The position of the full-width mid-deck oiltight bulkhead must be carefully determined and is normally below the minimum draught level to ensure an oil pressure which is less than the external sea water pressure. The cargo lines, cargo vent and inert gas lines, access trunks and fixed tank cleaning machines would be positioned in the lower tank. Access trunks would be led to each lower tank forward and aft and could be used for various fittings and services, thus keeping the upper tank clear.
A consortium of European shipyards have developed the E3 tanker design, so called because it is European, Economic and Environmentally-friendly. The vessel has a cargo capacity of 2 million barrels and a maximum dead weight of 295,100 tonnes. The length is 318 m, breadth 57 m, depth 31.1 m and draught 22m. Ballast capacity is 106,100 m3, about a third more than required under Marpol, which will enable the vessel to sail in all weathers without storm ballast.
The structural arrangements in the cargo tanks provide two longitudinal bulkheads inside a double-skinned hull. The double sides are 4 m wide, creating wing tanks of 13.94 m and a centre tank of 21.12 m, in the 57 m beam. The double bottom height is 3 m and the effective tank depth 29.2 m. Both protective boundaries exceed Marpol requirements. The tank length of 32 m means no swash bulkheads are needed and there and there are 24 cargo and two slop tanks.
Numerous variants are being proposed by shipowners and naval architects around the world. One further patented design will be mentioned, which is the ‘Coulombi Egg’ mid-deck tanker (Figure 8.10 ). Cargo oil is carried in the upper and lower centre tanks and the lower wing tanks. Only upper wing tanks are used for water ballast. The cargo in the lower wing tanks is considered to be protectively located and a hydrodynamic automatic cargo transfer system is provided which works on the same principle as the mid-deck grounding protection provided by hydrostatic forces. A reception tank is provided to receive oil which is forced out of a holed cargo tank. Furthermore, the construction and stiffening arrangements for the structure are considered to be much simpler than other proposed double-hull or mid-deck designs.
The designs which are ultimately adopted will take into consideration building costs, steel weight, corrosion protection, tank cleaning and maintenance considerations, in addition to the regulatory requirements of IMO as outlined in the MARPOL 73/78 Convention.
Framing at ends
Beyond the cargo tank length the vessel may be transversely or of combined framing construction and must have certain additional strengthening fitted. A deep tank or tank is often fitted forward of the cargo tank space. Where transverse framing is employed, solid floors are fitted at every frame space. Intercostal side girders of depth equal to the floors are also fitted in line with every other bottom shell longitudinal in the deep tank space. The deep tank is fitted with web frames not more than five frame spaces apart. A centerline bulkhead must also be fitted, unless the main longitudinal bulkheads extend through the deep tank. With longitudinal framing, transverses are fitted in the deep tank not more than 3 m apart. Intercostal side girders are also fitted either side of the centerline. On larger vessels
Oil Tankers, Bulk Carriers, Container and RO-RO Ships
the cargo tank structure may extend into the deep tank itself. Panting and pounding arrangements are also necessary and will be similar to those described in Chapter 5.
All modern tankers now have the machinery space and accommodation located aft. Web frames are fitted not more than five frame spaces apart in the machinery space, with fixed or portable beams across the casing opening. Transverse framing of the bottom is usual in the machinery space and construction is similar to that mentioned in Chapter 5. Transverse or longitudinal framing of the sides and deck may be used from the machinery space to the after end of the ship. Deck longitudinals must extend into the machinery space a distance equivalent to one-third of the ship’s breadth. Panting arrangements are also fitted in the after peak, as described in Chapter 5.
Superstructures
These are of much the same construction as described in Chapter 5. The load line rules require protective housings around openings in the freeboard and other decks and a forecastle extending 7 per cent of the ship’s length from forward. Because of a tanker’s high bending stresses extra care must be taken with discontinuities at the superstructure ends.
General
Cofferdams are fitted between oil tanks and other compartments and must be at least 760 mm wide. Pumprooms or water ballast tanks may, subject to certain conditions, be accepted instead of cofferdams. Special arrangements are necessary in tankers because of the reduced freeboard to clear the decks of water. Open rails are fitted for at least half the length of the weather deck. Solid bulwarks are usually fitted only at the forecastle and around the superstructure.
Hatches
Access to the cargo tank spaces is by oiltight hatches. Circular or oval shapes are usually employed with coamings at least 225 mm high. Steel covers with suitable oiltight fastening arrangements are usual, (Figurs 8.11(a)and8.11(b)). Patented covers of other approved materials are also available. Other tanks and cofferdam spaces may have similar hatches or manholes for access (Figure 8.12).
Ventilation
Ventilation arrangements are fully described in Chapter 10.
Inert gas plants
Inert gas plants are being fitted to an ever-increasing number of tankers to improve their operational safety. The plant provides an inert gas blanket over the surface of the cargo to stop the build-up of flammable vapours which might lead to explosions.
A typical system is shown in Figure 8.13. The plant uses exhaust gas which is drawn from the boiler flue uptakes, where available, or from a separate combustion chamber. The gas enters a scrubbing tower via a water seal which is circulated by sea water. The gas is cooled, solids and unwanted gases are scrubbed out and it then