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LIFE EXTENSION OF A FIXED OFFSHORE PLATFORM STRUCTURE BASED ON
MONITORING RESULTS
S. Copello, RINA Services, P. Castelli, Edison.
This paper was presented at the 11th Offshore Mediterranean
Conference and Exhibition in Ravenna, Italy, March 20-22, 2013. It
was selected for presentation by OMC 2013 Programme Committee
following review of information contained in the abstract submitted
by the author(s). The Paper as presented at OMC 2013 has not been
reviewed by the Programme Committee.
ABSTRACT
The structure of the existing offshore platform Vega A, operated
by Edison in the Sicily Channel, has been subject to a reassessment
process in order to extend its operating life beyond the original
design life. Such requalification analysis has been focused on a
fatigue verification of the jacket structure with the target of
life extension, as well as other reassessment issues such as the
actual status of structural components, present topside
configuration, etc., all considerations aimed to eventually update
a proper inspection and maintenance plan, everything considered as
normal practice in the offshore field where the number of existing
platforms subject to reassessment process due to expiration of the
original design life is increasing. What is peculiar in this case
is the availability of a large amount of significant information
recorded during the occurred service life of the platform by the
monitoring system mounted on the structure since early phases of
installation, which has definitively increased the level of
reliability in the new structural assessment. In particular, it has
been possible to re-evaluate the platforms response to
environmental loads (the governing loading for structural safety)
whose characterization has been reviewed and updated according to a
large amount of wave, wind and current data measured on site for a
long term and, what is more, to calibrate the calculated dynamic
response, which is the basis for the fatigue assessment, with
respect to the actual jacket accelerations continuously recorded on
field by relevant monitoring devices. In the following the
different steps of the reassessment process carried out through the
calibrated structural response are described, by highlighting how
the monitoring effort, along with a proper maintenance, has
facilitated the achievement of the goal of life extension.
INTRODUCTION
The Vega field, operated by Edison, is located at approximately
12 miles South from the southern coast of Sicily. It includes a
fixed platform, Vega A, and a floating storage offloading unit
(FSO), located at 1,5 miles from the platform, and connected to the
platform through sealines. The FSO is moored to the seabed through
an arc-yoke articulated system composed of (Figure 1):
- A column connected to the sea bottom and extending above sea
water level; - A yoke connecting the column tip to the FSO bow
tanker beam.
The offshore facility was installed in August 1987 with the FSO
unit being the 250,000 DWT converted tanker Vega Oil. Due to
international double hull requirements, in July 2008 the FSO Vega
Oil was disconnected and replaced in September 2009 by the
converted 110,000 DWT Aframax tanker named Leonis. In relation to
the FSO substitution, a new yoke has been designed and installed so
that both the FSO and the SPM have been subject to renewal of
classification by RINA, to comply with the operator requirement for
a field life extension.
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In the same field, the structural design of the Vega A platform
(Figure 2) was originally certified by RINA for an operating life
of 25 years, thus an assessment of the fixed platform has been also
required by the operator in order to extend the jacket life beyond
its original design life, according to a prescribed target of
further 25 years of service.
Fig 1: Vega field arc-yoke mooring system Leonis FSO
Given that target, RINA has carried out, as normal practice
/Ref. 1/, the structural analyses of the jacket with the main
purpose of conducting a fatigue assessment of the tubular
connections and consequently to define the relevant inspection and
maintenance plan to comply with for the extended service. To this
aim, an updated structural model of the Vega A jacket has been
built by integrating original design data with the information
relevant to the present status of the platform (such as topside
weight distribution) and data collected during the past service
life via inspection campaigns carried out and, what is more, via a
monitoring system which has been installed on the structure since
the early phases of its operating life.
