![st separation system with the ESD system [19]. Rectangular with dot line in Figure 1 presents the component of ESD system such as PSV (Pressure Safety Valve), ESD valve, PSD (Pressure ShutDown) valve, FO (Flow Orifice), PSI A (Pressure Safety Indicator/Alarm) and PSE (Pressure safety sensor). Equipment is expressed in P&ID with symbol and identification letters defined from American National Standard 'Instrumentation Symbols and Identification' [20]. Control panel (CLU: Control Logic Unit) is connected all of the ESD components.](https://www.researchgate.net/publication/301269975/figure/fig1/AS:361428235112448@1463182614448/st-separation-system-with-the-ESD-system-19-Rectangular-with-dot-line-in-Figure-1_Q320.jpg)
Available via license: CC BY 4.0
Content may be subject to copyright.
Design Optimization of ESD (Emergency ShutDown) System
for Offshore Process Based on Reliability Analysis
Jeong-hoon Bae
1
, Sung-chul Shin
1,a
,Byeong-cheol Park
1
and Soo-young Kim
1
1
Department of Naval Architecture and Ocean Engineering, Pusan National University, Busan, South Korea
Abstract. Hydrocarbon leaks have a major accident potential and it could give significant
damages to human, property and environment.To prevent these risks from the leak in design
aspects, installation of ESD system is representative. Because the ESD system should be
operated properly at any time, It needs high reliability and much cost. To make ESD system
with high reliability and reasonable cost, it is a need to find specific design method.In this
study, we proposed the multi-objective design optimization method and performed the
optimization of the ESD system for 1st separation system to satisfy high reliability and cost-
effective.‘NSGA-II (Non-dominated Sorting Genetic Algorithm-II)’ was applied and two
objective functions of ‘Reliability’ and ‘Cost’ of system were defined. Six design variables
were set to related variables for system configuration. To verify the result of the optimization,
the results of existing design and optimum design were compared in aspects of reliability and
cost. With the optimization method proposed from this study, it was possible to derive the
reliable and economical design of the ESD system.
1 Introduction
1.1 Motivation
As more offshore plants are installed around the world, more accidents related to the offshore plant
areoccurring. Since 1995, the number of accidents related to the offshore plants for oil production has
reached several hundred a year and a lot of people have been also injured or lost their lives[1].
Especially, most of offshore plants which are designed to drilling, production, retrieve, refine the oil
are closely related to the flammable hydrocarbon gas in high temperature and high pressure.
Since the accident in Piper Alpha[2], the offshore plant industries recognized importance of safety
from accident of hydrocarbon and fire/explosion in offshore plant. So, to reduce the many accidents
and risks, various attempts have been made such as rule revision and creation of safety division. UK
put the onus on the operator to identify the major hazards and to reduce risks with The Offshore
Safety Case regulations[3]. The HSE (Health and Safety Executive) also created the ‘Offshore Safety
Division’ and discussed the revision or verification of rules for safety. The NPD (Norwegian
Petroleum Directorate) founded ‘Regulations relating to management in the petroleum activities’ in
2001 for safety [4].
a
Corresponding author : scshin@pusan.ac.kr
DOI: 10.1051/
C
Owned by the authors, published by EDP Sciences
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There are a lot of approaches to satisfy safety in offshore plant. One is to reduce the ‘Probability’
of accident from human and organizational factors, system failure, natural disaster, etc. The other is to
reduce ‘Consequence’ severity of such an event when it occurs with visual alarms, fire suppression
system or a process shutdown [5]. From these aspects, the ESD system is very important to reduce
‘Consequence’ of accident as shutdown release of hazardous material. If the ESD system doesn't work
and fail to shutdown when there is release of hydrocarbon in offshore plant, this failure could cause of
fire/explosion disaster. So the ESD system is required to design with high reliability to avoid failure in
dangerous situation.
From reliability aspects, there are two international safety authorities governing SIL (Safety
Integrity Level), IEC (International Electrotechnical Commission) 61508 and IEC 61511. 61508
governs the functional safety of electrical, electronic and programmable electronic safety systems e.g.
