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In-service performance of emergency shutdown valves and
dependent operational relationships in the offshore oil and gas
industry
Keith Cameron
a
, Andrew Lewis
a
, Diogo Montalv~
ao
b
, Mohammad Reza Herfatmanesh
a
,
*
a
School of Physics, Engineering and Computer Science, University of Hertfordshire, United Kingdom
b
Department of Design and Engineering, Faculty of Science and Technology, Bournemouth University, United Kingdom
article info
Article history:
Received 6 December 2022
Received in revised form
26 April 2023
Accepted 13 June 2023
Keywords:
Emergency
Shutdown
Valve
Safety instrumented systems
Closure time
IEC-61511
abstract
Industrial process plants use emergency shutdown valves (ESDVs) as safety barriers to protect against
hazardous events, bringing the plant to a safe state when potential danger is detected. These ESDVs are
used extensively in offshore oil and gas processing plants and have been mandated in the design of such
systems from national and international standards and legislation. This paper has used actual ESDV
operating data from four mid/late life oil and gas production platforms in the North Sea to research
operational relationships that are of interest to those responsible for the technical management and
operation of ESDVs. The first of the two relationships is between the closure time (CT) of the ESDV and
the time it remains in the open position, prior to the close command. It has been hypothesised that the
CT of the ESDV is affected by the length of time that it has been open prior to being closed (Time since the
last stroke). In addition to the general analysis of the data series, two sub-categories were created to
further investigate this possible relationship for CT and these are “above mean”and “below mean”. The
correlations (Pearson's based) resulting from this analysis are in the “weak”and “very weak”categories.
The second relationship investigated was the effect of very frequent closures to assess if this improves
the CT. ESDV operational records for six subjects were analysed to find closures that occurred within a
24 h period of each other. However, no discriminating trend was apparent where CT was impacted
positively or negatively by the frequent closure group. It was concluded that the variance of ESDV closure
time cannot be influenced by the technical management of the ESDV in terms of scheduling the oper-
ation of the ESDV.
©2023 Southwest Petroleum University. Publishing services by Elsevier B.V. on behalf of KeAi
Communications Co. Ltd. This is an open access article under the CC BY license (http://creativecommons.
org/licenses/by/4.0/).
1. Introduction
ESDVs are commonly part of safety instrumented systems
(SIS), which are employed to safeguard process plants, automat-
ically detecting potentially hazardous events and making the
plant safe. SIS are subject to the guideline IEC-61511, “Safety
instrumented systems for the process industry”. In addition,
ESDVs are used for major infrastructure pipelines and in the UK
fall under the requirements of, “The pipelines safety regulations
1996, Statutory Instrument 825”. In both regulatory frameworks,
an ESDV is subject to a periodic, often annual, proof test against a
target closure time.
In performing this proof test, test failures can cause additional
assessments of suitability to be required. These instances result in a
loss of production until the failed ESDV can be brought back into
service. Often, on a re-test after a failure, the performance is found
to be adequate and the ESDV is returned to service, with no changes
made. On these occasions, it is not clear what caused the test fail-
ure, as subsequent closure tests have been carried out successfully.
This occurs with no servicing or replacement of parts being carried
out. Due to the nature of the ESDV, it is a major operation to replace
it and often replacements can have an associated lead time of
*Corresponding author.
E-mail address: m.r.herfatmanesh@herts.ac.uk (M.R. Herfatmanesh).
Peer review under responsibility of Southwest Petroleum University.
Production and Hosting by Elsevier on behalf of KeAi
Contents lists available at ScienceDirect
Petroleum
journal homepage: www.keaipublishing.com/en/journals/petlm
https://doi.org/10.1016/j.petlm.2023.06.004
2405-6561/©2023 Southwest Petroleum University. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd. This is an open access article under the CC
BY license (http://creativecommons.org/licenses/by/4.0/).
Petroleum 9 (2023) 613e620
several months, dependent on the components and material grades
needed.
The subject of this research project is the North Sea oil and gas
industry. After over thirty years in operation, many of the offshore
platforms are nearing the end of their production lives. However,
many of these platforms are still in operation and are in an operating
lifetime region where the reliability of emergency shutdown valves
is difficult to quantify. Research project participants are interested in
the reliability and operation of ESDVs, specifically in the area of
proof testing. Consequently, two research questions have been
formulated. Firstly, is the closure time (CT) of the ESDV affected by
the length of time that it has been open prior to being closed, i.e., the
time since last stroke (TSLS)? Secondly, do multiple ESDV closures in
succession improve the CT? Is has been hypothesised that frequent
movement of the ESDV can be beneficial against performance
degradation mechanisms such as galling and stiction.
