The transient analysis of multi-phase Pipelines is primarily performed for flow assurance purposes i.e. to assure the adequacy of design to transfer wellfluids economically from the reservoir to the export line over the life of a field. The technical data considered in the study as well as the stage in project life cycle at which this study is performed, both have significant bearing on many aspects of design. These can potentially impact the overall project schedule.

Generally, the transient analysis of multi-phase pipelines using OLGA software is performed during early stages of project such as FEED which lacks the level of details required for accurate prediction of transient effects. When such details are included at later stages such as EPC, the transient results are found to be quite different impacting the procurement of long lead items and thus the project schedule.

Design of equipment such as slug catchers, separators, chemical dosing packages and pumps etc, the adequacy of equipment in brownfield projects and decision on pipeline route grading especially in cases of pumped systems are dependent on pipeline transient analysis results. For example, conventionally the slug catcher is designed based on the slug volumes predicted by transient analysis during early stages (FEED) and includes some margin. However, transient simulations with more detailed pipeline profile information provide significant differences in estimated slug volumes and increased back pressures at source. The differences in results for various actual project and potential impact on project schedule are presented in this paper.

Typically, a transient simulation model includes wellhead topside piping, infield pipelines and a common manifold connection to the inlet of slug catcher to analyze the slugging behavior of wellfluid lines and the total slug volumes at the inlet of slug catcher. These simulation models incorporate the detailed geometry of all infield pipelines as per geo-physical survey data and topside piping.

Generally, the transient simulation of pipelines during FEED are not modeled with large number of XY data of pipeline route profile, as the software takes enormous time to simulate the various transient scenarios (i.e. normal operation, turndown, ramp up and pigging). However, due to the detailed geometry of pipelines and topsides employed during EPC, the estimated slug volumes / back pressures results have significant differences, sometimes as high as 50%. This may lead to bottleneck or inadequacy of slug handling capacity for existing slug catchers if not designed with adequate margin.

The objective of this paper is to highlight the significance of multi-phase pipelines transient analysis using OLGA during EPC in finalizing the design and operating philosophy of the facility and thereby impact on project schedule in procuring long lead items based on NPCC experiences.

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