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.

The problem investigated is the break of a high‐pressure pipeline carrying natural single‐phase gas which may condensate (retrograde) when the pressure drops. Single‐phase non‐ideal gas is assumed using a general‐ ized equation of state. Taking advantage of the choked massflow condition, the break is split into a pipe flow problem and a dispersion flow problem, both solved using a finite difference control volume scheme. The transient flow field from the pipeline break location is expanded analytically, using an approximation of the governing equations, until ambient pressure is reached and matched to the corresponding gas dispersion flow field using as subgrid model a jet box with a time‐varying equivalent nozzle area as an internal boundary of the dispersion domain. The turbulence models used for the pipe and dispersion flow fields are an empirical model of Reichard and the k–ϵ model for buoyant flow respectively. The pipe flow simulations indicate that the flow from the pipeline might include dispersed condensate which will affect quantitatively the mass flow rate from the pipeline and qualitatively the gas dispersion if the condensate rains out. The transient dispersion simulation shows that an entrainment flow field develops and mixes supersaturated gas with ambient warmer air to an unsaturated mixture. Because of the inertia of the ambient air, it takes time to develop the entrainment flow field. As a consequence of this and the decay of the mass flow with time, the lower flammability limit of the gas–air mixture reaches its most remote downstream position relatively early in the simulation (about 15 s) and withdraws closer to the break location.

Abstract. Platform-mounted shut-down valves for offshore pipeline systems, are vulnerable to fire and explosion hazards. Should this primary barrier fail, the full inventory of the pipeline could be consumed in the fire. A secondary subsea barrier would reduce substantially the loss of pipeline inventory, but also expose the platform to a new hazard. Valve failure could lead to the formation of gas or oil plumes which could be carried towards the platform. The choice of subsea barrier location, requires a trade-off between these safety concerns. The paper presents one prerequisite for such a trade-off; an analysis of the potential gas loss for different secondary-barrier positions and valve-closing rates when a guillotine break occurs at platform deck level. The paper gives a detailed study of the flow characteristics close to the valve and over the length of the segment between the barrier valve and the rupture. Particular attention is paid to the boundary conditions at the valve. These have been formulated based on the method of characteristics. Computer simulations using the simplifying assumption of isothermal flow have been compared to the adiabatic case. The differences between these cases have been found to be small. Problem Discussion. We consider the situation depicted in Fig. 1. The subsea pipeline is connected to the process equipment on the platform through a vertical segment. In the case of a fire or explosion on the platform, a possible pipeline failure would be at deck level, as indicated by the symbol R. At the broken end a counterpressure of about one bar exists. Two subsea barrier valves, one upstream at location A, the other downstream at location B, are installed to prevent a major loss of gas in case of flowline rupture. It is assumed that the barrier valves start to close at the same time as the guillotine rupture occurs. The aim of the present investigations is to determine the mass loss and flow variables in the pipeline. A good physical discussion of the pipeline break problem without valve is given by [1],[2] and a more detailed mathematical treatment is developed in [3],[4]. The length L of the pipeline segment between the valve and the broken end as well as the valve closing time TClose are considered to be two dominant parameters influencing the rate of outflow. The range of lengths L considered varies from an upper limit of 2000m to a lower limit imposed by the riser height (or ocean depth). The closing time is chosen in the technically realizable range of 40 s to 100 s. Two cases are of interest, one representing flow towards the platform (A) and one away from it (B). The pressure level will in general be higher with flow away from the platform. More important is the pressure gradient in the steady state. For Case B, the break leads to a flow reversal from the steady state direction.