Method and device for continuous monitoring of dynamic loads

For continuous monitoring of dynamic loads, including stresses and strains in large hull structures for vessels (5), a strain measurement system (2) is employed with fiber optic cables which connect optical strain sensors (7; 16) at different points in the hull structure. Optical signals for detection of stresses and strains are distributed to the strain sensors (7; 16) from an optical transmitter (11). The strain measurement system (2) is connected via a central monitoring unit (10) to a computer-implemented control system (1) which in turn is connected to a display and data presentation unit (4) and possibly other measurement systems (3). In a first operating mode, strain values during loading and unloading of the vessel (5) are detected by the strain sensors (7; 16). The control system (1) generates a curve which shows an average strain, and an alarm signal if the average strain exceeds a predetermined threshold which indicates an unacceptable level of strain. In a second operating mode, when the vessel (5) is underway, strains are continuously detected by the strain sensors (7; 16). An alarm signal is generated if the average strain exceeds a predetermined threshold which indicates an unacceptable level of strain. A strain measurement system and a fiber optic strain sensor (16) are also used with this method.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention concerns a method for continuous monitoring of dynamic loads, 
including stresses and strains in large hull structures for vessels, 
according to the introduction to claim 1. The invention also concerns a 
strain measurement system for implementing the method, comprising an 
optical transmitter unit, an optical receiver unit and a fiber optic cable 
network, wherein the transmitter unit and the receiver unit are provided 
in a central monitoring unit which is connected via an interface with a 
computer-implemented control system which in turn is connected to a 
display and data presentation unit and possibly other measurement systems. 
Finally, the invention also concerns a temperature-compensated, 
polarimetric, fiber optic strain sensor for use in a strain measurement 
system. 
2. Description of the Related Art 
It is a well known fact that large hull structures, such as both ship and 
aircraft hulls, are frequently exposed to substantial, dynamic loads. 
These loads can result in damage to the hull and lead to fatigue and 
fracture, often with disastrous consequences. Load damage to hull 
structures normally only becomes apparent in connection with a regular 
service and inspection carried out by the application of various 
non-destructive test methods. The problem is that load damage to a hull 
structure can occur when the vessel is underway and develop with 
disastrous consequences before it can be discovered in the normal 
inspection. 
From British patent application GP 2238112 (U.S. Pat. No. 5,038,618) a 
method is known for detection and monitoring of strains in large, 
technical structures by means of fiber optic cables which are attached to 
the structure. An optical signal is transferred along the cable to an 
optical receiver. By analysing the optical signal any changes with regard 
to length and/or changes in the length of the cable can be shown. The 
change in the cable length is found by determining phase changes on one or 
more modulation frequencies, by means of interferometry or pulse time 
measurement. By means of this method a ship's hull or the like can be 
monitored in order to indicate strains, but bending or twisting in various 
beam-like structures can also be revealed. 
The method according to this patent publication substantially only provides 
the opportunity of monitoring global strains, i.e. the relative strains 
between two points which form the respective end points of the fiber optic 
detection cable. Even though the awareness of global strains is naturally 
important in large, dynamically-loaded structures, a more localized strain 
monitoring is required in order to be able to follow the development of 
the progress of stress or strain and not least at those individual points 
in a structure which in all probability will be more vulnerable to strain 
than other points, and a local strain monitoring will therefore be 
essential in order to maintain safety. At the same time data from a number 
of local strain sensors, possibly in connection with one or more global 
strain sensors and a system which records the forces to which the 
structure in its entirety is exposed, can be employed in order to provide 
a complete strain history for the structure, and this can be done with 
high resolution both in the temporal and spatial domains. 
This, however, is based on the assumption that the strain system fulfils 
specific technical requirements which as yet have not been easy to 
implement in known, fiber optic strain measurement systems, whether they 
are based on phase change, interferometry or pulse time measurement. 
Problems with regard to accuracy and resolution become particularly 
critical if advantageous data from a large number of local sensors 
arranged in a very large structure, such as, e.g., a large bulk carrier or 
a tanker, have to be collected and transferred to a central monitoring 
unit in real time. 
