Friction Factor in High-Pressure Gas Pipelines in the North Sea

Abstract

Field data from capacity tests of long-distance gas pipelines in the North Sea have been used to model the friction factor in internal coated pipelines. The field measurements were compared to an experimental investigation of the friction factor in coated pipes performed in a high-pressure flow loop. A good correlation between the experimental data and the field data was obtained. It was found that the friction factor in the coated gas pipelines could be modelled by the Colebrook-White correlation with an equivalent sand-grain roughness of 1.5 µm. The results showed that internal coatings effectively reduce the pressure drop in gas pipelines and thus increase the transport capacity. Introduction Internal coatings have been used with success in gas pipelines since the 1950's1. Economic studies2 show that the typical pay-back time for the investment in internal coating is 3–5 years due to improvements in pipeline hydraulics. It is well known that internal coatings reduce the friction in gas pipelines and therefor reduce the operating cost of compressors. In Norway 35 % of offshore generated power3 is used for gas export compressors. The use of coatings to reduce the operating costs is therefore important. In addition the coatings protect the pipe wall against corrosion and reduce the need for maintenance of the pipeline1. The aim of the present study was to study the friction factor in coated gas pipelines. In Norway internal coating is used in the export pipelines which transport gas from the Norwegian shelf to continental Europe. Our objective was to establish more accurate friction factor correlations for coated gas pipelines to predict the capacity of large diameter pipelines accurately. The flow in offshore gas pipelines is characterized by high Reynolds numbers (Re~1×107) due to the low viscosity and the relative high density at typical operating pressures (100–180 bar). From the classical Colebrook-White friction factor correlation4 it is seen that minute irregularities on the pipe wall will have a significant effect on the friction in the pipeline at high Reynolds numbers. However, the measurements from which the Colebrook-White correlation was developed, reached a Reynolds number of Re=1×106 as maximum, one decade lower than what is typically encountered in offshore gas pipelines. Our aim has been to investigate the influence of roughness on the friction factor in high Reynolds number flow of natural gas in pipelines. To characterize hydraulic roughness of pipelines the equivalent sand-grain roughness has traditionally been used. The concept refers to the rough pipe experiments of Nikuradse5, and as common practice the hydraulic properties of a pipeline are compared to Nikuradse's work to arrive at the equivalent roughness. Equivalent sand-grain roughness of coated pipes is also given in handbooks6,18, but the values cited vary significantly. Here, data from internally coated pipelines in the North Sea are presented, which hopefully will improve the understanding of the hydraulical properties of coated gas pipelines. In addition to presenting the equivalent sand-grain roughness of coated pipes, the study also presents directly measured values of the wall roughness of test pipes and pipeline pipes. Ultimately, it should be possible to correlate the friction factor in pipes with the directly measured wall roughness. In order to develop such correlations, the authors7 have studied the influence of roughness on the friction factor (at Reynolds numbers above 1×106) in pipes of varying wall roughness using high-pressure natural gas. This study present result from measurements of friction factors in gas pipelines and two experimental pipes.

Reconsideration of the friction factor data and equations for smooth, rough and transition pipe flow

“…Statistical parameters of (38) show that it correlates the experimental results very well and ensure that the calculations of friction factor can be done with verifiable accuracy. Furthermore, we have connected (38) with the equation for laminar flow (4) through a simple expression (39), which enables the computations in the critical zone of the flow for various relative roughness of the pipe. While (4) covers the range range Re = 2000 ÷ 35.5·10 6 and Rr = 0 ÷ 0.0333.…”

“…As a result, one can only expect to obtain accurate results using the Moody diagram if k s is known for the surface of interest and the flow is fully rough. Commercial pipes are characterized with random natural roughness, and in recent years two parameters were found to be of the special interest [29]: root-mean-square roughness height (k rms ) and skewness of the roughness elevation distribution (s k ). Recently published research such as [16,[29][30][31] show that k s is 1.5 to 6 times greater than k rms and that pretty simple correlations in form of k rms s s k f k ;…”

“…This is the reason that more than a few research teams tried to provide experimental data for Re > 10 6 . For example, in [39] it was stated that the PKN correlation for smooth pipes (8) predicts too low values of the friction factor in case of high pressure natural gas flow, especially at high Reynolds numbers. Flack and Schultz [32] commented that all recent experimental results gathered in range Re > 10 6 show that the Moody diagram overestimates the pressure drop in the transitional flow regime for honed and commercial steel pipes (from an engineering standpoint Moody diagram provides a conservative estimate of pressure drop for a given roughness).…”

“…Altshul' s equation (24) has to be modified since the exponents on Re and Rr are not equal in (29) and (33), so for transitional pipe flow the following correlation was obtained 0.22 7 . 1 Re 6 .…”

Role of polymers in the provision of cost-effective, corrosion-resistant, high-pressure pipework with complex geometry

“…Examination of natural gas pipelines that have been in use for over 20 years showed no deterioration in the coating during this time. In addition, the coatings have been shown to increase the gascarrying capacities of pipelines by virtue of reducing the friction losses in the pipe [2]. However, in the application of interest to the authors, the pipe runs are so short that any increases in capacity are unlikely to make a significant contribution to the overall flow regime.…”