Fault Tree Analysis

What is fault tree analysis?

Fault tree analysis is a systematic approach of identifying the main cause of an event, such as a failure event, with the use of a fault tree diagram.

Overview

A key to uplifting the overall performance of a plant is to increase reliability and availability. While assets ideally run as planned, there will always be unforeseen events that can cause unplanned downtime or near misses that would have caused adverse breakdowns. These failure events may only be symptoms of a more serious problem, and without getting to the root cause, you can only expect the issues to recur. Without proper investigation and resolution, it won’t be a question of “if” the issues will happen again, rather a question of “when” they will haunt you in the future.

To get to the actual cause of a problem, as opposed to just addressing the symptoms, root cause analysis (RCA) methods can help make the process more systematic. Performing fault tree analysis is a visual exercise that can help to lay out the succession of events that could have caused a breakdown.

Fault tree analysis employs a top-down approach – it begins with a basic idea or a general event, which then branches out to go into more specific detail. From the topmost scenario, preceding events that may have caused the outcome are mapped out through a fault tree diagram.

Typical logic gate symbols. Images from Quality Digest.

The diagram ends with the most basic causes of the failure. The last layer would contain the root cause for the breakdown, and can provide valuable insight on corrective action.

A typical fault tree diagram resembles the following:

Sample fault tree analysis diagram. Image from Accendo Reliability.

Why use fault tree analysis?

Data, together with the technical expertise and experience from the maintenance teams, provide invaluable insights to why failures occur. Fault tree analysis procedures provide a framework of how to systematically transform available information into a concrete plan of action. The goal is to identify the most basic cause of a particular fault.

Aside from being a visual tool to qualitatively describe causes of failure, fault tree analysis can also be used to quantify the probability that a particular event will occur. That is, if it is combined with probability and statistical concepts. With techniques such as the method of cut sets (MOCUS), failure events can be predicted quantitatively based on calculations from available data.

Fault tree analysis vs. FMEA

Reliability is one of the key characteristics sought after by an organization. It can define the overall performance of a plant’s operations. Naturally, a lot of methods are being developed to increase reliability and reduce failures. Two methods that are usually brought up when talking about failure identification are fault tree analysis and failure mode and effects analysis (FMEA).

Looking at each procedure, these two seem like exact opposites – fault tree analysis uses a top-down approach starting with a failure event, while FMEA employs a bottom-up approach starting with all potential failure modes and ending with their effects to the system performance.

Fault tree analysis identifies the combination of dependent or independent conditions that will lead to a certain failure event. FMEA on the other hand, identifies all potential failure events, to understand their impact to the operating conditions of the plant.

How these methods approach the issue might be fundamentally different, however, studies suggest that there is great value in using both together. Using fault tree analysis and FMEA as part of maintenance culture allows for more data-driven decisions.