Structural Integrity Monitoring Progress
One-Day Seminar: 12th June 2001
UCL NDE Centre, Dept of Mechanical Engineering, London.
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Structural Integrity Monitoring ProgressOne-Day Seminar: 12th June 2001 |
Selected Abstracts
Life Extension of Pressure Vessels and Pipelines in Corrosive Service Through Continuous Corrosion Monitoring
(John R. Lilley. AEA Technology plc)
Pressure vessels and pipelines are retired from service when their design corrosion allowances have been consumed. The actual retirement or minimum allowable thickness is usually derived from a simplistic code calculation such as ANSI B31g. However, this approach tends to be conservative due to its generic nature. Component life can, under favourable circumstances be significantly extended through developing an understanding of the corrosion process(es), removing some of the conservatism embodied in the code calculation and by monitoring predicted corrosion rates continuously throughout the period of extended lifetime.
The process involves an investigation in the first instance to identify the nature and stage of advancement of the corrosive mechanism(s) present. This is followed by a detailed inspection, preferably supported by corrosion maps to identify the worst case locations for subsequent detailed monitoring. Ultrasonic probes are permanently attached to these locations which are then interrogated, typically on a weekly basis, to determine the remaining wall thickness at each point. The thicknesses are stored digitally for each probe and are presented graphically with extrapolations to determine the remaining life available until the minimum allowable wall thickness threshold has been exceeded. The probes used are rated for continuous use up to 260 degrees centigrade and cabling is used to allow the ultrasonic instrumentation to be sited remotely.
The minimum allowable wall thickness is calculated initially from the code (e.g. ANSI B31g). If this gives an undesirably short remaining life, various levels of fracture mechanics approaches can be used (i.e. treat the wall loss as a crack) and/or various levels of sophistication of finite element modelling to determine the actual minimum wall thickness required to contain the operating pressures. In advanced cases, the ultrasonic corrosion maps can be fed into the finite element models to predict the stresses around individual corrosion pits. The resultant minimum wall thickness can be very different from the initial calculation and this can dramatically extend the life of equipment, especially if measures can be taken to inhibit the corrosion process. The effectiveness of inhibition is monitored through the ultrasonic wall thickness measurements. As the probes are permanently attached, readings are always taken at exactly the right place and the accuracy of measurement is very high.
This approach has been used successfully to significantly extend the life of pressurised equipment. The paper discusses an example of a pressure vessel and a water injection pipeline.
Structural Health Monitoring with Smart Fibres
(Michael Hill-King - Sales & Marketing Manager, Smart Fibres)
Structures are designed for purpose, usually against proven and accepted codes. However, there are situations when a structure is novel and codes do not exist - these structures require health monitoring. Other structures degrade, are damaged or are required to meet requirements in excess of the original specification; these structures require replacing, reinforcing and often monitoring.
From an original project that qualified design data and provided real time health monitoring for super yacht masts, fibre optic technology is now available for a broader range of applications. This presentation will first describe the initial project, in the harsh environment of racing yachts where carbon fibre masts and booms are monitored during sailing trials. The presentation will then address the technology used, highlight some of the advantages and then illustrate several current applications of the technology for monitoring real structures.
Smart Fibres Ltd continue to pioneer technology implementing a fibre optic strain sensing system to revolutionise current structural load monitoring methodology, with major implications structural health monitoring systems.
TURBOlife™ - A Gas Turbine Life Monitoring System
(David Green, Department Manager - Asset Management Software, AEA Technology)
The industrial gas turbine can experience a wide variety of stop/start cyclic conditions. To maintain hot component integrity with ill-defined duty cycles, manufacturers often use the concept of equivalent operating hours. Each type of cycle is conservatively allocated an equivalent number of full power operating hours which are subtracted from the design life to estimate remaining life.
As an alternative to the equivalent operating hours approach, a series of software modules have been developed which together provide an accurate estimate of hot component life usage based on actual duty cycles rather than assumed worst case. Using continuously available measurements of pressure and temperature along the gas path, the modules link together to cover the thermodynamics, aerodynamics, solid mechanics and damage mechanics calculations necessary to arrive at a continuous assessment of component damage.
