An ‘incident’ usually occurs when a ‘system’ breaks down, as most systems have multiple ‘barriers’ that prevent failure; for example, we will be injured in an automobile accident if:

  • we are involved in an accident;
  • our reactions fail to avoid the accident;
  • our automobile protective devices (air bags, etc.) fail; and,
  • etc.

When a pipeline fails due to external corrosion, the corrosion protection ‘system’ has failed (Figure 1):

  • the coating has failed;
  • the cathodic protection has failed;
  • our inspections (internal and external) have failed to detect the corrosion;
  • etc.
Figure 1. Corrosion Failures are Prevented by ‘Systems’ made up of ‘Barriers’ [1, 2].

But… why does this ‘system’ break down? The barriers preventing the failure, are not perfect: they will have faults (holes), Figure 1. A failure from corrosion will occur if all the faults in the barriers line up. It is like ‘Swiss cheese’… and is called the ‘Swiss cheese model’ [1, 2].

Failure will only occur if all the faults in the barriers line up. Today we look at the engineering barriers, but others, such as management, culture, etc.

Figure 2. Barriers Preventing Incidents.


  1. J Reason, ‘Too little and too late: a commentary on accident and incident reporting systems’, in: Schaaf van der, et al. (Eds.), Near miss reporting as a safety tool, Butterworth Heinemann, Oxford. 1991.
  2. J Reason, ‘Managing the Risks of Organizational Accidents’, Ashgate Publishing Limited. 1997. ISBN 1 84014 104 2.