Prescriptive / Non-Prescriptive Process Safety
Previous posts in this series have defined the term ‘Process Safety Management’. Those posts include:
In this post we discuss the distinction between prescriptive and non-prescriptive standards.
Safety management programs can be divided into two broad categories: prescriptive and non-prescriptive. Prescriptive programs, which are often developed and enforced a regulator or a standards-setting body, are what most people think of when safety is discussed. For example, OSHA (the U.S. Occupational Safety and Health Administration) has the following rule to do with ladders:
All rungs shall have a minimum diameter of three-fourths inch for metal ladders, except as covered in paragraph (b)(7)(i) of this section and a minimum diameter of 1 1/8 inches for wood ladders.
A statement such as that is quite clear and uncompromising; it is also easy to follow. A person installing a ladder at an industrial site need not think about the basic principles of ladder design. Nor does that person have to carry out any type of risk analysis. He or she simply has to follow the rule as written.
Another example of prescriptive standards for offshore facilities comes from API Recommended Practice 14C. This standard, which is to do with the prevention of vessel over-pressure, states,
The safety system should provide two levels of protection to prevent or minimize the effects of an equipment failure.
Although more nuanced than the OSHA rule to do with ladders, the above sentence is also prescriptive. It does, however, allow for some non-prescriptive judgment. For example, it does not specify the types of protection. Most designers will protect against over-pressure using a combination of safety instrumentation and a mechanical device such as a pressure relief valve, but this approach is not actually a requirement.
The other style of risk management is non-prescriptive. A non-prescriptive rule states, ‘Do whatever it takes on your facility not to have accidents’. The only measure of success is success. Companies are not provided with specific and detailed rules telling them what to do; instead managers at these companies are called upon to take the appropriate actions needed to keep their facilities safe. The basic idea behind this approach is that the companies that operate sophisticated facilities such as offshore platforms and drilling rigs are the ones who know the process and equipment the best, so they are the best qualified to decide what needs to be done to achieve safe operations.
Non-prescriptive standards are, of necessity, performance-based because there is no external specific standard against which they can be assessed. Non-prescriptive standards are aspirational.
One consequence of this approach is that PSM regulations are often quite short. For example, OSHA’s process safety standard is only about ten pages long.
The non-prescriptive/performance-based approach to safety management means that there are no universally correct answers as to what needs to be done. What is appropriate in one location may or may not be appropriate in another. All that is required is that programs be in place, and that they be adhered to and that they work. (In this regard, PSM is similar to ISO 9000 and other quality standards, which also require that companies set their own standards, and then adhere to them.)
The non-prescriptive approach can be illustrated by the following quotation from OSHA’s Process Safety Management standard to do with Mechanical Integrity. It states,
The employer shall establish and implement written procedures to maintain the ongoing integrity of process equipment.
The standard provides no specific requirements regarding the amount of detail or the content of those procedures.
Advantages of the non-prescriptive/performance-based approach in a process safety context include the following.
Industry can quickly capture what has been found by experience to work well — there is no need to wait for regulators to catch up with the latest technology.
The causes of major events are typically complex, and involve a series of low probability causes. It is very difficult to write standards and regulations to cover such a wide variety of complex trains of events.
There is no way that a regulator can develop prescriptive standards for the many and varied industries that are covered.
One difficulty with a performance-based approach to safety is that it is never possible to be ‘in compliance’. The only truly acceptable level of safety is perfection — an environment in which injuries and fatalities never occur. Yet, no matter how well run a facility may be, a zero-accident rate is an unattainable goal. Risk can never be zero. Indeed, if a facility operates for long enough, it is certain — statistically speaking — that there will be a serious accident. Therefore management has to determine a level for ‘acceptable safety’ ― a difficult topic that we will discuss in a later post.
In practice, process safety management programs combine the prescriptive and non-prescriptive approaches. For example offshore safety is often managed through the use of Safety Cases; a non-prescriptive approach. However, many actual Safety Cases make reference to the American Petroleum Institute’s Recommended Practice 14C, a mostly prescriptive standard.
