Some subsea installations have a pipeline end manifold (PLEM), which connects a flowline with another subsea structure or joins a main pipeline with a branch pipeline. The PLEM can incorporate tie-in points with other components such as isolation valves, diverter valves and sensor arrays. Some PLEM designs incorporate facilities for launching pipeline pigs—devices used to clean or monitor the inside of a pipeline.
When a reservoir does not have sufficient energy to produce the fluids from one subsea component to the next, a subsea boosting pump may be installed. Boosting pumps function as a seafloor artificial lift system, increasing both flow rate and recovery by reducing backpressure on the reservoir.
Other recent advances in subsea processing are used to enhance field economics. Seafloor separation and reinjection of produced water can alleviate constrained topside water handling capacity while supplementing reservoir energy through waterdrive. Subsea gas compression, including wet gas compression, can improve viability of certain marginal developments.
Flow to the Surface
Flowlines tie one or more fields back to a production facility—a shore-based processing facility or fixed production platform in shallower waters—but in deeper waters, a semisubmersible, spar and floating production, storage, and offloading vessel (FPSO) is used. The flowlines do not necessarily trace a straight course from wellhead to platform but may bend to avoid obstacles such as existing subsea infrastructure or natural obstructions such as underwater seamounts or canyons. As it follows the topography of the seafloor, the flowline climbs gradually from the colder, deeper reaches of the field upward through relatively warmer, shallower waters before reaching the production facility.
Water depth affects temperature, which can adversely impact flow between the subsea tree and the production facility. Upon exiting the well-head, warm produced fluids may encounter deepwater temperatures approaching 2°C [36°F] at the seafloor. Heat transfer between the produced fluid in the pipeline and the surrounding seawater can cool the fluid to the point that gas hydrates start to form. The change in fluid temperature beyond the tree influences the operator's thermal management strategy. At some fields, chemicals such as methanol [CH3OH] or monoethylene glycol [C2H6O2] are injected into the system to keep the wellstream flowing then recovered on the surface and reused. Some operators use electrically heated flowlines; others use foam-insulated pipe. Some operators bury the flowline beneath the seafloor for insulation, but flowlines at certain fields require no additional heat or insulation at all. The chemistry and rheology of the produced fluids ultimately dictate which methodology is adopted.
Production flowlines run from the manifold to structures that are linked to risers that direct the flow to the production facility. Risers transport produced fluids from the seafloor to the surface production facility. Like flowlines, many risers are insulated against cold seawater temperatures. They offer a measure of flexibility to withstand subsurface water currents or movement of the floating facility.
The surface processing facility provides power, control, communication and chemical injection services back to the subsea production system. These services are transmitted through a subsea distribution system using umbilicals. Multiple steel and thermoplastic conduits are often bundled together with hydraulic lines, chemical injection lines, power conductors and fiber-optic cables to form a single integrated umbilical (Figure 2). These flexible conduits require sophisticated materials and manufacturing techniques to withstand deep-ocean currents, pressures and temperatures. Power conductors provide electricity for subsea equipment and system sensors. Hydraulic lines are used to open and close subsea valves. Fiber-optic lines instantly relay sensor information and control commands between the sea-floor and the surface. Some umbilical lines pump chemicals into the production stream. Umbilicals directly or indirectly service nearly every component in the subsea production system and are critical to operating the field. The lines typically run from the surface processing facility to an umbilical termination assembly (UTA) on the seafloor, from which services are distributed throughout the field.