Study Of Pipe-soil Interaction Of Touchdown Zone On Steel Catenary Riser

Steel catenary riser (SCR) is widely applied and extensively investigated due to its engineering significance in offshore petroleum industry. As a hot spot of fatigue on steel catenary riser, touchdown zone (TDZ) experiences repeated impact from seabed. On axial and lateral direction, the main effect of soil resistance is friction, which may lead to serious fatigue wear on the pipe surface. The calculation of frictional resistance mainly depends on the coefficient of friction, which changes over a large range because of the uncertainty of soil structure. In the current applications in practical cases and numerical simulations, this coefficient is generally extracted by experience or by standards and is set from 0.6 to 0.8. As the coefficient may change with different types of soils and the range of the empirical value is so wide, the accuracy of fatigue life prediction is diverted in a large scale.In this study, five typical soil samples with different particle distributions are applied according to the circumstances of China sea geotechnology. Six riser models with different diameters and weights, and a set of experiment equipments are designed and constructed. Riser models are dragged on the soil samples to measure friction coefficients and over 600 sets of experiments are conducted to get adequate data for further analysis. The results of this experiment is analyzed statistically to find a general relationship that reflects the influence of the effective size of soil particles, model weight and model diameter on probability distribution of friction coefficient, quantity of friction coefficient and variation coefficient of friction coefficient. It is found that friction coefficient increases when the effective size of soil particle grows, but decreases when the weight of riser model increases. A brief model is proposed to explain the phenomenon in this experiment. A curve of the range of friction coefficient that varies with the effective size of soil particles is plotted to offer a reference for riser calculation. More accurate friction coefficient can be found by merely realizing the information of local soil particle size distribution.