Offshore jacket platforms have been widely used in offshore oil and gas exploitation under hostile ocean environments. Structure damages, which may cause the change of natural frequencies, are unavoidable due to the severe environmental loads. If the natural frequencies of a damaged structure become close to the ambient vibration frequencies, excessive vibration of the platform would occur. In order to reduce the vibration level, a structural modification procedure of reassigning the natural frequencies of the platform is desired. In this paper, a structural modification algorithm aiming at reassigning natural frequencies of an offshore jacket platform is newly developed. The modifications are demonstrated either by installing a tuned mass damper at the deck, or by modifying the stiffness of floor members, of the platform. For verifying the developed algorithm, numerical studies based on finite element models are conducted for a simplified jacket platform. Numerical results indicate that the developed algorithm can properly assign the natural frequencies of the modified platform based on the preset frequencies.
Oil and gas are the major sources of energy in our society. Twenty percent of these hydrocarbons are recovered from reservoirs beneath the seabed [Moan, 2005]. Steel jacket-type platforms, the most common kind of offshore structures, have been widely used in offshore oil and gas exploitation. These platforms are subjected to various kinds of environmental loading, such as wind, waves, current, ice and so on. Eventually, if the natural frequencies of the damaged platform fall into the frequency range of the dominant excitations, excessive vibration of the platform can follow. The excessive vibration not only endangers the structure integrity but also causes the discomfort to people working on the platforms. One solution to remedy this problem is to reassign the structure's natural frequencies, keeping them away from those of ambient loads.