SUMMARY OF THE INVENTION 
A principal object of the present invention therefore is to provide a 
method and a technology which eliminate the disadvantages of the known 
strain measurement systems and which permit continuous monitoring of 
forces which affect, for example, a ship's hull during loading, unloading 
and when underway. Thus it is a first object of the present invention to 
be able to monitor movements in frames and hull. During loading, unloading 
and when underway the ship's hull structure will be exposed to different 
forces and stresses and these will be dependent on the effect which 
impacts and shocks during loading and unloading have on the ship's 
structure, and whether a uniform load profile is maintained during loading 
and unloading. This in turn depends on whether the individual cargo holds 
are loaded or unloaded according to a fixed plan. Movement in frames and 
hull will further be dependent on the weather conditions when the ship is 
underway and whether the cargo shifts. 
A second object of the present invention is to monitor vibrations which 
occur in the ship's hull or the like due to, e.g., engine vibrations, 
vibrations as a result of impact during loading and unloading, vibrations 
as a result of heavy seas when underway and general shock effects. It will 
be of special interest to be able to determine the general vibration 
spectrum in order, amongst other reasons, to avoid resonances in the hull 
structure, since such resonances may well lead to permanent damage to the 
structure. 
A third object according to the present invention is to be able to record 
any shock effects which may, amongst other things, be due to blows from 
equipment during loading and unloading, shifting of the cargo in rough 
seas or the impact of waves on the ship's side. 
A fourth object of the present invention is to be able to record twisting 
along the ship's longitudinal axis during operation. In heavy seas the 
ship's hull can experience substantial twisting, while there is little 
knowledge as to whether twisting is a problem during loading or unloading. 
A fifth object of the present invention is to be able to monitor stresses 
and strains caused by temperature gradients in the hull during normal 
service. Ships' plates can be exposed to substantial tensile stresses due 
to temperature gradients and it is well known that the elasticity of steel 
is linked to the temperature. 
Finally a sixth object of the present invention is to be able to record 
strains which are due to the formation of cracks, since these can affect 
the hull's rigidity and lead to changes in the stress diagrams. Such 
cracks can occur along hatches and coamings in cargo ships, while crack 
formation along the frames has been shown to be particularly dangerous. 
These in turn can lead to a reduction in the hull's rigidity along the 
side and, combined with lack of maintenance, damage incurred during 
loading and shifting of cargo, could result in parts of the ship's side 
being smashed in and the ship being wrecked. Maintenance of the ship is an 
important factor in this connection. 
The above-mentioned and other objects are achieved with a method which is 
characterized by features which are presented in claims 1-8, a strain 
measurement system which is characterized by the features which are 
presented in claims 9-18, and a temperature-compensated, polarimetric, 
fiber optic strain sensor which is characterized by the features which are 
presented in claims 18-21. 
The invention will now be explained in more detail in connection with 
embodiments of the method, the strain measurement system and the fiber 
optic strain sensor according to the invention, with reference to the 
attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates schematically the fiber optic strain measurement system 
2, this system being connected to a control system 1 which is implemented 
in a central computer. The control system is also connected to a central 
display and data presentation unit 4. In addition the control system can 
also be connected to other measurement systems or sensor systems 3. 
FIG. 2 illustrates how the strain measurement system 2 according to the 
invention is implemented in a large bulk carrier 5a. A number of strain 
sensors 7a are provided at specific points in the cargo holds 6, on the 
decks or on the frames. In addition there are also provided on the ship's 
hull global longitudinal strain sensors 7b for detection of global strains 
in the hull's longitudinal axis. In addition there is provided in the 
ship's bow a triaxial accelerometer 8 which may well be based on the same 
sensor types which are employed in the other strain sensors. The 
accelerometer 8 in the bow detects in three axes the accelerations to 
which the ship is exposed when underway. All the strain sensors 7 are 
connected via a fiber optic cable network 9 to a central monitoring unit 
10 on board the ship, this central unit 10 being simultaneously assigned 
to the control system 1 implemented on a computer in order to record and 
process information concerning the dynamic state of the ship's hull. 
FIG. 3 illustrates the strain measurement system 2 according to the present 
invention provided in a tanker 5b, the system being arranged in a similar 
fashion to the embodiment in FIG. 2. 
