Unit tour. Conduct a screening-level walk through the unit, to spot-check effectiveness of SHE-related management systems and to identify SHE hazards and operational issues not previously pinpointed by the team. Such a tour typically takes 2–4 hr. During this phase, the team must:
1. Test supporting systems in place to ensure transient operations are completed in a safe and effective manner;
2. Scan the unit for general process safety issues within the scope of the review; and
3. Identify any potential human factors issues that could potentially contribute to a significant consequence event. Check features such as:
• labeling of important equipment and lines;
• location of crucial valves;
• arrangement of valves that need to be operated in critical sequences;
• placement of manual control valves and their associated local meters;
• whether equipment and lines can be located and safely isolated during an emergency; and
• whether environmental factors, such as visibility and access, have been adequately addressed in the design.
It may be preferable to schedule this tour later in the study, to better assess issues identified as a part of the procedure review. The team should consider any issue with potential to contribute to release of a highly hazardous material as a finding.
Documentation. The reporting phase of the TOH process involves documenting the scope of the review, team composition, documentation reviewed, redlined copies of all procedures reviewed, as appropriate, identified follow-up items and associated risk assessment, as applicable. Include following lists in the final documentation of the completed HAZOP:
• team members, responsibilities and years of experience;
• procedures reviewed;
• incident investigations scrutinized; and
• other documentation examined, as appropriate — e.g., summary of MOC metric data, facility siting studies, process flow diagrams, P&IDs, MSDSs, electrical area classification drawings, safety relief review studies, and SHE critical equipment lists.
ExxonMobil has rolled out the TOH method at refining and chemicals manufacturing operations worldwide. More than 90% of manufacturing sites globally have finished initial TOH application. The TOHs completed to date are identifying findings of significance and providing value to the business. In addition to recommended procedural controls, application of the TOH methodology has determined the need for, and recommended, potential additional hardware and software hazard controls. The TOH methodology truly is more than just a procedures review.
ExxonMobil Refining and Supply and Chemical Companies have systematized the application of the TOH methodology through their global manufacturing operations integrity management system practice, ensuring a unit HAZOP specifically focused on transient operations is completed after the second HAZOP cycle. Additionally, global reliability system elements include milestone-driven application of the TOH methodology during turnaround planning and specific abnormal and non-routine operations.
In late 2008 approximately 1,200 findings from 27 completed TOH studies were analyzed for common findings, with the learnings then captured and communicated, as appropriate, through the organization. The ultimate goal is to enhance organizational knowledge so risks associated with process hazards can be consistently controlled to acceptable levels across the business.
A Valuable Tool
The TOH methodology can serve as a powerful supplement to traditional HAZOPs. Its focus on infrequently performed operations that require an increased level of human interaction with the process addresses situations that generate 50+% of medium- and higher-risk process safety incidents. The outcome is more-complete and easier-to-follow procedures for managing the process through transient states; increased operator awareness of hazards, design controls and the potential consequences of mal-operation; and experience in applying procedural controls that can be applied beyond those procedures covered in the TOH process.
Scott W. Ostrowski and Kelly M. Keim are process safety engineering associates for ExxonMobil Chemical Co., Baytown, Texas. E-mail them at Scott.W.Ostrowski@ExxonMobil.com and Kelly.K.Keim@exxonmobil.com.
1. Duguid, I. M., "Analysis of Past Incidents in the Oil, Chemical and Petrochemical Industries," Loss Prevention Bulletin, No. 142, p. 3, Institution of Chemical Engineers, Rugby, U.K. (1998).
2. Duguid, I. M., "Analysis of Past Incidents in the Oil, Chemical and Petrochemical Industries," Loss Prevention Bulletin, No. 143, p. 3, Institution of Chemical Engineers, Rugby, U.K. (1998).
3. Duguid, I. M., "Analysis of Past Incidents in the Oil, Chemical and Petrochemical Industries," Loss Prevention Bulletin, No. 144, p. 26, Institution of Chemical Engineers, Rugby, U.K. (1998).
4. "Process Safety Management of Highly Hazardous Chemicals," 29 C.F.R. § 1910.119(e)(6), U.S. Occupational Safety and Health Administration, Washington, D.C. (2008).