Production Inflow Control Devices (ICDs). It is applied across all industries and IEC 61511 governs
the functional safety of safety instrumented systems and itis applied in the process industries. In 2000
year in Norway, OLF (The Norwegian oil industry association) tried to issue a guideline on the
application of IEC 61508 and IEC 61511 in the Norwegian Petroleum Industry [6]. OLF also has
defined the procedure and requirements of the ESD for offshore plant in their "Technical Safety" of
‘NORSOK STANDARD S-001’ [7]. DNVestablish ‘OFFSHORE STANDARD DNV-OS-E201: Oil
and Gas Processing Systems’ and to provide an internationally acceptable standard of safety for
hydrocarbon production plants and LNG processing plant by defining minimum requirements for the
design, materials, construction and commissioning of plant[8].
1.2 A literature review
For high reliability of the offshore system, reliability analysis is necessary in the early stage of design.
There are a lot of domestic and overseas studies related to the reliability analysis. As for the overseas
studies, there was a research that suggested the simplified technique of reliability analysis and applied
it to the offshore plant mooring system for the optimal [9]. There was also a study on the fatigue
reliability analysis in the structure based on the analysis of various scenarios related to the structural
fatigue for the extension of lifetime of the offshore plant [10]. But they are focused on structural or
fatigue reliability of system. It is differ from functional safety of electrical, electronic, programmable
electronic safety-related systems or safety instrumented systems for the process industry sector such
as the ESD system.
As the overseas study directly related to the reliability analysis of the ESD system, FTA (Fault
Tree Analysis) was used to define the failure rate of system component as the lower level and enhance
the reliability of the system based on the HAZOP (HAZard and OPerability)[11]. SINTEF
(Norwegian: Stiftelsen for industriell og teknisk forskning)
studied reliability of subsea BOP systems
for deepwater application[12]. Detailed failure statistics for the various BOP systems were analyzed
and presented in the US GOM OCS (Outer Continental Shelf).Ram K. et al studied impact of
reliability or the number of emergency shutdown devices on flare relief system and analyzed related
factors for sizing of individual relief valves protecting equipment or process or system [13].This paper
highlighted several concerns such as standards, reliability, safety and offers practical advice to those
facing relief system design decisions.A.C. Torres-Echeverrıa et al studied about multi-objective
optimization for safety instrumented systems of chemical reactor system with three objective
functions reliability, STR (Spurious Trip Rate) and cost[14].Theyappliedthe reliability
modelstooptimizationofdesignandtestingof safety instrumented systems. The models for optimization
have been integrated, together with a Life cycle Cost model, as objective functions in to a multi-
objective genetic algorithm.FaresInnal et al also studied safety and operational integrity evaluation
and design optimization of safety instrumented monitoring systems with two objective functions
reliability and STR [15].
In domestic studies, there was a study about design of the flight control system. Reliability of the
system was analyzed and the method of improving reliability through simulation was proposed
[16].There was also another research in the field of fire prevention. The design of the system can be
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verified whether it is proper to the SIL through the reliability analysis of fire/explosion safety device
of Ethyl Benzene process [17]. In offshore industry, Bae J. H. et al performed reliability analysis of
the ESD for supporting design of LNG bunkering [18].
This study was focused on not only method of design optimization for offshore process but also
practical design by selecting ESD products on the market.Totally 22 types of ESD components were
investigated from valve companies and online. In order to design closer to practical system, Existing
system ‘Heidrun (TLP)’, has been operating in Norwegian Sea since 1995, was selected to optimize
design of ESD system and to compare its results.The multi-objective design optimization was
performed with two objective functions of ‘Reliability’ and ‘Cost’. ‘Reliability’is based on PFD
(Probability of Failure rate on Demand)values from reliability analysis and ‘Cost’is composed of
purchase cost, proof test cost, loss of production and etc. Design variables were set to six practical
variables for configuration of system.To verify improvement of the design, the results of Heidrun
design and optimum design was compared in aspects of reliability and cost.