Fig 2: Vega A Platform
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DESCRIPTION OF THE PLATFORM AND MODEL UPDATING
The submerged structure of the Vega A platform is a 8-legs steel
jacket structure in a water depth of about 138m, framed by seven
plans, each at elevation -120.8, -96.0, -75, -54.0, -33, -13, +7m
with respect to the mean water level, starting from a rectangular
base measuring 70m x48m. up to the top side of 50mx18 m. The pile
legs are tubular members with variable section from 2000mm outside
diameter (OD) x 50mm wall thickness (WT) from the bottom to 1700mm
OD x 50mm WT at the upper levels, provided with horizontal bracings
at each plan and rows tubular brace members between the plans
themselves. The jacket is fixed to the ground by 20 foundation
piles (3 for each of the 4 leg corners and 2 for the internal
legs), whose outside diameter is 2590 mm, penetrating 63.5m below
the sea bottom, while the connection between piles and legs is made
by sleeves filled by concrete grouting in the pile-sleeve section
interface. The jacket of the platform is completed by 2 boat
landings and other appurtenances such as bumpers, casings and
risers that are not structural members but are to be considered in
the model because their contribute to the hydrodynamic loading, as
well as the launch runners used during the platform installation
phase (carried out by launching) and still present along some
jacket legs. For the appropriate evaluation of the structural dead
loads (weight and buoyancy) it is worth noting that along the 2
lowest jacket plans sections the legs are still flooded, due to
launching design procedure. Finally, for the appropriate model of
the jacket hydrodynamic response, the specific presence of the
sacrificial anodes on the structural members has been taken into
account.
By considering all available up-to-date information relevant to
the present conditions of the platform, an updated structural space
frame 3D model of the Vega A jacket has been carried out by RINA
licensed software NSO (Figure 3). In particular the jacket geometry
and material from as built drawings /Ref. 2/ and structural steel
specification /Ref. 3/ has been validated by fabrication data book.
As far as the foundations model is concerned, the pile-soil
interaction has been modelled by appropriate characterization of
the soil layers along the pile depth as well as pile tip bearing
capacity, all deduced by original soil geotechnical report /Ref. 4,
5/ and validated by installation driveability analysis /Ref.
6/.
.
Fig 3: Vega A Platform Structural Model
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In order to complete the updated model of the platform so to
reflect the present conditions as much as possible, both the
generated structural weight of the jacket (about 17820 tons) and
the topside modules masses and positions have been validated by
their values available from project final weight control report
/Ref. 7/ in addition to information obtained from topside survey
and operators record, from which, e.g., it is reported that the
drilling rig equipment has been present on the platform up to the
end of the year 2002. Relevant mass has been considered in the
dynamic and fatigue analyses carried out accordingly. In order to
complete the updated model of the jacket, very important
information have been obtained from inspection reports available
/Ref. from 8 to 12/ for all the surveys carried out for the
submerged structure during the service life of the platform, in
terms of general and close visual inspection outcomes,
particularly:
- Wall thickness measurements; - Cathodic protection
measurements; - Marine growth measures and cleaning policy; -
Non-destructive examination of the welded joints.
THE MONITORING SYSTEM
Since the early phases of its service life, on the platform it
was installed a monitoring system, which, even if subject to
modifications and upgrading through the years, has continuously
provided the following data from 1988:
- Directional wave motion; - Current velocity and direction; -
Sea level variation: - Wind velocity and direction; - Air
temperature.
The present system, installed on Vega-A by the company DEAM srl
at the end of 2001, is made by: - Wave meter, based on the
measurement of water column pressure; - Sensors for measurement of
wind velocity (anemometer) and direction; - Current meter, to
record the 2-components current velocity; - Sensors for acquisition
of meteorological data. Inside these categories, there are
different timeframe for measurements, i.e. the meteorological
parameters are detected any ten minutes inside each hour, while
waves, sea level and current are detected along 17 minutes each
hour. All data collected on the platform, whether following a first
elaboration on the platform (reduced data), or by rough data, are
sent to the Department of Civil Engineering (DICeA) of the
University of Florence, which has the role of validating the
samples and following statistical interpretation of the data
relevant to the environmental parameters /Ref. 13/. As regards to
wave characterization, that post processing phase provides:
- Significant wave height; - Maximum wave height; - Significant
wave period; - Associated period to maximum wave height; -
Zero-crossing period; - Peak period; - Wave incoming direction.
