Evaluation of Performance Models for Wet Gas Compressors
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The Norwegian petroleum industry has played an important role in the country’s industrial development since the early 1970s. At present, however, the largest oil and gas fields on the Norwegian continental shelf have been developed. To maintain its position and production rates in the future, the petroleum industry rely on improved recovery from existing fields and the development of smaller, remoter and more technically complicated discoveries. A major contribution to accomplishing this will be made by the development of new and cost-efficient technology. One consequence of technological progress in recent years is that a greater proportion of the processing equipment is being installed subsea. Ormen Lange and Snøhvit are gas fields developed on the basis of subsea solutions, where wellstream processing is located on land. These discoveries are characterised by high gas fractions with relative small amounts of liquid. Gas containing less that 5% liquid on a volume basis is denoted wet gas. To maintain production rates as well pressure decreases, boosting the wellstream is essential. This has therefore created a big incentive in recent years to develop subsea boosting technology. Subsea compression is also applicable for boosting existing gas fields to increase tail production and in development of marginal fields where subsea boosting can be utilised in transportation of wellstream to nearby processing facility. Technology development for subsea compression is currently based on two alternatives: compressing the unprocessed wellstream and subsea separation with compression based on traditional gas compressors and pumps. This thesis evaluates performance prediction models for wet gas compression. No commercial available wet gas compressors exist at present, but several concepts for such equipment have been tested. No performance and test standards currently exist for wet gas compressors. To ensure nominated flow under varying fluid flow conditions, a complete understanding of compressor performance is essential. Present evaluation methods for compressor performance fail when evaluating the compression of wet gas. The development of a wet gas compressor performance model has been based on the polytropic approach given in ASME and ISO standards, and a full review of this method is given. Assuming a polytropic compression path, the method is based on averaged gas properties of inlet and outlet condition. Depending on how this polytropic compression analysis is implemented, the review has revealed up to 4% deviation in polytropic head and efficiency for some specific compressors. This adds an extra uncertainty in compressor performance verification. Even though the API 617 allows up to 4% deviation, some compressors have to meet a more stringent demand, such as 2% at the Snøhvit gas liquefaction plant. Existing computer technology permits a direct integration of the compression path where the variation in actual gas properties along the path is included. This method eliminates the averaging of gas properties which the Schultz procedure includes. Direct integration is of special interest when evaluating wet gas compression. Phase changes along the compression path are included, enabling a detailed prediction to be made of the actual volumetric flow rate for the various compressor stages. This thesis reports the implementation of the direct integration procedure for wet gas performance prediction. The procedure enables generic wet gas compression to be studied, which forms the foundation for performance analysis with variations in operation conditions and fluid properties. The polytropic procedure in its current form does not permit a direct comparison of wet and dry gas conditions owing to the influence of a high-density liquid phase in wet conditions. Correction methods to account for these changes are presented. These correction parameters are verified against the results from a wet gas compressor test. The compressor was tested on fluid conditions typically encountered in a North Sea gas and condensate field. The main contributions of this work are presented in four international papers contained in the appendix.