Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)
© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8
Monitoring the 352 meter long Monaco floating pier
M. de Wit & G. Hovhanessian
Advitam, Vélizy, France
ABSTRACT: Again limits have been pushed further with the realization of the key element of the extension
of Condamine port at Monaco, a 352 m long and 163 000 tons semi-floating pier.
The highly pre-stressed reinforcement concrete structure with a design life of 100 years is attached to the
main land abutment with a very complex and 770 tons steel ball-joint system while the other end of the pier it
is secured with two sets of fixed anchor chains to the seabed.
This exceptional project is a mix of building techniques, mechanical engineering, and offshore works: it
includes several world records and, particularly, the spectacular connection of the ball joint system.
All these design breaking records are possible thanks to the evolutions in civil design & construction methods.
In this context another evolution is of great help to allow confirming that the structures are behaving like expected
by the calculation models: the monitoring tools. New technologies for the monitoring of structures are powerful
tools to better understand the behavior and make sure that structure remains in good health over time.
In this paper we will review the structural health monitoring system that is installed for this extraordinary
structure.
1
INTRODUCTION
The first goal of a monitoring system is to provide a
confident, accurate and time related measurement of
specified parameters. The monitoring system installed
on the floating pier has several goals:
•
Asses and follow carefully the behavior of the structure and its key structural equipments as the steel
ball joint
• Establish health rules representatives of structural
behavior in various situations (high loads, bad
weather or sea conditions, earthquake. . .)
• Monitor ageing effects over the years
• Compare real behavior of the structure with
expected behavior coming from the theoretical
models.
technologies sensors. System includes existing information coming from the harbor meteorological system
and information coming from newly installed sensors.
A total of 72 parameters are monitored continuously
every 250 ms.
2
DESCRIPTION OF THE FLOATING PIER
The Floating Pier is made of pre-stressed concrete and
built in Algesiras (Spain).
In addition to its primary function, i.e. protection of
the outer harbour and the harbour itself, the breakwater
will provide berthing space for liners on its off-shore
and harbour sides, 360 car park places on the inside
over 4 different levels and a dry-dock.
Provide real time alerts to security services or traffic
control service in case usage restriction is required
because of particular situation.
The particularities of the floating pier are that it is
subjected to several very different load cases, impacting various part of the structure. Then the parameters to
monitor are widespread and very variable. As a consequence, the monitoring system installed on the floating
pier is diversified in terms of technologies. Very different type of sensors and technologies have been
used and merged into one unique information system:
analogical sensors, GPS, radar, ultrasonic and optical
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Its dimensions are impressive: 160,000 tons displacement, length 352.5 m, width 28 m on surface
and 44 m at the bottom (which is provided with two
8 m stabilising wings for preventing pitch and roll
movements).
It is anchored to the platform area by the abutment
caisson, to which it will be attached by an enormous
complex steel articulation.
The articulation is a metal structure that weighs
about 700 tons. Specially designed to allow rotation
and to resist up to 10 000 tons of horizontal and vertical
loads, it will articulate the connection of the floating breakwater to the abutment caisson. The floating
pier anchoring system is constituted by: eight mooring heavy chains connected at the harbour entrance
extremity five of which (those towards the open sea)
are 500 m long and the other three 100 m long.
3
MONITORING SYSTEM
the geodesic coordinates are precisely known. This
receiver is permanently calculating its position in the
GPS mode. In real time, the system is comparing
the “true” -fixed- coordinates and the “read” coordinates calculated from the satellites and spreaded of
errors (ephemerids, non-isotropic layers, multipaths
of waves. . .). The comparison of the “true” and the
“read” coordinates leads to an error vector. Another
GPS station is installed on the “moving part”, at the
other side of the floating pier where the yaw is maximum. Both GPS receivers are connected to a GPS
server. The fixed receiver is sending in real time the
error vector and the server is using this error vector to
enhance the accuracy of the mobile station. This differential GPS system can be applied only if the mobile
and the fixed GPS receivers are not too far away and if
the error vector is calculated in real time and applied
in the same time the positioning vector of the mobile
station. Thus, an accurate GPS system needs a high
speed communication network.
