GB2549166A

GB2549166A - A marine mooring bollard integrity detection system and method

Info

Publication number: GB2549166A
Application number: GB1700884.8A
Authority: GB
Inventors: Stuart-Cole Simone; Edward Martin James
Original assignee: Reece Innovation Centre Ltd
Current assignee: Reece Innovation Centre Ltd
Priority date: 2016-01-18
Filing date: 2017-01-18
Publication date: 2017-10-11
Also published as: GB201700884D0; WO2017125730A1

Abstract

A system is disclosed for determining the integrity of a marine mooring bollard possibly located on a quayside or a marine vessel. It comprises one or more sensors which may be coupled to the bollard and/or a supporting structure such as a plinth. The system may further comprise an impulsive source, for emitting a first frequency signal into the bollard, and a data acquisition module, for processing a second frequency signal based on the first signal. A frequency response may thereby be determined via modal analysis, or the amplitudes of the signals may be compared. A method is also disclosed, comprising providing the above devices in order to determine the integrity of the bollard. The impulsive source may be a calibrated hammer, a shaker, or a tow line of a vessel under heave, and may comprise an in-built accelerometer. The sensor(s) may be attached with a screw coupling mechanism, and may be one of: a multi-axis sensor, an accelerometer, a velocity meter, a magnetometer, an inclinometer, and a multi-axis inclinometer. A method is also disclosed for determining the integrity of the bollard, comprising the step of mounting one or more sensors.

Description

A MARINE MOORING BOLLARD INTEGRITY DETECTION SYSTEM AND METHOD

FIELD OF THE INVENTION

The present invention generally relates to a system and method for determining the integrity or condition of a marine mooring bollard. The present invention relates to monitoring of a part of a structure that may not generally be visible for inspection. It provides a means whereby a marine mooring bollard or other 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

Despite the development of many new materials since the installation of the original utilities network in the mid-19th century, timber is still the global material of choice for telegraph poles due to its abundance, cost effectiveness, ease of handling, insulating properties and low environmental impact. Whilst there are many timber treatment options available that serve to water-proof a wooden structure and therefore extend its lifetime, as a biological material, wood will still eventually be subject to decay due to rot, fungi and insects. After onset, rot or decay can advance rapidly inside a wooden structure, often with no visible external indicators, and, due to increased moisture levels, is particularly prevalent at, or below, ground level. As a result, inspection personnel are often unable to diagnose its presence and so unable to predict the inevitable failure of the structure.

Whilst modern telegraph poles have an average life expectancy in excess of 50 years, many factors, such as applied load, location specific ground conditions and varying timber quality can result in early onset decay and subsequent unexpected failure. Pole failure can cause serious network disruption, which is costly and can damage a company’s reputation in an industry that is already extremely competitive. More seriously, the high number of telegraph poles located in well-populated areas means that the risk to human life as a result of pole failure is great. For these reasons, utility companies are keen to employ techniques that afford an operator an accurate assessment of the condition of a pole. One of the techniques currently commonly employed is a simple acoustic test method, whereby an operator skilled in the art strikes the pole with a hammer and listens to the sound of the impact. Based on past experience, the operator judges whether the pole has been subject to decay and if so, to what degree. The operator is subsequently responsible for advising whether the pole is fit for purpose or if it needs replacing. This technique is estimated to provide the correct result approximately sixty five percent of the time and as such, incidents resulting from defective poles having been deemed safe are not uncommon. Furthermore, the cost of replacing healthy poles due to their having been incorrectly diagnosed as defective are huge, with utility companies needlessly spending millions of pounds per year.

