Bending of X65 Offshore Steel Pipes

Abstract

This thesis is part of an ongoing research program between SIMLab and Statoil about impact loads on X65 offshore pipelines and it is a continuation of previous work.Offshore pipelines are frequently impacted by accidental loads, e.g. trawl gear or anchors. Such loads may cause severe damage to the pipe and a complex stress-strain history locally in the impacted area.Fracture have previously been found in pipes dynamically impacted. Quasi-static bending of similar pipes with the same boundary conditions have been conducted as part of this thesis. The pipes were examined by metallurgical investigation afterwards, but no sign of fracture was found.Finite Element Analysis (FEA) of the experiments has been carried out to investigate if it is possible to determine fracture by using the Cockcroft-Latham fracture criterion. To carry out analyses on a highly refined mesh, it has been investigated if it is possible to utilize a technique known as submodeling. The analyses did not succeed in predicting fracture.Offshore pipelines are often pressurized and this influences the stress-strain history when impacted. As a continuation of previous work it has been conducted quasi-static bending of pressurized and axially loaded pipes. The internal pressure reduced the amount of denting in the impacted zone and increased the stiffness of the pipe with respect to transverse loading.FEA of the experiments has been conducted to investigate if it is possible to recreate the application of internal pressure and horizontal loading. The force-response and deformation of the pipe was found to be described quite well. Horizontal loading had minor importance on the plastic deformation locally in the impacted zone.Offshore pipelines are often pressurized and this influences the stress-strain history when impacted. As a continuation of previous work it has been conducted quasi-static bending of pressurized and axially loaded pipes. The internal pressure reduced the amount of denting in the impacted zone and increased the stiffness of the pipe with respect to transverse loading.FEA of the experiments have been conducted to investigate if it is possible to recreate the application of internal pressure and horizontal axial loading. The force-response and deformation of the pipe was found to be described quite well. Horizontal axial loading had minor importance on the plastic deformation locally in the impacted zone.