Good question recently from Josh Wickham of GPA. He’s not the first to ask this and there have in fact been a number of official requests for interpretation from the Standards Australia committee ME-038 (responsible for AS 2885 ). Josh’s question relates to design of scraper traps for which the client has already specified the materials, which happen to be linepipe for the barrels:
My query is what dictates the toughness requirement for these vessels, and does the fracture control plan (and hence the toughness requirements) then apply to this application? AS 2885.1 is vague on this matter – clause 4.8.2 (with amendment 1) states “the fracture control plan shall only apply to the linepipe as defined in fig 4.1. It shall not apply to accessories.” Accessories doesn’t appear to be defined – but figure 4.1 shows scraper facilities and main line valves as part of the ‘pipeline’ and therefore infers assemblies are part of the fracture control plan.
Quite right that there is some unfortunate wording in the standard here – it’s one of the things that needs attention in the next revision.
The short answer is that no, the fracture control plan is not meant to apply to pipeline assemblies. Note that “pipeline assembly” is a defined term, although it is defined in Clause 5.9.1 rather than the Definitions section:
Pipeline assemblies are elements of a pipeline assembled from pipe complying with a nominated Standard and pressure-rated components complying with a nominated Standard or of an established design and used within the manufacturer’s pressure and temperature rating.
So that includes mainline valves and scraper station piping, and a variety of other things ranging from simple branch connections up to slug catchers (if you wish to design them to AS 2885 rather than a piping code).
Josh goes on to ask:
… I would have expected that given that the intent of fracture control design is to select sufficiently tough materials to arrest crack propagation, and the fact that the connection between the vessel and the pipeline is a flanged connection, then a crack initiating on the vessel side would be arrested by the flange? If this is the case – then should the fracture control plan really apply?
This is definitely on the right track. Why do we need fracture control plans? Mainly to control propagating fracture. Clause 4.8.1 defines four pipeline failure modes: leak (without rupture), rupture (full bore opening, but arrested), propagating brittle fracture, and propagating tearing or ductile fracture. Just to be quite clear, propagating failures in the past have run for hundreds of metres, unzipping the pipe and completely opening up the trench over that distance. Generally a very nasty form of failure.
Does that sound as if it’s relevant to a scraper trap? Clearly not, nor to any other type of pipeline assembly (unless perhaps you have a really long slug catcher).
Josh continues with a third question:
If the fracture control plan does not apply, but the client would like to maintain the design temperature rating of the scraper vessel…could you then use the approach of the pressure piping standards – AS 4041 (section 2.11) or ASME B31.3 (section 323.2.2) which give guidance on impact testing requirements to meet a minimum design temperature (using graphs based on material wall thickness etc)?
At this point I’m going to stop expressing opinions because I’m not a materials engineer and will rapidly get out of my depth. However the approach suggested by Josh sounds entirely reasonable. Comments welcome from those with more expertise!