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Keeping Tabs on the Effects of AAR

The concrete structures of NB Power Generation’s 672-MW Mactaquac Generating Station are affected by alkali-aggregate reaction (AAR). To monitor the project’s condition, NB Power collects data from hundreds of instruments installed in the powerhouse, at the intake, and on the sluiceway. The utility shares its experience with managing the data from these instruments to effectively monitor the complex behavior of this AAR-affected structure.

By Phillip A. Gilks, John P. Fletcher, David W.B. Jewett, and Thomas H. May

The Mactaquac Generating Station is in New Brunswick, Canada, about 20 kilometers upstream of the provincial capital of Fredericton on the Saint John River. The station is the largest hydroelectric generating facility in the Maritime Provinces; it was constructed in stages between 1964 and 1980. The plant consists of a 500-meter-long zoned embankment dam that spans the original river bed. At the north end of the dam is a five-bay gated sluiceway. A second concrete structure, separated from the main dam and sluiceway by a rock island, includes another five-bay spillway in line with the generating station’s intake structure. The intake structure feeds water through six, 8.8-meter-diameter steel-lined concrete penstocks. The powerhouse is located directly downstream of the intake structure and houses six vertical shaft generators powered by Kaplan turbines. Each unit has a rated capacity of 112 MW, for a total plant capacity of 672 MW.

The concrete structures and the first three units were constructed from 1964 to 1968. Unit 4 was added in 1974, and Units 5 and 6 were completed in 1979 and 1980, respectively.

Even before construction was complete, there was evidence of distress in the concrete structures. As early as the mid-1970s, project owner NB Power Generation noted an opening of a construction joint in the powerhouse substructure. By the early 1980s, leakage through horizontal joints in the spillway, intake, and diversion sluiceway structures was also evident. At that time, NB Power launched studies to investigate the possibility that a geological phenomenon such as rock swelling was causing the distress.

In 1985, Spillway Gate 10, adjacent to the intake structure, was found to be obstructed by interference between the gate roller boxes and the liner plate at the back of the gate slots. This was caused by displacement of the spillway end pier toward the gate opening in excess of 25 millimeters. About this time, NB Power, through the investigation that started in the early 1980s, ruled out geological causes. By early 1986, the utility concluded the concrete structures were being affected by alkali-aggregate reaction (AAR).

AAR is a chemical reaction that takes place between the alkalis in the cement and certain rock used for aggregate. The product of the reaction is a silica gel, which has a high affinity for water. The silica gel deposits in the pores of the concrete, where it absorbs water and swells. The swelling of the gel results in expansion of unrestrained concrete and stress buildup in areas where the concrete is restrained.

Since 1985, NB Power has undertaken various innovative remedial measures to mitigate the effects of the concrete expansion on the structures and equipment of the generating station. These have included the cutting of slots within the concrete structures, using a diamond wire saw, installation of post-tensioned anchors and bar anchors, modifications to existing electro-mechanical equipment, and extensive water control and structural grouting.

Using instruments to understand behavior

The design and implementation of the many remedial measures to mitigate the effects of the concrete expansion required a thorough understanding of the behavior of the structures suffering from the effects of AAR. To gain this understanding, NB Power installed hundreds of instruments. Data collected from these instruments, along with stress measurements and survey and visual observations, are used to calibrate finite element models, facilitate engineering analyses, and predict future behavior.

Installing the instruments

At the time of project construction, NB Power – in keeping with the basic instrumentation requirements of the day – installed a series of relief wells, piezometers, and inclinometers. In the early 1980s, the utility began installing instrumentation in the foundation rock and within the concrete substructure to monitor movements. The instrumentation system was periodically upgraded over time; by the mid-1990s, more than 400 instruments were in place.

Upgrading the system

The amount of data being collected and the review and analysis procedures were becoming overwhelming. The data analysis and management software developed for the project by the University of New Brunswick Geodesy and Geomatics Engineering Department (then Survey Engineering) was DOS-based and becoming obsolete. The manually collected data for each instrument was stored in separate computer files; within each file, only the reduced values were retained. The raw readings, comments, or history could not be stored in the instrumentation file. NB Power technicians manually plotted the data from the instruments into graphs every three months, and then sent the graphs to consultants for review.