2 The Emergency ShutDown system
In this thesis, the ESD system of 1st separation system in TLP at Heidrun oil field was selected for
target system because it could be applicable more practically for optimization.The 1
st
separation and
related line has high pressure and temperature conditions with hydrocarbon material. It could
havehigh risks of fire/explosion accident. So, these separation systems are required to controlled and
monitored in all process functions on the topsides as well as Fire & Gas and the ESD for the entire
FPSO. The P&ID (Piping & Instrument Diagram)of the ESD system is as shown in Figure 1[19].
Figure 1.1
st
separation system with the ESD system [19].
Rectangular with dot line in Figure 1 presents the component of ESD system such as PSV
(Pressure Safety Valve), ESD valve, PSD (Pressure ShutDown) valve, FO (Flow Orifice), PSI A
(Pressure Safety Indicator/Alarm) and PSE (Pressure safety sensor). Equipment is expressed in P&ID
with symbol and identification letters defined from American National Standard ‘Instrumentation
Symbols and Identification’ [20]. Control panel (CLU: Control Logic Unit) is connected all of the
ESD components.
S-FO
S-PSV
S-Compressor
S-HP
S-Sand
S-Drain
S-Crude
S-Jet
S-LP
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3 Reliability Analysis
Reliabilityis defined by IEC 50 (191) as‘the ability of an entity to perform a required function under gi
ven conditions for a given time interval’and it is usually expressed in failure rate, MTTF (Mean Time
To Failure), SIL (Safety Integrated Level) and etc. [21].
To perform reliability analysis for the ESD system, shutdown procedure is as follows;
1. If overpressure is detected by the sensors during separating operation, the main pump
related to the 1
st
separator is stopped immediately.
2. The PSD/ESD control logic send shutdown signal to final elements.
3. Final elements shutdown system to prevent further accidents from occurring.
3.1 PFD and Failure scenarios
Nine failure scenarios of overpressure were defined for reliability analysis (PFD calculation) as
referred to ‘Component structure’[22] and The Norwegian Oil Industry Association[6].The PFD of the
E/E/PE safety-related system is determined by calculating and combining the average probability of
failure on demand for all the subsystems which provide protection against a hazardous event [22].
The failure scenario ‘Flare FO’ is related to the failure of two flow orifices, two pressure safety
indicators installed in the line to flare header and CLU for control. If there is the overpressure in line,
CLU should order to open the flow orifice. Once one of two flow orifices operates normally in failure
situation, this scenario is success as shown in Figure 2. In similar way to define scenarios such as
‘Flare FO’, theother eight scenarios were defined as shown Figure 3to Figure 10.
Figure 2.
Scenario - Flare FO.
G
Figure
3.
Scenario - Flare PSV.
Figure
4.
Scenario
–
Compressor
Figure 5. Scenario – HP (High Pressure) flare header
Figure 6.Scenario - Sand cleaning
Figure 7.Scenario - Drain header
Figure 8.Scenario - Crude heater Figure 9.Scenario - Jet water pump
Figure 10.Scenario – LP (Low Pressure) compressor
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3.2 Calculations of PFD and SIL
For, reliability analysis, failure data and MTTR (Mean Time To Repair)were referred from ‘OREDA
(Offshore and Onshore Reliability Data) 2009’[23]. From nine failure scenarios with failure data of
components, PFD and SIL were calculated as shown in Table 1.
Table 1.The results of reliability analysis (PFD and SIL).
