WO1982004319A1

WO1982004319A1 - Structure deflection measurement method and apparatus

Info

Publication number: WO1982004319A1
Application number: PCT/GB1982/000153
Authority: WO
Inventors: Gordon Bryce Fraser
Original assignee: Gordon Bryce Fraser
Priority date: 1981-05-27
Filing date: 1982-05-27
Publication date: 1982-12-09
Also published as: GB2101305A; GB2101305B

Abstract

A method and apparatus for measuring structural deflection of sea-going vessels and structures employing a laser beam. A laser beam is transmitted to a reflector (2) mounted on the structure at a location to be monitored. The beam reflected from the reflector (2) is detected and compared with a reference beam to determine a path difference between these beams from which the structural deflection at the location monitored can be determined. By relating these measurements to time, stress and fatique effects can be monitored.

Description

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^STR^UCTUREDEFLECTIONMEASUREMENTMETHOD_AND_APPARATUS

TECHNICAL FIELD:

The present invention relates to a method and apparatus for monitoring deflections in sea-going vessels and structures such as, for example, ships, tankers and oil rigs.

BACKGROUND ART:

During the past few years, structural failures in ships and oil rigs have caused heavy financial losses, extensive loss of life and considerable environmental destruction through pollution.

A three year programme of investigation was carried out by the inventor using structural performance .data and computers. The investigation revealed that most of the approaches to ship and oil rig design have been based on empirical methods and that ships' hulls have never been practically tested for factors such as, for example, strength, resilience and fatigue. It is believed that this situation exists f_{or a} number of reasons, including simple resistance to change. However, it would appear that the primary reason for this situation is that those measurement techniques which have been available are insufficiently accurate to be practicable. Up till now, the principle stress measurement technique employed has been the use of strain gauges which are highly sensitive to the environment and are therefore unsuitable for accurate in-service testing of ships and oil rigs.

Large tankers, in particular, come under severe stressing during loading and unloading. The sequence of loading and unloading such tankers is therefore crutial in order to avoid unacceptable stresses being developed with the possibility of factures occurring in its structure. However, tanker piping systems, not designed to cope with all the different shoreside installations they might encounter, may severely restrict the process of safe loading and unloading which may lead to unsuitable loading and unloading sequences being used. Furtfier, the practice of transporting mixed cargoes of crude oil in tankers originally designed to carry only one type of oil, have made stress calculations additionally complicated and more critical then ever.

"Loadicators" are employed in some ships to enable the ships Officers to calculate the stresses set up in the ships structure during loading/unloading. These loadicators are basically programmed calculators containing data as to acceptable stress levels at. various key locations in the ships structure. However, this data is based on the original design factors of the ship and therefore does not allow for the inevitable weakening of the hull through age, through fatigue caused, for example, by incorrect loading over a long period, and through corrosion and atructural damage. Further, these loadicators do not provide the Officers with enough information as to how the hull is responding at any moment to changes in loading.

In addition, there is at present no effective means of monitoring the stress levels developed in the structures of ships and oil rigs at sea. Thus, it is not possible to determine whether critical stress levels are being approached and whether defensive action can be taken or whether the ship or oil rig should be abandoned. Similar problems arise in monitoring the thermal stresses set up in the hull of tankers carrying heated oil due to temperature differentials. Recent developments in laser optic science and electronics have now made it possible to develop measurement and control techniques which, when interfaced with small computers, achieve greater accuracy and versatility than has previously been possible. For example, with such systems, accuracies of one micron in 60 metres or more are easily achieved. Laser measurement systems are, in the main, based on experiments carried out by A.A. Michelson in the 19bO's. The measurement and control systems of the present invention are based on laser interferometry.

An object of the present invention is to provide a method and apparatus for the direct measurement of structural deflections of sea-going vessels and structures with a view to establishing and monitoring safe working structural stress levels.

