1. technology and computing

ACMC/SAMPE Conference on Marine Composites
Plymouth, 11-12 September 2003 (ISBN 1-870918-02-9)
APPLICATION OF OPTICAL FIBRE SENSORS FOR MARINE
STRUCTURAL MONITORING
Crispin Doyle and Chris Staveley
Smart Fibres Ltd, C3, Centennial Court, Easthampstead Road,
Bracknell RG12 1YQ, United Kingdom
Email: [email protected]
web: www.smartfibres.com
ABSTRACT
This paper addresses issues relating to structural monitoring in composite-hulled craft
using optical fibre sensors.
Fibre optic sensors are attractive for a wide range of structural load monitoring
applications. Due to their small size, they can be readily embedded into composite
materials. Other advantages include their multiplexing capability and immunity to
electro-magnetic interference. Results from an installation of fibre-optic sensors in a
38 m free-standing composite yacht mast are presented, demonstrating the potential of
this technology in a maritime environment.
INTRODUCTION
As in many other fields of engineering, the advantageous properties of Advanced
Fibre-Reinforced Composites (AFRC) have encouraged their use in structural marine
applications. However, AFRC structures have traditionally been more costly than
steel or aluminium and this has tended to restrict their use to smaller vessels, leisure
and competition craft. Much of this cost has been from relatively labour-intensive
production processes which meant that larger vessels were for special applications
only, such as minesweepers, which are some of the largest ship hulls made entirely
from glass-fibre reinforced plastic (GFRP). Newer production processes, such as
lower-cost variants of resin infusion employing some flexible tooling, enable large,
high-quality AFRP components to be built at much reduced cost. Still, the use of
composites for the structures of bigger vessels is relatively new. In-service
monitoring may be the key to gaining confidence and overcoming the relative lack of
experience of long-term performance. The availability of accurate, real-time strain
and load information from composite ships’ structures under real loading conditions
can help naval architects and engineers to refine their designs to make more efficient
use of the particular mechanical properties offered by fibre-reinforced composites.
Moreover, the availability of strain measurement technology that is cheap and reliable
enough to ‘fit and forget’, rather than expensive equipment suitable only for shortterm testing, offers the possibility of continuous Structural Health Monitoring (SHM)
of a vessel. This could potentially lead to improved safety and reduced operating
costs by providing advance warning of structural deficiencies and allowing conditionbased maintenance to be implemented.
One such system available currently is HULLMOS (HULL MOnitoring System). It is
an integrated strain and motion (acceleration) measurement system offered for SHM
on large ships, bulk carriers for example [1]. The equipment consists of one or more
ACMC/SAMPE Conference on Marine Composites
Plymouth, 11-12 September 2003 (ISBN 1-870918-02-9)
sensitive, low-frequency accelerometers to record the motion of the ship and powered
long-gauge (100 mm or 1700 mm) strain sensors. The transducers are large and
unwieldy, but appropriate to the scale of the monitoring application. Sensors are
bolted on to attachment plates welded to the hull or deck and connected with shielded
cables to a data acquisition and processing unit on the bridge. Drawbacks are that a
significant part of the cost of this system is in the installation and the long runs of
power and signal cable that are needed. Furthermore, the attachment method does not
lend itself to composite structures.
An alternative to conventional electrical sensors is provided by devices based on
optical fibres. More than twenty years of research has demonstrated that optical fibre
sensors (OFS) have certain properties which give them the potential to be smaller,
lighter, more robust and more sensitive than other types. The particular features that
suggest their application for strain monitoring in composite-hulled ships are:
a) small size, small enough to be embedded in the structure during manufacture
without unduly affecting its mechanical properties [2], [3].
b) operation unaffected by water,
c) resistance to corrosion,
d) resistance to electromagnetic interference (EMI), enabling long cable runs
without the need for signal conditioning or heavy shielded analogue cables and
e) ability to be multiplexed i.e., to operate many sensors, typically up to around 100
but in some cases many more, on a single optical fibre.
Optical fibre sensors and systems have tended to be more expensive than existing
strain gauges, resistance strain gauges (RSG), for instance. However, the relative cost
of OFS per sensor is reduced as the sensor count goes up (by making use of the
multiplexing advantage) and if long leads are needed. Cross-over of technology and
expertise from the telecommunications industry is an important support for the
development of OFS and will enable costs to come down further in the future.
