Glossary
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Probability of Failure Per Hour PFH
Definition(s)
Probability of Failure Per Hour PFH
average failure frequency as 3.1.23 in the functional safety standard terminology (e.g. IEC 61508[2] or IEC 61511[3]) Note 1 to entry: The old meaning “Probability of Failure per Hour” is obsolete and replaced by “average failure frequency”. Nevertheless PFH is still in use to keep the consistency with the previous versions of functional safety standards. Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsAverage Failure Frequency
Definition(s)
Average Failure Frequency
w(t1,t2) , w T ( ), w average value of the time-dependent failure frequency over a given time interval
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsFailure Frequency
Definition(s)
Failure Frequency
3.1.22 unconditional failure intensity w(t) conditional probability per unit of time that the item fails between t and t+dt, provided that it was working at time 0
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsVesely Failure Rate
Definition(s)
Vesely Failure Rate
conditional failure intensity λV(t) conditional probability per unit of time that the item fails between t and t+dt, provided that it was working at time 0 and at time t
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsAsymptotic Failure Rate
Definition(s)
Asymptotic Failure Rate
3.1.20 λ as limit, when it exists, of the failure rate λ(t) when t goes to infinity
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsAverage Failure Rate
Definition(s)
Average Failure Rate
3.1.19 λ (t1t2 ), λ (T) average value of the time-dependent failure rate over a given time interval
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsSteady State Availability Asymptotic Availability
Definition(s)
Steady State Availability Asymptotic Availability
3.1.17
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsAverage Probability of Failure on Demand
Definition(s)
Average Probability of Failure on Demand PFDavg
3.1.16 average unavailability as per 3.1.12 in the functional safety standard terminology (e.g. IEC 61508[2]) Note 1 to entry: “Failure on demand” means here “failure likely to be observed when a demand occurs”. PFDavg encompasses both the failure occurred before the demand and the failure occurring due to the demand itself. Then this term needs not to be mixed up with the probability of a failure due to a demand (see 3.2.13). Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsProbability of Failure on Demand
Definition(s)
Probability of Failure on Demand PFD
3.1.15 unavailability as per 3.1.12 in the functional safety standard terminology (e.g. IEC 61508[2]) Note 1 to entry: “Failure on demand” means here “failure likely to be observed when a demand occurs”. This encompasses both the failure occurred before the demand and the failure occurring due to the demand itself. Then this term needs not to be mixed up with the probability of a failure due to a demand (see 3.2.13). Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsAverage Unavailability
Definition(s)
Average Unavailability
3.1.14
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsAverage Availability
Definition(s)
A(t1,t2)
Average Availability
3.1.13 〈measure〉 average value of the availability A(t) over a given interval [t ,t ] 1 2.
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsUnavailability
Definition(s)
Unavailability
3.1.12 instantaneous unavailability point unavailability U(t) 〈measure〉 probability for an item not to be in a state to perform as required at a given instant Note 1 to entry: The unavailability is the complementary of the availability: U(t)=1− A(t ) Note 2 to entry: The unavailability is called “Probability of Failure on Demand” (PFD) by the standards related to the functional safety of safety related/instrumented systems (e.g. IEC 61508[2]). Note 3 to entry: Note 3 to entry: When dealing with safety U(t) is generally small compared to 1 and this property is used to develop approximated formulae (see Clause 7). Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsInstantaneous Availability
Definition(s)
Instantaneous Availability
3.1.11 point availability A(t) probability for an item to be in a state to perform as required at a given instant Note 1 to entry: In this Technical Report the word “availability” used alone stands for “instantaneous availability”. Note 2 to entry: This is a time-dependent parameter. Note 3 to entry: No matter if the item has failed before the given instant if it has been repaired before. Note 4 to entry: For non-repairable items, Availability and Reliability are identical. Note 5 to entry: When dealing with safety, A(t) is generally close to 1 and this property is used to develop approximated formulae (see Clause 7). Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsFailure Probability Density
Definition(s)
Failure Probability Density 3.1.10
f(t) 〈measure〉 probability for an item to fail between t and t+dt
Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsUnreliability
Definition(s)
Unreliability 3.1.9
F(t) 〈measure〉 probability for an item to fail to perform a required function under given conditions over a given time interval [0, t] Note 1 to entry: F(t) is also the probability that the time of the first failure tf is lower than t: F(t)= P(t _ t) f . This is in relationship with the occurrence of the first failure. Note 2 to entry: F(t) is the cdf (cumulative distribution function) of the time to the first failure tf of the item. It ranges from 0 to 1 when t goes from 0 to infinity. Note 3 to entry: The unreliability is the complementary of the reliability: F(t)=1−R(t) Note 4 to entry: When dealing with safety, F(t) is generally small compared to 1 and this property is used to develop approximated formulae (see Clause 7). Note 5 to entry: Unreliability is better to communicate than MTTF. Source: ISO/TR 12489:2013(E) Reliability modelling and calculation of safety systems. Global StandardsSafety Function
Definition(s)
Safety Function
Functions that are related to overload protection i.e. break-away system (see [6.7]), automatic/manual protection systems (see [6.8] and [6.9]), protection against movements outside operational limitations - i.e. limit switches, physical stops (see [6.10.2]), protections against dangerous gangway movements i.e. emergency stop function (see [6.10.3]). Source: Offshore Gangways, DNVGL-ST-0358, DNV GL, September 2017. Global StandardsSafety Function
3.1.6
function which is intended to achieve or maintain a safe state, in respect of a specific hazardous event Note 1 to entry: This term deviates from the definition in IEC 61508–4 to reflect differences in process sector terminology. Source: ISO/TR 12489:2013(E). Global StandardsDangerous State
Definition(s)
Dangerous State
state of the process when safety is not achieved Note 1 to entry: A dangerous state is the result of the occurrence of a critical dangerous failure (3.2.4, Figure B.1).
