将以下英文翻译成准确的中文: The well consists of a wellhead, which supports the well casing in the ground, and a pod, which contains valves to control the flow and to shutoff the flow in the case of an emergency or a leak in the riser. This pod is called a “submerged Christmas tree”, or simply a “wet tree”. Subsea wells are expensive, but not as expensive in deepwater as placing a platform at the site. If a subsea well ceases to produce, or if its rate of production falls below economic limits, it is necessary to bring in a mobile drilling unit to remove the tree and perform the workover. This can be an extremely expensive operation and if the outcome of the workover is in doubt, the operator may choose to abandon the well instead. Because of this, much of the oil and gas in reservoirs produced through subsea trees may be left behind. Subsea wells may also result in lower reservoir recovery simply because of the physics of their operation. The chokes and valves placed in a subsea tree result in a pressure drop in the flow of oil or gas. When the well formation drops below a certain threshold, production ceases to flow. The difference in cut-off pressure between a subsea well and a surface well can be as much as 1000 psi vs. 100 psi [OTRC, 2002]. These facts motivated operators to seek floating platforms, which could support Christmas trees at the surface, “dry trees”. Fixed and compliant platforms were safe for this kind of production because they could protect the well casings from the environment. Floating platforms generally had too much motion to protect the wells during extreme storms. A group of engineers in California invented a floating system in the early 1970s, which could be tethered to the sea floor, effectively making it a tethered compliant platform [Horton, et al 19761. This gave rise to what is called the Tension Leg Platform (TLP) [Horton, 1987]. The first commercial application of this technology, and the first dry tree completion from a floating platform, was the Conoco Hutton TLP installed in the UK sector of the North Sea in 1984 [Mercier, et a1 1980]. Dry trees are possible on a TLP because the platform is heave-restrained by vertical tendons, or tethers. This restraint limits the relative motion between the risers and the hull, which allows for flowlines to remain connected in extreme weather conditions. The deep draft Spar platform is not heave-restrained, but its motions are sufficiently benign that risers can be supported by independent buoyancy cans, which are guided in the centerwell of the Spar. Both the Spar and the TLP designs are discussed in more detail in Chapter 7. Today, many deepwater fields in the Gulf of Mexico are being developed by a combination of surface and subsea wells. Operators are able to develop a number of smaller marginal fields by combining subsea production with hub facilities [Schneider, 2000; Thibodeaux, et a1 2002]. There is a growing trend towards third party ownership of the floating facilities, which opens the possibilities of several operators sharing production through one facility [Anonymous, 2003]. A consequence of this is that floaters may be designed with excess capacity for a given reservoir, in effect adding an “option cost” into the facility investment banking on future tiebacks from additional reservoirs.