FIG. 4 illustrates the principle of the strain measurement system 2 
according to the invention. A light source in the form of a laser 11 in 
the central unit 10 transmits an optical signal through a fiber optic 
cable network 9 which employs a polarization-maintaining fiber. In 
branches 12, 13, 14 of this network 9 there are provided optical strain 
sensors 7 in the form of polarimetric sensors 16. The polarimetric sensor 
16 consists of an optical fiber 16b which is mechanically decoupled from 
the actual hull structure, thus preventing it from being influenced by 
vibrations, strains or stresses therein, and a corresponding, equally long 
piece of fiber 16a which is mechanically coupled to the structure and 
constitutes the strain detecting fiber. The polarization-maintaining fiber 
is also employed in the sensor fibers. These fiber optic polarimeters 16 
are temperature-compensated in that they comprise two spliced, double 
refractive, optical fibers 16a, 16b of the same type and of the same 
length. In the splice the fibers are rotated 90.degree., thus causing the 
phase shift between the two polarizations formed in the first fiber 16a to 
be reversed in the second fiber 16b, and thereby ensuring that all 
perturbations apart from strain are cancelled in the double refractive 
optical fibers. This in turn means that phase shifts caused by temperature 
or hydrostatic pressure in turn are cancelled. Since one 16a of the two 
fibers 16a, 16b in the polarinmeter 16 is connected to the structure, 
while the other fiber 16b is decoupled from strains in the structure, the 
strain will be connected to the optical signal which passes through the 
polarimeter 16, even though the other perturbations are cancelled out. The 
strain sensor 7 will advantageously employ the linear part of the 
polarimeter's response. Without external compensation for temperature 
variations the acceptable linear response range is 30.degree. which in the 
embodiment can correspond to a relative strain of 2000 .mu..epsilon., i.e. 
2000 ppm. The output on the polarimetric strain sensors 16 is connected to 
a through-going fiber in the fiber optic cable network 9 and passed to one 
or more detectors 17a which are provided in the central unit 10. 
A possible design of the transmitter unit 11 for the strain measurement 
system 2 is illustrated in FIG. 5. This transmitter unit 11 first of all 
comprises the main laser or the exciting laser 11a as shown in FIG. 4. 
This constitutes the transmitter laser 11a which supplies the optical 
signal to the fiber optic cable network 9 in the strain measurement system 
and preferably employs a high-power, highly stable, diode-pumped Nd; YAG 
laser. This emits at 1319 nm. The laser 11a can advantageously include an 
optical insulator (not shown) in order to reduce the sensitivity to 
feedback light. The laser will be in operation all the time, apart from 
during any calibration which may be performed. 
In order to calibrate the strain measurement system, as illustrated in FIG. 
5, an additional laser 11b can be included which emits on an entirely 
different wavelength from the main or transmitter laser 11a. This 
reference laser 11b which thus works on a substantially different 
wavelength from the main laser 11a still lies within the valid wavelength 
range for the optical fiber which is used in the strain sensors 7; 16. A 
suitable wavelength for the emission can be 1250 nm. A semiconductor laser 
with distributed feedback is preferably used as reference laser 11b. The 
reference laser 11b is connected via an optical coupler (not shown) and an 
optical fiber to an intensity reference 18 and to a reference polarimeter 
19. During calibration, as mentioned, the main laser 11a is switched off, 
and the reference laser 11b can be used to calibrate the operation of the 
main laser 11a, including frequency shifts or mode jumps. Mode jumps can 
lead to errors in the measurement results from the fiber optic strain 
sensors 7; 16 and the demonstration of errors during the measurements can 
be used to allow the strain measurement system to generate an error code 
which is transferred to the control system. In case a time multiplexed 
detection system is chosen, a high frequency intensity modulator 20 must 
be included between the main laser 11a and the strain sensors 7;16. The 
fiber optic network 9 is illustrated in more detail in FIG. 6. The 
transmitter unit 11 and the receiver unit 17, i.e. the main laser 11a and 
the optical detectors 17a, are naturally provided as before in the central 
unit 10. A branch network 9 based either on polarization-maintaining 
optical fibers or single-polarization fibers runs from the transmitter 
unit 11. Not shown optical couplers distribute the light to the individual 
branches 12, 13, 14, 15. The network's topology is of greater importance 
the greater the number of sensors. If only seven sensors are used, there 
is little difference between a ladder structure or tree structure with 
respect to the available optical effect. When the strain sensors 7 are 
based on polarimetric sensors 16 as in the present embodiment, 
polarization maintenance will be the most critical parameter in the choice 
of the optical cable network. The optical couplers too must be 
polarization-maintaining. If the number of sensors increases, more 
couplers will also be required, in which case it will be advantageous to 
use a tree structure and not a ladder structure. 
