For offshore structures composed of welded steel members, the use of structural random analysis procedures are recommended in order to conveniently represent the environment load and the structure's response. In this paper, some joints of a jacket type offshore platform were studied in order to determine the fatigue damage by means of three structural random dynamic analysis approaches. One of them is based on the use of stresses RAO's obtained through a dynamic frequency-domain analysis. The second approach allows the obtaining of stresses time-histories from a timedomain analysis, where the sea-state spectra are simulated. The third approach is a hybrid one, as the sea-state are simulated and response is obtained on frequency-domain. The fatigue damage was calculated through a rainflow counting cycles method applied directly on stresses time histories, and through the Rayleigh distribution for stresses spectra. Results are presented and commented.
The tendency of oil and gas exploitation on fields located at deep waters demands the use of offshore structures concepts that are very sensitive to the environmental loads dynamic effects. The dynamic response assumes great importance, mainly for the fatigue damage verification. Offshore structures are subjected to several types of loads during their life that may be static or time-varying ones. When installed, the action of environmental phenomena like wind, current and sea waves, that have a time-varying random behavior, generates stresses fluctuations on the material. In the case of the welded joints of jacket type platforms, this process contributes to the fissures development and propagation, possibly leading to the collapse for fatigue damage. Two basic different approaches are generally used by designers in order to perform the fatigue verification: a) One of them is based on deterministic procedures, where environmental phenomena are represented by deterministic loads, the stresses are obtained from a deterministic dynamic analysis and the fatigue damage is calculated through a deterministic fatigue analysis.
