Prediction on safety length of free spanning pipeline near seabed

Applying the lift force distribution along a free spanning pipeline near seabed obtained by the modified potential method, the displacements and stresses of the pipeline with steel-concrete structure were investigated by the pipe dynamics equation. Using the finite element analysis and iterative method, the effects of concrete stiffness, constraints in span shoulders and current velocity on the maximal displacement and stress were analyzed. The stiffness failure and strength failure were discussed when the spanning lengths extend. The safety lengths zone schemes were drawn under different gap ratios and current velocities. The results suggest that the stiffness failure takes place first when the spanning length increases and the strength failure then may occur after the pipeline touching seabed. The gap ratios and constraints at the span shoulders are important factors in the allowable spanning length. The static critical allowable length should be considered firstly when the current velocity is relatively small.