l would like to know,in case a marine engineer working on petrolier ship has a contract for 4months,for how long this contract can be extended and whats the reason of the extension?thanks

Instead of telling the marines ask a marine?

If the ships Engineer was offered a 4 month contract it is probably dependent on the Ships contract, in this case expected to last four months. If the Ships contract is extended and the Officer has performed well he will probably be asked to extend as well. He may also be asked to extend if the company values his ability and expects he may contribute to their requirements when off contract.

Hope this helps.