Soil Resistance to Lateral Sliding of Marine Pipelines

Abstract

ABSTRACT Pipeline lateral sliding tests show that the Coulomb friction model is adequate for specifying minimum pipe weight for lateral stability on sand but not for pipelines on soft clay. A finite element model was developed to predict resistance to sliding on soft clay. Friction coefficients derived from the test data are recommended for use as upper limits in design against lateral sliding on sand. To determine resistance to sliding on soft clay, the finite element model-or a similar procedure-should be used. INTRODUCTION New data show that the Coulomb friction model is adequate for specifying minimum pipe weight to prevent lateral sliding of marine pipelines on sand but not on soft clay. The fact that pipes do not settle into sand but do settle into clay accounts for this different sliding behavior. A finite element model was developed to predict resistance to sliding on soft clay. Friction coefficients derived from smal1scale and large-scale tests on sand (simulating ocean floor conditions) essentially were independent of submerged pipe weight, a requirement for the Coulomb friction model to be applicable (Fig. 1). Friction coefficients derived from soft clay tests were not independent of pipe weight. Soil resistances to sliding predicted by a finite element model correlated well with the soft clay data. This model included the effects of nonlinear soil properties and large displacement geometric nonlinearities. The friction coefficients for sand are considered to be the best available and are recommended for use as upper limits in design against lateral sliding. These data should be used only when the ratio of vertical to horizontal components of hydrodynamic force are approximately 3:4. To design pipelines for use on soft clay, we recommend that a procedure similar to the finite element model be used to determine soil resistance to sliding. If such a procedure is not available, these data can be used as a guide to choose design friction coefficients for use with the Coulomb friction model. For specific applications, consideration should be given to soil strength, pipe diameter, submerged weight, and pipe surface condition. EXPERIMENTAL DATA Small- and large-scale tests on sand and large-scale tests on soft clay were performed to develop data on pipeline lateral sliding. Tests were conducted on a submerged soil bed with hydrodynamic loads simulated by externally applied gravity loads (Fig. 2). Loads were applied incrementally up to the sliding load, and vertical and horizontal displacements were measured after each increment. Table I gives a summary of the tests. Tests on Sand Data from the small-scale tests show that the friction coefficient between pipe and sand is approximately constant (Fig. 3A), so Coulomb friction adequately describes the pipe-sand interaction.