Cancellation of a contract or policy of insurance issued under
authority of the Act shall not become effective otherwise than as
provided by 33 U.S.C. 936(b); and notice of a proposed cancellation
shall be given to the Office and to the operator in accordance with the
provisions of 33 U.S.C. 912(c), 30 days before such cancellation is
intended to be effective (see section 422(a) of part C of title IV of
the Act).