Helidecks
Variable
I've inspected a few hundred helidecks in my time in different parts of the world but have yet to come across a 'sectionalised helideck', presumeably with removeable deck plates.
Not sure to what standard it was built to but it certainly doesn't sound like it meets the requirements of either the ICAO Heliport Manual, or, for that matter CAP 437. It doesn't look like the authors of these documents have contemplated a 'loose plated deck' so one cannot say that it is "Non Compliant" but somewhere along the line there has been a goof-up because the risk assessment conducted at the design stage should have considered the 'worst-case' downwash effects.
Under JARs the helicopter operator (NOT the rig operator) is responsible for ensuring that the landing areas meets with the Helicopter Operators standards as set out in either their Vol A or Vol B or possibly their Vol C. If there is a problem then liability falls upon the helicopter operator and not the rig operator. Thats why all North Sea companies have a method for at least an annual inspection of helideck facilities.
Outside the highly regulated world of Europe there can be found a motley collection of rules and regs with a similarly motley standards of regulatory oversite. My advice is 'user beware' and if you don't like it then advise the owner that ops are suspended until the problem is fixed. After any incident you will be deafened by the noise of escaping stakeholders and the slowest, most exposed and least powerful/influential party will have the Donkey's Tail pinned to their backside.
For your SMS risk assessment you will just have to accept that there will be little or no other data available if the deck design is unique or nearly so. For this platform the event has already occured so the probability of it happening again on that rig is going to be high unless changes are made. If the platform is an 'import' has such an event occurrred elsewhere?
Rgds
G