GB2543114A

GB2543114A - Determination of a physical condition of a pole-type structure

Info

Publication number: GB2543114A
Application number: GB1603756.6A
Authority: GB
Inventors: Edward Martin James; Stuart-Cole Simone; Kitching Alistair
Original assignee: Reece Innovation Centre Ltd
Current assignee: Reece Innovation Centre Ltd
Priority date: 2016-03-04
Filing date: 2016-03-04
Publication date: 2017-04-12
Also published as: GB201603756D0

Abstract

A system and method for determining a physical condition of a pole-type structure (which may be a bollard) is described. The system comprises: an impulsive source (for example, a hammer with in-built accelerometer) for emitting a first frequency signal into the pole-type structure; one or more sensors (for example, accelerometers), coupled to the pole-type structure for detecting a second frequency signal; and a data acquisition module, for processing the second frequency signal based on the first frequency signal. A sensor for determining a physical condition of a marine mooring bollard is also described.

Description

DETERMINATION OF A PHYSICAL CONDITION OF A POLE-TYPE STRUCTURE

Field of the invention

The present invention generally relates to a system and method for determining the condition of a pole-type structure. In particular the present invention relates to monitoring of a part of a structure that is not generally visible for inspection. It provides a means whereby a pole-type structure can be monitored and the presence of a poor state or poor condition of the pole-type structure can be determined.

Background of the invention

Whilst for centuries, ships and other seafaring vessels have moored up in harbours, on quaysides and in shipyards using marine bollards, until recently, very little attention has been paid to the integrity of these bollards or their surrounding supporting structures.

The expected lifetime of a mooring bollard is in excess of 40 years, but as the size and weight of commercial marine vessels has increased rapidly in recent decades, it is highly likely that older bollards are poorly equipped to withstand the loads regularly exerted upon them. As bollards age they are subject to corrosion, and mechanical fatigue and additionally, their foundations are often subject to erosion and scouring. Further, although newer bollards may be designed with these huge loads in mind, there are many factors involved in the installation of the bollards that, if not properly considered, may result in a defective mooring with a lower capacity than intended.

There have been a number of cases over the last decade of bollards failing whilst ships are moored to them, all of which have resulted in damage to the quayside and the vessel in question. Some of these incidents have resulted in fatalities, providing evidence that a bollard that is damaged, or that has a damaged foundation, presents a serious danger to quayside workers and civilians alike.

Any structure with foundations that cannot be easily observed has the potential to fail with catastrophic consequences. For example, in the marine industry mooring bollards can be torn from their base causing damage to marine vessels and injury or death to personnel. In the offshore wind industry the expense involved in recovering failed offshore wind turbines can be huge. In the construction industry buildings with defective foundations are liable to subside or be deemed not fit for purpose.

Existing techniques for testing the integrity of a mooring bollard rely on exerting a large force on a mooring bollard, which has the potential to be extremely damaging. The traditional method of testing the safety of a mooring bollard is to use a tugboat far out at sea to apply a huge load to the bollard via a towline. If the bollard is still secure after a maximum load has been applied it is deemed safe. Due to the time and cost involved in performing the procedure described above, it is rarely implemented and is certainly not suitable for performing routine condition assessments. WO20I5114380 Al discloses a Bollard Load Test System (BLT), a device that operates on the quayside, using a hydraulic ram and a torque rope to recreate typical pull loads on a bollard. The strength of the bollard is measured using a pressure transducer fitted to the hydraulic cylinder. Whilst this may be a more convenient and standardised test than the aforementioned tugboat test, the test is still potentially destructive by nature and as such, is unlikely to be adopted as standard by the industry.

However, there remains no adequate means of assessing or monitoring the condition of a bollard that has been adopted en masse by the global marine industry, and there is a real requirement for the development of a solution that is portable, cost effective and above all non-destructive. In order to address this issue, there is a growing interest within the marine industry in developing a technique that can reliably and accurately assess the condition of a bollard and its foundations in a non-destructive manner.

The present invention seeks to provide a means for monitoring the condition of a pole-type structure using a non-destructive technique. This invention has applications in a number of industries, in particular, applications are found in the marine industry for mooring bollard condition assessment; in the offshore wind industry for turbine body condition assessment; and the construction industry for building foundation assessment.

