Accidents system factors

ABSTRACT Four hundred and sixty seven coal gas explosion accidents that occurred in China between the years of 1950 and 2000 were investigated through statistical methods so as to review the overall situation and provide quantitative information on coal gas explosion accidents. Statistical characteristics about accident-related factors such as space, time, gas accumulation reasons, gas grade, ignition sources, accidents categories, and accident economic loss were analyzed. Some special conclusions have been achieved. For example, most gas explosion accidents were found to have concentricity on the space-time and hazard characteristics. Such results may be helpful to prevent coal gas explosion accidents. Moreover, comments were made on APS (Accident Prevention System) and safety culture. In conclusion, countermeasures were proposed in accordance with the results of statistical studies, including the change of safety check time. 

Causality models based on event chains (or dominos or layers of Swiss cheese) are simple and therefore appealing. But they are too simple and do not include what is needed to understand why accidents occur and how to prevent them. Some important limitations include requiring direct causality relationships, subjectivity in selecting the events to include, subjectivity in identifying chaining conditions, and exclusion of systemic factors. 

The third set of causal factors is only indirectly related to the events and conditions, but these indirect factors are critical in fully understanding why the accident occurred and thus how to prevent future accidents. In this case, the systemic factors include the owner of the ferry (Townsend Thoresen) needing ships that were designed to permit fast loading and unloading and quick acceleration in order to remain competitive in the ferry business, and pressure by company management on the captain and first officer to strictly adhere to schedules, also related to competitive factors. 

Several attempts have been made to graft systemic factors onto event models, but all have important limitations. The most common approach has been to add hierarchical levels above the event chain. In the seventies, Johnson proposed a model and sequencing method that described accidents as chains of direct events and causal factors arising from contributory factors, which in turn arise from systemic factors (figure 2.7) [93]. 

In addition, there does not seem to be any reason for assuming that initiating failures are mutually exclusive and that only one starts the accident, except perhaps again to simplify the mathematics. In accidents, seemingly independent failures may have a common systemic cause (often not a failure) that results in coincident failures. For example, the same pressures to use inferior materials in the foundation may result in their use in the jacket and the deck, leading to a wave causing coincident, dependent failures in all three. Alternatively, the design of the foundation—a systemic factor rather than a failure event—may lead to pressures on the jacket and... 

Most accidents in well-designed systems involve two or more low-probability events occurring in the worst possible combination. When people attempt to predict system risk, they explicitly or implicitly multiply events with low probability— assuming independence—and come out with impossibly small numbers, when, in fact, the events are dependent. This dependence may be related to common systemic factors that do not appear in an event chain. Machol calls this phenomenon the Titanic coincidence [131]. ... 

Escaping from the Whack-a-Mole trap requires identifying and eliminating the systemic factors behind accidents. Some common reasons why safety efforts are often not cost-effective were identified in chapter 6, including ... 

Perform in-depth incident and accident investigations, including all systemic factors. Assign responsibility for implementing all recommendations. Follow up to determine whether recommendations were fully implemented and... 

In addition, accidents and incidents should be treated as opportunities for learning and investigated thoroughly, as described in chapter 11. Learning will be inhibited if a thorough understanding of the systemic factors involved is not sought. 

The accident was thoroughly investigated including, to the Navy s credit, the systemic factors as well as the technical failures and deficiencies. Deep sea photography, recovered artifacts, and an evaluation of the Thresher design and operational... 

Most accident analyses stop at this point, particularly in that era. To their credit, however, the investigation continued and looked at why the technical deficiencies existed, that is, the management and systemic factors involved in the loss. They found deficient specifications, deficient shipbuilding practices, deficient maintenance practices, inadequate documentation of construction and maintenance actions, and deficient operational procedures. With respect to documentation, there appeared to be incomplete or no records of the work that had been done on the submarine and the critical materials and processes used. 

In addition, Bill Johnson used a scroll as a normally expected event and an oval as a satisfactory event. The normally expected event distinguishes events that are typically a part of any system, such as change and normal variability. The satisfactory event describes events that may be accident causal factors but are a necessary part of the operation, like functional (part of the system) people or objects in the energy channel. Also, in addition to using the traditional transfer symbol (a triangle), the MORT chart includes capital letters as drafting breaks and small ellipses as risk transfers (Fig. 18-3). 

The purpose of MORT analysis is to provide a systematic tool to aid in planning, organizing, and conducting an in-depth, comprehensive accident investigation (or inspection, audit, or appraisal) to identify those specific control factors and management system factors that are less than adequate and need to be corrected to prevent recurrence of the accident (or to prevent other undesired events). 

Nonfunctional If the persons or objects in the energy path were not there as a necessary part of the system, they are nonfunctional. This class generally includes visitors, passers-by, the great American public, intruders, and perhaps even organizational members with no requirement to be at the accident location when the accident occurred. Factors to be evaluated include... 

The root causes of most accidents are found during an analysis of the management system factors (Fig. 18-19). This major branch of the chart addresses the why issues. Specific factors and branches to be analyzed include... 

Step 3 Identify one important systemic factor that has played a key role in the occurrence of accident/incident under consideration. This factor is the result of team consensus and serves as an initial point for all further investigations. 

Chapter 11 of this text discusses the use of fault tree analysis in determining system reliability, failure potential, and even accident cause factors through examination of specific or general fault paths. Additional information on the application and use of probability values in fault tree analysis is also provided in Chapter 11. 

By moving beyond specific, low-level details of accidents and incidents to consider broader oiganizational and system factors, these techniques enable commonalities between different accidents to be identified and problem areas to be prioritized and addressed. 

This analysis is broadly applicable to industrial accidents as well. There are both latent (system) factors and active (individual) factors which can be identified in most if not all accidents. It is thus quite misleading to suggest that a certain proportion of accidents can be attributed to unsafe acts by workers and another proportion to unsafe conditions or systems in which the work is carried out. 

Heinrich, at the time, was an insurarKe investigator, who based his research on examining accident reports completed by company supervisors prior to 1931. He concluded from his research that approximate 90% of all accidents were caused by the unsafe acts of workers. It would be interesting to know if those compare supervisors had adequate skills and knowledge of the work system to identify the true accident causation factors, or if they simply adopted a blame mentahty when accidents occurred. 

Experience has shown that there have been occasions when hazards and potential accident causation factors have gone undetected because the workplace was not inspected under normal working conditions. There should not be an underestimation of the possible benefits that can be realized from having workplace inspections conducted by people who do not regularly work in the area. People carrying out inspections who are unfamiliar with a particular workplace or work system are forced to ask questions of the locals. The resulting exchange of information can lead to the identification of opportunities to improve safety levels overlooked by the locals because of either familiarity or complacency with their own work environment. 

It is possible to take atty of the accident causation factors and break them down further in order to closer examine the influence that particular factor is having on the work system. This second tier analysis further increases knowledge about the work system through developing a better understanding of the individual processes that make up the system. It is in this area of investigation and analysis where we come closest to discovering tme accident causation. 

Once the various types of energy affecting the system have been identified, the ETBA worksheet should be completed. Figure 9.1 shows a sample ETBA worksheet. The information recorded on the completed ETBA worksheet can then be used to perform subsequent analyses (PHL, PHA, etc.) along with their related reports. In some cases, depending upon the level of detail desired, the ETBA itself may provide an adequate amount of information to be included in the final PHA. In fact, since hazardous events can usually be associated with some type of energy transfer and, since accident causal factors typically involve the absence of controls or the failure of existing barriers and. 

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