Optical Sensors
Many different types of OFS exist, working on many different principles: intensity
modulation, interferometry, polarization effects, refractive index changes,
reflectometry and so on. One relatively mature type which appears to be particularly
attractive for use in SHM of composites is the Fibre Bragg Grating (FBG). An FBG
is a periodic modulation of the refractive index in the core of an optical fibre which
forms a wavelength-selective mirror having maximum reflectivity at the Bragg
wavelength , given by:
=2neff










where neff is the effective refractive index of the mode propagating in the fibre and 
is the FBG period. Equation (1) implies that the reflected wavelength  is affected
by any variation in the physical or mechanical properties of the grating region. For
example, strain on the fibre alters and neff, via the stress-optic effect. Similarly,
changes in temperature lead to changes in neff via the thermo-optic effect and in an
unconstrained fibre,  is influenced by thermal expansion or contraction. This
situation is expressed in Equation 2, where the first term on the RHS gives the effect
of strain on  and the second describes the effect of temperature.
ACMC/SAMPE Conference on Marine Composites
Plymouth, 11-12 September 2003 (ISBN 1-870918-02-9)
(1-) + (+ξ)T
(2)
where is the change in Bragg wavelength, and  are respectively the
photoelastic, thermal expansion and thermo-optic coefficients of the fibre,  is the
change of strain and T is the temperature change. For a typical grating written in a
silica fibre and with B ≈ 1550 nm, sensitivities to strain and temperature are
approximately 1.2 pm/ and 10 pm/°C respectively.
To use an FBG as a sensor, it is illuminated by a light source with a broad spectrum
and the reflected wavelength is measured and related to the local measurands of
interest. Shifts in the Bragg wavelength can be monitored by using an interferometer,
optical filters or a narrowband swept light source and because information about the
measurands is encoded in the wavelength of the reflected light, FBG sensors are
immune to drifts and have no down-lead sensitivity. Multiple gratings in a single
fibre can be addressed by Wavelength-Division Multiplexing (WDM), in which each
FBG has a different wavelength, or Time-Division Multiplexing (TDM), in which a
pulsed light source is used and sensors are identified by the time of arrival of their
reflections. Spatial multiplexing, i.e., separating FBGs on different fibres is another
way to increase the number of sensors that a single unit can address.
Marine Applications of Optical Sensors
The greatest interest in fitting OFS to ships has been shown by the military,
particularly by the US Navy, who hope to use SHM to improve maintenance
programs and increase the operational availability and survivability of their ships.
Resistance strain gauges have been fitted to some ships to prove the validity of
computer models of damage accumulation, but current work is on systems based on
OFS. Immunity to EMI and drift, multiplexability and compatibility with shipboard
fibre-optic data networks are seen as important advantages of OFS.
A WDM system based on a dispersive spectrometer has been trialled on a propeller
from an LPD17 amphibious assault ship [4]. Four arrays of 6 gratings were affixed to
the structure and strains were recorded during a series of static and low-frequency
dynamic loading tests.
These authors also used an improved FBG system during rough sea trials on the
experimental British trimaran RV Triton [5]. Fifty-one sensors multiplexed onto four
fibres were installed, with co-located resistance strain gauges (RSG). The intrinsic
cross-sensitivity of FBG to strain and temperature was overcome by mounting them in
pairs in a so-called ‘Flat Pack’ configuration, in which two FBGs were carried
between polymer films, one intimately bonded for strain transfer and the other, loose
in a cavity formed in a piece of photo-etched brass, responding to temperature only.
This enabled a temperature-compensated FBG strain gauge to be handled and fitted in
much the same way as a conventional RSG. The packaged FBG were installed in the
same time as RSG, but were much quicker to set up and troubleshoot and needed
fewer, much less bulky cables. The authors emphasised the importance of planning
and preparing sensor arrays in the laboratory before performing such a complex
installation.
ACMC/SAMPE Conference on Marine Composites
Plymouth, 11-12 September 2003 (ISBN 1-870918-02-9)
The US and Norwegian Navies have carried out joint research on OFS for composite
ships under the Composite Hull Embedded Sensor System (CHESS) programme.
Results from the first sea trial carried out during this work are presented in Kersey et
al. [6]. Three arrays, each of four FBG, were used to measure transient hull strains in
a prototype Surface Effect Ship (SES), a catamaran minehunter made from GFRP.
Sensor arrays were mounted on the inside skin of the GRP/foam sandwich hull skin.
A combination of WDM and interferometry was used to address the gratings, giving
strain resolution of better than 1  at a 2.5 kHz sampling rate. One array was
addressed at a time by manually selecting the appropriate connectorised cable. Data
were recorded under different sea state conditions, from SS0 to SS6. At low sea states
(SS0 to 2) RMS strains of 5 to 30  were measured, but transients of up to 1000 
were noted at SS5/6. The authors noted that these transients had rise times of 0.01 s
or more, indicating that a sampling rate of 200 Hz was adequate for monitoring events
such as wave slamming.
The same system was employed during simulated wave slamming experiments [7].
