The DSV class hierarchy

The survey's crewed submersibles are classed by depth rating into three tiers. The classification is operational rather than strictly engineering; vessels at the top of one tier are not, in practice, interchangeable with vessels at the bottom of the next.

DSV-1 — bathyal work

Rated to 3 000 m. The most numerous class. Used for routine observation work along the Trench of Avals and similar bathyal sites, and for liaison runs to the Vault of Hours when conditions permit. Crew of two to four; mission length up to eighteen hours.

DSV-2 — abyssal observation

Rated to 6 000 m. Fewer in service. Required for the lower bathyal and the abyssal proper, and for the rare survey runs to the trench floors that fall short of hadal. Crew of two; mission length up to twelve hours; significant pre-mission certification.

DSV-3 — hadal expedition

Rated to 11 000 m. Two in regular service, one in slow refit. Used for hadal trench expeditions and other research programmes that justify the cost. Crew of one or two; pilot certification carries a separate licence.

Why the tiers are operational

A vessel approaching the bottom of its rated depth is, by convention, considered to have used most of its margin for the day. A DSV-1 at 2 800 m is a competent boat; a DSV-1 at 2 950 m is a tired one. The survey schedules to the tier above the depth, not to the rating.