able to trade after 5 April 2005, (for ships delivered on or before 5 April 1982 or earlier) or after their anniversary date in 2005 (for ships delivered after 5 April 1982). Category 1 oil tankers, (commonly known as Pre-MARPOL tankers) include oil tankers of 20,000 tonnes deadweight and above carrying crude oil, fuel oil, heavy diesel oil or lubricating oil as cargo, and tankers of 30,000 tonnes deadweight and above carrying other oils, which do not comply with the requirements for protectively located segregated ballast tanks.

Category 2 oil tankers, which have some level of protection from protectively located segregated ballast tank requirements will be phased out according to their age up to 2010. The year 2010 is also a final cut off date for Category 3 oil tankers which are generally smaller oil tankers. Category 2 oil tankers (commonly known as MARPOL tankers) include oil tankers of 20,000 tonnes deadweight and above carrying crude oil, fuel oil, heavy diesel oil or lubricating oil as cargo, and oil tankers of 30,000 tonnes deadweight and above carrying other oils, which comply with the protectively located segregated ballast tank requirements. Category 3 oil tankers are oil tankers of 5,000 tonnes deadweight and above but less than the tonnage specified for Category 1 and 2 tankers.

Revised phase-out schedule:

Category of oil tanker Date or year for phase-outCategory 1 - oil tankers of 20,000 tonnes deadweight and above carrying crude oil, fuel oil, heavy diesel oil or lubricating oil as cargo, and of 30,000 tonnes deadweight and above carrying other oils, which do not comply with the requirements for protectively located segregated ballast tanks (commonly known as Pre-MARPOL tankers)

5 April 2005 for ships delivered on 5 April 1982 or earlierAnniversary date in 2005 for ships delivered after 5 April 1982

Category 2 - oil tankers of 20,000 tonnes deadweight and above carrying crude oil, fuel oil, heavy diesel oil or lubricating oil as cargo, and of 30,000 tonnes deadweight and above carrying other oils, which do comply with the protectively located segregated ballast tank requirements (MARPOL tankers), and

Category 3 - oil tankers of 5,000 tonnes deadweight and above but less than the tonnage specified for Category 1 and 2 tankers

5 April 2005 for ships delivered on 5 April 1977 or earlierAnniversary date in 2005 for ships delivered after 5 April 1977 but before 1 January 1978Anniversary date in 2006 for ships delivered in 1978 and 1979Anniversary date in 2007 for ships delivered in 1980 and 1981 Anniversary date in 2008 for ships delivered in 1982Anniversary date in 2009 for ships delivered in 1983Anniversary date in 2010 for ships delivered in 1984 or later

_____________________________________________________________________ Basic Ship Construction

HPH/07

79