The wall thickness of a steel pipeline is the most relevant factor in the pipeline's capacity to withstand the loads imposed during installation and under operating conditions, as well as a considerable factor affecting pipeline costs. Collapse and local buckling check are used for wall thickness selection in ultra-deep water applications of large diameter line pipes. The external pressure combined with bending load causes the pipeline to develop ovalisation buckling. Minimum wall thickness requirements are strictly linked to the material specifications. Two parameters are very important for the cross section capacity to sustain the external pressure:

  • the cross section ovality (maximum value of fabrication ovality allowed by DNV OS-F101 is 1.0 and 1.5% at pipe ends and body, respectively) and

  • the specified minimum yield stress through the pipe wall thickness (usually a reduction of 15 and 7% is to be expected in the hoop compressive SMYS with respect to the tensile SMYS for steel pipes manufactured by the U-O-E and U-O process, respectively).

The increase compressive yield strength in the circumferential direction, as well as the reduction of the maximum cross section ovality during regular mass production of UOE line pipes, will allows reducing the required steel wall thickness. The effect of such optimized line pipe mechanical and geometrical characteristics [8, 9] on the pipeline strength under the combined loading conditions i.e. in combination with external pressure loads, is addressed in this paper. Relevant loads considered for the local buckling check are bending moment/curvature and axial forces on the pipe shell. The effectiveness of the design equations of relevant Codes and Standards (API, ISO, and DNV) in light of the results of FE analyses that accounts for the mentioned optimized line pipe characteristics is discussed.

INTRODUCTION

Dedicated international codes, particularly the most recent issues, suitably cover design conditions and sizes envisaged for deep water pipeline projects.

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