The Coulombi egg tanker (CET) is a single hull mid-deck tanker alternative to double-hull tankers, designed to reduce oil loss in cases of collision and grounding. The CET was designed by Anders Björkman and developed under Heiwa Co.
In cross-section, the CET displays high-wing ballast tanks port & starboard, with oil cargo tanks beneath & inclusive of the turn of the bilge. The longitudinal bulkheads which form the inboard boundary of the wing cargo and ballast tanks extend from the main deck to the bottom shell. The area inboard of the longitudinal bulkheads are divided horizontally to form upper and lower main cargo tanks. The mid-tank decks in the wings and central cargo area are situated to ensure adequately low internal pressures (relative to the surrounding seawater).
The CET design was created in 1989, and accepted by IMO as a valid double-hull alternative in 1997 (Marpol 73/78 13F). The current reference design  includes trunking for the main cargo tanks (void when loaded, rising seawater in bottom damaged tank displaces oil into the trunks), side coffer dams, downturned mid-decks in the wings, and similar access trunking for the lower wing tanks. These modifications came after concerns of the reliance on active systems.
When the lower or outer cargo tanks are damaged, hydrostatic pressure forces the oil (which is less dense than seawater) up into trunking, the high ballast tanks, or both. Non-return valves prevent captured oil from escaping.
A CET carries less ballast area, with the ballast tanks situated in the area most susceptible to collision damage. When damaged, draught and list are anticipated to be less than in a double-hull tanker (as no bouyancy is lost, such as when a double-hull void is flooded), a consideration which may prevent a grounded tanker from becoming stuck. The lower ballast tank area limits the amount of coatings and corrosion liable to occur. This arrangement has very low overall oil loss in event of groundings.
While accepted by IMO, the USCG has not approved this type of vessel for operations in United States waters, as it fails to meet zero emission requirements (in case of grounding, a small amount of oil will be lost - in a double-hull vessel, this will be contained within the depth limits of the bottom tanks). There are also some concerns over structural complexity and the use of voids/trunking, which can increase cost & decrease cargo capacity.