WO2025008037A1

WO2025008037A1 - Inspection of bolted connections in a wind turbine

Info

Publication number: WO2025008037A1
Application number: PCT/DK2024/050158
Authority: WO
Inventors: David ÖSTERBERG; Pedro Manuel LIMA CARNEIRO CAVALEIRO
Original assignee: Vestas Wind Systems AS
Current assignee: Vestas Wind Systems AS
Priority date: 2023-07-04
Filing date: 2024-07-02
Publication date: 2025-01-09
Anticipated expiration: 2026-01-04

Abstract

A method and system for scheduling inspection of a bolted connection on a wind turbine, the bolted connection comprising a bolt under tension. At a first sample time, a first tension parameter of the bolt is determined, having an associated upper uncertainty bound, and, at a second sample time, a second tension parameter of the bolt is determined, having an associated lower uncertainty bound. Thereafter, a linear relationship is determined based on the upper uncertainty bound of the first tension parameter and the lower uncertainty bound of the second tension parameter. Based on that linear relationship, a third sample time is determined in the future, at which a further tension parameter is determined. Based on the third sample time, an inspection event is scheduled. A benefit of the invention is that inspection events are scheduled in an adaptive manner in a way that is responsive to the difference in values of those measurements, but which takes account of the inherent uncertainty in such measurements without compromising safety. In this way, inspection events can be scheduled less frequently that is currently the case which presents a much- reduced maintenance cost which still ensuring the integrity of the bolted connection.

Description

INSPECTION OF BOLTED CONNECTIONS IN A WIND TURBINE

Technical Field

This disclosure relates to systems, apparatus and methods adapted to schedule inspections of bolted connections in wind turbines.

Background

Wind turbines are complex structures that require regular maintenance and inspection to ensure their safe and efficient operation. One critical aspect of wind turbine maintenance is the inspection and maintenance of bolted connections which are used to join various components of the wind turbine together. For example, typically bolted connections are used at the flanged interface between a foundation of a wind turbine and the lowermost tower section in order to provide a secure connection of the wind turbine to the foundation. Bolted connections also feature in the connection between a hub and the blades of a wind turbine, and also at the interface between tubular tower sections of the wind turbine. Such connection arrangements are known to the skilled person.

To achieve a high-strength connection, it is known to pre-stress each bolt in the bolted connection. This may be achieved by using a machine to elongate the bolt. This technique achieves a particularly strong bolted connection which may be suitable in bolted connections between the foundation and the wind turbine tower, for example, n

It is also known that bolted connections can ‘relax’ over time due to settling in mating surfaces, material creep, and turning back of nuts, in the event nuts are used, which reduces the tension in the bolted connection. For this reason, organizations responsible for maintaining wind turbine installations implement set schedules for checking the tension of bolted connections to ensure that the connection is sound. If one of more bolts in the bolted connection is found to be out of tension tolerance, action can be taken to retension that bolt to ensure the performance of the bolted connection. This is typically done by re-applying the tension using the same type of tools as used in installation.

Tension checking of bolted connections is a time consuming and physically demanding maintenance task, particularly when considering the number of bolted connections in a single wind turbine and factoring in the number of wind turbines in a typical wind farm, which may number in the tens to the hundreds.

One known approach is to provide indicia or markings on bolted connections so that visual inspection can reveal whether nuts have moved relative to the bolt which will reduce tension in the bolt. A wrench can be used to re-torque the bolt. However, the rotational position of the nut does not accurately reflect the tension within the bolt.

An approach proposed by EP3040701 is to use an ultrasonic system to test tension in bolted connections, whereby the system is a permanent installation for ongoing monitoring. However, such a system could be very costly, and may be prone to failures, thereby requiring further diagnostic action. Furthermore, ultrasonic measurement systems are subject to uncertainty in their measurements.

A solution to reduce the maintenance burden of bolted connections in wind turbine installation is therefore desirable. It is against this background that the invention has been devised.

