The Definition of Bow-Tie Analysis

Definition of Bow-Tie Analysis

Bow-tie analysis is a visual risk assessment method that shows how a hazard could get out of control, what could cause that to happen, what harm could follow, and which barriers stand in the way. The diagram is named for its shape: causes fan out on the left, consequences fan out on the right, and they meet at a central "top event" in the middle, like the knot of a bow tie.

The method is widely used in process safety, oil and gas, chemicals, mining, aviation and other high-hazard industries. It matters because it turns a complex risk into a single picture that operators, engineers and managers can all read. That makes it easier to see which barriers are carrying the most weight, where protection is thin, and what needs to be inspected, tested and maintained to keep major accidents from happening.

How Bow-Tie Analysis Works

Core Elements of a Bow-Tie Diagram

• Hazard: Something with the potential to cause harm, such as a flammable liquid under pressure, a suspended load or a vehicle moving on site.
• Top event: The moment control of the hazard is lost, for example "loss of containment of hydrocarbons." No harm has occurred yet, but it can follow.
• Threats: The causes on the left side that could trigger the top event, such as corrosion, overpressure or operator error.
• Consequences: The outcomes on the right side, such as fire, explosion, toxic release or injury.
• Prevention barriers: Controls between each threat and the top event that stop it from occurring, such as relief valves, inspection programs or interlocks.
• Mitigation barriers: Controls between the top event and each consequence that reduce its severity, such as gas detection, emergency shutdown or fire suppression.
• Degradation factors and controls: Conditions that can weaken a barrier, such as poor maintenance or bypassing, and the measures that keep the barrier working. Some practitioners call these escalation factors.

Building a Bow-Tie

Bow-ties are usually built in a facilitated workshop. The team picks one hazard and one top event, lists the credible threats and consequences, then adds the barriers on each pathway. Each barrier is then assigned an owner and checked against agreed quality criteria. Guidance from the Center for Chemical Process Safety (CCPS) describes valid barriers as effective, independent and auditable, meaning each one can stop the sequence on its own and can be verified.

Bow-Tie Analysis in Process Safety Practice

Recognized Guidance

No OSHA regulation requires bow-tie analysis by name. The most widely cited reference is the 2018 concept book Bow Ties in Risk Management, published jointly by CCPS and the UK Energy Institute, which sets out common terminology and good practice. Bow-ties are often used alongside other methods to support process safety management (PSM) programs under 29 CFR 1910.119, which require a process hazard analysis (PHA) using a recognized methodology such as HAZOP, what-if or fault tree analysis.

How Bow-Ties Relate to Other Methods

• HAZOP and PHA: Detailed studies identify scenarios, and bow-ties summarize the most serious ones in a format that is easier to communicate.
• Fault and event trees: The left side of a bow-tie resembles a simplified fault tree, and the right side resembles a simplified event tree.
• Layer of protection analysis (LOPA): LOPA can add numbers to the barriers shown on a bow-tie when a quantitative estimate is needed.
• Risk registers: Bow-ties can be linked to a risk register so each major risk has a visual barrier map.

Good Practices for Bow-Tie Analysis

• Keep one top event per diagram: Mixing several loss-of-control events makes the diagram confusing and the barriers harder to judge.
• Be specific about barriers: List real hardware, systems and defined human actions, not vague items such as "training" or "be careful." Training usually supports a barrier rather than acting as one.
• Prefer stronger controls: Use the hierarchy of controls to favor engineered barriers over procedures alone.
• Assign barrier owners: Give each barrier a named role responsible for its testing and upkeep.
• Use barriers as indicators: Track barrier health, such as overdue inspections or impaired alarms, as a leading indicator.
• Review after changes and incidents: Update the diagram when equipment, procedures or staffing change, and compare actual incidents against the barriers that should have worked.

Example of Bow-Tie Analysis

A chemical distribution site stores flammable solvents in bulk tanks and transfers them to drums. The safety team runs a bow-tie workshop with operators and maintenance staff on the top event "loss of containment during drum filling." On the left, they list threats such as tank overfill, hose failure and a vehicle striking the transfer line. On the right, they list consequences such as a pool fire, worker exposure and a release to the drain.

When the team adds barriers, they find that two threats depend on a single operator check with no independent backup. They add a high-level shutoff and vehicle barriers, assign owners for each, and set inspection intervals. The completed diagram is posted in the control room and used in shift briefings, so new operators can quickly see which controls matter most.

Frequently Asked Questions: Bow-Tie Analysis

Is bow-tie analysis required by OSHA?

No. OSHA does not require bow-tie analysis by name. It is a recognized good practice that can support PSM, hazard analysis and risk communication.

What is the difference between a threat and a hazard?

A hazard is the source of potential harm, such as stored energy or a toxic chemical. A threat is a specific cause that could release or lose control of that hazard.

Is bow-tie analysis qualitative or quantitative?

It is mainly qualitative. Teams can add frequency or probability data, often through LOPA, when they need a numerical risk estimate.

How many barriers should each pathway have?

There is no fixed number. The right number depends on the severity of the consequence and the strength of each barrier, which is why barrier quality matters more than barrier count.