Abstract: Because of the widely adopted laying technologies, such as J-laying method or S-laying method in deep sea, untrenched pipelines often stay in shallow-embedded state. In this case, lateral displacements may occur due to loading of waves, currents or thermal buckling in practice, while seabed is in limit state subject to the combined load of pipeline self weight and horizontal expansion force. Therefore, in order to accurately evaluate the stability of shallow-buried pipelines under this combined loading condition, the envelopes of bearing capacity of shallow-buried pipelines in vertical-horizontal(V-H) load space and the main influencing factors in sand seabed are studied by finite element numerical analysis method. A series of model tests in laboratory is conducted to verify the reliability of numerical results. The results show that the failure envelopes expand significantly but their shapes remain unchanged with the increase of internal friction angle φ and relative embedment depth z/D. Based on this, fitting formulas are established to describe the changing laws of parameters Vmax and μ0 depend on φ and z/D correspondingly. In practice engineering, the parameters of seabed soil and the possible load combination can be incorporated into the envelope analysis to evaluate the on-bottom stability of pipelines.

Key words: shallow-embedded pipeline, failure envelopes, pipe-soil interaction, model test