Summary of the invention

According to a first aspect there is provided a system for determining a physical condition of a pole-type structure, the system comprising an impulsive source for emitting a first frequency signal into the pole-type structure, one or more sensors coupled to the pole-type structure for detecting a second frequency signal, and a data acquisition module for processing the second frequency signal based on the first frequency signal.

According to a second aspect there is provided a system for determining a physical condition of a supporting structure of a pole-type structure, the system comprising an impulsive source for emitting a first frequency signal into the pole-type structure, one or more sensors coupled to the supporting structure of the pole-type structure for detecting a second frequency signal, and a data acquisition module for processing the second frequency signal based on the first frequency signal.

According to a third aspect there is provided a system for determining a physical condition of a pole-type structure and a supporting structure of the pole-type structure, the system comprising an impulsive source for emitting a first frequency signal into the pole-type structure, one or more sensors coupled to the pole-type structure and the supporting structure of the pole-type structure for detecting a second frequency signal, and a data acquisition module for processing the second frequency signal based on the first frequency signal.

The pole-type structure may be one of: a marine mooring bollard; an offshore wind turbine; and a building foundation.

The supporting structure may be a plinth and/or the ground upon which the pole-type structure is mounted.

The one or more sensors may be one of: a multi-axis sensor; an accelerometer; a velocity meter; a magnetometer; an inclinometer; and a multi-axis inclinometer.

The one or more sensors may be attachable to the pole-type structure via a magnetic coupling mechanism or a screw coupling mechanism.

The first sensor may be mounted on the base of the pole-type structure and a second sensor may be mounted on the support structure.

The one or more sensors may be permanently mounted.

The data acquisition module may be a handheld, portable device that is optionally arranged to operate using telemetry.

The impulse source may comprise an in-built accelerometer arranged for detecting the first frequency signal for processing by the data acquisition module.

The impulsive source may be mounted on or near the pole-type structure and may be one of: a calibrated hammer; and a calibrated shaker, that may or may not be calibrated.

The first and/or second frequency signal may be one of: a frequency swept signal; and a mono frequency signal.

The pole-type structure may be located at a quayside or on a marine vessel.

According to a fourth aspect there is provided a method for determining a physical condition of a pole-type structure and/or a supporting structure of the pole-type structure, the method comprising the steps of providing an impulsive source, for emitting a first frequency signal into the pole-type structure, providing one or more sensors coupled to the pole-type structure and/or a supporting structure of the pole-type structure for detecting a second frequency signal, and providing a data acquisition module for processing the second frequency signal based on the first frequency signal.

The method may determine the physical condition of the pole-type structure and/or the supporting structure using a non-destructive technique.

The processing of the second frequency signal based on the first frequency signal may determine a frequency response of the pole-type structure and/or the supporting structure via modal analysis and the physical condition may be assessed based on interpretation of the second frequency signal that is processed.

The processing of the second frequency signal based on the first frequency signal may comprise comparing amplitudes of the first frequency signal and second frequency signal and the physical condition may be assessed based on an amplitude spectra of the second frequency signal.

The physical condition may be determined at regular intervals and/or a time lapse evolution analysis may be performed.

The one or more sensors may be multi-axis sensors and the processing of the second frequency signal based on the first frequency signal may decomposes the second frequency signal into an arbitrary azimuthal direction.

The method according to the fourth aspect may use the system of any of the first, second or third aspects for assessing the integrity of any given structure that consists of one or multiple fixed and free ends and/or assess the integrity of surrounding foundations or fixings of the structure.

According to a fifth aspect there is provided a sensor for determining a physical condition of a marine mooring bollard.

According to a sixth aspect there is provided a system and/or method substantially as herein described with reference to, and as shown in, the accompanying drawings.

Further particular and preferred aspects of the present invention are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with the features of the independent claims as appropriate, and in combination other than those explicitly set out in the claims. Each aspect can be carried out independently of the other aspects or in combination with one or more of the other aspects.

