Inspection can reduce risk mainly by reducing the probability of failure. Probability of failure of an in-service damage mechanism is affected by three factors:
1. Type of damage.
2. Damage rate.
3. Probability of detection of the existing inspection program.
Damage can be of type:
2. Surface cracking
3. Subsurface cracking
4. Microfissuring/micorvoid formation
5. Metallurgical changes
6. Dimensional changes
8. Material properties changes
Inspection method differs in their effectiveness when applied to different type of damage. Dye penetrant technique maybe effective when used to detect surface cracking, but it can’t detect subsurface cracking. Depends on the mechanism, above damage type can be occurred uniformly or locally. Sulfuric acid corrosion will result in uniform thinning at low velocity, while at high velocity it turns into a localized one. Uniform damage can be detected by inspecting any relevant part of the surface. When it comes to localized damage, application area of the inspection technique will affect the probability of detection. This effectiveness will affect the level of confidence on damage rate data obtained from inspection measurement.
Performing inspection on entire surface of an equipment is unpractical and will render high cost. Usually inspection is performed on a certain portion of the pressure containment boundary. Therefore the damage rate data obtained from inspection is sample data. Because damage rate is a statistical variable, there is a likelihood that exist damage rate value higher than measured.
Accuracy of an inspection technique is varied depending on operator skill and test conditions. Accuracy can be measured by repeating test on flaws of known dimension and recording the result. A distribution is then fitted into the data and a probability of detection curve is obtained.
All factors mentioned above contributes to uncertainty of the damage rate data. This uncertainty will increase probability of failure.
Inspection program is developed in order to reduce this uncertainty by selecting more effective inspection method, determining the area of test, and controlling inspection intervals. Increasing level of confidence on damage rate data will reduce the risk value associated with a particular damage mechanism.
The concept of risk reduction can be employed to optimize inspection program by:
1. Increasing inspection frequency or upgrading to more effective method if the risk reduction is insufficient.
2. Downgrading the level or decreasing the inspection frequency if no significant risk reduction gained from higher level of inspections.
While probability of failure can be reduced using better inspection program, consequence of failure can be reduced through the use of detection, isolation and mitigation system. Because the value of consequence of failure is a function of quantity of released fluid during leak, effectiveness of leak detection and isolation system will affect the value of consequence of failure. Mitigation system reduce consequence of failure by reducing the damage severity caused by post-leak event.