Editing Safety engineering (section)

==Analysis techniques==
Analysis techniques can be split into two categories: [[Qualitative research|qualitative]] and [[Quantitative research|quantitative]] methods. Both approaches share the goal of finding causal dependencies between a [[hazard]] on system level and failures of individual components. Qualitative approaches focus on the question "What must go wrong, such that a system hazard may occur?", while quantitative methods aim at providing estimations about probabilities, rates and/or severity of consequences.

The complexity of the technical systems such as Improvements of Design and Materials, Planned Inspections, Fool-proof design, and Backup Redundancy decreases risk and increases the cost. The risk can be decreased to ALARA (as low as reasonably achievable) or ALAPA (as low as practically achievable) levels.

Traditionally, safety analysis techniques rely solely on skill and expertise of the safety engineer. In the last decade [[Model-based systems engineering|model-based]] approaches, like STPA (Systems Theoretic Process Analysis), have become prominent. In contrast to traditional methods, model-based techniques try to derive relationships between causes and consequences from some sort of model of the system.

===Traditional methods for safety analysis===
The two most common fault modeling techniques are called [[failure mode and effects analysis]] (FMEA) and [[fault tree analysis]] (FTA). These techniques are just ways of finding problems and of making plans to cope with failures, as in [[probabilistic risk assessment]]. One of the earliest complete studies using this technique on a commercial nuclear plant was the [[WASH-1400]] study, also known as the Reactor Safety Study or the Rasmussen Report.

====Failure modes and effects analysis====
{{Main|Failure mode and effects analysis}}
Failure Mode and Effects Analysis (FMEA) is a bottom-up, [[inductive reasoning|inductive]] analytical method which may be performed at either the functional or piece-part level. For functional FMEA, failure modes are identified for each function in a system or equipment item, usually with the help of a functional [[block diagram]]. For piece-part FMEA, failure modes are identified for each piece-part component (such as a valve, connector, resistor, or diode). The effects of the failure mode are described, and assigned a probability based on the [[failure rate]] and failure mode ratio of the function or component. This quantization is difficult for software ---a bug exists or not, and the failure models used for hardware components do not apply.  Temperature and age and manufacturing variability affect a resistor; they do not affect software.

Failure modes with identical effects can be combined and summarized in a Failure Mode Effects Summary. When combined with criticality analysis, FMEA is known as [[Failure Mode, Effects, and Criticality Analysis]] or FMECA.

====Fault tree analysis====
{{Main|Fault tree analysis}}
Fault tree analysis (FTA) is a top-down, [[deductive reasoning|deductive]] analytical method.  In FTA, initiating primary events such as component failures, human errors, and external events are traced through [[Boolean logic]] gates to an undesired top event such as an aircraft crash or nuclear reactor core melt. The intent is to identify ways to make top events less probable, and verify that safety goals have been achieved.

[[File:Fault tree.svg|thumb|A fault tree diagram]]

Fault trees are a logical inverse of success trees, and may be obtained by applying [[de Morgan's laws|de Morgan's theorem]] to success trees (which are directly related to [[reliability block diagram]]s).

FTA may be qualitative or quantitative. When failure and event probabilities are unknown, qualitative fault trees may be analyzed for minimal cut sets. For example, if any minimal cut set contains a single base event, then the top event may be caused by a single failure. Quantitative FTA is used to compute top event probability, and usually requires computer software such as CAFTA from the [[Electric Power Research Institute]] or [[SAPHIRE]] from the [[Idaho National Laboratory]].

Some industries use both fault trees and [[event tree]]s. An event tree starts from an undesired initiator (loss of critical supply, component failure etc.) and follows possible further system events through to a series of final consequences. As each new event is considered, a new node on the tree is added with a split of probabilities of taking either branch. The probabilities of a range of "top events" arising from the initial event can then be seen.

=== Oil and gas industry offshore (API 14C; ISO 10418) ===
The offshore oil and gas industry uses a qualitative safety systems analysis technique to ensure the protection of offshore production systems and platforms. The analysis is used during the design phase to identify process engineering hazards together with risk mitigation measures. The methodology is described in the [[American Petroleum Institute]] Recommended Practice 14C ''Analysis, Design, Installation, and Testing of Basic Surface Safety Systems for Offshore Production Platforms.''

The technique uses system analysis methods to determine the safety requirements to protect any individual process component, e.g. a vessel, [[Pipeline transport|pipeline]], or [[pump]].<ref name=":0">API RP 14C p.1</ref> The safety requirements of individual components are integrated into a complete platform safety system, including liquid containment and emergency support systems such as fire and gas detection.<ref name=":0" />

