Hydrocarbon Processing - April 2022 - 87
Process Controls, Instrumentation and Automation
Use Case 4: Diagnosing cause and
effect to find the needle in the haystack.
Poor performance is sometimes
isolated to single controllers, and the
need for retuning or valve maintenance
can be quickly determined. In more
complex situations, poor performance is
widespread, with a single problem propagating
across many controllers. Before
the problem can be addressed, the controller-which
is the root cause of the
variability-must be identified.
Root cause identification can be tedious,
particularly in processes with recycle
streams, heat integration and features
that mask cause and effect relationships.
Causality algorithms mathematically
quantify cause and effect relationships
among signals, revealing relationships
obscured on time series trends. Without
analytics to guide troubleshooting and
narrow the potential root causes, control
engineers may spend many hours
performing invasive process tests and inspecting
data trends.
In this use case, a plant-wide oscillation
lasting for about 2 hr persists through the
measurements and controllers designated
with blue and red circles (FIG. 6). The
dataset of oscillating controller process
variables (FIG. 7) is input to the causality
analysis. The directional arrows in the
generated causal map (FIG. 8) guide the
engineer toward controller LC2 causing
the related oscillations in TC1 and TC2.
Combining the causality results with process
dynamics and connectivity knowledge,
the engineer agrees that LC2 is a
suitable candidate for the root cause and
confirms this with additional plant testing.
Complementing analytics tools for
monitoring, diagnostic tools like causality
can streamline tedious investigations, enabling
the full range of control workflows.
Good things come in threes: Process
data + control engineers +
analytics. Exemplified by the four use
cases spanning single controllers to
many and analytics from monitoring to
diagnostics, advanced analytics is an invaluable
component for control and automation
success. By integrating control
engineer analytics within a comprehensive
and extensible analytics platform,
the collaborative value with other operations
personnel is greatly enhanced,
enabling a superior approach vs. nichefocused
control analytics packages. FeaT13
21.
Condenser
Condenser
22.
6.
FI1
4.
1.
PC1
2.
FC3
Steam
Condenser
7 & 8.
FC1
Feed
FIG. 6. Chemical process exhibiting plant-wide oscillations. Red circles designate cycling
measurements and blue circles designate cycling controllers.
P11 9.
T11
3.
LC1
5.
17.
LC3
T12
10.
FC4
12.
T14
Decanter
LC2
19.
FC5
18. F12
T15
11.
13.
TC1
T16
15.
FC6 14.
Steam
Condenser
Condenser
F15 20.
Product
FC8
23.
PC2 16.
Separator
Decanter
27.
26.
T18
T17
Condenser
F14 24.
F13 29.
TC2
25.
28.
PI2
30.
FC7
Steam
FIG. 7. Oscillating controller data over a 2-d period. With all controllers cycling
at a similar frequency, the oscillation source is not readily apparent.
ture-laden advanced analytics, applied
from a process control perspective and
utilizing the engineer's expertise and
process understanding, can be used to
quickly uncover process value hidden in
historian data and to accelerate process
control benefits.
JOHN COX is a Principal Analytics
Engineer with Seeq Corp.
He earned BS and MS degrees
in chemical engineering and
has an industrial background
in chemical process control.
FIG. 8. Causal map generated for the
dataset in FIG. 7 accurately guides the
user towards controller LC2 as the
root cause of plantwide oscillations.
Hydrocarbon Processing | APRIL 2022 87
Column 3
Reboiler
Column 2
Column 1
Condenser
Reboiler
Hydrocarbon Processing - April 2022
Table of Contents for the Digital Edition of Hydrocarbon Processing - April 2022
Contents
Hydrocarbon Processing - April 2022 - Cover1
Hydrocarbon Processing - April 2022 - Cover2
Hydrocarbon Processing - April 2022 - Contents
Hydrocarbon Processing - April 2022 - 4
Hydrocarbon Processing - April 2022 - 5
Hydrocarbon Processing - April 2022 - 6
Hydrocarbon Processing - April 2022 - 7
Hydrocarbon Processing - April 2022 - 8
Hydrocarbon Processing - April 2022 - 9
Hydrocarbon Processing - April 2022 - 10
Hydrocarbon Processing - April 2022 - 11
Hydrocarbon Processing - April 2022 - 12
Hydrocarbon Processing - April 2022 - 13
Hydrocarbon Processing - April 2022 - 14
Hydrocarbon Processing - April 2022 - 15
Hydrocarbon Processing - April 2022 - 16
Hydrocarbon Processing - April 2022 - 17
Hydrocarbon Processing - April 2022 - 18
Hydrocarbon Processing - April 2022 - 19
Hydrocarbon Processing - April 2022 - 20
Hydrocarbon Processing - April 2022 - 21
Hydrocarbon Processing - April 2022 - 22
Hydrocarbon Processing - April 2022 - 23
Hydrocarbon Processing - April 2022 - 24
Hydrocarbon Processing - April 2022 - 25
Hydrocarbon Processing - April 2022 - 26
Hydrocarbon Processing - April 2022 - 27
Hydrocarbon Processing - April 2022 - 28
Hydrocarbon Processing - April 2022 - 29
Hydrocarbon Processing - April 2022 - 30
Hydrocarbon Processing - April 2022 - 31
Hydrocarbon Processing - April 2022 - 32
Hydrocarbon Processing - April 2022 - 33
Hydrocarbon Processing - April 2022 - 34
Hydrocarbon Processing - April 2022 - 35
Hydrocarbon Processing - April 2022 - 36
Hydrocarbon Processing - April 2022 - 37
Hydrocarbon Processing - April 2022 - 38
Hydrocarbon Processing - April 2022 - 39
Hydrocarbon Processing - April 2022 - 40
Hydrocarbon Processing - April 2022 - 41
Hydrocarbon Processing - April 2022 - 42
Hydrocarbon Processing - April 2022 - 43
Hydrocarbon Processing - April 2022 - 44
Hydrocarbon Processing - April 2022 - 45
Hydrocarbon Processing - April 2022 - 46
Hydrocarbon Processing - April 2022 - 47
Hydrocarbon Processing - April 2022 - 48
Hydrocarbon Processing - April 2022 - 49
Hydrocarbon Processing - April 2022 - 50
Hydrocarbon Processing - April 2022 - 51
Hydrocarbon Processing - April 2022 - 52
Hydrocarbon Processing - April 2022 - 53
Hydrocarbon Processing - April 2022 - 54
Hydrocarbon Processing - April 2022 - 55
Hydrocarbon Processing - April 2022 - 56
Hydrocarbon Processing - April 2022 - 57
Hydrocarbon Processing - April 2022 - 58
Hydrocarbon Processing - April 2022 - 59
Hydrocarbon Processing - April 2022 - 60
Hydrocarbon Processing - April 2022 - 61
Hydrocarbon Processing - April 2022 - 62
Hydrocarbon Processing - April 2022 - 63
Hydrocarbon Processing - April 2022 - 64
Hydrocarbon Processing - April 2022 - 65
Hydrocarbon Processing - April 2022 - 66
Hydrocarbon Processing - April 2022 - 67
Hydrocarbon Processing - April 2022 - 68
Hydrocarbon Processing - April 2022 - 69
Hydrocarbon Processing - April 2022 - 70
Hydrocarbon Processing - April 2022 - 71
Hydrocarbon Processing - April 2022 - 72
Hydrocarbon Processing - April 2022 - 73
Hydrocarbon Processing - April 2022 - 74
Hydrocarbon Processing - April 2022 - 75
Hydrocarbon Processing - April 2022 - 76
Hydrocarbon Processing - April 2022 - 77
Hydrocarbon Processing - April 2022 - 78
Hydrocarbon Processing - April 2022 - 79
Hydrocarbon Processing - April 2022 - 80
Hydrocarbon Processing - April 2022 - 81
Hydrocarbon Processing - April 2022 - 82
Hydrocarbon Processing - April 2022 - 83
Hydrocarbon Processing - April 2022 - 84
Hydrocarbon Processing - April 2022 - 85
Hydrocarbon Processing - April 2022 - 86
Hydrocarbon Processing - April 2022 - 87