In 1998, NB Power undertook a project to modernize the whole instrumentation system, with the goal of reducing the intensity of the labor required to collect the measurements and streamline the processing, storage, and transmission of the data.

The project involved three steps. In the first step, NB Power – with the support of engineering consultants Hatch Energy (formerly Acres International) and Groupe RSW – reviewed all field instruments and assigned priorities related to their purpose and critical importance.

The next step involved a review of each instrument, with the aim to improve instrument reliability and to ease the acquisition of the data. As part of this review, NB Power considered the possibility of installing a largely automated system. In the end, however, NB Power determined that the environment at most instrument sites was so harsh that regular operator intervention was required to ensure the correct operation of the instruments. Also, the data was being collected to assist in engineering analysis, so real-time data collection was not required. Ultimately, the utility determined the cost of maintenance of such a system outweighed the benefits. Therefore, a select number of instruments were connected to remote reading stations, while most remain to be read at the instrument site.

The final step involved implementing a state-of-the-art data collection and management system. The system, called DamSmart for Windows, a product of URS Corporation, stores all data in a relational database and utilizes modern computer-based tools to optimize the handling, interpretation, and presentation of data.


Figure 1: This screen shot illustrates how data from six extensometers in one borehole is displayed. In this case, the fluctuations in the extensometer measurements reflect changes caused by temperature variations, while the slope of the plots identifies the rate of expansion due to alkali-aggregate reaction.
Click here to enlarge image

The following sections describe details about various aspects of the instrumentation system modernization.

Modernizing hardware

The first step in modernization was to review the suite of instruments, prioritizing them so that the level of maintenance and management effort would be commensurate with the value of the data provided. To achieve this, each instrument was given a priority based upon the importance of the measurements. The highest priority instruments were the first to be upgraded, while the lowest priority instruments received minimum levels of maintenance and were generally decommissioned if they failed to function properly. In no case was the quality of the data compromised.

Today, about ten years after the modernization program, the data is collected from most of the instruments through an electronic interface, thereby reducing the risk of operator error. An operator is still required to visit each instrument site to connect to it. At the same time, the instrument is checked for visible damage and its operating condition is confirmed.

Modernizing software

NB Power considered a number of alternatives for the replacement of the existing series of spreadsheets and DOS-based programs. The preference was to obtain a commercially available package that would simplify the process while utilizing a supported program. To that end, NB Power elected to purchase DamSmart for Windows. DamSmart was selected based upon its designed purpose as a data management tool for use in the performance monitoring of dams, tunnels, and other civil infrastructure and its demonstrated ability to automate instrumentation data collection, reduction reporting, and plotting for multiple projects on a desktop PC. Furthermore, the program is capable of preparing analytical plots and data reports and simplifying the communication of the results.

Converting data

The first step in implementing the upgrade was to convert existing data so that the historical information would be seamlessly available. The original data was stored in an ASCII file format. Each instrument had its own separate file. Field data was collected with data recorders and downloaded to a PC. It was then run through an adjustment program, which stored the reduced data into the ASCII instrumentation files. The raw readings from the field were not saved. Meanwhile, important information about each of the instruments was maintained in a separate, index spreadsheet that included location, coordinates, date of installation, etc. All the ASCII files were imported into DamSmart, including data from obsolete instruments. The information in the index spreadsheet was also imported and stored with the corresponding instrument. While DamSmart is designed to store both raw and reduced data, the raw data could not be retrieved. Therefore, this field was left blank for historical measurements.

DamSmart has the ability to manage multiple projects, so the structural instrumentation was divided into three projects based upon location, i.e., powerhouse, intake, and sluiceway.

With the historical data conversion complete and the various instruments initialized in DamSmart, the next step was to begin the process of collecting and storing new data into the DamSmart database.

Currently, the majority of the data is collected electronically, using a field PC. The raw data is stored in Excel spreadsheets in the field PC. In the office, the Excel spreadsheets are uploaded to a desktop PC that has DamSmart installed. They are converted to *.dat files before being imported to Dam Smart. During the import process, the raw data is reduced using correction factors supplied by NB Power. Both raw and reduced data are stored.

