Directions: Each passage in this section is followed by a group of questions to be answered on the basis of what is stated or implied in the passage. For some of the questions, more than one of the choices could conceivably answer the question. However, you are to choose the best answer, that is, the response that most accurately and completely answers the question, and blacken the corresponding space on your answer sheet.
Oil companies needs offshore platforms primarily because the oil or natural gas the companies extract from the ocean floor has to be processed before pumps can be used to move the substances ashore. But because processing crude (unprocessed oil or gas) on a platform rather than at facilities onshore exposes workers to the risks of explosion and to an unpredictable environment, researchers are attempting to diminish the need for human labor on platforms and even to eliminate platforms altogether by redesigning two kinds of pumps to handle crude. These pumps could then be used to boost the natural pressure driving the flow of crude, which, by itself, is sufficient only to bring the crude to the platform, located just above the wellhead. Currently, pumps that could boost this natural pressure sufficiently to drive the crude through a pipeline to the shore do not work consistently because of the crude’s content. Crude may consist of oil or natural gas in multiphase states—combinations of liquids, gases, and solids under pressure—that do not reach the wellhead in constant proportions. The flow of crude oil, for example, can change quickly from 60 percent liquid to 70 percent gas. This surge in gas content causes loss of “head”, or pressure inside a pump, with the result that a pump can no longer impart enough energy to transport the crude mixture through the pipeline and to the shore.
Of two pumps being redesigned, the positive-displacement pump is promising because it is immune to sudden shifts in the proportion of liquid to gas in the crude mixture. But the pump’s design, which consists of a single or twin screw pushing the fluid from one end of the pump to the other, brings crude into close contact with most parts of the pump, and thus requires that it be made of expensive, corrosion-resistant material. The alternative is the centrifugal pump, which has a rotating impeller that sucks fluid in at one end and forces fluid out at the other. Although this pump has a proven design and has worked for years with little maintenance in waste-disposal plants, researchers have discovered that because the swirl of its impeller separates gas out from the oil that normally accompanies it, significant reductions in head can occur as it operates.
Research in the development of these pumps is focused mainly on trying to reduce the cost of the positive-displacement pump and attempting to make the centrifugal pump more tolerant of gas. Other researchers are looking at ways of adapting either kind of pump for use underwater, so that crude could be moved directly from the sea bottom to processing facilities onshore, eliminating platforms.英语作文
【在百度搜索更多与“美国法学院入学考试LSAT阅读理解真题19(答案)”相关英语作文】