In this article7 sections
Commissioning systems and subsystems are the pieces a plant is divided into so that it can be completed, tested and handed over one piece at a time instead of all at once. A subsystem is the smallest part of a process or utility system that can be tested and operated as a unit, and every line, valve, instrument, piece of equipment and cable on the plant belongs to exactly one. The division is drawn on the P&IDs and single-line diagrams as a set of coloured boundaries, often called the systemization markup, and recorded in a register that assigns every tag to its subsystem. From then on completion is tracked per subsystem. Check sheets, punch lists, certificates and handover packages are all issued against it, which is why mechanical completion is granted system by system.
This note covers what the markup shows, where the boundaries go and what the register carries. The certificates issued along the way are covered in mechanical completion to commissioning and the loop checks in the commissioning loop check plan.
Why construction areas become commissioning systems
Construction builds by area, because that is how crews, cranes and scaffolding are organised. Commissioning cannot test an area. It tests a function, such as instrument air reaching every consumer or a pump moving liquid from one vessel to another, and a function crosses area boundaries. Systemization is the translation between the two, and the point in a project where construction starts completing work in the order commissioning needs it instead of the order it was built.
API Recommended Practice 1FSC, Facilities Systems Completion Planning and Execution, describes this systems completion approach for oil and gas facilities. Owners and contractors write their own procedures on top of it, so the names below vary by project while the structure holds.
| Level | What it is | Example |
|---|---|---|
| Unit or facility | The plant or the part of it under one handover | Unit 34 |
| System | A function of the plant | 34-10, instrument air |
| Subsystem | The smallest part of a system that can be completed, tested and operated as a unit | 34-10-02, air dryers |
In this example, instrument air splits into three subsystems, 34-10-01 for the compressors and receivers, 34-10-02 for the dryers and 34-10-03 for the distribution header. The numbering is illustrative. Each project fixes its own format in its completion procedure, and the format appears on every certificate, check sheet and punch item afterwards.
How a boundary is drawn
Each subsystem is highlighted in its own colour along its lines and equipment, and a boundary marker, usually a flag or a symbol carrying the subsystem number, sits wherever a line passes from one subsystem into the next. Lines that continue onto another sheet carry their colour and subsystem number across the off-page connector, so the markup on the receiving sheet picks up where the first one ended.
A boundary goes at a valve, a spectacle blind or a removable spool, somewhere the plant can actually be isolated. A subsystem is pressure tested, flushed and later energised while its neighbours may still be under construction, and a boundary drawn at a point that cannot hold pressure or be spaded off gives commissioning a line on paper and no separation in the field.
| Item at or near a boundary | Where it usually goes | The rule behind it |
|---|---|---|
| Boundary isolation valve | One side only, by the procedure's rule | Assigned once, so it is tested once |
| Spectacle blind or spool | With the valve it pairs with | The isolation point and its blind are one boundary |
| Instrument on a process line | The subsystem of the line it is connected to | Where its process tapping is, not where it is drawn |
| Control or shutdown valve | The subsystem of the line it sits in | Its actuator and position feedback go with it |
| Utility branch to a consumer | The utility subsystem, up to the consumer's isolation valve | The consumer's side belongs to the consumer |
| Motor | The subsystem of the machine it drives | Tested and run together |
| Switchboard feeder | The power distribution subsystem that energises it | Energised before the motor it feeds |
| Field cable | The subsystem of the field device it serves | Checked with the device it connects |
| Control and shutdown system cabinets | Control system subsystems of their own | Powered and tested ahead of the process |
Procedures differ on which side a boundary valve belongs to. Some give it to the upstream subsystem. Others give it to the subsystem commissioned first, so that the valve is already tested and able to isolate the next subsystem while that one is still being built. Both work. Leaving the valve on both sides, or on neither, does not.
Vendor packages usually become subsystems of their own, bounded at the skid edge, with the interconnecting piping and cable on the project side. On a brownfield project, tie-in points are natural boundaries between new scope and the operating plant, and the tie-in number and the boundary marker sit at the same place.
