Dynamic Behavior of Deep‐Ocean Pipeline

The design of bottom‐laid deep‐ocean pipelines requires estimates of the dynamic stresses and motions that result from a random wave environment. A 3‐D finite element algorithm is presented that estimates the large displacements of pipelines sliding on irregularly contoured marine sediments and subjected to episodic hydrodynamic loads. Nonlinear elastic‐plastic springs simulate transverse and axial bottom resistances for both cohesionless and cohesive sediments. Inclusion of a hydrostatic pressure differential across the pipe wall modifies axial stresses and influences the flexural stiffness of the finite element model through the geometric stiffness matrix. Hydrodynamic inertia and nonlinear viscous drag forces are represented by a relative‐motion form of the Morison equation. The equations of motion during each episode of extreme wave activity are integrated using the Newmark method of implicit integration. Within each time step, a Newton‐Raphson scheme of iteration is used to satisfy equilibrium while accurately predicting the nonlinear path‐dependent response. An updated Lagrangian formulation shifts the reference configuration of the pipeline and separates rigid‐body movements from pipeline deformations. This provides a large‐deflection, small‐strain, transient analysis of the pipeline motion, which is capable of computing finite‐amplitude motions that have been accumulated over the random wave episodes.