Figures

The present invention will now be more particularly described, with reference to the accompanying drawings, in which:

Figure I is a schematic showing a typical bollard;

Figure 2 is a schematic of a plan view of a bollard showing the geometry of accelerometers during data acquisition;

Figure 3 is a schematic of a plan view of a bollard describing the x, y and z impact positions of the hammer;

Figure 4 illustrates raw data acquired from three accelerometers (1,2 and 3), placed around three bollards (A, B and C) for x-data acquired with an x-direction source impact;

Figure 5 illustrates a discontinuity between a bollard and accelerometer 2;

Figure 6 illustrates raw data acquired from three accelerometers (4, 5 and 6), placed on the base of three bollards (A, B and C) for x-data acquired with an x-direction source impact;

Figure 7 illustrates an air gap between the base of bollard C and the concrete plinth upon which it is mounted;

Figure 8 illustrates a frequency response function of each bollard (A, B and C) calculated using accelerometer 6 for x-direction data positioned on each bollard base; and

Figure 9 illustrates a frequency response function of each bollard (A, B and C) calculated using accelerometer 6 for x-direction data.

Description

The development of a system that could assess the condition of structures with foundations that are otherwise not easily observed could serve as a tool for personnel to assess the condition of the structures to prevent potentially catastrophic consequences, reduce costs and improve safety for personnel. As will be described below, the present invention incorporates the measurement of an impact force into a data processing stage when determining the condition of a pole-type structure such as a mooring bollard. The addition of this step into the process of analysing vibrational modes excited in a pole provides an accurate assessment of the condition of the pole and it’s foundation that is unaffected by an operator’s skill and experience. This provides for a more reliable determination of the condition of the pole-type structure.

Figure I shows a typical bollard structure, however it should be noted that bollards are available in a wide range of materials and designs. The bollard comprises a bollard base. The bollard and bollard base are mounted on the ground which may be a concreted quayside.

Two methods of performing condition assessment measurements on marine mooring bollards using acoustic techniques are described below. The first method relates to an active monitoring system and the second method relates to a passive monitoring system. Both methods involve placing one or more sensors on and/or around a bollard. The physical condition may be an integrity of the pole-type structure or its supporting structure on which the pole-type structure is mounted. The same system can be used to assess the integrity of the bollard foundation and/or fixings surrounding the bollard.

Method I: Active Monitoring Systems

An experimental set-up for performing condition assessment measurements on marine mooring bollards is described in Figure 2. In this example, three bollards were measured for demonstration purposes. The bollards were labelled A, B and C and were known to be from different eras of quayside construction. Bollard A was the oldest bollard with an estimated age of more than 40 years, bollard B was “middle aged” with an estimated age of less than 30 years, and bollard C was the youngest bollard with an estimated age of less than 5 years. During each test, three tri-axial accelerometers were coupled to the base of each bollard and three further tri-axial accelerometers were coupled to the ground surrounding the bollards.

Each accelerometer or sensor may be acoustically couple-able to the marine mooring bollard or the one or more accelerometers or sensors may be coupled to a bollard base. A first frequency signal may be an input signal from the impulsive source, for example the impulse imparted into the bollard. A second frequency signal may be an output signal to be detected by the one or more accelerometers or sensors. The method described herein comprises the step of analysing amplitudes of the second frequency signal that correspond to acoustic reflections between the mooring bollard, supporting structure such as the concrete plinth upon which the bollard is mounted, and the ground. Signal processing techniques allow for the calculation of the frequency response of the bollard using both the input signal and detected/recorded signal, allowing for a more accurate assessment of the frequency response of the bollard, whilst removing any inconsistencies between measurements due to varying impulse forces. The first frequency signal may be emitted in a pre-determined direction, e.g. x-direction. A calibrated hammer with an inbuilt accelerometer was used to impart an impulse through the bollard at three points on the base whereby the directions of impact were in the x, y and z direction respectively, as described schematically by Figure 3. For illustration purposes, the discussion herein will focus solely on the x data acquired from each accelerometer when the bollard was subject to an impact in the x direction. However, it should be understood that x, y or z data may be acquired from each accelerometer and each bollard may be subject to an impact in any of the x, y or z directions. In the example described in Figure 2, the plane perpendicular to the waterside is labelled x, the plane parallel to the waterside is labelled y and the plane perpendicular to the ground is labelled z.

