Free-Spanning Submarine Pipeline Response to Severe Ground Excitations: Water-Pipeline Interactions

Abstract

Submarine pipelines are usually laid unburied. Free spanning may be caused in the line by seabed unevenness, topology changes, scouring, or sand waves. In seismically active areas, the free spannings are prone to severe ground excitations. This article deals with the seismic performance of the submarine pipeline free spanning, and the water/pipeline interaction during the event is its focal point. For this, a numerical finite-element model has been used. Results from a conventional added mass approach have been examined against those from a more elaborate coupled acoustic-structural model. With the coupled system the surrounding seawater, pipe body, seabed, and free spanning have all been incorporated into the model. Both random earthquake and harmonic excitations have been considered. A time domain incremental dynamic analysis (IDA) method has been used to evaluate the pipeline performance. Effects from type, frequency, intensity, and direction of excitation and the free-span length on the pipeline response have been investigated. In most cases, the added mass approach has been found to provide more conservative predictions (in some cases up to 60%) for the free-spanning seismic response in comparison to those obtained from the acoustic method. For the earthquake excitations, the predictions obtained from each of the two methods have been more consistent as compared to the results obtained for harmonic excitations.