shipboard duty vs service abroad

shipboard duty and service abroad both are nouns.

Word	Noun	Adjective	Verb	Adverb
shipboard duty	Yes	No	No	No
service abroad	Yes	No	No	No

As nouns, shipboard duty and service abroad are synonyms defined as:

Other synonyms of shipboard duty include sea-duty.

shipboard duty (noun)	service abroad (noun)
naval service aboard a ship at sea	naval service aboard a ship at sea