Barriers

What is a barrier

A barrier is a defence system to reduce critical effects of unwanted events. Other nicknames are safeguards, safety function and protective systems. Barriers are related to the current situation and consist of one or more barrier elements. A barrier description should include:

• What scenarios to prevent
• How does it work (step-by-step)
• Barrier element description
• Barrier element performance
• Barrier element performance testing
• Barrier element operational conditions
• Barrier elements dependability
• Limitations to the operational system when a barrier is non-functional
• Unwanted effect of the barrier

Unwanted effects of the barrier could be premature activation of the barrier, resulting in e.g. production stop or cut drill pipe. A barrier for preventing burglars to break into your house could prevent evacuation during a fire. There could be one or more barriers for the same unwanted event. The three barrier categories are pre-accident barriers, accident barriers and post-accident barriers.

Pre-accident barriers

Pre-accident barriers are introduced to avoid accidental events after an unwanted event. These barriers will either result in full recovery, stabile or labile control. First priority is full recovery. Second priority is stabile or labile control. Reduced functionality is accepted in these situations. Some pre-accident barriers are taking over the failed function. Pre-accident barriers could be active and passive. Active barriers include:

• Detection
• Diagnosis
• Action

Each of the above functions could be executed by technology or humans. Detection of failure could thus be sensors or human observations like vibration, noise, smell, smoke, leakage and alarm. The operator or the technical device must then make a correct diagnose. Options for actions must be clarified and the selected action must be performed. Pre-accident barriers are the first protection layer after an unwanted event.

Technical redundancy, organisational redundancy and human recovery will normally be pre-accident barriers but not necessarily. It depends on the situation. They will be pre-accident barriers if an accident is prevented. Technical redundancy as dual braking circuits in an automobile is a passive pre-accident barrier. Technical redundancy as dual wiring to the cigarette lighter in an automobile would not be a pre-accident barrier since no accident is prevented upon a cable failure. It is just an action for increased reliability of the power supply. Organisational redundancy is one kind of human recovery. Organisation redundancy requires at least two humans while human recovery can also be achieved when humans are working alone. A general introduction to technical redundancy, organisational redundancy and human recovery follow:

• Technical redundancy
• Organisational redundancy
• Human recovery

Pre-accident barriers from traffic and oil production follow:

• Pre-accident traffic barriers
• Pre-accident production barriers

Accident barriers

Accident barriers are introduced to reduce the consequences of the accidental event after the event and become true if the pre-accident barriers fail. Accident barriers take place immediately after the accident. Accident barriers could be active and passive. Active accident barriers are not taking over the failed function but limit the consequence. Active barriers typical include:

• Detection (automatic by sensors or manual by humans)
• Decision (CPU, relay, human pushes the emergency button)
• Action (e.g. automatic or manual valve closure)

Passive accident barriers are embedded in the design and independent of the active accidental barriers.

• Accident traffic barriers
• Accident production barriers

Post-accident barriers

Post-accident barriers are also introduced to reduce the consequences of the accidental event and become true if the accident barriers fail. Post-accident barriers are in other words the same as the first phase in emergency preparedness and include:

• Emergency preparedness plan describing responsibilities, routines and recovery
• Emergency communication system
• Emergency light
• Emergency power
• Fire and evacuation alarm
• Evacuation (like lifeboats, rafts and escape ways)
• Fire-fighting brigade
• First aid
• Medical treatment

Emergency preparedness plans should be established based on an analysis of different accidental scenarios with consequences on personnel, environment and materials. Efficient handling of an emergency situation requires good routines and training. It may also be costly. The emergency preparedness analyses should include practical issues that require people with experience. The analysis for a fire evacuation should e.g. include issues like:

• Restricted view due to smoke
• Reduced view due to tears
• Difficult to breathe
• Hot smoke
• Hot door handle
• Availability to the escape doors, etc

The second phase in emergency preparedness is recovery back to normal. Actions in this phase are not barriers but still a critical phase for the companies involved in the incident. This phase includes actions like:

• Press conference
• Campaigns to regain market position if the market reputation is damaged
• Investigation

Also this phase require plans and training to be handled correct and efficient.

Further discussion

A barrier system could be separated or integrated into equipment. Examples are:

• Totally independent of normal equipment functionality (like the air bag in a car)
• Using parts of the normal equipment functionality (like the antilock brakes in a car)
• Being a part of the normal equipment functionality (like the brakes in a car)

The braking system in a car has two main functions. The first function is to safely bring the vehicle to a stand-still anytime. The second function is to guarantee the shortest possible stopping distance in emergency braking situations like when having an elk on the road.

The quality of the barriers and the dependability between different barriers are well as important as the number of barriers. The quality of the barrier is found by qualifying the barrier the same way as other functional equipment. Independence between humans and technical barriers requires humans with theoretical and practical knowledge about the technical barriers. That includes how humans shall act if the technical barrier is down due to e.g. maintenance. Also aim at independence between more technical or between more human barriers. Even qualified barriers require regularly inspection, monitoring and maintenance to function properly in the long term.

The barrier system for the well is called the ‘well barrier’. This system includes both active barrier elements (like DHSV and master valves) and passive barrier elements (like casing, tubing and production packer). Some well barriers are “pre-accident production barriers”. All well barriers are “accident production barriers”. This is described above. More details about the well barrier independent of pre-accident, accident, active and passive are given below.

• Well barriers

Influencing factors are often mixed with barriers (barrier definition dependent). Examples of risk influencing factors from the traffic is often defined as barriers. Examples are described below.

• Influencing factors in traffic