The Basic Structure of Operating Procedures
During the Sleipner Field Project, SAT developed the Node Diagram and a structure for operating procedures which has become widespread in the oil and gas industry. Operating procedures may be divided into four groups:
- Normal start-up after a major turnaround and normal shut-down for a major turnaround.
- Trip Restart procedures covering restart after shut-down levels APS, ESD, PSD3 and PSD4. Black Start is a special case of APS restart, i.e. total black-out with potential gas in ventilated areas.
- Routine operations procedures covering procedures for service and special operations for each system.
- Maintenance procedures covering process, utility and electrical equipment.
The Node Diagram
Start-up and shut-down or trip and restarts are operations which should be handled safely and efficiently. This may be achieved if the elements of operations are standardized and well known to all operators. Standardized operations means that most cases of start-up and shut-down or trip and restarts follow the same procedure route between operating levels. An operating level is a stable operating status like:
- Black platform status
- Emergency power only in operation
- Essential power in operation (and SAS system and utilities)
- Main power in operation (and associated utilities)
- Wellhead, oil/condensate treatment and export in operation (gas flaring)
- Gas conditioning and export in operation
- Gas injection (if relevant) in operation
In the case of a start-up (after a turnaround) or a restart (following a trip), the platform systems are started in a sequence that allows the operating levels to be reached in this order. The order of start-up of individual systems between the main operating levels must typically follow some sequence (by design). SAT developed the start-up diagram (also known as the Node Diagram) to enable us to map the total sequence of system start-ups and activities.
Start-ups or restarts of all systems are performed between the main operating levels in the Node Diagram. A node is a system procedure. The procedure structure is basically the same for all systems. First there is a system Status which forms the basis for the procedure. Then Preparations for Start-up follows. (Status and Preparations are in the Industry often labelled as Prerequisites). Finally the actual start procedure is introduced. Each node has a unique number and name, which identifies its place in the diagram, and to which Node Diagram it belongs. Since start-ups and restarts will differ due to different individual plant status prior to start-up or restart (e.g. inert or pressurized or depressurized process), each defined case must have its dedicated Node Diagram. The structure of the various Node Diagrams for a plant will essentially be identical, but the procedures for some of the systems will be different. See sample Node Diagrams below.
The electric power systems and the main process systems represent the main operating levels. They are therefore treated differently from the support systems in the Node Diagram. For these systems, the Status and Preparations for Start-up are separate nodes. This means that they can be carried out in parallel with support system start-ups. When all the systems supporting each electrical power system have been started, the power system can be started. Similarly, the main process can be lined up (status and preparations) and await pressurizing and start-up with hydrocarbons when all process support systems are in operation. The support system nodes contain Status, Preparations for Start-up and Start.
The procedures in the nodes collectively describe all systems interdependencies, and steps and actions necessary to start the plant. The Node Diagrams with associated nodes and procedures allow early “on paper” operation of the plant. This is of great value since it represents:
- Overall plant systems verification
- Operating procedures verification
- Early training capability in plant operations, e.g. through simulator training
The most extensive of the plant restarts is the Black Start. This type of restart has to consider the following:
- The installation has been shut down for some time.
- All ignition sources have been isolated.
- All UPS batteries are discharged.
- Compressed air is unavailable
- The main process has been depressurized.
- There may be HC gas in mechanically ventilated (safe by ventilation) areas.
Black Start is normally the first case to develop. The Black Start Node Diagram will then be the basis for all subsequent start-up and restart cases. As explained above, the different Node Diagrams will normally be identical in the sequences and structure, while the individual system procedures differ depending on the system status prior to start-up.
During initial start-up of offshore installations there is a shut-down system test. The final step in this test is to initiate and verify an APS which shall result in a complete black-out. The restart can then be carried out using a Black Start procedure. The process plant is inert and ready to receive HC fluid. There is no HC gas in any ventilated areas. The full procedure should still be carried out to verify the full Black Start procedure for the plant.
The Black Start procedures will form the basis for:
- Shut-down for Turnaround and start-up after Turnaround
- Trip-restart cases.