A numerical model for geometrically nonlinear analysis of long free spanning offshore pipelines enhanced with buoyancy modules

Despite a detailed oil and gas pipeline route assessment, the emergence of highly long free spans induced by seafloor roughness in deep waters, accompanied by significant gaps between the pipeline and the seafloor, can sometimes be unavoidable. In those cases, cost-efficient span correction can be achieved by attaching buoyancy modules along certain sections of the free span. This paper provides a simple numerical technique for static analysis of submarine pipeline free spans enhanced with buoyancy modules and resting on span shoulders of nonequal elevations. The entire pipeline is modeled as a single continuous segment considering geometrical nonlinearity induced by large deformations. The solution technique is based on consistent minimization of the total potential energy of the system discretized as a Riemann sum and the solution of the resulting algebraic system of nonlinear finite difference equations with a Newton-Raphson technique. The feasibility of the proposed technique is demonstrated through several pipeline free span scenarios. The effects of buoyancy module section length, its distribution, effective axial tension, span shoulders soil stiffness, and span shoulders elevation gap are considered. The proposed technique provides a fast and accurate alternative to special-purpose finite element commercial software, particularly throughout the intermediate stages of the design process.