Sixteen FBG were multiplexed on three fibres and used to make dynamic
measurements at 1 or 2 kHz on a GFRP panel impacting flat on water surface. The
wavelength-division scheme limited the strain range per sensor to ±2300  for a 4grating array and between ±670 and ±1200  for an 8-grating array. This turned out
to be insufficient for some of the tests. Nevertheless, the authors concluded that FBG
provided a definite weight and convenience advantage over conventional electrical
strain gauges, due solely to the multiplexing of sensors and the light weight of optical
fibre vs. analogue copper wire. Further, it was thought that valuable experience had
been gained which could be put to good use in implementing a full-scale installation
on an operational vessel.
Improved hardware was installed in another GFRP-hulled fast SES, the patrol boat
KNM Skjold, and strains measured under operational conditions were compared to an
FE model of the vessel and design rules prescribed by Det Norske Veritas (DNV) [8].
The strain data showed the DNV recommendations to be sometimes conservative and
sometimes to underestimate loads, a useful result which demonstrates the value of
SHM and could aid in the understanding of the behaviour of SES, which can be
difficult to reproduce numerically or in scale model trials. In a parallel paper, the
authors concentrate more on details of the sensor interrogation scheme [9]. They
describe a method for increasing the effective strain range per channel by taking
advantage of a priori knowledge derived from extensive FEA about the relative
amplitudes and phases of strain at different locations.
In all of the works discussed above, the same advantages are quoted for OFS over
RSG, namely: EMI immunity resulting in higher SNR, corrosion resistance,
multiplexing to allow higher sensor counts with greatly reduced cable requirements
and suitability for fire rating/explosive environments.
ACMC/SAMPE Conference on Marine Composites
Plymouth, 11-12 September 2003 (ISBN 1-870918-02-9)
SMART FIBRES’ EXPERIENCE: SMART MAST INSTALLATION AND
DEMONSTRATION
The data presented here are from sea trials that were carried out on the 27 m yacht
Jacquelina, which has a free-standing 38 m CFRP AeroRig® mast and boom fitted
with FBG arrays.
Sensor Locations
Figure 1. Schematic diagram of a typical optical fibre sensor layout on an AeroRig®. Fibre positions
within the cross-section of mast and boom are also shown.
Figure 1 shows sensor locations for the Jacquelina. Twelve optical fibres were
embedded in the mast, eight carrying arrays of four FBG to measure strains of ± 3500
 and four with single FBGs to measure strains of up to ± 15000  A further eight
arrays of low-range FBG were embedded in the boom. All the optical fibres were
placed between two unidirectional plies parallel to the fibre direction to minimise
perturbation of the host material. They exited through protective plastic tubing at the
base of the mast, were spliced onto Kevlar-jacketed fibres and terminated with
standard telecommunications connectors. Embedding the FBG in the mast rather than
bonding them on the surface serves to protect them from damage and avoids the
ACMC/SAMPE Conference on Marine Composites
Plymouth, 11-12 September 2003 (ISBN 1-870918-02-9)
problem of loss of strain sensitivity caused by moisture-induced debonding. It is also
possible to use embedded sensors to monitor temperature changes and residual strain
build-up during cure.
The maximum bending loads were expected to be at the base of the mast and they
were calculated from estimated wind loadings on the sails and the mast itself. The
combination of point and distributed loads can be estimated from wind tunnel testing,
but this can never exactly reproduce real sailing conditions, hence certain assumptions
have to be made about the loads on the mast. Therefore, similar to reference 8, data
from strain sensors were to be used to provide real feedback to test the validity of
design assumptions.
Optoelectronic Hardware
The Smart Fibres Optical Fibre Strain Sensor Systems (OFSSS) combine wavelengthdivision and spatial multiplexing. A schematic diagram of a representative unit is
given in Figure 2 below. Each of 4 channels is addressed simultaneously by a single
swept source, consisting of a broadband superluminescent diode and a scanning
Fabry-Pérot filter and light reflected from the FBGs is picked up by photodetectors.
The scan generator signal determines the position of the scanning filter and hence the
wavelength of light that is output at a particular time, therefore by relating the timing
of the reflected light pulses to the scan generator signal, the Bragg wavelengths of all
of the sensors can be derived during a single scan. Depending on the equipment used,
a full wavelength sweep takes between 2 and 0.004 seconds and a single channel may
have up to 128 FBG. Converting Bragg wavelengths to strain, data presentation and
storage are performed by an embedded PC. The equipment used in the trials
presented here had a resolution of 10 , but in newer units this is improved to 1 .
e
h
1
b
a
d
t1
2
3
4
t2
t3
t4
I
1



time
g
c
f
1  

Figure 2. Schematic and operating principle of WDM equipment. Key: a) light source, b) scanning
filter, c) scan generator, d) coupler network for channels 1-4, e) FBG arrays, f) photodetectors, g)
processor and h), time-varying output of the detector on channel 4, showing times ti converted into
Bragg wavelengths i.
