Minimising the risk of catastrophic failure
Risers are made by welding multiple lengths (approximately 12m long) of linepipe together, either in the field or long before installation (in the latter case, they are reeled onto a spool and reeled off at the point of use). Whichever installation method is used, the structural integrity of the girth (circumferential) weld is of particular significance, since girth welds are subjected to cyclic (fatigue) loading and are typically uninspectable once installed. Leakage or failure of a riser could, of course have disastrous consequences for human safety, for the environment, and for the
cost-effectiveness of the field.
Although the weld (an example of which is shown in Figure 2) is made by an automated process and to a very high quality, it should be recognised that any welding process has the potential to introduce flaws, and the ability of non-destructive testing (NDT) to detect, size and, if necessary, reject such flaws needs to be critically considered. In practice, failure of the girth welds is avoided by careful attention to the quality of materials, welding and inspection, backed up by an ECA, which is a quantitative analysis of the relationship between flaw size, materials properties and the loading on the riser. In this way, it is possible to ensure that flaws that might escape detection during NDT will not grow large enough to lead to failure of the riser during its design life, as illustrated in Figure 3.
Working in accordance with the relevant standard
The calculations for this case were carried out using BS 7910 (‘Guide to methods for assessing the acceptability of flaws in metallic structures’). This standard, backed up by substantial validation data and recognised by both industry and safety authorities, provides the means to carry out complex ECA calculations in a reproducible manner. The document contains a range of information, including stress intensity and reference stress solutions for cracked bodies (including pipes), fatigue crack growth data, residual stress profiles and information on inspection reliability. This allows the analyst to carry out an assessment with minimal reference to sources outside it, although information such as applied stresses and materials properties (fracture toughness and tensile properties) are, of course, generated specifically for the project.
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