An experimental study of pressure drop in pipes at high Reynolds numbers has been carried out to determine the effect of roughness on the transport capacity of natural gas pipelines. In a high pressure flow loop the pressure drop was measured in 19.7 ft (6 m) long test pipes with 5.9 in (150 mm) inner diameter. The measurement of pressure drop of a coated pipe and a bare steel pipe showed the drag reducing effect of pipeline coating. The measurements were performed at Reynolds numbers which are typical for subsea pipelines in the North Sea (Re - 107). At a Reynolds number of 1 x 107 the friction factor for the coated pipe was 31 % lower than for the steel pipe, which corresponded to an increase in the transport capacity of 21 %. Roughness values obtained by direct measurements on the pipe wall were compared to the roughness values obtained from flow tests (equivalent sand-grain roughness &). For the steel pipe the measured roughness parameter R, (mean peak to valley height) corresponded well with,&. In the coated pipe the k, was not easily defined because the flow never reached fully rough flow conditions at the maximum Reynolds number.
Internal coatings have been used with success in gas pipelines since the 1950's [l]. Economic studies  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 the pipeline and therefor reduce the operating cost of compressors. In Norway 35 % of offshore generated power  is used for gas export compressors. The use of coatings to reduce the operating cost of compressors is therefor important. In addition the coatings protect the pipe wall against corrosion and reduce the need for maintenance of the pipeline [l]. Also, pigging operations are improved. The aim of this study was to study the friction factor of coated pipes. In Norway internal coatings are used in the export pipelines which transport gas from the Norwegian shelf to continental Europe. The object has been to establish more accurate friction factor correlations for coated gas pipelines and thus being able to predict the capacity of large diameter trunk lines accurately. The flow in offshore gas pipelines is characterized by large Reynolds numbers (Re - 107) due to the low viscosity and the relative high density at typical operating pressures 1450 - 2610 psi (100 - 180 bar). From the classical Colebrook-White friction factor correlation  it is seen that even very 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 1 x 106 as maximum, one decade lower than what is typically encountered in offshore gas pipelines.