The transport of hydrocarbon fluids in a multiphase manner has now become commonplace in both onshore and offshore pipeline transportation systems. Although respectively necessitated by different business drivers, the practice creates an additional dimension of complexity when it comes to the design and operation of such systems. The paper aims at presenting the current state of play in technology that is available to the pipeline engineer to design pipelines that will, at the onset or during later life, be operated under multiphase flowing conditions. It will attempt to provide a roadmap of what technology to use to address the various challenges of hydrodynamic as well as thermo-chemical flow assurance. In particular, phenomena will be addressed that include wax deposition, hydrate and asphaltene formation, terrain induced and severe slugging. Two-phase and three-phase transport will also be addressed.
The analysis of multiphase flow phenomena is not a novelty by any means. Early analyses, [1], [2], [3], [4], and [5] focused on understanding the hydraulic behavior of the mixed fluids as they traveled in pipelines of various configurations, be they horizontal or inclined. The behavior, in terms of pressure drop and liquid hold-up (a measure of the amount of liquid in a particular pipeline cross-section on an instantaneous basis), was characterized in terms of appropriate equations that an engineer would employ providing data input that related to fluid properties, pipeline configuration, and flowrates for the respective fluid phases. The equations incorporated empirical terms that derived from specific experimental configurations and, therefore, lacked ubiquity. To this day, flow analysts must exercise caution when using such formulae to ensure appropriateness and applicability. The economics of offshore exploration in the early eighties drove the evolution of production systems incorporating satellite wells tied-back to a host platform - Figure 1. Such systems enabled production from off-set well locations using advanced sub-sea systems - Figures 2, and 3, without the necessity for additional, and substantial, infrastructure to process the fluids. The satellite production approach does, however, necessitate that the fluids be transported in a commingled multiphase manner and, as a result, delivered a new dimension in operational complexity as will be discussed in this paper.
The early nineties saw exploration and production traverse into deepwater (>1500ft WD) and, with the discovery of reserves in commercially viable quantities, the challenge evolved to determining the economic feasibility of developing the field(s) and transporting produced hydrocarbons to shore and on to refineries for processing. The cornerstone of such feasibility became what is now termed flow assurance defined as production operation that generates a reliable manageable and profitable flow of fluids from the reservoir to the sales point. Being able to assure the flow of production generally falls into two categories of challenge, namely: Hydrodynamic Chemical - Thermodynamic
The laying of pipelines and flowlines over undulating terrain results in pipeline configurations that incorporate inclined sections of pipe as well as dips. Both these elements result in flow pattern complexity when it comes to multiphase flow systems.