Summary of the Invention

According to a first aspect of the invention, there is provided a method for scheduling inspection of a bolted connection on a wind turbine, the bolted connection comprising a bolt under tension. The method comprises at a first sample time, determining a first tension parameter of the bolt, having an associated upper uncertainty bound, and, at a second sample time, determining a second tension parameter of the bolt, having an associated lower uncertainty bound. Thereafter, a linear relationship is determined based on the upper uncertainty bound of the first tension parameter and the lower uncertainty bound of the second tension parameter. Based on that linear relationship, a third sample time in the future is determined, at which a further tension parameter is determined. Based on the third sample time, an inspection event is scheduled.

A benefit of the invention is that inspection events are scheduled in an adaptive manner in a way that is responsive to the difference in values of those tension measurements but which takes account of the inherent uncertainty in such measurements without compromising safety. In this way, inspection events can be scheduled less frequently than is currently the case which presents a much-reduced maintenance cost while still ensuring the performance of the bolted connection.

The tension parameters in the above context may be a direct measurement or may be a statistical quantity calculated from a plurality of measurements. The sample times as referred to above may also be expressed as calendar dates at which the tension parameters are determined/measured.

The invention extends to and therefore also embraces a system for scheduling inspection of a bolted connection on a wind turbine, the bolted connection comprising a bolt under tension. The system comprises a measurement system adapted to determine, at a first sample time, a first tension parameter of the bolt, having an associated upper uncertainty bound, and to determine, at a second sample time, a second tension parameter of the bolt, having an associated lower uncertainty bound. The system further comprises a computerised planning system adapted to: determine a linear relationship based on the upper uncertainty bound of the first tension parameter and the lower uncertainty bound of the second tension parameter, and, based on the linear relationship, determine a third sample time for determining a further tension parameter; and schedule an inspection event based on the determined third sample time.

Preferred and/or optional aspects and features of the invention are set out in the appended claims.

In another aspect, there is provided a method, for example a computer-implemented method, for scheduling inspection of an item or system to be tested. The item or system may be any item or system having an attribute which degrades or reduces or increases over time at a non-linear rate. For example, the process may apply to oil contamination of a hydraulic system in which contamination increases over time, or to surfaces which wear over time to a critical level, thereby becoming thinner. The method comprises at a first sample time, determining an attribute or parameter of the item or system, having an associated upper uncertainty bound, and, at a second sample time, determining a second attribute or parameter of the item or system, having an associated lower uncertainty bound. Thereafter, a linear relationship is determined based on the upper uncertainty bound of the first attribute/parameter and the lower uncertainty bound of the second attribute/parameter. Based on that linear relationship, a third sample time is determined in the future, at which a further attribute/parameter should be determined. Based on the third sample time, an inspection event is scheduled.

Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all examples and/or features of any example can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

Brief Description of the Drawings

The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a front view of a typical horizontal-axis wind turbine to which the example of the invention may apply, in which an inset panel demonstrates a bolted connection;

Figure 2 is a system view of a system for scheduling an inspection event of a bolted connection in accordance with an example of the invention;

Figure 3 is a flow chart embodying an example of the invention for scheduling an inspection event of a bolted connection in a wind turbine;

Figure 4 is a chart illustrating tension of a bolted connection compared to time, which shows the scheduling of bolt inspection events.

Detailed Description

A specific embodiment of the invention will now be described in which numerous features will be discussed in detail in order to provide a thorough understanding of the inventive concept as defined in the claims. However, it will be apparent to the skilled person that the invention may be put into effect without the specific details and that in some instances, well known methods, techniques and structures have not been described in detail in order not to obscure the invention unnecessarily.

In general terms, the examples of the invention described here provide systems and corresponding methods for scheduling inspection events for bolted connections based on previously measured tension measurements of the bolted connections in way that is responsive to the difference in values of those measurements, but which takes account of the inherent uncertainty in such measurements without compromising safety. In this way, inspection events can be scheduled less frequently than is currently the case which presents a much reduced maintenance cost while still ensuring the performance of the bolted connection.