Three data sampling and display options were available, a real time strain overview
with, averaged strain data overlaid on a representation of the mast on a lap top
computer, continuous long-term monitoring with peak strain data recorded at 5 Hz for
later analysis and a ‘Black Box’ option of 5 minutes’ worth of storage at 100 Hz.
ACMC/SAMPE Conference on Marine Composites
Plymouth, 11-12 September 2003 (ISBN 1-870918-02-9)
Sea Trials of the Smart Strain Sensor System
Sea trials of the Jacquelina ‘Smart Mast’ installation occurred in wind strength force
4 to 6 with a moderate to rough sea state. The sea trial ran through various sailing
procedures including tacking and reefing.
1200
1000
Strain (microstrain)
800
600
400
200
0
-200
-400
-600
-800
-1000
24:30
24:45
25:00
25:15
25:30
25:45
26:00
Time (MM:SS)
Figure 3. Comparison between video footage and optical strain sensor record during a tacking
manoeuvre.
Optical strain sensor readings in the mast during tacking are presented in Figure 3.
Peak strains of around 1000  were recorded from which operating levels of stress
can be determined. The results show good correlation between pairs of sensors
located opposite each other on port and starboard sides of the mast. As the tacking
manoeuvre is completed, the plot becomes inverted as load is applied from the reverse
side.
The still images from the video footage at the top of the Figure follow the progress of
the manoeuvre. From left to right, the images show respectively the boom on the
starboard side, then aligned centrally along the boat and finally to the port side. The
video was shot looking forward in the direction of travel from a position just behind
the helm. The relative position of the boom can be judged using the fixed canopy
framework as a reference.
ACMC/SAMPE Conference on Marine Composites
Plymouth, 11-12 September 2003 (ISBN 1-870918-02-9)
500
400
Strain (microstrain)
300
200
100
0
-100
-200
-300
-400
-500
35:00
35:15
35:30
35:45
36:00
36:15
36:30
36:45
37:00
Time (MM:SS)
Figure 4. Strains due to wave slamming.
Optical strain sensor readings in the mast during wave slamming events are depicted
in Figure 4. Each wave event is indicated by a corresponding strain peak with good
mirroring between FBG tension and compression pairs. Strain maxima were found to
coincide with the impacts of waves on the vessel as recorded by video camera.
SUMMARY
Structural health monitoring offers important benefits to ship builders, owners and
operators. Information about the response of a vessel under real loading conditions
can be used to refine design guidelines. Of particular relevance to the marine
applications of AFRP, the ability to test new materials and construction techniques
may support their introduction and proliferation. A reduction in ownership cost
results from the ability to implement a condition-based maintenance schedule and
increased safety follows from the ability continuously to monitor structural integrity.
An important requirement for effective SHM is an accurate, reliable, affordable strain
data and OFS have been demonstrated to fulfil this need, even in the demanding
marine environment. The trials conducted with sensors in a CFRP mast confirm that
FBG are particularly suited for use with composite materials.
ACKNOWLEDGMENTS
The authors acknowledge the help and support of former employees of Smart Fibres
Ltd and the MAST consortium: British Aerospace Military Aircraft and
Aerostructures, British Aerospace Sowerby Research Centre, Carbospars Ltd.,
Pendennis Shipyard Ltd, Aston University and Egli Consultants Ltd. MAST was part
funded under the UK DTI/EPSRC LINK Photonics Programme (Project. No.:
YAG/08/02/1054)
REFERENCES
1. HULLMOS, offered by R. Rouvari OY (Finland) and Hydrographic and Marine Consultants BV
(Holland)
ACMC/SAMPE Conference on Marine Composites
Plymouth, 11-12 September 2003 (ISBN 1-870918-02-9)
2. A. Paolozzi, M. Ivagnes and U. Lecci, ‘Qualification tests of aerospace composite materials with
embedded optical fibres’, Proc. 2nd International Workshop on Structural Health Monitoring,
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sensors’, Smart Materials 2000, Proc. SPIE Vol. 4234, 2000, 134-143
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deck slamming experiments with a FRP sandwich panel using a network of 16 fibre optic Bragg
grating strain sensors’, Composites: Part B, Vol. 31, 2000, 187-198
8. A. E. Jensen, J. Taby, K. Pran, G. Sagvolden and G. Wang, ‘Measurement of global loads on a fullscale SES vessel using networks of fiber optic sensors’, Journal of Ship Research, Vol. 45 (3) Sept.
2001, 205-215
9. K. Pran, G. B. Havsgård, G. Sagvolden, O. Farsund and G. Wang, ‘Wavelength multiplexed fibre
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