Up to a decade ago fixed offshore platforms in the Gulf of Mexico were inspected below the water line only after extreme loading from hurricanes or ship collisions. However. increased investment in new platforms has led to oil companies introducing regular inspection routines. and some countries have laid down regulations for instrumentation and inspection of offshore install~ tions. to ensure safe operation and a feedback of data to designers. The conventional techniques used for inspection of offshore structures include: (a) visual inspection by divers and use of underwater television with video and sound recording; (b) magnetic particle and ultrasonic crack detection. methods. These all require diver access. and are feasible in moderate water depths and in areas with long and predictable periods of calm weather. Neither of these conditions is met in the northern North Sea. There is a demand for inspection methods which will monitor the performance of platforms under any weather conditions. and. in particular under storm conditions. The methods must give unambiguous warning about failures which could endanger the installations. Furthermore. they should depend on only a small number of transducers all positioned above the waterline for easy access and maintenance. 1 The purpose of this work was to investigate a method of detecting damage from changes in the natural frequencies of a structure. The main objective was to investigate what changes in natural frequencies would occur when a primary load-carrying memner became detached from a typical fixed offshore platform. The platform design was a four-leg. K-braced platform for which approximate dimensions were available. and which operated in 70 metres of water. A 1/20th scale model was also designed and the structural analysis programme ICES STRUDL II used to compute the natural frequencies and mode shapes of both the full scale and model platforms. Before STRUDL II was applied to the quite complex offshore platform. analyses were carried out on two simple structures to gain expe:ience in using the programme. Natural frequencies and mode shapes were computed and compared with measured values and with analytical solutions where applicable. This preliminary work also gave experience in dynamic response testing. The sensitivity of the natural frequencies of the platform was investigated by removing a single diagonal in one K-panel. and again computing the natural frequencies of the structure. The brace was then replaced and the computations were repeated with other single braces removed in turn. The reductions in the frequencies with each member removed could then be calculated. There were large changes in some of the modes and the pattern of the frequency reductions gave an indication of the location of the f~ilure. 2 The computational study was repeated for the model platform and frequency reductions similar to those for the full scale platform were found. The model platform was built and sinusoidal force excitation was applied to determine its natural frequencies andlmode shapes. Comparison of computed and measured frequencies was generally good. Two K-braces in different planes and at different elevations were severed and repaired in turn; the changes in natural frequencies compared well with the computed predictions. In the third stages of the project a dedicated Time Series Analyser became available and a number of tests were carried out with random excitation. The results for frequencies and mode shapes were in good agreement with those obtained with sinusoidal excitation. This demonstrated the feasibility of obtaining natural frequencies and mode shapes from response to ~andoM excitation. It was concluded for the project that changes in natural frequencies could provide a viable method of detecting gross damage in offshore structures. 3
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:463526 |
| Date | January 1978 |
| Creators | Loland, Olaf |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/6191/ |
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