Determination of Actual Wall Roughness Using Operational Data

The "N.V. Nederlandse Gasunie" is a company which buys, sells and transports natural gas. The company operates a gas transmission network of over 6000 miles length. The network is divided in two distinct parts: a high pressure transmission system and a low pressure system which has more the nature of a distribution network. The high pressure system has a total length of 2500 miles and is operated at pressures between 500 and 1000 psi. The diameter of most pipelines vary from 32 to 46 inch. Virtually all of the pipelines in this part of the system are internally coated. The low pressure network is operated at pressures from 180 to 500 psi and consist mainly of pipelines with much smaller diameters and generally without internal coating. For planning and dispatching purposes Gasunie relies heavily on simulation models for gas transmission. To be able to calculate the pressure drop these models require an estimate for the wall roughness of the pipelines. About 20 years ago a measurement of the wall roughness of a main transmission line has been-performed by Gasunie. This resulted in the value of about 160 microinch (4 micrometers). In order to prevent over estimating the actual transport capacity the value 490 microinch (12 mu) has been chosen as the standard value to be used for all calculations where the friction factor for coated pipelines is involved. For uncoated pipelines 735 microinch (18 mu) is used as a standard. This value is not based on our own measurements. Probably the recommended value from monograph no 9 of the American Bureau of Mines was chosen. Recently at a number of locations in the Gasunie network the measured pressures appeared to be lower than expected from model calculations, implicating that the actual transport capacity may be lower than assumed. These observations resulted in a renewed interest in the actual wall roughness of our pipelines. Internal fouling is obviously a possible cause for the decreased transport capacity. This raised questions about the effect of dry pigging on the transport capacity of a pipeline. Therefore we decided to measure the actual wall roughness on a number pipeline sections. Some of these measurements were combined with a cleaning operation. The high pressure system is equipped with an extensive telemetry system. An overwhelming quantity of measured data is available. However it appeared hard to extract reliable data for the wall roughness from this information. One of the problems is that the main part of the gas delivered by Gasunie is used for heating. The system is designed to be able to satisfy the demand at extreme low temperatures. This means that in summer time the pressure drop in the system is 'neglectable. In the Dutch climate even most winter days are so moderate that the demand is low compared to the design capacity. Therefore the opportunity for wall roughness measurements is restricted. Furthermore the possibilities to control the gasflow specially for wall roughness measurement are restricted.