The hammer with an inbuilt accelerometer that was used to impart an impulse through the bollard may be calibrated or uncalibrated. The hammer is used as an impulsive source. The impulsive source may comprise an in-built accelerometer. The impulsive source may be mounted on the bollard (as in this example) or on the bollard base or ground. The impulsive source may be a hammer or shaker. The bollard is an example of a pole-type structure and the bollard base or ground may be considered to be supporting structures for the bollard or pole-type structure.

The raw data acquired from accelerometers I, 2 and 3 and shown in Figure 2 are displayed in Figure 4 for each bollard A, B and C. The impulsive source is respectively mounted on each bollard to impart an impulse into each bollard. The detecting accelerometers are mounted on the ground surrounding each bollard. The accelerometers record the amplitude of acoustic waves in the x-direction and reaching the location of each accelerometer from the impulse through each bollard. In this configuration a high amplitude signal recorded by the accelerometers indicates a bollard having acceptable or solid foundations where the bollard is well fixed. In contrast, a low amplitude signal would indicate a discontinuity between the ground and the source of impact on the bollard for a bollard having poor foundations.

For each bollard, accelerometers 1, 2 and 3 may produce similar signals as the impact energy radiates through the bollard and into the surrounding concrete. Whilst this is the case for bollard A and bollard C, bollard B displays a far lower amplitude signal on accelerometer 2 than on accelerometers I and 3 (dash-boxed region). This would suggest some discontinuity in the ground lying between this sensor and the source of impact e.g. a crack, fracture or weakness. A photograph of bollard B captured with the data acquisition confirms the presence of a discontinuity between the bollard (at the source of impact) and accelerometer 2, Acc2 as displayed in Figure 5.

It may be reasonable to expect to observe differences in the data acquired from bollards A and C due to their large age difference. However, the condition assessment confirms that the foundations of each bollard A and C are of similar quality despite the difference in ages of the bollards.

Figure 6 displays the raw data acquired from accelerometers 4, 5 and 6 positioned on each bollard base A, B and C as shown in Figure 2. The impulsive source is respectively mounted on each bollard to impart an impulse into each bollard. The detecting accelerometers are mounted on the bollard base. The accelerometers record the amplitude of acoustic waves in the x-direction and reaching the location of each accelerometer from the impulse through each bollard. In this configuration a low amplitude signal indicates a bollard that is well fixed and having acceptable foundations.

It can be observed that bollards A and B display low amplitude signals across all three accelerometers, indicating that both are well fixed in position and thus exhibit very little movement upon impact. In contrast, bollard C produces a “ringing” type signal in accelerometers 4 and 6 indicating that there is a high degree of movement in the x direction which in turn, suggests that bollard C is poorly coupled to its foundation when compared to A and B (dash-boxed region). This result is somewhat counterintuitive as bollard C is the youngest and so it could be expected to have the most rigid coupling to its base of the three bollards A, B and C. Closer inspection of the features of bollard C reveals that there is an air gap between the base of the bollard and the concrete plinth upon which it is mounted and this air gap is displayed by the arrows in figure 7. The presence of this air gap confirms the assertion that bollard C is incorrectly mounted. Further, the amplitude of this “ringing” signal is higher on the signal recorded by accelerometer 6 than that recorded by accelerometer 4, suggesting that the air gap is bigger on the side of the bollard where accelerometer 6 is mounted.

Whilst the raw data acquired by the accelerometers is demonstrably rich in information regarding the structure and integrity of a mooring bollard, in order to develop the technique described into one that produces a fast and reliable result that can be easily interpreted by an untrained operator, it is necessary to develop a more quantitative method of assessing the data.

One way of doing this involves the determination of the resonance characteristics of the structure after the application of the impulsive force. This determination of the frequencies at which a structure resonates is known as “modal analysis”. A particularly useful technique for performing modal analysis on a given data set has been reported by Halvorsen and Brown [I] where the technique is not only based on exciting the structure with an impulse, but that actually utilises the impulsive input data during the analysis.

The complete mathematical approach is not reproduced herein, however it can be summarised by equation I, below:

Equation I where Guv(f) is the cross-spectrum between the input signal (hammer) and the output signal (accelerometer) and Guu(f) is the power spectrum of the input.