2. Background
The analysis and quantification of critical valve failure rates have
long been of interest in the oil and gas industry for the prediction of
problems that would impact plant uptime and production [1], such
as subsea isolation valves. Similarly, failure modes of other safety
critical valves such as pressure relief valves have been previously
investigated [2]. In addition, environmental factors and the impact
that they have on a SIS, as well as safety critical equipment and
developing hazards have been studied [3].
In the consideration of environmental factors, there have been
methods proposed for estimating probability of failure on demand
(PFD) that account for the environmental conditions [4]. The
importance of barriers such as SIS against identified hazards, with
the assessment of their adequacy [5], is also of interest to industry
regulators.
Wider society also has a concern about the operation of these
safety systems, as exact consequences of failure to prevent hazards
can only be partially estimated, as considered by Boakye [6]. This
consideration affecting the health, shelter and environment capa-
bilities described by Boakye brings forward the realisation and il-
lustrates the potential of hazard consequences for oil and gas
operations.
The importance of maintenance and proof test of ESDVs cannot
be understated, the study conducted by Zhu [7]challengesthecur-
rent system of functional test in a review of shutdown system
workflows. Similarly, there is a large amount of interest in predictive
maintenance systems using Industry 4.0 technologies [8]. Also, in the
validation of proof tests of logic solvers, a checklist is proposed to
ensure that all aspects of the logic solver and the ESDV interface are
adequately covered [9]. In the assessment of the performance of
safety systems carried out by Andrews and Fecarotti [10], some mi-
nor recognition of the problem of degradation in valve performance
during service is given. In this study [10], the degradation of closure
time (CT) is linked to the mean time to failure (MTTF). Among the
proposed risk indicators against major hazards for offshore in-
stallations [11], over half of the indicators are based on ESDV and
valve technology. Similarly, as a future concept for dynamic barrier
management, use of real operating data, including includes sensor
data and condition monitoring, may be applied to ESDV technology
[12]. This approach is also proposed for intelligent sensors by Mkhida
et al. [13], however, it was reported that a potential weakness exists
in the IEC-61508 standard where a potentially high rate of safe
failures does not lead to a lower PFD result.
Condition monitoring of safety barriers with reliability degra-
dation due to ageing is investigated by Xing et al. [14 ]. Where a case
study regarding a high consequence scenario is covered. The
performance impact of degradation due to ageing is equally
applicable to ESDVs. Xing et al. concluded that time dependant
failure is a key influencer of the dynamic behaviour for business
continuity and production uptime. This is not possible with the
existing level of sensors in current ESDV equipment. However, this
is perhaps an area of future work, with the potential levels of sensor
integration possible with Industry 4.0.
While safety instrumented systems, such as ESDVs, are in the
operational and maintenance phase of the functional safety life-
cycle, there is a recognition that the assumptions made and the
methods used in the design and engineering phase may not be
appropriate [15]. In the study carried out by the industry regulator,
The UK Health and Safety Executive (HSE) [16], the focus has been
on the riser ESDV. Riser ESDVs are intended to isolate flow to the
offshore platform from production flowlines. These riser ESDVs are
very similar to the ESDVs used in normal service to protect the
processes on the offshore platforms, commonly referred to as
topside ESDVs. This research was carried out with reference to KP4
which is a UK HSE initiative to mitigate the effects of aging offshore
assets. As well as this industry approach, KP4, other methods for
condition assessment and useful life prediction, beyond the original
design lifetime have been proposed [17 ]. The operation of ESDVs in
degraded service is also of interest to those operating this type of
equipment in other industries, such as national infrastructure [18].
Specifically for ageing of oil and gas fields, there is particular
concern that age related failures may add to the demand rates for
safety instrumented systems [19]. In the initial phases of oil and gas
lifecycle operations, such as drilling, the management of essential
service valve characteristics has been investigated using Industry
4.0 technologies [20]. Likewise, the intelligent diagnosis of process
equipment items such as transfer pumps have been reviewed [21],
generally on a larger scale in the management for oilfield in-
vestments [22] using Industry 4.0 technologies.
