Optimum Pipeline Inventory

The gas transport system on the Norwegian continental shelf (NCS) consists of more than 6 600 km of pipelines and is the largest offshore gas transport system in the world. In 2004, dry gas exports from the NCS totalled 77.7 billion scm . The network is operated at very high pipeline pressure, up to 210 bar, and there are few booster stations along the way. Combined with the low ambient temperature, the high pressure provides the opportunity to store much gas in these long pipelines. A major challenge is to operate the pipelines at optimum inventory. This implies a delicate balance between high pressure and appurtenant high compressor energy consumption and environmental emissions versus lower pressure and the risk of loss in gas sales. A minimum quantity of gas is required in a pipeline for the initial pressurization of the system. The quantity is defined with an assumed pressure for each pipeline, and is known as linefill. Linepack is a quantity of gas in excess of linefill, which is necessary in order to make deliveries as requested and to provide operating tolerances, not least to cope with shutdowns upstream of the pipelines. A high linepack means higher pressure into the pipeline, but it also provides high flexibility and safety. In order to find the optimum inventory level, i.e. the total amount of gas in the pipeline, simulation in a pipeline simulation software and statistical analysis are used. For a given pipeline, the minimum inventory curve as a function of the steady state flow in the pipeline is found by the means of simulation. Historical flow rate values from the past two years are analyzed, and expected changes in the flow into and out of the pipeline are found by means of statistical analysis. On this basis, a recommended inventory curve is found for both the original and the increased capacity in the pipeline. The scope of the work is to increase energy efficiency and reduce environmental emissions by developing a recommended inventory level as a function of the flow in the pipeline. This paper reports on the analysis of one of the pipelines on the NCS. It describes the method used to analyze the flow/inventory level in the pipeline and the resulting recommended inventory curve. Furthermore the paper reports on experience from operation of the pipeline after implementation of the recommended curve. Keywords: Linepack, Pipeline inventory, Pipeline optimization, Pipeline operation.