To provide context for the invention, Figure 1 shows a typical horizontal axis wind turbine 2, that includes a nacelle 4, mounted on top of a tower 6, which supports a front facing rotor 8 comprising a plurality of blades 10. Although not shown in Figure 1 , the rotor 8 is connected to a powertrain or drivetrain housed within the nacelle 4. The drivetrain comprises components required to convert rotation of the rotor 8 into electricity, including a generator, a gear system and a controller, although these components are not shown in Figure 1 or described in detail for brevity. Although a horizontal-axis wind turbine is shown in Figure 1 , which is a common configuration of wind turbine, it should be noted that the invention may be applicable to other wind turbine configurations.

Typically, the tower 6 may be made from steel or concrete, the choice of which depends on many factors such as expected loading conditions, hub height, and location, to name a few examples. Hybrid towers of concrete and steel are also known.

The tower is constructed from annular or tubular tower sections 20, as can be seen in Figure 1. Due to the slight tapering of the tower 6, each of the annular tower sections 20 are slightly frustoconical, in this example and can be considered as entirely, or mostly, made of steel which is a common material of construction for wind turbine towers.

In the illustrated wind turbine 2, there are two tower sections, comprising a lower tower section 22 and an upper tower section 24. Note that the tower 6 may comprise more than two tower sections, for example between three and ten tower sections. Each of the tower sections 20 are connected together by way of a flanged connection or ‘coupling’ 25. A portion of a flanged connection 25 between the lower tower section 22 and the upper tower section 24 can be seen in the inset panel in Figure 1.

The flanged connection 25 comprises an upper tower flange 26 and a lower tower flange 28 which are adjacent one another so as to fit together in a butted interface.

The upper and lower tower flanges 26, 28 define respective through-bores that are aligned to provide a bolt hole 30 that extends through both flanges 26,28. The bolt hole 30 accommodates a bolted connection 32.

The bolted connection 32 includes a bolt 34 and a nut 36. The bolt 34 in this example is a headed bolt, having a shank 38. The shank 38 has a threaded section 39 onto which the nut 36 is screwed in a conventional manner to bear against a washer 37. In other examples the bolted connection 25 may include a stud bolt, rather than a headed bolt, in which upper and lower nuts are screwed onto correspondingly threaded ends of a double-headed shank. Still further applicable bolted connections may be anchor bolts and bolt/nut arrangements at the fixing between a wind turbine tower and a foundation of a wind turbine installation. Note that bolts can be used in blind holes where a stud or headed bolt is threaded into a threaded hole in a component.

It will be noted at this point that the bolted connection 32 shown in Figure 1 illustrates a single bolt, although in practice a bolted connection would comprise a circular array of bolts arranged circumferentially around a circular flanged connection of the wind turbine. It should be noted, for the avoidance of doubt, that the term ‘bolted connection’ should not be considered limited to the specific bolted connection as shown in the illustrated example.

It should be noted that such a bolted connection 32 typically is pretensioned or preloaded to increase the effectiveness of the bolted connection and its resistance to cyclical loading. The process of applying pretension to bolted connections is a known technique in this regard. It is also known that the pretension or preload induced in such bolted connections may reduce or ‘settle’ over time, for example by elongation of the bolt shanks or turning back of the nuts. From a safety perspective, bolted connections in wind turbines are inspected regularly to ensure that the bolted connection is sound and to guard against an undetected excessive loss of pretension. Typically, such inspection events may be scheduled on a yearly basis. However, this requires a significant maintenance burden so it would be desirable to schedule bolt inspection events more intelligently to reduce maintenance effort whilst ensuring excellent safety of the bolted connections.

Figure 2 illustrates a bolt inspection system 40 inspecting the bolted connection 32 to determine the tension in the bolt and whether that tension is acceptable. In turn, the bolt inspection system 40 is configured to schedule the next inspection event in an adaptive manner, thereby making efficient use of maintenance resources.

In overview the bolt inspection system comprises a tension measuring system 42, a data store 44 and a computerised planning system 46.

The computerised planning system 46 may be a conventional general-purpose computer which may be housed at an appropriate location. As such, the computerised planning system 46 may comprise at least one processor 48, memory 50 and a suitable humanmachine interface or HMI 51. The processor 48 may be programmed to process and/or execute digital instructions stored in memory 50. Processor 48 may be any electronic device or circuit programmed and/or otherwise configured to perform the process steps and algorithm details described herein.