Quantitative analysis of degradation in SIS components, such as
ESDV, is reported by Zhang, Barros and Liu [23]. Wu [24] extended
the methods available from IEC-61508, including specific methods
for the ESDV time dependent failures. Furthermore, leakage is an
issue that affects most types of valves, including the quarter turn
ball valve that is normally used for ESDV service. Several methods
are available to estimate the leakage rate, such as built up back
pressure testing and non-destructive acoustic recognition testing
[25]. This is of interest in maintaining the overall integrity and
operation of the ESDV. The quarter turn ball valve is used in
blowdown applications, as referred to in the simulation presented
by Shafiq[26] and also in the comparison in fire protection capa-
bility for pressure vessels on offshore oil and gas installations [27].
This reinforces the case that this type of ESDV and the quarter turn
ball valve as its basis, is used extensively in safety applications
across the oil and gas industry.
Socio-essential energy infrastructures, such as oil and gas op-
erations which are needed to fuel the de-carbonisation of the future
are the subject of the work by Alderson et al. [28]. This study as-
sesses the impact of climate driven causes to improve hazard out-
comes. Their findings indicated that novel training exercises,
including the use of simulations and games with increasing expo-
sure to surprise elements, help strengthen adaptiveness and
improve outcomes. This becomes increasingly relevant to SIS in mid
and late equipment life, as climate change potentially accelerates
the equipment ageing process and, in some cases, may bring about
failure types that have not been previously encountered.
It is therefore evident that a comprehensive understanding of
the mid and late life operation of the ESDV as an operating part of
any safety instrumented function is of great importance to oil and
gas industry operators, maintainers and regulators.
K. Cameron, A. Lewis, D. Montalv~
ao et al. Petroleum 9 (2023) 613e620
614
3. Objectives
In order to research the factors that affect proof testing, it is
important to understand the behaviour and performance of the
ESDV in the industrial setting. For this goal, two specific research
questions have been formulated, namely:
(1) Is the closure time of the ESDV dependent on the time that it
has remained in the open position prior to the command to
close?
(2) Do frequent successive closures over a short number of hours
improve the closure time?
Although the performance of the opening/closure of the ESDV
can contain useful information such as indicating problems with
the actuator (e.g., leakage), for these research questions the open-
ing stroke performance has been omitted for brevity, also the
closing stroke is of most interest in safety applications.
4. Methodology
An important aspect for the safe continued operation of ESDVs is
the comparison of the actual operating time to the pre-determined
target closure time. This is the time available for the ESDVto close in
order to prevent the envisaged hazard. Target closure times are
often set using estimation from records of plant operation, calcu-
lated using process simulation software or arbitrarily set in perfor-
mance standards. When set inperformance standards, these closure
time targets are assigned to cover a large number of different ESDV
applications and sizes, so the target is suitably easy to meet, in some
cases less than 1 min for full closure. Critical, high consequence,
safety applications tend to have calculated target closure times set,
often with a difficult to meet target due to the convergence of
developed system pressure and equipment design limitations in the
scenarios considered. In this study, target closure time for each of
the ESDVs considered was not provided by the project participants.
This has a limitation on the output from this study, as pass and fail
criteria for each ESDV cannot be stated, which is of principal interest
to the operators of process operations and safety systems. Instead,
this study has concentrated on investigating the relationship be-
tween the closure time and the time spent in the open position, and
also the effect on closure time of frequent closures.
