ABSTRACT

During the last decades significant savings have been achieved for development of gas condensate fields by advances in the ability to transport untreated well fluid over long distances. Multiphase pipelines longer than 100 km are now not uncommon, and transient multiphase simulations have been an important factor in enabling these developments. This paper considers important elements in the modelling of gas condensate pipelines, both for steady state and for dynamic operations, such as rate changes and pigging. Among elements discussed is how to best create a pipeline profile based on the data available, the importance of the flow model, sensitivities to pipe inclination angle, flow rate and pipe diameter and the importance of boundary conditions for transient operations. The results of a set of steady state and transient are used to show the importance of proper modelling of the pipeline profile. In addition liquid handling during such transient operations as rate changes and pigging operations are discussed. The simulation results have been obtained using the transient multiphase simulator OLGA 2000 [1].

INTRODUCTION

The traditional gas condensate development would have a process facility close to the field, and the fluids would be exported through a single phase gas and a single phase liquid line. This involved expensive process facilities at remote locations, often off-shore, and multiple pipelines. As the ability to design two phase (gas and hydrocarbon liquid) pipelines advanced it was sufficient to dehydrate the fluid close to the field, and the hydrocarbons were transported in a multiphase pipeline. This gave significant saving due to a simpler and smaller process facility and a single pipeline (MLNG in Malaysia, BLNG in Brunei, Sable Island in Canada and Nam Con Son in Vietnam). The last step in the development has been the ability to design three phase pipelines (gas, hydrocarbon liquid and water). This completely removes the need for the process facility, which are replaced by simple wellhead platforms (Huldra and Troll in Norway, South Pars in Iran, Ras Laffan and Qatar Gas in Qatar and Goldeneye in the UK) or complete sub-sea developments (Mensa and Canyon Express in the US, Scarab/Saffron in Egypt, TOGI, Snøhvit and Ormen Lange in Norway). The three main issues considered when designing gas condensate systems are liquid handling, pressure drop and hydrate prevention, and in this paper the main focus will be on liquid handling. Liquid handling is important because it determines the sizing of the slug catcher, the plant liquid processing capacity and the pipeline. In addition liquid handling is closely tied to how the pipeline can be operated, thereby strongly affecting the profitability of the development. Even with proper pipeline modelling there are still uncertainties in the simulations results. The paper summarizes some of the experiences from modelling of gas condensate pipelines, strengths and weaknesses in the models, and what uncertainties can typically be expected.

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