Top of line corrosion (TOLC) is typically a concern in the first few kilometres of wet gas pipelines where water in the warm gas condenses on the cold pipe walls. With the introduction of a subsea tieback to existing infrastructure, the changing fluid composition and temperature profiles may increase condensation in sections not previously expected to have condensation. Accurate prediction of the water condensation rate (WCR) becomes essential to support reliable corrosion modelling. Transient flow simulation of pipeline operating conditions and detailed heat transfer modelling is required to calculate the WCR. This calculation is complicated because the mass of water condensation is very small compared to the fluid mass in the pipeline, and sensitive to glycol that is often present in the aqueous phase for hydrate management purposes. This paper introduces a method to calculate WCR by using detailed transient modelling of the pipeline operating conditions. The fluid thermal hydraulic behaviour and hydraulic pressure drop in the pipeline are considered in the model. The fluid composition in the pipeline and glycol component in the aqueous phase are calculated by using a PVT software package. A few sensitivity studies will also be presented. The implications of Equation of State (EoS) and transient flow module on WCR calculation will be quantified. The WCR sensitivity results will be analysed based on varying inlet temperatures, glycol concentrations, and pipeline heat transfer coefficients. A WCR calculation method will be recommended for TOLC modelling.
The objective of pipeline drying during pre-commissioning is to remove residual water left in the pipeline after dewatering and desalination operations. Removing the residual water mitigates corrosion and hydrate formation and aids quicker delivery of product to required dryness. The common pipeline drying methods are vacuum drying and convection drying. The convection drying method blows dry air through the pipeline to remove the residual water. Its disadvantages are an inability to adequately dry complex-shaped pipeline networks, significant equipment footprint and expelling air noise during the convection drying operation. The vacuum drying method can achieve low dewpoints particularly for complex-shaped pipeline networks and the equipment footprint can also be smaller than for the convection drying method. Therefore, it is advantageous when facing space restrictions for equipment. This paper introduces a dynamic integrated model to simulate the pipeline drying operation. This model considers vacuum pump performance and gas saturation condition in the pipeline during the drying operation. The modelling results can be used to determine the vacuum drying suitability, predict the drying operation duration and identify opportunities to improve the pipeline drying efficiency, such as vacuum pump performance, dry gas injection and convection dry air flow rate. It also demonstrates where vacuum drying is unlikely to be feasible, i.e. low ambient temperature conditions, and methods for identifying such. An optimisation case study is also presented. The drying duration can be reduced significantly by integrating vacuum drying with dry gas injection. This combined methodology can thus significantly improve the pipeline vacuum drying efficiency, which reduces the project cost and improves and de-risks scheduled and simultaneous operations.
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