Many factors can affect the performance of ESDVs in service and
these include aspects of design and operation. For instance, the
valve type and actuation type, especially where the cleanliness of
the hydraulic system needs to be maintained to a high standard to
prevent clogging of exhaust ports. Aging assets generally give rise
to concerns with supportability and obsolescence, none more so
than in safety applications involving ESDVs. Large valve sizes, as
employed in riser valve service, can also cause specific in-service
issues due to large breakout torques required and the vulnera-
bility of components at this point in the process to erosion and
friction from production fluid sand. The problem of large breakout
torques is also present in high pressure applications. Additionally,
high differential pressure can negatively affect valve seals. The role
of ongoing operational maintenance and the application of lubri-
cation while in service are also factors to be considered, but this is
believed to have negligible impact on the overall performance, due
to the high percentage uptime of these production systems and the
scarcity of skilled personnel to carry out greasing and flushing
tasks. On reflection of these general issues affecting ESDV perfor-
mance, they can be considered in this study as factors affecting the
in-service performance of ESDVs in the following ways. Ageing of
ESDVs is a factor in this study as all the subjects have been assumed
to have been in service for in excess of 25 years, which in many
cases is outside the normal service life of this type of industrial
equipment. Industry participants that provided data for this study
have not provided any accompanying service records for these
ESDVs, so no information was available on any historic failures and
subsequent repair. In terms of any mandated testing of ESDV
operation, it is understood from the project participants that
generally the ESDVs from the plants providing the operational data
have been tested at a six-month full closure regime, though this
might not be applicable to all ESDVs universally. Process fluids can
have a major impact on the ESDV performance and reliable oper-
ation over time. Based on the data shown in Table 1, it is evident
that the majority of the ESDVs (four out of six) are in hydrocarbon
gas service. This is considered a “clean”service by industrial stan-
dards within the field of upstream oil and gas, indicating that the
service is not highly abrasive or erosive and the fluids would not
carry much entrained sand or solids. The remaining two ESDVs are
employed in riser valves for which they produced oil service. These
may be considered to have a higher amount of entrained solids that
would have a negative impact on the ESDV internal parts and seals,
as well as inducing friction when accumulated in closely mated
sealing surfaces and internal moving parts. All the subject ESDVs
come from early North Sea era offshore platforms, as such they will
benefit from a good level of environmental protection from natural
elements such as salt laden air, sea spray and weather. This is in
opposition to the more modern and current design philosophies for
offshore platforms of minimal and open structures, designed for a
large amount of natural ventilation. Finally, ESDVs investigated in
this study are part of the wellhead, separation and gas processing
modules in a typical offshore oil and gas processing plant, their
duty of operation would only be to intervene in the case of a process
upset or for process isolation purposes.
The data analysed has been collected from four separate oil and
gas producing offshore platforms in the North Sea, the average age
of which is thirty-three years. A total of 479 specific ESDV datasets
have been received from the project participants. In each case, in-
formation has been exported from the process plant control system
into a format that is suitable for analysis.
For the initial research question, the analysis focuses on a small
number of ESDVs to provide a greater level of specific detail as
shown in Table 1. These ESDVs were selected from a larger group
which provided the most complete datasets and were considered
the most credible. The analysis is then carried out for a greater
number of ESDVs and the general results are reported.
Table 1
ESDVs employed for detailed analysis of valve CT vs TSLS.
ESDV No. Size (in) Pressure class Process media Valve type Actuation Valve age (years in service)
142 6 1500# HC gas Ball valve Pneumatic 33
186 6 1500# HC gas Ball valve Pneumatic 28
160 2 600# HC gas Ball valve Pneumatic 33
171 5.125 5000# Riser Ball valve Hydraulic 33
169 6 600# HC gas Ball valve Pneumatic 33
134 8 600# Oil Ball valve Pneumatic 33
K. Cameron, A. Lewis, D. Montalv~
ao et al. Petroleum 9 (2023) 613e620
615
The pressure class stated for each valve in Table 1 defines the
range of operating pressures and the maximum pressure that the
specific piping system can work within, referring to ANSI/ASME
B16.5. In addition, the process media listed in Table 1 are described
below:
HC gas ehydrocarbon gas phase fluids, minimally processed (1/2
stages of separation) condensate bearing gas.
Riser eunprocessed well fluids mainly liquid with evolving gas
phase.
Oil eliquid phase, minimally processed (1/2 stages of separa-
tion), with evolving gas.
All the ESDVs analysed are installed with open and closed po-
sition limit switches which are used to show the status of the ESDV
in the plant control system. The status of these limit switches are
date/time stamped and logged and it is this information that is the
basis for the data analysis. As part of the pre-processing routine,
data was made uniform in structure and content prior to the onset
of the investigation.
The first step taken in screening of the input data is to sort
tagged information into separate files. Spreadsheet files were used
together with Matlab scripts to ensure consistency and reduce
human errors in this operation. Fig. 1 shows the type of information
that has been provided by the project participants, this data is in
the format exported from the process plant control system, with
date and time stamps, tag and equipment state.
Once the data for a specific tagged ESDV was placed in one file,
the data was then sorted chronologically, and duplicate rows were
removed. It was at this point that the data was used to provide the
closing time from the open to close status by a closure detection
algorithm. The algorithm detects a character string containing a
confirmed “open”position as the start of the closing sequence.
