The integrity of subsea pipe girth welds is routinely ensured through Engineering Criticality Assessments (ECAs) based on fracture mechanics principles. In order to capture the whole design life, the ECA must cover loadings that the pipeline is subjected to during both installation and operation.
Stress based semi-analytical methods using Failure Assessment Diagrams (FAD) have been used extensively for ‘run of the mill’ assessments as far as C-Mn steel pipe with overmatching weld is concerned. These stress based methods are captured in depth in industry codes such as BS7910 and DNV-OS-F101.
In recent years however, with industry seeking to transport more corrosive fluids, the use of pipe containing a corrosion resistant alloy (CRA) layer is becoming increasingly popular. Together with the CRA layer, anti-corrosion weld consumables material would be used at the pipe joints to protect the whole pipeline from corrosion. Some of these weld consumables, especially at high temperature, partially overmatch or even undermatch the parent pipe material, resulting in difficulties in application of conventional FAD approach. Industry has turned to numerical methods to fill in gaps which are not covered by the FAD based approach. However no codified approach/standard exists for executing numerically based ECAs.
This paper attempts to outline the FAD and numerical approaches being used by industry. The different approaches will be assessed for their range of application, limitations of execution and comparison of results obtained. The study is carried out for the case of a typical external surface flaw in a 12″ pipe.