Memory 50 may include any non-transitory computer usable or readable medium, which may include one or more storage devices or articles. Exemplary non-transitory computer usable storage devices include conventional hard disk, solid-state memory, randomaccess memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), as well as any other volatile or non-volatile media. Non-volatile media include, for example, optical or magnetic disks and other persistent memory, and volatile media, for example, also may include dynamic random-access memory (DRAM). These storage devices are non-limiting examples; e.g., other forms of computer-readable non-volatile storage media exist and include magnetic media (e.g., disk or tape), compact disc ROM (CD-ROMs), digital video disc (DVDs), other optical media, any suitable memory chip, cartridge or flash RAM (e.g., such as a portable USB flash drive), or any other medium from which processor 48 can read. In general, memory 50 can store instructions executable by processor 48 and also can store data to be used when executing the instructions. The HMI 51 may be any suitable interface for a user to engage with the computerised planning system 46 and may include a data entry system such as a keyboard or touch screen by way of non-limiting example, and a display such as an LCD screen. Such features are well understood by a skilled person so will not be discussed further, for the sake of clarity.

The data store 44 may be suitably configured to store large quantities of data and may include various kinds of mechanisms for storing, accessing, and retrieving various kinds of data, including a hierarchical database, a set of files in a file system, an application database in a proprietary format, a relational database management system (RDBMS), and so on. The data store 44 may be hosted on a cloud-based hosting service and in such a case, and as shown here, is in communication with the computer system 46 and the tension measuring system 42 over a telecommunications network 52. The telecommunications may be any suitable network, for example a wide-area network (WAN) such as the internet. For this purpose, the computerised planning system 46 includes a suitable telecommunications interface 54 for communicating over the network 52 with the data store 44. It should also be noted that although in the illustrated embodiment the data store 44 is separate to the computerised planning system 46, it is envisaged that in other examples the data store 44 may comprise a physical part of the computerised planning system 46 and not provided in the Cloud, as shown here.

The tension measuring system 42 may comprise a suitable system for measuring the tension of the bolted connection 32. In one example, the tensioning measuring system 42 may apply a non-contact technique, e.g in the style of a non-destructive testing technique for determining the tension in the bolted connection 32. Such tension measuring systems are known in the art. An example of a suitable tension measurement system is the ‘Bolt-Check’ product as available from R&D Test Systems A/S. Such a system is also described in W02020/108721 , the contents of which are incorporated herein by reference. Since a suitable tensioning measurement system is available commercially, a full discussion of such a system will not be provided here.

However, as a general overview, the tension measuring system 42 is configured to apply an ultrasonic measurement technique to quantify the elongation of the bolt of the bolted connection 32 and therefore to calculate tension. In this respect, discussion herein concerning calculation of the ‘tension’ in a bolt also encompasses calculation of parameters from which the tension of the bolt can be inferred, for example from elongation of the bolt.

The tensioning measuring system 42 comprises an ultrasonic probe 60 and a user device 62. The user device 62 may be any suitable general purpose computer system which is operable to interface with the ultrasonic probe 60 in order to conduct tension measurements of the bolted connection 32. As such, the user device 62 includes a suitable processing system 64, onboard memory 66 and a suitable HMI 68. Attributes of these components are comparable with those described with respect to the processor 48, the memory 50 and HMI 51 of the computerised planning system 46 described above, so further discussion will not be provided here.

For the purposes of communication, the user device 62 is provided with a network interface 70 for communicating with the data store 44 over the telecommunications network 52. Any suitable network interface 70 would be appropriate for achieving network communications with the network 52 either wirelessly or via a wired connection.

Having described the functional components of the illustrated example of the invention, the discussion will now proceed to describing a method in accordance with an example to the invention which provides a technically beneficial process of scheduling bolt tension inspections. In this discussion, reference will be made to Figure 3, which illustrates an example method 100 according to the invention and Figure 4 which illustrates the result achieved by the method.

