# Oil and Gas Pipeline Fault Tree Analysis

The fault tree method was developed in the early 1960s at the Bell Telephone Laboratories for performing an analysis of the Minuteman Launch Control System in regard to safety [12]. Nowadays, the method is widely used around the world for conducting various types of reliability and safety studies. The method is described in detail in Chapter 4.

Here, this method’s application for performing oil-gas long pipeline failure analysis is demonstrated through two examples presented below [13].

Example 11.1

Assume that an oil-gas pipeline failure can occur due to any of the following five events: misoperation, third-party damage, pipeline with defects, pipeline with serious corrosion, and material with poor mechanical properties. The occurrences of two of these events are described below:

• • The event “Pipeline with serious corrosion” can occur due to the occurrence of the following events: Pipeline with poor corrosiveness resistance and corrosion. In turn, the event “corrosion” can be either due to internal corrosion or external corrosion.
• • The event “Pipeline with defects” can be either due to pipeline with initial defects or pipeline with construction defects.

By using the fault tree symbols given in Chapter 4, develop a fault tree for the top event “oil-gas pipeline failure”.

A fault tree for the example is shown in Figure 11.3. The single capital letters in Figure 11.3 denote corresponding fault events (e.g., A: Misoperation, B: Pipeline with defects, and C: Pipeline with serious corrosion).

Example 11.2

Assume that in Figure 11.3, the occurrence probabilities of independent fault events A, D, E, F, G, /,7, and К are 0.15,0.14,0.13,0.12, 0.11, 0.10,0.09, and 0.08, respectively. With the aid of Chapter 4, calculate the occurrence probability of the top event T: oil-gas pipeline failure.

The probability of the occurrence of event H is

where

P(J) is the probability of occurrence of event J.

P(K) is the probability of occurrence of event K.

FIGURE 11.3 A fault tree for the top event: Oil-gas pipeline failure. Similarly, the probability of the occurrence of event В is

where

P(F) is the probability of occurrence of event F.

P(G) is the probability of occurrence of event G.

The probability of the occurrence of the intermediate event C is The top event T (oil-gas pipeline failure) probability of occurrence is

Thus, the occurrence probability of the top event T (i.e., oil-gas pipeline failure) is 0.5100. Figure 11.3 fault tree with given and calculated fault event occurrence probability values is shown in Figure 11.4.

FIGURE 11.4 Redrawn Figure 11.3 fault tree with given and calculated fault event occurrence probability values.

# Common Cause Failures Defense Approach for Oil and Gas Industry Safety Instrumented Systems

Safety instrumented systems (SIS) in the oil and gas industrial sector generally function in the low demand mode, which means that regular inspection and testing are absolutely necessary for revealing their failures. Past experiences over the years clearly indicate that common cause failures’ occurrences are a serious threat to SIS reliability and may result in simultaneous failures of redundant parts/units and safety barriers [14-16].

Thus, a common cause failure may simply be expressed as any instance where multiple units/components/parts malfunction due to a single cause [16]. Some of the causes for common cause failures’ occurrence are common manufacturer, design deficiency, common external power source, operation and maintenance errors, external normal environment, and external catastrophe.

The common cause failures defense approach described below for oil and gas industry SIS focuses on three key aspects presented below [15].

• • To use the insight of failure causes for selecting efficient mechanism to defend against common cause failures’ future occurrences.
• • To highlight common cause failures and their associated causes on the basis of failure reports.
• • To avoid introducing common cause failures during inspection and function testing-related processes.

## Common Cause Failures Defense Approach

The approach is based on six function testing and inspection tasks. These tasks are scheduling; preparation, execution, and restoration; failure reporting; failure analysis; implementation; and validation and continuous improvements. Thus, the approach is composed of six tasks based on checklists and analytical methods, such as operational sequence diagrams (OSD), influence diagrams, and cause-defense matrices. The six tasks are described below [15].