I realised recently that my understanding of pipeline fracture control had become out of date; in fact it was probably never up to date. Fracture control is a difficult topic. The main concepts are deceptively simple but as soon as you get into the detail it becomes complex and potentially confusing, and many pipelines require the level of detail that opens up that scope for confusion. The situation is not helped by the fact that a proper fracture control plan is required only infrequently (how many new pipelines do you design every day?) so it is not a routine activity.
I’m pretty sure I’m not alone in struggling to get my mind around a lot of this stuff. In fact I have feeling that very few people indeed have a confident understanding but few want to admit that they struggle with it (including me). So it seems a suitable topic for a series of posts. I’m taking a risk of exposing myself as lacking expertise here, but I’m sure someone will correct me if I get anything seriously wrong and perhaps we will all get a better grasp of this infuriating subject.
I’ll start with the real basics, perhaps well-known to almost everyone but worth setting out anyway. There is not much below that is not in AS 2885.1, but expressed somewhat differently.
Steel is usually ductile, but not always. I was alerted to that at an early age when my father (who was a naval architect) talked about the WWII liberty ships, mass-produced cargo carriers that suffered brittle fractures in the cold North Atlantic. Wikipedia says about 2700 of these ships were built and 1500 suffered cracking problems including 12 that snapped in half without warning. So the first step in preventing fracture is to ensure that the steel remains ductile at operating temperature, but there is a lot more to it than that.
If a pipeline suffers a some insult that causes a through-wall defect (hole or crack) there may be three outcomes depending on the length of the defect, the operating conditions and the properties of the pipe:
- Leak, when the defect remains stable in size.
- Rupture with arrest, which occurs when the defect is too large for the remaining steel to resist the hoop stress and the pipe peels open for its full diameter but the fracture does not propagate very far.
- Propagating fracture, when the rupture is not arrested but the crack continues to run for a considerable distance, up to many kilometres in the worst cases; propagating fractures may be either brittle or ductile.
(I have already glossed over through-wall versus part-wall defects but for the time being let’s keep things simple.)
The objectives of fracture control for pipelines include:
- Maximise the likelihood that a defect will remain stable and not progress to a rupture; this is achieved by providing adequate toughness in both the pipe body and the longitudinal weld (fracture initiation control).
- Ensure that brittle fracture can never occur by providing sufficient toughness in the steel at the minimum temperature the pipe will encounter (brittle fracture control); the drop weight tear test is used to assess whether fracture at the minimum design temperature is brittle or tearing (ductile).
- Ensure that a ductile fracture will be arrested rather than propagating, again by providing sufficient toughness (tearing fracture control); toughness for tearing fracture control is assessed by the energy absorbed in the Charpy impact test.
In future posts I hope to address some of these areas in more detail.