I can't immediately think of a major US naval deployment in the North Sea outside the big NATO exercises every 1-2 years. Were you thinking of a specific shipping lane?
US keeps the 6th fleet in Europe / Mediterranean sea. It costs a pretty penny to defend Europe's seas - about 40 ships,175 aircraft and 21 thousands people.
Scandinavia (like most countries; it's not specific to them) benefits from a relatively piracy-free world, and a constrained Russia. This takes a giant amount of money to achieve, and it's mostly funded by the US taxpayer.
https://www.c6f.navy.mil/ and https://en.wikipedia.org/wiki/United_States_Sixth_Fleet