A Comparison of Simplified Engineering and FEM Methods for On-Bottom Stability Analysis of Subsea Pipelines

Abstract

This paper presents a comparison between simplified engineering and FEM (Finite Element Method) methods for on-bottom stability analysis of a subsea pipeline. The simplified engineering method is first used to assess the absolute on-bottom stability of empty and filled pipelines under different scenarios. The calculations of the hydrodynamic loads for three scenarios, i.e. steady current alone, regular waves alone and combined regular waves and current, are implemented in MATLAB code. The drag and lift coefficients are determined based on Keulegan-Carpenter number, Reynolds number and surface roughness of the pipelines. Only the friction force is considered in the simplified methods. In order to achieve the absolute stability, the vertical (lift force/submerged weight<1) and horizontal (in-line force/friction force<1) criteria need to be fulfilled at same time. Time-domain dynamic on-bottom stability analysis is performed by PONDUS for the same cases. The results of water particle velocity, hydrodynamic force, lift force and soil resistance force are compared between the simplified engineering and advanced FEM methods. Their results are in good agreement for the cases, which fulfills the absolute on-bottom stability criterion. For the cases which the pipelines will move under the combined wave and current loadings, the soil resistance force predicted by the simplified engineering method is different from that of the FEM method. The study shows that for engineering purpose the simplified engineering method could be used to check the absolute on-bottom stability of the pipeline, whereas the more advanced FEM method needs be performed when the pipeline is allowed to move within a limited distance.