For decades, the practice of separating the tasks of production engineering and process engineering has given rise to the well-known distinction between the upstream and downstream petroleum technologies. Process engineers base their input data on the results of the production engineering analysis and strive to make adjustments in facility operations to meet the field production requirements. On the other hand, production engineers are mainly concerned with maximizing production, often ignoring the operational constraints of a gas processing facility. Often, the sole means of creating a buffer zone that interfaces with the downstream plant equipment is the working volume of surface storage facilities that admittedly offers a limited degree of freedom. Hence, the necessity of a computational tool that dynamically integrates reservoir performance, well deliverability, surface network analysis and process engineering becomes imperative as optimization of production imposes itself as an indispensable condition especially in view of recent high oil prices.
To illustrate the application of this integrated modeling approach, a floating Liquefied Natural Gas (LNG) facility is examined as a case study. The study investigates how the output of the LNG plant is affected when a series of decisions are made regarding various aspects of production (e.g. tubing sizes, choke pressure values, flow rates, separator pressure etc) and vice versa, for a desired plant feed stream, which upstream quantities are amenable to manipulation. The conclusions can be used to identify potential system bottlenecks, to properly size downstream plant equipment and to swiftly ameliorate final production. Furthermore, an optimization is carried out to maximize production at the most economical cost.
Historically, offshore gas fields located at long distances from the marketplace have remained undeveloped due to high transportation costs. However, increasing demand for natural gas combined with lowered processing and transportation costs associated with improvements in LNG technology and infrastructure have renewed interest in these fields1–3. Field development economics are made more attractive by the prospect of consolidating the process and storage facilities as close as possible to the well locations, just as FPSO (Floating Production Storage and Offloading) vessels have been employed to process and store liquids produced from remote offshore oil fields. Still, with FPSO's, associated gas production is typically flared, as gas storage requires volumes far exceeding the capacity of conventional facilities. To store the gas on floating facilities, liquefaction is required, with the product then transported via shuttle tankers to onshore terminals.
While a floating LNG plant has yet to be commissioned in the field, a number of conceptual design projects have been proposed1. One of the key challenges unique to an offshore floating facility is the requirement to accommodate process equipment and facilities in a very limited deck space. For example, the processing capacity of a floating LNG facility may be subject to constraints including liquids handling capacity, operating ranges of dehydration units and scrubbers, and power availability to the compressors used in the refrigeration process. The capacity of the plant will also be affected by the composition and arrival temperature of the fluids.