GB2366382A - Remote monitoring of structure condition - Google Patents

Abstract

Offshore oil and gas exploration and production companies maintain large open lattice structures that are mostly underwater. To extend the design life of these structures requires a method to monitor a structure so as to give advance warning before a structural member 18, 19 or 20 should fail. The method proposed here measures the vibration response at a number of points 17 distributed around the structure to forces input 15 in the acoustic range up to about 20kHz at a much smaller number of points. The time of arrival, magnitude and phase of the pulses (11, 12, 13 and 14, Fig. 2) at the response points 17 change in a manner that is sufficient to indicate the presence of a substantial crack in one of the members and is sufficient to indicate the location of that defect. The influence of changes in supported mass is removed by effectively isolating the variable mass with additional actuator groups placed between the supported mass and the main structure actively controlled so as to make this cross-section behave as a perfect reflector.

Description

2366382 STRUCTURE CONDITION REMOTE MONITORING APPARATUS AND METHOD This

invention relates to structure condition remote monitoring apparatus and method and concerns in particular remote means of detecting incipient failure in a structure.

Offshore oil and gas exploration and production companies maintain large open lattice structures that are mostly underwater. To extend the design life of these structures requires a method to monitor a structure so as to give advance warning before a structural member should fail. This method needs to assess the whole structure on a regular and frequent basis. It needs to give warning before failure of a member is complete. It needs to operate in a manner that keeps costs to a minimum. It needs to be reliable, not missing any member failure and not giving false alarms. The invention proposes such a method. There exist several methods to monitor offshore structures that are either in use or have been suggested elsewhere for use.

Most offshore steel structures have hollow structural members designed to be watertight. Flooded member detection can give early indication of a crack. The detection of water inside the member involves expensive and hazardous diver intervention, and so reduces the frequency at which such inspections are carried out. The alternative of fitting pen-nanent sensors to every member is very costly.

Growing cracks are known to emit noise in the range of 100kHz, and acoustic emission is a method of listening for such noise. A large number of sensors has to be deployed for this method to have any chance of working.

Water waves subject offshore structures to excitation. The response of the structure to the excitation can change if a member cracks. The frequency input from the waves is generally below I Hz, and local member structural response is well above that. The force input cannot be measured, so the only parameters to compare are frequency and damping, not magnitude or phase of response. The overall structure frequency response in the lowest modes may be closer to the excitation frequency but the method can be insensitive even to complete severance of a single member. Offshore structures are deliberately designed to have redundant numbers of members; a single member can be removed without the structure collapsing. Such redundancy makes the response to water wave excitation highly insensitive to failure of individual members. The method is thus of limited use.

Ultrasonic reflection techniques that are used on land to assess pipelines and similar structures can be used underwater. The presence of water is an advantage as it improves the coupling between the transducers and the steel. The high frequencies used at over 100kHz give good resolution in detecting defects but limit the range because of rapid attenuation of the waves. Every weld has to be -3individually tested, and the steelwork has to be carefully cleaned by divers. This is a lengthy, dangerous and expensive operation.

Ultrasonic refraction wave techniques, sometimes called Lamb waves, work in the 50kHz to IMHz range, well beyond the human audible range. The waves travel along the wall of cylindrical structures with a range of a few metres. The possible operating coverage along the structure can be ten or twenty metres. The high frequency gives the opportunity for good defect resolution. The input excitation is in the form of a few cycles of a single frequency pulse to reduce the effect of dispersion of the waves. The frequency of the pulse is chosen to optimise the detection. The method could be sensitive to changes in the mass supported by the structure. The input pulse has to be exactly repeated so that the response wavefon-ns can be time averaged to reduce the affect of background noise. Again this is a hazardous and expensive operation involving divers, although fewer access points are required than is the case with ultrasonic reflection techniques.