The method 100 begins at step 102 at which a set of initial conditions are established. It should be appreciated that the initial conditions discussed here may be varied from the specific examples given which will influence the exact functionality of the method 100. However, the purpose of the initial conditions is to strike a balance between scheduling bolt inspection events at time intervals to achieve a reduction in maintenance burden whilst avoiding any compromise to the safety case of the bolted connection 32.

In overview, the initial conditions may comprise 1) a value for minimum acceptable pretension (MAP), 2) a value for a minimum sampling period, 3) a value for measurement uncertainty and 4) a value for the initial or starting sample period and therefore a date/schedule for a second measurement. The initial conditions may be initialised upon activation of the method as an application running on the computerised planning system 46. Optionally, the computerised planning system 46 may be configured to enable adjustment of the initial conditions so that a user may influence the precise functionality of the method as desired.

Once the initial conditions of the algorithm have been set, the process proceeds to step 104 where a first measurement of bolt tension is taken.

This step may be the first measurement that is taken of bolt tension following installation of the wind turbine. However, this is not critical and it may instead be the first measurement following reinitialization of the method following a re-tensioning of the bolted connection 32, for example.

The user device 62 receives data from the probe 60 and calculates the tension in the bolted connection 32. That tension measurement is then stored by the user device 62 into the data store 44 in the Cloud 52. As an alternative, the tension measurement may be stored in the user device 62 in local memory 66 to be accessed at a later time for further processing and calculation.

Suitably, the first tension measurement may be stored in the data store 44 by way of a suitable data structure including such data as required to store and identify the required data for future use. For example, the tension measurement may be stored with a unique index value to aid future retrieval of said data from the data store 44, a time stamp, and uncertainty values associated with the tension measurements, as will be discussed later. Other data items may also be stored such as the system identification item or code of the tension measuring system 42 that has carried out the tension measurement, such data possibly being useful for calibration or verification purposes.

Once the first tension measurement has been taken, a calendar date for a second tension measurement may be set. This may be achieved in various ways. Optionally, the date for next inspection may be determined by the user device 62. However, it is envisaged that the process will be most appropriately coordinated by the computerised planning system 46. Thus, at this processing step, the computerised planning system 46 communicates with the data store 44 and identifies that a first tension measurement has been stored, together with an identification of the date/time of measurement. In response, the computerised planning system 46 determines a calendar date for a second inspection of the bolted connection 32. At this point, the computerised planning system 46 may schedule the date for second inspection internally within its onboard memory 50 so it can be accessed by appropriately trained maintenance personnel. In addition, or alternatively, the computerised planning system 46 may communicate the date of second inspection to the data store 44. Still alternatively the user device 62 may access the date of second inspection date from the data store 44 or it may communicate with the computerised planning system 46 directly. With reference to the initial conditions discussed above, the inspection date may be based on the ‘starting sample period’ of the initial conditions. Note that the exact date an inspection takes place may differ from what has been scheduled, based on availability of a maintenance crew. Note also that the first and second dates for taking the respective first and second tension measurements may be determined at the same time as part of setting the initial conditions. In the above context, it should be appreciated that in the case where there are many such bolted connections to inspect, the computerised planning system 46 may determine different inspection due dates for different groups of bolted connections depending on how best to manage maintenance resources and activities.

At this point, reference is made to Figure 4 which provides a visual representation of the results of the method 100, with ‘time’ on the X-axis and ‘bolt tension’ on the Y-axis. Here, the X-axis is in unit of years from installation, and spans a time frame in excess of 30 years, which corresponds to the expected design life of a typical wind turbine. The Y-axis is expressed as a percentage of bolt yield strength, with 100% tension corresponding to the yield strength of the bolt. The first tension measurement P1 is therefore a percentage of the yield strength, for example 80% to 90% of the yield strength. It should be noted that the values shown here are merely exemplary.

As can be seen, the first tension measurement is indicated by P1 on Figure 4 at time T 1 and is also shown in enlarged detail on the inset panel.

