Effect of Ovality on the Collapse Pressure of a Subsea Pipeline at High Outer Diameter-to-Wall Thickness Ratios

Subsea pipelines are primarily used in the transportation of oil and gas from the excavation site to the oil refinery. These pipelines are usually put under immense internal and external ambient pressures during their operation. The pipelines are therefore designed to account for the supported pressure and to ensure lower in-service failure probability. However, manufacturing issues arising from precision of machine tools and residual stresses in raw materials tend to cause the physical pipes to be geometrically imperfect. Imperfections including ovality (out-of-roundness), uneven wall thickness and longitudinal eccentricity (conical sections) may render the original design unsafe during the operation of the pipes. Experimental observations indicate that the collapse of pipeline pressure can be potentially overestimated if the geometrical imperfections are not considered in the design. Finite element analysis (FEA) that includes the geometrical imperfections in the pipeline model gives a better estimation of the collapse pressure. This study is aimed at modeling the ovality of a selected subsea pipeline and investigating its effect on the circumferential pressure distribution on the internal and external walls of the pipes using FEA. The collapse pressures of the pipeline are determined as a function of its outer diameter-to-thickness (D/t) ratios as well as the ovality. The type of ovality considered is full ring with 3%, 5% and 7% ovality. The FEA is performed using the Abaqus™ (ver. 6.12) software package and the collapse pressure for six D/t ratios between 30 and 80 (at a stepwise increase of 10) are determined based on the von Mises yield criterion of AISI 1009 carbon steel. Simulation results indicate that the collapse pressure reduces with the D/t ratio for all percentages of ovality. It is also observed that greater ovality gives smaller values of collapse pressure. For example, the collapse pressures of the pipe with D/t = 30 are 9.08 MPa, 8.64 MPa and 7.91 MPa for 3%, 5% and 7% ovality of the full ring type, respectively. The simulation results are compared against analytical results obtained using relevant formulas from two standards, i.e. BS 8010-3 and API 1111. The discrepancies between the simulation and API 1111 analytical results reduces for pipes with D/t = 60 and higher. The lowering of the collapse pressure would lead to unpredicted failure of pipelines if the effect of initial ovality in the pipe is not considered in the geometrical model for FEA. It is therefore imperative that ovality of pipes be kept to a minimum. Buckle arrestors may be placed on the pipe to limit the effect of ovality on the collapse pressure of the pipe.