Engineers Australia

Equivalent absolute lateral static stability of on-bottom offshore pipelines

Y. Tian and M.J. Cassidy

Abstract

Although Dynamic Lateral Stability Analysis (DLS) is highly recommended for analysing offshore pipeline stability by authoritative recommended practice, namely DNV (2007) and PRCI (2002), it is still limited in its practical applications due to its complexity and because the software required is not widely available. In contrast, Absolute Lateral Static Stability (ALSS) analysis, in which the critical state is that the hydrodynamic loads on a pipe segment do not exceed the soil resistance, is still widely used in industry design. It is usual for ALSS analysis still to be based on the concepts of simplistic Coulomb friction model and the Morrison equation to account for soil resistance and hydrodynamic loading, although both are criticized for their conservatism and less theoretical basis. This paper presents a suite of new design charts with tabulated data using the Fourier method and pipe-soil models based within a plasticity framework for evaluating hydrodynamic loads and soil resistance, respectively. The results are presented as equivalent soil friction factors and hydrodynamic coefficients using the ALSS framework. These can be consulted by pipeline designers to give extra insight into an offshore on-bottom analysis without running the numerically complex DLS.