REVIEW OF 'REVIEW OF STRUCTURAL MONITORING'
by FUGRO (OTO 97 040)
(Report available from the Research Strategy Unit, Bootle. Fax: 0151 951 3098)
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REVIEW OF 'REVIEW OF STRUCTURAL MONITORING'by FUGRO (OTO 97 040) |
This report contains the results of a study undertaken for the UK Health & Safety Executive, completed in 1998. Structural monitoring is considered to be the process whereby response characteristics of a fixed offshore jacket structure are measured (either continuously or at regular intervals), with a view to comparing the measured characteristics with a previously reported baseline.
Several monitoring projects of this type were carried out offshore in the late 1970's, monitoring changes to the natural frequencies (and mode shapes) as a result in any reduction in the stiffness of the structure. Since then the design of jackets has evolved and structural instrumental technology has developed significantly.
The early projects are briefly reviewed in the report. Several platforms were monitored with different levels of instrumentation. These included Forties A and B, Heather, Piper Alpha, Ninian South and Northern and Magnus. A number of general conclusions are drawn from these early studies, particularly relating to the effectiveness of the measurement of natural frequencies in detecting member severance. It is concluded that normal variations in the platform natural frequencies are small, of the order of 1%, which provides a baseline for any measurement of change due to structural effects. The projects also measured the effect of member severance, at various locations within the structure. Details are given of 14 separate measurements on a range of structures, due to severance of members in different locations.
In many locations the measured change is small (1-3%), but for severance of key diagonal members the change was of the order of 10% in frequency. The overall conclusion was that the loss of a diagonal in a K braced framed structure results in a frequency change of 9.5-11.5% (detectable), whereas a similar loss in an X braced framed structure would result in changes of only 1-2% (not-detectable).
The report also reviews current developments in technology relevant to structural monitoring. Sensors are highly sensitive and accurate devices, well proven in the offshore environment. Data collection has changed with the introduction of cable-less underwater sensor packages, which are battery powered and data is transmitted by hydro-acoustic telemetry. However it is claimed that the use of batteries limits the operating time for the equipment, and the introduction of the receiver into the water, when data is to be collected may require manual intervention. Data processing has improved considerably with the development of computers. A standard desk-top PC can perform the necessary data acquisition, signal processing and archiving. Such systems can also require no operator intervention. Communication technology has also improved, enabling data to be transmitted onshore automatically.
The report also considers developments in structural analysis, and in particular ultimate strength behaviour. Safety issues are normally concerned with maintaining sufficient residual strength in the structure to enable repairs to be undertaken before the overall integrity of the installation is compromised. Some experimental and analytical studies have shown that member severance does not normally lead to collapse. However current published data available does not in general associate stiffness changes with changes in ultimate strength.
The report suggests that it may be possible to assess the suitability of s structure for monitoring in terms of its redundancy after the occurrence of damage. It is concluded that platforms will be suitable for monitoring if any member failure or combination of member failures which places the structure in an unsafe condition would result in a change of stiffness of about 3% or more. It is considered that many North Sea platforms would fall in this category. However there is a need for a consistent approach to evaluating system reserve, in the context of stiffness. On this basis as long as the stiffness does not change significantly during monitoring then there is an acceptable level of redundancy remaining in the structure. Structural monitoring would also provide early warning of any single or multiple member severances, as it can be performed continuously.
Further work is recommended to evaluate certain platforms for which early monitoring was carried out (e.g. Ninian Northern), and in particular to perform redundancy analyses on these jackets to assess whether a relationship between redundancy and suitability for monitoring can be established. Also a review of the results from experimental and analytical studies associated with redundancy could possibly establish stiffness changes associated with redundancy levels.
Overall it is concluded that a proven link between stiffness and redundancy could provide a technique which would be low cost addition to the range of techniques available for in-service inspection of platforms.