Disclosure of the invention: According to one aspect of the present invention, there is provided a method of measuring structural deflections of sea-going vessels and structures wherein a laser beam is transmitted to a reflector mounted on the sea-going vessel or structure at a location to be monitored, the beam reflected from said reflector is detected and compared with a reference beam to determine a path difference between said reflected and reference beams from which the displacement of the reflector and hence the structural deflection at said location can be determined.

In one embodiment of the present invention, the path difference data is fed to a computer together with information as to ambient air conditions. A corrected value of reflector displacement is computed and compared and/or displayed with permitted maximum - 4 -

deflection data. The path difference data fed to the computer is preferably related to the passage of time allowing the speed of deflection to be computed and/or deflection over a period of time to be monitored and recorded on, for example, magnetic tape.

In an alternative embodiment of the present invention, the path difference determined is corrected to allow for ambient air conditions prior to being fed into the computer.

The laser beam may comprise a single frequency light beam or a two frequency light beam. In the latter case a Doppler signal is compared with a reference signal to give the Doppler frequency shift which is a measure of the velocity of the reflector. Reflector displacement is obtained by integration of this velocity measurement.

In a preferred embodiment, the output of the laser is amplitude modulated and a phase detector compares the relative phase of amplitude modulation in the reflected beam with that transmitted. Such amplitude modulated beams have the advantage of being less likely to cause eye damage than the unmodulated beam, if viewed directly for any length of time. In an alternative embodiment the laser beam is frequency modulated and the difference in phase of the reference and reflected beams is detected to give the path difference.

In another preferred embodiment, a plurality of retro-reflectors are employed and spaced from each other along the structure to be investigated. In this case, the laser scans the retro-reflectors in a predetermined sequence to give the overall deflection of the structure. According to a further aspect of the present invention there is provided an apparatus for measuring structural deflection of sea going vessels and structures comprising a laser; a reflector mounted on a support structure adapted for mounting at a location to be monitored on the sea-going vessel or structure; a sensor for detecting reflected light from the reflector, and means for comparing the reflected beam with a reference beam to obtain a path difference from which displacement of said reflector, and hence the structural deflection at said location, can be determined.

In an embodiment of the present apparatus, a plurality of reflectors are provided each mounted on a calibrated support on which the reflector can be moved both horizontally and vertically and clamped. The reflectors are preferably rectro-reflectors mounted on gimbals so that they remain vertical despite deflection of the structure on which.their support is mounted. The supports may be adapted for permanent fixing in a desired position, or be provided with disengagable clamping means. Preferred clamping means are energisable magnets.

An embodiment of the present invention further comprises a computer for computing the actual structural deflection from the wave path difference and comparing this with comparable stress limit data, together with means for displaying and/or recording the output of the computer. Additionally, warning signal means may be provided to indicate when safety limits have been exceeded and/or operate control means for altering loading/unloading conditions.

BRIEF DESCRIPTION OF THE DRAWI GS:

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: ^' OMPI - 6 -

Figure 1 illustrates the principle of interferometric measurement;

Figure 2 illustrates an apparatus according to an embodiment of the present invention; Figure 3 illustrates an embodiment of the present invention for measuring hogging/sagging of a sea-going vessel;

Figure 4 illustrates the arrang ent shown in Figure 3 in the direction of arrow A, and Figure 5 illustrates a support for a reflector. The high intensity and temporal coherence of laser beams offer advantages in interferometric measurement over those of conventional light sources. An interferometric method based on Michelsons interferometer will first be described in order to outline the basic operation of interferometric laser distance measuring systems. Variations on this system which provide better stability under conditions of atmospheric turbulence and attenuation will be described later.

