Architecture of the oil tanker

Individual compartments are referred to by the tank number and the athwartships position, such as "one port", "three starboard", or "six center.

"[1] A cofferdam is a small space left open between two bulkheads, to give protection from heat, fire, or collision.

These tankers did not extend high above the water line, allowing Hydrostatically Balanced Loading (HBL), so relatively little oil was spilled in case of bottom damage.

The downside is designs based on MARPOL spill more oil when damaged than a preMARPOL tankers.

[needs update] After the Exxon Valdez disaster, public outcry became so strong that authorities were forced to come with preventive measures.

In 1998, the Marine Board of the National Academy of Sciences conducted a survey of industry experts regarding the pros and cons of double-hull design.

Some of the advantages of the double-hull design that were mentioned include ease of ballasting in emergency situations,[9] reduced practice of saltwater ballasting in cargo tanks decreases corrosion,[10] increased environmental protection,[10] cargo discharge is quicker, more complete and easier,[10] tank washing is more efficient,[10] and better protection in low-impact collisions and grounding.

[10] The same report lists the following as some drawbacks to the double-hull design, including more expensive to build,[11] higher canal and port expenses,[11] ballast tank ventilation difficult,[11] ballast tanks need continual monitoring and maintenance,[11] increased transverse free surface,[11] more surfaces to maintain,[11] explosion risk in double-hull spaces if vapor detection system not fitted,[12] cleaning mud from ballast spaces a bigger problem.

In grounding events of this type, a mid-deck design overcomes this by eliminating the double-bottom compartments that are void with air.

The United States Coast Guard does not allow this design to enter US waters, effectively preventing it from being built.

[18] Fuel oil itself is very difficult to ignite, however its hydrocarbon vapors are explosive when mixed with air in certain concentrations.

[19] The purpose of the system is to create an atmosphere inside tanks in which the hydrocarbon oil vapors cannot burn.

[22] Thus, as air replaces the inert gas, the concentration cannot rise to the lower flammable limit and is safe.