At any rate polarization-maintaining couplers and polarization-maintaining 
optical fibers should be used in a branch network Instead of standard 
polarization-maintaining fibers, single-polarization fibers can preferably 
be used and the same type of fiber can be used as a fiber optic polarizer 
at the output on each polarimetric sensor 16. An optical standard fiber 
can thereby be used in the data acquisition network which leads from the 
sensors 16 to the receiver unit 17. If the strain measurement system 
according to the invention does not employ multiplexing in the acquisition 
part of the network, this leads to lower costs. In FIG. 6 the sensors 16 
are shown with the fiber optic polarizer 16a, 16b and delay loops 16c 
provided in the actual sensor which is located in a not shown housing. The 
optical couplers can preferably also be provided in the sensor housing. 
The strain measurement system 2 according to the invention can employ a 
multiplexed or non-multiplexed return network, i.e. the acquisition part 
of the fiber optic cable network 9, or the system can comprise a partially 
multiplexed acquisition network. A strain measurement system 2 according 
to the invention without time multiplexing is illustrated in FIG. 7. The 
difference with a strain measurement system according to the invention 
substantially lies in systems with time multiplexing and systems without 
time multiplexing. An entirely spatial multiplexed system as in FIG. 7 has 
the advantage that all the optical power can be exploited, even though the 
noise problems can be greater than in time multiplexed systems. In 
addition to this many individual receiver channels have to be calibrated 
and if the number of strain sensors 16 is large, large quantities of 
optical fiber are required in the cable network, thus incurring high 
costs. FIG. 8 illustrates the strain measurement system 2 according to the 
invention based on time multiplexing in the return network. This uses 
considerably less fiber in the cable network, with the result that a 
number of different sensor systems can virtually employ one standard 
cable. A significant advantage with the time multiplexed solution is that 
all the channels use the same signal processing electronics, thus giving 
relatively little deviation in the response between the individual 
sensors. Due to optical power loss with the use of many sensors it can be 
advantageous to divide the strain measurement system according to the 
invention between several separate time multiplexed acquisition networks. 
It may be advantageous to employ a combination of time multiplexing and 
spatial multiplexing, especially in strain measurement systems with a 
large number of sensors, as illustrated in FIG. 9. The receiver unit 17 is 
based on the use of photodiodes 17a, preferably of the PIN type In the 
receiver 17 it is advantageous to employ a transimpedance amplifier as an 
operational amplifier 17b. 
FIG. 10 illustrates schematically the design of a polarimetric, fiber optic 
strain sensor 16 according to the invention. The strain detecting fiber 
16a is mechanically coupled to the structure via adjustable couplers 20 
and on one end of the housing 21 by a 90.degree. splice joined to the 
mechanically decoupled, optical fiber 16b which otherwise has exactly the 
same properties as the mechanically coupled optical fiber 16a. The sensor 
fibers 16a, 16b are placed in a watertight housing 21 which is preferably 
filled with a viscous liquid so that the fibers' acceleration sensitivity 
is as low as possible. The sensor fibers 16a, 16b are preferably installed 
and attached to support plates of metal or of another material which is 
compatible with the ship's hull material. The strain-sensitive fiber is 
naturally prestressed through the attachment to the support plates. 
FIG. 11 illustrates a practical embodiment of a sensor housing 21 for the 
polarimetric strain sensor 16 according to the invention. The sensor 
housing 21 with the support plates for prestressing of the 
strain-sensitive fiber is rigidly connected to attachment devices 22 which 
can be welded or rigidly fastened in some other way to the ship's hull at 
the measuring point at which the sensor has to be located, thereby 
ensuring that stresses and strains in the hull are transferred to the 
sensor. 
FIG. 12 shows a section through the sensor housing 21 in FIG. 11, viewed 
from above and from the side respectively, with the sensor device and the 
attachment of the sensor fibers illustrated. 
It should be understood, however, that even the structural design of the 
sensor housing and the arrangement of the sensor fibers can be implemented 
in a number of different ways within the scope of the present invention. 
As illustrated in FIGS. 12a and 12b, an embodiment of the polarimetric 
sensor 16 according to the present invention is based on a mechanical 
device which permits a purely axial displacement of two elements which are 
connected with each other by means of strain-detecting 16a and 
temperature-compensating fibers 16b respectively. The sensor consists of 
two cylindrical components which form a male main part and a female main 
part which are fastened via a ball joint device to two bolts 22 which in 
turn are connected to a structural element in the hull at the measuring 
point. The elements of the main part can be freely displaced axially away 
from each other and towards each other as a reaction to a change in the 
distance between two bolts 22 which provide attachment to the structural 
element. The elements of the main part are located in a protective casing 
and attached to each other by means of the fibers as described above. The 
strain and temperature-compensation fibres 16b, 16a are attached via fiber 
fastening blocks to the main part elements. Adjustment of the relative 
distance between the main part and thereby the tension in the strain fiber 
16a can be achieved by means of adjustment bolts. The sensor 16 can 
therefore be adjusted after having been installed, thus obtaining a 
prestressing which provides a mid-stress response in the strain fiber 16a. 
