Flexibility monitoring of offshore platforms

Abstract Flexibility monitoring has been proposed as an ambient vibration monitoring technique for detecting the occurrence and location of damage on steel jacket offshore platforms. The technique is based on the heuristic observation that the fundamental mode shapes of platforms closely resemble the deflections of a shear beam caused by a concentrated load at its free end in the associated direction. This paper presents a theoretical framework for evaluating the flexibility monitoring concept. Detailed studies are performed with the following three objectives: 1. 1. To investigate the nature of the relationship between the flexibility ratios used in damage detection that are obtained from mode shape measurements and the true flexibility ratios for a structure derived from its stiffness properties. 2. 2. To assess the effects of both random and bias errors arising from inaccuracies in the data measurement systems and parameter estimation algorithms on the reliability of damage detection. 3. 3. To assess through a modal sensitivity analysis: (i) the influence of the structural framing system on the damage detection ability, (ii) the damage detection capability under multiple member failures, and (iii) the ability to discriminate between structural damage and changes in foundation stiffness and structural mass. The paper also includes a brief introductory review of the history of vibration monitoring research in order to provide the necessary perspective within which to view the present development.