The invention described here has advantages over existing methods and suggestions. More specifically, the invention proposes that this be accomplished by introducing into the structure at several points on the structure vibration waves in the acoustic frequency range and monitoring simultaneously the response to these waves at a limited number of points on the structure. Compression waves and bending waves are to be separated out. The ratio of the Fourier transform of force input at one point to the Fourier transfon-n of acceleration measured at another point can be called the impedance spectrum between the -4two points. There may be several paths between the two points along which waves may travel. A defect such as a crack that comes into being along one of those paths will alter the measured impedance spectrum. Tracking the compression wave impedance and bending wave impedance between several points allows the defect to be located on the structure.

Changes in the supported mass can also cause changes in the measured impedance spectrum. Since such changes are not of interest in this context, the invention provides means of preventing them.

-5In one aspect, therefore, the invention provides Apparatus usable to detect the origin, location and growth of defects in a large structure, which Apparatus comprises:

1. several electromechanical or mechanical actuators which can introduce force into the structure in the acoustic frequency range; 2. on each actuator there is a load sensor to measure the force introduced into the structure; 3. several accelerometer groups which can measure the bending wave and compression wave response at any cross-section of the structure; 4. an electronic system to control the sequential action of the actuators and the time history of force they introduce; 5. an electronic system to capture the measured forces and responses, and store these in an archive; 6. a system to form the frequency domain impedance spectrum ratio of forces and responses; 7. a system for correcting the impedance ratios for any changes in supported mass; 8. a system to transform the frequency domain impedance spectrum ratios into time domain unit impulse responses; and 9. a system to compare the latest set of unit impulse responses with a previously measured set and relate any changes between the two sets to the growth of defects on the structure', 10. a means of providing a warning to the operator of the existence of a potential failure of a member and the location of the potential failure point.

In a second aspect the invention provides a Method using the Apparatus of this invention, to detect the origin, location and growth of defects in a large structure, in which Method:

1. forcing means are distributed around the structure to introduce controlled forces into the structure; 2. vibration measurement locations are distributed around the structure; 3. forces are introduced into the structure in the acoustic range; 4. the vibration response of bending and compression waves is stored for future recall together with the forces measured; 5. the bending and compression wave impedance spectra between every shaker and every response point are calculated by taking the ratio of the Fourier transform of force input at one point to the Fourier transform of vibration measured at the response point; 6. the wave impedances are corrected if necessary for changes in the mass supported by the structure; 7. the wave impedance spectra are converted into the time domain by an inverse Fourier Transform, and referred to as unit impulse time responses; 8. the various unit impulse time responses are compared with original values obtained on the intact structure, and any changes are related to the size and location of defects in the structure; and 9. warning is provided of any defects that are growing with time and an indication given of their location.

The force may be input by various means, including an electro-mechanical device in which case the time history of the force input can be conveniently chosen to be a pulse, a sinusold whose frequency sweeps through the acoustic range, a random signal or some other preferred time history. Other means can be used to input force to the structure. The force is preferably input above the waterline and in an axial direction along the vertical member but neither preference is necessary.

A large number of measurements can be repeated to average out random background noise.

The response is most conveniently measured by the use of accelerometers. Such accelerometers can produce an electrical output. Alternative accelerometers can produce a variation in a light signal passed down optical fibre cable. More than one accelerometer can be placed at a given cross-section of the structure and their responses combined to separate out the bending and compression waves. This separation can be done in place or remotely by post processing.

By choosing to limit the frequency to the human audible acoustic range of below 20kHz the present method extends the reach of the waves to hundreds of metres instead of tens of metres. The penalty paid for lower attenuation, and therefore greater reach, is poorer resolution of defects. The present invention is able to detect cracks extending over at least twenty per cent of the member cross-section. It will not detect small defects of the order of size that typical ultrasonic methods will find but the prime objective is to detect cracks which are large enough to be a threat to structural integrity and early enough to be able to repair them before complete severance occurs.