Hydrocarbon Processing - April 2022 - 88
Hydrocarbon Processing - April 2022 - 88A
Hydrocarbon Processing - April 2022 - 88B
Hydrocarbon Processing - April 2022 - 89
Hydrocarbon Processing - April 2022 - 90
Hydrocarbon Processing - April 2022 - Cover3
Hydrocarbon Processing - April 2022 - Cover4
Hydrocarbon Processing - April 2022 - GP-1
Hydrocarbon Processing - April 2022 - GP-2
Hydrocarbon Processing - April 2022 - GP-3
Hydrocarbon Processing - April 2022 - GP-4
Hydrocarbon Processing - April 2022 - GP-5
Hydrocarbon Processing - April 2022 - GP-6
Hydrocarbon Processing - April 2022 - GP-7
Hydrocarbon Processing - April 2022 - GP-8
Hydrocarbon Processing - April 2022 - GP-9
Hydrocarbon Processing - April 2022 - GP-10
Hydrocarbon Processing - April 2022 - GP-11
Hydrocarbon Processing - April 2022 - GP-12
Hydrocarbon Processing - April 2022 - GP-13
Hydrocarbon Processing - April 2022 - GP-14
Hydrocarbon Processing - April 2022 - GP-15
Hydrocarbon Processing - April 2022 - GP-16
Hydrocarbon Processing - April 2022 - GP-17
Hydrocarbon Processing - April 2022 - GP-18
Hydrocarbon Processing - April 2022 - GP-19
Hydrocarbon Processing - April 2022 - GP-20
Hydrocarbon Processing - April 2022 - GP-21
Hydrocarbon Processing - April 2022 - GP-22
Hydrocarbon Processing - April 2022 - GP-23
Hydrocarbon Processing - April 2022 - GP-24
Hydrocarbon Processing - April 2022 - GP-25
Hydrocarbon Processing - April 2022 - GP-26
Hydrocarbon Processing - April 2022 - GP-27
Hydrocarbon Processing - April 2022 - GP-28
Hydrocarbon Processing - April 2022 - GP-29
Hydrocarbon Processing - April 2022 - GP-30
Hydrocarbon Processing - April 2022 - GP-31
Hydrocarbon Processing - April 2022 - GP-32
Hydrocarbon Processing - April 2022 - GP-33
Hydrocarbon Processing - April 2022 - GP-34
Hydrocarbon Processing - April 2022 - GP-35
Hydrocarbon Processing - April 2022 - GP-36
Hydrocarbon Processing - April 2022 - GP-37
Hydrocarbon Processing - April 2022 - GP-38
Hydrocarbon Processing - April 2022 - GP-39
Hydrocarbon Processing - April 2022 - GP-40
Hydrocarbon Processing - April 2022 - GP-41
Hydrocarbon Processing - April 2022 - GP-42
Hydrocarbon Processing - April 2022 - GP-43
Hydrocarbon Processing - April 2022 - GP-44
https://www.nxtbook.com/gulfenergyinfo/gulfpub/HPI-Market-Data-2023-v3
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2024-v2
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2024
https://www.nxtbook.com/gulfenergyinfo/gulfpub/gas-processes-handbook-2022_v2
https://www.nxtbook.com/gulfenergyinfo/gulfpub/gas-processes-handbook-2022
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hp_202212
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2023-v2
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2023
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hp_202211
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202210
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202209
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202208
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202207_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202207
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202206_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202206
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202205_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202205
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202204_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202204
https://www.nxtbook.com/nxtbooks/gulfpub/hp_032022
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202203
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202202
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202201
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202112
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202111
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202110
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2022_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2022
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202109
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202108
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_processes_handbook_2021_v2
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_processes_handbook_2021
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202107
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202106
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202105
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202104
https://www.nxtbook.com/nxtbooks/gulfpub/hp_ebooks
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202103
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202102
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202101
https://www.nxtbook.com/nxtbooks/gulfpub/catalyst_handbook_2020_v2
https://www.nxtbook.com/nxtbooks/gulfpub/catalyst_handbook_2020
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202012
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202011
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202010
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2021
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202009
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202008
https://www.nxtbook.com/nxtbooks/gulfpub/refining_processes_handbook_2020
https://www.nxtbook.com/nxtbooks/gulfpub/refining_processes_handbook_2020_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202007
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202006
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202005
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202004
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202003
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202002
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202001
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201912
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201911
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2020_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2020
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201910
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201909
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201908
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201907
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201906
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201905
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201904
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201903
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201902
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201901
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_2018_v2
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_2018
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201812
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2019_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2019
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201811
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201810
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201809
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201808
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201807
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201806
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201805