In order to address the issues outlined above, in recent years there have been huge efforts within the utility industry to develop techniques that can accurately assess the condition of a telegraph pole. Existing techniques can assess the condition of a structure, such as a wooden utility pole, fixed in the ground at one end, however these rely on operator experience. These techniques range from: drilling into the pole foundation, where the resistance of the pole to the drill is used to estimate the amount of decay present; to ultrasonic methods, where ultrasound waves are used to produce a visual image of the inside of the pole. These analysis techniques may predict the health of the structure, however, many of the existing techniques are inaccurate and more importantly, most are destructive to the pole itself, even if in only a minor way. To the best of the Applicant’s knowledge none of the commercial systems available are independent of operator skill and experience. As such there remains no adequate means of assessing or monitoring the condition of a telegraph pole that has been adopted en masse by the global utilities industry. The development of a device that is portable, cost effective and capable of a rapid and accurate condition assessment without damaging the structure and which is independent of the operator’s skill and experience would have applications in a number of industries.

In the utilities industries telegraph poles, used by utility companies to support their extensive overhead cable networks, are predominantly made of timber. The timber telegraph poles are subject to rot and decay, particularly at, or below, ground level rendering most pole degradation invisible to inspection personnel. Failure of the poles can lead to injury or loss of life, network disruption and expensive asset damage. The development of a system and/or tool that could reliably and repeatably assess the condition of telegraph poles at, or below, ground level could serve as a tool for utilities industry personnel to effectively check the condition of the telegraph poles to improve safety to personnel, minimise network disruption and reduce costs associated with asset damage.

Any structure with foundations that cannot be easily observed has the potential to fail with catastrophic consequences, whether in the utility industry, marine industry, offshore wind industry or the construction industry. 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. 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.

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.

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. W020I5II4380 AI 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

The present invention generally relates to a system and method for determining the condition of a pole-type structure. The present invention relates to monitoring of a part of a structure that may not generally be 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.

According to an example 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.

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

The impulsive source may be one of: a calibrated hammer; and a calibrated shaker. The calibrated hammer or calibrated shaker may further comprise an in-built accelerometer. The in-built accelerometer may be arranged for detecting the first frequency signal for processing by the data acquisition module.

The first frequency signal may be an input signal, and the second frequency signal may be an output signal to be detected by the one or more sensors.

The one or more sensors may be one of: one or more accelerometers; one or more velocity meters; and one or more magnetometers.

The one or more sensors may be mounted within a detachable collar. The detachable collar may further be adjustable such that the collar is attachable to pole-type structures of varying diameters.

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

In some embodiments a first sensor is mounted at an exposed base of the pole-type structure and a second sensor is mounted towards an exposed distal end of the pole-type structure and longitudinally aligned with the first sensor. The second sensor may be mounted at a distance of two meters from the first sensor.

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

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

The data acquisition module may be arranged for analysing vibrational modes that are excited in the pole-type structure.

The pole-type structure may comprise one or more fixed ends and/or one or more free ends.

According to an example there is provided a method for determining a physical condition of a 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 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 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. Further, the frequency response may be calculated via modal analysis.

The physical condition of the pole-type structure determined may correspond to a part of the structure that is at, or below, ground level or forms part of a foundation of the structure.

The method may further comprise the step of analysing amplitudes of the second frequency signal that

The physical condition of the pole-type structure may be determined at regular intervals and/or a time lapse evolution analysis may be performed.

The method may comprise the steps of providing a first sensor mounted at an exposed base of the pole-type structure and providing a second sensor mounted at a distance of two meters from the first sensor and towards an exposed distal end of the pole-type structure and longitudinally aligned with the first sensor.

The method may comprise processing the second frequency signal based on the first frequency signal by comparing amplitudes of the first frequency signal and second frequency signal.

According to an example there is provided a method and/or system substantially as herein described with reference to, and as shown in, Figures 1-7.

According to an example there is provided a device for determining a physical condition of a pole-type structure, the device comprising a sensor coupled to the pole-type structure for detecting a frequency response of the pole-type structure.

The device may be mounted within a detachable collar. The detachable collar may be adjustable such that the collar is attachable to pole-type structures of varying diameters.

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

According to an example there is provided a pole condition sensor for determining a physical condition of a pole.

The pole condition sensor may be acoustically couple-able to the pole.

According to an example there is provided a calibrated hammer for impact testing comprising a holding device and an impact hammer, wherein the holding device comprises a pivot mechanism and an adjustable clamp to secure the impact hammer to the holding device.

