Simulation of Thick-walled Submarine Pipeline Collapse Under Bending and Hydrostatic Pressure
Authors: Al-Sharif A. M. , Preston R. 
Published: June 2014

Abstract
The problem of submarine pipeline buckling or collapse as a result of bending and external pressure is investigated by numerical modeling using finite element analysis. The model takes into account the initial variability of material properties, the effect of cold-work on the pipe material properties and initial geometric imperfections. It is capable of simulating the nonlinear behavior, and structural instability due to the combined effects of bending and pressure. The solution algorithm and verification against experimental result are presented. In additional deterministic model for collapse under combined pressure and bending based on measured stress - strain behavior and pipe geometry is derived. Resultsfrom both finite element and deterministic models for different parameter sensitivities are examined.

Introduction
The Ornan-India pipeline (OIP) will be laid in water as deep as 3500 m. As a result, during pipe laying and in subsequent service the pipe will experience a high external pressure that will constitute a threat to the structural integrity of the pipeline. Moreover, bending stresses induced during construction and pipe manufacturing residual stresses are known to reduce the pipe resistance to collapse. In order to address these and other issues, and as part of the overall pipeline design development process, Oman Oil Company (OOC) instigated a collapse testing and analysis program which began in March 1994.
From the test program, the following conclusions were obtained:The UOE manufacture process degrades the collapseresistance of line pipe1Thermal aging produces a recovery in collapse resistance(2)
Due to the complexity of the mechanism of failure under external pressure, there is a level of uncertainty even today regarding the geometrical and material relative factors that influence the final critical collapse state. Moreover, published work does not support the application of existing theoretical models for pipes with the characteristics required for OIP. ?4)This paper discusses the techniques used to develop, verify, and utilize numerical and analytical collapse models suitable for use in the design of large diameter thick wall pipe for deepwater use.

Wall Thickness Considerations in Deepwater Pipeline Design
For gas pipelines in deep water internal pressure design considerations can be secondary to external pressure considerations, since during pipe laying and in subsequent service, the pipeline may not always be internally pressurized to resist the external pressure tending to collapse the pipe. For a gas pipeline therefore, the design should be such that the pipe can withstand the full external pressure with atmospheric internal pressure without collapsing. Figure 1 shows the typical magnitude of OIP pip wall stresses relative to yield stress under these different loading conditions.
For a thick wall pipe, the external pressure produces a similar pattern of circumferential stresses as produced by internal pressure but with the sense reversed (i.e. compressive instead of tensile), When the stress exceeds a critical value, a thick wall pipe will buckle by progressive plastic deformation (i.e. bifurcation type failure).