The force may be input by various means, usually along a line parallel with the structural member - for example, by means of an electromechanical device, in which case the time history of the force input can be conveniently chosen to be a pulse, a sinusold whose frequency sweeps through the acoustic range, a random signal or some other preferred time history. Other means can be used to input force to the structure. For instance, a hammer can be instrumented with a force sensor to measure impulses introduced by a hammer blow. Various piezo-electric and piezoresistive materials such as PZT and PVDF can be used to excite the structure in the chosen frequency range. These haw the attraction of being robust and relatively cheap to apply. The difficulty with these lies in calibrating them so that the force is known.

Structures that are a few hundred metres in height may be able to be excited by shakers that are all above the waterline. This will have the advantage of reducing the cost of their installation. In these instances the excitation would be near the top of each of the main members and there will be more accelerometer groups than actuator groups. Taller structures, or structures with several gross changes in cross-sectional area down the main members, may need to be excited at some additional points below the waterline. These additional points of excitation are likely to be on the main members. With large members it will be difficult to excite compression waves with a single shaker. In principle this does not matter but it can be achieved by mounting three or more shakers at one cross-section at equal distances around the circumference. Each is excited in turn. The sum of the impedances formed from each in turn will be the impedance due to a compression wave alone. It would be feasible to excite all the shakers at one location together and in phase, so that the net result was that of an excitation along the centreline of the pipe.

The response and force points on the structure are reciprocal, so that impedances can equally well be obtained by reversing the force and response points. The implication is that it is possible to have the response points above the waterline, and use remotely operated vehicles (ROV) to apply forces underwater. The limitation in this approach is that the forces are less to be well controlled, which is a preferred aspect of the method.

The number of locations for sensor points will depend upon the design of the structure, but it is expected that only of the order of one tenth of the members will need to have sensors mounted on them. Numerical analysis is perfon-ned to decide on the required density of coverage of the structure with accelerometer groups to ensure defects can be located with the required accuracy.

At each location for measurement of response there will be a group of sensors chosen so as to easily separate out the bending and compression waves. For example, there could be four accelerometers diametrically opposite each other at one cross-section and measuring in an axial direction along the member. The addition of all four responses will subtract out the bending waves, leaving only the compression wave response. The subtraction of opposite accelerometers will remove the compression waves, leaving only the bending wave response in the two planes. The use of both types of wave is important because their response to a fracture will differ, which increases the information available when seeking to identify the location of the fracture.

The term "impedance spectrum" has been defined as the ratio of the Fourier transform of force input at one point to the Fourier transform of resulting acceleration measured at another point. If the resulting frequency domain impedance spectrum is transfon-ned back into the time domain the result is the response to a unit impulse. Any variation in magnitude, phase or frequency content of the input force excitation is removed by the operation. Either the unit impulse response can be obtained in repeated excitations, and the result averaged in time, or the frequency domain impedance spectrum from repeated -11excitations can be averaged in the frequency domain before transforming into the time domain. The consequence will be that random noise will be averaged out. The unit impulse response consists of the pulse that travels along the direct path from excitation to response, followed by a succession of pulses that travel by other paths. If one of the paths should be removed by complete severance of a member then the corresponding arrival pulse from that path will disappear. Equally, if the member is not severed but instead is fractured part way through then the corresponding arrival pulse will be reduced in magnitude, and there will be a new arrival pulse corresponding to the new path formed by reflection from the fracture and its time of arrival is then known.

The purpose of the structure is to support equipment whose weight is borne by the structure. If the mass of that equipment should vary significantly then the measured impedances can be altered. It is proposed to remove the effect of such changes in supported mass by installing additional response and actuator groups as close as possible to the junction between the supported mass and the main structure. These actuator and response groups form units that can be actively controlled, as described in "Active Control of Vibration" by C.R.Fuller, S.J.Elliot & P.A.Nelson, Academic Press 1996, to ensure that the motion in these response groups is held at zero. In doing so the impedance at these points is being constrained to be that of a perfect reflector and so the influence of the mass beyond the reflector is no longer observed in response units on the main structure. The active control can be applied in real time while other actuator groups are operated or else virtual active control can be applied. With virtual active control the -12additional actuator groups and additional response groups are operated separately in the same manner as all other actuator and response groups. It can then be calculated what drive signal at the additional actuator groups would hold the response at the additional response groups to zero. These additional responses would be added to the other measured responses.

