Structural integrity monitoring of a hydro-elastic model of a jacket platform

Marshall, Mervin Allan (1990) Structural integrity monitoring of a hydro-elastic model of a jacket platform. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (37MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

As a result of the increasing requirement for structural safety of offshore drilling platforms, the need for monitoring techniques capable of detecting damage of subsurface structural components increases. Since it is not possible, because of limited time constraints, to carry out tests on an actual operating platform in the ocean, an alternative solution is to experiment on a reduced model of an offshore structure. Very few experiments using a truly hydro-elastic model of a jacket platform have been published. -- This treatise deals with a detailed design and construction of a hydro-elastic model of a typical four-legged jacket offshore platform to carry out structural integrity monitoring. Both an indepth theoretical and experimental analysis were carried out. Experiments were conducted in a wave tank equipped with a pseudo random wave making facility. To obtain the necessary modal parameters (e.g., resonant frequencies, and damping ratios) from the ambient response data, the recently developed Marple algorithm was used with the very powerful Maximum Entropy Method (MEM). Regarding structural integrity monitoring, structural damage was simulated by saw cutting the member(s) under consideration. In the theoretical analyses, this was done by making the member(s) inactive in the analyses. The detection of damage was based on the changes in resonant frequencies compared with the intact structure. -- Resonant frequencies were determined for the intact and damaged structure and the results (i.e., experimental and theoretical) were compared to measure the influence of the damaged member(s) on the resonant frequencies of the structure. By cutting an inclined member in a k-braced panel to simulate damage of the member, a 39% decrease in the resonant frequency of the 2nd flexural mode was measured on the spectral density function of the wave induced vibration from the hydro-elastic model. The theoretical results showed a 29% decrease. (Theoretical results from the prototype structure showed similar trends.) -- Based on the reported results and evaluations, it was concluded that the resonant frequency changes clearly indicated the structure had been damaged. Besides, the theoretical analyses showed that the global modes most affected from the damage depended on the location of the damaged member(s).

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