Best Practice for Risk Based Inspection

1.4. DEFINITIONS
Within this report, any unintentional release of stored energy and/or hazardous
contents from a pressure system or containment constitutes a failure. Failure usually
involves a breach in the containment boundary and a release of contents into the
environment. In extreme cases, stored energy may be released as a high pressure jet,
missiles, structural collapse or pipe whip and contents may be flammable and/or
toxic.
The probability of failure is the mean frequency or rate with which the specified
failure event would be expected to occur in a given period of time, normally one
year.
The consequence of failure through the unintentional release of stored energy and
hazardous material is the potential for harm. Duty Holders have a responsibility to
assess the potential harm to the Health and Safety of employees and/or the public,
and to the environment from pollution and other damage. They may also
legitimately consider the consequences of failure on their business, such as the costs
of lost production, repair and replacement of equipment and the damage to of the
company reputation.
The risk of failure combines the probability of failure with a measure of the
consequences of that failure. If these are evaluated numerically, then the risk is
defined as the product of the probability of failure rate and the measured
consequence. There can be different risks for different measures of consequence.
Despite this definition, risk is often assessed qualitatively without this formal
factoring. In this situation, risk is the combination of the qualitatively assessed
likelihood and consequences of failure and is often presented as an element within a
likelihood-consequence matrix. (Within this report, ‘probability’ is used in
association with quantitative assessments and ‘likelihood’ is used in association
with qualitative assessments of risk).
1.5. CAUSES OF FAILURE
Root causes of failure of pressure systems, tanks and other containers include:
• Inadequate design and/or material for the loading and operating environment.
• Incorrect and/or defective manufacture.
• Unanticipated in-service deterioration such as corrosion or fatigue cracking.
• System errors in operation or maintenance or over-pressure protection.
• Malfunction of instrumentation, control systems or feed and utility supplies.
• Human factors including deliberate damage.
• External events such as fire, impacts or storms.
An integrated integrity management strategy will contain measures that address and
mitigate the possibility of these root causes of failure. Design reviews,
manufacturing quality assurance, operating training, and systems analyses are
examples of such measures. In-service inspection is a backstop to prevent failure
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