Then, character strings containing the "travel" or "moving" condi-
tion were searched for as a successive condition, together with the
confirmed “closed”condition to calculate the closure time. This
three-state successive search provides the most stringent rule set in
detecting full ESDV open to close stroke. Two-state successive
searches were used in the development of the final algorithm,
however in this case the possibility of the ESDV not being fully open
at the start of the sequence does exist, therefore this two-state
successive search pattern was not used, and the three-state suc-
cessive pattern was selected instead.
The algorithm ran continuously to process all the available
tagged data collected from the plant control system. In total, 479
individual ESDV datasets were identified. The first step in pro-
cessing this data was to assess the overall confidence for each
particular ESDV dataset. This involved quantifying several aspects
of the data for comparison amongst the datasets collected. The
factors assessed are:
(1) A set minimum number of open/closing transitions, seven
(Considered a minimum to provide a meaningful
correlation).
(2) The number of transitions recorded in excess of 600 s
(Considered too long to be a credible ESDV closure).
(3) An average non-zero transition time.
The content of the datasets may be affected by factors such as
power blackouts affecting the recording equipment or loose com-
ponents or connections in the limit switch assemblies. The result of
this confidence screening process is that from the initial 479
datasets, items with zero confidence factors were removed. This
meant that a total of 33 ESDV datasets remained for analysis. Of
these 33 datasets, six datasets with the highest confidence factors
have been taken forward for detailed analysis as shown in Fig. 2.
The confidence factor was determined by calculating the ratio of
closing transitions to opening transitions since this referred to the
completeness of the dataset.
Once the datasets are assessed for confidence, the analysis aims
to address the research questions introduced earlier, namely:
(1) Is the closure time of the ESDV dependent on the time that it
has remained in the open position prior to the command to
close?
(2) Do frequent successive closures over a short number of hours
improve the closure time?
To address the first point, from the confirmed “open”and the
“travel”or “moving”states, it is possible to determine the time
period that the ESDV has been in the open position prior to closing,
referred to as TSLS. For a given ESDV dataset, the series of TSLS
values is correlated against the series of closure times. Pearson's
assessment of correlation is used as the basis for this part of the
Fig. 1. Typical input SOE (sequence of events) data from pla nt control system.
K. Cameron, A. Lewis, D. Montalv~
ao et al. Petroleum 9 (2023) 613e620
616
analysis. Values approaching 1 are considered as having a strong
correlation, values approaching 0 are considered as having no
correlation and values approaching -1, having a strong inverse
correlation. The implementation of Pearson's correlation coefficient
is shown in Eq. (1).
r
ðA;BÞ¼ 1
N1X
N
i¼1
A
i
m
A
s
A
B
i
m
B
s
B
(1)
A
i
¼Input array independent variable, TSLS.
B
i
¼Input array dependent variable, closure time.
N¼Number of observations
m
A
¼Mean of A
s
A
¼Standard deviation of A
m
B
¼Mean of B
s
B
¼Standard deviation of B.
As part of the analysis, the closing stroke times have been
divided into two groups, above mean closure times and below
mean closure times. This has been done to assess the sensitivity of
each of these groups in the correlation analyses, (i.e., do higher
closure times or lower closure times have a greater correlation to
TSLS?).
For the analysis on the second question relating to the effect of
successive closures over a short number of hours, the records
within the dataset are analysed to search for date/timestamps that
have a close proximity. This is defined as within a 24 h period of the
previous closure, and this can result in extended series of closures
within multiples of 24 h periods.
Several assumptions have been made to carry out the above-
mentioned analysis which are listed below:
(1) The effect of process line pressure is negligible as these
plants are continuously operated and only shut down for
annual or biennial maintenance, during which time valves
are not used.
(2) None of the ESDVs analysed have received major mainte-
nance that would improve their performance and that the
ESDVs have not been replaced during the analysed time span.
(3) None of the ESDVs are fitted with equipment that carries out
online partial closure testing. These partial closures are
automatically discounted by the detection algorithm.
(4) Closures with excessively long closure times, longer than
600 s, are not valid and these have been removed from the
dataset.
This analysis approach has been selected largely due to the
volume and type of data received from the project participants.
The data has required careful handling and transposition to make
it suitable for calculation purposes, while preserving the fidelity
and digital system artefacts attached to the ESDV records (status
tags). The individual ESDV transition records have run into hun-
dreds of rows; thus, an automated scripted solution has been
created and utilised within Matlab. This has been split into multi-
stage processing to ease verification of the data processed within
each stage.