Source: ISO/TR 12489:2013(E). Global StandardsSafe State
Definition(s)
Safe State
state of the process when safety is achieved Note 1 to entry: Some states are safer than others (see Figures B.1, B.2 and B.3) and in going from a potentially hazardous condition to the final safe state, or in going from the nominal safe condition to a potentially hazardous condition, the process may have to go through a number of intermediate safe-states. Note 2 to entry: For some situations, a safe state exists only so long as the process is continuously controlled. Such continuous control may be for a short or an indefinite period of time. Note 3 to entry: A state which is safe with regard to a given safety function may increase the probability of hazardous event with regard to another given safety function. In this case, the maximum allowable spurious trip rate (see 10.3) for the first function should consider the potential increased risk associated with the other function.
Safety Integrity
Definition(s)
Safety Integrity
ability of a safety instrumented system to perform the required safety instrumented functions as and when required Note 1 to entry: This definition is equivalent to the dependability of the SIS (Safety Instrumented System) with regard to the required safety instrumented function. Dependability, being often understood as an economical rather a safety concept, has not been used to avoid confusion. Note 2 to entry: The term “integrity” is used to point out that a SIS aims to protect the integrity of the operators as well as of the process and its related equipment from hazardous events. Source: ISO/TR 12489:2013(E), Global StandardsSafety Integrity
The attribute of an information system when it performs its intended function in an unimpaired manner, free from deliberate or inadvertent unauthorized manipulation of the system. From: CNSSI 4009. Source: NICCS™ Portal Cybersecurity Lexicon, National Initiative for Cybersecurity Careers and Studies (https://niccs.us-cert.gov/glossary) as of 11 November 2015, Global StandardsDependability
Definition(s)
Dependability
ability to perform as and when required Note 1 to entry: Dependability is mainly business oriented. Note 2 to entry: IEC/TC 56 which is the international “dependability” technical committee deals with reliability, availability, maintainability and maintenance support. More than 80 dependability standards have been published by the IEC/TC56. In particular, it is in charge of the international vocabulary related to those topics (IEV 191[14]) and also of the methods used in the reliability field (e.g. FMEA, HAZOP, reliability block diagrams, fault trees, Markovian approach, event tree, Petri nets). Note 3 to entry: The production availability is an extension, for production systems, of the classical dependability measures. This term is defined in the ISO 20815[16] standard which deals with production assurance and relates to systems and operations associated with drilling, processing and transport of petroleum, petrochemical and natural gas. The relationship between production-assurance terms can be found in Figure G.1 of ISO 20815[16].
[SOURCE: IEC 60050 −191].
Source: ISO/TR 12489:2013(E), Global StandardsWLCPF
Definition(s)
WLCPF
Well Life Cycle Practices Forum. Source: Oil & Gas UK, Guidelines on subsea BOP systems, Issue 1, July 2012, Global StandardsWell Intervention Operation
Definition(s)
Well Intervention Operation
Well intervention operation is well servicing operations conducted within a completed wellbore.
Source: Norwegian Oil and Gas Association, Guideline No. 135, Recommended Guidelines for Classification and categorization of well control incidents and well integrity incidents, Rev. 4, 27 June 2017, National or Regional StandardsWell Intervention Operation
An operation in which a well is re-entered for a purpose other than to continue drilling or to maintain or repair it. Regulation 2, DCR. Source: Oil & Gas UK, Guidelines on subsea BOP systems, Issue 1, July 2012, Global StandardsWell-examiner
Definition(s)
Well-examiner
In this document “well-examiner” covers any individual team, department or company providing well examination services as described by Regulation 18, DCR. Source: Oil & Gas UK, Guidelines on subsea BOP systems, Issue 1, July 2012, Global StandardsUPR
Definition(s)
UPR
upper pipe ram Source: Oil & Gas UK, Guidelines on subsea BOP systems, Issue 1, July 2012, Global StandardsUBO
Definition(s)
UBO
under-balanced operation. Source: Oil & Gas UK, Guidelines on subsea BOP systems, Issue 1, July 2012, Global StandardsPressure Containment Barrier
Definition(s)
Pressure Containment Barrier
The well is defined in terms of its pressure containment boundary. Any equipment that is vital to controlling the pressure within the well is therefore covered. This would include downhole pressure-containing equipment and the pressure-containing equipment on top of the well such as blowout preventers or Christmas trees, but excludes well control equipment downstream that can be isolated from the well by valves. Examples of where the well ends are:- above the top blowout preventer (BOP) in the BOP stack and outside the choke and kill valves;
- downstream of the swab and production win valves of a Christmas tree;
- at the top of the wireline stuffing box of a wireline BOP.