3.1 The movements of the structure
3.2 Monitoring the stability of the structure
Like a boat, the floating pier has pitch, roll and yaw.
The pitch and roll are continuously monitored with
sensitive tilt meters connected to the data acquisition
system. By installing tilt meters at each side of the
structure, not only the roll and the pitch are monitored
but the torsion and the flexion of the structure too. The
information of the torsion and the flexion can provide very useful information to the owner regarding
the efficiency of the steel ball.
Monitoring the yaw could not be realized with
any pointing measurement system like laser meters
or other pointing systems (total stations, image
analysis. . .): First, because the sightline of the measurement system will be continuously interrupted by
boats and yachts entering or exiting the port, secondly
because of the morning fog decreasing drastically the
accuracy or liability of such measurements, and thirdly
because of the huge movement freedom of the structure: up to 12 m towards the see or towards the port.
And the weather-related quality of measurement was
an unacceptable parameter for the monitoring system,
especially because the most sensitive and interesting
data appears when the weather is bad.
It has been chosen to add a Global Positioning System (GPS) and accelerometers on the structure. The
accelerometers can measure the effect of the movements, when the GPS measures directly the movement.
The combination of these 2 types of measurements
is very interesting to asses some specific behaviors
or effects like the influence of the anchors chain
on the movements. Nevertheless, a classic GPS system cannot offer itself the sufficient accuracy to
give valuables data to asses the movements, therefore a Differential GPS system has been installed :
A GPS receiver is installed on a fixed point where
Each side of the floating pier is constituted by 8 ballasts
tanks that are precisely filled to ensure the structure’s
stability and flatness. The ballast tanks are completely
waterproofed from outside and inside the structure and
between themselves. The humidity in the ballast is
very high and very critical for many common sensors.
Rugged radar sensors have been used to monitor the
high of the ballast and their variation, if any. Depending on the ballasts, some are filled with 1 m of biocide
water and other with 15 m. The main goals of the sensors installed are to follow the variation of these levels
for each ballast.A sudden variation of the ballast would
indicate a failure into the superstructure.
3.3 Monitoring the superstructure’s strain
Usually, boats or floating barge are designed to be
resistant for any phenomena coming in front of the
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structure: swell, waves. . . etc. In the case of this floating pier, most of the swell and waves are coming from
the East, i.e perpendicularly to the structure, causing
a longitudinal strain in the structure. In addition, the
pitch of the floating pier also has some influence in the
strain of the structure, due to its length. Also a specific
attention has been given to the strain in the vicinity of
the submarines edges of the structure, where many turbulences and venturi effects take place. The strain is
measured all along the structure into the floating pier
(for longitudinal strain) and under the floating pier
with long base optical sensors specifically designed to
be water-resistant, completely designed in composite
materials. All of these sensors are connected together
in bus with the optical fiber and connected to the data
acquisition system.
Long-base fiber optic strain sensor
3.4
Monitoring the steel ball
Attaching to floating pier to the abutment and giving 3
degrees of freedom (pitch, roll and yaw) to the structure, the steel-ball is the neuralgic center of the floating
pier. It weights 770 tons, for a diameter of 8 m at its
largest part, and is designed to act like a fuse in case of
seism by separating the structure from the abutment.
The steel ball is attached to the floating pier, where
the socket is attached to the abutment. Both of the parts
are attached with stressing bars. A complete monitoring system has been installed on the anchorage of
the bars to follow their tension. The system has been
installed when the structure was already attached to
the abutment, so no de-tensioning under 10% of the
bars were allowed for the installation, which meant
measuring the tension with load cell was impossible.