Customizing data analysis

While DamSmart has the ability to view data and issue alarms when measurements exceed established tolerances (among other things), NB Power required the ability to do more detailed analysis of the data. The engineering consultants who use the data had very specific expectations and demands that could only be met by custom programming. NB Power staff, with the support of co-op students from the University of New Brunswick Department of Computer Science, wrote the custom programs.

The first of two main applications was written to allow the consultants and the owner’s staff to view data from extensometers, plumb lines, micrometers, and other instruments. This application uses Visual Basic (VB) as the programming language. A custom configuration script identifies the exact data that need to be withdrawn from the database and exports that data into an Excel spreadsheet for analysis. Once in the spreadsheet, the data can be displayed and manipulated to extract the necessary information to complete the engineering analyses. Figure 1 is an example of how the data is displayed. Project engineers can use this information to design appropriate remedial measures and, by extrapolating these graphs, they can predict future behavior of the structures.


Figure 2: This plot was generated using data from an inverted pendulum to show tilt of a structure. As this plot shows, the structure is moving left over time, with the greatest movement occurring in September.
Click here to enlarge image

The second application uses similar VB programming and configuration scripts, but is designed to display inverted pendulum data. These instruments measure the profile of a borehole and provide lateral locations at a number of points along the length of the vertical borehole casing. DamSmart requires that the inverted pendulum measurement at each elevation be treated as a separate instrument. This viewer compiles the data from the group of instruments that makes up a single inverted pendulum and plots the data in a rational way. Figure 2 shows the tilt of one of the piers between February 2007 and January 2008. This data gives information about the movement of the structure, which designers can use to perform a structural analysis.

Turbine-generating unit instrumentation, measurements

While the mass concrete and structural movements have been instrumented and measured since the early 1980s, the hydro turbine-generating units only became extensively instrumented and measured starting in 1995. Up to that year, the usual suite of maintenance measurements and observations gave hints that not all was normal. The abnormalities showed up in such areas as the need to re-plumb the turbine-generator shaft every few years.

The turbine-generating units’ embedment concrete is expanding three-dimensionally, and the net displacements are asymmetrical, depending on concrete thickness. This causes the unit components to suffer displacements and distortions far exceeding the original designer’s intent. Some of these effects are non-uniform inclination (of the entire unit block), non-uniform radial distortions, varying degrees of vertical displacements, and other effects and interferences. These movements result in direct differential movements of the embedded components such as the generator main support bracket soleplates, generator stator soleplates, turbine stay ring, and steel liners of the runner chamber and draft tube. This, in turn, affects such parameters as generator stator circularity and concentricity and the alignment and clearances of rotating parts of the units. Extraneous loading and distortions of the stay vanes and steel-lined water passages are typical. Additional loading and clearance issues occur at the anchors supporting the generator main bracket and stator soleplates, at the studs that connect the stay ring and headcover flanges, and at the anchorage of the steel-lined discharge ring and draft tube. Figure 3 shows the results of how discharge ring circularity changed between 1995 and 2007.

While the AAR effects were realized some years after the original turbine-generator installations, the measurements taken up to 1995 were not sufficient to capture the information required to determine what interventions were needed to continue reliable unit operation. In 1995, the existing unit measurements were extensively supplemented. This involved the establishment of a comprehensive set of 27 field measurement parameters that, when totaled, add up to more than 1,000 individual manual measurements per unit. Several of the parameters require manual rotations of the unit, to capture the measurements and repeat rotations to verify quality of the measurements.

Measuring instruments consist of off-the-shelf and fully customized instruments. The off-the-shelf instruments include long-length inside micrometers (up to 15 feet), vernier calipers, feeler gages, dial indicators, and optical leveling equipment. The inside micrometers and vernier calipers have site-customized ends to allow efficient measurement between field-installed stainless steel balls. Fully customized equipment includes a portable rigid pole with Perm-a-PlumbĀ® laser attached for accurately measuring relative changes in verticality between two points. (This device is used to monitor “racking” of the upper to lower parts of the stay ring).

The 27 measurement parameters are normally measured in the “as found” conditions of the unit every three years (the three-year cycle coincides with the regular “unwatered” unit maintenance outage schedule). Interventions for unit adjustments or repairs are usually necessary, so specific parameters are re-measured as required during the outage, including the acceptable “as left” conditions of the unit.