The holding device and/or impact hammer may comprise a spirit level.

The holding device may further comprise a frame and the impact hammer may comprise a shaft which is secured to the frame of the holding device.

The frame of the holding device may comprise two or more tangs.

The adjustable clamp may be moveable within slots of the holding device.

The calibrated hammer may further comprise angular displacement gauges and/or stops.

According to an aspect there is provided a marine mooring bollard integrity detection system, for determining the integrity of the marine mooring bollard, the system comprising one or more sensors.

The system may comprise a data acquisition module, for processing a second frequency signal based on a first frequency signal, wherein the one or more sensors are coupled to the marine mooring bollard for detecting the second frequency signal.

The system may comprise an impulsive source, for emitting a first frequency signal into the marine mooring bollard. A first sensor may be mounted on the marine mooring bollard and a second sensor may be mounted on a supporting structure of the marine mooring bollard.

The supporting structure may be a plinth and/or the ground upon which the marine mooring bollard is mounted.

The one or more sensors may be coupled to the supporting structure and the integrity may be determined for the supporting structure.

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 via a magnetic coupling mechanism or a screw coupling mechanism.

The one or more sensors may be permanently mounted.

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 marine mooring bollard and may be one of: a calibrated hammer; a shaker, that may or may not be calibrated; and a tow line of a vessel under heave.

The marine mooring bollard may be located at a quayside or on a marine vessel.

According to an aspect there is provided a method for determining integrity of a marine mooring bollard and/or a supporting structure of the marine mooring bollard, the method comprising the steps of providing an impulsive source, for emitting a first frequency signal into the marine mooring bollard, providing one or more sensors, coupled to the marine mooring bollard and/or a supporting structure of the marine mooring bollard 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 integrity of the marine mooring bollard 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 marine mooring bollard and/or the supporting structure via modal analysis and the integrity 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 integrity may be assessed based on an amplitude spectra of the second frequency signal.

The integrity 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 decompose the second frequency signal into an arbitrary azimuthal direction.

The method may use the system 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 an aspect there is provided a method for determining the integrity of the marine mooring bollard comprising the step of mounting one or more sensors.

The method may further comprise the step of attaching a tow line from a vessel to the marine mooring bollard and using a heave factor to determine the integrity of the marine mooring bollard.

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.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present disclosure will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example only, features of the present disclosure, and wherein:

Figure I is a schematic showing a field experiment designed to assess the condition of a telegraph pole according to an embodiment of the invention;

Figure 2 illustrates raw data acquired from; the hammer at the point of impact and from both accelerometers a and b shown in Figure I;

Figure 3 illustrates a frequency response of a telegraph pole that was calculated using data acquired from accelerometer b shown in Figure I at a distance of 1.5 m from the ground;

Figure 4 illustrates resonant frequency estimates for a wooden telegraph pole;

Figure 5 illustrates frequency responses calculated using data acquired from accelerometer a shown in Figure I for each of the different measurement heights shown;

Figure 6 illustrates data acquired from each repeat measurement from accelerometer a shown in Figure I;

Figure 7 is a schematic showing a field experiment designed to assess the condition of a telegraph pole according to a further embodiment of the invention;

Figures 8A-E illustrate a holding device for an impact hammer in a first operating position;

Figures 9A-C illustrate the holding device with the impact hammer in the first operating position attached to a telegraph pole;

Figures I0A-E illustrate the holding device for an impact hammer in a second operating position;

Figures I IA-C illustrate the holding device with the impact hammer in the second operating position attached to a telegraph pole;

Figure 12 is a schematic showing a typical bollard;

Figure 13 is a schematic of a plan view of a bollard showing the geometry of accelerometers during data acquisition;

Figure 14 is a schematic of a plan view of a bollard describing the x, y and z impact positions of the hammer;

Figure 15 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 16 illustrates a discontinuity between a bollard and accelerometer 2;

Figure 17 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 18 illustrates an air gap between the base of bollard C and the concrete plinth upon which it is mounted;

Figure 19 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;

Figure 20 illustrates a frequency response function of each bollard (A, B and C) calculated using accelerometer 6 for x-direction data; and

Figure 21 illustrates a mooring bollard under heaving.

