In any engineering project — whether a pressure vessel, structural steel frame, tank or piping skid — drawings serve a specific audience at a specific stage of the project. Two of the most commonly misunderstood drawing types are the General Arrangement (GA) drawing and the Fabrication (or shop) drawing. They look similar to the untrained eye, but they serve fundamentally different purposes and are consumed by entirely different people in the project chain.
Getting this distinction wrong costs money: procurement managers ordering material from a GA drawing alone, or site teams trying to weld from a GA, are common sources of rework and NCRs in oil & gas projects across Saudi Arabia and the GCC.
What is a General Arrangement (GA) Drawing?
A GA drawing communicates the overall layout and relationship of components within an assembly or within a facility. It answers the question: "Where does everything sit, and what does the assembly look like as a whole?"
Key contents of a GA drawing typically include:
- Overall envelope dimensions (length, width, height)
- Key reference dimensions — centreline-to-centreline, nozzle orientations, base bolt circle
- Equipment tag numbers and item references
- Nozzle schedule (size, rating, face type, orientation)
- Tie-in points for piping, electrical conduit entries and instrument connections
- Weight data (empty, operating, test)
- Codes and standards referenced
- Material specifications (high level — e.g., CS to ASTM A36, internals SS316)
GA drawings are typically issued at the design review and procurement stages. They are submitted to the client, process engineer and civil/ structural engineer so each discipline can check spatial co-ordination. They are not sufficient for fabrication on their own.
What is a Fabrication (Shop) Drawing?
A fabrication drawing — also called a shop drawing or detail drawing — is prepared specifically for the workshop fabricator. It provides every dimension, weld detail, material grade, tolerance and finish requirement needed to manufacture a component without requiring any further engineering input.
Fabrication drawings typically include:
- Exact plate/section dimensions with machining tolerances
- Weld symbols to AWS D1.1, ASME IX or applicable standard
- Joint preparation details (bevel angle, root gap, back gouging)
- Bill of Materials (BOM) with item marks, quantities, material grades and heat treatment
- Surface treatment and painting specifications (e.g., SSPC-SP10, primer type, DFT)
- NDE requirements by joint (RT, UT, PT percentages)
- Dimensional inspection hold points
- Handling and marking instructions for site assembly
A fabrication drawing must be self-contained. The welder in the shop should not need to refer back to the engineer to understand what to make. This is why fabrication drawings are significantly more detailed — and time-consuming to produce — than GA drawings.
When Each Drawing Type is Used in a Typical EPC Project
Front End Engineering Design (FEED) / Concept
Only GA drawings or sketch arrangements are needed at this stage. The focus is on confirming spatial envelopes and process connections — not construction detail.
Detailed Engineering
GA drawings are formally issued for client review (IFR/IFA). Once approved, the engineering team develops fabrication drawings in parallel with material procurement. On fast-track projects, fabrication drawings may be issued in packages as equipment is approved.
Construction and Fabrication
Issued-for-Construction (IFC) fabrication drawings govern what is built. Any field deviation is captured as a redline mark-up and incorporated into as-built drawings after construction — a process described in our guide on as-built drawing preparation for brownfield projects.
Summary: Key Differences at a Glance
- Audience: GA = client/design team; Fabrication = workshop fabricator
- Detail level: GA = envelope dimensions + nozzle schedule; Fabrication = every dimension, weld and material
- Stage: GA = design review / procurement; Fabrication = construction
- Sufficiency: GA drawings cannot be used for fabrication; fabrication drawings supersede GAs in the workshop
SLETEC prepares both drawing types for equipment, structural steel, storage tanks and piping skids. Our 2D CAD drawing services cover GA and layout drawings, while our fabrication drawing services deliver fully detailed shop drawings with weld symbols, BOM and surface treatment specifications — ready for workshop use and client document control submission.