In offshore pipeline engineering, the stability of the pipeline is critically affected by the degree of penetration into the seabed. Ideally, the pipeline should be embedded into the seabed by at least one diameter, eliminating direct hydrodynamic loading and minimising risk of scour. The embedment may be achieved by trenching, or by selfburial under the combined weight of the pipeline and the effects of cyclic lateral loading. In order to be able to predict such self-burial, and to assess the lateral stability of partially embedded pipelines, a finite element technique has been developed, based on (a) small strain analysis of incremental penetration, and (b) automatic re-meshing and interpolation of field quantities (stresses, strains and material properties) at frequent intervals. In this way, the analysis can follow large deformation of the seabed during penetration of the pipeline. This paper describes the analysis procedure and presents results for different non-homogeneous soil conditions. Results are compared with analyses without re-meshing, with classical plasticity solutions, and with experimental data for pipeline penetration into uncemented calcareous soil.
Offshore pipelines must be designed to resist hydrodynamic loading from seabed currents, and also to remain stable against buckling under axial forces arising from temperature changes. Stability of the pipeline is generally achieved by partial burial below the seabed. The last question is critical, since if it can be shown that the pipeline will tend to embed further under limiting lateral stability, then the required embedment under self-weight alone may be relaxed. Current design rules for pipeline embedment and lateral stability are based on a combination of empirical data, and traditional bearing capacity methods. The effects of the changing geometry of the seabed adjacent to the pipeline during pipeline penetration are difficult to quantify accurately.