Figure 1 is a schematic diagram showing such an interferometer. A laser beam - is split into two parts by a beam splitter 1 to produce a measurement beam"^and a reference beam R. The measurement beam travels to a reflector 2 on the structure whose displacement is to be measured. The reflector 2 is usually a corner cube reflector which provides an accurate return of the beam. The return beam γ and reference beam R are combined at the beam splitter 1 and travel to a detector 3. The combined beams form a interference pattern. The amplitude of the light at the detector 3 depends on the phase difference between the reference beam and the measurement beam which in turn depends on the - 7 -

difference in the optical path that the two beams have travelled. The phase difference S is given by 5 = 2ks Cos θ where k = 2 ?r /fi and ^ = wavelength; s=the path difference, and θ=the angle between the common axis of the beam and the direction of observation which, in this case, is zero. When the moving part travels one half wavelength of light, the total difference in optical path goes through one wavelength and the fringe pattern obtained will go through one period, corresponding to a change from light to dark to light at the position of the detector. Thus, the motion of the moving part leads to amplitude modulation of the light which is sensed by the detector.

BEST MODE OF CARRYING OUT INVENTION:

In the embodiment of the present invention illustrated in Figure 2, electronic circuitry 5 counts the period of amplitude modulation detected by an interferometer 4 and feeds this information to a computer 6 which calculates the distance through which the structure has moved and compared this with acceptable deflection values determined by the required stress limits. A real time clock 8 is connected to the computer to allow the measurements to be related to the passage of time and the velocity of displacement to be ascertained.

The output of the computer 6 is transmitted to a display unit where it is displayed digitally or graphically by, for example, a print out or cathode ray tube. The output of the computer may also or alternatively be fed to a magnetic tape recorder for subsequent analysis.

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The distance obtained by the interferometric measurement is the optical path length, which differs from the physical path length by a factor equal to the index of refraction of the air, and this is dependent on humidity, temperature and pressure variations. Corrections for the index of refraction of air are necessary to obtain good accuracy. Systems which automatically sense the variation in ambient air pressure, temperature and humidity and calculate the appropriate correction ^" values have been developed. In the embodiment of Figure 2, the output of such adetector system 7 is fed into the computer in order to provide accurate determination of physical path lengths. Various interferometric arrangements for measuring direction of motion have been developed and can be used as the interferometer 4, In one such system, two detector are provided which respectively collect light from regions of the fringe pattern where the phase difference of the interfering beams differs by Λ /2. The relative phase of the amplitude modulation viewed by the two detectors will therefore be different depending on the direction of the reflector motion and this is used to determine the correct displacement.

In another such system, the laser beam is circularly polarized and split into a." reference beam and measuring beam. A series of reflectors of the reference and measuring beams result in their being of opposite circular polarization when they are combined. These beams combine to form a linear polarization vector whose orientation will depend on the relative phase of the two circularly polarized beams and which therefore rotates as a function of reflector position.

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The above interferometric method of distance measurement is sensitive to variations in the intensity of laser light. If the light beam intensity changes because of air turbulance, shifts in laser output, or air turbidity, improper fringe counting can result in measurement error. In an alternative embodiment of the present invention, the interferometer 4 is a two frequency laser system which overcomes these shortcomings. In this system, the Doppler shift of the beam reflected from the moving reflector is measured.

The laser emits light or two slightly different frequencies fl and f2 with different polarization properties allowing the beam to be split into these two frequencies. The frequency f2 travels to a fixed reflector and frequency fl to the movable reflector whose displacement is to be measured. Light reflected from the moving reflector has a frequency shifted by an amount A f, where:

(/*-" being the velocity of motion of the reflector and c the velocity of light. The beams reflected from the fixed and movable reflectors are combined and produce an amplitude modulation of the light, of modulation frequency f2-(fl — Δfl) . A reference signal f2 - fl is generated and is fed with the modulation frequency to a converter which extracts Af1 and hence

can then be integrated to obtained linear displacements. A two frequency system can also be adapted to measure pitch and yaw.

In yet another type of interferometer 4 the laser beam is amplitude modulated and the phase of the reflected light beam is compared with that of the emitted beam. The difference in phase occurs

_. OMPI - 10 -

because of the finite time required for the light to travel to the movable reflector and return to the detector. The phase shift 0 is related to the total path length L by the equation 0 = 2?f(2n !>/ f\ g )

n is the group index of refraction.