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying diagrammatic Drawings in which:

Figure I shows how in a semi-infinite continuous homogeneous medium one subsurface sensor and two surface excitations can identify to position of a defect.

Figure 2 shows some example response measurements.

Figure 3 shows the overall structure standing in water with attachments of shaker groups and accelerometer groups.

Figure 4 shows the attachment of three electro-mechanical shakers I to the top of a main member of the structure.

Figure 5 shows the attachment of accelerometers 4 to one of the members of the structure.

Figure 6 shows the attachment of an additional actuator group 33 and additional response group 34 to a member 19 near the junction between the supported mass and the main structure.

To illustrate how detection of such changes in pulse arrivals at only a small number of points on the structure can determine the exact location of the defect, consider the method applied to a semi-infinite continuous medium such as the earth and illustrated in Figure I in which is shown the surface 6 of the earth 23. Two surface excitation points 5 and 32 -14and a single sub-surface sensor 9 can reduce the indeterminacy in the location of a defect 7 within the body to just two possible positions. A second sub-surface sensor or a third excitation point can then give a unique location of the defect. The damage point time of arrival is known and for one excitation point 5 and one response point 9 the damage must lie on an ellipse 8 formed with the excitation point 5 and response point 9 as foci and the path length known from the time of arrival. The location is then known to be on that ellipse. With a second excitation point 32 a second ellipse 30 can be drawn and the damage point 31 will lie where the two ellipses intersect. A real open lattice structure is more complicated having wrap-around symmetries and other complications but the principle remains the same.

The means whereby the complete severance of a member can be detected from the unit impulse response is illustrated in Figure 2 taken from tests on a scale model structure in which is shown the undamaged unit impulse response I I and the response 12 after a cut around fifty per cent of the perimeter, the response 13 after a cut around ninety per cent of the perimeter and response 14 after complete removal of the member on the path of the first direct arrival pulse. The first direct arrival pulse progressively reduces in magnitude and disappears when completely removed. The next arrival pulse is more complex due to a bending path and progressively changes as the damage is increased. The third arrival is a second direct path and progressively changes with increasing damage.

A specific embodiment of the invention is now described by way of example with reference to the accompanying explanatory sketches of the apparatus in operation shown in Figures 3, 4 and 5.

In Figure 3 is shown the overall structure standing in water 21 with attachments of shaker groups 15 and accelerometer groups 17. The shaker groups have coincident accelerometer groups. The top mass of the structure 16 is supported by the cross-braced structure. The structure has main legs 19, or members, which are usually the most significant members and close to vertical. There are usually horizontal members 18 and other diagonal crossbraces 20. There will usually be other horizontal members and cross-braces in the unrepresented third dimension of the Figure. The whole structure in this case is supported within the seabed 22. The shaker groups 15 are close to the top of the main members and above the waterline. The accelerometer groups 17 are shown mounted on the main members below every level of horizontal braces. The exact distribution of shaker groups and accelerometer groups will differ according to the exact geometry of the structure.

In Figure 4 is shown the attachment of three electro-mechanical shakers I to the top of a main member of the structure. In the Figure the shakers are inertial and so react against their own mass. They act on the main member through a stinger 3 pushing against a pad 2 on the member. Three shakers are shown equally disposed around the member so that equal inphase forces can be applied to generate a compressive wave in the member. At the point of input 2 of each force is also measured the response with an accelerometer 29.

