FRACTURE CONTROL - OFFSHORE PIPELINE CURRENT STATUS OF FRACTURE ASSESSMENT FOR PIPELINES LIMITATIONS AND THE NEED FOR DEVELOPMENT

摘要

Recent development plans envisage the exploitation of very deep offshore reservoirs as well as transport of hydrocarbons at temperature and pressure conditions far more severe than in past projects. Technical feasibility of such projects requires higher material utilisation, and the design guidelines need to be improved to allow for the new design conditions. Fracture assessment methods have been used in the evaluation of pipeline integrity for several years. In particular, the verification of acceptable defect sizes for installation and operational loads are now widely used and assessment methods are referenced in pipeline standards and guidelines. However, design guidelines are still missing the calibrated safety factors and stringent design format required to let the fracture failure mode be consistent with the other failure modes in the pipeline design such as bursting, local buckling and fatigue. The Fracture Control Offshore Pipelines Project is a Joint Industry Research and Development Project, whose objective is to study the behaviour of defected girth welds in pipelines subject to construction and operational loads ever experienced before. Due to the envisaged high loading condition and the high costs of recent offshore pipeline projects it is important, with an accurate defect assessment analysis, to avoid delays caused by unnecessary repairs or failures because of flaws that should have been detected and repaired. The final objective is the development of specific design criteria in the form of a design guideline to be used in the verification and design of offshore pipelines against the fracture/plastic collapse failure of a defected girth weld. The design criteria are based on the application of reliability methods to calibrate the partial safety factors in compliance with the safety philosophy established by DNV OS-F101 and will include the rational application of new NDT techniques. The JI Project is carried over 5 years and has started in 2002. The JI project is sponsored by the industry (BP, ENI Norge, Hydro and Statoil) and by the Norwegian Research Council. This paper describes the current status of existing fracture assessment procedures for pipelines with particular attention to their limitations and the needs for development and a brief overview of the results obtained in the project so far as well as the challenges to be solved in the project.