As discussed above the second inspection date may be one of the initial conditions that is set at step 102. Once the second inspection date has arrived, a second measurement is taken, as indicated at step 106. Here it should be noted that the second inspection should take place as close to the scheduled second inspection date as possible. However, it may be the case that it is not possible for an inspection to take place precisely on the scheduled date, for various reasons. The scheduled date can also be considered a ‘due date’. For example, it is possible that weather conditions may prevent sea-going vessels from ferrying maintenance personnel to an offshore wind turbine. In other circumstances it may be the case that a maintenance event is scheduled at the wind turbine for other purposes and, as such, it may be preferable for the user device 62 to flag to the maintenance personnel that the bolted connection 32 should also be inspected. Therefore, the system permits some flexibility in the precise date at which inspection is carried out.

It is considered acceptable that an inspection of the bolted connection is carried out as close to the second inspection date as possible, within acceptable constraints. Note that, optionally, if a tension measurement is taken earlier than the set inspection date, then it is envisaged that the method will replicate the measurement data and apply it to the date that has been set for inspection. Moreover, in the event that the tension measurement is a later tension measurement, and not the second tension measurement, then in the event that the internal between the instant tension measurement and the previous tension measurement is less than a minimum time period, then the algorithm may take into account a previous one or more tension measurements when calculating the next inspection date/due date.

The second measurement is illustrated on Figure 4 at P2 which is recorded at time T2. As before, the measurement P2 is stored in the data store 44 by the user device 62 transmitting the measurement data with appropriate date information over the communications network 52.

At step 108, suitable data validity checks are carried out to check the tension measurement provides valid data. One such validity check is to compare the tension measurement to the previous tension measurement to determine whether the subsequent tension measurement is greater than or less than the previous tension measurement, subject to appropriate uncertainty bounds for each tension measurement. If the subsequent tension measurement is determined to be greater than the previous tension measurement then the data is considered to be invalid for the purposes of the algorithm, and the process flow steps back to step 102. In effect, therefore, second tension measurement P2 is re-identified as the previous tension measurement.

The effect of this step is to detect circumstances where the bolted connection 32 may have been retightened in the interval between the two tension measurements P1,P2. This may be the case where there is a maintenance event in the intervening period the maintenance team has increased the tension on the bolted connection 32 without a specific instruction to do so. In his instance, therefore, the second tension measurement will be greater than the first tension measurement. In such a situation, the process can effectively be ‘reset’.

If the tension measurement taken at step 106 is considered to be valid, then the method proceeds to step 110 where further calculations are carried on the two tension measurements that have been taken. However, and has been shown in Figure 3, there is an option step 109 which takes into account timing for the second of the two tension measurements under consideration.

The principle of method step 109 is to take account for the second tension measurement P2 taking place before the time of the scheduled inspection date that has been set previously so that the second tension measurement P2 will be treated as having been taken at a minimum sample interval. As discussed above, the minimum sample interval is one of the initial conditions that are set at step 102.

Once two consecutive samples have been measured and validated, the proves moves to step 110 at which a relationship line or curve is calculated based the values of the two previous tension measurements P1 ,P2 and which takes into account certainty bounds associated with the two tension measurements. Pictorially, this can be appreciated by viewing Figure 4.

In Figure 4, the two consecutive tension measurements are seen at P1 and P2; the tension measurement P2 is lower than P1. This represents an initial ‘settling’ of the bolted connection 32 following its initial installation.

In broad terms, the method step 110 involves determining a linear relationship between the two data samples P1 and P2 which takes account of the inherent uncertainty in the data samples. The determined linear relationship can then be used to schedule an appropriate next inspection event for the bolted connection 32.

More specifically with reference to Figure 4, it can be seen that a linear relationship R1 is determined between the two tension measurements P1 and P2. The linear relationship R1 interpolates beyond the data points P1, P2 and represents a worst-case scenario for the continued de-tensioning of the bolted connection. Based on this understanding, the point at which the linear relationship R1 intersects a minimum acceptable pretension line, or ‘MAP’ line, can be predicted as the appropriate point to set a next inspection date or ‘due date’ for the bolted connection 32. However, simply extrapolating between data points P1 and P2 to the intersection of the of the MAP line would provide an optimistic assessment of the recommended next inspection date. Therefore, the invention takes into account that the tension measurements P1 and P2 that are acquired by the tension measurement system 42 have a level of uncertainty associated with them. As such, a more accurate determination of the next inspection date can be determined when the uncertainty inherent in the tension measurement data is taken into account.