In terms of hypothesis testing, these two research questions are
investigating the alternate hypothesis. The null hypothesis has
been established through interviews with experienced ESDV field
technicians. The null hypothesis is that the closure time of ESDVs is
negatively affected when the ESDV is kept in the open position for
extended periods. The alternate hypothesis investigated in this
study is that CT is independent of TSLS.
5. Results
5.1. Relationship between CT and TSLS
In analysing the dependence of the closing stroke time on the
time period that the ESDV was held in the open position prior tothe
close command, reference is made to the TSLS value that is related
to each of the closures. In order to test for a relationship between
the CT and the TSLS, Pearson's correlation coefficient was used to
assess the relationship between the data series representing CTand
TSLS. The TSLS value series plotted against CT for ESDV142 is shown
in Fig. 3 as a typical example.
Considering the convention used by Evans [29] to describe the
absolute measure of Pearson's correlation coefficient, shown in
Table 3, the quantitative information in Table 2 has been supple-
mented with a description.
In the summary of the information in Table 2, there is mainly a
weak or very weak correlation between the closure time of the
ESDV and the time that the ESDV was in the open position prior to
the command to close. There is one isolated instance of a ‘moderate’
relationship with the 0.53 value for above mean closures for ESDV-
160, however this does not help provide a finding applicable to the
wider group. The same application, actuation media and pressure
class are present in other ESDV datasets which produce much lower
correlation results. In addition, the general correlation and the
below mean correlation remain within the ‘very weak’and ‘weak’
Fig. 2. ESDV data set confidence factor.
Fig. 3. CT vs TSLS for ESDV142 valve.
K. Cameron, A. Lewis, D. Montalv~
ao et al. Petroleum 9 (2023) 613e620
617
categories. Therefore, the isolated instance for ESDV-160 is not
considered further.
Considering the data from the other ESDV datasets, the general
correlation coefficients calculated for the larger group of non-zero
confidence factor datasets (33) are shown in Fig. 4. This demon-
strates that there are low levels of correlation between these pa-
rameters for a larger group of ESDVs. The group of ESDVs analysed
in this case consisted of the thirty-three non-zero confidence factor
ESDVs from four different offshore platforms.
As evident in Fig. 4, the level of correlation between closure time
and TSLS remains low across the larger group of ESDVs. Most values
remain in the classifications of “weak”or “very weak”. There are
two outliers, indicating opposite correlation, direct and inverse,
however when considered together they do not bring any mean-
ingful finding to the overall correlation relationship. Therefore,
detailed analysis of the results demonstrates no relationship be-
tween the ESDV closure time and the time that the ESDV has been
open before being commanded to close (i.e. TSLS). Thus, the
response to the first research question is negative. The correlations
between the closure time and TSLS are almost exclusively weak or
very weak, demonstrating little or no meaningful relationship. As
such, based on the range of ESDVs investigated, it could be
concluded that ESDVs do not benefit from any periodic closure or
partial movement to maintain a target closure time.
5.2. Relationship between CT and successive closures
The next research question was whether successive closures
over a short number of hours can improve the ESDV closure time.
The same subjects used in the previous section were analysed
regarding the frequency of successive closures. The algorithm that
was used for the analysis was modified to seek closure times that
occurred within a 24 h period of each other. For each ESDV the
frequent closure series were generated and recorded as depicted in
Table 4.
The 3D stem plots of the frequent closure groups for each ESDV
are also shown in Figs. 5e10. The x-axis shows the month/year, y-
axis shows the closure order and z-axis shows the CT.
In review of Figs. 5e10, it can be seen that in each closure order,
there is not a consistent trend on CT, the CT is not consistently
increasing or decreasing through the closure order. Considering
this finding, this helps to corroborate the findings of the previous
section, where no appreciable levels of correlation between CT and
TSLS were found.
Table 2
Correlation values between CT and TSLS.
