ZeroAlpha,
Hompy's rather brief reply is accurate.
As you state, that job is worth $x over y years, meaning it has been contracted out to the current operator for that time. Now, barring any serious breaches of that contract by the operator that will void the terms as wirtten and automatically result in cancellation, the next time this job is up for grabs is at the end of the term.
THEN it will go out to re-tender and be up for grabs to whoever puts the best offer on the table. Under certain circumstances a party in a contract can withdraw early by mutual agreement, thus opening up the job for re-tender, but generally the job is secure to the contractor for the duration of the term.