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201804
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201803
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201802
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201801
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201712
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201711
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201710
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201709
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201708
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201707
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201706
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201705
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201704
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201703
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201702
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201701
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2017mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201612
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201611
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201610
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201609
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201608
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201607
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201606
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201605
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201604
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201603
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201602
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201601
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201512
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201511
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201510
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2016mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201509
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201508
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201507
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2015tmsd
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201506
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201505
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201504
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201503
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201502
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201501
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201412
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201411
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2015mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201410
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201409
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201408
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201407
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014petrochemicalprocesses_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014petrochemicalprocesses
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201406
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201405
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201404
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201403
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014calendar
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201402
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201401
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201312
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201311
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201310
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201309
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201308
https://www.nxtbook.com/nxtbooks/gulfpub/hp_srsguide2013
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201307
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201306
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201305
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201304
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201303
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201302
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201301
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201212
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201211
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201210
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201209
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201208
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201207
https://www.nxtbook.com/nxtbooks/gulfpub/hp_srsguide2012
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201206
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201205
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201204
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201203
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201202
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201201
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201112
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201111
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201110
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201109
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201108
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201107_srsguide
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201107
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201106
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201105
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201104
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201103
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201102
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201101
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201012
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201011
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201010
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201009
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201008
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201007
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201006
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201005
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201004
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201003
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201002
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201001
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200912
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200911
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200910
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200909
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200908
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200907
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200906
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200905
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200904
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200903
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200902
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200901
https://www.nxtbookmedia.com