A modular approach is normal for fabrication of an offshore oil and gas project. The substructure Jacket is fabricated separately and the topside is fabricated separately.
Topside elevations are first fabricated on the ground. This is to avoid welding in elevation. Temporary supports are placed under main beams to prevent excessive deformations under own dead load. Before installation of columns and braces each elevation is a flexible plane structure. Some of the piping, electrical, instrument, and vessels supports may be fabricated with the primary and secondary members. Plating is also placed. Sometimes deck penetrations including piping, electrical, instrument, gutters, drains, etc. are punched and actual cutting is left during installation of related crossings.
Topside fabrication requires a very good coordination between engineering, procurement, and yard teams. The best condition is when equipment of each elevation arrives at the time that elevation is ready.
Also, the fabrication tonnage capacity of the yard that have necessary facilities and experience in the construction of such facilities dictate the fabrication methodology.
The scope of work could be broken down to offer options which is usually dictated by schedule, quality and economic considerations.
E2P MAGNUS GLOBAL TECH can assist the Company or contractor who wish to undertake such constructability studies so that most viable option is selected.