Work permits for offshore staff working in EU countries

I am having a lot of difficulty in getting information about what sort of work permit or visa our staff may need.

Basically we have a number of contracts in Denmark and the Netherlands to do offshore maintenance work on wellheads. Our offshore technicians are UK residents and they go to one of these two countries for 2 to 3 weeks, staying either in a hotel and travelling to the platform daily or staying on the platform, then returning to the UK. We have some staff who will go regularly and other who may go occasionally or rarely. They don't have an address in the country they are going to as they are not 'living' there.

There is a lot of excellent information for workers coming into this country to work, but finding out the rules for our category of worker is very difficult. I have trawled a lot of different websites, but the various options seem to apply to people taking up either short-term or long term residency in the EU country. It has also been not possible to send an email or phone to clarify. I did find a phone number for the Netherlands, but since I don't speak Dutch it was impossible to know what was being said on the recorded message, so I just kept pressing 1 until I got a real person who I asked if they spoke English. They said someone would call me back, but would not give me an email address, name or number. So I wait!

Any advice would be appreciated.

Many thanks