manrow,
Simplistically, the principles are the same but the reference heights, and rates of climb (as opposed to gradients) are different.
As VTA has recommended, take a look at Annex 6 Part III; however, the references he has provided are from Attachment A - Example (at the back of the Annex); reading the whole of this attachment is recommended.
Note 1 in the attachment provides some guidance on the application of performance for offshore operations.
Note 2 with respect to performance in congested areas.
The basic principles are covered in the first part of Attachment A; the mechanics in the Example.
Jim