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Similar process is undertaken for the structural behavior, which
has been monitored since the commissioning of the platform by
installation of strain gauges and accelerometers on some structural
bracings. At present, 6 accelerometers are employed for
accelerations measurements (6 linear and 3 angular). In total, 17
sensors are present on the platform, in the positions indicated in
Figure 4, with the characteristics shown in the table reported as
Figure 5, where it is reported the duration, the frequency and the
number of samples for any set of data collection.
Fig 4: Sensors on Vega A Platform
Fig 5: Sensors properties
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UPDATING OF ENVIRONMENTAL DATA AND STATIC ANALYSIS
After completion of the updated structural model, and
propaedeutical to the following dynamic and fatigue analysis to be
performed for platforms life extension, a static analysis has been
carried out in relation to the two typical design conditions that
are to be verified according to the international standard API code
used for fixed offshore structures /Ref. 14/:
- Operating condition (i.e. with wave, current and wind loading
characterized by 1-yr return period);
- Extreme condition (i.e. with wave, current and wind loading
characterized by 100-yrs return period);
This latter condition has been particularly analyzed to check
the accuracy of the model in comparison with the original design
results /Ref. 15/, in terms of global loads resultant at the
structures base or pile loads, by taking into account of course the
occurred and modeled variation on environmental actions and weight
distribution. Indeed a first important benefit for the structure
obtained from the monitoring results has been the reduction of the
environmental loads acting on the platform: for instance in the
following Table 1 the values of wave heights (for both operating
and extreme conditions) adopted at design stage as a result of the
former meteomarine study /Ref. 16/ are reported in comparison with
the new values obtained as main outcome of the elaboration of more
than 20 years of recorded data on the platform /Ref. 17/. Also, new
data are more refined with respect to the incoming direction
(original design referred to 4 sectors only); from the comparison
it can be seen how new data are more homogeneous and with a
significant reduction in extreme values, which eventually results
in a favorable contribution for the purpose of extending the
platforms life. Furthermore, from the specific analysis of wave
heights raw data /Ref. 18/, it has been possible to get an
appropriate calibration of the theory that best represent the
actual values of recorded wave velocities and acceleration,
specifically the adoption of a 3rd order wave theory better
describe the wave parameters distribution than the Stokes 5th order
theory used in the original design and, in any case, the
theoretical prediction overestimated the actual recorded values.
That conservative assumption does result in a margin of safety
actually present in the designed structural members, safety gained
for the present reassessment analysis. Similar comparison and
observations can be obtained from the analysis of Table 2 as
regards to prediction of wind velocity.
Tab. 1: Maximum values of wave heights comparison
Operating Conditions Extreme Conditions Direction () Hmax design
(m) Hmax 2012 (m) Hmax design (m) Hmax 2012 (m)
0 3.7 6.5 5.5 6.5 30 7.2 7.5 60 7.0 8.4 90 10.3 7.3 15.5 13.0
120 7.9 15.2 150 6.1 11.3 180 10.3 7.1 15.4 12.8 210 7.3 12.3 240
7.3 12.1 270 11.4 8.8 17.1 15.0 300 9.6 15.8 330 9.7 15.7
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Tab. 2: Maximum values of wind velocities comparison
Operating Conditions Extreme Conditions Direction () v
design (m/s) v 2012 (m/s) v design (m/s) v 2012 (m/s) 120 27.3
21.1 43.4 31.0 150 27.3 21.7 43.4 36.2 210 27.3 26.5 38.0 39.5 240
27.3 30.7 48.7 41.5 300 27.3 25.5 48.7 34.2
The static analysis carried out for both environmental and
operating conditions showed non critical elements with respect to
the structural members and joints checks performed according to API
requirements. These results are shown in graphical form (e.g. in
Figure 6 it is reported the output for member checks in extreme
conditions) in terms of unity check (UC) of the structural
components, which can be represented as the ratio between the loads
demand and the resistance capacity of each component (by also
accounting for appropriate safety factors according to the rules
/Ref. 14/), therefore resulting in a safe state if UC < 1.