DETAILED DESCRIPTION

Example embodiments are described below in sufficient detail to enable those of ordinary skill in the art to embody and implement the systems and processes herein described. It is important to understand that embodiments can be provided in many alternate forms and should not be construed as limited to the examples set forth herein.

Accordingly, while embodiments can be modified in various ways and take on various alternative forms, specific embodiments thereof are shown in the drawings and described in detail below as examples. There is no intent to limit to the particular forms disclosed and as well as individual embodiments the invention is intended to cover combinations of those embodiments as well. On the contrary, all modifications, equivalents, and alternatives falling within the scope of the appended claims should be included. Elements of the example embodiments are consistently denoted by the same reference numerals throughout the drawings and detailed description where appropriate.

The terminology used herein to describe embodiments is not intended to limit the scope. The articles “a,” “an,” and “the” are singular in that they have a single referent, however the use of the singular form in the present document should not preclude the presence of more than one referent. In other words, elements referred to in the singular can number one or more, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, items, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, items, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein are to be interpreted as is customary in the art. It will be further understood that terms in common usage should also be interpreted as is customary in the relevant art and not in an idealized or overly formal sense unless expressly so defined herein.

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 telegraph pole. 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 an embodiment of the invention, as an experimental set-up of a field experiment designed to assess the condition of a telegraph pole. Two accelerometers a and b were coupled to a 6 meter (m) high telegraph pole at a height of 1.75 m and 2 m from the ground respectively. A calibrated hammer with an inbuilt accelerometer was used to impart an impulse through the pole at varying distances from the accelerometers, shown as impact points.

Data was acquired from impact /excitation points at: 0.25 m, 0.5 m, 0.75 m, I m, 1.25 m and 1.5 m and 2.25 m above the ground. Three repeat experiments were performed at each measurement position and three poles across three different locations were assessed (poles 1-3). The data acquired from the calibrated hammer and from the two sensor accelerometers (a) and (b) are displayed in Figure 2, where the data is plotted as a function of arrival time. The hammer acts as the impulsive source and is used for emitting a first frequency signal into the pole. The two sensor accelerometers (a) and (b) are coupled to the pole and used to detect the reflected signal or second frequency signal from the pole.

The data acquired from the hammer (top) shows a clean, narrow band impulse as one would expect from a hammer impact. The accelerometer (bottom) data show two clear events measured by both accelerometers (a) and (b) with a clear lag in arrival time between the two accelerometers observed for both. The arrival time interval between the two accelerometers for event I was 0.3 milliseconds (ms) corresponding to a wave speed of 833 meters per second (m/s) in the pole, and for event 2 was 0.2 ms, corresponding to a wave speed of 1250 m/s in the pole. Taking the average of these values, the speed at which the impulse travels through the pole can be estimated to be approximately 1040 m/s, a value that agrees well with the speed of sound in wood as reported in the literature. The time interval between event I and event 2 was recorded as 8.0 ms for accelerometer (a) and 8.6 ms for accelerometer (b), corresponding to distances of 8.3 m and 8.9 m respectively, suggesting that event 2 is a reflection off the top of the pole. The arrival of event 2 at accelerometer (a) before arriving at accelerometer (b) supports this assertion. A strong reflection from the top of the pole implies that the pole is not subject to decay and has a strong foundation, and it is the presence of this reflection that acts as a first and rapid indicator that the pole in question is fixed at one end. This reflection can be further utilised in that its amplitude can be related directly to the degree to which the pole is fixed, i.e. a high amplitude is indicative of a well fixed pole whilst a lower amplitude would suggest some degree of movement at the apparently fixed end. The evolution of the condition of a pole’s foundations can be tracked using this relationship in conjunction with a time lapse analysis technique, whereby the data is assessed at regular intervals and the changing amplitude of this reflected signal is used to track the changing conditions. As such, the foundation can be assessed quickly from interpretation of the amplitude of the reflections observed from each end of the pole, i.e. top and base reflections and the degradation of the pole can be tracked. A second, more quantitative method of assessing the pole’s condition is to determine 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”, and whilst there are a number of methods reported in the literature for performing modal analysis on a given data set, a particularly useful technique was reported by Halvorsen and Brown (') which is not only based on exciting the structure with an impulse, as described above, but that actually utilises the impulsive input data during the analysis.

