Structural integrity monitoring of a fixed-bottom frame tower

As a consequence of the increasing necessity for structural safety of drilling platforms, the need for monitoring techniques capable of detecting early damage of subsurface structural components increases. -- The purpose of this study is to provide theoretical and laboratory experimental evidence that a predictable relationship exists between physical damage of certain structural components, and changes in the resonant frequencies of a structure measured at the deck level. To achieve this objective, experiments were carried out on a k-braced model of an offshore tower using random vibration analyses, and the results were compared to those determined from a finite element model of the structure (free vibration STRUDL II analyses). -- Natural frequencies were determined from the intact and damaged structure and the results, experimental and analytical, were compared to measure the effects of the damaged member(s) on the resonant frequencies of the structure. By removing an inclined member in a k-braced panel to simulate damage of that member, a 22.75% decrease in the natural frequency of the 2nd sway mode was measured on the transfer function of the experimental model; the theoretical results indicated a 16.02% decrease. Investigations into the effect of a member cracking showed a 9.2 % decrease in the excitation frequency of the 2nd sway mode, when a k-brace member was cut halfway across at one joint on the experimental model. -- From the results obtained, it was concluded that the changes in natural frequencies clearly indicated that the structure had, in fact, undergone damage. Moreover, the theoretical results showed that the mode(s) most affected depended on the location of the damaged member(s). -- It is recommended that although the technique appears promising, it should not be viewed as a panacea for solving the problem of early detection of subsurface structural damage with great certainty since certainty, in such a complex environment as the ocean, can rarely be assured. Nonetheless, if the structural integrity monitoring technique is applied in conjunction with conventional detection procedure, the effectiveness of early damage detection will be enhanced