Fig 6: Outcome of structural members check in extreme
condition
In particular, the following maximum values of the UC factor
resulted for the platforms structural components in the two
analyzed conditions:
- UC = 0.942 for the members in operating condition (for a
compression + bending + hydrostatic collapse limit state);
- UC = 0.708 for the members in extreme condition (for a
compression + bending + hydrostatic collapse limit state);
- UC = 0.303 for the joints in operating condition (for punching
shear check); - UC = 0.489 for the joints in extreme condition (for
punching shear check); - UC = 0.339 for the pile bearing capacity
under compression + bending.
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DYNAMIC ANALYSIS AND VALIDATION ACCORDING TO MONITORING
RESULTS
The dynamic analysis of the platform in the present conditions
has been performed to evaluate the natural periods and modal shapes
of the structures. In particular the calculated first 3 frequencies
have been compared to the values of the same dynamic parameters
obtained as main outcome from the monitoring of the structure and,
consequently, a proper calibration of the model is reached. A
prediction model of the structural dynamics as close as possible to
the real response is of paramount importance for the following
fatigue spectral analysis, which is driven by the vibration
analysis results and is the definitive assessment to be performed
for the life extension of the jacket. The jacket and the topside
models adopted for the dynamic analysis have been the same ones
used for the static analysis, apart from the pile section below the
mud-line which cannot be treated for the dynamic response analysis
with the full non-linear foundation model used for the static
in-place analysis. The foundation pile behaviour must be linearized
by evaluating the characteristics of a special pile-stub element.
This element simulates the pilesoil interaction during the dynamic
analysis, taking into account for the characteristics of axial load
transfer along the pile shaft and the pile response to lateral
loads. The dynamic mass model carried out includes:
- Jacket masses accounting for spatial orientation and
distribution; - Topside masses accounting for spatial orientation
and distribution (structures,
equipment, pedestal cranes and live loads); - Appurtenances
(such as risers and boat landings); - Added and entrapped masses
below the sea level; - Marine growth weight; - Mass of the
piles.
The dynamic analysis has been carried out through eigenvalues
(natural periods) and eigenvectors extraction, for a sufficient
number of modes of vibration, so that a minimum 90% mass
participation is achieved. As mentioned above, the results obtained
from the dynamic analysis carried out on the updated structural
model have been eventually validated in relation to data from
monitoring, which, in turn, reflects the actual behavior of the
structure. From the final analysis of the monitoring data (carried
out on yearly basis), the structural response derived by the
records of the deck accelerations is very useful for the purpose of
definition of the global dynamic behavior. In particular the
outputs of the 9 accelerometers (both linear and angular) presently
installed on the structure have been processed to get average
spectral response on a 3-months basis. The comparison between
elaborated average spectra and raw data collected during the past
platforms life shows a substantial stability in the dynamic
response of the structure, which means that no significant
variation in both mass and stiffness distribution have occurred
along the lifetime. From the examination of the elaborated spectra
/Ref. 13/ it can be drawn that prevailing harmonics are reported
with a frequency of about 0.45Hz in x-direction and 0.50Hz in
y-direction. This latter is slightly higher than the value of
0.48Hz reported in 2001, but equal to the value reported from 2002
to date. By the way, such peak values of the spectral response are
quite higher than the values estimated at design stage in the
platforms seismic analysis (i.e. fx = 0.300Hz and fy = 0.316Hz,
/Ref. 19/), probably due to underestimation of the foundation
stiffness in the design soil-structure prediction model. Given the
actual natural periods obtained from the monitoring, the present
dynamic analysis has been calibrated accordingly.
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In order to reach the same vibrations response, the sensitivity
of the natural periods to the following parameters, which were
uncertain or affected to some extent by model assumptions, has been
investigated:
- Marine growth thickness; - Contribution of the conductors to
global structural stiffness; - Foundation stiffness,
with the final determination of the structural frequencies as
reported in the following Table 3:
Tab. 3: Evaluated structural frequencies
Shape of vibration Frequency from monitoring (Hz)
Calculated frequency (Hz)
1st mode 0,45 0,42 2nd mode 0,50 0,51 3rd mode 0,78 0,78
The corresponding 3 modes are reported in the following Figure
7.
