Numerical simulation of submarine pipelines in dynamic contact with a moving seabed

Abstract

The dynamic response of submarine pipelines to earthquake-generated vertical seabed motions is examined with the aid of a finite element model. A relaxation algorithm is adopted in order to overcome the problem of unknown dynamic seabed reaction forces. Rotational rigidity is assumed at the boundary points, which are allowed to move vertically in unison with the random seabed oscillations. For the interior nodes, the description of the kinetic energy loss, resulting from pipeline-seabed impact, is approached through the use of a restitution coefficient of 0.5, which represents an intermediate collision mode between a perfectly elastic and an inelastic one. Fundamental system frequencies are determined by the use of support positions obtained from a static analysis of the unilaterally constrained structure. The structural damping matrix is approximately evaluated according to Rayleigh's method. Response spectra to a strong motion vertical acceleration earthquake record provide an initial guideline into the pipeline stability. Peak dynamic and static bending stresses are calculated for a case study involving two submarine pipeline crossings. Complete, partial and lack of pipeline gravel cover is assumed in the analysis. A seabed profile alteration, due to probable soil liquefaction, is furthermore imposed, in order to investigate the subsequent effect upon the dynamic pipeline stresses.