Generally, laser interferometers provide measurement of displacement from an arbitrary zero, rather than an absolute measurement of distance. Thus, the instrument reading is set to zero at the initial position of the movable reflector, prior to deflection of the structure and its motion is measured relative to this preset zero. However, there are systems on market such as, for example, the "Hewlett-Packard 3850A industrial distance meter" which measure absolute distance. The Hewlett-Packard meter is also self-correcting for ambient air conditions based on an operators presetting. Figure 3 shows an arrangement of laser head 10 and rectro-reflector 2 which is suitable for monitoring the hogging'sagging of a ship during loading/unloading by an embodiment of the method according to the present invention. The laser head 10 is mounted to be horizontally and vertically movable, on a deck mid-line of the ship. Each retro-reflector 2 is mounted on a calibrated support 16, and the supports 16 are located spaced apart along the deck mid-line as shown in Figure 3. These supports 16 are removably clamped to the deck of the ship by energisable magnets 17. The horizontal and/or vertical positions of the rectro-reflectors on the supports 16 are staggered in relation to each other and appear as shown in Figure 4 when viewed in the direction of arrow A in Figure 4.

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OMPI - 11 -

The laser scans the rectro-reflectors 2 prior to loading the ship so as to obtain a zero reading for each reflector 2. During loading, the laser repeatedly scans the reflectors in a predetermined sequence. The reflected beams are returned to the laser head 10 where they are received by a receiver lens surrounding the laser's transmitter lens and compared with a reference beam as previously described.

Each rectro-re lector 2 is mounted on its respective support 16 on gimbals. This ensures that the reflectors always lie in a vertical plane despite rotation of the supports 16 during structural deflections, so as to return the beam to the receiver lens.

By computing the horizontal distance moved by each rectro-reflector, located at a predetermined height above the deck, the angle of inclination of the deck can be obtained by simple geometry i.e. θ = L/H where L is the horizontal displacement, H is the height of the rectro- reflector above the deck and θ is the inclination of the deck in radians.

In an alternative embodiment, an interferometer is associated with each reflector and the measurements are fed back to a computer for analysis.

The above described arrangement can also be used in determining fatique behaviour and limits. In this case, the velocity of displacement of the reflectors is determined as stress waves propagate through the ships structures.

Torsional deflection of a hull may be determined by locating the supports 16 with rectro-reflectors 2, spaced apart along one side of the ships deck and scanning each rectro-reflector 2 by means of the laser 10 located on the other side of the ship. In this case, of course, the rectro-reflectors 2 can all be located at the same height H above the deck. - 12 -

INDUSTRIAL APPLICABILITY Applications of the present invention include the following:

1. Ship and oil rig structural quality control during building, while afloat or in dry dock. Actual length, angle, straightness and squareness of the ship or oil rig structure can be measured, computed and checked against constructural drawings, and records can be permanently stored for subsequent comparison with in-service data. 2. Direct measurement of longitudinal and transverse resiliance of ship and oil rig structures after building, and subsequent establishment and monitoring of safe working structural deflection can be re-checked at periods during the vessel or oil rigs life and any structural deterioration caused by, for example, corrosion or fatique, can be accurately noted and the stress limits adjusted accordingly. Further, such measurement provide an indication as to when repairs to the structure are necessary. 3. Monitoring ship or oil rig structural deflections during transient loading conditions allowing stress and fatique effects to be monitored and computed and also for the automatic control of loading by, for example, direct control of the loading valves of a tanker. 4. Monitoring ship or oil rig structural deflections in a seaway. Such a system may be used as a voyage recorder to provide a permanent record for subsequent analysis in the manner of an aeroplane flight recorder. This will then help in determining the reason for any structural failure and consequently in future structural design.

Claims

CLAIMS :

1. A method of measuring structural deflections of sea-going vessels and structures wherein a laser beam is transmitted to a reflector mounted on the sea-going vessel or structure at a location to be monitored, the beam reflected from said reflector is detected and compared with a reference beam to determine a path difference between said reflected and reference beams from which the displacement of the reflector and hence the structural deflection at said location can be determined.

