The German offshore area where these windparks are built and the routing to them is mostly airspace Golf. In German air law you have to stay clear of clouds at all times and no IFR is allowed. There are plans to change that, as our neighboors England, Denmark and the Netherlands. Currently these operations are mostly conducted "VIFR" with IFR certified crews and helicopters. Some call this a temporary "grey" solution, others call it highly illegal. Its about time that the German authorities adopt the standards of our neighbouring countries, so companies can safely and legally implement IFR procedures like ARA's to these installations. This also answers your question why all operatos require you to have an instrument rating.
EASA NPA 2013-10
4.1.1 Definition of offshore operation
The Agency considers that offshore operations are all flights over open sea areas to a location in the sea in accordance with the following amended definition of offshore operations.
‘Offshore operations’ means a helicopter operation that has a substantial proportion of any flight conducted over open sea areas to or from an offshore location for the purpose of:
(a) support to offshore oil, gas and mineral exploration, production, storage and transport;
(b) support to offshore wind turbine and other renewable energy sources;
(c) support to marine lights; or
(d) sea-pilot transfer.