There is not much that could be added to 'fermrib's' description although this statement merits a comment:
Although the H won't provide precise visual cues about the boundaries of the OFS, at least it helps to know what is the helideck (half) sector where any landing path can be performed with the helicopter entirely within the the OFS.
Clearly the whole helideck area (TLOF, FATO and OFS) are clear of obstacles; the OFS should be 210 degrees and not, as stated, 'half'.
As has often been shown on PPRune, not all offshore pilots know, or understand, the logic of the helideck markings. One of our most experienced members has just demonstrated that.
To answer 'fermrib's' question, the orientation and markings of the helideck should be known to the pilot before the sortie starts. Those of us who have the experience of many offshore landings will know that the cues to the approach direction and the final path to the helideck will not be the on-deck markings but general cues. With this in mind, pilot must be aware that the critical marking is the Touchdown/Positioning Marking (TD/PM) which will ensure that the helicopter is free of obstacles when the helicopter is placed correctly (as you have stated).
The modern trend, even for PC1 heliports (helipads), is for the landing surface (TLOF) to be less than the overall dimensions of the helicopter (the 'D'). Because of this, it is critical that designers ensure that the FATO is completely free of obstacles as required by Annex 14 and CAP 437 (which will soon become the basis for the ICAO helideck guidance) and pilots understand the relevance of, and correctly position in accordance with, the TD/PM.
Jim