Effects of Pipe Internal Surface Roughness on Decompression Wave Speed in Natural Gas Mixtures

Abstract

The control of propagating ductile (or tearing) fracture is a fundamental requirement in the fracture control design of pipelines. The Battelle two-curve method developed in the early 1970s still forms the basis of the analytical framework used throughout the industry. GASDECOM is typically used for calculating decompression speed, and idealizes the decompression process as isentropic and one-dimensional, taking no account of frictional effects. While this approximation appears not to have been a major issue for large-diameter pipes and for moderate pressures (up to 12 MPa), there have been several recent full-scale burst tests at higher pressures and smaller diameters for which the measured decompression velocity has deviated progressively from the predicted values, in general towards lower velocities. The present research was focused on determining whether pipe diameter was a major factor that could limit the applicability of frictionless models such as GASDECOM. Since potential diameter effects are primarily related to wall friction, which in turn is related to the ratio of surface roughness to diameter, an experimental approach was developed based on keeping the diameter constant, at a sufficiently small value to allow for an economical experimental arrangement, and varying the internal roughness. A series of tests covering a range of nominal initial pressures from 10 to 21 MPa, and involving a very lean gas and three progressively richer compositions, were conducted using two specialized high pressure shock tubes (42 m long, I.D. = 38.1 mm). The first is honed to an extremely smooth surface finish, in order to minimize frictional effects and better simulate the behaviour of larger-diameter pipelines, while the second has a higher internal surface roughness. The results show that decompression wave speeds in the rough tube are consistently slower than those in the smooth tube under the same conditions of mixture composition and initial pressure & temperature. Preliminary analysis based on perturbation theory and the fundamental momentum equation indicates that the primary reason for the slower decompression wave speed in the rough tube is the higher spatial gradient of pressure pertaining to the decompression wave dynamics, particularly at lower pressure ratios and higher gas velocities. The magnitude of the effect of the slower decompression speed on arrest toughness was then evaluated by a comparison involving several hypothetical pipeline designs, and was found to be potentially significant for pipe sizes DN450 and smaller.

Decompression wave speed in CO2 mixtures: CFD modelling with the GERG-2008 equation of state

“…These are based entirely on assumptions of one-dimensional homogeneous-equilibrium fluid flow. In these models the effects of friction, heat transfer, and pipe diameter can be considered, which is particularly relevant for smaller diameter and longer pipelines where friction could lead to a range of complex effects on local flow conditions, temperature, and pressure within the pipeline [24,[26][27][28][29]. CFD-based techniques involve discretising the governing partial differential equations of fluid flow.…”

Investigation of the Effects of Pipe Wall Roughness and Pipe Diameter on the Decompression Wave Speed in Natural Gas Pipelines

“…The detailed experimental conditions and results have been given in Ref. [9]. The shock tube experimental results have shown clearly that the decompression wave is slowed down in a shock tube with a rough inner surface relative to that in a smooth tube under the same (or very similar) conditions.…”

“…Shock tube tests were carried out at the TransCanada Pipeline Gas Dynamics Test Facility in Didsbury, Alberta, Canada [9]. The tests were designed to understand and quantify the effects of pipe diameter and wall friction on the decompression wave speed.…”

“…However, full scale burst tests are prohibitively costly, except for major projects. Botros et al have experimentally investigated the decompression wave speed using a small diameter NPS 2 shock tube [9,10]. Phillips and Robinson have reported decompression wave speed measurements using an NPS 6 shock tube [11].…”

“… Recent shock-tube tests conducted by Botros et al [9] have shown that the decompression wave speed decreases as the non-dimensional wall roughness (/D) is increased. GASDECOM-type models cannot include the effects of wall roughness and pipe diameter on the decompression wave speed.…”