Scenario
Component
Type
Failure rate
(per10E+6hours)
MTTR
(hours)
PFD
SIL
RequiredSIL
S-FO
Sensor
PSI
4.20E-07
4
1.21E-03
SIL 2
SIL 2
Logic unit
CLU
2.85E-05
6
Final element
FO
4.23E-06
8
S-Flare PSV
Final element
PSV
8.47E-06
8
1.48E-04
SIL 3
SIL 2
S-
Compressor
Sensor
PSI
4.20E-07
4
1.73E-02
SIL 1
SIL 2
Logic unit
CLU
2.85E-05
6
Final element
PSD
1.90E-05
17
Sensor
PSE
4.10E-07
4
Logic unit
CLU
2.85E-05
6
Final element
PSD
1.90E-05
17
S
-
HP flare header
Sensor
PSE
4.10E-07
4
9.89E-03
SIL 2
SIL 2
Logic unit
CLU
2.85E-05
6
Final element
ESD
2.58E-05
16
S
-
Sand cleaning
Sensor
PSE
4.10E-07
4
9.89E-03
SIL 2
SIL 2
Logic unit
CLU
2.85E-05
6
Final element
ESD
2.58E-05
16
S-Drain
Sensor
PSE
4.10E-07
4
8.66E-03
SIL 2
SIL 2
Logic unit
CLU
2.85E-05
6
Final element
PSD
1.90E-05
17
S-Crude
heater
Sensor
PSE
4.10E-07
4
9.89E-03
SIL 2
SIL 2
Logic unit
CLU
2.85E-05
6
Final element
ESD
2.58E-05
16
S
-
Jet water pump
Sensor
PSI
4.20E-07
4
8.66E-03
SIL 2
SIL 2
Logic unit
CLU
2.85E-05
6
Final element
PSD
1.90E-05
17
S-LP compressor
Final element
PSV
8.47E-06
8
1.48E-04
SIL 3
SIL 2
From, the results of reliability analysis, scenario ‘S-Compressor’ has lowest SIL 1 and scenarios
‘S-PSV’, ‘S-LP’ have high SIL 3. Except these 3 scenarios, allscenarios have SIL 2. Even if ‘S-FO’
has already SIL 2, it has more chance to reduce cost with higher PFD value in SIL 2 range.If the ESD
system for 1
st
separation system in offshore plant is required to minimum SIL 2 as referred from The
Norwegian Oil Industry Association[6], þS-Compressor’ is needed to improve design to meet SIL 2
from SIL 1, while S-PSV’ and ‘S-LP’ are needed to simplify design to make SIL 2 from SIL 3 for
reducing cost.
4 The design optimization of the ESD system
4.1 Definition of Optimization problem
The purpose of this design optimization is to find design variables that make minimum value of
objective function. It means optimized design has high reliability with reasonable cost for the ESD
system. NSGA-II is selected for optimization algorithm.
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4.1.1Objective function
ė Objective function ‘Reliability’
Objective function of ‘Reliability’ (
1
) is estimated from each scenarios’ as equation (1).
1
= (1)
ė Objective function ‘Cost’
Objective function of ‘Cost’(
2
) is calculated from ‘Product cost’, ‘Replace cost’, ‘Proof test cost’
and ‘Loss of production’ of the ESD systems on the following equation (2).
2
=
+
+
+
(2)
ė
is product price of sensors, logic unit and final element for installation at first
time.
ė
is cost of replacement during lifetime thatdepends on MTTF.
ė
is calculated based on times of proof test during lifetime, test cost of labor[24]
for one equipment and number of equipment.
ė
is loss of production from downtime duringproof test[25]. It estimated with
WTI crude oil price $57 (April 17, 2015), production ‘65,000bbl/day’ at ‘Heidrun’
oilfield[26] and test time ‘1 hour’[27].
4.1.2 Design variables
From a reliability point of view, system is generally consist three parts; sensor, logic unit and final
element. As shown in Table 2, six design variables were set to the number of redundancy at each part,
type of sensor and final element, proof test interval.Database for design space was created including
information of products as MTTF and price. Eight types of sensors and fourteen types of final
elements were investigated from brochure of product[28] andonline market[29]. Failure data is
referred to ‘OREDA 2009’ data for sensors, logic unit and final elements.
Table 2. Design variables and space.
