The Corona virus is among us for a long enough period of time for companies to have set up their own Covid-regulations. For specific information regarding the Covid policy of companies you can reach out directly to these companies.
For the rules from the Dutch Government (including travel & potential quarantine restrictions) you can visit the site of the Dutch Government here. The State Supervision on the Mines (responsible for safe working offshore) has more information here on their site.
The rules for helicopter flights from Den Helder Airport can be found here on their website.