In Figure 5 is shown the attachment of accelerometers 4 to one of the members of the structure. The sensors are shown in a group of four equally disposed around the circumference of the member. This arrangement allows the addition of all sensor responses to obtain the in-line component of acceleration and the difference of opposite pairs of sensors to obtain the rotational acceleration about two planes at right angles to each other.

The shakers in each actuator group are excited in turn by a sine wave that rapidly and steadily increases in frequency from a few hertz up to a few kilohertz. Such anexcitation is known as a chirp. The excitation may instead consist of a pulse whose width is of the order of a few milliseconds. The response to each excitation is recorded at all accelerometers. The excitation is repeated sufficient times to average out background noise and the repetition count is expected to be in the order of fifty or more. Every input force and every response is Fourier transforined. into the frequency domain. The impedance at every response point relative to every input force is formed as the ratio of input force to response. The impedances formed as a result of repeated excitations at the same point are averaged to reduce background noise.

The impedances are corrected for any change in supported mass 16 in the following manner. Additional actuator 33 and response 34 groups are provided at, or close to, the points of attachment of the support structure to the supported mass 16. The additional actuator groups 33 are driven at the same time as the actuator groups 15 in such a manner that there is no net motion at the additional response groups 34. Enforcing no net motion -17at the additional response groups 34 constrains the cross-sections at points of attachment of the support structure to the supported mass 16 to be perfect reflectors. Thus, since the points of attachment to the mass 16 are kept stationary, changes to the mass 16 can have no influence on the behaviour of the structure. By using directional response groups 33 at the points of attachment of the support structure to the supported mass 16 it is possible to bold the impedance of the cross-section at the point of attachment of the support structure to the supported mass 16 to be any other constant value, of which a perfect absorber would be one choice. Directional response groups have two groups of sensors in close proximity. This increases the amount of hardware and complexity of the system. Using directional additional support groups is not a preferred method.

It is not strictly necessary to drive the additional actuator groups 33 at the same time as the actuator groups 15. By operating each actuator in each additional actuator group 33 in the same manner as other actuator groups 15 and measuring the response in each sensor of each additional sensor group 34 in the same manner as other response groups 17, it may be determined what drive signals would have cancelled all motion at the additional response groups 34. By determining the motions produced at all other response groups 17 by these required additional drive signals and summing these with the motions produced by the actuator groups 15, the same result of active control can be achieved by postprocessing.

The impedances formed as a result of three or more excitations in a group on the same member at one cross-section are averaged to obtain the impedance due to a force input in- _18line with the main member. So the impedances formed at point 4 due to excitations at 1, 25 and 26 are averaged. The impedances formed at four response points at one crosssection are averaged to forin the impedance corresponding to a wave reaching that crosssection in-line with the member at the response group. We will call these the compressive impedances. So, for example, we average the impedances at points 4, 24, 27 and 28 to forin the compressive impedance at cross-section 4-24-27-28. The impedances of pairs of sensors in the same group that are opposite each other are subtracted to form the impedance due to bending waves reaching that cross-section. Respective responses from the other pair of sensors in the group are subtracted to form the bending wave impedance about the other axis. So, for example, the impedances at points 4 and 24 are subtracted to form the bending wave impedance about the line orthogonal to 4-24 and the impedances at points 27 and 28 are subtracted to form the bending wave impedance about the line orthogonal to 27-28.

The impedance spectra are Fourier transformed back into the time domain to form the unit impulse response functions. The time of arrival and magnitude of waves in the impulse response function are compared with corresponding quantities in the impulse response functions taken originally on the structure when it was presumed to be undamaged. The changes in these quantities in the various impulse response functions are used to detect the onset of failure and its location based on the known paths through the structure. Warning is given to the operator in the event that any failure is detected.

-19Subsequent to failure being detected and repaired a new set of reference undamaged impulse responses are taken from which to make later comparisons.

