With regard to an employee working while on an elevated pipe rack, please be advised that such a situation to be in compliance with OSHA fall protection regulations if 1) the employee is protected at all times by an adequate fall arrest system (body belt/harness and lanyard), 2) a safe means of access and egress is provided, and 3) the work can be performed with reasonable ease without a platform.
then it would be acceptable provided 100% fall protection is maintained and proper access and egress are maintained.
another issue is anytime that you have to access something and transfer from, around or climb over a competent person should evaluate the scenario and 100% tie off should be maintained to eliminate the chance of a fall.
dave