ESDV General correlation
coefficient
Correlation
description
Above mean closure
time coefficient
Correlation
description
Below mean closure
time coefficient
Correlation
description
ESDV142 0.031462 Very weak 0.12431 Very weak 0.026146 Very weak
ESDV186 0.14433 Very weak 0.22120 Weak 0.16889 Very weak
ESDV160 0.049306 Very weak 0.53693 Moderate 0.37208 Weak
ESDV171 0.17234 Very weak 0 - 0.21790 Weak
ESDV169 0.062095 Very weak 0 - 0 -
ESDV134 0.19699 Weak 0.28239 Weak 0.11155 Very weak
Fig. 4. General correlation between CT and TSL S for multiple ESDVs.
Table 3
Description of Pearson's correlation values by Evans [29].
Pearson's R value Description of correlation
0.00 to 0.19 Very weak
0.20 to 0.39 Weak
0.40 to 0.59 Moderate
0.60 to 0.79 Strong
0.80 to 1.00 Very strong
Table 4
Frequent closure groups.
ESDV Number of frequent
closures series detected
Maximum number of closures
from frequent closure groups
ESDV142 602 14
ESDV186 67 7
ESDV160 35 3
ESDV171 9 3
ESDV169 58 7
ESDV134 6 4
Fig. 5. Frequent closure groups for ESDV142 valve.
K. Cameron, A. Lewis, D. Montalv~
ao et al. Petroleum 9 (2023) 613e620
618
6. Discussion &conclusions
This paper aims to address two questions that are prominent in
the management of barriers to major accident hazards, barriers
such as emergency shutdown valves. These questions are:
(1) Is the closure time of the ESDV dependent on the time that it
has remained in the open position prior to the command to
close?
(2) Do frequent successive closures over a short number of hours
improve the closure time?
In response to the first question, maintaining a target closure
time is the prime measure of confidence and fitness for service for
an ESDV, either for pipeline riser valve service or as part of a SIS
under IEC-61511. Under both regimes, the CT is demonstrated in a
proof test, and it is in the scheduling and management of proof
testing that the outputs of this investigation are thought to be
valuable.
Fig. 6. Frequent closure groups for ESDV186 valve.
Fig. 7. Frequent closure groups for ESDV160 valve.
Fig. 8. Frequent closure groups for ESDV171 valve.
Fig. 9. Frequent closure groups for ESDV169 valve.
Fig. 10. Frequent closure groups for ESDV134 valve.
K. Cameron, A. Lewis, D. Montalv~
ao et al. Petroleum 9 (2023) 613e620
619
In the investigation between CT and TSLS, the aim was to
discover if there is a stable and reliable relationship that may
become the basis for maintaining a target CT. If successfully
demonstrated, the specifics of a single ESDV may then be investi-
gated for detailed analysis. Based on the results of this study, where
there is at best a consensus of a “weak”,or“very weak”relationship
between CT and TSLS, it can be concluded that there is not a clear
alternative to the current regime of scheduling proof testing based
on probability calculations.
The second research question is whether successive closures
over a short number of hours improve the closure time. From the
initial group of six ESDVs, groups of closures taking place over a 24-
h interval have been found from the operating records available.
The intent in this part of the investigation was to determine if there
was a significant trend or profile impacting CT. According to the
results presented in Figs. 5e10, it is apparent that there is not a
consistent relatable trend through the order of closures. This in-
formation is considered useful in dealing with proof test results
that do not meet the target as further closures, without other
intervention/maintenance activities, do not necessarily indicate
better results.
In most of the ESDVs studied in this investigation (4 out of 6),
there is a variance in the CTover the time that the ESDV is in service.
The results presented in this paper show that the technical man-
agement of the ESDV is not able to influence the CT performance by
manipulating the frequency of operation of the ESDV. Therefore, it
can be concluded that CT variance of the ESDV depends on factors
outside of those that the technical management of the ESDV can
influence. Whilst the factors that do affect the CT are of interest to
all in the sphere of operation and verification of major accident
hazard barriers, the findings of this study demonstrate that these
do not include the frequency of operation of the ESDV.
However, it should be stressed that the findings of this study
require further qualification before this information can be used to
inform decisions on the operational management and testing of
ESDVs. Firstly, ESDVs should be managed and tested within their
legislative area, either pipeline regulations or functional safety.
Secondly, the findings stated in this paper should not be used to
extend test frequencies beyond the boundaries of existing accepted
and mandated practices.
Abbreviations
CT Closure time
ESDV Emergency shutdown valve
IEC International electrotechnical commission
MTTF Mean time to failure
PFD Probability of failure on demand
SIS Safety instrumented system
SOE Sequence of events
TSLS Time since last stroke
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