What is a double-hulled tanker?
A double-hulled oil tanker has cargo tanks separated from the outer side and bottom shell by protective double-side and double-bottom tanks or spaces that do not carry oil cargo. In the arrangement shown here, the side spaces and double bottom are segregated ballast spaces, while inner hull and inner bottom plating form the side and lower boundaries of the cargo region.
What are double-bottom tanks?
Double-bottom tanks occupy the vertical space between bottom shell plating and inner bottom plating. Floors, the centre girder, side girders, and longitudinals support the surrounding plating. The double bottom is a structural region containing tanks or spaces; it is not another name for the outer bottom shell.
How do you read an oil tanker cross section?
Start at the side and bottom shell, move inward through the wing and double-bottom spaces, and then locate the inner hull and inner bottom enclosing the cargo tanks. Bulkheads divide tank spaces; web frames and floors support the boundaries but are not complete cargo-tank boundaries themselves. Compare the external terminology in the ship hull bottom and keel guide.
Learning outcomes
- Separate cargo oil tanks from segregated wing and double-bottom ballast spaces.
- Identify the inner bottom, inner hull, transverse bulkheads, and longitudinal bulkheads as tank boundaries.
- Distinguish primary supporting members such as floors, girders, and web frames from secondary longitudinals.
Begin with spaces, then reveal structure
Internal structure is easiest to understand in two passes. First identify the volume occupied by each tank or void. Then hide or section those volumes and inspect the plating and supporting members that create their boundaries.
| Term | What it defines | 3D location |
|---|---|---|
| Cargo oil tanks | Oil-tight spaces inside the protected cargo region. | Open |
| Wing ballast tanks | Segregated spaces between side shell and inner hull plating. | Open |
| Double-bottom tanks | Spaces between bottom shell and inner bottom plating. | Open |
| Inner bottom plating | The upper boundary of the double bottom and lower cargo boundary. | Open |
| Longitudinal bulkheads | Fore-and-aft oil-tight boundaries dividing cargo spaces. | Open |
| Web frames and floors | Deep transverse members supporting plating and longitudinals. | Open |
How the structural system is organised
Double sides and double bottom
IMO describes a double-hull oil tanker as having cargo tanks protected through the cargo length by double sides and double-bottom spaces used for water ballast or as void spaces. This arrangement provides separation between cargo oil and the outer shell. It does not mean that every space between two plates is interchangeable.
Bulkheads create boundaries
Transverse oil-tight bulkheads divide the cargo region into tank bays and limit liquid movement along the ship. Longitudinal oil-tight bulkheads divide the breadth into centre and wing cargo spaces in the training arrangement. The collision bulkhead is a different watertight boundary near the bow and is not a cargo-tank wall.
Primary members support secondary members
Floors, centre and side girders, web frames, and deck transverses are deep primary supporting members. Longitudinals attached to the deck, inner bottom, side shell, and bulkheads support local plating between primary members. Brackets improve load transfer at intersections; openings require careful structural detailing.
Check your understanding
What separates cargo oil from the sea in a double-hull tanker?
Double-side and double-bottom spaces intervene between the cargo region and outer shell; inner hull and inner bottom plating form the protected cargo boundaries.
Are all internal spaces cargo tanks?
No. Ballast tanks, cofferdams, voids, machinery spaces, and fore and aft spaces have different functions and boundary requirements.
What is the difference between a bulkhead and a web frame?
A bulkhead is subdivision plating and may be watertight or oil-tight. A web frame is a deep supporting member and is not normally a complete tank boundary.