-20

Claims (18)

1. I Apparatus to detect the origin and growth of defects in a large structure, which Apparatus comprises: several electro-mechanical or mechanical actuators which can introduce force into the structure in the acoustic frequency range; on each actuator there is a load sensor to measure the force introduced into the structure; several accelerometer groups which can measure the bending wave and compression wave response at any cross-section of the structure; an electronic system to control the sequential action of the actuators and the time history of force they introduce; an electronic system to capture the measured forces and responses, and store these in an archive; a system to form the frequency domain impedance spectrum ratio of forces and responses; a system for correcting the impedance ratios for any changes in supported mass; a system to transforin the frequency domain impedance spectrum ratios into time domain unit impulse responses; a system to compare the latest set of unit impulse responses with a previously measured set and relate any changes between the two sets to the growth of defects on the structure; and a means of providing a warning to the operator of the existence of a potential failure of a member and the location of the potential failure point.
2. 2. Apparatus as claimed in claim I in which the force is input by piezoresistive or piezo-electric material.
3. 3. Method as claimed in any of the preceding claims in which the force is introduced at each loading point by actuators in a group so disposed that the resultant force is axial to the loaded member,
4. 4. Method as claimed in any of the preceding claims in which the force is input as a wide frequency band pulse.
5. 5. Method as claimed in any of claims I to 3 in which the force is input as a sinusoid whose frequency sweeps through the frequency range.
6. 6. Method as claimed in any of claims I to 3 in which the force is input as a random time signal.
7. 7. Method as claimed in any of claims I to 3 in which the force is input as a sequence of a few cycles of single frequency signals in the audio frequency range.
8. 8. Apparatus as claimed in any of the preceding claims in which the actuators are near the top of each vertical member.
9. 9. Apparatus as claimed in any of claims I to 7 in which actuators are used at general positions not limited to the top of the vertical members.
10. 10. Apparatus as claimed in any of the preceding claims in which the actuators are not pen-nanently installed but operated from a remote operated vehicle.
11. 11. Method as claimed in any of the preceding claims in which the response at chosen locations is measured by accelerometer groups so disposed that combinations of summation and subtraction of accelerometer responses separates out the axial compressional waves and the bending waves.
12. 12. Apparatus as claimed in any of the preceding claims in which there are more accelerometer groups than actuator groups
13. 13. Apparatus as claimed in any of claims I to 11 in which the positions of actuator groups and response accelerometer groups are reversed so that there are mom actuators than response accelerometer groups.
14. 14. Apparatus as claimed in any of the preceding claims in which the responses consist of strain measurements rather than acceleration measurements.
15. 15. Apparatus as claimed in any of the preceding claims in which additional actuator and response groups are installed close to the points of attachment of the support structure to the supported mass.
16. 16. A method as claimed in the preceding claim 15 in which the correction for changes in supported mass are made by effectively isolating the variable mass by actively controlling additional actuator groups placed between the supported mass and the main structure so as the make this cross-section behave as a perfect reflector.
17. 17. A method used to detect the origin, location and growth of defects in a large structure., in which Method: forcing means are distributed around the structure to introduce controlled forces into the structure; vibration measurement locations are distributed around the structure; forces are introduced into the structure in the acoustic range; the vibration response of bending and compression waves is stored for future recall together with the forces measured; the bending and compression wave impedance spectra between every shaker and every response point are calculated by taking the ratio of the Fourier transform of force input at one point to the Fourier transform of bending and compression vibration measured at the response point; the wave impedances are corrected if necessary for changes in the mass supported by the structure; the wave impedance spectra are converted into the time domain by an inverse Fourier Transform and referred to as unit impulse time responses; the various unit impulse time responses are compared with original values obtained on the intact structure and any changes are related to the size and location of defects in the structure; and warning is provided of any defects that are growing with time and an indication given of their location.
18. 18. A method used to detect the origin, location and growth of defects in a large structure as claimed in any of the preceding claims and substantially as described hereinbefore.