Fig 7: First 3 modes of vibration of Vega-A jacket.
FATIGUE ANALYSIS AND CHECKS
A stochastic spectral fatigue analysis has been performed to
evaluate the fatigue damage, at the welded tubular connections of
the jacket through the following calculation steps:
- Stress range transfer function; - Environmental load spectrum;
- Stress response spectrum; - Fatigue damage evaluation.
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The wave response analysis has been used to determine the system
transfer function. This approach assumes that an infinite train of
repeatable wave form are stepped through the structure and the
response is established. In order to accurately define all the
peaks and valleys inherent the sub-structure response transfer
function, a sufficient number of frequencies and the corresponding
wave heights are to be selected. The selection of such frequencies
is based on the dynamics of the structure, thus the more realistic
is the dynamic response, the more reliable is the fatigue
assessment. The wave data for the fatigue check is provided on a
statistical basis, where the normal parameters are the significant
wave height and the zero up-crossing period, as detailed in /Ref.
17/. Then, Jonswap energy spectrum with the peak enhancement factor
appropriate for the site has been used. Each connection of the
jacket, that is each welded tubular joint, has been checked at 8
points around the circumference of the joint. The stress
distribution all around the tubular joint connections has been
defined considering hot spot stresses calculated on the basis of
parametric formulation of the stress concentration factors
available in literature for tubular joints, whereas the S-N curve
for the evaluation of the fatigue life, applicable for tubular
connections as well, is available from API rules /Ref. 14/. The
evaluation of fatigue damage, and the corresponding calculation of
the fatigue life of each tubular joint of the jacket, has been
performed by comparing the summation of damages relevant to the
various stress range sets, following the Miner-Palmgren model, with
the allowable S-N curve. The results of the fatigue assessment, in
terms of fatigue life for each connection, are reported in the
following Table 4, for the connections with the lowest fatigue
lives (lower than 200 years): all the jacket joints satisfy the
requirement of fatigue life greater than 50 years (25 years of
target extension life multiplied by the safety factor 2, adopted
for joint connections in and below the splash zone, according to
/Ref. 14/).
Tab. 4: Fatigue analysis results
Node Chord Brace Life(years) Side 448 1391-1392 2235 87 Chord
454 1369-1370 2224 113 Chord 448 1391-1392 2232 113 Chord 201
937-938 2199 116 Chord 454 1369-1370 2223 118 Chord 201 937-938
2200 120 Chord 673 1806-1807 1805 121 Brace 647 1809-1810 1805 123
Chord 463 1387-1388 1363 129 Chord 463 1387-1388 1345 131 Chord 464
1389-1390 1354 149 Chord 682 1794-1795 1994 153 Chord 681 1796-1797
1997 168 Chord 682 1794-1795 1993 170 Chord 464 1389-1390 1362 171
Chord 184 559-560 2171 172 Chord 184 559-560 2170 177 Chord 666
1811-1812 3936 193 Chord
As shown in Table 4, the most critical joint in terms of fatigue
life (87 years) is located at elevation -33m below the sea
level.
