Treg,
Apart from talking to the team at HCA, you could do no better than to visit the CAA site and pull down "CAA Paper 2004 - Helicopter Turbulence Criteria for Operations to Offshore Platforms":
http://www.caa.co.uk/application.asp...detail&id=1390
Alternatively approach the International Chamber of Shipping and ask about their "Guide to Helicopter/Ship Operations" (although this is in the process of being amended, the staff at ICS will point you towards the experts).
Jim