Content of PetroWiki is intended for personal use only and to supplement, not replace, engineering judgment. SPE disclaims any and all liability for your use of such content. In the context of subsea, a crown plug is a plug that fits in the bore of a subsea tree to serve as the secondary barrier against reservoir pressure.
Many hundreds of subsea wells are currently in service worldwide. Subsea wells may be installed individually, in clusters, or on a template where the reservoir fluids from all the wells are channeled to a manifold that is tied back to a host platform. A simple template arrangement is shown in Figure 1. Often wellheads and wet trees are designed as "diverless" and more recently "guidelineless" because they can be installed, maintained, and repaired either by remote control using equipment that does not need guidelines or tools that are wire guided from a vessel. Figure 1 shows a single-well diverless subsea production system.
How these and related factors affect subsea processing design are discussed below. The value of subsea processing is determined primarily by reservoir characteristics and water depth. Well productivity index (barrel per psi drawdown or PI), which is a function of reservoir permeability, is one of the keys. A high PI will leverage the reduced backpressure provided by subsea processing to higher production rates. This can have enormous economic implications for low-pressure reservoirs in deep water.
As oil and gas production moves into deeper water, the cost of surface production platforms becomes prohibitively high. The industry has found that surface facilities must be kept to a minimum and shared by satellite fields to be commercial. Subsea processing is a key toward a cost-effective, "hub-and-spoke" development (Figure 1), allowing the industry to operate successfully in deeper water. Subsea processing refers to the separation of produced fluids into gas and liquid--or gas, oil, and water--for individual phase transport and disposal (in the case of water). The liquid stream can be pumped to a central facility for final processing.
In natural settings, such as the ocean bottom, when buried organic matter decomposes to methane and dissolves in water, clathrates form at temperatures greater than 277 K (4 C or 39 F). Biogenically produced methane in dissolved water forms hydrates very slowly, because of mass-transfer limitations. Over geologic time, the total enclathrated methane in the oceans has been estimated at 2.1 1016 standard cubic meters (SCM)--twice the energy total of all other fossil fuels on Earth. The amount of hydrated methane in the northern latitude permafrost is relatively small (7.4 1014 SCM), within the error margin of ocean hydrate estimates. Figure 1 shows world hydrate deposits in the deep ocean and permafrost, most of which were determined by indirect evidence such as seismic reflections called bottom simulating reflectors (BSRs).
Flow assurance, by definition, focuses on the whole engineering and production life cycle from the reservoir through refining, to ensure with high confidence that the reservoir fluids can be moved from the reservoir to the refinery smoothly and without interruption. The full scope of flow assurance is shown in Figure 1. Flow assurance matters specific to subsea tieback systems are shown in Figure 1. Flow assurance is sometimes referred to as "cash assurance" because breakdown in flow assurance anywhere in the entire cycle would be expected to lead to monetary losses. A few specific flow assurance issues are discussed next.
Subsea processing using subsea separation and pumping technologies has the potential to revolutionize offshore oil and gas production. When combined with relatively mature subsea production technologies (see subsea chapter on well systems, manifold, pipeline, power and control umbilical, and so on), it can reduce development cost, enhance reservoir productivity, and improve subsea system reliability and operability. Over the period from 1970 to 2000, millions of dollars have been spent to develop subsea separation and pumping systems. But because of unresolved technical issues, along with a lack of confidence and clear understanding of the costs and benefits, industry has not rushed to deploy the technology on a commercial basis. However, as the industry moves into remote deep and ultradeep water, various degrees of subsea processing are becoming more common. In deep water, the technology can enable hydrocarbon recovery from small reservoirs that are subeconomic by conventional means, ...
Subsea processing using subsea separation and pumping technologies has the potential to revolutionize offshore oil and gas production. Between 1970 and 2000, millions of dollars were spent to develop subsea separation and pumping systems. But because of unresolved technical issues, along with a lack of confidence and clear understanding of the costs and benefits, industry did not rush to deploy the technology on a commercial basis. However, as the industry has moved into remote deep and ultradeep water, various degrees of subsea processing are becoming more common. The benefits of subsea processing have been recognized for several decades.