Figure 8 displays the frequency response function calculated, using equation I, from the data acquired from each bollard test (A, B and C). The data acquired from each bollard A B and C in the x-direction looks similar for all three bollards in the lower frequency regions (below 200 Hz). However, there are some clear differences in the data acquired from bollard C for higher frequencies (between 200 Hz and 1,000 Hz), most notably the presence of a high amplitude resonance peak at 264 Hz (dash-boxed region). This high amplitude resonance peak can be attibuted to the aforementioned poor coupling between bollard C and it’s foundation. Using the active monitoring system above in combination with the frequency response technique [I] provides a more clearly identifiable set of features that can indicate a mooring bollard in a poor or unacceptable state.

In an effort to demonstrate the reliability and repeatability of the described active monitoring approach, Figure 9 displays the frequency response spectra calculated using data acquired from three repeat measurements of each bollard A, B and C. The data acquired clearly show that the differences observed in the frequency response of bollard C are real and repeatable features. Thus, the technique described herein produces extremely reliable and accurate data, irrespective of variations in the hammer impact force.

More extensive data proccessing can be performed on the data acquired using the techniques described herein, whereby the x, y and z components of the data recorded by each accelerometer can be decomposed into an arbitrary azimuth such that weaknesses or deficiencies in the bollards foundations can be more specifically identified with regards to their location or directionality.

Bollards with a high degree of integrity and a strong coupling to their fixings produce notably different modal analysis results than those with poor structural integrity and poor coupling to their quayside fixings. The different features observed in the modal analysis results allow for the development of a measurement metric that enables the user to produce a quick and easy to interpret result that accurately describes the condition of the bollard and its ability to withstand the required loads.

The evolution of the structural integrity of a bollard and/or the condition of its foundations can be tracked using the techniques described above in conjunction with a time lapse analysis technique, whereby the data is acquired, assessed and compared at regular intervals affording the user the ability to chart the degradation of a bollard and predict when, for example, the bollard will require a downgrading of its maximum allowed load or when it will require full replacement As such, a given bollard can be assessed at regular intervals and time lapse evolution analysis can be performed, whereby successive data sets are compared in order to track the degradation of the bollard affording a prediction of its useable lifetime.

Method 2: Passive Monitoring Systems

An alternative embodiment of the active bollard integrity measurement system is a passive system whereby one or more dual axis inclinometers, with measurement axes parallel to the mounting plane and orthogonal to each other, are permanently mounted onto the bollard. Any axial inclination of said bollard, as a result of its normal use, is recorded. These data can be decomposed to provide the azimuth of the movement recorded and thus can be used to identify any weaknesses or defective areas in the foundations with regards to their location or directionality. This system, in conjunction with the aforementioned time lapse technique can be used to chart the deterioration of the bollard foundations or structural integrity. A device is also disclosed for determining a physical condition of a pole-type structure and/or a supporting structure of the pole-type structure, the device comprising a sensor coupled to the pole-type structure and/or a supporting structure of the pole-type structure for detecting a frequency response of the pole-type structure.

The present invention provides the advantages of a device and/or system that is portable, cost effective and capable of rapid and accurate condition assessment of a pole type structure without damaging the structure. Further advantages are provided since the condition assessment using the device and/or system of the present invention is independent of the operator’s skill and experience, since inconsistencies between measurements using varying impulse forces are removed. Signal processing techniques allowing for the calculation of the frequency response of the pole using both the input signal and measured signal, allowing for more accurate assessment of the frequency response and removing and inconsistencies between measurements due to varying impulse forces, i.e. the present invention incorporates measurement of an impact force into the data processing stage and hence does not rely on the skill and experience of an operator.

Whilst the invention has been described herein with reference to a marine mooring bollard, the present invention has applications in a number of industries. For example, the invention is applicable to the determining of a physical condition of other pole-type structures including, but not limited to turbine bodies in the offshore wind industry, and building foundation assessments in the construction industry.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiments shown and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

References [I] Halvorsen, W. G., Brown, D. L., Impulse Technique for Structural Frequency Response Testing; The Journal of the Acoustical Society of America; 05/1978; 63(SI).

