On an operating plant, structural modification drawings must show three things clearly: what exists, what will be removed, and what will be added. All of it has to rest on existing-condition information that someone has verified. The detailing itself resembles new-build work. What makes brownfield different is the uncertainty of existing records, limited site access and the cost of rework during a shutdown.
Why is brownfield drawing work different?
- Records drift. Original drawings rarely reflect every later change, and maintenance modifications may never have been recorded.
- Access is restricted. Permits, live process areas, offshore logistics and shutdown windows limit how often anyone can go and measure.
- Rework is expensive. Steel that does not fit during a shutdown can extend the outage. Research on offshore brownfield work notes that where accessibility is restricted and rework is expensive, accurate front-end design matters, and that laser scanning with 3D modelling can improve modification work.
- Interfaces are live. Existing piping, cable trays and equipment cannot simply be moved to suit new steel. This is typical of onshore oil and gas facilities and offshore installations alike.
How are existing conditions established?
Existing-condition information comes from several sources of differing reliability:
| Source | Typical reliability | Use |
|---|---|---|
| Laser scan / point cloud, registered and controlled | High for geometry captured; blind spots possible | Modelling existing steel, clash checks, tie-in locations |
| Dimensional survey of critical tie-points | High for measured points | Final fit-up dimensions |
| Recent as-built drawings or models | Medium; depends on how well they have been maintained | Starting reference |
| Original design or IFC drawings | Low to medium | Member sizes, intent |
| Site photographs and sketches | Supporting only | Clarification |
Reliable workflows combine scan data with traditional measurement of critical tie-points. One survey provider describes recording critical points by direct measurement "as these are required to guarantee successful fit up". The detailer should not treat unverified records as fact. Discrepancies are raised as queries.
Existing, removed and new work conventions
Modification drawings commonly distinguish:
- Existing to remain: shown in a lighter line weight or a distinct layer or colour.
- Existing to be removed: shown dashed or hatched, with removal notes.
- New work: shown in full line weight, with marks and fabrication information.
- Temporary works and supports: shown only where in scope, and with responsibility stated.
The convention should follow the client's drawing standard. Each drawing should state which survey or record revision it is based on.
Tie-ins and site-fit allowances
Where new steel connects to existing members, decide in advance:
- whether connection holes are shop-drilled from survey data, or site-drilled;
- whether site-fit plates, slotted holes or extra length ("site trim") are provided;
- who checks the existing member's capacity for new loads. This is an engineering task, not a detailing one;
- whether hot work is permitted, because it affects bolted versus welded tie-ins.
Updating as-built records
After installation, the record should reflect what was actually built:
- 1Collect red-line mark-ups or post-installation survey data from site.
- 2Update the drawings and model to match the verified information.
- 3Remove or archive superseded information, and mark the drawings as as-built or record status per the client procedure.
- 4Issue the updated register.
The updated record becomes the starting point for the next modification. That is why brownfield sites benefit from keeping models current.
Responsibility split
| Task | Typically owned by |
|---|---|
| Design of the change and checking existing capacity | Client or EPC structural engineer |
| Survey and field validation | Survey contractor or site team |
| Modification drawings and model updates | Detailer / drafting team |
| Installation and red-line mark-ups | Construction contractor |
| As-built updates | Detailer, from authorised and verified field data |
PetroHill updates records from authorised, verified field information. Who owns field validation is clarified before work begins. See also our notes on existing plant rehabilitation documentation.
Frequently asked questions
Can you work from a point cloud we already have?
Generally, yes, where the scan is registered, its coordinate system is defined and the formats are compatible with the agreed software.
Do you perform laser scanning?
Our scope is drawing and model production. Scanning and site survey are provided by the client or a survey contractor unless separately agreed.
What if the scan and the old drawings disagree?
The discrepancy is logged and raised. Where it matters for fit-up, it is resolved by targeted field measurement.
Next step
For replacement steelwork, equipment-support changes and record updates, see brownfield structural modifications and as-built documentation, or discuss your modification scope.
Related services
References
Technical sources consulted. Standards are paid documents; confirm clauses against the licensed edition named in your project specification.
- Laser scanning and 3D modelling in brownfield offshore facilities: Case studies · ResearchGate (paper abstract) (opens in a new tab)
