operational submarine duty

(i) while attached under competent orders to a submarine, while serving as an operator or crew member of an operational submersible (including an undersea exploration or research vehicle), while undergoing training preliminary to assignment to a nuclear-powered submarine, while undergoing rehabilitation after assignment to a nuclear-powered submarine, or, in the case of a member qualified in submarines, while attached as a member of a submarine operational command staff whose duties require serving on a submarine during underway operations—

(I) during one calendar month: 48 hours, except that hours served underway in excess of 48 as a member of a submarine operational command staff during any of the immediately preceding five calendar months and not already used to qualify for incentive pay may be applied to satisfy the underway time requirements for the current month;

(II) during any two consecutive calendar months when the requirements of subclause (I) of this clause have not been met: 96 hours; or

(III) during any three consecutive calendar months when the requirements of subclause (II) of this clause have not been met: 144 hours;

(ii) while receiving instruction to prepare for assignment to a submarine of advanced design, or

(iii) while receiving instruction to prepare for a position of increased responsibility on a submarine.