Claims

1. A system for determining a physical condition of a pole-type structure, the system comprising: an impulsive source, for emitting a first frequency signal into the pole-type structure; one or more sensors, coupled to the pole-type structure for detecting a second frequency signal; and a data acquisition module, for processing the second frequency signal based on the first frequency signal.

2. A system for determining a physical condition of a supporting structure of a pole-type structure, the system comprising: an impulsive source, for emitting a first frequency signal into the pole-type structure; one or more sensors, coupled to the supporting structure of the pole-type structure for detecting a second frequency signal; and a data acquisition module, for processing the second frequency signal based on the first frequency signal.

3. A system for determining a physical condition of a pole-type structure and a supporting structure of the pole-type structure, the system comprising: an impulsive source, for emitting a first frequency signal into the pole-type structure; one or more sensors, coupled to the pole-type structure and the supporting structure of the pole-type structure for detecting a second frequency signal; and a data acquisition module, for processing the second frequency signal based on the first frequency signal.

4. A system according to any of claims I -3, wherein the pole-type structure is one of: a marine mooring bollard; an offshore wind turbine; and a building foundation.

5. A system according to any of claims 2-3, wherein the supporting structure is a plinth and/or the ground upon which the pole-type structure is mounted.

6. A system according to any preceding claim, wherein the one or more sensors are one of: a multi-axis sensor; an accelerometer; a velocity meter; a magnetometer; an inclinometer; and a multi-axis inclinometer.

7. A system according to any of claims I -3, wherein the one or more sensors are attachable to the pole-type structure via a magnetic coupling mechanism or a screw coupling mechanism.

8. A system according to any of claims 2-3, wherein a first sensor is mounted on the base of the pole-type structure and a second sensor is mounted on the support structure.

9. A system according to any preceding claim, wherein the one or more sensors are permanently mounted.

10. A system according to any preceding claim, wherein the data acquisition module is a handheld, portable device that is optionally arranged to operate using telemetry.

I I. A system according to any preceding claim, wherein the impulse source comprises an in-built accelerometer arranged for detecting the first frequency signal for processing by the data acquisition module.

12. A system according to any preceding claim, wherein the impulsive source is mounted on or near the pole-type structure and is one of: a calibrated hammer; and a calibrated shaker, that may or may not be calibrated.

13. A system according to any preceding claim, wherein the first and/or second frequency signal is one of: a frequency swept signal; and a mono frequency signal.

14. A system according to any preceding claim, wherein the pole-type structure is located at a quayside or on a marine vessel.

15. A method for determining a physical condition of a pole-type structure and/or a supporting structure of the pole-type structure, the method comprising the steps of: providing an impulsive source, for emitting a first frequency signal into the pole-type structure; providing one or more sensors, coupled to the pole-type structure and/or a supporting structure of the pole-type structure for detecting a second frequency signal; and providing a data acquisition module, for processing the second frequency signal based on the first frequency signal.

16. A method according to claim 15, wherein the method determines the physical condition of the pole-type structure and/or the supporting structure using a non-destructive technique.

17. A method according to claim 15, wherein the processing of the second frequency signal based on the first frequency signal determines a frequency response of the pole-type structure and/or the supporting structure via modal analysis and the physical condition is assessed based on interpretation of the second frequency signal that is processed.

18. A method according to claim 15, wherein processing the second frequency signal based on the first frequency signal comprises comparing amplitudes of the first frequency signal and second frequency signal and the physical condition is assessed based on an amplitude spectra of the second frequency signal.

19. A method according to claim 15, wherein the physical condition is determined at regular intervals and/or a time lapse evolution analysis is performed.

20. A method according to any of claims 15-19, wherein the one or more sensors are multi-axis sensors and the processing of the second frequency signal based on the first frequency signal decomposes the second frequency signal into an arbitrary azimuthal direction.

21. A method according to any of claims 15-19 using the system as claimed in any of claims 1-14 for assessing the integrity of any given structure that consists of one or multiple fixed and free ends and/or assess the integrity of surrounding foundations or fixings of the structure.

22. A sensor for determining a physical condition of a marine mooring bollard.

23. A system and/or method substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.