After installation and calibration the sensor can thereby measure the 
axial strain in the structural element, when this element is exposed to 
various load conditions. The strain is then measured as the distance 
between the supporting bolts 22 increases or decreases. An increase in the 
supporting bolt distance increases the strain in the strain fiber, while a 
reduction in the distance reduces the strain. The actual sensor housing 21 
can be filled with a highly viscous fluid or a gel with good heat transfer 
properties, thus improving the heat stability of the sensor. In addition, 
as mentioned above, this means that the sensor becomes less sensitive to 
accelerations. As mentioned, the actual sensor housing 21 is designed as a 
watertight unit and can therefore be exposed to unfavourable marine 
environments without a reduction in efficiency or accuracy. In the 
illustrated example the actual sensor fibers 16 have a length of 
approximately half a metre, but in addition as illustrated in FIGS. 
13a,b,c, a delay loop 16c of several meters can be integrated in the 
sensor housing between the sensor fibers 16a, 16b and the output fiber. 
Basically, the strain measurement system according to the present invention 
could also have been based on other types of optical strain sensors than 
the polarimetric sensors here preferred. For example Bragg grating sensors 
could have been used which in principle are a narrow band reflector, where 
the center wavelength is altered when the sensor is exposed to strain. A 
system based on Bragg grating sensors is excited by means of a laser, but 
both the laser and the grating appear to be most suited to a multiplexed 
system, which at present is extremely expensive to implement if a large 
number of sensors require to be used. Systems based on Bragg grating 
sensors, however, have several advantages with regard to resolution and 
dynamic range and can employ a standard optical communication fiber. In 
addition the sensors can be made very small. 
A further advantageous design will be a sensor in which the sensor element 
is rigidly connected to the structural element which is exposed to 
strains, either directly or indirectly. It will be an advantage if the 
sensor is designed in a small size. 
The strain measurement system according to the present invention could also 
be envisaged implemented by means of fiber optic interferometers based on 
ordinary white light. Strains can then be recorded by means of standard 
Doppler interferometry. However, there is a problem when using fiber optic 
interferometers that great distances between the measuring points combined 
with the wish for high resolution greatly reduces the signal/noise ratio. 
It is an advantage, however, that fiber optic interferometry does not 
require the use of a polarization-maintaining fiber, thus enabling a 
standard communication fiber to be employed in the fiber optic cable 
network The use of fiber optic interferometers in the strain measurement 
system according to the present invention can therefore be an alternative 
if the requirement for resolution and dynamic range is a minor 
consideration and if there should be a wish to reduce the system costs. 
The strain measurement system according to the present invention works in 
two modes, the first operating mode recording strains during loading and 
unloading and measuring all relevant strains continuously. Via the control 
system average strain can be displayed simultaneously on the display unit 
and updated every minute. Should unacceptable strain levels occur during 
loading and unloading, an alarm signal can be activated. In a second 
operating mode the strain measurement system according to the invention 
measures all relevant strains continuously when the vessel is underway. At 
the same time the display unit displays a curve for average strain which, 
for example, is updated every five minutes or according to one's choice 
within an interval of 1-60 minutes. A curve is also displayed 
simultaneously which shows the strain's standard deviation, and this curve 
can similarly be updated every five minutes or according to one's choice 
at intervals of 1-60 minutes. Should an unacceptable level of strain 
arise, an alarm signal can be activated. On the basis of given 
predetermined criteria, the control system can simultaneously record the 
worst strain sequences within a period of for example 20 minutes, i.e. 
strain sequences in which the strain for example is greatest or exceeds a 
predetermined threshold. Up to a hundred of these sequences can be stored 
in the control system. The control system will also be capable of 
recording statistics for load sequences and make these data available when 
it is time for service and inspection. Such load sequences can for example 
be categorised within a range of 50 .mu..epsilon. (relative strain in ppm) 
and shown in histogram form in an operator-defined time interval, for 
example day, week, month or year. 