A spreadsheet (or workbook containing several spreadsheets) is used to analyze the data. One analysis spreadsheet or workbook exists for each of the 27 measurement parameters.

Originally, the analysis spreadsheets were designed to be populated manually. With the introduction of DamSmart, it was possible to create a program that would retrieve raw data from DamSmart and place it in the appropriate spreadsheet automatically. To accomplish this, NB Power staff built spreadsheet “templates” for each of the 27 measurement parameters. The templates are empty of any data, but already contain all the required analysis calculations and graphs. Not only do the templates accept any one set of measurement data, but, in fact, are populated with the entire historical set of measurements for the measurement parameter being analyzed. In this way, graphs showing up-to-date historical trends are automatically plotted each time the data is processed in the spreadsheet. The spreadsheet template, once populated, is called an analysis spreadsheet; ready for saving, reviewing, printing, and presenting.


Figure 3: This polar plot of discharge ring circularity shows how Unit 2 at Mactaquac has been affected by alkali-aggregate reaction. Measurements taken in 1995 and again in 2007 illustrate the change in discharge ring circularity.
Click here to enlarge image

The transfer of data from the DamSmart database to the spreadsheet template is performed by VB scripting program developed by NB Power. As with the structural viewers, this program accesses configuration files, which allocate the data from the database to the proper cell locations in the spreadsheet templates.

The program was also designed to enable selection of any two measurement sets taken on different dates, so that those direct comparisons can be made in the analysis spreadsheet.

Managing unit measurements

Initially, the processing of the measurements was a manual operation – the field-collected measurement sets were faxed to external engineering personnel, where manual entry into custom, complex analysis spreadsheets was performed. Timely analysis was usually not achieved. Any desire to compare specific measurement sets collected on two different dates was time-consuming. Errors due to oversights in inputting data and typing errors tended to creep into the analysis.

With the introduction of DamSmart in 2000, the data processing was completely revamped and automation of the data management and analysis was introduced. In this process, DamSmart is used primarily as a repository for the measurements. The original analysis spreadsheets were retained and separate programs were developed to extract data from DamSmart and populate these custom spreadsheets.

The process flows as follows:

  1. ) Plant personnel fill in measurements and associated data in field data forms, which are reviewed for completeness by the maintenance supervisor and then a copy is forwarded to the site engineering group;
  2. ) The measurements/data are typed into “input spreadsheets,” which resemble the field forms. These contain background programming to alert if a measurement appears to be outside of a pre-determined tolerance range;
  3. ) When the input spreadsheet is saved, a data file is automatically created, ready for uploading to the DamSmart database;
  4. ) Using a computer terminal with the DamSmart software on board, the user imports the data file into the database. Because the data is analyzed by companies external to NB Power, the database is made available on an FTP site; and
  5. ) Analysis spreadsheet templates are populated with data, which is reduced; calculations performed; and graphs plotted for analysis and reporting.

Results: nine years later

Since its installation in 2000, DamSmart has become an integral part of the data management system at Mactaquac Generating Station. By bringing historical data into a new, coordinated data management system, the security of this information is assured, while new data can be added through a modern and reliable interface.

The instrumentation data at Macta-quac is reviewed and summarized on a regular basis. The objective of these instrumentation data reviews is to monitor any change in behavior of the structures.

The cost of implementing the latest instrumentation data management system has been considerable. Dam- Smart itself was of modest cost, with the owner purchasing the software for several of its employees and one license for each of its two major engineering consultants. However, owing to the unique nature of the instrumentation required to monitor the complex behavior of this AAR-affected structure, the normal package of data analysis programs was not adequate to meet all of NB Power’s demands. The utility developed and implemented a variety of custom data analysis programs. The main cost was the labor required to prepare these programs.

Acknowledgment

The authors thank Kan Liu, a computer science student at the University of New Brunswick, for doing the programming work needed to coordinate the monitoring instruments at Mactaquac with the DamSmart program.


Phillip Gilks is manager of hydro, John Fletcher is senior civil technician, Dave Jewett, P.Eng., is process controls and information technology specialist, and Tom May is mechanical technical specialist with NB Power Generation. Phillip is responsible for management of the project to gather information on concrete condition at 672-MW Mactaquac, John and Tom are responsible for collecting and analyzing the instrumentation data, and Dave is responsible for the software used to manage the data.


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