In the design of pipelines, various risk evaluations are undertaken to assess the threat to adjacent pipeline, structures and the surrounding population. This paper addresses how to assess the magnitude of a partial rupture in a high vapor pressure [HVP, Y-grade (combined ethane, propane, butane from fracking) liquid] pipeline with very high Charpy energy. The methodology is applicable to pipelines containing other fluids as well, but substantial development was needed to assess the example case presented.
In this assessment it was found that a very quick arrest could occur due to the specific design variables, material toughness, and saturation pressure of the product, but the methodology to assess the length and opening area of such a partial rupture had to be developed. Based on past partial rupture pipe burst tests, the length of crack propagation first proposed by Maxey was extended to much tougher pipe cases that had more limited axial crack extension. The Maxey work had actual toughness to minimum arrest toughness values of up to ∼2, whereas the ratio for this sample case was 11. Data from a number of recent and past pipe burst tests with limited ruptures were used to further extend the Maxey correlation to cases of much shorter crack extensions during a rupture. Rupture length was determined as a function of design variables, material toughness and saturation pressure, enabling extension of this approach to a broad range of applications to either natural gas or high vapor pressure (HVP)/Y-grade liquid pipelines.
Once the rupture length is known, then the crack-opening area of the resulting rupture is needed for blowdown calculations and hazard assessments. Some data were available from nuclear piping tests in the UK by CEGB, but a significant amount of additional data were added for the determination of the opening area in the shorter rupture length region of interest.
Additionally, a small survey of service failures was conducted to determine the size of the crater from such ruptures.
Once the opening area was known, then the PipeTech© software was used to determine the static pressure at the rupture mouth, and the mass flow rate. Additional analyses were used to determine the effect of the mass flow rate on creating a dynamic pressure from the impingement of the exhausting fluid on an adjacent parallel pipeline.
For the case of an adjacent pipeline, this peak dynamic force was used in a FE analysis including the loss of soil support on the adjacent pipe from the crater created. It was determined that for cases where rupture length is limited by toughness, the loads on adjacent piping can be quite small, even with a number of conservative assumptions applied, suggesting that for the conditions explored, close spacing between adjacent pipelines can be tolerated without having to include the rupture of the smaller pipe in the hazard zone region. This analysis did not involve a thermal or debris analyses, although the opening areas calculated could be used for more precise evaluations of those concerns.