Regulation 13F of Annex I of MARPOL 73/78 deals with prevention of oil pollution in the event of collision or stranding. Describe a double hull type of construction and state if any other type of construction provides equivalent protection.

Regulation 13F of MARPOL Annex I applies to oil tankers (crude and product tankers of 5,000 DWT and above) ordered after 6 July 1993 or delivered after 6 July 1996 (and, for larger tankers of 20,000 DWT and above carrying heavy grades, other dates - under the phase-in to 2026), requiring the entire cargo tank length to be protected by a double hull/ double bottom or other protection of an equivalent level so that, in the event of collision or stranding, the release of oil is reduced.

Double hull construction:

• A double hull tanker has an outer hull (shell) separated from an inner cargo tank boundary (inner hull) by a longitudinal void space on each side, plus a double bottom void beneath the cargo tanks. The side void is typically about 2.0 m wide (for ships of 5,000-20,000 DWT the void is around 1.9-2.0 m; for larger ships the width increases), and the double bottom height is typically around 2.0 m (but ranging); the exact values are set by Regulation 19 (damage stability) and the Reg 13F/13G requirements. The void/ballast tanks in the double skin and double bottom provide the protective space between the cargo tank and the sea. In the event of grounding or collision, the outer hull (and double bottom/side) is breached, but the cargo tanks (separated by the void) remain intact, so little or no oil is released.
• The outer skin, the inner/boundary bulkhead, the longitudinal girders and transverse framing of the double skin, and the bulkhead/deck head are arranged so that the damage to the skin does not penetrate the cargo tank. The design must permit access for inspection of the double hull (through cofferdams/void tanks) and must satisfy the subdivision/damage stability (Regulation 19/25).
• In practice, the double bottom and double sides allow ballast water/generally watertight cargo-free space, and the ship can carry segregated ballast in the wing/double bottom tanks in compliance with the segregated ballast requirement.

Whether any other type of construction provides equivalent protection:

Regulation 13F (and its adoption via the amendments) provides that an engineering design alternative may be accepted if shown to provide "equivalent protection" to the environment as a double hull - namely that, in the event of collision or stranding, the release of oil would be no greater than that of a double-hull design. Examples considered equivalent by MEPC/an adverse review:

• Mid-deck tanker design (a specific type of crude tanker with a "mid-height deck" arrangement to limit the oil released at stranding).
• Crude oil tankers with a high-level "COW" and the use of "mid height deck", or designs that rely on a combination of a double bottom plus a mid-deck to keep the released oil within acceptable bounds.
• The "double sides with mid-deck" configuration where the design is justified by the appropriate standard (the equivalence accepted by MEPC based on damage hold analysis).

As a general rule, the only construction accepted as providing equivalence by the IMO/MEPC is the mid-deck tanker concept (and associated designs) that demonstrate, through a "tanker modification/equivalence" process, that oil outflow in collision/grounding is not greater than a double-hull. In practice the vast majority of new tankers are built with a true double hull. Any equivalent design must be approved by the flag Administration/IMO and the equivalence shown by an appropriate risk/outflow analysis.

The result of Reg 13F is that virtually all oil tankers trading today must be double hulled (or an IMO-approved equivalent) by the phase-in dates, considerably reducing oil outflow in case of collision or grounding.