The first stage of the analysis identifies individual process components, these can include: flowlines, headers, [[pressure vessel]]s, atmospheric vessels, [[Industrial furnace|fired heaters]], exhaust heated components, pumps, [[compressor]]s, pipelines and [[heat exchanger]]s.<ref name=":1">API RP 14C p.vi</ref> Each component is subject to a safety analysis to identify undesirable events (equipment failure, process upsets, etc.) for which protection must be provided.<ref name=":2">API RP 14C p.15-16</ref> The analysis also identifies a detectable condition (e.g. [[high pressure]]) which is used to initiate actions to prevent or minimize the effect of undesirable events. A Safety Analysis Table (SAT) for pressure vessels includes the following details.<ref name=":2" /><ref name=":3">API RP 14C p.28</ref>
{| class="wikitable"
! colspan="3" |Safety Analysis Table (SAT) pressure vessels
|-
!Undesirable event
!Cause
!Detectable abnormal condition
|-
|Overpressure
|Blocked or restricted outlet

Inflow exceeds outflow

Gas blowby (from upstream)

Pressure control failure

Thermal expansion

Excess heat input
|High pressure
|-
|Liquid overflow
|Inflow exceeds outflow

Liquid slug flow

Blocked or restricted liquid outlet

Level control failure
|High liquid level
|}
Other undesirable events for a pressure vessel are under-pressure, gas blowby, leak, and excess temperature together with their associated causes and detectable conditions.<ref name=":3" />
[[File:Vessel_level_instrumentation.jpg|thumb|Vessel level instrumentation]]
Once the events, causes and detectable conditions have been identified the next stage of the methodology uses a Safety Analysis Checklist (SAC) for each component.<ref>API RP 14C p.57</ref> This lists the safety devices that may be required or factors that negate the need for such a device. For example, for the case of liquid overflow from a vessel (as above) the SAC identifies:<ref>API RP 14C p.29</ref>

* A4.2d - High level sensor (LSH)<ref name="ISO-14617-1:2005">{{cite web |title=ISO 14617-1:2005 Graphical symbols for diagrams — Part 1: General information and indexes |url=https://www.iso.org/standard/41838.html |publisher=[[International Organization for Standardization]]}}</ref>
** 1.  LSH installed.
** 2.  Equipment downstream of gas outlet is not a flare or vent system and can safely handle maximum liquid carry-over.
** 3.  Vessel function does not require handling of separate fluid phases.
** 4.  Vessel is a small trap from which liquids are manually drained.
[[File:Vessel_pressure_instrumentation.jpg|thumb|Vessel pressure instrumentation]]
The analysis ensures that two levels of protection are provided to mitigate each undesirable event. For example, for a pressure vessel subjected to over-pressure the primary protection would be a PSH (pressure switch high) to shut off inflow to the vessel, secondary protection would be provided by a [[Safety valve|pressure safety valve]] (PSV) on the vessel.<ref>API RP 14C p.10</ref>

The next stage of the analysis relates all the sensing devices, shutdown valves (ESVs), trip systems and emergency support systems in the form of a Safety Analysis Function Evaluation (SAFE) chart.<ref name=":1" /><ref>API RP 14C p.80</ref>
{| class="wikitable"
! colspan="4" rowspan="2" |Safety Analysis Function Evaluation  (SAFE) chart
|Close inlet valve
|Close outlet valve
|Alarm
|-
|ESV-1a
|ESV-1b
|
|-
|Identification
|Service
|Device
|SAC reference
|
|
|
|-
| rowspan="5" |V-1
| rowspan="5" |HP separator
|PSH
|A4.2a1
|X
|
|X
|-
|LSH
|A4.2d1
|X
|
|X
|-
|LSL
|A4.2e1
|
|X
|X
|-
|PSV
|A4.2c1
|
|
|
|-
|etc.
|
|
|
|
|-
|V-2
|LP separator
|etc.
|
|
|
|
|}
X denotes that the detection device on the left (e.g. PSH) initiates the shutdown or warning action on the top right (e.g. ESV closure).

The SAFE chart constitutes the basis of Cause and Effect Charts which relate the sensing devices to [[Shut down valve|shutdown valves]] and plant trips which defines the functional architecture of the [[Plant process and emergency shutdown systems#Process shutdown (PSD)|process shutdown]] system.

The methodology also specifies the systems testing that is necessary to ensure the functionality of the protection systems.<ref>API RP 14C Appendix D</ref>

API RP 14C was first published in June 1974.<ref>{{Cite book|chapter-url=https://www.onepetro.org/conference-paper/SPE-7147-MS|chapter=Impact of API 14C on the Design And Construction of Offshore Facilities|doi=10.2118/7147-MS |access-date=7 February 2019|title=All Days |year=1978 |last1=Farrell |first1=Tim }}</ref> The 8th edition was published in February 2017.<ref>{{Cite web|url=https://global.ihs.com/doc_detail.cfm?document_name=API%20RP%2014C&item_s_key=00010460|title=API RP 14C|access-date=7 February 2019}}</ref> API RP 14C was adapted as ISO standard ISO 10418 in 1993 entitled ''Petroleum and natural gas industries — Offshore production installations — Analysis, design, installation and testing of basic surface process safety systems.''<ref>{{Cite web|url=https://www.iso.org/standard/38067.html|title=ISO 10418|access-date=7 February 2019}}</ref> The latest edition of ISO 10418 was published in 2019. <ref>{{Cite web|url=https://www.iso.org/standard/55440.html|title=ISO 10418|access-date=2 January 2025}}</ref>