Most of the times, platforms are design in modules ( these could be segregated by deck level, weight or may be harzadous area classification (based on equipment/piping). The major reason for designing platforms in this way is to ease fabrication & erection. When the various module are then transported to sea for laying/installation, that’s when the word “hookup” comes in play. Similing means connecting the various module together; piping, electrical cables, structure members etc. To achieve this, “pup piece” which can also be referred to as hookup pieces matching the required piping, electrical, structural members etc

Another instance for hookup is connecting piping to equipment, instrument to piping etc. You should have heard of instrument hookup design/drawings.

As an Electrical Engineer..i will answer based on electrical testing and commissioning.. The electrical acceptance testing and commissioning of electric power systems is essential to the start up of any electrical system for the first time, regardless of its size, type or industry. It is a very special occurrence and poses some unique challenges to electrical personnel. So this commissioning will take place after we install the equipment at plaform. And before we start-up and running the equipment, this commissioning must done first.

hope this answers will help you..