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 excitation

Free Span Analysis for Submarine Pipelines

“…For interaction of the soil and pipe, the definition of the friction coefficient has been used [39]. e assumed friction coefficient for the pipe and soil bed has been considered as 0.3.…”

The Application of the Coupled Acoustic-Structural Approach (Casa) Method on the Free Vibration of Submerged Structures

“…The results showed that the wetted frequencies were lower than the dry frequencies due to added mass effects. Zeinoddini et al (2012) focused on the seismic behavior of a freespanning submarine pipeline. They used a numerical finite-element model and compared its results to those obtained through a traditional added mass approach and a more complex CASA method.…”

Analytical fragility curves for assessment of the seismic vulnerability of HP/HT unburied subsea pipelines

“…However, to the knowledge of the authors, the seismic studies of high pressure/high temperature (HT/HP) pipelines so far has been limited to buried and trenched pipelines [24]. A subsea HT/HP pipeline can undergo large bending stresses and strains due to lateral/upheaval buckling or free-spanning [25,26,27,28]. If the compressive strain in the pipe wall exceeds a certain limit, structural instability, in the form of local buckling/wrinkling may occur [29].…”