As a worst-case scenario, as well as a means to easily incorporate the intuitive belief that large diameter can mean higher consequence, pipe failures can be modeled as having opening (hole) areas equal to the cross-sectional area of the pipe—a guillotine rupture. This provides a consistent way to compare the maximum hazard zones from equipment of varying sizes and operating pressures. However, a rupture is a very rare event and can lead to over-conservatism and associated misunderstandings of true risk. It will also not recognize the differences that influence hole size and therefore will not ‘reward’ those components less susceptible to large hole size and ‘punish’ those that are more susceptible.
Including, in the assessment, the various potential hole sizes adds more robustness and realism to the analysis. The leak size probabilities—derived from hole size and other factors—can offset a consequence potential that would otherwise be modeled as being higher. For example, a smaller diameter line that is more prone to rupture can exceed the consequence potential of a larger line that is vulnerable only to small leaks. So, the larger line may actually carry less consequence potential.
The hole size is related to the failure mode, which in turn is a function of pipe material, stress conditions, and the failure mechanism. Failure modes can be categorized in different ways, such as: pinhole, large holes, ruptures; tearing, cracking, etc. Interrelationships among many factors determine the likely type of pipeline leak/rupture (hole size) for any failure scenario.
One intent of including hole size in estimating consequence potential is to identify components more likely to fail in a catastrophic fashion. Material toughness, including the implications of joints which may have greatly reduced toughness equivalents, is a key determinant of catastrophic failure potential in some scenarios. Where pipe material toughness is constant, changing pipe stress levels or initiating mechanisms will discriminate components more susceptible.
As an extreme example of catastrophic failure mode, an avalanche failure is characterized by rapid crack propagation, sometimes for thousands of feet along a pipeline, which completely opens the pipe, sometimes violently launching fragments into the air. (See discussion under Cracking). A crack will move at the speed of sound through a material. If the crack speed is higher than the depressurization wave—where pressure is the driving force creating the failure stress—then cracking continues. When the depressurization wave passes the cracking location, the driving force is lost and cracking halts.
Product compressibility and the level of pressurization play a role in crack length. Less compressible products can have relatively fast depressurization speeds. In other words, on initiation of the leak, the pipeline depressures quickly with an incompressible fluid. This means that usually insufficient energy is remaining at the failure point to support continued crack propagation.
A compressed gas, due to the higher energy potential of the compressible fluid, can promote significantly larger crack growth and, consequently, leak size. This is because the stored energy in a compressed fluid is relatively slow to release, allowing continued pressure on a crack that is opening.
Material toughness and thickness can each reduce crack speed. Crack arrestors take advantage of this. A crack arrestor is designed to slow the crack propagation sufficiently to allow the depressurization wave to pass. Once past the crack area, the reduced pressure can no longer drive crack growth. A more ductile or thicker material (stress levels are reduced as wall thickness increases), sometimes used intermittently along a pipeline, can act as a crack arrestor.
Given this model of crack growth, main contributing factors to an avalanche failure include low material toughness (a more brittle material that allows crack formation and growth), high stress level in the pipe wall (especially when at the base of a crack), and an energy source that can sustain rapid crack growth (usually a gas compressed under high pressure).
A hole size probability distribution can be generalized from research and/or an examination of past releases. This provides insight into what hole sizes have more often been associated with what types of failure mechanisms and pipeline characteristics—ie, incident frequencies typically show corrosion causing smaller holes and mechanical damage causing larger.
While useful as a calibration tool for populations of components, care should be taken to ensure that a statistical analysis does not introduce an inappropriate bias into assessing the spill size for a specific scenario. The subject pipeline being assessed may behave in ways drastically different from the population underlying the summary statistics.
Component Materials
Material types and their various failure modes are important aspects of a risk analysis and contribute to the PoF (exposure, mitigation, resistance) and CoF assessments. While especially important in addressing the widely different materials often encountered in an older distribution systems, for example, it is also useful in addressing more subtle differences in pipelines of basically the same material but operated under different conditions. For example, a higher strength steel pipeline may have slightly less ductility than Grade B steel and, when combined with factors such as changing stress levels and crack initiators, this raises the likelihood of an avalanche-type line break.
An important difference lies in materials that are inherently prone to more consequential failure modes. A large leak area is often created by the action of a crack in the pipe wall. A crack is more likely to activate in a higher stress environment and is more able to propagate in a brittle material; that is, a brittle pipe material is more likely to fail in a fashion that creates a large leak area—equal to or greater than the pipe cross-sectional area.
Stresses
Material stress levels in a component are a main determinant in the probability of a larger hole size. Stress is often expressed as a fraction of SMYS. For many years, 30% SMYS has been used as a discrimination point between leak and rupture. This level changes as defect size increases, with large defects susceptible to generating large failure areas at low stress levels. This is not a hard rule however. While rare, ruptures at lower stress levels have also been documented.
Initiating mechanisms
The role of initiating mechanisms in failure potential is discussed in . Their role in influencing hole size is briefly noted here.
Shorter defects under less stress tend to fail as leaks. As defects get longer and stresses increase, rupture becomes more likely. Weld seam anomalies, which can be relatively long, often fail as ruptures.
Damage type is another consideration a failure mechanism such as corrosion is often characterized by a slow removal of metal and is often modeled as producing smaller leak sites, whereas cracking and third-party damage initiators often have a relatively higher chance of leading to large opening.
GRI report
GRI-00/0232 Leak Versus Rupture Considerations for Steel Low-Stress Pipelines
Results indicate the leak to rupture transition for corrosion defects in the low-wall-stress pipeline system can be taken as 30 percent of SMYS, a value that is conservative in comparison with in-service incidents. Work on the threshold for delayed mechanical damage is incomplete, but is currently taken to be 25 percent of SMYS. The main conclusion of the study is that thresholds for the transition from leak to rupture are consistent with the current regulatory provisions for low-wall-stress pipelines.