Thus by means of the method and strain measurement system according to the 
invention a wholly continuous monitoring is obtained of the loads to which 
the vessel hull is exposed. For example average strain in the last elapsed 
minute could be recorded every minute, and statistical parameters as 
standard deviation, top to top in the same cycle, minimum and maximum top, 
and the number of dynamic passages through zero over for example the last 
5 minutes can be recorded every 5 minutes. 
The strain measurement system 2 will therefore be capable of monitoring the 
absolute strain to which the vessel or ship 5 is exposed. A triaxial 
accelerometer 8 in the vessel's bow is also included in the total system, 
and as mentioned this is preferably in the form of a fiber optic sensor 
which is included in the strain sensor system. The strain measurement 
system 2 also comprises an uninterrupted power supply (UPS) which enables 
the system to operate for at least 20 minutes if a power cut occurs. The 
strain measurement system according to the invention should be able to be 
coupled to load computers, particularly with a view to calibration 
purposes. If the system is inadvertently shut down or breaks down, it will 
start up again automatically and continue recording. The system should 
also be capable of recalibrating all the strain sensors 7 against a known 
load state. The strain measurement system according to the present 
invention is essentially a purely optical system, apart from the system's 
central monitoring unit. Any extra electronic equipment apart from the 
central unit or the control system should comply with standard EX 
regulations and be sea water-resistant. 
Between the strain measurement system 2 and the control system 1 there is 
provided an interface which permits data from each strain sensor 7; 16 for 
example to be sampled at a frequency of 10 Hz and converted to a digital 
signal. The data from all the sensors 7; 16 are then transferred 
sequentially to the control system's computer in real time and in 
accordance with a predefined protocol. If the data from the strain sensors 
7; 16 are not reliable, a separate digital line goes to low, acting as a 
validity check. An interface between the control system and the strain 
measurement system 2 is used to signal that the system should be switched 
on or off or to give a command for calibration of the system. 
The actual control system 1 handles the sampled data and decides which 
events should be stored. The control system 1 includes a real time clock 
and an uninterrupted power supply, thus giving all the data recording a 
correct time indication, while at the same time any break-down of the 
system is detected and recorded. The uninterrupted power supply must be 
able to run the real time clock until the ordinary power supply returns. 
The control system 1 will also monitor all incoming data in order to 
detect errors in the individual strain sensors. 
The strain measurement system 2 according to the invention is designed for 
a resolution in each sensor element of 5 .mu..epsilon. or right down to 2 
.mu..epsilon., while the dynamic range for each sensor element should be a 
minimum of 2000 .mu..epsilon.. Furthermore the strain measurement system 2 
can handle fiber optic cable lengths of up to 600 meters and at least 7 
independent strain sensors and preferably up to 25 independent strain 
sensors. Furthermore the strain measurement system according to the 
present invention can record accelerations of at least 2 G with a cross 
sensitivity of less than 20 .mu..epsilon./G in all three axial directions. 
The strain measurement system according to the present invention should 
also be able to work correctly up to a minimum of 5 bar and with a cross 
sensitivity of not less than 40 .mu..epsilon./bar. If an error condition 
arises, the aim of the strain measurement system according to the present 
invention is that it should be capable of correcting itself and supplying 
correct data in the course of 5 seconds. The error condition is reported 
to the control system 1 and it is a precondition that such error 
conditions should not occur more than 5 times during one hour's operation. 
The aim is that the system according to the invention should not be out of 
operation for more than 5 seconds per day. It is naturally assumed that 
the system's specifications should be capable of continuous further 
improvement in order to keep pace with the relevant technical development. 
It should be understood that the method and the strain measurement system 
according to the present invention are not exclusively restricted to large 
hull structures for ships such a bulk carriers and tankers, but that they 
can naturally be employed on other types of ship, offshore structures or 
for that matter on aircraft and spaceships, as well as land-based vehicles 
or generally speaking any large structures which are continuously exposed 
to dynamic loads. 
By means of the strain measurement system according to the present 
invention load sequences and strain data can be transferred from the ship 
to a land-based, central data recording system, and the transfer can, 
e.g., be performed via satellite. The land-based system can comprise a 
large data base which can store the load and strain history of the vessel 
or the monitored object through a number of years and thereby manifest 
long-term tendencies with regard to the stresses and strains to which the 
vessel or ship is exposed. These data can then be used to evaluate the 
need for repair, replacement or improvements and finally also for 
estimating the life span of the vessel.