Research On Seismic Design Methods For Subsea Pipelines: Extreme Design

Engineering practice shows that the ultimate stresses in subsea pipelines calculated by current seismic design methods are much larger than the real stresses induced by earthquakes. Moreover, the calculated stresses are nearly unrelated with the geometric parameters of the pipelines such as the diameters, wall thickness or cross-sectional areas. The stresses also have nothing to do with the properties of the covering soil on the pipe. Such problems have frustrated the pipeline designers.A comprehensive investigation on soil-pipeline interaction during earthquakes is performed in this paper. It is found that the ultimate shear strength of the soil surrounding subsea pipelines decreases greatly because of its submerged unit weight and nearly saturation of water content, and the elastic constraint zone of the soil acting on the pipelines is apparently decreased, which force the constrained soil into plastic slippage state. A criterion is developed to predict the constraint state between soil and the pipelines. The results demonstrate that the elastic constraint range of the soil on the pipeline is quite small and the plastic slippage of the soil takes place soon after the elastic state. It is strongly recommended that the constraint state of the soil be determined by the developed criterion before calculating the seismic stress of pipeline. The seismic stress can be calculated on the basis of the plastic slippage theory if plastic constraint state of the soil is predicted. The method is applied to engineering calculations for a subsea pipeline project. The results show that increasing the wall thickness of the pipeline and decreasing the buried depth are two effective measures for decreasing the ultimate seismic stress. The increase of the outer diameter of the pipeline does not have apparent effect on the decrease of the stress because of the increase of the constraint of the soil on the pipeline. The work of this paper plays an important role in seismic design of subsea pipelines, seismic risk assessment, residual strength and service life estimation of the pipelines.Four approaches for calculating the ultimate constraint force of unit length are proposed based on the above theory and respectively named the unit length frictional force method, the shear strength method, the amend method and the weighting average method. The developed methods are applied to two engineering projects, and the results are comparatively discussed, especially with the results from the present methods. The results demonstrate that the ultimate seismic stress methods developed reflect the soil-pipeline interaction during earthquake waves, and solve the problems existing in the present methods.