During initial design stages for new pipeline projects, significant emphasis is often placed on selecting the proper line size to meet the peak production targets. While peak production is critical, often driving the entire economics of a project, defining the true operating envelope of the pipeline is a key component to the operability of the system. Assessing turndown performance from a hydraulic standpoint to ensure proper liquid handling capabilities, as well as from a thermal standpoint to ensure arrival above the wax deposition/hydrate formation region, is crucial for multiphase pipeline design. In addition, designing multiphase pipelines to accommodate future expansion, particularly in remote areas, requires a range of potential operating conditions to be considered, potentially impacting current operating/design procedures.
The term ‘flow assurance’ is one that has grown immensely in popularity over recent times, due in large part to the progress of the oil and gas industry into frontier environment. Loosely defined, ‘flow assurance’ is the analysis of the entire production system to ensure that fluids will continue to flow over the life of the field. Primarily, deepwater projects rely heavily on ‘flow assurance’ to ensure that the large capital investments that are required for these projects will result in adequate reserve recovery to result in a positive return on investment. Other extreme environments where ‘flow assurance’ issues come to the forefront are long-distance tiebacks, hub/spoke systems where new fields tie into existing infrastructure, and marginal fields. However, regardless of the environment, all multiphase pipeline system has a certain degree of ‘flow assurance’ that is required, be it at the design stage or operation stage. During initial concept feasibility screening, the choice of line sizes and insulation requirements are strongly driven by ‘flow assurance’ concerns. For oil systems, the thermal aspects which dominate in selecting a pipeline design are to avoid hydrate formation and/or wax deposition. With regards to hydraulics, terrain-induced slugging is exacerbated for largerthan-necessary pipeline diameters. Thus, designing for future capacity may make sense from a capital expense perspective, but this additional flow area may lead to severe operational problems. Likewise, gas/condensate systems are very sensitive to line size when it comes to turndown ratios, rampup feasibility, and slug catcher requirements. Multiphase pipeline design should take into account a range of steady state and transient scenarios in order to define the overall operating envelope that the system will function within. The purpose of this paper is to present a case study of two fields: Omega Oil and Gamma Gas (actual names are omitted for confidentiality reasons). The steady state and, more importantly, the transient behavior of these systems will be discussed to illustrate the role of flow assurance in pipeline design. For the Omega Oil field, the primary focus will be on the thermal aspects of the system. For the Gamma Gas field, the hydraulic behavior of the system will be evaluated.
