PIPE-SOIL INTERACTIONS FOR A PIPELINE WITH STRAKES ON A FURROWED SEABED

Although significant pipe-soil interaction information is now available in the industry for shallowly-embedded, cylindrical pipelines on a flat seabed, there is little information available for subsea pipelines with vortex-induced vibration (VIV) suppression stakes on a furrowed seabed. A heavily furrowed seabed may have multiple free spans that are subjected to seabed currents and potential VIV. To suppress VIV at the free spans, stakes are often applied to the pipeline over the entire length of the furrowed area. At the tops of the furrows, the pipeline will penetrate further into a soft clay seabed due to the weight of adjacent free spans. Significantly different pipe-soil interactions are possible in these areas, especially with the presence of VIV suppression strakes. Accounting for these differences is essential when assessing thermal expansion issues, such as global lateral buckling and pipeline walking. This paper presents the findings for a test case of a pipeline with strakes on a furrowed seabed and provides suggestions for a user subroutine to be used for a pipeline system with beam elements in a finite-element analysis. The CEL method is first used to assess the impact of stroked-pipeline penetration into a flat, soft-clay seabed, then to assess lateral and axial pipe resistances at different penetration depths. This information is used to develop a user subroutine for beam elements to further assess the pipeline on a furrowed seabed, in which the effects of vertical pipeline load and penetration on the lateral soil resistance are included. The test case also considers a typical analysis for a pipeline subjected to elevated temperature and pressure cycles.