The complete mathematical approach is reported in the literature ('), 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 3 displays the frequency response calculated for the pole, using equation I, from the data acquired from pole 3, accelerometer b, with the hammer impact/excitation occurring at a distance of 1.5 m above the ground, i.e. 0.25 m from accelerometer b.

This frequency response data can be compared to the expected resonant frequencies for a bar fixed at one end and free at the other end, which can be approximated using the equations:

Equation 2 Equation 3

Where n is the vibrational mode of interest, a is the bar thickness, Y is the Young’s Modulus, L is the bar length and D is the density of the material.

Figure 4 displays a plot of an estimate of the first fourteen harmonic frequencies for a wooden telegraph pole 3, if fixed at one end, as calculated using equations 2 and 3.

If the frequency response spectrum of Figure 3 is compared with the expected harmonic frequencies for pole 3 as displayed in Figure 4, it is observed that the peak values agree well with the theoretically predicted values, suggesting that pole 3 is well fixed at its foundation, and therefore lending support to the conclusion drawn previously based on the reflected signal observed. It should be noted that these predicted harmonics (Figure 4) are based on estimated parameters only and that as a result, there are likely to be small inaccuracies in the data. It is assumed that with correct parameter data the correlation between the measured and predicted natural frequencies will be even greater.

Having established that the measurement technique and signal processing algorithms are effective, the geometry of the system components can be optimised. Figure 5 displays a comparison of the frequency response functions calculated using the measurements acquired from each experiment, where the point of impact/excitation varies between experiments.

Whilst all the frequency response spectra are highly similar in nature, there are some discrepancies between them that suggest an optimal measurement geometry exists. The peak at approximately 3.2

Hz, which corresponds to the first harmonic as per Figure 4, has better definition when the impact point is within close range of the accelerometer position. An impact point of 1.25 m, 1.5 m and 2.25 m produces a markedly better amplitude for this first harmonic than the impact points that lie further from the accelerometer. Of these three data sets, the data acquired when the point of impact was 2.25 m, i.e. above the accelerometers, begins to differ from the other spectra at higher frequencies and for this reason, this measurement position has been deemed unacceptable. The data in Figure 5 leads to the conclusion that that point of impact must lie below the accelerometer and within 0.5 m of it.

The result is a repeatable experimental procedure that can be performed with ease due to there being no requirement to climb the pole or to ensure that the point of impact is an exact distance from the collar containing the sensors. The sensors may be located within a detachable collar that is adjustable such that the collar can be attached to poles of varying diameters and thereby ensuring that the sensors are well coupled to the pole.

One of the benefits of using this particular experimental approach, where the input signal is recorded and incorporated into the data processing stage, is that the resultant data are always specific only to the pole, and that the impact excitation, i.e. the force of the hammer swing, is accounted for and therefore has no bearing on the resultant data. As such, the operator performing the measurement, irrespective of height, strength, or any other physical qualities, will have no bearing on the quality of the resultant data.

In an effort to further demonstrate the repeatability of this experimental approach, Figure 6 displays the frequency response spectra calculated using data acquired from pole I, accelerometer a, over three different repeats. The data clearly show that the technique described above produce extremely repeatable data and support the assertion that the characteristics of the impact/excitation have no bearing on the quality of the resultant frequency spectra.

Figure 7 shows a further embodiment of the invention comprising an additional single or vertically separated pair of accelerometers (or velocity sensors) that are mounted near to the base of the exposed utility pole above ground level, and oriented in-line longitudinally with the sensors previously described (with reference to Figure I) and located typically around 2m above ground level. The amplitude of the accelerometer signals near to the base of the pole is compared with the amplitude of the accelerometer signals around 2m above the ground.