Moreover, the narrow space between bars and caps was
another argument to forget the load cells. An ultrasonic
measurement method has been used to answer to these
needs. This ultrasonic measurement method gives the
tension into the bars. Widely used in the nuclear field
for measuring the tension in pre-stressed anchors bolts,
the system has been adapted to the needs of simultaneous measurement with many ultrasonic sensors. The
principle is to send an ultrasonic wave into the bar and
measure precisely the time of receiving the ultrasonic
echo. Thanks to a very specific algorithm, the tension
into the bars can be calculated with a 2% accuracy.
The tension in the stressing bars is a great indicator
of the efficiency of the steel ball : any increase of the
tension shows that the steel ball is transferring a part
of the movements onto the structure instead of rolling.
Ultrasonic sensor
A total of 6 bars M120 are monitored permanently with
the ultrasonic measurement system from the ball side,
and 16 bars M52 are continuously under control from
the abutment side.
The floating pier being designed for 100 years, it is
important to pay a specific attention to the fatigue and
the mechanical wear of the steel ball and its socket.
The mechanical wear is measured by very sensitive
displacements sensors pointing at the surface of the
steel ball. They are installed to point exactly towards
the center of the steel ball, this particular disposition
of the sensors allowing to measure any longitudinal
and radial wears without the need of a compensation
of the roll and yaw effects.
Displacement sensors
3.5 The data acquisition system
Regarding the diversity of sensors used on this monitoring system, and thus the multiplicity of protocols,
the data acquisition system includes 2 layers of data
acquisition units:
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•
The first level is in charge of powering if needed, filtering, formatting, time stamping and publishing the
data. There are called secondary data acquisition
units, or field data acquisition unit.
• The second level, highest in terms of layer, is in
charge to analyze the data, record it if needed, and
communicate with the outside of the measurement
network. It is called the supervisor.
The secondary data acquisition units are connected to
the supervisor with an optical Ethernet network. To
be consistent for the analysis, all the secondary data
acquisition unit are synchronized by the supervisor.
The supervisor is itself synchronized to a time server
of an atomic clock, trough the web. All the data acquisition are done at frequency of 4 Hz, and after time
stamping, published in “Ethernet variables” for the
supervisor. The data acquisition frequency is sufficient
regarding the period of the interesting phenomena taking place on the structure even if the bandwidth of the
optical network allow to increase in large portion the
acquisition frequency.
The supervisor receives the data coming from the
field acquisition unit, and analyze them. Every sensor
is setup with 4 levels of alarms (2 high alarms and 2
low alarms). Whenever a sensor passes one threshold,
all the data are recorded into a file with a buffer of
some minutes before the event happens.
In addition to this alarms recordings, the supervisor
records periodically all the sensors of the system, and
stores it into a file. These files give a “snapshot” of the
structure even when everything is going well.
The system offers the possibility to the owner to
create its own “health rules”. It could be worthwhile
for example to have a value of the global tension in the
bars of the steel ball, and not only of the 20% of those
instrumented. Or we can consider that a roll of 1◦ is
not acceptable with normal weather condition, when
it is a day of big swell or wind. For these consideration, the system allows the owner to create, manage the
“health rules” and to setup himself more sophisticated
alarms.
The last point of the system, which contribute to
the particularity and the complexity of the monitoring system, is the private connection to the Traffic
Control Center and the Firemen station of Monaco:
In some specific cases detected by the monitoring
system, especially when safety of the people are not
ensured anymore into the structure, the system will
send an alert to the firemen station. It is understandable that the complexity of the system is greatly
increased; false alarms would be unacceptable in such
situations.
4
CONCLUSION
The structural health monitoring system designed for
and installed on the floating pier allows the maintenance department of Monaco public works, thanks to
very simple indicators, to remotely and continuously
make sure of the good condition of the structure.
Health rules defined from the seventy-five key
points spread everywhere in the floating pier are
permanently monitored by the data acquisition system.
Thanks to the smart acquisition system, data is organized and relevant data is accessible by people involved
in the maintenance, as for example the consultant in
charge of the analysis of the structural behavior or the
traffic control service.
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