Forewarned is forearmed. Nowhere is this old saying more true than it is for process safety. To secure the leading indicators it needs to tell further in advance what bad actors might potentially cause safety issues, RasGas Co. in Doha, Qatar, began a demand on safety system (DOSS) project about nine months ago on the seven liquid natural gas (LNG) production trains that make about 37.1 million tonnes of LNG per year from the 95 kilometers of pipelines, wellhead platforms and onshore facilities serving its huge, offshore North Field in the Arabian Sea.
"DOSS determines the frequency and quantity of the demands against the process parameters operating on the verge of safe operating limits (SOL), and provides an added opportunity to identify and correct weakness in the control systems," says Navid Farooqi, safety instrumented system (SIS) engineering advisor at RasGas, who spoke on Sept. 12 at Invensys' Foxboro and Triconex Global Client Conference 2013 in San Antonio, Texas. "DOSS monitors the performance and flaws in process designs, instruments, always-in-demand DCSs with continuous control, and operator responses."
Farooqi reports that RagGas started its DOSS program by seeking and defining initiators that were or might get triggered, and by determining the criticality of the loops where they were located. Next, it analyzed events to identify bad actors and is addressing applicable issues to rectify any flaws in its overall loops.
To secure data for its DOSS, Farooqi and his colleagues developed DOSS key performance indicator (KPI) reporting procedures and scorecards. These reports include SIS process trip indicators such as valid signals from process high-high (HH) and low-low (LL) alarms; activation of mechanical shutdown events, such as turbine over-speed trips and mechanical trips; and activation of mechanical shutdown events. Exclusions include alarms due to startups, preventive maintenance or overrides; initiators due to routine tests or planned shutdowns; machine monitoring system (MMS), chattering, duplicate or bad I/O alarms.
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To generate DOSS reports, engineers and operators at RasGas use a "Consequential Alarm Report" utility in the alarm management system in its Plant State Suite (PSS) software. "This utility generates a report of defined alarm tags, which have HH, LL or shutdown block names, such as UZ, as part of their tagnames," explains Farooqi. "Corresponding to every alarm, whether it's HH or LL, is a set of consequential alarms. This includes all the HH, LL or UZ alarms that appeared one minute before or after any event. The same is repeated for all HH and LL alarms. We must generate reports with well-defined alarm tags to make sure we're not getting any fake data."
Farooqi adds the DOSS reports also include:
- A database of safety integrity level (SIL) loops, which is a collection of SIL references generated through RasGas's SAP system. This database is a reference to assign the SIL level against each initiator in the DOSS report.
- Segregation of initiators categorized based on their asset, SIL level, number of times an initiator get triggered, and type of initiator, such as process HH or LL alarms or manual shutdown, such as by pushbutton or mechanical shutdown from over-speed or vibration.
- Filters configured in the report. For example, the initiator must have the UZ as "following" against it; all the alarms must be "state" alarms, which are valid alarms coming from the TMR system; data removed due to planned shutdown; chattering alarms removed because they appear more than three times; and data removed due to LNG loading tests offsite.
The DOSS KPI reports also summarize and present their results on scorecards with user-friendly spreadsheets, bar charts and pie charts. These scorecards also group all applicable events by number of occurrences, initiator counts and if they're at SIL-1, SIL-2 or SIL-3 levels. Farooqi says the reports and scorecards make it easy to see any bad actors.