Structural monitoring of offshore platforms using impulse and relaxation response

Monitoring offshore platforms, long span bridges, high rise buildings, TV t owers and other similar structures is essential for ensuring their safety i n service. Continuous monitoring assumes even greater significance in the c ase of offshore platforms, which are highly susceptible to damage due to th e corrosive environment and the continuous action of waves. Also, since a m ajor part of the structure is under water and covered by marine growth, eve n a trained diver cannot easily detect damage in the structure. In the pres ent work, vibration criterion is adopted for structural monitoring of jacke t platforms. Artificial excitation of these structures is not always practi cable and ambient excitation due to wind and waves may not be sufficient fo r collecting the required vibration data. Alternate methods can be adopted for the same purpose, for example, the application of an impact or a sudden relaxation of an applied force for exciting the structure. For jacket plat forms, impact can be applied by gently pushing the structure at the fender while relaxation can be accomplished by pulling the structure and then sudd enly releasing it using a tug or a supply vessel in both cases. The present study is an experimental investigation on a laboratory model of a jacket p latform, for exploring the feasibility of adapting vibration responses due to impulse and relaxation, for structural monitoring. Effects of damage in six members of the platform as well as changes in deck masses were studied. A finite element model of the structure was used to analyze all the cases for comparison of the results as well as system identification. A data acqu isition and analysis procedure for obtaining the response signatures of the platform due to the: impulse and relaxation procedure was also developed f or possible adoption in on-line monitoring of offshore platforms. From the study. it has been concluded that both impulse and relaxation responses are useful tools for monitoring offshore jacket platforms. The present work fo rms the basis for the development of an automated, on-line monitoring syste m for offshore platforms, using neural networks. (C) 2000 Elsevier Science Ltd. All rights reserved.