Nonlinear Seismic Analysis of Free Spanning Submarine Pipelines: Effects of Pipe-Water Interaction

Abstract

The seismic response of free spanning submarine pipelines has received little attention in the literature. This paper deals with this subject and its main focus is on water-pipeline interaction during the event. The added mass approach provides a simple water-pipeline interaction model which is commonly used for the dynamic analysis of submarine pipelines. With free spans, however, earthquake components normal to the seabed also contribute to the dynamic pressures inputs to the pipeline. The line, therefore, becomes subject to loads which might be overlooked in a simplified added mass approach. To provide a more accurate estimation for the water-pipeline interaction, the pipe body, the sea bed, the free-spanning and the surrounding water have been incorporated in a numerical finite element model. The accuracy of the model has been verified by correlating the dynamic characteristics of the model with some analytical solutions. The models have then been analyzed under both harmonic excitations and extreme ground motion records. The site effects have been ignored. Both the horizontal and vertical excitations have been considered. A non-linear incremental direct integration dynamic analysis approach has been employed. Deviations up to 14% have been observed between the model predictions and those from the simple added mass method. The maximum deviation has been found to occur when the frequency contents of the excitation has been close to the first natural frequency of the free spanning pipe.