The system is installed on two gas turbines at a gas compressor station where it provides valuable information for managing the asset . Maintenance costs are reduced as decision can be made on an 'as needed' rather than an 'as scheduled' basis. Cost/benefit decisions concerned with 'sweating the asset' can be made where the gas turbine operation can more readily be allowed to follow periods of high or low gas prices.
Structural Integrity Monitoring - The Key to Implementation of Novel Strengthening Technology
(Paul Hill, Technical Manager, DML Composites and Alan Kenchington, Director, Structural Statics Ltd)
The current economic climate has prompted a shift in all industries away from the replacement of unsatisfactory structures to repair. This shift has catalysed the development of more cost-effective repair technologies. DML Composites have pioneered the development and application of carbon fibre reinforced polymers as a repair and strengthening technique for metal structures. The implementation of this technology has been based on validation testing at materials, component and full scale. The results of the full-scale work have relied on Structural Monitoring to confirm that performance at this scale matches design, and Structural Statics Ltd have provided the key technology to gather the data required.
This presentation will cover three specific examples of strengthening, detailing how structural monitoring has provided the assurance of success. The examples are:
The monitoring employed and the results of the strengthening will be discussed.
Testing of the London Millennium Footbridge
(Tim Armitage, Ove Arup)
In order to support the development of a passive damping installation for the London Millennium Footbridge and number of test programmes have been undertaken. These test programmes have involved testing of several of the mechanical components that make up the passive solution and full scale tests on the bridge itself.
This presentation will describe aspects of the component tests undertaken at several UK test facilities in order to evaluate the performance of many of the mechanical components proposed for the passive damping scheme.
The presentation will also describe the planning and execution of a series of prototype tests that were conducted in December 2000 in order to validate the passive damping scheme in-situ on the bridge.
ASRANet
(Nina Baker, ASRANet Coordinator)
ASRANet is EPSRC-funded network to encourage the integration of advanced structural analysis (ASA) with structural reliability analysis (SRA). With nearly 120 members world-wide and covering a broad range of industries and academic research interests, the network has developed quickly and is looking forward to some serious work in the coming year. The Network is run by a steering group and is organised into 8 industry-related special-interest groups and 3 generic groups. The Network is aiming to become the first port-of-call for information on current and recent research. It will be prioritising research proposals and advising the EPSRC on what their priorities should be in order to help industry integrate ASA & SRA. To do this the Network is seeking information from industries and researchers of work in ASA (e.g. non-linear FE Analysis) and SRA. Particularly sought, is research that includes validating data from real life structures.
Structural Monitoring of fatigue cracks
(W D Dover and N Tantbirojn, UCL NDE Centre)
Structural monitoring of fatigue cracks with ACPD has been routinely conducted for many years, in the laboratory, using spot welded connections. Array ACPD probes have also been attempted but with limited success due to probe contact problems. More recently ACFM monitoring has been attempted with more success, as electrical contact is not necessary. The presentation will describe recent work involving fatigue tests on high strength steels using ACFM array probes to monitor crack growth.
Conductivity Coefficient and Heat Flow Characteristics Evaluations for Assessing Concrete
(Ramiz Delpak and Ching Wen (Rita) Hu, University of Glamorgan)
This paper discusses the studies conducted in using an in-situ and non-conventional method for determining thermal conductivity coefficient "k" in cementitious and other materials, used for civil engineering construction.
The same method of k evaluation was also used to determine the thermo-mechanical properties in specimens with controlled levels of fabric damage.
Concrete specimens with cylindrical geometry were cast which were then subjected to different compressive loads. The load was applied at different stages with identifiable levels of damage due to non-revesible levels of stress. ATA (active thermographic assessment) method was used to partially heat the samples. The steady-state and transient heat flow patterns were recorded through IR thermography. In addition to thermal measurements, a record was kept of the density and the axial stress in the test-piece.
Thermo-mechanical properties of the specimen were determined using a comparison (or matching) technique from repeated sets of FE analyses. Using this method the changes in material thermal properties, such as thermal conductivity, could be related to the state of the fabric distress. It was then possible to observe the full variation range in material properties from the "pristine" condition to "near failure" state. There are key indicators which emerge from the data, to show clear relationship in variation between the factors influencing heat flow characteristics and the state of material integrity.
It is suggested that the proposed method could be developed to be a standard form of Non-destructive Evaluation (NDE), in order to assess damage in the built environment infrastructure by using remote and non-contact sensing.