ABSTRACT

Gassco supplies Norwegian natural gas to the European market through nearly 5,000 miles of large-diameter high-pressure subsea pipelines. In 2007 3·106 MMSCF of gas were exported from the Norwegian Continental Shelf (NCS). During the winter, demand for gas usually exceeds the estimated transport capacity of the pipelines. More accurate modeling of the flow can lead to improved use of available network capacity. The pipelines in Gassco's network are typically 200 - 560 miles long, and the gas temperature is only measured at the inlet and outlet. Consequently, the calculated gas temperature along the pipeline depends on the accuracy of the assumed ambient temperature and the estimated heat transfer. This paper mainly focuses on heat transfer modeling, and how this affects the estimated gas temperature. The importance of a correct total heat transfer coefficient for different conditions has been studied, and the most important parameters associated with this coefficient have been identified. A recommendation regarding which parameters to focus on under different conditions such as different burial depths and flow rates is given.

INTRODUCTION

The Norwegian gas is transported in seven large diameter sub-sea pipelines to United Kingdom and continental Europe, covering around 15 % of the European natural gas consumption. The transportation network is operated by the state-owned company Gassco. The Norwegian export pipelines are between 200 and 560 miles long, and have diameters up to 44 inches. Pressure transmitters, flow meters and temperature measurements are only located at the inlet and at the outlet. To know the state of the gas between those two points one has to rely solely on computer models and simulators, which are very important in order to obtain optimal operation of the pipelines. The computer models are used for general monitoring of the gas transport, providing estimated arrival times for possibly unwanted quality disturbances and ‘pigs’, predictive simulations when the operational conditions changes and for transport capacity calculations. The transport capacity is usually made available to the shippers of the gas many years in advance, and accurate calculations early in a pipeline's lifetime are appreciated and valuable. High accuracy in the transport capacity calculations is important to ensure optimal utilization of invested capital in the pipeline infrastructure. The calculations need to be as close to, but not higher than, the true capacity as possible. This will ensure optimal utilization of invested capital. As soon as a pipeline is built, the true capacity is determined by the diameter, length, available inlet compression, gas temperature and other physical parameters. A lot of effort is put in to estimate this capacity figure exactly. In 2004 a research program was launched to optimize the gas transport modeling involving high flow rates, high pressures, large diameters and very low roughnesses. The R&D Foundation Polytec and Gassco have worked together for several years to optimize the gas transport modeling. In the project several subtasks have been conducted or are ongoing; to improve the friction factor correlation, viscosity measurements and implement new viscosity correlation, use of more accurate ambient temperatures and more accurate modeling of heat transfer. This paper will focus on work performed related to heat transfer. After realising that the simulation tool used in the project did not model the heat transfer for partially buried pipelines, a literature survey was conducted to identify an appropriate model. It seems that very few published articles discuss this topic [1, 2]. An analytical model [1] was identified, able to

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