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Designing a process system capable of separating oil, water, gas, and any solids, traditionally relies upon the application of sophisticated spreadsheets, based upon empirical information. The aim of the design is to meet the process operating conditions, by optimising the type, location and size of the internals within the vessel. These spreadsheet calculations provide fundamental information on the overall operating conditions within a separator vessel. However, the calculations also assume that the flow in the vessel is uniform in nature whereas, in reality, the flow field in the vessel will be strongly three-dimensional and highly turbulent.

To account for the true nature of the flow, therefore, Computational Fluid Dynamics (CFD) is used to simulate the flow conditions in the vessel, thereby providing greater detail and accuracy. These CFD predictions can be evaluated against the spreadsheet calculations to ensure the validity of the process design methods. Coupling the CFD solutions with Finite Element Analysis (FEA) of the mechanical design features of the separation internals enables the integrity of the systems to be guaranteed.

A CFD software package is simply a representation of the laws of physics in mathematical form so that the governing equations can be solved numerically. Visualisation software provides the means to interpret these results and understand the flow behaviour.

CFD enables the nature of the flow in the vessel to be illustrated so that the interaction between the separation internals and the different flow components can be understood. For example, introducing bubbles and droplets into the CFD model at any chosen position enables their trajectories to be predicted as they are transported by the turbulent flow through the vessel. Agreement between the CFD predictions and the spreadsheet results generates confidence in the design since this essentially provides an independent verification of the alternative methods.

Modelling the complex three-dimensional flow interactions with the vessel internals can provide a visual indication of the weaknesses in the process design. This greatly simplifies the trouble-shooting process.

To these ends, working closely with the client, we can offer:

  • Trouble-shooting
  • Design optimisation – reducing size and weight of separation equipment
  • Process optimisation – increasing capacity of separation equipment
  • Process optimisation – increasing efficiency of separation equipment
  • Evaluation of revamps - before and after studies
  • Anti-sloshing arrangements for Floating Production Units (FPUs)
  • Viability analyses
  • Feed studies.

To enhance our in-house CFD capabilities, Zeta-pdm purchased 100% of the shareholding of CFD specialist and consultancy company Flow Dynamics AS.

Case Studies

Slug Catcher Flow Assurance

Gas production systems may operate in the "slug flow" multiphase regime in which large volumes of liquid are interspersed with the gas. Under these conditions, the arrival of the liquid slugs can be detrimental to separation performance and the mechanical stress levels are greatly increased. In the example shown below, Zeta-pdm was asked to analyse the failure of a slug catcher vessel. Inspection showed that the slugs had caused mechanical failure of the separation internals. Subsequently, a simulation study yielded a vastly improved understanding of the flow behaviour, enabling Zeta-pdm to recommend internals better able to withstand severe operating conditions.

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