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UPDATING OF IMR AND RELIABILITY BASED INSPECTION PLAN
Notwithstanding the engineering assessment results, the life
extension of the jacket shall be subject to the provision of an
appropriate inspection, maintenance and repair (IMR) plan, which,
in turn, can be updated by using the newly performed fatigue
analysis, as well as the outcomes of the inspections actually
carried out. Indeed, during the jacket past service life, besides
the monitoring, the platform has been subject to regular
maintenance and inspection activities, as usual for offshore
platforms. The same joint (at node 448) which has been found as
most critical in the present assessment has been subject to
non-destructive examination by MPI (magnetic particle inspection)
in the 1993 with no cracks detected. In general, 23 out of 110 main
structural connections have been inspected by NDE in the offshore
campaigns carried out from 1989 to 2011. A specific IMR plan is
considered for the appropriate maintenance of the jacket, reporting
the schedule of close visual inspections of welded joints as well
as other controls such as wall thickness measurements, cathodic
protection measurements and marine growth cleaning. During the past
years most of the planned MPI have been replaced by surveys
performed by flooded members detection (FMD) method, particularly
for the deeper jacket structural plans (i.e. plans at el. -54m and
-75m) where the NDE inspection is more difficult and expensive. All
the inspected joints did not reveal defects inside the welds, apart
from a micro crack detected during the 1989 inspection on a
connection of the plan at -13m depth. In 1990 the same weld was
subject to further MPI without showing any defect. In relation to
the extension life issue, a new IMR plan has been developed to be
applied to the jacket structure starting from the year 2013. The
new updated IMR plan is based on the above discussed jacket
analysis, particularly the fatigue analysis results have been
considered to select critical and representative joints to be
inspected according to the new schedule, which is covering the
period of extended life. Moreover the actual inspections outcomes
can be considered and fully combined with the fatigue evaluation at
each jacket joint by using a reliability approach, which allows the
planning of future inspections on a rational basis. In particular,
by using the reliability approach, the probability of failure or,
correspondingly, the safety margin, with respect to the fatigue
limit state can be expressed by an index, properly called safety
index , whose evaluation can be determined in closed analytical
form by using, e.g., the lognormal format for the different
statistical parameters which contributes to the fatigue life
evaluation for a given tubular joint of an offshore platform /Ref.
20/. Such reliability evaluation allows to determine the safety
margin with respect to the fatigue failure as a function of time;
thus, it is possible to represent the trend of this safety margin,
which, of course, is decreasing with increasing age of the
structure (green line in Figure 8). The same margin can be updated
by using the inspection events /Ref. 20/, which, in any case,
represents a factor of knowledge on the structure, reducing
uncertainty and, therefore, increasing the safety margin,
particularly in case that no cracks are detected. For instance, the
safety margin evaluated for the above reported critical joint 448
is reported as basic case (green line) by supposing that it will
reach the safety target (i.e. the minimum allowable safety, red
straight line in Figure 8) at the time of its planned next NDE
inspection, that is in the year 2015. However, by considering that
the same joint was inspected in the 1993 with positive outcome, the
safety margin is consequently increased (yellow line) in such a way
that it could be actually not inspected till the year 2027;
moreover, by actually inspecting the joint with positive result in
planned 2015, relevant index will be further updated (pink line) by
maintaining the safety margin far greater than the minimum required
one: it is highlighted how the reliability evaluation provides
further confidence on the interpretation of the engineering
assessment results for planning future inspection scheduling.
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Fig 8: Fatigue reliability index evaluation
CONCLUSIONS
The common process of reassessment of an existing offshore
platform, in order to extend its operating life, which typically
implies a structural reanalysis of the jacket with particular
attention paid to the fatigue issue, is significantly improved by
availability of monitoring data, gathered by a measurement system
installed on the platform since the early phases of its operating
life. The large amount of significant information recorded has
eventually increased the level of reliability in the new structural
assessment, by allowing:
- The revision of statistics of wave, wind and current data with
relevant reduction of the characteristic meteomarine loadings
acting on the platform;
- The calibration of the estimated structural dynamic response,
which is the basis for the fatigue assessment of the jacket welded
joints, with respect to the actual jacket accelerations
continuously recorded on field by mounted accelerometers.
No critical situations have been highlighted by updated
structural analyses carried out; in particular the performed
fatigue assessment has shown that the jacket original design life
can be extended up to the requested target of 25 years, provided
that the operator will continue with the regular inspection and
maintenance measures carried out during the whole platforms service
life. To this aim a rational updating of the IMR plan has been
prepared on the basis of both new engineering analyses and
inspections results.
ACKNOWLEDGEMENTS
The authors would like to thank all the Edison and RINA
colleagues, based in Siracusa and Genoa respectively, as well as
the personnel of DEAM and professors and researchers at DICeA of
University of Florence, without whose effort and contribution the
job described in the paper wouldnt have been possible and, above
all, it wouldnt have been possible to reach the goal of extending
the operating life of the Vega A platform.
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REFERENCES
/1/ RINA, Guidelines for Requalification of Existing Offshore
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/20/ R. Facciolli, C. Ferretti, R. Piva, S, Copello, System
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