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The paper presents structural health monitoring (SHM) system, dedicated to marine structures. The considered system is based on the fibre optic technique with Fibre Bragg Grating (FBG) sensors. The aim of this research is recognition of possible practical applications of the fibre optic techniques in selected elements of marine structures. SHM and damage detection techniques have a great importance (economical, human safety and environment protection) in the wide range of marine structures, especially for ships and offshore platforms. In the paper monitoring system of the Horyzont II and Dar Młodzieży ships and offshore oil platform is presented. Practical implementation of safety system based on optical sensors meets several difficulties. There has been installed the FBG system and its measurement results have been compared with classical techniques, e.g. piezoelectric accelerometers. The investigations have been performed for undamaged and damaged structure. Different types of failures have been modelled and tested. Damage detection ability has been specifying on the base of static and dynamic structural characteristics.

a b s t r a c tWe discuss the non-linear theory of thin shells expressed in terms of displacements of the shell reference surface as the only independent field variables. The formulation is based on the principle of virtual work postulated for the reference surface. In our approach: (1) the vector equilibrium equations are represented through components in the deformed contravariant surface base, and using the compatibility conditions the resulting tangential equilibrium equations are additionally simplified, (2) at the shell boundary the new scalar function of displacement derivatives is defined and new sets of four work-conjugate static and geometric boundary conditions are derived, as well as (3) for prescribed shell geometry all non-linear shell relations are generated automatically by two packages set up in MATHEMATICA. The displacement boundary value problem and the associated homogeneous shell buckling problem are generated exactly without using any additional approximations following from errors of the constitutive equations. Both problems are extremely complex and available only in the computer memory. Such an approach allows us to account also for those a few supposedly small terms which may be critical for finding the correct buckling load of shells sensitive to imperfections. This approach is used in the accompanying paper by Opoka and Pietraszkiewicz [Opoka, S., Pietraszkiewicz, W., 2009. On refined analysis of bifurcation buckling for the axially compressed circular cylinder. International Journal of Solids and Structures, 46,[3111][3112][3113][3114][3115][3116][3117][3118][3119][3120][3121][3122][3123] to perform the refined numerical analysis of bifurcation buckling for the axially compressed circular cylinder.

The lifetime prolongation of the old industrial structures is an important economic issue. Damage detection techniques can be helpful to warn about any structural deterioration and to assure sufficient time for counteraction activities. In the paper, the results of strain measurements on scaled leg model of offshore oil platform with different damage cases are presented. The considered damage cases were yielding, chord cutting and brace-chord detachment located in the bottom part of the model. Yielding was detected mainly by the sensors located in the lower part of the structure. On the other hand, brace-chord detachment was detected by all sensors on the structure. Therefore, the global character of the considered damage indicator is proven at least for some important class of damage cases. Due to transient response during brace-chord detachment and interpretation problems of results for partial chord cutting, the necessity of continuous monitoring was evident. The time point of detachment occurrence can be easily detected using difference or cross-correlation technique. The change in static-strain levels together with the detection of transient signal seems to be a simple and an effective damage indicator based on interrogation of thermo-mechanical strains.

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