Oil and gas production handbook An introduction to oil ... - ABB Group

order. Safety needs to be documented in order to pass information
to different engineering disciplines.
For the oil and gas industry, safety standards comprise a set of corporate,
national and international laws, guidelines and standards. Some of the
primary international standards are:
• IEC 61508 Functional safety of electrical/electronic/programmable
electronic safety-related systems
• IEC 61511 Functional safety - Safety instrumented systems for the
process industry sector
A safety integrity level is not directly applicable to individual subsystems or
components. It applies to a safety function carried out by the safety
instrumented system (end-to-end: sensor, controller and final element).
IEC 61508 covers all components of the E/E/PE safety-related system,
including field equipment and specific project application logic. All these
subsystems and components, when combined to implement the safety
function (or functions), are required to meet the safety integrity level target of
the relevant functions. Any design using supplied subsystems and
components that are all quoted as suitable for the required safety integrity
level target of the relevant functions will not necessarily comply with the
requirements for that safety integrity level target.
Suppliers of products intended for use in E/E/PE safety-related systems
should provide sufficient information to facilitate a demonstration that the
E/E/PE safety-related system complies with IEC 61508. This often requires
that the functional safety for the system be independently certified.
There is never one single action that leads to a large accident. It is often a
chain of activities. There are many layers to protect against an accident, and
these are grouped two different categories:
• Protection layers – to prevent an incident from happening. Example:
rupture disk, relief valve, dike.
• Mitigation layers – to minimize the consequence of an incident.
Example: Operator intervention or safety instrumented system (SIS)
An SIS is a collection of sensors, controllers and actuators that execute one
or more SIFs/safety loops that are implemented for a common purpose.
Each SIF has its own safety integrity level (SIL) and all sensors, controllers
and final elements in one SIF must comply with the same SIL, i.e., the end-
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