Design variable
Unit
Range
The number of redundancy - sensor
Number
0, 1, 2
The number of redundancy - logic unit
Number
0, 1, 2
The number of redundancy - final element
Number
0, 1, 2
Type of sensor
Type
1~8 (8 types of products)
Type of final
element
For blowdown
Type
1~2 (2types of products)
For shutdown
Type
1~12 (12 types of products)
Proof test interval
Year
1~3
4.1.3 Constraints
The topside process in offshore plant is not extremely dangerous such as nuclear plant or has not very
severecondition such as deepwater subsea well operation. Therefore generally SIL 2 is proper for
offshore topside process. The Norwegian Oil Industry Association[6] also suggested minimum SIL 2
for the ESD system related to separation system. Constraints were set to SIL 2 and it has range of
10
−3
≤ PFD < 10
−2
by PFD value.
4.2 The results of the optimization
4.2.1 ‘S-FO’ - Blowdown operation
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Population of NSGA-II was set to 40, generation was 1,000 andcalculation time was 9.4s for
optimization. Figure 11 is Pareto-frontier results from the optimization of scenario ‘S-FO’. In this
study, we focused on optimum design which has minimum cost in SIL 2. This means among the
alternatives which satisfied SIL 2(
10
−3
≤ PFD < 10
−2
), lowest cost alternative ‘FO’could be chosen as
shown in Figure 11.For ‘S-FO’ - Blowdown operation in ‘To flare header’ line, It should have
equipment for blowdown system such as flow orifice. So, type of the final element in ‘S-FO’ was
fixed to flow orifice and optimization was performed with the other design variables type of sensor,
the numbers of redundancies and proof test interval.
G
Figure 11.Pareto-frontier of scenario ‘S-FO’ and optimum alternative
Details of alternative ‘FO’ (0.006348, 1.38e+6) are as shown in Table 3.
Table 3.Optimum alternative of ‘S-FO’
Design variable
Value
Details
Sensor
0
No redundancy
Logic unit
1
1 redundancy
Final element
0
No redundancy
Type of sensor
2
Pressure indicator ‘P Series’
Type of final element (blowdown)
2
Flow orifice
Proof test interval
3.000
3 years
4.2.2Summary of the results include other eight scenarios
The total results and comparisons of optimization results to Heidrun system are as shown in Table 4.
Table 4.The total results and Comparison of optimization results.
Heidrun
Optimum
Scenario
PFD
SIL
Cost($)
PFD
SIL
Cost ($)
S-FO
0.0012
SIL 2
4,220,513
0.0063480
SIL 2
1,379,840
S-Flare PSV
0.0001
SIL 3
4,035,682
0.0048284
SIL 2
1,384,680
S-Compressor
0.0173
SIL 1
4,220,463
0.0098441
SIL 2
1,452,460
S-HP flare header
0.0099
SIL 2
4,047,008
0.0065769
SIL 2
1,383,100
S-Sand cleaning
0.0099
SIL 2
4,047,008
0.0065769
SIL 2
1,383,100
S-Drain
0.0087
SIL 2
4,047,008
0.0065769
SIL 2
1,383,100
S-Crude heater
0.0099
SIL 2
4,047,008
0.0065769
SIL 2
1,383,100
S-Jet water pump
0.0087
SIL 2
4,047,008
0.0065769
SIL 2
1,383,100
S-LP compressor
0.0001
SIL 3
3,975,550
0.0044857
SIL 2
1,353,610
Total cost
36,687,248
12,486,090
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Every scenario is optimized to meet the minimum SIL 2 and total cost of final design also
decreased $24,191,186 from origin design.
4.3Discussion
As shown in Figure 12, all PFD values of scenarios are in the range of SIL 2 and this means they
satisfied the required reliability through the optimization.Although SIL of the scenario ‘S-FO’ is the
same as SIL 2 before the optimization, PFD is increased up to about 0.005 for reducing cost by design
modification. In case of the scenario ‘S-FO’, redundancy was removed and another element among
the database that has lower PFD was selected to reduce the cost of system in SIL 2.PFD of‘S-PSV’
and ‘S-LP’ scenarios were also increased and their SIL was degraded to SIL 2 from SIL 3 to reduce
the cost. To design system with higher reliabilityneeds more cost because they need generally high
quality products and complex system.But from the results PFD and cost as shown Figure 12, it was
possible to improve reliability andreduce cost simultaneously.