At this point, it should be noted that the MAP line is set as an initial condition as discussed above. In addition, the uncertainty or ‘uncertainty bounds’ associated with the measurements is also set at the initial condition stage. The precise uncertainty that is appropriate for the tension measurement is dependent on the tension measuring system 42. For example, the tension measuring system 42 may be certified as providing a measure of tension in the bolt to an accuracy of +/- 5%, by way of illustration only.

This is illustrated in Figure 4. Note that the first tension measurement P1 has an associated upper uncertainty bound P1 U and a lower uncertainty bound P1L. Similarly, the second tension measurement P2 has an associated upper uncertainty bound P2U and a lower uncertainty bound P2L. The upper and lower certainty bounds are determined based on the specific tension measurement and their associated uncertainty values. It will be appreciated that the linear relationship R1 is determined based on the upper uncertainty bound P1 II of the first tension measurement P1 and the lower uncertainty bound P2L of the second tension measurement P2. Determining the linear relationship R1 from the uncertainty bound values in this way builds in a safety factor to the calculation compared to simply determining a linear relationship based on the initial tension measurements.

The linear relationship, or ‘model’, or ‘curve’ or ‘line’, may be determined using a conventional least squares regression technique, as would be understood by the skilled person. Note that such a technique may be used to find a best fit line between more than two measurements, If appropriate. For example, if several tension measurements happen to be taken at relative short time intervals, then a plurality of those measurements may be taken into account in the regression calculation. Once the linear relationship R1 has been calculated in step 110 the method can move to step 112 in which the next inspection/due date is calculated based on the determined linear relationship R1 between the previous two tension measurement P1 ,P2 taking into account their associated uncertainty values.

To calculate the next inspection date, the linear relationship R1 is used as a basis for determining the crossing point of the MAP line, which provides a sample time, marked here as T3. This sample time T3 represents the next sample time at which a tension measurement should be made by the tension measuring system 42.

Once the sample time T3 has been determined by the computerised planning system 46, it can be used to schedule the next inspection date, at which a further sample/measurement can be taken, and stored in the data store 44 and communicated to the user device 62 for maintenance scheduling purposes.

Once the next inspection date has been set at step 112, the process loops back to step 106 for the taking of the next tension measurement, and the entire process is repeated. As can be seen in Figure 4, the next tension measurement is indicated at P3, so a linear relationship R2 is calculated based on the uncertainty bounds associated with tension measurements P2 and P3. The consequent linear relationship R2 extrapolates from the data points P2 and P3 and crosses the MAP line at a time period labelled as T4 on Figure 4. T4 is then set as the next scheduled maintenance event. Note that if the time period between T2 and T3 is less that a minimum time period, then the algorithm may take into account previous tension measurements as well when calculating the next inspection date/due date.

Various modifications may be made to the specific embodiments that have been discussed above with reference to the accompanying figures. Some variants have already been discussed but others would be apparent to the skilled person. Therefore, the scope of the invention should be determined from the appended claims rather than with reference to the specific examples discussed in this text.

In the above discussion, references have been made to the first tension measurement P1 and the second tension measurement P2. It will be appreciated that the two measurements P1 and P2 are the first two measurements taken by measuring system 42. However, in the context of the inventive concept, the first and second measurements may be any two consecutive tension measurements that are made in a series of such tension measurements.

Claims

1. A method for scheduling inspection of a bolted connection (32) on a wind turbine, the bolted connection comprising a bolt under tension, the method comprising: at a first sample time (T1), determining (104) a first tension parameter (P1) of the bolt, having an associated upper uncertainty bound (P1 U), at a second sample time (T2), determining (106) a second tension parameter (P2) of the bolt, having an associated lower uncertainty bound (P2L), determining (110) a linear relationship (R1) based on the upper uncertainty bound (P1 U) of the first tension parameter (P1) and the lower uncertainty bound (P2L) of the second tension parameter (P2), and, based on the linear relationship (R1), determining (112) a third sample time (T3) in the future for determining a further tension parameter (P3); and scheduling (112) an inspection event based on the determined third sample time.

