It's a ship that's used to drill for oil, natural gas out in the oceans or seas of this world...
To the OP... Could you please provide us with more information in order for us to give you some better possibilities as to why this is happening?
First problem I have to mention is no mention of any specific quality controls taken into account... in order to prevent for the possibility of different types of corrosion you have so far encountered...
Now, is this vessel completed as of yet, or is this still an ongoing build that has reached whatever stage it's currently @??? Because if it's still an ongoing build, then it's not too late for a corrosion review or audit to be performed in order to mitigate the current situation but since I'm going by what little information you so far have provided, then I can only guess what details are involved with this specific project, and I don't guess too well so I'm just going to defer to you providing us with much more details than presently available.
Respectfully,
Henry