If the pole is intact and has a robust subsurface structure within its foundation, then the base of the pole will be fairly rigidly held by its mechanical constraints. If the base of the pole or the pole foundation is damaged, then the base of the pole will be more free to move. The amplitude of the accelerometers located at the pole’s base will be higher for a decayed pole and foundation than for the case where the pole is intact.

The accelerometers shown in Figure 7 can be used to separate up and down going acoustic waveforms. The amplitude of the up going signal that has reflected from the base of the pole and its foundation can be used to determine the reflection coefficient from the base of the pole. A high reflection amplitude from the base of the pole is indicates that the wood is structurally sound, while a weak reflection would indicate decay in the wood.

The acoustic signal from the impact hammer propagates to the top of the utility pole. Some of the energy is reflected from the top of the utility pole back down the pole. Some of the energy reaching the top of the pole, however, couples into the cables at the top of the pole and excites an elastic wave in the cables which propagates to the adjacent utility poles where it is reflected back to the instrumented pole, where it couples back into the pole and travels down the pole to be detected by the mounted sensors. The delay of this signal can be used to evaluate the tension in the cables attached to the utility pole.

The impulsive source may be a calibrated source that can deliver a calibrated blow. For example, this may be achieved using a hammer on a pendulum, a pneumatic hammer that is actuated or fired using compressed air, or via other means such as using a spring. In all cases the calibrated source can deliver an impact force with high repeatability.

The impulse imparted by a calibrated hammer may be controlled via the use of a holding device, designed to release the hammer in such a way that each impact will be fully repeatable. A holding device with an impact hammer is shown throughout Figures 8-11. Ensuring that the impulse imparted is identical in every case allows direct comparisons to be made between different structures. The holding device comprises; a frame with a handle, a pivot mechanism and an adjustable clamp/assembly to secure the hammer handle and/or shaft to the main frame/handle. Figure 8A shows a side view of the holding device and impact hammer in a first operating position. Figure 8B shows a view from the back of the holding device and hammer. Figure 8C shows an angled view from the back of the holding device and hammer in the first operating position. Figure 8D shows a view from the top of the device from the handle of the holding device down towards the impact hammer. Figure 8E shows a view from the bottom of the device from the impact hammer up towards the handle of the holding device.

Figures 9A-C illustrate the holding device with the impact hammer in the first operating position attached to a telegraph pole. The frame can either be handheld against the structure to be tested, or alternatively it can be strapped to the structure to be tested by means of a suitable banding device. The frame comprises one or more tangs or points (not dissimilar to the points on crampons) - in the example of Figures 9A-C, four points/tangs/points are utilised. These points ensure that the hammer / frame assembly will not move in any orientation when in use.

The pivot between the frame and the hammer clamp uses suitable bearings to minimise static and dynamic friction. The pivot axis is adjustable, via for example wing-nuts, such that the pivot axis can be moved further towards or further away from the structure to be tested - this allows for different diameter structures to be tested with excellent repeatability. The pivot axis sits in slots that allow the impact hammer to be translated or adjusted before being clamped into position for the impact testing. This allows the user to set and ensure that the hammer head itself, when at rest in the first position, is completely normal to the structure impact point, i.e. when the face of the hammer impacts the structure there is zero angle between the face of the hammer and the structure.

To further ensure that the frame is correctly mounted it has the addition of one or more spirit levels (as shown in Figure 8D) - these allow the frame to be set to be true to the structure or the Earth if they are different. The pivotable impact hammer also has spirit levels attached (as showing in Figure 9A) - this ensures that the hammer can be released from a 90 degree to the structure orientation with great repeatability. Figures Ι0Α-Ε illustrate the holding device for an impact hammer in a second operating position in an active positon for impact testing. Angular displacement gauges and/or stops can also be fitted to allow the hammer to be released from different angles as required - again with great repeatability. Figures I IA-C illustrate the holding device with the impact hammer in the second operating position attached to a telegraph pole.

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.