2. The method of Claim 1 , wherein: if the time interval between the first sample time (T1) and the second sample time (T2) is less than a predetermined minimum time interval, adjusting (109) the second sample time (T2) to match or exceed the minimum sample interval.

3. The method of Claim 1 , wherein: if the time interval between the first sample time (T1) and the second sample time (T2) is less than a predetermined minimum time interval, identifying one or more previously measured tension parameters to take into account in the determination of the linear relationship (R1).

4. The method of any one of Claims 1 to 3 wherein: determining (112) the third sample time (T3) includes identifying the time point at which the extrapolation falls below a minimum bolt tension threshold (MAP).

5. The method of any one of the preceding claims, wherein: the step of determining (104) the first tension parameter (P1) of the bolt and/or the step of determining (106) the second tension parameter (P2) are conducted using an ultrasonic measurement system (42).

6. The method of any one of the preceding claims, wherein: if the second tension parameter (P2), and optionally the lower uncertainty bound (P2L) thereof, is greater than the first tension parameter (P1) and, optionally, the upper uncertainty bound (P1 U) thereof, then it is identified that the second tension parameter (P2) is an invalid value.

7. The method of Claim 6, wherein in the event that the second tension parameter is identified as an invalid value, the method further comprises re-determining the second tension parameter at a later date.

8. The method of any one of the preceding claims, wherein step of determining (110) the linear relationship determining (R1) based on the upper uncertainty bound (P1 U) of the first tension parameter (P1) and the lower uncertainty bound (P2L) of the second tension parameter (P2) includes applying a linear regression technique.

9. The method of any one of the preceding claims, wherein the linear relationship is based on the first tension parameter (P1), the second tension parameter (P2) and one or more further tension parameters.

10. The method of any one of the preceding claims, wherein the determined first tension parameter (P1) and the determined second tension parameter (P2) are recorded in a computerised data store (44).

11. The method of Claim 10, wherein in recording the first and second tension parameters (P1 ,P2) said parameters are communicated to the data store (44) over a telecommunications network (52).

12. The method of Claim 10 or Claim 11 , wherein the steps of: determining a linear relationship (R1) based on the upper uncertainty bound (P1 U) of the first tension parameter (P1) and the lower uncertainty bound (P2L) of the second tension parameter (P2), based on the linear relationship (R1), determining (112) a third sample time (T3) for determining a further tension parameter (P3); and scheduling (112) an inspection event based on the determined third sample time (T3), are performed by a computerised planning system (46) that retrieves the determined first tension parameter (P1) and the determined second tension parameter (P2) from the data store (44) over a telecommunications network (52).

13. The method of any one of the preceding claims, wherein the step of scheduling (112) an inspection event includes recording the inspection event in a computerised planning system (46).

14. The method of any one of the preceding claims, further comprising outputting the scheduled inspection event to a computerised user-device (62).

15. A system for scheduling inspection of a bolted connection (32) on a wind turbine, the bolted connection comprising a bolt under tension; the system comprising a measurement system (42) configured to: to determine (104), at a first sample time (T1), a first tension parameter (P1) of the bolt, having an associated upper uncertainty bound (P1 U), and to determine (106), at a second sample time (T2), a second tension parameter (P2) of the bolt, having an associated lower uncertainty bound (P2L), the system further comprising a computerised planning system (46) configured to: determine (110) a linear relationship (R1) based on the upper uncertainty bound (P1 U) of the first tension parameter (P1) and the lower uncertainty bound (P2L) of the second tension parameter (P2), based on the linear relationship (R1), determine (112) a third sample time (T3) in the future for determining a further tension parameter; and schedule (112) an inspection event based on the determined third sample time (T3).

16. The system of Claim 15, wherein the measurement system (42) comprises an ultrasonic probe (60) adapted to measure the tension in the bolted connection (32).

17. The system of Claims 15 or 16, wherein the computerised planning system (46) is adapted to communicate with the measurement system (42) over a telecommunications network (52).

18. The system of Claim 17, wherein the measurement system (42) and the computerised planning system (46) are adapted to communicate with a data store (44) over the telecommunications network.