2. A method according to claim 1, wherein the path difference data is fed to a computer to compute the displacement of the reflector.

3. A method according to claim 2 , wherein the path difference data is fed to the computer together with information as to ambient air conditions and a corrected value of the reflector displacement is computed.

4. A method according to claim 2, wherein the path difference data determined is corrected to allow for ambient air condition's prior to being fed into the computer.

5. A method according to claim 2, 3 or 4, wherein the reflector displacement computed is compared in the computer with permitted maximum deflection data.

6. A method according to claim 2, 3, 4, or 5 wherein the path difference data fed to the computer is related to the passage of time.

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7. A method according to claim 6, wherein the rate of structural deflection is computed.

8. A method according to claim 6, wherein the structural deflection is monitored over a period of time.

9. A method according to claim 8, wherein the structural deflection with time monitored is recorded for subsequent analysis.

10. A method according to any preceding claim, wherein the laser comprises a single frequency light beam.

11. A method according to claim 1 or 2, wherein the laser comprises a two frequency light beam and a Doppler signal is compared with a reference signal to give the Doppler frequency shift from which the velocity of the structural deflection is obtained.

12. A method according to claim 11, wherein the structural deflection is determined by integration of the velocity obtained.

13. A method according to claim 1 or 2, wherein the output of the laser is amplitude modulated and a phase detector compares the relative phase of amplitude modulation in the reflected beam with that transmitted.

14. A method according to claim 1 or 2, wherein the laser beam is frequency modulated and the difference in phase of the reference and reflected beams is detected to give the path difference.

15. A method according to any preceding claim, wherein a plurality of reflectors are employed, spaced from each other along the structure to be investigated, and the laser - 15 -

scans the_. reflectors in a predetermined sequence to give the overall deflection of the structure.

16. An apparatus for measuring structural deflection of sea going vessels and structure comprising a laser; a reflector mounted on a support structure adapted for mounting at a location to be monitored on the sea-going vessel or structure; a sensor for detecting reflected light from the reflector, and means for comparing the reflected beam with a reference beam to obtain a path difference from which displacement of said reflector, and hence the structural deflection at said location, can be determined.

17. An apparatus according to claim 16, wherein a plurality of reflectors each mounted on a support structure adapted for mounting at a location to be monitored, is provided.

18. An apparatus according to claim 16 or 17, wherein said support structure comprises a calibrated support on which the reflector can be moved both horizontally and vertically and clamped.

19. An apparatus according to claim 16, 17 or 18, wherein the or each reflector is rectro-reflector mounted on gimbals so that it remains vertical despite deflection of the structure on which its support is mounted.

20. An apparatus according to any of claims 16 to

19, wherein the or each support is adapted for permanent fixing in a desired position.

21. An apparatus according to any of claims 16 to 19 wherein the or each support has disengageable clamping means.

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22. An apparatus according to claim 21, wherein the clamping means comprise energisable magnets.

23. An apparatus according to any of claims 16 to 22, wherein a plurality of reflectors each mounted on a support structure adapted for mounting at a location to be monitored and a plurality of interferometers are provided each interferometer being associated with a respective reflector.

24. An apparatus according to any of claims 16 to 23, further comprising a computer for computing actual structural deflections from the wave path differences and comparing this with acceptable stress limit data.

25. An apparatus according to claim 24, wherein a real time clock is connected to the computer to allow structural deflections to be related to the passage of time and the velocity of these deflections to be ascertained.

26. An apparatus according to claim 24 or 25, further comprising means for displaying the output of the computer.

27. An apparatus according to any of claims 24 to 26, comprising means for recording the output of the computer.

28. An apparatus according to any of claims 24 to 27, further comprising warning signal means responsive to an output of said computer to indicate when safety limits have been exceeded.

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29. An apparatus according to any of claims 24 to 28, wherein warning signal means responsive to an output of said computer are provided to operate control means for altering loading/unloading conditions when specified limits have been exceeded.

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