SAS,
That's because what you're referring to regarding offshore decks, is mostly related to performance Class 2, though even for that RFM graphs give credit for wind and temperature. Very few helicopters are capable of operating Class 1 from an offshore deck with more than a few passengers. Many pilots will call committed at the point of rotation from an offshore deck, but that doesn't mean they can fly away, it just means that they're committed to continuing the take off in an attempt to gain sufficient airspeed to at least fly level, or even better start to climb before they enter the water. Under most jurisdictions, even Class 2 operations require that a twin engine aircraft must be capable of Class 1 performance at some point, often 1000 feet above the take off elevation.
TDP and CDP, as has been stated are basically the same thing. The DP is the essence; it's a decision point used for Class 1 operations. It's the only point at which in a multi-engine helicopter, the pilot may either decide to reject the take off, or continue it in the event of the failure of a critical power unit (after all the helicopter may have more than 2 engines and it may be able to climb on one or more engines after any one fails). If an engine fails before TDP/CDP the take off must be rejected and after it must be continued. In many companies this will be further modified inasmuch as there may be considerably more runway length available than that necessary for the relatively short accel/stop distance required by many helicopters (the distance required to accelerate to TDP speed and then safely recect and come to a stop on the prepared surface). If the Vtoss is low (CDP/TDP speed is usually related to Vtoss and a lower TDP speed means less space is required to safely reject the take off), the accel/stop distance will be correspondingly shorter. When there is a very long runway ahead the pilot may still be able to safely land back on even after TDP, though my experience with pilots under training shows that many less experienced pilots may find this difficult to achieve because we usually don't train much for this and it's a difficult thing to judge. You'd be surprised at the number of pilots who would have overrun the runway had I not restored an engine when under training. At least then pilots have an experience to relate to should it happen again for real. The TDP/CDP is really only critical when operating from a runway which is equal to the accel/stop distance for the weight of the aircraft in nil wind for a given temperature. This may well be the case when operating from small heliports, not fixed wing airfields.
Pauses for breath, has another cup oif strong coffee and goes off for a little sleep............
