For Fugro Aberdeen, the standard contract is for 175 days. There are two contracts offered, Guaranteed (consolidated) days or "pay as you earn".
Once you work over 175 days you get paid 1/175 of your salary in addition to all other allowances (as your salary is calculated over 175 days). There is no "double bubble". Basically your salary is calculated over 175 days, any day you work over that they just pay you salary for those extra days. All offshore allowances etc. remain the same on all days worked even when over 175.
Note the contracts are not for a fixed number of days (i.e. they do NOT state you must work 175 or you only have to work 175). They say you have to make yourself available as and when required by the Company.