Why not add ‘the’ before ‘North’ in the sentence: So it comes as a surprise to learn that giant fish are terrifying the divers on North Sea oil rigs

Question

Why not add ‘the’ before ‘North’ in the sentence:

So it comes as a surprise to learn that giant fish are terrifying the divers on North Sea oil rigs.

If there is not ‘oil rigs’, I think it is no need to add the ‘the’ before ‘North’ too.

Answer 1

When place names that usually have "The" are used as modifiers in phrases, they usually drop the "the":

Atlantic trade

Pacific islands

United States envoy

Amazon rainforest

Mediterranean coast

Answer 2

Normally when referring to oceans or seas you would use the word "the". For example, you would write:

The Titanic sunk in the Atlantic Ocean.

rather than:

The Titanic sunk in Atlantic Ocean.

However, in your example "North Sea" is not being used to refer to the sea itself. It is being used as a descriptor of the oil rigs. I.e. these were not just regular oil rigs, but specifically North Sea oil rigs. That being the case, we have to instead determine whether "oil rigs" (with a descriptor) should get the word "the" in front of it. And that would depend on whether the sentence is referring to (North Sea) oil rigs in general, or some set of specific (North Sea) oil rigs.

In this case the point of the sentence is that divers on North Sea oil rigs in general are getting terrified, rather than that divers on some specific North Sea oil rigs are getting terrified; thus, the word "the" is not used.

Answer 3

To extend Alex's excellent answer

the is used before a noun to highlight it is that exact item

the door to the left of the room

There is only 1 door, on the only left of the exact room we are discussing

but there are many "north sea oil rigs" and this sentence is discussing all of them.