PFD values of eight scenarios except ‘S-Compressor’ could not reached close to boundary of SIL
2 and SIL 1 as shown in Figure 12. It means they could have still more possibilities of improvement
with reduction of cost. Despite convergence of optimization in this study, to reach near the ideal
optimum point ‘boundary of SIL 2 and SIL 3’ was difficultbecause there were discrete design
variables such as type of element and the number of redundancy. One of the methods of improve the
result of optimization is to adding various elements for increasing database in order to make design
space almost continuous.
G
G
Figure12.PFD and cost comparison of the results.
As comparison of design variables of Heidrun and optimum as shown in Table 5, all of test
intervalsare increased and all structures of S-L-F (Sensor-Logic unit-Final element) are changed.
Number in S-L-F column of Table 5 means the number of element in each scenario. The number of
sensor and final element are modified to the same as one except ‘S-PSV’ and ‘S-LP’. It seems they
tried to decrease the number of redundancy for reducing cost of each scenario.From the results of
‘Type (sensor)’ in Table 5,‘1: Pressure safety indicator’ and ‘2: Pressure safety Sensor’are considered
suitable for the ESD system in this study.
Table 5.Comparison of design variables of Heidrun and optimum.
Operation
mode
Scenario
Heidrun
Optimum
S-L-
F
(structure)
Type
(sensor)
Type
(final
element)
Proof test
interval
S-L
-F
(structure)
Type
(sensor)
Type
(final
element)
Proof
t
est
interval
Blowdown
S-FO
2
2
2
1
2
1.000
1
2
1
2
2
3.000
S-PSV
0
0
2
-
1
1.000
0
0
1
-
1
3.000
Shutdown
S-Comp.
1
1
1
1
3
1.000
1
2
1
2
3
2.999
5+.
5+.
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1
1
1
1
2
1
S-HP
1
1
1
2
8
1.000
1
2
1
1
2
3.000
S-Sand
1
1
1
2
12
1.000
1
2
1
2
2
3.000
S-Drain
1
1
1
2
8
1.000
1
2
1
2
2
3.000
S-Crude
1
1
1
2
8
1.000
1
2
1
2
2
3.000
S-Jet
1
1
1
2
12
1.000
1
2
1
2
2
3.000
Blowdown
S-LP
0
0
2
-
1
1.000
0
0
1
-
1
3.000
From Table 5, the number of logic unitforsix scenarios ‘S-Comp.’, S-HP’, ‘S-Sand’, ‘S-Drain’, ‘S-
Crude’, ‘S-Jet’ are increased to two from one. We can estimate that this reason from graph of PFD
comparison as shown in Figure 12. All of six scenarios’ PFD values are decreased and this means
redundancy of logic unit was be used for reduction of PFD.
5 Conclusions
In this study, following were carried out in order to attain final goals.
ė Reliability analysis of the existing ESD system foroffshore process was performed with
defined scenarios and failure data.
ė The multi-objective design optimization was performed with defined two objective
functions of ‘Reliability’ and ‘Cost’. Six design variables and ‘SIL 2’ constraints were
defined. Optimum design was selected from Pareto-frontier and it satisfied both reliability
SIL 2 and cost reduction.
ė In order to designcloser to practical system, existing system was selected to optimize
design of ESD system. Database for design space was also created including information
of product on the market.
With these results, more practical method of design optimization was proposed for the ESD
system of offshore process and it could be applied to other similar process.One of the methods of
improve the result of optimization is to adding various ESD elements for increasing database and
makes design space almost to be a continuous.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the
Korean government (MEST) through GCRC-SOP (No. 2011-0030671).
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