The markup is an agreed document
The commissioning team prepares a first pass and then agrees it with construction and operations. Construction has to finish work to the subsystem boundaries, and operations has to accept each subsystem when care, custody and control are transferred, so all three sign the marked-up set. It is controlled like any other issued document, with the signed originals kept and every later change issued under revision.
The markup also carries the sequence. Subsystems are ranked for completion, and the order generally follows what the rest of the plant depends on: power and safety systems, utilities such as instrument air, cooling water and nitrogen, then the control systems, then process subsystems. A process subsystem that is complete before its utilities cannot be commissioned, so an out-of-order priority buys nothing.
Systemization is revised through the life of the project. Design changes add tags and move lines, owners change the first objective, and a subsystem that turns out too large to complete as a unit gets split. Each revision of the markup has to reach the register the same day, because the register is what the completion work is issued against.
What the subsystem register carries
| Column | What it holds |
|---|---|
| Tag | The line number, equipment number, instrument tag, valve tag or cable number |
| Tag type and discipline | Line, equipment, instrument, valve, electrical or cable; the discipline that installs it |
| System and subsystem | The one subsystem the tag belongs to, and its parent system |
| Drawing reference | The P&ID or single-line diagram, and its revision, where the tag is marked |
| Check sheets required | The installation and pre-commissioning check sheets the tag type calls for |
| Completion status | Installed, mechanically complete, pre-commissioned, commissioned |
| Open punch items | Outstanding items against the tag, by category |
| Handover package | The turnover package the tag is delivered in |
The register is built from the same documents the plant is built from. Instruments come from the instrument index, lines from the line list, equipment from the equipment list and cables from the cable schedule, and the systemization markup supplies the subsystem for each. Every tag in exactly one subsystem is the rule the register exists to enforce. A tag in none is never given a check sheet; a tag in two is checked twice or holds up two certificates.
How the subsystem drives completion
| Stage | Issued per subsystem | Closes with |
|---|---|---|
| Construction | Installation check sheets for every tag, a punch list | Mechanical completion |
| Pre-commissioning | Loop checks, flushing, pressure tests, cable tests, equipment run-ins | Ready for commissioning |
| Commissioning | Functional tests and commissioning procedures | Ready for start-up |
| Handover | The turnover package, with certificates, test records and as-built drawings | Transfer of care, custody and control |
The names of the stages and certificates vary by owner and contractor. What does not vary is that each one is issued against a subsystem, and that a subsystem's status is only as complete as the tags the register assigns to it.
Where systemization goes wrong
A late tag is never assigned. An instrument added at a P&ID revision after the markup was signed exists on the drawing and in the index but in no subsystem. No check sheet is generated for it, and it is found at start-up, uninstalled or untested.
The markup stops at the sheet edge. A line is coloured to the off-page connector on one sheet and picked up in a different colour, or not at all, on the next. The register then carries the same line in two subsystems, or half of it in none.
The boundary cannot isolate. A boundary drawn at a flange with no provision for a blind, or at a valve that does not hold, leaves no way to test one subsystem while the next is open.
Instruments follow the drawing, not the process. A transmitter drawn on one sheet but tapped into a line of another subsystem is assigned by sheet, loop checked with the wrong subsystem and certified against a system it does not measure.
Temporary connections are unmarked. Flushing spools, temporary fills and bypasses used during pre-commissioning cross boundaries. Unless they are marked and tracked, they are removed late or not at all.
Who owns it
The commissioning manager owns systemization, the commissioning leads maintain the markup and the register, construction and operations countersign each revision, and document control holds the signed set. Completion status, punch items and handover packages are all reported per subsystem from then on.
The systemized P&IDs are only as current as the P&ID revision they were marked on. When the design changes after the boundaries are signed, the register has to follow tag by tag, or the completion status of a subsystem describes a plant that no longer exists. For the certificates themselves, see mechanical completion to commissioning; for the checks carried out per loop, see the commissioning loop check plan.