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 12 shows atypical 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 13. 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 14. 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 I 3, 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 13 are displayed in Figure 15 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 16.

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 17 displays the raw data acquired from accelerometers 4, 5 and 6 positioned on each bollard base A, B and C as shown in Figure 13. 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 18. 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 4, below:

Equation 4

Figure 19 displays the frequency response function calculated, using equation 4, 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 20 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.

Figure 21 illustrates a mooring bollard under heaving. A heave test may be carried out for a moored vessel whilst putting heave stresses on the mooring bollard. As an alternative to, or addition to, the hammer impact testing described herein, the heave stresses exerted by a moored vessel upon the mooring bollard may be used to indicate a structural integrity of the mooring bollard. An accelerometer may be attached to the mooring bollard to indicate a structural integrity of the mooring bollard, where movement of the mooring bollard beyond an acceptable threshold indicates a weakness in the mooring bollard. An alerting mechanism or alarm may be attached to the mooring bollard and/or accelerometer that alerts dock personnel if movement of the mooring bollard is outside an acceptable threshold. For example, an alarm may sound when the accelerometer readings extend beyond a threshold level to warn dock personnel of the potential dangerous situation on the quayside. 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.

For the purposes of this description a marine mooring bollard is considered to be an example of a pole-type structure. Whilst the invention has been described herein with reference to a utility pole or telegraph pole and 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. For example, this may include building structures such as a wall or brick wall, concrete or reinforced concrete. Condition monitoring may be achieved over a period of time providing a time-lapse indication of the condition of the structure.

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 marine mooring bollard integrity detection system, for determining the integrity of the marine mooring bollard, the system comprising one or more sensors.

2. A system as claimed in claim I, comprising a data acquisition module, for processing a second frequency signal based on a first frequency signal, wherein the one or more sensors are coupled to the marine mooring bollard for detecting the second frequency signal.

3. A system as claimed in claim I or claim 2, comprising an impulsive source, for emitting a first frequency signal into the marine mooring bollard.

4. A system according to any preceding claim, wherein a first sensor is mounted on the marine mooring bollard and a second sensor is mounted on a supporting structure of the marine mooring bollard.

5. A system according to claim 4, wherein the supporting structure is a plinth and/or the ground upon which the marine mooring bollard is mounted.

6. A system as claimed in claim 4 or claim 5, wherein the one or more sensors are coupled to the supporting structure and the integrity is determined for the supporting structure.

7. 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 multiaxis inclinometer.

8. A system according to any preceding claim, wherein the one or more sensors are attachable via a magnetic coupling mechanism or a screw coupling mechanism.

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

10. A system according to any of claims 2-9, 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 of claims 3-10, 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 of claims 3-11, wherein the impulsive source is mounted on or near the marine mooring bollard and is one of: a calibrated hammer; a shaker, that may or may not be calibrated; and a tow line of a vessel under heave.

13. A system according to any of claims 2-12, 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 marine mooring bollard is located at a quayside or on a marine vessel.

15. A method for determining integrity of a marine mooring bollard and/or a supporting structure of the marine mooring bollard, the method comprising the steps of: providing an impulsive source, for emitting a first frequency signal into the marine mooring bollard; providing one or more sensors, coupled to the marine mooring bollard and/or a supporting structure of the marine mooring bollard 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 integrity of the marine mooring bollard and/or the supporting structure using a non-destructive technique.

17. A method according to claim 15 or claim 16, wherein the processing of the second frequency signal based on the first frequency signal determines a frequency response of the marine mooring bollard and/or the supporting structure via modal analysis and the integrity is assessed based on interpretation of the second frequency signal that is processed.

18. A method according to any of claims 15-17, 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 integrity is assessed based on an amplitude spectra of the second frequency signal.

19. A method according to any of claims 15-18, wherein the integrity 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 15-20 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 method for determining the integrity of the marine mooring bollard comprising the step of mounting one or more sensors.

23. A method according to claim 22, further comprising the step of attaching a tow line from a vessel to the marine mooring bollard and using a heave factor to determine the integrity of the marine mooring bollard.